GROUNDJWATER RECHARGE HYDROLOGY

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GROUNDJWATER RECHARGE HYDROLOGY

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ARS 41-161 December 1970

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

PREFACE

This publication presents in rather general terms the current thinking on artificial ground-water re charge-what is its place in the hydrology of a basin-where can it be effective-how can it best be accomplished—what is physically required to maximize its economic value-how can its effects best be predicted?

Artificial recharge means replenishment of the ground-water storage through works provided primarily for that purpose. The source for artificial recharge is surface water in excess of current needs for which there is no surface storage available.

The engineer is concerned with problems of water control, distribution, water conditioning, and physical works required to make recharge successful. The soil scientist and geologist are interested in the control and prediction of where and when recharged waters reach the water table and their movement within it. The economist, resource manager, or attorney will be concerned in general with the control and administration of large-scale artificial-recharge projects within a ground-water basin. Even a single landholder can realize significant benefits from artificial recharge through simple procedures of water control.

Therefore, it is hoped that this pubHcation can define operational artificial recharge for this broad group of interested individuals and at least introduce the technical problems that may be encountered.

325557

ACKNOWLEDGMENTS

This publication contains considerable information and data developed at the Agricultural Research Service field station at Fresno, Calif., where laboratory and field studies of ground-water recharge have been carried on for a number of years. It is published under a cooperative agreement with the California Department of Water Resources. The State has provided substantial financial support to the ground-water research project of the Agricultural Research Service station, as well as technical advice and reviews of the project. Special acknowledgment is due to Helen J. Peters, Ground-Water Engineer; Raymond C. Richter, Supervising Engineering Geologist; and Albert J. Dolcini, Principal Engineer, Water Resources.

CONTENTS

Page

Glossary of terms v

Chapter I. Introduction: Ground-water recharge in the hydrologie cycle 1 The ground-water reservoir 1 Auditing ground-water storage 2

Inputs 2 Outputs 2 Storage 2

Artificial recharge in basin water management 2

Chapter II. The geology of ground-water basins: Relation of geology to ground-water recharge 4

Alluvial deposits 5 Eolian deposits 8 Glacial deposits 8 Fractured and porous rock systems 8

The ground-water reservoir 8 The base of the ground-water reservoir 8 Economical pumping lift and prevention of intrusion 9 Lateral Umits of ground-water storage 9 Upper Umit of storage reservoir 9 Artesian aquifer storage 10 Storage in consolidated rocks, Umestones, and volcanics 10

Geophysical methods ^0 Exploratory wells 10 Down-hole resistivity and potential logging 10 Seismic surveys 12 Gravity-meter survey 12 Magnetic surveys • • 12

Questions for the geologist 12

Chapter III. The surface and ground-water hydrology of artificial recharge: Definition of hydrology 1^ Trends in water development 14 Surface storage ^^ Surface-water conveyance 1^ Ground-water storage 1^ Ground-water flow ^^ Recharging ^0 Transfer of water to the water table 22 Questions for the hydrologist ^^

ii

Page

Chapter IV. Recharge through surface soils: Water intake rates 24

Effect of particle size and distribution 24 Effect of pore size distribution 25 Effect of soil structure and aggregation 25 Effect of chemical constituents 25 Effect of clogging and particle realinement 27 Effect of compaction and cultivation 28

Location of recharge site 28 Soil stratification 28 Soil profile exploration 28 Existing perched water tables 29

Field measurement of soil intake rates 29 Methods of assessing recharge rate:

Pilot recharge areas 30 Infiltrometers 30 Soil cores 31

Site selection vs. engineering design 31 Questions for the soil scientist 31

Chapter V. The apphcation of ground-water flow theory to artificial recharge: Need for theoretical analysis 33

Definitions 33 Heat flow vs. ground-water flow 33 Ground-water mounds resuhing from recharge 34 Specific capacity 40 Aquifer tests 43

Questions for the hydrologist 43

Chapter VI. Methods for artificial recharge 44 Basins 44 Ditches or furrows 46 Flooding 47 Natural stream channels 47 Pits and shafts 48 Injection wells 49

Chapter VII. Water quality 51 Physical characteristics affecting water quahty 51 Chemical constituents affecting water quality:

Dominant cations 51 Dominant anions 52 Other cations 52 Other anions 52

Biological factors affecting water quality 52 Water microbiology 52 Soil microbiology 53

Salt balance and ground-water recharge 53 Questions for the chemist 54

iii

Page

Chapter VIII. Benefits from artificial recharge: The problem ^^ Benefits ^^

Relief of overdraft ^^ Use of ground-water basin as reservoir and distribution system ^'

Costs Experience and data of Los Angeles Flood Control District ^^

Facilities Operational problems

Literature cited

iv

GLOSSARY OF TERMS

1. AQUICLUDE—A geologic formation so impervious that, for ail practical purposes, it completely obstructs the flow of ground-water (although it may be saturated with water itself), and completely confines other strata with which it alternates in deposition. A shale or very impervious tight clay is an example.

or An areally extensive body of saturated but relatively impermeable material that functions as an upper or lower aquifer boundary and does not yield appreciable quantities of water to wells or to adjacent aquifers.

2. AQUIFER—A permeable geologic formation that stores and transmits water.

3. AQUIFER SYSTEM-A heterogeneous body of interrelated permeable and poorly permeable material that functions regionally as a water-yielding hydraulic unit. It comprises two or more interconnected aquifers separated by laterally discon- tinuous aquitards that locally impede ground-water movement but do not greatly affect the overall hydraulic continuity of the system.

4. AQUIFUGE—A rock that contains no interconnected openings and, therefore, neither absorbs nor transmits water. A massive hard granite is an example.

5. AQUITARD—A rather impervious and semiconfin- ing geologic formation that transmits water very slowly in comparison to the aquifer. Over a large area of contact, however, it may permit the passage of large amounts of water between adjacent aquifers that it separates from each other. Clay lenses interbedded with sands, if thin enough, may form aquitards.

or A body of saturated material of relatively low permeabihty that impedes ground-water movement and does not yield freely to wells, but which may transmit appreciable water to or from adjacent aquifers and, where thick enough, may function as an important ground-water storage unit.

6. COEFFICIENT OF STORAGE, also DRAINABLE OR FILLABLE VOlE^The volume of water an aquifer releases from or takes into storage per unit

surface area of the aquifer per unit change in the component of head normal to that surface. The volume of water (measured outside the aquifer) thus released or stored, divided by the product of the head change and the area of aquifer surface over which it is effective, correctly determines the storage coefficient of the aquifer. For an ideal artesian or confined aquifer, regardless of its atti- tude, the water released from or taken into storage, in response to a change in head, is attributed solely to compressibiUty of the aquifer material and of the water. Although rigid Umits cannot be established, the storage coefficients of artesian aquifers may range from about 0.00001 to 0.001. In nonartesian or unconfined aquifers, the storage coefficient is equal to the specific yield of the material.

7. CONFINED AQUIFER (or ARTESIAN AQUI- FER)-Theoretically an aquifer in which the water is separated from the atmosphere by impermeable material. Because of its orientation in the verfical plane and overburden pressures, a well that penetrates it can have a static water level above the bottom of the upper confining bed. In reahty confined aquifers are open to the atmosphere or other unconfined aquifers and it is here that they receive their recharge. Changes in head in pumping wells result from changes in pressure within the aquifer rather than storage changes. Confined aquifers exhibit only minor changes in storage and so act as conduits from zones of recharge to those of discharge.

8. GROUND-WATER RESERVOIR-An aquifer or aquifer system in which ground-water is stored. The water may have entered the aquifer by artificial or natural means.

9. GROUND-WATER STORAGE CAPACITY-The reservoir space contained in a given volume of deposits. Under optimum conditions of use, the usable ground-water storage capacity volume of water that can be alternately extracted and replaced in the deposit, within specified economic limitations.

10. HYDRAULIC CONDUCTIVITY-The proportion- ahty constant (K) between the volumetric flow (Q) through a unit cross-sectional area (A = 1) and the loss in hydraulic head (Ah) per unit length (L) of aquifer.

11. PERCHED GROUND-WATER-Ground water sup- ported by a zone of material of low permeability and located above an underlying main body of ground-water with which it is not hydrostatically connected.

12. POROSITY-That portion of a soil or aquifer not occupied by sohd particles. It is usually expressed as a ratio of voids to total volume or as a percent by volume.

13. SOIL—The natural accumulation of mixed geologic and biologic materials on the surface of the earth in which land plants grow.

14. SPECIFIC RETENTION-The amount of water retained in a geologic formation after it has been drained by gravity. It is expressed as the ratio of the volume of water that, after being saturated, a formation will retain against the pull of gravity to its own volume. The ratio is usually given as a percentage. Specific retention in unconsoUdated materials ranges from about 5 percent in the grain size range of coarse sand to boulders, to about 30 percent in sandy clay.

15. SPECIFIC YIELD-A measure of the water drained from an aquifer by the force of gravity. It is

expressed as the ratio of the volume of water that a formation, after being saturated, will yield by gravity to its own volume. The ratio is usually given as a percentage. Specific yield in unconsoUdated materials ranges from about 2 percent in clay to 35 percent in coarse sand, gravelly sand, and fine gravel.

16. TRANSMISSIBILITY-TransmissibiMty is the rate of flow of water, at the prevaiHng water temperature, in gallons per day, through a vertical strip of the aquifer 1 foot wide extending the full saturated height of the aquifer under a hydrauUc gradient of 100 percent. Some hydrogeologists-geohydrologists have proposed the term "transmissivity" as the characteristic of the aquifer to transmit ground- water, and "transmissibihty" as the volume of water transmitted.

17. UNCONFINED AQUIFER-A water-transmitting geologic formation that is directly accessible to the atmosphere through open spaces in permeable material. The water table serves as the upper surface of the zone of saturation. This upper surface undulates in form, depending on locations of recharge and discharge, pumpage of wells, and permeabiUty, Rises and falls in the water table correspond to changes in the volume of water in storage in the aquifer.

vi

Ground-Water Recharge Hydrology' By

W. C. Bianchi and Dean C. Muckel^

CHAPTER I. INTRODUCTION

GROUND-WATER RECHARGE IN THE HYDROLOGIC CYCLE

Ground-water functions as a stored fresh-water reserve, buffering the rapid changes in the transfer of surface water within the earth's hydrologie cycle. Before man's intervention, cycHc changes in ground-water storage through natural recharge were significant in magnitude but relatively small in comparison to total storage capacity of most major ground-water basins. Man's use of stored ground-water by modern well and pumping techniques has, in a very short geologic time, gready decreased the available ground-water supplies in many areas. This, coupled with the control and diversion of surface water for irrigaUon and domestic uses, has changed patterns of natural recharge and further increased the rate of ground-water depletion.

Many ground-water basins are in a state of overdraft because of man's modificarion of the historic regional water balance. If the re-establishment or conservation of the stored ground-water of a basin is to man's socio- economic advantage, then artificial recharge is one mechanism he may wish to apply to accomphsh this. The decision must be made whether to attempt to use ground-water reservoirs as cyclic storage or to mine the stored water.

THE GROUND-WATER RESERVOIR

Ground-water storage has certain advantages over surface storage. It can have nearly perfect horizontal availability, whereas surface water requires a distribution

Soil and Water Conservation Research Division, Agricultural Research Service, U.S. Department of Agriculture.

^Respectively, soil scientist, Fresno, Calif., and Chief, North- west Branch, Boise, Idaho.

works. Subsurface storage is free from evaporation loss. Underground water may be relatively immune from degradation in the event of nuclear attack and may provide a reliable water supply in case surface water supphes are sabotaged.

Disadvantages of ground-water storage when compared to surface storage include the fact that vertical availabihty consumes energy, as the water must be pumped, whereas energy can be produced from surface storage. Ground-water storage is susceptible in the long run to chemical pollution from surface and subsurface sources of water-soluble salts and minerals. Most surface storage reservoirs are far removed from basin salt sinks and are not likely to degrade in quality while in storage, other than through evaporation and changes in inflow quality.

Most important from a water-management view is the contrast in the responsiveness of the two reservoirs to demands for storage and delivery. Rates of delivery from surface storage can be directly controlled by man and are limited only by the available water source and size of the distribution system he wishes to engineer. Delivery from ground-water storage can be increased by increasing the number of wells tapping it, but eventually the yield to the well field will be limited by the rate of ground-water transfer within the reservoir. For delivery purposes, surface storage is instantaneously available, as is evidenced by its use in flood control. However, even if a distribution system were present for recharge injection into each well in the well field, the delivery rate to storage could not exceed the delivery from storage for very long, due again to the physical limits of the underground reservoir. In other words, a surface reservoir may cycle through its entire storage in 1 year, whereas the ground-water storage of a large basin may take many hundreds of years to exchange through recharge.

AUDITING GROUND-WATER STORAGE

The first approach to the analysis of an area's water problems is to balance the hydrologie budget of a basin. This is done by setting up a simple bookkeeping analysis of water inflow, outflow, and storage. This same approach apphes to ground-water storage also.

INPUTS

Recharge is the general term denoting all inputs into the ground-water reservoir. This includes:

1. Streambed percolation. 2. Deep percolation of rainfall. 3. Subsurface inflow. 4. Deep percolation resulting from irrigation; waste

water and floodwater disposal; seepage from cess- pools, septic tanks, water supplies, and sewage conduits; discharge of industrial cooling waters and wastes; and artificial recharge.

Inputs 1, 2, and 3 are considered natural recharge. Most engineering structures are designed to conserve surface water and therefore tend to Umit or minimize natural ground-water recharge. Such measures include:

1. Lining of stream channels and concentration of surface runoff by flood-control works.

2. Discharge of sewage and industrial wastes to saline waters through closed sewage-disposal systems.

3. Sealing of natural-recharge areas with impervious sidewalks, streets, airports, parking lots, and buildings.

4. Storage, diversion, and export of local surface- runoff waters that might otherwise percolate naturally in stream channels or on the alluvial floodplains.

The surface spreading of irrigation waters, cooling water, or other wastes is considered as incidental recharge because ground-water replenishment is generally incidental to the primary function of these works. In some places, such inputs more than compensate for decreased natural recharge, as evidenced by drainage problems that develop in local areas or even large agricultural basins where there is inadequate ground- water discharge. While artificial recharge denotes a planned introduction of surface water into the ground- water storage reservoir, the procedures or methods involved can be accompUshed independently of, or in conjunction with, natural and incidental recharge.

OUTPUTS

Outflow from the ground-water reservoir includes:

1. Subsurface outflow. 2. Pumpage and deep-rooted vegetative use. 3. Irreversible changes in storage capacity; subsidence

due to extraction of fluids, sea-water intrusion, wetting of dry formations.

4. Direct surface discharge from springs, tile drainage, canal bank seepage, artesian wells, etc.

Where water is pumped for any length of time, the surface discharge of ground-water storage will disappear. The amount of subsurface flow depends considerably on the boundaries of the ground-water basin. Generally, where overdraft is present, there may be little or no subsurface outflow through physical boundaries. If this situation persists, the irreversible secondary effects of sea-water intrusion or subsidence due to extraction of fluids will gradually diminish the storage capacity. This loss of storage capacity often goes unrecognized because of the more immediate and obvious secondary effects of salty water or structural damage to surface faciUties.

STORAGE

Artificial recharge is not the only way that ground- water storage can best be controlled for the overall conservation of the water resources of a basin. Because of the large area and limited delivery rate, subsurface inflow or outflow of ground-water through a physical or political boundary can be minimized by concentrating pumping withdrawals near these boundaries. With- drawals of ground-water and its subsequent discharge into the surface distribution system have effectively relieved agricultural drainage problems in many areas.

ARTIFICIAL RECHARGE IN BASIN WATER MANAGEMENT

Artificial ground-water recharge has been used successfully as a water management tool to help meet regional water requirements. Recharge can be used to:

1. Maintain or augment the natural ground-water to preserve it as a continuing economic resource, that is, maintain or raise water levels to avoid increased water-well construction costs and pumping costs.

2. Prolong the economic use of the natural ground- water until a surface water supply is available.

3. Combat adverse conditions such as intrusion of sea water and local saline waters where caused by overdraft.

4. Provide subsurface storage for local or imported surface waters or both.

5. Provide a subsurface distribution system v^here an economy has developed on ground-water.

6. Provide for dilution of waste waters prior to reuse, and provide the throughflow required for basin salt balance.

7. Reduce the rate of land subsidence due to extraction of fluids and thus minimize damage to engineering structures sensitive to minor move- ments of the land surface.

In any given water management area, one or all of the above purposes might be served by recharge operations. The methods used and locaHties for recharge will vary with the intended purpose. Quite obviously, if natural recharge is reduced or is not adequate to balance pumping withdrawals, artificial recharge is one way of balancing ground-water storage. If surface water must be imported to prevent an overdraft from developing, then reduced pumping can be combined with increased recharge to balance storage. The location of pumping fields relative to recharge areas can be quite critical when sea-water intrusion is involved.

At times it is advantageous to use stored ground- water to the extent that overdraft develops. This might be the case while an imported surface-water source is being developed. In the interim, procedures that protect natural recharge or create artificial recharge will extend the useful Hfe of the ground-water supply. Once the surface supply has been developed, the available storage that has been created can be used through expanded artificial-recharge operations.

Frequently an urban area will develop using wells and pressure-distribution systems. Such systems are expensive and often are not interconnected. If overdraft forces the area to turn to a surface water source, major revision in the main distribution system as well as the construction of water-treatment facilities may be necessary. The physical makeup of the ground-water basin may be such that water can be artificially recharged near the wells. The water percolating from these areas will be distributed through underground aquifers in sufficient quantity to satisfy part or all of the demands.

Artificial recharge can be effective in controlling sea-water intrusion—a subsurface pollution source. Also, ever-increasing amounts of ground-water pollution from surface sources are becoming evident. Where shallow ground-water causes drainage problems, a certain amount of throughflow is required to maintain soluble salt concentrations within crop tolerance levels. The same can be applied to the expanding practice of reuse from ground-water storage of water for industrial and urban purposes, but the tolerance limits may be controlled by human, rather than plant, standards. Even- tually these Umits will have to be maintained by dilution or throughflow within the ground-water body. This can be accomphshed in part by the proper appUcation of artificial recharge.

Artificial recharge can be an effective tool in the management of a ground-water basin's resources. How- ever, the benefits derived depend greatly on all the variables (economic, political, geologic, physical, etc.) that are associated with each individual management unit. In the following chapters we will attempt to present in a general way the manner in which these variables affect recharge and how they are evaluated in the field.

CHAPTER II. THE GEOLOGY OF GROUND-WATER BASINS

RELATION OF GEOLOGY TO GROUND-WATER RECHARGE

Determining whether artificial ground-water recharge is physically possible in a basin or water management area requires an analysis of the geologic nature and structure of the area. The geologic environment affects the four-step sequence necessary to make artificial recharge work. These steps are:

1. An efficient intake point or area. 2. Subsurface transmission to the point of discharge. 3. Subsurface storage at the point of discharge. 4. An efficient point of discharge.

This sequence can best be illustrated by a flow diagram as shown in figure 1.

The arrows in the recharge sequence indicate a flow of water (steps 1, 2, and 4), while storage (step 3) is merely the presence of a fillable void. Limitations in any one of the flow steps will control the rate in the recharge sequence. The question is: can the intake (step 1) and transmission (step 2) deliver the required flow to storage at, or associated with, the location of discharge?

Ground-water occurs in permeable geologic formations known as aquifers. These formations have voids that permit appreciable water to move through them under ordinary field conditions. Ground-water reservoir and water-bearing formation, bed, stratum, or deposit are commonly used terms for aquifers. An aquiclude is an impermeable formation that may contain water but is incapable of transmitting significant water quantities.

INTAKE RECHARGE PONDS

/A\ \

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WATER TABLE

DISCHARGE

WELL FIELD

i'^

STORAGE

Jf Jr

^^^~^ X X X X X X X XXX

INTAKE ^ DISCHARGE

V TRANSMISSION

STORAGE

Figure 1 .-Sequence of steps in ground-water recharge.

4

Clay is an example. An aquifuge is an impermeable formation that neither contains nor transmits water. SoHd granite belongs in this category.

In ground-water basins where aquifers are of recent alluvial origin, the recharge capacity is controlled by the layering of the material. It is directly related to the porous granular nature, and mode of deposition of the soils at the surface and aquifers beneath the surface, because it is through them that the water must flow and in which it is stored. These are in turn related to the geologic origin of these materials, the grading or sorting in particle sizes associated with the transport processes of deposition of the layer, the conditions at the time of deposition, and secondary chemical changes that have occurred in place. Water is transmitted best in the larger pores of the coarser sand and gravel aquifer layers. These materials have a high hydraulic conductivity. The smaller, more numerous pores of sandy aquifers will provide greatest volume for storage, thus having a high specific yield or fillable void. The silts and clays contain a large volume of very small pores, but since little flow can occur in them (very low hydraulic conductivity) they act as confining layers and provide little storage. Thus a distinction can broadly be drawn between sediments that transmit water, those that store it, and those that impede flow almost entirely.

Only a detailed study of the geology of an area can yield specific answers as to how useful recharge will be in any given ground-water basin. Every ground-water basin is a unique product of many overlapping geologic processes, which of themselves vary in duration and intensity. However, some of the broad characteristics associated with the geologic origin of ground-water basin sediments can help, in the preliminary analysis, to select the recharge method or, in some instances, to exclude artificial recharge from consideration.

ALLUVIAL DEPOSITS

Nearly all the major ground-water basins in the western United States are located in deposits of recent alluvium. That is, the surface soils and subsurface aquifers were deposited in their present position when fresh water transported parent materials from adjacent mountain ranges. Such deposition occurred in recent geologic times (1_3),^ and so the pattern of placement has been altered only to a minor degree by other geologic forces such as faulting, glaciation, volcanism, etc. Although physical movement, other than subsidence, may have ceased, chemical changes continue

Underscored numbers in parentheses refer to Literature Cited, p. 154.

primarily in the surface soil due to continual weathering, surface cultural practices, and vegetational processes.

Alluvial Fans.-As a stream discharges into a valley from a mountain canyon, it carries a load of suspended fines and coarse rock materials. The selective deposition of these materials, based on sudden changes in stream velocity, forces the stream to shift its channels to one side, then another. The pattern assumed by these deposits is fan shaped, with its apex at the canyon exit. A typical radial and cross-sectional view of such a fan is shown in figure 2. Changes with time in the velocity, duration, and uniformity of flow control the uniformity of particle size, area, and thickness of the beds of the graded sediments that make the best aquifers in the fan. in ñgure 2 the idealized disposition of the sediments has been depicted by taking the extremes in grain size. The gravels, which make up the most transmissive aquifers, were deposited during periods when streamtlow was confined to the channels and velocities were swift. The clay or finest sized particles, which make up the aquiclude or aquitard, were deposited in still or slowly moving water. During floodflows, great amounts of poorly sorted and mixed sized material were provided as a matrix for the above extremes. Because of their poor uniformity, these materials function poorly as aquifers.

The stream's velocity is always highest in the narrow confines and steep slopes of its mountain canyon. As it discharges onto the apex of the fan, the channel width is no longer confined, and the stream velocity decreases. The coarse gi-avels fall out here, and the resultant flattening in slope further decreases the velocity. As the stream flows down toward its flood plain and drops more sediment, the velocity is further decreased. More water is lost by seepage out of the stream channel into the ground-water body of the fan. Eventually the water flows out of the flood plain onto the valley fill. As the stream wanders over the fan, it deposits sediments in a seemingly random pattern. Interconnection of gravel beds is more probable near the apex of the fan, while interconnection of clay lenses is more likely at the toe of the fan. This is illustrated in cross sections A and B in figure 2.

This idealized picture shows where the distribution of surface soil textures and profiles best suited for artificial recharge on an alluvial fan can be expected. Regardless of the method of artificial recharge, the recharge intake rates should be highest on the coarsest, most uniform fan deposits. The chances of locating major layers of such deposits at the surface or in the profile increase as one approaches the apex of the alluvial fan. This is also nearest to the natural water source for recharge to the fan's aquifers. The surface and subsurface profiles are dominated by finer textured sediments at the toe of the

Figure 2.-Cross-sections of an alluvial fan, showing geology.

fan, and this can limit surface intake or vertical flow into the aquifers.

Transmission of recharged water should also be most rapid at the apex because of the abundance of coarse sediments and maximum interconnection between such deposits. At the toe of the fan, these stream-channel deposits should be individually smaller in cross-section and fmer in grain size. These water-transmitting beds are more isolated, and a cross section of the fan will be dominated by water-retarding lenses of fines, which may cut the coarser beds off from the rest of the aquifer network. However, the total deposits are considerably deeper, and the aquifers are finer grained, so more water is actually stored at the toe than at the apex of the fan.

Valley-Fill and Flood-Plain Deposits.-The major val- leys were formed by drastic shifts in the earth's rock crust. These depressions have filled with sediments from deposition of peripheral streams. A significant break in slope occurs between the fan deposits and the valley floor. Here the depositional environment shifts from that of the rapidly moving water of a stream channel to

the slow broad expansive movement of a meandering stream or the still water of a lake or marsh. Periodically, fines can settle broadly across the relatively flat expanse of the vaUey floor. This valley-fill type of ground-water system is generalized in figure 3. If the lake changes in depth because the valley's discharge point changes, or subsidence of valley-fill occurs as overburden builds, the fine-textured layers can, in effect, lap up on the fine- textured impermeable strata of the toe of the fan or interfan deposits. This creates the physical confining boundaries necessary to transmit pressures from the higher intake zones to lower lying aquifers in the valley-fill material. Drilling into these confined aquifers produces artesian wells where the stafic water level rises above the level of the upper confining bed. Because these aquifers are under pressure, the water pumped from them does not come direcfly from storage next to the well. It is transmitted from recharge areas on the fan and, to a smaller degree, is forced out by subsidence of the aquifer-aquiclude system. Thus artificial recharge at the artesian aquifer's intake, if located, will be of no immediate consequence to the well's production. Sur-

face recharge at the well may never reach the aquifer because of the aquiclude.

Shallower intervening layers of clay can also prevent vertical water movement and cause perched water tables to develop (fig. 4). Such shallow layers in the valley fill result in poor soil drainage created by incidental

recharge from irrigation developments. Where such conditions develop, recharge by surface methods is impractical.

There may be significant areas of interfan alluviation between major rivers. Here the sediments associated with floodwater deposition of poorly sorted materials are

PIEZOMETRIC GRADE LINE WATER TABLE WELL;),

^ ARTES! AN__W EU X ^^^^^^^^^

Figure 3.-Cross-section of a valley-fill formation, showing geology and indicating intakes to confined and unconfined water tables.

^'f^xxxxxxxxxxxJ^* %T^ BEDROCK

Figure 4.-Cross-section showing perched water tables on impeding soil layers.

7

mixed with the outwash of smaller drainages. Such deposits seldom allow a proper recharge sequence because of the absence of well-graded aquifer layers and

poor continuity.

EOLIAN DEPOSITS

Another sedimentary process leading to graded deposits of granular rock and soil material is wind erosion. Exposed and buried dunes and the loess soil deposits in

Nebraska are associated with the deposition of wind- borne particles of quite uniform fine sand to silt. The area and depth of dune deposits can be significant near seacoasts or near unconsoUdated sedimentary materials or desert areas. While these fine-grained materials do not transmit water rapidly over great distances, their lack of stratification and comparatively large storage capacity offset this restriction. Dune deposits and alluvial sands serve very effectively as recharge reservoirs and aquifers in many locations. In the finer loess deposits the intake and transmission of water are quite limited because of pore size, even if strafification is absent. Vertical jointing is frequently present in depth; however, such jointing is very unstable, and surface cultivation will Umit its effect

on water intake.

GLACIAL DEPOSITS

A sediment-transporting process that results in other large areas of unconsoiidated deposits over the earth's surface is glaciation. In this process the material picked up by the ice during the advance of the glacier is dumped-unsorted-as the ice melts. This has occurred during several ice ages, causing several overlapping deposits. During the interglacial periods, streams from the melting ice sort through these materials, but because of the flat broad expanse of area glaciated, the grading of these deposits is quite random and localized. No extensive drainage systems developed to lay down the extensive fans and flood-plain deposits.

Ground-water has been utilized in the sorted parts of these deposits, where it can be located. The predictable ground-water supplies in the glaciated areas usually are associated with older buried alluvial deposits. To reach these buried aquifers with surface recharge would also be only a random possibility, because the upper unsorted mass of clay and boulders limits the intake part of the recharge sequence.

FRACTURED AND POROUS ROCK SYSTEMS

Other large bodies of water-bearing deposits are cemented sandstones, fractured limestone, and porous lava deposits. Water is stored and transmitted in the

voids in otherwise nonporous rock. In volcanic rocks these voids are created by fracturing and venting of gases during cooUng of the mohen rock. In limestone and other more porous rocks, fractures are caused by shifts in the earth crust and the subsequent widening of these fissures due to water soluüon. The productivity of wells in these deposits varies within extreme limits, depending upon the amount of fracturing surrounding the wells. In relation to their total volume, the storage capacity of these materials is low, but water can be transmitted over long distances through interconnected fissures from large solution cavities. Therefore the storage and transmission parts of the recharge sequence can be met, but only on a selective basis. Intake rates depend on gaining access to the fissure network, which may be somewhat problematical.

THE GROUND-WATER RESERVOIR

Even a preliminary analysis of the value of artificial recharge in ground-water basin management requires a definition of the physical boundaries, hydraulic function, and accessibility of the ground-water reservoir. The geologist will be concerned with defining its geometry, continuity, and geographic location relative to the discharge and transmission requirements of the recharge sequence. The physical barriers to water flow, limits in storage capacity, area, and distribution must be determined as a first step.

THE BASE OF THE GROUND-WATER RESERVOIR

The base of the ground-water storage in alluvial deposits may take several forms. The most common of these is an impermeable basement rock or bedrock on which porous sediments have accumulated. The depth to this boundary can be approximated from surface geologic features, the type of rock, and a knowledge of the geologic forces that have created the basin. Geo- physical techniques can accurately define the position of the basement rock. It may be a sharp boundary, or it may be a gradual decrease in storage and/or water- transmitting capacity over considerable depth.

Bodies of saline water may also determine the storage basement. Because of its greater density, saline water may be trapped in porous marine sediments beneath fresh water. In island or coastal hydrology there may currently be a direct contact with the ocean. It is important to recognize that such a boundary must be taken above the salt water-fresh water contact, otherwise pumping will cause the salt water to rise locally into wells. A great deal of theory is available to the geologist for the prediction and control of these phenomena.

8

Intermediate impermeable layers or aquicludes above bedrock can act as lower boundaries to storage. Such layers are common in valley-fill deposits and are associated with perched water tables, as shown in figure 4. There can be several of these, one above another, with any one controlling the recharge through it to the others. The geologist can locate these layers and map them from well-drillers' logs, well depth vs. static water table observations, and again by geophysical methods. For example, artesian aquifers have an upper confining layer. If this is sufficiently thick and impermeable, it can contain water under considerable pressure beneath it, and it will also be the base limit of water storage above it. At the other extreme, and most difficult to recognize, will be where a layer in the unsaturated profile above an existing water table acts as the storage base only after surface recharge is provided. Such shallow perching layers have no influence on the ground-water hydrology of an area until they are required to transmit recharge water vertically. These are the layers on which agricultural drainage problems develop when the storage above them is satisfied by deep percolation from overirrigation. Any shallow water table not otherwise expected should make the geologist suspect the existence of such a perching layer.

ECONOMICAL PUMPING LIFT AND PREVENTION OF INTRUSION

The base of available storage can also be defined by the economics of pumping ground-water. When pumping lifts increase to a point where the pumping costs exceed the value of the water, an economic lower boundary is established to the storage reservoir. The hmits of an economical lift vary widely, depending on the economics of the development and use of ground-water in the basin.

The base can also be established by salt-water intrusion into the fresh water as the fresh-water table is lowered. Or, as in artesian aquifers, the subsidence of the ground surface may cause economic damage.

LATERAL LIMITS OF GROUND-WATER STORAGE

The horizontal limits of the ground-water storage body can be defined by the same physical conditions as the basement; that is, bedrock, saltwater contacts, and impermeable layers as they rise up on the fan deposits or pinch out in the profile. A lateral salt-water boundary may move horizontally into the basin. Salt-water intrusion is not easily reversed at a horizontal boundary because the density difference causes surface-recharged fresh water to float on top of the salt water, rather than displace it. Injection recharge wells have been used with

some success to stem seawater intrusion. Even so, considerable amount of storage capacity may be lost for many years when salt water intrudes along a horizontal front (23).

Political boundaries can also define, the effective boundaries of ground-water storage of a water management unit, where storage is a property right of the surface area unless adjudicated. Many major political boundaries are generally related to surface hydrological features such as ridge crests, shorehnes or center lines of streams. Some of such boundaries may bear no relation to the subsurface ground-water hydrology of the storage unit being recharged. This is even further complicated by personal property subdivisions within the larger units. The ground-water geologist should provide a picture of how these boundaries do or do not correspond with the physical limits ground-water storage. Discrepancies between the physical water-storage and political boundaries then are a legal problem.

UPPER LIMIT OF STORAGE RESERVOIR

The uppermost limit in exisfing ground-water storage is the first water table encountered beneath the ground surface. This water table can be a^^r even above ground surface in swamps, ponds, or lakes. It is the upper limit of the zone of saturation in the aquifer. As illustrated in figure 4, if beds of impermeable clay occur beneath one another, several water tables may be penetrated by a single well, each having a separate saturated thickness as defined by its particular limits. Water may actually cascade down from upper water tables to the water surface in the well. This multiple aquifer situation is often difficult to define hydraulically because a well, penetrating several of these squifers, can provide a conduit through which water can be transmitted between aquifers. Such interaquifer transfer will buffer out the response of the individual aquifers to recharge and make the system as a whole react locally as a single aquifer. A measurement of the water table in a well under the above circumstances can be a very poor estimate of the upper limit of ground-water storage. Techniques are available for determining whether water is fiowing within non-pumping wells, and down-hole geophysical measurements can indicate if the profiles contain the aquifer-aquiclude sequence necessary to produce these conditions.

If salinity excludes the use of a shallow perched water table, then the upper Hmit becomes the next fresh-water table below it. Such zones of poor-quality water within the profile require that the wells penetrating them be cemented or cased off through them to prevent contamination. Obviously, attempts at artificial recharge of water on a broad scale through these salinized aquifers,

even if possible, would result in the degradation of the recharged water quality.

ARTESIAN AQUIFER STORAGE

Water yielded to wells in artesian aquifers comes predominantly from flow within the aquifer and orig- inates at an intake or recharge region that may be many miles distant. A small amount of water is released from storage next to the well as the pressure diminishes and the aquifer-aquiclude system irreversibly collapses under the load of the overburden above it. Therefore, the artesian aquifer is a transmitting conduit from an area of recharge where storage changes can be affected by recharge (Fig. 3). This recharge area may be difficult to isolate for artificial recharge, as it may be hidden or overridden by gross influences of the basin's ground- water table.

STORAGE IN CONSOLIDATED ROCKS, LIMESTONES, AND VOLCANICS

The regional evaluation of storage in fractured rocks is at best a statistical problem. Each well has its own characteristic storage network which may or may not be directly connected with nearby wells. And so recharge location becomes problematical also. The yield and available storage of individual wells can vary greatly, from practically zero to many times the yield and storages of wells located in alluvial and sedimentary deposits. The capacity of recharge areas will follow this pattern also.

GEOPHYSICAL METHODS

While there is no substitute for field experience in planning and control of the water resources of a specific basin or water-management area, the complexities of current and future water developments require that estimates be made of their limiting factors and of resources not previously developed. Such information is necessary for protection against outside encroachment, improving internal efficiencies, and evaluating the requirements for, and results of, importation of water into the basin. Ground-water is a major component of the water resources of a basin but, then, so is the entire finable ground reservoir if recharge can be accomplished. Even in the most highly developed basins, information on the quantity of available ground-water storage is sparse, and interpretations are from statistical inference rather than engineering definition. Such an engineering definition of the character and response of the ground- water reservoir should be a major part of water resource planning. To this end the geologist has available many excellent tools for providing a quantitative measure of

location, bounds, capacity, and internal structure of this reservoir.

The most direct method for observing what is beneath the surface at a given point is by boring a well. Other geophysical techniques then provide a means by which the physical characteristics observed at one point in the basin can be inferred over a greater area. The most widely used methods require very refined measurements of four physical properties of the earth's crust: (1) Elec- trical conductivity, (2) seismic wave transmission, (3) gravitational field modifications, (4) magnetic field distributions.

EXPLORATORY WELLS

The bench marks to which all exploratory geophysical methods are referenced are the test wells or core holes drilled under the supervision of the field geologist. Cores collected at specific depths in the profile show the kind and position of the significant materials that determine the size and nature of the ground-water reservoir. Physical, chemical, and mineralogical analysis of these samples provide the basis for interpretations on the geologic origin and processes of deposition for specific aquifers and perching and confining units. This information can be related to their area and eventually to how they affect the hydraulics of the reservoir. Values for the water-transmitting and storage properties of these units can be assigned after testing cores in the laboratory. From these, estimates can be made of the response of the formations to artificial recharge of the storage.

Bore holes obviously are the only direct way of assessing the nature of subsurface geologic formations. The development of a uniform recording method and a local library of commercial water well-drillers' logs can be invaluable in extending test-well observations throughout the basin.

DOWN-HOLE RESISTIVITY AND POTENTIAL LOGGING

This technique consists of lowering a series of electrodes to specific distances into an uncased test hole filled with drill-mud. A controlled electrical potential (voltage) applied to these electrodes produces an electrical current. The^flow of this current depends on the resistance of the formation within the potential field between the electrodes. As the electrodes are lowered into the mud-filled hole, the resistance depth is plotted (or logged) on a recorder. In fresh-water formations the resistance measured between the electrodes at any point in depth is related to the salt concentration of the formation water, the relative porosity and degree of

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Saturation of the formation, the depth of mud penetration (fluid-transmit ting properties of the formation), and the specific formation resistivity (related to formation density). The information from this depth log defines the position and thickness of the major units in the profile. A second record logged at the same time by the equipment is the spontaneous-potential or formation- potential log. This provides an independent comparison of the relationship between the difference in the salt concentration of the drill-mud and that of the formation water.

The electric log provides a record or "fingerprint" of the profile. Figure 5(10) shows the first 120 feet of a well log from a deep well in the western part of the San Joaquin Valley. The log of the first 70 feet shows the typical effect of the subsidence of drilling mud in the bore hole. Below this the formation resistivity and spontaneous potential then are determined by the characteristics of the formation. At a depth of 100 feet the spontaneous potential decreases, indicating the dense clay layer shown by the core permeability profile on the right. This clay layer caused a perched water table to form in response to incidental recharge from the irrigation. By comparison with other logs, formations can be correlated throughout the basin so that thickness of the aquifers can be calculated and total storage predicted. The electric log is widely used to provide the information necessary to match the perforation of well casings to the most productive fresh-water zones of the profile.

SEISMIC SURVEYS

The most widely recognized surface geophysical method is the seismic survey. Here the elastic nature of the different subsurface layers is measured from the velocity of transmission of compressional waves produced by an explosion at or near ground surface. The time of arrival of the seismic waves both refracted and refiected by subsurface layers can be recorded by a line of microphone-type detectors laid out for some distance from the point of detonation. These phones are tuned to the long wavelength of seismic radiation, and their signal is amplified and recorded on a strip chart. From wave-propagation theory, the vertical location of the major formation boundaries can be calculated and subsequently correlated for the basin. Because of the narrow range of seismic velocities in porous unconsolidated sediments, only the most extensive and thick beds can be identified in most deep alluvial basins. The method is excellent, however, for predicting bedrock configuration.

If exploratory work for oil has been done in the basin, seismic information on the shallower ground-

water body may be available from the oil company. Miniature seismic units can be used to explore depths of less than 60 feet for layers that might influence the surface recharge intake rate.

GRAVITY-METER SURVEY

The gravity meter measures very small differences in the earth's gravitational field at adjacent locations due to variations in the density and thickness of the overburden. By referencing gravity measurements to a known profile, the configuration of bedrock can be constructed in reasonable detail.

MAGNETIC SURVEYS

Aside from the earth's magnetic field, rocks themselves exhibit magnetism. Different rock types have different magnetic properties. A magnetometer can sense these differences in magnetic field at considerable depth. Because sedimentary rocks have very little magnetiza- tion, and igneous rocks are strongly magnetized, it is possible to identify irregularities, such as intrusions and dikes, in the bedrock configuration from the horizontal distribution of magnetic intensity. Faulting of the bedrock can be recognized, in certain cases, if the variation in magnetic field across the fault is significant. Again it is necessary to anchor these observations to bore-hole data, for the magnetic-field distribution can also be controlled by changes in the bedrock properties.

QUESTIONS FOR THE GEOLOGIST

To assess the potential value of recharge in a basins water management, the geologist should provide the answers to the following questions:

A. What is the origin of the water-bearing sediments of the basin? 1. How actively and how long have sorting

processes been operating in the geologic past? 2. Have these processes been of significant inten-

sity and duration to produce the required conditions for recharge?

3. What, if any, overlapping depositional processes have been active?

4. Where can they influence the recharge sequence?

B, Do the characteristics and extent of these sediments meet the requirements of the recharge sequence? 1. What specific units in the basin's sediments

limit and/or function as aquifers, perching or confining layers, etc.? Can they be mapped and profiled?

12

2. What quantitative evaluation can be given to the hydraulic properties of these units?

3. Do these estimates check hydrologically with existing data on recharge, discharge, and storage change?

C. What defines the ground-water storage reservoir?

1. What are its horizontal and vertical dimensions, and its potential and available storage capacities?

2. What internal stratigraphie units determine its function, capacity, and accessibility for recharge?

3. Where and how will future cyclic water use influence the reservoir, its dimensions, and its function?

D. What gaps exist in the geologic definition of tlie ground-water basin, and how should they be most efficiently closed? 1. What techniques should be used and for what

purpose? 2. What existing information has been reviewed

and what new sources found?

E. How should the geologic data be presented to make it most useful in the planning and engineering of the recharge sequence?

13

CHAPTER 111. THE SURFACE AND GROUND-WATER HYDROLOGY OF ARTIFICIAL RECHARGE

DEFINITION OF HYDROLOGY

Hydrology is defined as that branch of physical geography that is concerned with the distribution, in space and time, of water on earth. Those subdivisions thai most affect the success of artificial recharge are the hydrology of surface runoff and that of ground-water. The hydrologistes task is to identify the amount and distribution of water within the ground-water basin and predict what effects changes in any one or a combination of the components will have on its hydrology. In artificial recharge the concern will be with defining the available surface and ground-water storage, its development potential, its accessibihty to users, and its response to the normal cyclic changes in water supply. What follows is a very general view of what information the hydrologist should provide the engineer so that artificial recharge may be properly evaluated.

TRENDS IN WATER DEVELOPMENT

Historically, excessive surface water has been the major concern to which man has directed his engineering. His efforts to get as close to his water supply as possible placed him in a position to be periodically flooded. Surface flood control and routing, therefore, were the first major water projects to develop, even in the semiarid areas of the West. Here the water's destructive energy was conserved and later released so as not to interfere with man's development oï lands with an existing adequate water supply. An obvious extension was to use the water's controlled energy for power generation. Then volumes of water were transmitted by gravity to lands of short supply. Eventually, when water and land values rose, it became feasible to develop surface-storage and distribution systems for the prime purpose of attempting to balance water storage and runoff against irrigation and water supply demands in water- short areas many miles from the watershed.

With the extension of water-distribution facilities came the development of efficient power generation and long-distance energy transmission. This meant that the energy lost at one point in the system could be transmitted with the water to increase its energy through pumping at another point, thus further extending the distance between the source and point of use.

Now we can see the development of systems that consume energy for the placement of water on lands or in surface storage above the source; also systems that raise water to areas where it is recharged downward again, thus expending energy for the sake of ground-

water storage alone. And finally, we can see the use of energy alone to produce usable water through the reclamation or desalinization of sea and waste waters.

Water development and demand exceed supply in many areas. Ground-water has been depleted or mined in areas where demand has exceeded the developed surface supply or where there is insufficient surface-water storage carryover to provide for short and long drought cycles. The substitution of ground-water for surface supplies will continue, but unless increased recharge is provided, it too will be limiting. Thus, couphng all available uncontrolled surface-water sources to the available ground-water storage appears to be a last stage in surface-water development. Such accompHshment requires an exact understanding of the comparative hydrology of the ground-water vs. surface-water reservoir characteristics.

SURFACE STORAGE

Because water in surface storage is at its highest economic potential and has the greatest use and control flexibihty, its development always takes precedence over ground-water storage through artificial recharge. Surface storage has not been fully developed in most areas. However, there would appear to be an upper geographic limit in dam-site availability.

Along with flood control, surface storage functions to match the yearly natural cycle of seasonal water supply to cyclic demand of use, and in particular to the largest use, irrigation. Thus, surface-storage capacity provides a time delay between the source and point of use. If sufficient capacity is available, it will also stretch availability through seasons of low supply. As seasonal demand expands, however, this reserve diminishes, and eventually the storage will be cycled through its available range each year.

Generally, in large watersheds and compound drainage areas, runoff in excess of existing surface storage will always occur and in significant amounts. It is impractical to design storage to catch all runoff High-intensity, short-du ration storms can occur at unpredictable intervals, and flood-control storage must be provided in advance of runoff.

Storm runoff is a frequent source of water for recharge. On large watersheds runoff will be controlled by flood-control dams. Important to the design of diversion structures at downstream recharge areas are the streamflow and water-stage hydrographs that might be expected below such structures. The curve A B C E G

14

TIME

Figure 6.-Effect of reservoir storage for flood control on streamflow, as shown in a Hydrograph at the dam site (12).

in figure 6 (J_2) is an idealized picture of the uncontrolled flood Hydrograph of a stream. At A, flow is at the base flow rate that is associated, for the most part, with subsurface flow into the stream channel from ground-water storage on the watershed. When rainfall commences and surface runoff reaches the channel, flow increases to a maximum, and as runoff decreases, the discharge recedes to base flow again. In order to decrease the peak flow, a control dam would be operated so that at B the desired channel flow would be controlled by storing water for the period B D E. At E, the gates, which gradually closed as head built in the dam, would now be gradually opened and channel flow maintained until F, at which time the dam would be empty and all storage would be released.

The hydrograph that might be expected at some downstream diversion in response to a flood peak controlled at some upstream point is idealized as curve C in figure 7.(12). Diversion and transmission structures would have to be designed for acceptance of all or part of such flows.

Curve A in figure 7 is the hydrograph that would be associated with the direct delivery of uncontrolled floodflow to recharge basins. The intensity and duration of the storm on the watershed determines the amplitude and wavelength of this curve, so the curves may vary over wide limits. To engineer structures for diverting all the uncontrolled floodflows is difficult. Usually diversion is made on the falling side of the hydrograph when control is assured and when the channel load of debris has decreased. This will be discussed in detail later.

A great deal can be done to delay flood runoff on the watershed above the surface-storage or recharge facility. Small check dams and stock ponds will provide a degree of control, but practices to preserve and improve the natural recharge on the watershed could greatly expand the base-flow period. Because of the shallow depth of porous sediments on the watershed, the methods used to recharge the ground-water basins are not directly ap- plicable, but the principle of increasing or maintaining surface soil intake to augment subsurface storage and flow is the same.

15

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CONTROLLED OUTFLOW AT DAM NATURAL INFLOW AT DAM

ROUTED TO DOWNSTREAM POINT

\ \ \ CONTROLLED OUTFLOW \ \

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Figure 7.-Hydrograph of effect of flood-control storage on delivery of surplus water to downstream recharge basins (12).

SURFACE-WATER CONVEYANCE

Existing surface-distribution systems are usually designed for maximum yearly discharges associated with seasonal irrigation use or floodflows. Therefore, distribution capacity always exists if water is available for artificial recharge. This then will be the primary system for conveyance of artificial-recharge water.

In areas where water management has advanced to the point that artificial recharge is of concern, the surface-water distribution and disposal system normally will be well developed. In the arid West, irrigation distribution systems, which service large areas, will provide the discharge capacities necessary to spread water available for recharge. Routing and water control is no problem because the procedures for irrigation are so similar to water spreading. The need to drain canals for necessary maintenance is the limiting factor in their use for off-season distribution of water for recharge. Getting water to areas with high recharge capacities may require the expansion of lateral ditches, but the biggest

problem, that of right-of-way, usually has already been solved if the sites selected have ever been irrigated.

In spreading floodwaters, water is diverted high on the natural watercourse, close to the mouth of the watershed. Once the water enters the permanent flood- control and surface-runoff control systems, it is often inaccessible for recharging because such structures are usually in the lows of the flood plain. Lifting water by pumping may be practical for the capture of natural runoff and should be considered in the future. Often irrigation-distribution systems are available for routing trash-free floodwaters. This water can be spread for recharge or be distributed to irrigated lands on the higher reaches of the flood plain.

GROUND-WATER STORAGE

The amount of ground-water stored on earth within 2,500 feet of ground surface is roughly 37 times that found in surface storage in all lakes, rivers, and dams (27). In recharge, however, we are not concerned with what is already in storage so much as the amount of

16

available storage space that has been drained by ground- water withdrawals or is naturally present above the water table. So, the resource developed by artificial recharge is the unfilled pore space within the sediments of the ground-water basin or watershed.

Not all the water present in the pores of an aquifer is available when the ground-water is extracted by pumping. The volume extracted is only a fraction of the

aquifer's total pore space, and this fraction depends on the particle-size and pore-size distribution of the aquifer as well as the time that is allowed for its drainage. Figure 8 (5) provides a generalized picture of how the total porosity, specific yield (available storage), and specific retention (unavailable storage) vary with the particle size of aquifer materials that might be present in any alluvial ground-water basin.

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The amount of available storage capacity in a surface reservoir is easily determined from the total volume behind the dam, less that in retention. Because the ground-water basin may contain all the texture ranges shown in fig. 8, in random layer sequences and thick- nesses, the problem of estimating available storage capacity above the current water table is difficult. Ground water already in storage beneath the water table is also difficult to approximate for this same reason. Ground-water extraction versus water-table depth records provide the most accurate estimates of available ground-water storage.

GROUND-WATER FLOW

Water-transmitting properties of aquifers are also related to particle-size and pore-size distribution. Figure 9 gives a general idea of the range of hydraulic conductivity for materials that might be found in any alluvial basin. Mathematically, hydraulic conductivity is the proportionality constant between the volumetric flow (Q) through a unit cross-sectional area (A = 1) and the hydraulic head loss (Ah) per unit length (L) of aquifer. The equation for the Hnear relationship is called the Darcy equation:

Q = KAf (1)

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Q = volumetric water flow

[L^ -ÍÍ2O/TI or [LVI]

A = aquifer cross sectional area perpendicular to flow

Ah = measured as fall in water table X elevation (Ah) per unit distance

(L) in the direction of flow (e.g., cm.-HsO/cm. orft.-HjO/ft.).

[^] " [T] So in the unit section of aquifer shown in ñgure 10, if the head loss is changed through a range, and the flow measured, the slope of a plot of discharge per unit area (Q/A) against the hydraulic gradient (Ah/L) will result in the hydraulic conductivity (K). Equation 1 can also be written

^ A L

where V is the velocity of the water through the unit aquifer cross section or what is sometimes called the Darcy velocity.

Example:

From flgure 8, the hydraulic conductivity of a mixture of washed sand and gravel making up an aquifer might be 100 ft^/day-ft^. What is the velocity through a unit cross section of this aquifer if the gradient in head is 2 ft./1,000 ft.?

V = Kf=.00xA, = 0.2 ft./day

In naturally occurring sediments, layering and tex- tural sequences in profile and area distributions in the plane of ground-water flow make a direct measure of flow velocities difficult. Field methods can provide inplace estimates of hydraulic conductivity (K) that lump the complexities of layering into a single value that applies to flow in the vicinity of the measurement. With enough such measurements, estimate of the ground- water flow system in a basin may be constructed. There is no gaging technique available for ground-water flow comparable to that for streamflow. Because of layering, hydraulic conductivity in the vertical direction is commonly several orders of magnitude smaller than that in the horizontal. As happens with a perched water table, a single layer can eliminate vertical movement completely.

The comparison of ground-water velocities and surface-distribution velocities is well illustrated by the common units of measurement for each. Open-channel flow velocity is measured in feet per second, while ground-water velocities are described in feet per day. Thus there is an 86,400-fold difference between the magnitudes of the two. Surface water is available rapidly from great distances while ground-water transfer is slow and in some cases nonexistent. It is possible to modify surface transmission capacities over wide ranges; however, the hydraulic conductivities of the natural sediments control the flow of ground-water in the basin. Man can alter the gradient locally, but the massive storage capacity of the basin and its horizontal scale prevent any great changes in overall gradients. Figure 11 (22) shows theoretically how slowly the cone of depression develops around a well pumping continuously for many years, at a rate of 100 g.p.m. from a saturated unconfined aquifer 100 ft. thick having a storage coefficient of 0.20 and a hydraulic conductivity of 1,000 g.p.m./ft^

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To manipulate basin-scale ground-water flow over short periods of time by gradient changes through pumping would be difficult. However, gradients can be controlled over Hmited areas of a basin by additions, as in the case of successes with sea water intrusion barriers through injection wells, or in the case of the ground- water depressions that develop under well fields in urban areas.

RECHARGING

The surface storage system is characterized by: 1. Limited storage capacity. 2. High delivery capacity and fast reaction time. 3. Temporary storage of the water to be recharged.

The ground-water storage reservoir has: 1. Great capacity. 2. Limited reaction time. 3. Limited contact with source of recharge water.

The surface conveyance system: 1. Provides high transmitting velocity and capacity. 2. Is controlled by man. 3. Is connected to surface-water source.

The ground-water transmission: 1. Is extremely slow in comparison to surface con-

veyance. 2. Cannot be modified.

The ideal recharge hydrology system would be to use surface conveyances to transmit source water to the closest area of maximum available ground-water storage in the basin, which is generally the area of maximum pumping withdrawals. This would bypass the restriction of horizontal ground-water transfer, and place recharge where storage capacity was available. The source water would be only that surface water for which no surface storage could be found.

With this model view of the problem at hand we can summarize the sources of water for recharge in terms of their accessibility to the ground-water storage available. In table 1 we have summarized in a general way the various sources of water, their geographic accessibility, availability in time, and predictabiHty as to amount. It becomes the task of the hydrologist to estimate the measurements of these items so that the engineer can design or check the capacities of the recharge and distribution systems from the source. The ground-water hydrologist should then determine how ground-water

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Figure 11.-Theoretical response of ground-water storage to continuous long-time pumping of a single well (22).

transmission might be used in the basin and what the loss in system response might be. The impHcations of such a loss could then be considered from both an engineering view and economic standpoint.

Recharge operations tend to use greater quantities of the more predictable sources of water as time goes on.

More expensive imported water is being spread, not just local runoff and waste. This is because of its high quality and ever-increasing availability to areas with severe ground-water deficits. As a result, refined studies of the hydrology of recharge and its engineering consequences are needed.

21

TABLE 1.-Comparison of water sources by avaüability for ground-water recharge

Source Time

Surface storage Periodic Natural runoff Periodic.... Imported surface water Periodic Waste water Continuous . Flood water Periodic.... Irrigation deep percolation Continuous

Predictability

Good. Poor.. Good. Good. Poor..

Proximity Amount

Peripheral Large Peripheral Moderate Distant Large Close Small Peripheral Small.

Good Close. Large.

TRANSFER OF WATER TO THE WATER TABLE

Barring any perching of the recharged water above the water table, the rate at which water is deUvered to ground-water storage under continuous recharge is limited by the surface intake rate of the spreading area. In most cases the source of water to the recharge area is periodic, thus providing for intervals of drainage of the unsaturated zone above the water table. This water does not become part of the ground-water storage available for pumping extraction until it enters the water table.

Thus if the unsaturated depth beneath a recharge area is several hundred feet thick, both the time that the recharged water is in transit downward to the water table and the quantity in storage above the water table can be considerable. This drainage through the unsaturated zone to the water table, especially when the area is large, as from deep percolation of irrigation water or rainfall, can be very significant in the overall hydrology of ground-water storage.

Water ponded on a recharge area penetrates downward in an open soil profile that is initially at specific retention as a definite wetting front. The moisture content is raised to very nearly saturation just behind this front. Not until this wet front reaches the water table does actual recharge begin. When surface spreading is stopped, drainage of the moisture in the profile above the water table, called the vadose zone, continues for some time. Figure 12 shows how slowly drainage occurs from 100 feet of profile observed beneath two experimental recharge areas. The water that was required to bring the first 100 feet of profile up to a maximum moisture content, and so transmit the required amount of recharge, took more than 2 years to drain. The quantity of water that was drained into the water table was 10.6 acre-feet/acre during this period. Thus vadose zone storage can be significant itself, and its drainage can delay completion of water delivery for several months after surface water-spreading has ceased.

QUESTIONS FOR THE HYDROLOGIST

So that engineers can design and operate required recharge systems most efficiently, the hydrologist should answer the following questions:

A. What are all the possible sources of water for recharge? 1. Can they be accurately defined?

a. In volume discharge? b. In time when water is available? c. In position within the basin?

2. Can they be more efficiently used through surface storage?

3. Can their availability in quantity and time provide the engineer enough latitude to design new facilities or route the water within the existing distribution system?

B. How can each source be most efficiently routed into ground-water storage? 1. How can ground-water transmission be most

effectively used? 2. Is surface-transmission capacity already avail-

able, and can it deliver the amount of water to sites of optimum recharge capability?

C. How can one minimize the transfer time lag between the point of recharge and point of discharge? 1. Is the vadose mositure transfer lag significant?

D. What procedures would make the most efficient use of available surface storage in conjunction with ground-water storage in the basin? 1. What are the ultimate limits to the water-

storage development in the basin? 2. What are the ultimate limits to water supply

from all sources? 3. Will artificial recharge contribute to extending

these limits? By how much? E. How can the hydrology be presented to make it

most useful in planning the function of artificial recharge in a basin's water development?

22

> 8-

O

O Cantua Pond

X Huron Pond

o 100 200 300 400 500 600 700 800 Time - days

Figure 12.-Rate of drainage of recharged water as observed in the 0 to 100 foot depth beneath two experimental ponds.

23

CHAPTER IV. RECHARGE THROUGH SURFACE SOILS

WATER INTAKE RATES

Large-scale artificial recharge is accomplished for the most part by surface water-spreading methods. In water-spreading operations the intake step in recharge sequence (fig. 1) is controlled by the flow of water through the first few inches or feet of soil just beneath the surface of the spreading area, basin, or pit. It is practical to excavate, cultivate, or otherwise engineer in these surface sediments only in hmited areas and to limited depths for the purpose of developing the most efficient ground-water recharge system. In a few cases excavation will allow direct recharge of a basin's ground-water storage, but for the most part recharge water will have to travel through an unsaturated mantle of porous soil above the main body to storage.

This mantle or soil profile may be a few inches thick if it lies on fractured bedrock, or tens or hundreds of feet thick if it is valley-fill. Regardless of thickness it is porous, unsaturated, and capable of transmitting water. The soil profile water-transmitting capacity and the area available are what determine the economic feasibihty and engineering design of the recharge facihties and water conveyance systems. The engineer seeks recharge sites that have sustained high intake rates and are close to a water source and to the eventual point of discharge within the ground-water reservoir. The location of such sites on suitable soil profiles is a field-survey problem deahng with the classification of specific locations according to the several factors that determine the magnitude and persistence of high intake rates under field recharge procedures.

The field conditions and procedures for most efficient recharge are the opposite of those for most efficient irrigation. In irrigation, the aim is to keep all the apphed water in a narrow depth zone near the soil surface where plant roots locate. In recharge, the objective is to transmit as much water through this zone as possible. The major apphcation conditions that distinguish recharge from irrigation are extended periods of flooding and the heavy sediment load that the water can carry if storm runoff is spread.

A soil profile to be evaluated for recharge should be classified as to:

1. The the water-transmitting capacity of the soil. 2. Variations in this capacity with depth in the

profile. 3. Expected changes in capacity in response to

sediment load and biological activity in the recharge water.

4. Expected changes in capacity due to the chemical constituents in the water.

5. Expected changes due to physical manipulations of the surface or subsurface.

6. Expected effects of chemical, physical, and vegetative treatments on the intake capacity.

When water is spread over a dry or drained soil, the rate that water enters its surface will vary with the length of time of flooding. In irrigation, this rate of entry is evaluated in terms of hours. In recharge, this period may be measured in days or months. The conditions that control the short-period entry (infiltration) are not the same as those over the long periods of recharge intake. The primary difference here is concerned with the magnitude and source of the physical forces causing entry of water into the soil. Dur- ing the first few minutes of infiltration the rate is determined by the very large gradient or driving force associated with the affinity that soil particle surfaces have for the water itself. Soon, however, the surfaces become wetted through a significant depth and the driving force is predominately that due to gravity. When gravity takes over, then the changes in intake rate will be primarily associated with the water-transmitting properties of a few inches of soil just beneath the surface. The physical, chemical, and biological conditions that control the water-transmitting character (permeabiUty or hydraulic conductivity) of this layer of soil over the period of recharge determine the recharge efficiency of a site.

EFFECT OF PARTICLE SIZE AND DISTRIBUTION.

Soils are classified according to particle size distribu- fion based on the percent of three size fractions-sand, silt, and clay. Table 2 indicates the range of diameters considered in the classification. This class description is called the texture of the soil. The name given to the soil-sand, silt, loam, clay loam, or clay-is determined

TABLE 2.-U.S. Department of Agriculture scale for particle-size analysis of soils

[Soil Conservation Service classification (20)]

Name of fraction Limiting mean

effective diameter

Fine gravel 1/2 inch to 2 mm. Sand:

Very coarse 2 mm. to 1 mm. Coarse 1 mm. to 0.5 mm. Medium 0.5 mm. to .25 mm. Fine. .... .25 mm. to .10 mm. Very fine .10 mm. to .05 mm.

Silt 05 mm. to .002 mm. Clay Less than .002 mm.

24

by the proportion of each particle size in its composi- tion.

It is impossible to evaluate field soil permeabiUties exactly from the soil texture, because field structure, particle aggregation, or even biological activity in the soil can override the influence of texture alone on permeability. However, texture is a starting point for classification of surface soil permeabihty in a ground-water basin.

High rates of water intake are required for efficient recharge. Therefore, high surface soil permeability is very important in site selection. Generally, high intake rates are associated with field soils that are single grained and contain a high fraction of the coarse grain sizes (sands and gravels). When all other influences are equal, the highest intake will be found in an idealized soil consisting of 100 percent of a single grain size, the coarsest to be found. Any gradation in particle size will fill pore space with smaller particles that will impede water flow and decrease the permeabihty. The poorest permeabihty would exist when particles are so graded that successively smaller sized particles pack within the pores of the next size. The ideal recharge soil would have relatively large particles of uniform size. Such conditions can be approached in surface or buried stream channels, dunes, and beach deposits. These areas can be of major significance in the artificial recharge, but usually they are small in area. At most sites, surface soils will have been subjected to variations in sedimentation, to weathering, and to soil-forming processes that produce a broad distribution of particle sizes.

EFFECT OF PORE SIZE DISTRIBUTION

The transmission of recharged water through the soil surface is affected by the particle size distribution and the continuity of the pores in the soil just as was true for aquifers. The larger and more continuous the pores in a unit volume of soil, the greater the permeability but the smaller the total porosity (fig. 8). A high recharge rate over an extended period of time depends on a high proportion of large pores that are contmuous and persistent through a long recharge period. Many attempts have been made to relate permeability, recharge, and infiltration rates to the measured total porosity. In general these equations relate permeability to some power of the total porosity. Baver (2), however, suggests an empirically determined porosity factor that is related to the volume of pore water not strongly influenced by capillary attraction to the soil's surface. The apparent volume of noncapillary pores is measured in a given soil sample by placing the sample in a core on a porous plate and measuring the volume that is displaced as the suction (moisture tension) is increased incrementally from 0 to 300 cm.-H2 0. The first inflection point of

the cumulative volumetric moisture release curve is taken as the moisture tension in the sample and volume moisture released from the noncapillary pores. The percolation rate is measured through these same samples and related to the porosity factor as shown in table 3. The porosity factor assumes the percolation rate to be directly related to the volume of large pores and inversely proportional to the tension (expressed as the log of the tension in cm.-H2 0)required to drain them. Notice the influence of compression on the rate and its independence from texture and so importance of structure.

SOIL STRUCTURE AND AGGREGATION

Because of the random deposition and in-place weathering, most soils contain the entire range of particle sizes. They also contain organic constituents left by plant growth, inorganic products of the precipitation of salts brought in with irrigation water, or evaporation products from previous high water tables. Not all soil particles are inert. The larger the fraction of the clay particles, the greater will be the surface area in a unit volume of soil. This surface area is capable of developing both attractive and repulsive forces toward other particles, depending upon the other chemical constituents present. The strength and direction of these forces control the spacing between the particles and, therefore, the soil's permeabihty to water. If the net effect is to draw the particles together in a combination that resists breakdown in the presence of water, then the soil is said to have good water-stable aggregate structure. Field soils can have a high fraction of clay, yet transmit water readily because of this aggregate structure (table 2). On the other hand, soils with good particle size distribution can be nearly impermeable if the small particles are kept dispersed by mutually repulsive forces.

Organic constituents in soils aie the most important contributors to the production of water-stable structure. The accumulation of biodegraded products of vegetation and roots in the soil pores, accompanied by the physical reahnement of the individual particles by drying, tillage, or root growth, glues the particles together into aggregates. Such aggregates themselves may contain few large pores, but the pores between aggregates will be similar to those between single-grained particles of like size.

EFFECT OF CHEMICAL CONSTITUENTS

The effect of dissolved inorganic constituents on the maintenance and magnitude of recharge intake rates is directly-related to the chemical quahty of the water to be spread. Only soils with high initial intake rates are

25

TABLE 3.-Percolation and noncapillary porosity of various soils.

Soil type Moisture tension

at flex point cm.-H2^

A

Noncapillary porosity at flex point, percent

of soil volume B

Percolation rate

cc./lO min.

Porosity factor

B/log A

Genesee silt loam: Sample 1 Sample 3

Cecil clay: Sample 1 Sample 2 Compressed

Davidson clay: A horizon B horizon B horizon, compressed

Chenango loam Compressed

Iredell sandy clay loam: A-1 horizon A-2 horizon B horizon

Paulding clay

Wooster silt loam.

Quartz sand: (40 to 100 mesh). . . . (40 to 60 mesh) .... (20 to 40 mesh) . . . .

35.7 31.6

44.7 56.2 63.1

44.7 44.7

100.0

141.0 112.2

50.1 56.2 35.5

39.8

398.0

35.5 31.6 17.8

14.7 13.0

12.0 13.5 5.5

6.0 8.0 6.0

11.0 6.0

9.0 13.0 9.2

11.5

10.0

25.0 22.0 22.0

205 137

127 136

6

17 28

4

50 2

65 131

36

93

10

850 675

1216

9.5 8.7

7.3 7.7 3.1

3.6 4.8 3.0

5.1 2.9

5.3 7.4 5.9

7.2

3.8

16.1 14.7 17.6

used for recharge, and quite soon after large-scale recharge begins, all but the most insoluble salts will be leached from the surface by the large volumes of water that pass through it. The soil then will be in chemical equilibrium with the water being recharged, in connection with the irrigation and reclamation of saline and alkali soils, the penetration of irrigation water into soils and the removal of soil salinity have been closely studied. Most commonly in recharge we will be interested in those conditions that can decrease an initially high intake rate, as the equilibrium is established with a water very low in dissolved salts. (Saline waste-water disposal is excluded.)

Dispersion of the clay mineral fraction of a soil results from the formation of a zone of polarized water molecules around the individual clay particles. The thickness of this zone of hydration depends on the charge of the cations i"*") that associate themselves with the negatively (-) charged surface of the clay. This association is called cation exchange. The cations that predominate in natural waters, and on soil, are calcium

(Ca++), magnesium (Mg++), sodium (Na+), and potassium (K+). Generally, monovalent cations (Na+ and K+) disperse soils because of their high level of hydration, and divalent cations (Ca++ and Mg++) flocculate, or weakly aggregate, because of their low hydration and the possibihty of sharing their double charge between clay particles. The dispersion of a soil when recharged with a natural water containing all these ions depends on a weighted ratio of the cations called the Sodium Ab- sorption Ratio, or SAR (25). Potassium (K+) is normally not present in large amounts, so

SAR^ Na+

v/(Ca++ + Mg++) 12

where Na+ Ca++, and Mg"*""*" are concentrations in water in milHequivalents per liter. When most soils come to chemical equilibrium with a water that has a SAR approaching 15, dispersion will begin to affect the structure of the soil and decrease intake rates.

26

The bicarbonate and carbonate anions HCO3' and COa" can also appreciably affect the chemical equilibrium by the precipitation of CaCO., and Mg CO3 in the soil, thus increasing the relative amount of Na +, This is particularly important under irrigation or in recharge, where frequent intermittent drying concentrates the soil solution at the soil surface. Normally where flooding is carried out over extended periods, Ca++ and C03 = concentrations will be in equilibrium with the water being spread.

Total electrolyte concentration can also affect the hydration of clays and, thus, intake rates. If the soil is not in equilibrium with the total salt concentration of the water, the intake will decrease as the soil solution concentration decreases. If poor-quality waters, high in dissolved salts, are recharged (as in waste-water disposal), the high electrolyte concentration will decrease clay hydration and dispersion and intake rates will remain high even if the SAR is above 15. Subsequent spreading of low-concentration, low-SAR water can seal the area through dispersion after the leaching of the surface.

EFFECT OF CLOGGING AND PARTICLE REALINEMENT

While the surface soil may rapidly come to chemical equihbrium with the quality of the water recharged througli it, it will never reach a physical equilibrium. Any individual soil particle is very slowly but continually subjected to forces that would reduce it to its lowest energy state. In other words, the only path is down.

Continued downward water flow through any soil provides internal transport of suspended as well as dissovled matter. Particle movement can result only in a progressive decrease in the size continuity and number of larger water-transmitting pores in a soil profile. The smaller particles are moved by the water until they become lodged or sieved out of the stream in restrictions between the larger particles; this in turn traps even smaller particles in succession.

There is a continuing supply of loose particles, even in well-aggregated soils. The most significant source is the suspended load of the recharge water itself. Figure 13 illustrates the manner in which suspended silts or clays can lodge on and in the surface of a sandy recharge area when floodwaters are spread. This thin layer of fine particles effectively sealed the soil surface.

Surface maintenance of the spreading basins breaks down the aggregates and releases the individual particles. Wetting and drying, freezing and thawing, and chemical weathering, all continuously subdivide the aggregates and soil particles. Provisions must be made for maintaining the open pore space in the soil of a recharge area if its capacity is to be maintained.

•'

*^ *».*

«**, Jh*r '. > Aï*:';'--,! j^y.

.-Zi-

Figure 13.-A. "Orange-peel effect," caused by the drying of a surface seal of silt and clay. When turbid floodwaters are spread, these sediments are filtered out in the upper surface of sandy recharge areas, where they clog the pores and reduce the water intake. B. A clogging zone of fine material filtered out just beneath the surface of a coarse surface soil in a spreading area.

27

EFFECT OF COMPACTION AND CULTIVATION

Soils are often cultivated or mechanically manipu-

lated to improve irrigation infiltration rates. Such practices improve water intake throughout a crop's root zone, but unless the soil has considerable water-stable structure, the long-time intake rate may actually be impaired. Plow pans, or slowly permeable zones, often develop just below the tillage depth in intensively cultivated areas. Surface traffic also causes soil compaction and soil structure degradation, particularly if the soil is at or near field moisture capacity when compacted. Even well- aggregated soils flow Uke a viscous fluid if subjected to vibration and loading. Single-grained sands also pack under loading to reduce the large pore size.

LOCATION OF RECHARGE SITE

In every major agricultural area the surface soils have usually been classifled and mapped, particularly if they are part of a major irrigation or reclamation development. These surveys describe the soil profiles to a depth usually not exceeding 5 feet. For the first stage of recharge site selection, these surveys provide a general picture of the texture, structure, salinity, and upper proflle of the soil in a ground-water basin. Field surveys are quite accurate in describing the area of the major soils mapped, and can provide good first estimates of the available area of soils with recharge potential. Such area estimates, along with a hydrologie estimate of the period and volume of available recharge water, will tell the engineer if the intake rate needed can be achieved on the area and type of soil described in the survey.

If surface recharge seems generally feasible, then the most effecient sites within the area should be identified. Most recent mapping is done from aerial photographs. These pictures provide numerous clues to the shallow geology of the recent alluviation of the basin. Coarse- gained soils, with high water-intake rates but low water-storage capacity and fertility, are indicated by the sparse vegetation they support. Dunes, old streambeds, and stream meanders can be identified quite readily

from air photos.

SOIL STRATIFICATION

Jn all ground-water basins where the storage capacity is large enough to be a signiflcant component in the basin's water management, the surface topography will be comparatively flat. Soil-forming factors of climate and vegation will have been uniformly active over broad areas of this comparatively flat surface. Within the basin, any area mapped as a single soil series can contain accumulations of the chemical and physical products of surface weathering in the upper few feet. If this zone of

accumulation is well developed, it will be much less permeable than the soil above it. When water is spread on these soils the initial intake rate will be determined by the permeabiUty of the upper soil layer until its storage capacity is satisfied. Then the intake will drop to a level nearly equivalent to the percolation rate through the lower zone, acting under the head that is ponded on it. This creates, on a microscale, a perched water table beneath the spreading basin in response to the down-

ward-moving recharge flow. The accumulation of cementing materials within

horizons of a soil profile can produce what are called hardpan layers. Hardpans can be so cemented with lime, siHca, iron oxides, or aluminum oxides that they are impermeable, and transmit water only through cracks and discontinuities. These developed profiles are often buried by subsequent alluvial depositions and may not be evident in the soil survey of an area. Even areas classed as deep alluvium often contain extensive clay and silt lenses that are not evident in the first 5 to 6 feet of soil profile but may restrict recharge rates. So, deep exploration is necessary at all proposed recharge sites.

SOIL PROFILE EXPLORATION

Soil surveys normally describe only the first 5 feet or so of proflle. This is deep enough for most agricultural purposes. Recharge intake may be affected by proflle restrictions at much greater depths.

From a pracflcal field survey standpoint, only a qualitative description can be expected from any indi- rect techniques of shallow-proflle exploration such as surface resisUvity and seismic methods. The problems are the same as those found in describing the basin's geology, only scaled down greafly. Core holes do allow direct observation and laboratory analyses of the material in the profile at a given location, but eventually this information must be extrapolated over the entire area being surveyed. The objectives of the field profile

survey, regardless of methods, are: 1. To determine the thickness and nature of the soil

to the top of the flrst layer that might restrict the downward flow of recharge water in the recharge

area. 2. To determine the area and thickness of the flrst

restricting layer in the spreading area and the surrounding property.

3. To distinguish the character of the restricflng layer from the material above it.

4. To flnd and analyze any other restricting layers as

in objectives 2 and 3 above. Jeíímg.-Probably the least expensive and most useful

technique for shallow soil profile exploration is jetting (25). Here higlvpressure pump is used to discharge water

28

through 3/8-inch or 1/2-inch pipe. The pipe is forced into the soil and successive lengths of pipe added as the water washes it into the profile. The profile can be logged by an experienced operator from the material washed out of the hole, the vertical force necessary to move the pipe downward, and the amount of water lost in the hole. In very coarse sands, 1/2-inch pipe discharg- ing 30 g.p.m. (gallons per minute) may be jetted to 50 feet with little difficulty. In fine-textured silts and clays, 3/8-inch pipe may be jetted to 100 feet with a discharge of 8 g.p.m. Cobbles will stop the pipe. Most hardpans can be penetrated by ramming the pipe through them.

Soil Auger.-The soil auger is a 4-inch diameter, 6-inch-long cyUnder with two hardened steel blades fixed at one end and a bracket for attachment to a 5-foot length of 1/2-inch metal electrical conduit at the other. Screw-in extensions to the conduit can be used to auger to depths around 20 feet. In unconsolidated sands the sides of the hole often slough in, preventing progress. Hardpans are seldom penetrated because of the hole diameter. The method provides a visual sample of the material encountered, and the soil profile can be logged quite accurately. The soil structure may be disrupted by sampling, but chemical and particle-size analyses can be made. The sampHng is slow and arduous and is practical only to shallow depths.

Mechanized Augers.-Augering machines use a ro- tating helical screw to cut and Hft the soil. This speeds up the sampüng,but retains the other disadvantages of the hand auger. Also, the samples angered are quite mixed, and some of this mixing is with materials from the sides of the hole.

Shallow Seismic Methods.-Seismic wave velocities can also be used to measure layer placement in shallow profiles. The physical principles are identical to those used in defining the geology of the basin. The equipment used is capable of locating only the first layer of material having a major increase in density, if it is v^îthin 45 to 50 feet of the surface. Hardpans are readily identified, as are completely open profiles of uniform density. Lateral discontinuities can be shown for layers that have sufficient density definition.

The equipment is similar in principle to geophysical equipment, only scaled down in sensitivity and price. The "shot" consists of a blow with a sledge hammer or a blasting cap discharge. All operations are carried on at the surface, and units are hand carried.

Surface-Resistivity Methods.-lf in the soil profile there is a zone differing greatly in its electrical- conducting properties from that in the material above or below it, such as a sahne water table or a drained gravel below or above fine-textured materials, the surface- resistivity method may apply. It is analogous in physical

principles to the down-hole geophysical method. An electrical field is estabUshed between two stationary electrodes. The shape of and resultant displacement of this field by the layering in the profile is measured by moving a second set of electrodes out along the ground surface. From the changes in slope of a graphical plot of the resistivity distance relationship, layering can be inferred. This method is of value where the layers are within a depth of 50 to 70 feet.

EXISTING PERCHED WATER TABLES

The first thing to look for at a potential recharge site is the depth and cause of the first water table. This can be found by noting the moisture saturation of the soil cores, by determining the position of the water table in the core hole, by jetting pipe to the top of the suspected perching layers, or by determining if differentials exist in the static heads in nearby wells. If a perched water table is present, the continuity and extent of the layer causing it can be traced through the area of interest with jetted observation wells. If a perched table is found to be present with only incidental recharge as a source of percolation, then the layer causing it will be the restriction hmiting the vertical movement of water artificially recharged from above.

No layer that might have a hydraulic conductivity less than the surface soil should be overlooked as a possible perching zone in the profile under a proposed spreading area. Although no perched water table may be apparent before recharge water is spread over the area, such layers can commence to function when vertical water movement is greatly increased with artificial recharge. If such layers are extensive enough and either quite thick or low in hydraulic conductivity they also can eventually limit the recharge rate of the spreading area. If the perched water table produced by recharge rises to the soil surface then the recharge rate is no longer controlled here but by percolation through the sublayer and fiow laterally on it away from the area's boundaries. A network of piezometers on these layers and observation wells in the actual water table will prove invaluable in predicting the function of a recharge area and should be considered as part of the cost of its construction.

FIELD MEASUREMENT OF SOIL INTAKE RATES

The necessary evaluations of intake rates fall into four categories:

1. Site selection by intake rate comparison, 2. Operational recharge rates for engineering design, 3. Extended-period rates for engineering design, and 4. Measurements of effect of surface treatment on

rates for maintenance and surface modification.

29

After a site has been selected as having recharge potential on the basis of soil surveys, estimated intake rates, and on-site inspection, some measure is needed of the field intake rate for the area, or the block of real property associated with the soil selected. The surface hydrology at the site will determine the type of intake evaluation, and accuracy needed. Five general situations evolve depending on whether the supply of water limits recharge, or recharge capacity limits deHvery rate from the source. At some sites the situations overlap somewhat.

Source Limited.-Uere, the estimated rate of soil intake over the available area exceeds the water delivery to the site. This may be the case when there is a dependable source of water available such as imported water, industrial waste water, off-season regulated flow from irrigation, surface storage, etc. Here the engineer can design to the known available delivery. He must know if additional area at the site can compensate for the expected long-term decrease in the soil's intake rate. If an excess of area for spreading is available, then only a few very coarse field measurements of soil intake are necessary to back up the initial estimates. Maintenance procedures necessary to stabilize the area over the long run of recharge should be considered.

Source and Intake Area Matching.-If economics dictate that no more area be used than is needed, then a critical estimate of intake is required in the field. The question is, "Can the recharge area's size and, therefore, cost be matched to water input?" Of major concern is the size of the area that can be stabilized by maintenance over the long run. Some projections as to the influence on the soil intake of long-term spreading must therefore be made.

intake Area Limited.-\n these instances, the maximum possible volume is recharged in a Hmited time; as for example, during storm runoff. The question here is, "What diversion structures and timing will be required to optimize the recharge over the available area?" Water will be available for short periods of time. On-site short-term storage for pretreatment of water and settle- ment of suspended load is desirable. Spreading-area maintenance will affect the intake rates so greatly that any measure of the proposed site's initial intake rate is meaningless. What is required here is some way of evaluating the effects of pretreatment of the water on intake rates.

Intake Rate and Area Both Limited.-ln this situation, plenty of water is available for spreading, but the capacity for acceptance is limited because of small area and low intake rate of the recharge area. Since not all of the available water can be used for recharge, careful estimates of the long-term intake capacity are needed for

planning actual water delivery that can be efficiently used. These areas might function best as settling and storage areas for more efficient sites.

Area Very Large but Intake Limited,-k very large area with very low intake rates can accommodate a large input if distribution is available. This would be the situation where recharge is supplemental to irrigation. Surface evaporation and transpiration of crops or vegetation use large quantities of water in an irrigation system. Therefore, the evaluation of intake rates by soil characteristics alone is useless. Recharge can best be estimated from the area available, the surface irrigation-water deliveries or rainfall that disappear into the area, and the estimates of how much of this was consumed by evaporation.

METHODS OF ASSESSING RECHARGE RATE

PILOT RECHARGE AREAS

The intake rate of soil is a dynamic and transitional quality that depends on the interplay of many physical, chemical, and biological reactions. An accurate prediction of the future intake from any current measurement is doubtful. Also, natural field variability makes it difficult to extend measurements from one point to another even when the soil is mapped as the same. In addition, the quality of the water eventually to be recharged affects the measurements. For these reasons, the rate can be measured accurately only during actual recharging.

Pilot recharge basins of 1 to 2 acres are not difficult to lay out if they are next to the source of water to be spread. Water can be metered onto the area, and the area can be put through simulated recharge cycles. Mainte- nance and surface treatments can also be programmed into the basin's operation. Because this method approaches the actual operational scale most closely, the resulting rate information is most reliable.

INFILTROMETERS

Any small but accurately controlled area of soil that is isolated for the measurement of water intake is called an infiltrometer. Commonly, metal cyhnders 6 to 12 inches in diameter are driven into the ground (fig. 14). Water is put into the cylinders, and the rate at which it enters into the soil surface is measured. The water entry with time can be monitored in many ways, but because of the small areas involved, the accuracy of the volume measurement is critical.

Larger ponds may be controlled by levees or metal sides. The area can be ponded and the fall in pound level measured with a hook gage. Volume changes in a storage tank connected to a float value-head control in the infiltrometer can be measured. If the area is large enough

30

Figure 14.-lnfiltrometers replicated to evaluate water intake rates over extended flooding periods.

or the intake high, a small in-line flow meter can be used.

Many elaborate systems have been devised to try to duplicate the physical conditions found in spreading water over a large area within the area of influence of the infiltrometer. One of these is measuring the intake inside of a larger flooded area in order to eliminate lateral flow influences. Regardless of how carefully they are installed, the placement of the cylinders or construction of the small ponds alters the natural structure of the soil over a significant area associated with the measurement. Also, where point values are to be integrated over a considerable area, only a large number of measurements statistically analyzed can yield a reliable rate.

Infiltrometers are of greatest use in evaluating the effects of soil treatments or surface modification on intake rates. Here the rate can be compared to the rate in a specific inflltrometer before treatment. Again, experiments should be statistically designed. Infiltrom- eter studies are best suited to comparisons of recharge intake rates. By careful standardization of field experimental procedures and repHcation of treatments, the method can indicate the relative performance of different sites, but it should be used only as a first approximation of the absolute rate of intake for a site.

SOIL CORES

The smallest sample size for determining the rate of water flow through a given soil surface is obtained by pressing a small-diameter cylinder into the surface and extracting a core of soil for study. These cores range from 2 to 6 inches in diameter. The cores are set up in the laboratory, a constant ponded head of water is established above them, and the volume of flow through

them is measured. The application and restrictions of this method are like those for infiltrometers.

Soil cores are the only practical way of evaluating the relative performance of deeper layers and lenses found in most soil profiles. Coring of deep soil profiles, as continuously as is operationally possible, and determining their permeability in the laboratory can identify subsurface layers that might restrict the flow of recharge to the water table or control the surface intake rate.

SITE SELECTION VS. ENGINEERING DESIGN

The entire physical, chemical, and biological balance of a soil is drastically altered when subjected to recharge operations. In soils with naturally high recharge rates, these alterations can result only in a decrease in these rates, even if careful attention is paid to surface maintenance and treatments.

The methods available for evaluating site performance can not adequately predict the long-term changes to be expected in soils under recharge. They should be considered only as a means of comparison between sites. The true measure of the site's performance can at best only be estimated by a pilot recharge experiment. The actual performance may be realized only after the area has been in operation for several years.

The first approximation of recharge rates and the area required can be made on the basis of soil texture, structure, and profiles gained from soil surveys and from generalized permeability data atttributed to the texture. The proper use of infiltrometers and soil cores can measure the initial intake rate of the major soil areas that show potential for recharge. The engineer or hydrologist should then consider whether the area is large enough to make recharge worth while in balancing ground-water withdrawals.

If recharge appears feasible, the intake evaluation for each site should be attempted. Even if the rates come from pilot recharge areas, they should be considered as maximums for long-term recharge. Designs of structures for diversion and recharge, and extent of areas to be used should be adjusted accordingly.

QUESTIONS FOR THE SOIL SCIENTIST

To assess the surface recharge capabilities and intake potentials of the soils in the ground-water basin, the soil scientist should answer the following questions:

A. What is the origin of the parent material, depositional environment, and extent of soil development in the major soils in the basin?

1. Are soils of suitable particle size distribution accessible for recharge?

31

2. Are the surface areas large enough to be useful for recharge?

3. Could soil-forming processes have modified the profiles enough to affect usefulness for recharge?

B. Ave the basin's soils mapped? 1. Are the air photos or soils maps of sufficient

detail for estimates of the area of potential recharge sites?

2. Can textual and profile data provide a first estimate of intake rates under recharge?

3. Do the data indicate any shallow restrictive layers?

C. Hov^ will soil stratification affect recharge rates and the subsurface disposition of storage? 1. What methods were used in the analysis? 2. How deep and how extensive are the major

restricting layers shown in the soil? 3. How thick are these layers? 4. How permeable are these restrictive layers, and

how will they react when artificial recharge water moves through them?

5. Are these restrictive layers the intake-limiting factor of the profile?

6. If so, could engineering procedures improve the rate? (For example, excavation of first restriction.)

D. How can a value closest to the actual intake rate of a given recharge site be provided? 1. How accurate are the laboratory and on-site

measurements used? 2. Can the conditions of recharge be simulated for

the rate evaluation? 3. Can any projections be made as to the long-

term intake rates after several years of operation?

E. What is the nature and soil structure at the site? 1. How might this soil structure change under

extended flooding? 2. How will the chemical quahty of the recharge

water affect the rates? 3. How much will the biological and sediment

load of the recharge water affect short- and long-term rates?

4. What procedures can be suggested for maintaining the surface structure of the recharge area?

How can the soils data best be organized to yield the necessary information on water intake in the recharge sequence? How should the soils data be presented to make it most useful to the overall picture of the basin's physical hydrology?

32

CHAPTER V. THE APPLICATION OF GROUND-WATER FLOW THEORY TO ARTIFICIAL RECHARGE

NEED FOR THEORETICAL ANALYSIS

Who benefits most from artificial recharge? Do recharge operations at point A influence water availabihty at point B and, if so, by how much? The answers require a definition, in space and time, of the position of the recharged water within the ground-water basin or water service area. The direct approach, measuring storage changes by monitoring a network of observation wells, is excellent for determining broad-scale, long-term reactions to artificial recharge practices. However, when the ground-water body is artificially recharged at isolated points in the basin, or if the basin is not under control of a single water-service unit or district, accurate definition of changes in storage in basin subunits and next to individual recharge areas may require an expensive observation network of wells and frequent monitoring. Therefore the first step is generally to use all available direct observational data as a base and apply idealized ground-water flow theories to estimate the storage distribution with time. Because of the shortage of observational ground-water data in most basins, this may be the engineer's only way of predicting the effects of ground-water withdrawals and recharge over the long run.

Historically, as the value and control of ground water increases, the need for its accurate description expands. Thus, changes of ever-smaller magnitude and over smaller areas will require definition. Currently, computer descriptions of basin ground-water flow and storage are based on the simplest theoretical descriptions of ground-water flow in aquifers. But, as the problems narrow to given areas in the basin, the more comphcated theories deahng with cones of depression around wells, ground-water mounds at recharge areas, and their interactions will enter into the hydrology of these areas and so into the resulting computer analysis. This chapter presents some of the theoretical work dealing with recharge and indicates its relationship to observed flow and storage of water during and after recharge.

r = radial distance from geometric center of well, in ft.

h = rise in water-table elevation at some point (x, y), in ft.

ho = rise in water table elevation at geometric center of recharge area (x = 0, y = 0), in ft.

s = fall in water table or drawdown at some distance r from a pumping well, in ft.

s^' = drawdown in the well, in ft.

Sf^ = drawdown in the aquifer adjacent to the well, in ft.

Q = discharge of well, in gallons per minute (g.p.m.)

D = saturated thickness of aquifer, static water table to base of aquifer, in ft.

K = aquifer hydrauUc conductivity, in

ft.^ sec. ft.îft.-H.G/ft.)

or

ft./sec.

V = specific yield or fillable pore space, in ft.^/ft.^

oc = aquifer constant

T KD . ^ ., = y = ^,mft.Vsec.

T = Transmissibility

= KD, in ft./sec.

i = recharge rate or volume of water per unit area entering soil surface, in ft.^ -H2 0/ft.^ sec. or in ft./sec.

R = ;^ is the rate of rise of water table if all water were retained in pore space beneath spreading area and above existing water table, in ft./sec.

DEFINITIONS

The following terms will be used in analysis:

t = time, in sec.

W = width of recharge area, in ft.

L = lengthof recharge area

X, y = coordinate distance away from geometric center of plot, in ft.

HEAT FLOW VS. GROUND-WATER FLOW

In problems related to ground-water flow and storage beneath a recharge area of given dimensions, the applied mathematician generally first looks to the mathematics of heat flow in solids for analytical solutions that might be successfully applied to ground-water flow. The analogy between the flow of heat in soHds and the flow of water in saturated soils and aquifers is a good one, and many practical problems can be solved with it. The

33

analogy is quite apparent when one compares the steady-state heat-flow equation with the Darcy equation for steady-state water flow through soils.

Heat-flow equation

AT H = KA ^

Darcy equation

Q = KA^

Thus the steady-state flow of heat through a conductor of cross section A is in proportion to the gradient in temperature AT/L, just as the steady-state flow of water in a sand-filled conductor, cross section A, is proportional to the gradient in hydraulic head Ah/L.

Darcy's steady-state equation finds excellent application in ground-water problems where the scale of the problems is such'* that flow can be considered as occurring through a uniform cross section under a constant gradient. However, in recharge, where the water table is changing its position with time, a steady state no longer exists. Then problems must be treated by mathematical methods that include time-dependent storage changes.

Heat-flow theory has been successfully applied to several transient ground-water flow problems. However, two major differences between the physical nature of the water and that of heat flow qualifies the analogy for transient problems. The heat capacity of a body is not restricted by its geometry; as heat is added there is a corresponding increase in temperature in the body until it melts. Water storage, or specific yield, is limited by the internal pore space of the aquifer. The geometric boundary of a ground-water body, defined by the water-table surface, rises as water is added. This rise significantly increases the dimensions over those of the analogous heat-flow system and will measurably influence the agreement of the heat-flow theory with the actual ground-water flow.

Generally the engineer is interested in estimating the position of the water table at any given time and given distance from the center of a recharge area during and after recharge. Mathematical solutions by the heat-flow analogy are excellent for this purpose, but certain concessions must be made for the differences between ground-water flow and ideal heat flow. These concessions are called the Dupuit-Forchheimer assumptions:

1. Flow within the ground-water body occurs along horizontal flow lines whose velocity is independent of depth.

2. The velocity along these horizontal streamlines is proportional to the slope of the free water surface.

In some physical situations in ground-water flow these assumptions can lead to errors in prediction, for example, flow into tile lines, flow right next to wells. In full-scale recharge operations these errors can be neglected (3).

GROUND-WATER MOUNDS RESULTING FROM RECHARGE

In the following sections we shall present the theoretical analysis by R. E. Glover^ of ground-water mound changes under the most common basin configurations found in recharge. These theoretical ground-water mound descriptions were field-tested in the alluvial sediments of the San Joaquin Valley. The predictions of the observed rise were well within engineering accuracy (3). The major problem in their application lies in the field evaluation of the aquifer properties K, D, V. This will always be the case, however, in any situation where theory is applied to ground-water phenomena in the field.

Spreading of a Mound During Continuous Recharge from a Square Area.-Figaie 15 is a dimensionless plotting of the rise at the center (h^) of a square recharge basin of width (W). The rise at the center of a circular recharge plot of the same area is identical to this. This chart may be used either to estimate the height of a mound or to determine the aquifer properties from observed data.

Example of the use of figure 15 to compute the height at the center (ho) of ground-water mound:

Recharge is applied evenly to a square plot 330 feet on a side at the surface intake or recharge rate (i) of 1 foot per day for a period of spreading (t) of 15 days. Compute the height of the center of the ground-water mound at the end of this period.

Suppose

K = 0.00015 ft./sec.

D = 100 feet

V = 0.15 (dimensionless)

R = — = rate of rise of mound if no lateral flow oc- V A curred

1.0 (0.15) (86,400)

= 77.16 X 10"^ ft./sec.

V

W = 330 feet

^Flow down-gradient through distances measured in thou- sands of feet, aquifer depth in tens of feet, and cross section in hundreds of square feet.

Glover, R. E. Mathematical derivations as pertain to ground- water recharge. Agr. Res. Service, USD A Mimeo. 81 pp. 1961.

34

Figure 15.-Dimensionless plot of the rise at the center (ho) of the mound beneath square recharge area (see example calculation page 34).

i = 1.0 foot/day

t =15 days

t = (15) (86,400) = 1,296,000 seconds

Rt = (77.16) (10)'^ (1,296,000) = 100 feet

^ ^ 330 _ 330 _ yjÄ~^ t V(4) (0.1) (1,296,000) 720

From fig. 15 for ^ ^ = 0.458, ^ = 0.207

So

ho = Rt(.207) = lOOx.207 = 20.7 feet

This is the estimated rise at the center of the plot.

Figure 15 also will be useful where the rise of the ground-water mound under the center of the plot is observed and it is desired to determine the aquifer constant ^. Suppose a rise of 20.7 feet is observed under the center of the square plot described above after the plot has been recharged at the rate of 1 foot per day for 15 days. It is assumed that V is known from a laboratory measurement to be 0.15.

Then

ho Rt

20.7 100 = 0.207

W From fig. 15, for ^ = 0.207 read ,

Rt V4 ex t 0.43

Then

y/4 oc t W

0.43

330 ^

0.43

4 ex t = 588,900; « =

= 0.114ft.Vsec.

767.4

588,900 (4) (1,296,000)

This figure is to be compared to oc = o.l ft.^/sec. with which we began, since the second computation retraced the steps covered in the first computation. The difference in the values is due to errors introduced in reading the charts.

The lateral spread of the ground-water mound can be calculated with the aid of fig. 16. For example:

For the same conditions of the previous example, what will be the height of the ground-water mound at

35

EDGE OF PLOT

(—)

Figure 16.-Dimensionless plot of the rise and horizontal spread of the mound beneath a square recharge area.

36

w the edge of the plot (—) when recharge has continued

0.15:

W 300

for 15 days? From fig. 16,

when W W

' 2'

h

V4 a t Rt

then

h = (100) (0.152) = 15.2 feet

If X = W, ^ = 0.08

h = (100) (0.08) = 8.0 feet

Spreading of a Mound During Continuous Recharge from a Rectangular Area.~The chart of fig. 17 is similar to that of fig. 16, but it apphes to a rectangular recharge plot whose length is twice its width. As before, the chart apphes to the conditions along the axis of x. The pattern is symmetrical with respect to the y axis. The chart applies to the half of the pattern to the right of the y axis.

The chart of fig. 18 applies at the center x = o, y = o of a rectangular recharge plot. The pattern of rise (h«) is shown for length to width (L/W) ratios of 1, 2, and 4 and for an infinitely long strip. As the (L/W) ratio grows large (L/W -> ^) compared to unity, the pattern of the ground-water mound, taken along a section joining the centers of the long sides, approaches the pattern that develops along a transverse section of an infinitely long strip.

As an example of the use of fig. 17 the shape of the ground-water mound, after recharge has been applied for several periods of time, will be computed. The conditions will be the same as for the example on page 34 except that (L/W) = 2. The recharge plot would then be 660 feet long and 330 feet wide. The manner of making the computations is shown in the following tabulation for time 1 day or 86,400 seconds.

Example: What if in the previous example the plot were a

rectangle L/W = 2, or W = 330 and L = 660. What would the water elevation be at the center at x = 0, W/4, W/2, 3W/4, W from the centerline after 1 day?

So:

R = 77.16 X 10"^ ft./sec.

1 day = 86,400 seconds

Rt = 77.16 X 10"^ X 86,400 = 6.667 ft.

\/4 o: t \/4(.01) (86,400)

From Figure 17:

1.775

X

W h

Rt -è-*' Feet

0.00 0.913 6.09 0.25 0.823 5.49 0.50 0.500 3.33 0.75 0.123 0.82 1.00 0.045 0.30

Recharge Hê//—Theoretically a recharge well that injects water into an aquifer can be considered as an upside-down well with the rise in the water-table elevation or increase in hydraulic head at any radial distance being numerically equal to the drawdown at the same distance had the well been pumped.

The mathematical relation that stems from heat-flow theory deaHng with the performance of a pumped well is

(8):

where Ui

2 Tí KD

\/4 a t

f du (2)

Ui

; U2

Q = well discharge (constant);

s = drawdown at some radial distance from the center of the well;

r = the above radial distance ;

t = the time since pumping started;

K, D, a = as previously defined.

The integral

/

du (3)

from u 1 = \/4 ex t

37

1.0-

EDGE OF PLOT

Figure 17.-Dimensionless plot of the rise and horizontal spread of the mound beneath a rectangular recharge area in which the length is twice the width.

38

Ä ^

O

E 5

S)

39

has been evaluated by Bittinger and is tabulated for

values of the dimensionless parameter

Water Movement by R. E. Glover (8). \/4at

in Ground-

Example: Suppose a well is pumped at the rate of 250 gallons

per minute and the aquifer properties are, as before

K = 0.00015 ft./sec. oc = ^ = o.l ft.^sec.

D = 100 feet

V = 0.15

Compute the drawdown (s) at a distance of 500 feet from the well after pumping has continued for 3 days.

Solution: To convert gallons per minute to cubic feet per

seond, multiply by 0.002228, then

Q = 0.557 ft.Vsec.

One day is 86,400 seconds. Then

t = (86,400) (3) = 259,200 seconds

\/4 oc t = Vl,036,800 = 1,018 feet

r 500

V4 oc t 1,018 0.492

From the table previously cited (8) the integral in (3) can be evaluated directly as 0.535.

Then, from formula (2)

_ (0.557) (0.535) _.. ^ '"(6.2832) (.015) - ^'^^^'''

or the ratio of discharge (g.p.m.) to feet of drawdown in the well. This characteristic is usually determined when the well is first developed, since it is necessary to match the pumping plant properly to the well characteristics. The specific capacity is not a constant, it varies with time of pumping, even if the rate is constant. But it depends most noticeably on the type of aquifer system the well penetrates. Figure 19 shows in a general way what miglit be expected during a specific-capacity test on the two extremes of aquifer configuration. If the well produces water from an ideaUzed artesian aquifer, the line relating discharge (Q) to drawdown in the well (s^v) will be, for practical purposes, linear until the drawdown is great enougli to dewater the aquifer. At this point the well commences to respond as if it were pumping from an unconfined water table aquifer, and the hne showing the Q to s,v relationship begins to curve. Most real aquifer systems are complexes of the two shown in figure 19, but available specific capacities can be valuable. For example, if a well with a specific capacity of 10 g.p.m./ft. and exhibiting artesian characteristics were recharged with clear water kept at heads above the original water table and equivalent to the range of pumping drawdown, it could be expected to recharge water at a capacity of 10 g.p.m./ft. of head rise in the well.

If the aquifer properties are known from well tests in the area, equation (2) can be used to estimate the specific capacity at recharge capacity of a well. By assuming values of Q and using the aquifer constants, the well drawdown (s^) may be estimated from equation (2) at an r equal to the radius of the well. However, there are entry or exit head losses as the water flows out of the aquifer into the gravel pack or well casing. That is, it is possible to calculate the drawdown at the edge of the aquifer nearest to the well, but the well and gravel pack may also contribute considerable head loss. The well efficiency (E^.) is defined as

F = ^ x= 100

SPECIFIC CAPACITY

Often it is important to estimate the recharge capacity of a proposed recharge well in an aquifer having known characteristics. In a pumped well, the well and aquifer capacity to produce water is combined into a simple experimentally determined factor called the specific capacity. It relates the drawdown in the well (Svv) to the associated discharge of the pump (Q) is defined by

specific capacity = Q/s^

where

s^ = drawdown in the well

Sr^ = drawdown in the aquifer adjacent to the well

Well efficiencies can range from practically nothing to nearly 95 percent, depending on correct construction, development, and maintenance.

Example: What is the approximate specific capacity of the well

described in the previous example if the well efficiency

40

CO

(/> Ï3

••3

CSI

o o

<

Tí

o I

CO Csl

i"-d'ß - 39ilVHDSia dwnd

41

is 80 percent and the well is gravel packed to 24 inches in diameter?

Let r = 1 foot

V4 oc t 1018 = 0.00098

s = (0.557) (6.639) (6.2832) (0.015)

= 39.2 feet

At 1 foot from the center of the well, there would be strong vertical components in the flow pattern. Since these are neglected in developing the formulas described herein, this latter computation would not be exact. It should be of the right order of magnitude, however.

So

_Q

39.2 0.80

250 49.0

= 49.0 feet

= 5.1 g.p.m./foot

With recharge wells the efficiency falls off quite drastically with time because of the filtration of suspended matter in the water within the gravel pack or in the aquifer material very near the well. Pumping can re-establish efficiency in part but never completely. This effect will mask all the other interactions that might be theoretically determined.

Equilibrium Calculation-Confined Aquifer-If 2í\VQ\\

is pumped or recharged for any length of time, an equilibrium condition in the cone of drawdown can be approached. This relationship has been derived by Thiem (26). In the current notation the Thiem equation for a confined aquifer is

r2

S2

Qloge 11

2 TTKD (4)

where

Si and S2 are drawdowns at two observation wells at

ri < r2 feet from the well in the aquifer

To apply this relationship one must assume a distance (r2) where the drawdown (S2) becomes insignificant; generally 1,000 feet is assumed, so

S2 ^ 0 at r2 = 1000

Example: Evaluate the previous example as if the well were in

an artesian or confined aquifer and at equilibrium.

K = 0.00015 ft./sec.

D = 100 feet

ri = 1 foot

ri = 1000 feet

Sl = 7

Sj = 0

Q = 0.557 ft.Vsec.

om equation (4)

n..7W. 1000

Sl = (6.2832) (100) (0.00015)

(0.557) (6.9077) (6.2832) (0.015)

= 40.82

because of symmetry the rise from recharge (h) would be the same magnitude as the drawdown (si), so if the well is 80 percent efficient

40.82 .n 1 f . Sw = —T^TT - 50.1 feet .80

_Q ^ 250 Sw 50.1

5.0 g.p.m./foot

Equilibrium Calculations-Unconfined Aquifer.-The equation suggested (26) for the unconfined aquifer

Qloge r

n [D + (D-Si)] (S1-S2)

where S2 -> 0 at r2 = 1000

(5)

compensates for the fact that the wetted depth increases or decreases in the vicinity of the well when flow is not confined. This quadratic can be solved for Si from the formula

Qloge '^

s.^ - 2 D s, + ,K ' = ° (6)

Example-Pumped well: Substituting previous values and using the quadratic

reduction formula,

Sl = 57.16 feet (the root 142.8 is excluded)

Sw = 71.45

Q/Sw= 3.50 g.p.m./ft.

42

If flow during recharge is unconfined, symmetry is not present, so equation (6) will take the form

= 0 Sl^

Qloge -^ -iJ-i ^

Example ;-Recharging well:

Sl = 34.78 feet

Sw ' 1^ = «- Q_ c = 5.75 g.p.m./ft. %^

So the fact that the water table buildup in the vicinity of the well appreciably increases the value for the wetted depth (D) during recharge and decreases it during pumping, yields an asymmetry in the h to s relationship for a well into the water table or unconfined aquifer.

AQUIFER TESTS

The most important application of the well theories is the evaluation of the constants defining the nature of the flow through the aquifers in the field. A great deal of literature is available on the procedures, problems, and interpretations of aquifer test methods (7, H, 26). These should be directly consulted before attempting these measurements in the field.

Basically the procedures involve inducing controlled discharges into or out of the ground-water body, and measuring resultant head changes with distance away from the well as a function of time. From such

information the theories introduced previously may be used to calculate the required coefficients. A program to perform and analyze well tests within a ground-water basin can be of inestimable value to future ground-water development in a basin.

QUESTIONS FOR THE HYDROLOGIST

Questions that the hydrologist should answer from ground-water flow theory are:

A. What are the properties of the aquifer materials in the basin? 1. What are the capacities for storage? 2. What are the capacities for transmission?

B. What is the expected response of the water table in the vicinity of the proposed ground-water recharge basins? 1. Will waterlogging occur in surrounding areas? 2. If so, what basin shapes might alleviate the

problem? 3. Will the intake rate be affected by subsurface

conditions, causing a rise of the water table to the surface?

C. Will the water recharged stay within the boundaries of the agency operating the recharge program? 1. What surface locations will be benefited di-

rectly by this recharge? 2. How soon will these areas benefit?

D. How can an aquifer property data collection program be established and justified to aid in future descriptions of ground-water recharge and basin water-use programs?

E. How should the ground-water hydrology be presented to make it most useful to an engineer estimating the results of the recharge sequence?

43

CHAPTER VI. METHODS FOR ARTIFICIAL RECHARGE

In the methods used for artificial recharge, two main factors influence infiltration: (1) Increase of wetted area and (2) length of time water is on the land. The term "water spreading" has long been used to describe recharge systems. As the term implies, water is diverted from natural channels and spread over adjacent porous lands, thus increasing the wetted area (14)(15).

There are six general methods of artificial recharge: (1) Basins, (2) ditches or furrows, (3) flooding, (4) use of natural stream channels, (5) pits and shafts, and (6) injection wells.

Other methods have been suggested but have not been used to any great extent—for instance, the overap- plication of water to irrigated fields to increase deep percolation. This implies either an excess water supply during the irrigation season or use of irrigated lands during the noncrop season. The suggestion has merit in that an existing irrigation system can be used for bringing water to the land and that a large area is involved. However, many factors must be evaluated before it can be confidently recommended. The effect on crops and soil when excessive amounts of water are applied for long periods of time, the costs involved, and attitudes of the landowners toward permitting their lands to be used for recharge purposes are among the unknown factors.

According to Richter and Chun (18), there were 276 active artificial recharge projects in California in 1958. Of these, the basin method constituted 149 projects or 54 percent of all projects. Modified streambed was used in 15 percent, ditches and furrows in 8 percent, pits in 7 percent, flooding in 4 percent, and injection wells in 12 percent. As to relative quantities of water recharged, the basin method handled 58.4 percent of all water reported to be recharged, modified streambed 29.5 percent, ditch and furrow 9.4 percent, pit 1.3 percent, well 1.0 percent, and flooding 0.4 percent.

BASINS

In this most common method of recharge, water is impounded in a series of basins formed by low levees. In general, levees follow ground surface contours, and are arranged so that the flow of water from upper into lower basins can be regulated. The size of individual basins generally depends on the slope of the land surface, except on relatively flat plains such as valley floors.

The objective of the basin-type project is to obtain the maximum ratio of wetted area to gross land area, commensurate with efficient operation and maintenance. This is particularly important where suitable locations are scarce or land values are extremely high.

Existing basins have an average use ratio of about 75 percent; however, in urban areas, the ratio in some instances has approached 90 percent.

The water moving into the ground, and away from the basin, moves laterally as well as vertically. The lateral movement perniits more water to be absorbed than could be accommodated in the vertical column underlying the pool. Thus, the greatest efficiency in small ponds is attained when the perimeter of the pond is large in relation to the area. While it is evident that a maximum condition would be attained with a long, narrow pond, considerations of service and maintenance must temper judgment in this regard. However, as basin area increases, this shape effect diminishes rapidly and may be disregarded since the lateral flow at the boundary will become a much lesser percentage of the total volume of water absorbed.

In general, greater flexibiÜty of operation and maintenance can be obtained by providing for standby basins. This is important in continuous spreading projects because it provides for continuity of operation when certain basins are removed from service for drying, maintenance, and rehabiUtation to enhance infiltration rates. Facihties can be provided to bypass water around basins temporarily out of service. In projects designed principally to spread storm waters, multiple basins are advantageous because the first of a series of basins can be used for settling silt. The desilting basin should be large enough to reduce the velocity of flow substantially, and its inlet and outlet facilities should be so located that short-circuiting is prevented.

In undeveloped areas, levees are frequently constructed by bulldozing native soils into place without detailed consideration of fill slopes or compaction. Irregular and guUied surfaces can be used with a minimum of preparation. However, in or near urban areas where seepage may damage private property, greater attention must be given to details of construction. In general, levees can be constructed with side slopes of 1-1/2:1, with an allowance for freeboard ranging from 1 to 3 feet, depending on the compaction of the levee material. Roads are usually constructed on levees to facilitate patroUing, inspection, operation, and maintenance.

The design of any multiple basin system must provide adequate control of flow between basins. Gated culverts of adequate size, strategically located through levees separating adjoining basins, have been used successfully, as have weirs and spillways. The flow of water in weir installations is usually controlled by flashboards. Weirs or drop structures constructed of concrete rather than

44

treated lumber are preferred, since treated lumber generally has to be replaced in 10 to 15 years. Consideration must be given to erosion caused by the increased velocity on the downstream side of interbasin structures (fig. 20, 21, and 22). If necessary, riprapping should be provided to control erosion. Figure 23 illustrates a plan of a typical basin-type recharge project, together with several types of interbasin structures. The photos in figure 24 show basins used by Los Angeles County Flood Control District.

Unless inflow is carefully controlled and supervised, a structure to facilitate the return of excess water to the stream should be placed at the lower end of the project.

1 «

Figure 21.-Rosedale - Rio Bravo Water Storage District, Balcers- field, Calif. Permanent drop structure in Goose Lake Slough. Concrete abutments are precast; welded metal boardways and catwalk are set in a poured-in-place concrete apron.

-í:**'

kl

Figure 20.-Berenda Slough in Chowchilla Water District, Chow- chilla, California. A. Permanent drop structure, which may be extended by a sand dam. Steel boardways are hinged at base. The top latch allows quick removal in time of flood or if trash blocks discharge, B. Downstream view of concrete wings and apron with riprap to protect channel's levees.

■.v'iri-j 1 T"^' jr~*- *^f^^

B . *:. •?•

Figure 22.-Arvin-Edison Water Storage District, Arvin, Calif. A. Interbasin drop structure. Steel grating provides access to boardways that control individual pond levels. B. Down-gradient side of drop. Sixteen-inch pipe carries water from drop through levee. Concrete apron and riprap extension stabilizes pond bottom during filling.

45

Baffle, as required

Figure 23.-Typical plan of basin-type recharge project.

Usually a weir is placed so as to direct the flow back to the stream.

The basin method has one advantage over other methods of spreading. Its surface storage capacity can be used to even out fluctuations of the inflow.

fe^'

^0^-

Figure 24.-A. Water-spreading ba.sins in Arroyo Seco Wash next to main stream channel near Pasadena, CaUf. B. Water- spreading basins in operation in Santa Anita Wash. (Photos courtesy of Los Angeles County Flood Control District.)

Experience has shown that maintenance is required. The relatively small levees or dikes may be eroded by wave action created by winds. This has been overcome in some places by planting grasses on side slopes. Levees are an ideal environment for burrowing rodents which, if not controlled, will cause leaks in and ultimate failure of the levees. For most efficient maintenance, the levees should be wide enough to support vehicle traffic.

For the basin surfaces, or infiltrating area, the need for maintenance varies widely. If silty flood waters are introduced into the basins, the silt will have to be removed periodically to maintain original infiltration rates. In a series of basins in which the uppermost ones act as settling basins, the requirement for removal of silt will vary from basin to basin. Scarifying, disking, or other mechanical manipulations sometimes improve the infiltration rate, at least temporarily. In general, if silt deposits are not great, the best approach is to permit vegetation to grow within the basin and keep traffic to an absolute minimum. Any traffic or soil disturbance should take place when the soil is dry; otherwise, severe compaction with loss of infiltration will resuh.

DITCHES OR FURROWS

The ditch or furrow method of water spreading uses relatively flat-bottomed ditches to transport water through the project and provide opportunity for percolation. In general, ditches and furrows are grouped into three basic types: (1) Contour, where the ditch follows the ground contour; (2) tree-shaped, where the main canal successively branches into smaller canals and ditches; and (3) lateral, where a series of small ditches

46

extend laterally from the main canal. The ratio of wetted to gross area is usually low in these projects, averaging about 10 percent. Consequently, this method is generally used only on relatively inexpensive sites. This method may combine well with the basin method where the natural ground slopes are too steep for economical stepped-basin construction.

The width of ditches generally ranges from 1 to 6 feet, depending on the terrain and the flow velocity desired. Slopes should be limited to those providing noneroding velocities. On very steep slopes, checks are used to minimize erosion and to increase the wetted area. A collecting ditch should be provided at the lower end of the site to return excess water to the stream channel. An advantage of the ditch system is that the ratio of the perimeter to wetted area is large, thereby permitting more lateral flow than in a basin system. Where infiltration is retarded by substrata less permeable than the surface soils, the same total recharge to the ground-water may be obtained with a system of ditches occupying a far less surface area than with a basin system occupying 100 percent of the surface.

FLOODING

The flooding method resembles a crude irrigation system in which the water is released to high points and permitted to flow downslope either without much control or confined to definite constructed areas such as border checks. Good results have been obtained on gentle slopes not cut by large gullies and ridges. Few such areas exist naturally, however. Often the land must be prepared to prevent the water from collecting in small streams and running off instead of spreading over the required surface. Small ditches or embankments may be constructed to divert the water from the shallow gullies to the higher ridges where it may spread in all directions, thereby wetting the slopes of the ridges as it again runs to the lower levels. By the generous use of these ditches or embankments, a large part or all of the area may be wetted.

Water may enter the uppermost point of the spreading area in a main canal and then be released to follow the guUies and controUing structures. Usually, however, the use of one main ditch, or perhaps several that meander over the highest ridges or circle the upper boundary, is desirable. From these ditches the water may be diverted at intervals. This method gives better control over the water and, if a dike or ditch fails, only a part of the spreading operation will be interrupted while repairs are being made.

The infiltration rate with the flooding method of spreading is higher than with other methods because the native vegetation and soil covering have been disturbed

less. However, a disadvantage of the flooding method is that the water is not so easily confined as by other methods. Suitable structures, such as embankments or ditches at the boundaries of the spreading area, are often necessary to prevent damage to adjacent lands by escaping water. As in any other system, the water should be controlled at all diversion points and at the entrance to the spreading grounds. The net amount of land actually wetted is usually much less than by the basin method. This may or may not be important, depending on the cost and availability of suitable lands.

It is often impractical to design a complete flooding system before the application of water. The main ditches, the diversion works, and the control device should be installed first. Then, with the application of water, the smaller training walls and ditches can be located and constructed to the best advantage. It is advisable to have at least one man on the grounds during spreading to patrol and inspect the ditches and embankments. Often such simple adjustments as placing a few shovelfuls of dirt in the proper place will divert sufficient water to increase the wetted area substantially. In this manner a well-developed spreading system may be made highly efficient at very low operating cost. Figure 25 illustrates a combinafion ditch-and-flooding system.

NATURAL STREAM CHANNELS

A popular method of artificial recharge is to increase the amount of water that would naturally percolate to ground-water in natural stream channels by varying two major factors: (I) The period of time water is available for seepage and (2) the wetted area of the streambed.

The natural length of time that water is available in the streambed is usually determined by the hydrologie characteristics of that stream and its watershed. The construction of dams for reservoirs on a stream increases the period of flow, extending through the months in which the streambed would normally be dry. Stream channel modification to increase the wetted area is another means of increasing stream seepage. Figures 26 and 27 illustrate methods of streambed modification whereby water is diverted to normally dry sand and gravel bars next to the main meandering stream. Such works are generally temporary and must be replaced after the larger flows have receded.

Another method of using streambeds is extending a low dam or weir across the bed where the stream has a very wide bottom caused by the meandering of the channel. The water behind the weir and spilling over it spreads out in shallow depth over the entire streambed and thereby increases the wetted area. Precaution should be taken not to create a hazard in time of flood by

47

Gate and measuring device ■Diversion structure

Supply ditch •

Alternate y^\ diversion, as

required

Measuring device

'-^Collecting ditch

_ _ re-bound check dams, Supply ditch'^ as required

Figure 25.-Typical plan of ditch-and-flooding recliarge project.

Figure 26.-Furrows made in sandbars or islands to increase wetted area of streambed and promote infiltration. Rio Hondo near El Monte, Calif.

backing up the water or diverting it out of its normal streambed. Practically the only operation cost in this type of spreading is for periodic inspection of the dam or weir.

Streambeds are generally the most porous part of an area. They are used extensively in southern California

Figure 27.-Furrows in sand and gravel Deas next to main stream channel, San Gabriel River near El Monte, Calif. (Photo courtesy of Los Angeles County Flood Control District.)

for percolating beds of imported water and other controlled flows. Besides having high infiltration rates, they cost nothing for additional land. Also, the ground- water is replenished over a long narrow strip with less danger of building up a ground-water mound that could restrict recharge in a large concentrated recharge area.

PITS AND SHAFTS

A pit or shaft, excavated into a highly permeable formation, is frequently ideal for facilitating ground- water recharge. Since cost of excavation and removal of excavated material is high, use of an abandoned excavation is most economical. In certain situations, pits and shafts also can be used effectively to reach material with higher infiltration rates by excavation of a relatively limited depth of overlying impervious material.

Pits used for recharge operations may be abandoned excavations for sand and gravel, borrow pits, or excavations planned specificaUy for water recharging. Aban- doned pits may need repair or modification. In long- abandoned sites, weathering and other factors may cause sloughing or caving of the sides. If abandoned pits have been used for trash disposal, debris should be removed.

The planned steep-sided pit is gaining in popularity. A principal reason is the small initial capital cost, to the recharging agency, of a well-designed and -constructed spreading area. In urban areas, sand and gravel operators have excavated this type of project to prepared plans and specifications as part of the purchase contract for the excavated sand and gravel. This procedure can help defray the cost of water-conveyance facilities. Another reason for the increased use of such basins is the higher resulting tolerance for silt. Silt usually settles to the bottom in the planned recharge pit, leaving the steep walls relatively unclogged and permitting continued

48

infiltration of water. For this type of project, provision should be made for rapid removal of silt from the bottom of the pit.

The initial cost of a pit is generally small, since the major faciUty required is a conveyance system for delivery of water to the pit. Although infiltration rates are high in sandy and gravelly material, relatively nonsilty water should be used unless the pits have steep sides, and unless provisions have been made for economical removal of silt from the pits.

Shafts can be used only to a limited extent, unless silt-free water is available. If storm waters are to be recharged through shafts, serious consideration should be given to special facilities for silt removal. Also, problems arising from algae and bacterial growth must be considered. These problems will be treated in greater detail under the section on injection wells.

INJECTION WELLS

Injection of water into abandoned wells or wells specifically designed for artificial recharge has been practiced for many years with varying degrees of success. The use of injection wells is confined largely to areas where surface spreading is not feasible because extensive and thick impermeable clay layers overlie the principal water-bearing deposits. They may also be economically feasible in metropolitan areas where land values are too high to use the more common basin, flooding, and ditch-and-furrow methods.

Many attempts to recharge ground water through injection wells have been disappointing. Difficulties in maintaining adequate recharge rates have been attributed to silting, bacterial and algae growths, air entrainment, rearrangement of soil particles, and deflocculation caused by reaction of high-sodium water with soil particles. However, the Los Angeles County Flood Control District in California has successfully operated injection wells as part of a large-scale field experiment to ascertain the feasibihty of creating and maintaining a fresh-water ridge to halt sea-water intrusion in the Manhattan-Redondo Beach area in Los Angeles County. Favorable injection rates have been maintained by chlorination and deaeration of the water supply, and by conducting a comprehensive well-maintenance program. In Texas, where attempts have been made to recharge through pumped wells, silty water has caused rapid decline in the intake rate. This has been partly overcome by pumping the recharge wells for 15 to 30 minutes per day to prevent accumulation of silt in the well.

The spacing of the injection wells depends on the range of influence of a well, which in turn depends on the amount of water to be recharged through the well and the acceptance rate of the aquifer. The acceptance

rate is a function of the aquifer permeability, the hydrauhc gradient, the length of casing penetrating the aquifer, and the number of casing perforations.

In general, it has been found that gravel-packed wells operate more efficiently and require less maintenance than do nongravel-packed wells. At Manhattan Beach, Calif., a 24-inch gravel-packed well with an 8-inch casing was found more desirable for recharging purposes. On Long island. New York, where cooling water is returned to the ground-water basin, a minimum casing size of 8 inches and a minimum packing width of 2 inches has been recommended. In addition, where water is being injected under pressure, it has been found that a concrete seal should be provided on the outside of the casing where it passes through the relatively impermeable cap, to prevent the upward movement of water along the outside edge of the casing (j_).

With respect to perforations, consideration should be given to using a device that makes horizontal louvered shts in the casing. These perforations are particularly advantageous in a predominantly sandy formation, since the movement of sand from the formation into the well, when injection pressure is relieved, is inhibited. Locating the casing perforations below the normal water table lessens the incidence of chemical incrustation. When recharge wells are installed near a pumping well, perforations of the recharge wells should be at an elevation somewhat different than those of the pumping well to increase the percolation path of the recharged water.

A well-header assembly is needed to bring the water to the recharge well and to regulate the flow of water into the well. In general, the water to be recharged should be supplied at a relatively constant pressure. It should not be allowed to fall freely into the well, as the resulting aeration greatly affects acceptance rates. The design of the assembly varies with the purpose of the project. Figure 28A shows a simple plan for recharge through active irrigation wells, used in the High Plains area in Texas to recharge stored natural runoff artificially. Figure 28B shows a typical recharge assembly used for injecting water under positive pressure to prevent the encroachment of sea water into fresh-water-bearing deposits at Manhattan Beach, CaHf.

If a long-term project is contemplated, treatment of water is imperative. Sediment must be almost completely removed, and the clear water should be treated with chlorine, calcium hypochlorite, or copper sulfate to prevent the formation of bacterial shme and algae. If the aquifer contains much clay and silt, water high in sodium cannot be used, because this will deflocculate the aquifer sediments and rapidly decrease the transmissibihty. Continued injection rates rarely exceed 0.5 ' cubic feet per second.

49

Metering device and totalizi

( \ Pump and motor

ffl ^Gate valve

ng recorder y fi^*^

Valve^ O N-X^

ste seal fV^

Chlorinating equipment

iPiezometric surface prior to recharge

(a) TEXAS HIGH PLAINS

DISTRICT TYPE

(b) LOS ANGELES COUNTY

PRESSURE TYPE

Figure 28.-Diagrams of typical recharge wells. A. Texas High Plains District type. B. Los Angeles County pressure type.

50

CHAPTER Vil. WATER QUALITY

Water quality concerns how the constituents of a given water differ from those of distilled water and how these constituents may influence the intended water usage (6). Water is continually changing chemically, physically, and biologically as it passes through the hydrologie cycle. Any alteration in the natural water balance, or any process that uses water, including artificial recharge, changes the character of the water.

PHYSICAL CHARACTERISTICS AFFECTING WATER QUALITY

All surface waters normally contain suspended inorganic and organic particles. These range in size from colloidal particles that stay suspended even in still water, through clay and silt particles kept suspended in comparatively slow-moving water, to very large materials that are moved only by fast water. Organic and biological constituents may stay in suspension not only because of their size but also because of their density and electrical charge. Ground-water is nearly free of suspended solids because of the filtering action and absorptive properties of soil material between the point of recharge and the water table. So any artificial-recharge method will also be a water-filter method.

The removal of suspended solids during the recharge of ground-water improves water quality for domestic and industrial use, but it seldom is important in irrigation. However, because the point of separation of water and soHds is at the recharge area's surface (or aquifer face in well recharge), the filtering action adversely affects the recharge intake rate. Where economical, the suspended solids are removed from the water before it goes into the recharge facility. This can be done by ponding the water until the solids settle out, or by treating it with flocculants—chemicals that aggregate small particles into others large enough to settle. The materials commonly used are alum—AI2 (804)3 • 18 H2O—and a group of water-soluble organic polymers called polyelectrolytes. Small amounts of these materials effectively clarify turbid waters (17).

Temperature is another important water-quality property of water from industrial and domestic cooling. Soils are poor heat conductors (good insulators), and the injection of warmer water into the ground-water body raises its temperature locally. The heat capacity of the large mass of material associated with ground-water has kept this from being noticed as a quality problem.

The rate at which water enters and flows through porous materials is also influenced by the viscous properties of the water, which depend on temperature and concentration of disolved constituents. Since re-

charge water should be low in disolved salts, temperature is of more concern. The effect is measurable, with intake increasing with increased water temperature.

CHEMICAL CONSTITUENTS AFFECTING WATER QUALITY

The disolved constituents in water determine the water quality, and they go where the water goes and at essentially the same rate of travel. Although specific constituents may change in concentration along the flow path, only under special circumstances will the total quantity of dissolved solids decrease with distance travelled. Surface types of artificial recharge will increase the flow path of the water and also its concentration of dissolved solids. In areas of natural recharge and profiles with high intake rates, the increase in dissolved solids comes from the weathering of rock and soil materials. At the other extreme, recharge of water through profiles that have been subjected to very small amounts of percolation, as in arid regions or irrigated soils of fine texture, it is probable that the water for recharge will pick up large amounts of soluble but undissolved salts. Here, then, the quahty of the recharge water can deteriorate drastically until sufficient water has passed through the profile to bring the soil into chemical equilibrium with the water applied.

DOMINANT CATIONS

The major cations that determine the chemical quality of a water are Na*" (sodium), K*" (potassium), Ca"^ (Calcium), and Mg^ (Magnesium). These cations occur in the soil profile as undissolved salts, absorbed on the surface of the exchange complexes (clay particles and organic matter), and in solution of films of soil water. Their relative abundance in the ground water is associated with the type of vegetative ground cover and the amount and nature of weathered soil, rock, and aquifer material over and through which the water has traveled.

The prediction of the relative proportions, concentration, and time of arrival of these ions at the water table v^th the recharged water has been attempted, but at best such interpretations depend on an accurate chemical analysis of the specific soil profile through which the water travels (4, 21). In general, where artificial recharge is most effective the clay content of the profile is low, and the water that reaches the water table should come to equilibrium with the constituent cation concentration of the applied water rather soon. However, where recharge is incidental to irrigation of soils high in clay, the actual concentration is best measured by field

51

sampling the soil solution periodically under the specific site (9).

The quality aspects associated with the above ions vary with the specific use to which the ground-water is to be put (6). For example, a high degree of hardness (related to the total Ca*^ and Mg^ in solution) is detrimental to industry (producing boiler-scale problems) and to domestic users in the effectiveness of soaps and detergents, but in agriculture it maintains water intake rates during irrigation. The quahty of surface water influences recharge rates greatly.

DOMINANT ANIONS

The dominant anions in natural surface and ground waters are SO4" (sulfate). Cl" (chloride), CO3" (carbonate) and HCO3" (bicarbonate). The SO4" and CO3" salts of Ca*^ and Mg^ are comparatively insoluble and under certain physiochemical conditions can precipitate from solution as the recharge water interacts with the soil solution. This affects the quality of the recharge water where irrigation of a Hmited zone of the profile has caused the precipitation and accumulation of considerable amounts of salts. Because of their low solubility these salts would continue to add to the total salinity of the recharge water for some time. The more soluble Cl" and S04^ salts of Nä^ and iC move more freely. They are more likely to contribute to total salinity of the recharged water when it penetrates subsurface accumulations in buried evaporite deposits or sahnized soil profiles. If analyses of the soils above the water table indicate these soluble constituents are present, they will eventually be delivered into the ground-water with the recharge water.

OTHER CATIONS

Some specific cations in low concentrations can have detrimental effects on humans, animals, and industrial applications (24). Arsenic, cadmium, chromium, selenium, molybdenum, and other heavy metals can accumulate in the soil profile above the water table either from natural sources or as the result of man's activities. These materials can be transmitted into the main ground-water body if recharge water comes through profiles containing them. Land use histories of potential recharge areas can be valuable in preventing such pollution, but a geologic interpretation of the parent material of the profile may be necessary to predict whether natural sources are present deep in the soil profiles.

OTHER ANIONS

The most important aniqn that can freely move into the ground water is NO3 ~ (nitrate). It is found naturally

in the soil root zone and can be deposited in the profile under arid conditions from sedimentary parent materials. Because it is an important plant nutrient, it is applied to the soil surface as a fertiHzer. All sources of nitrogen fertilizer (NH3, urea, (NH4)2 SO4, etc.) can eventually be oxidized to nitrate by aerobic nitrifying bacteria that are present in all well-drained soils. Since the nitrate ion, hke the chloride ion, is not absorbed by the soil material, it travels with and into the ground- water with the recharged water. In deliberate artificial recharge operations, unless extremely large amounts occur in the recharge water or as native nitrate in the soil, the amount of nitrate usually found in soil profiles is lost by dilution with the large volume of water passing through. It can, however, be significant when the volume of flow through the profile is small, but area large, as in incidental recharge.

The less mobile ions BO4" (borate) and PO4'' (phosphate) are important to irrigation water quahty. They are readily fixed in the soil. Phosphate forms relatively insoluble calcium salts. These seldom reach the main ground-water body, but can reach significant concentration in shallow water tables in soil profiles derived from parent materials containing them.

BIOLOGICAL FACTORS AFFECTING WATER QUALITY

All surface waters contain biological populations that vary in makeup depending on the relative adaptability of the organisms to the environment. Surface soils also are active from both a microbiological and a botanical and zoological standpoint. The ground-water is essentially free from biological population; thus recharge provides a very effective means of removing the biological population from the water. In the process there are some important features that can influence the efficiency of artificial recharge.

WATER MICROBIOLOGY

Along with the suspended inorganic silt and clay particles, various types of bacteria and algae are found in surface water. The size of these organisms is such that they are quite effectively filtered out as the water moves through the soil or aquifer. In the process they are just as effective in plugging or clogging the open pore space that transmits water as clay and silt particles. Therefore the improvement in water quality is again at the expense of the recharge rate. However, this accumulation of living and dead cellular material filters out submicro- scopic virus populations that may have contaminated surface waters.

52

As was the case with suspended clay and silt, these organisms can be flocculated with chemical treatment. They can also be removed to some extent by treatment with chemical oxidizers such as chlorine, or aeration before spreading as is done in sewage treatment. Being chemically reactive, these organic materials can be removed in place from the soil pores with oxidizers.

SOIL MICROBIOLOGY

Surface soils contain a large active population of bacteria, fungi, and animals that exist on the large supply of organic material. Flooding soils for recharge can have a great effect upon this population, which depends on atmospheric oxygen and nitrogen for its metaboHsm and sunlight for energy. Flooding removes the oxygen source from the environment and shifts the population balance toward those organisms that are able to use other sources of oxygen in their metabohsm. The most important effect on percolating water quality is the increase in acid products that increase the solubility of soil minerals and salts such as carbonates. The aerobic atmospheric nitrogen-fixing organisms and plants give way to anaerobic denitrifying organisms that reduce the nitrate to nitrogen gas in the soil and water passing

through. The actual magnitude of these effects is related to the specific site and recharge flooding procedure.

SALT BALANCE AND GROUND-WATER RECHARGE

Salt balance in a ground-water basin is the relation between the amount of salt leaving the basin and the amounts entering and produced within the basin. When the amount leaving is greater than the amount entering, the salt balance is considered to be favorable. Con- versely, when more salts enter than leave, the balance is unfavorable, and it is probably only a matter of time until the water quality is impaired.

The following sketch (fig. 29) diagrams schematically some of the relationships in salt balance of a ground- water basin (16). In this sketch the relative importance of inflowing water source is indicated by the v^dth of the arrow. The relative importance of inflowing salt sources are in the following order of decreasing importance:

Ci = crop water (irrigation), imported, well, and irrigation return waters; highly variable in chemical constituents.

Zone of aeration H«Water table Zone of saturation

GW

Figure 29.-Unit section of ground-water basin, showing factors that determine salt balance.

53

Hi = human contribution through sewage, urban and industrial wastes, fertiHzation, and soil reclamation practices.

li = imported water, usually low in salts. Ij = irrigation return waters, variable quahty.

GWi = ground-water flowing into the unit section. Rj = recharge water, of equal or better quality

than the original ground-water. M = salt contribution to the system, from solution

of soil minerals and residual salts above the water table.

Pi = precipitation as rain and snow, adds relatively little salts to the system.

Outflowing water and salts are; ETQ = water lost by évapotranspiration (leaving salt

behind). So = surface runoff waters, rivers and man-made

surface drainage systems, coupled with soil- drainage systems where required. These faciH- ties should be located and constructed so they are the major method of mass salt export.

WQ = drainage wells pumping from the unit section and exported from the basis solely for the purpose of salt balance control. Such salt discharge wells would be located and screened or perforated at depths required to maximize reduction of ground-water salinity.

GWQ = ground-water outflow from the unit section. CQ = salt removed by crop export.

Although it is not indicated in the diagram, the unit section of the basin has a salt-storage capacity in the water filling the available pore space, and any salt- transfer concept must consider this capacity. Also, leached salts will accumulate in dissolved form in perched water tables beneath irrigated areas. Water recharged into these water tables will dilute the concentration of the perched water, but this salinized water then may be carried into the main body of ground water, affecting its quality. The rise of a saHne water table into the root zone or to the soil surface also creates conditions for further salt concentration through water loss from evaporation and plant use.

Research has fairly well established the upper salinity limits for soils and waters, to be used for crops, industry, and human consumption. But solution of the inventory and salt-balance problem hinges on the adequacy of the descriptions of the previously listed sources. With good data it is possible to develop a mathematical model for salt balance paralleling water balance in a ground-water basin, and eventually to project the future salt balance of the basin.

Recognition of the need to preserve a favorable salt balance in the basin is an important first step in

preserving the water quality. Certain general conclusions as to the effect of recharge operations on water quality can be suggested. A salt export point or sink should be provided, separate from the ground-water body. Only the highest quality water should be used in artificial recharge, thus providing for the dilution of sahnity already in the ground-water body. Water from wells, or surface supplies from parts of the basin where water quaHty is poor or notably deteriorating, should be used in areas where surface or subsurface discharge from the basin provides salt export. Surface sources of imported salts should be provided with direct surface connection to salt-export points. Incidental recharge should be minimized, or if it is encouraged, it should be on profiles containing the least salt and with sufficient surface water supplies so that recycling of soil water in the root zone can be minimized and thus keep the vadose zone relatively free of salt accumulations.

Control of the salt balance of a basin requires monitoring of surface and ground-water quahty, sam- pHng of the salt in the profile above the water table, records of surface imports of salts and exports in drain waters and by other means, and other measurements of the water balance of the basin. It is no small program, but, to protect this important resource, it will eventually be necessary.

QUESTIONS FOR THE CHEMIST

Questions for the water-quality chemist, geochemist, and soil chemist regarding the maintenance of ground- water quality:

A. What specific water-quality criteria are economically important to industry, agriculture, and domestic use in the basin? 1. How are they affected by water passing

through the basin? B. What and where are the sources of all possible

waterborne constituents that now, or could in the future, adversely affect water quahty? 1. Can such sources be isolated, limited, or

accumulated? 2. Can recharge be used to dilute their effect? 3. Can they be removed or routed to a discharge

point? C. What water treatments or engineering procedures

will maximize recharge rates with existing water sources?

D. What measurements must be made in soil profiles to insure that recharged waters are of the highest quahty? 1. What will be the quality of the water that

passes through a given soil profile?

54

E. Does a favorable salt balance exist in the basin? F. What procedures will both optimize water-use How can it be attained? efficiency and insure a favorable salt balance? 1. How can it be monitored? G. How can the water quality and basin sah balance 2. How well can hs components be evaluated? be described to make it most useful in showing 3. How and where will artificial recharge best the engineering effects of artificial recharge on the

improve it? basin's water-resource development?

55

CHAPTER VIII. BENEFITS FROM ARTIFICIAL RECHARGE

THE PROBLEM

The benefits of artificial recharge are extremely difficult to accurately evaluate in most instances because

of the complex nature of the general hydrologie equation of ground-water reservoirs. This equation may be expressed as follows: (15)

Surface inflow ( Surface outflow plus ( plus

Subsurface inflow ( Subsurface outflow plus ( plus

Precipitation on surface plus

- equals - ( Consumptive use (évapotranspiration losses) ( plus

Artificial importation of water and sewage or of ( Artificial exportation of water and sewage or of sewage ( sewage

( plus ( Changes in storage

Artificial recharge by spreading or use of injection wells may increase the inflow side of this equation if the water that is recharged is imported, or artificial recharge may reduce the outflow side of the equation if water or sewage that ordinarily leaves the area is captured and spread within the confines of the ground-water reservoir. In either case the ultimate effect is an increase in the amount of water going into underground storage.

If the ground-water basin has a large capacity so that the water table can be drawn down without danger of deficiency, and if it will fill during wet periods from natural percolation, there is little benefit to be obtained from arfificial recharging other than to keep the water table shghtly higher during dry periods. When artificial recharge is practiced, the net increase in water stored is the difference between the amount recharged and that which otherwise would have percolated into the underground basin if the runoff had been allowed to remain in the stream channel. Water imported to an area for recharge purposes is almost entirely a net gain, although some minor losses by evapotranspiraüon are bound to occur.

A major difficulty in measuring the effects of artificial recharge is that water from artificial recharge loses its identity as it enters the underground, where it mingles with water coming from natural recharge, subsurface inflow, deep penetration of irrigafion water, and other sources. The net effect on the water table, which may be either rising or failing because of the other factors in the general hydrologie equation, may not be represented solely by well measurements or depth to water table.

Basin-wide evaluations may give fairly accurate indi- cations of the benefits of artificial recharge. However, local pumping concentrations may cause overdrafts in

one part of a ground-water basin while a surplus exists in another part of the same basin. Individual pumpers may therefore have different opinions as to whether an overdraft exists or whether artificial recharge is beneficial. The user who pumps in a region where the water table fluctuates widely or is far below ground surface may think that any added extraction that increases either the fluctuation or pumping lift is an overdraft and any added recharge would be beneficial. Another pumper, located near the point of outflow of a ground-water reservoir where both fluctuation and hft are small, may consider that lowering the water table results only in reducing the outflow or the amount of water that otherwise would be wasted, and therefore artificial recharge within the basin would have no beneficial effect. An extreme view is that of the pumper who is in a position to extract economically the last water in a reservoir and who considers that no overdraft exists as long as he can pump what he needs, no matter how many others are forced to go elsewhere or go without. To him artificial recharge is unnecessary.

BENEFITS

The consideration of any plan to replenish ground water artificially presupposes the desirability or neces- sity of either augmenting the existing water supply or using the underground storage capacity for storage and distribution of local and imported water supplies (18,_i).

An evaluation of net benefits is necessary for a decision on whether or not to use artificial recharge. Net benefits of a surface water system can be measured directly as the amount of revenue obtained as a result of the project. However, when deaHng with ground-water replenishment, the benefits may not be as tangible. In

56

many instances, the organization conducting the operation will realize, at best, only partial direct benefit. Furthermore, the problem may be complicated by factors of ground-water hydrology and quahty that are exceedingly difficult to evaluate accurately in money.

In general, the benefits to be derived through artificial replenishment of ground-water basins may be broadly grouped into two categories: (1) Relief of overdraft on the ground-water basin, and (2) use of ground-water basins as reservoirs and distribution systems.

RELIEF OF OVERDRAFT

There are certain calculable benefits that are immedi- ately apparent where artificial replenishment is con- ducted to relieve overdraft on a ground-water basin. These include: A possible decrease of energy charges for pumping as a result of a reduction of pumping hfts, the prevention of possible capital expenditures for deepen- ing of wells and the lowering of pumps, and the prevention of possible premature abandonment of wells.

Benefits that would be difficult to calculate can be derived where replenishment of an overdrawn basin can prevent seawater intrusion, the release of deep-seated connate brines, or the possible dewatering of the basin or parts of it. Any one of these could result in the partial or complete failure of the underground basin to yield a continued supply of water. Some measure of this benefit might be derived from a determination of the cost of replacing the lost facihty with an equivalent surface system. However, the value of the ground-water basin as an emergency supply is inestimable.

To show maximum benefits from artificial recharge, however, the ground-water basin must have the geologic and hydrologie characteristics that will permit the infiltration and transmission of the water required to relieve the overdraft. If these characteristics are not adequate for the purpose, some combination of ground- water basin development and surface-water distribution system must be developed. In this case, the net benefit of recharge to a given area would be evaluated as a difference between the cost of supplying the total water needs by a surface system from an available water supply, and the cost of a system to supplement the ground-water supply with water from the same available water supply.

USE OF GROUND-WATER BASIN AS RESERVOIR AND DISTRIBUTION SYSTEM

The benefits of using a ground-water basin as a reservoir for the storage and regulation of surface supplies can be measured by the saving in cost of

equivalent surface-storage reservoirs and related facilities. This saving will be particularly great where there are few, if any, natural reservoir sites, and surface water storage may be exceedingly expensive.

For the use of a ground-water basin as a reservoir to be beneficial, geologic and hydrologie conditions must favor the desired storage and regulation. Enough ground- water storage capacity must be available or developed to meet the probable needs for regulation of both local water and imported supphes. The aquifers must have sufficient transmissibility to permit movement of the spread water from the point of replenishment to the point of extraction. The usable capacity of the ground- water reservoir can be developed by planned extractions of the ground-water during periods of deficient supply and subsequent replenishment during periods of surplus surface supply, in much the same manner as a surface reservoir would be operated.

One of the largest benefits to be derived is the saving in cost of developing equivalent usable capacity in a surface-storage reservoir. A second large but calculable benefit when using the aquifer as a distribution system is the savings derived in terms of the difference of cost of a surface distribution system to supply part or all of the demands with due regard to peak requirements. An additional benefit from the use of a ground-water basin is the saving, in water that would be lost by evaporaüon from a surface reservoir. This could be computed as the cost of the water saved. An aspect that is very difficult to evaluate is the high degree of protection from contamination that is characteristic of ground water. This immunity, together with elimination from danger of destruction of reservoir structures and the wide dispersion of outlet facilities that can be developed, makes the ground-water basin of value as an emergency supply, particularly in the event of nuclear warfare.

One of the more important disadvantages is the excess loss of water due to consumption of water by plants where they can reach the ground-water table. Filling up storage to regulate imported supplies when it may be needed later to conserve local runoff is another disadvantage.

COSTS

The cost of artificial recharge should be evaluated to determine the financial feasibility of and economic justification for a proposed project Q). Financial feasibihty refers to the abihty of the project beneficiaries to repay the cost of the project. Economic justification, expressed in terms of a cost-benefit ratio, permits comparison of alternative projects to select the most economical project.

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Expenditures for land and easen-ients, engineering and construction of facilities, water or rights to water, and operation and maintenance generally make up the total cost of a project. For existing projects, these costs vary greatly with (1) purpose, (2) method of spreading, (3) quantity and quahty of water and regimen of flow, (4) surface and subsurface conditions, (5) location of the artificial recharge project, and (6) standards and requirements of the agencies involved in spreading operations.

The cost of land and easements usually forms a large proportion of the total cost of the project. It includes the cost of surveys, maps, title search, and acquisition. In some cases, legal and court fees are involved when pubUc agencies must use condemnation procedures. In urban areas, additional expense is often involved in rezoning a recharge site.

The cost of the land varies with location and the time at which the land is purchased. As a result of the present trend of urbanization and inflation, cost of land increases considerably with time. Natural resources on lands purchased for artificial recharge projects can be used to reduce the apparent total cost of the project. This is particularly evident when the sand and gravel on many project sites in and near populated areas are sold to companies dealing in building materials.

The largest part of the total cost of a project is generally the engineering design and construction of facihties to (1) divert water from streams, (2) convey water to and from the recharging area, (3) measure the amount of flow, (4) contain water within and control flow through the recharging area, and (5) operate and maintain the facilities efficiently and safely. Engineering costs include preUminary studies, field surveys and maps, laboratory tests, designs, plans and specifications, and construction inspection and control. These costs are greatly affected by the standards and requirements generally dictated by the type of development in and around the project area. In urban areas, the need for levee compaction, fencing, and other protective measures has added significant costs to the total investment in spreading facihties.

The cost in developing a spreading area is closely related to the efficiency with which the area is used. Thus, the cost per acre of a basin development with an average area efficiency ratio of about 70 percent is considerably more than the cost of a ditch-and-furrow project with a ratio generally less than 10 percent.

The total cost of a spreading project is affected by the type and size of related facihties such as structures for diversion, equipment for treatment of water and measurement of flow, and conduits for conveying the water from source to the spreading project and for returning unused water to the main stream. When costs

of these facihties are large in proportion to the facilities in the spreading area itself, the cost per unit area is considerably higher than for projects in which cost of the related facilities is not a major item.

Operation and maintenance costs include such items as rent, utilities, taxes, insurance, and legal fees. Others may be cost of silt removal, operating personnel, patrolling, cleaning, and repair of facihties. As the amount of water spread during the operation period increases, the cost per unit volume of water spread usually decreases. Thus, the cost for spreading a unit volume of water is expected to be less during wet periods than during dry periods. However, unit costs may be great, even when a large amount of water is spread, due to inefficient use of personnel and spreading grounds. In general, the operation is most efficient when the spreading project is operated at design capacity for a long period of time.

EXPERIENCE AND DATA OF LOS ANGELES FLOOD CONTROL DISTRICT

The Los Angeles County Flood Control District has probably more experience and more complete records than any other . single agency carrying on recharge projects. For nearly the past 40 years the District has used various types of water to replenish the coastal ground-water basins of southern Cahfornia. The Dis- trict's experience and cost data should prove valuable to anyone contemplating artificial recharge (19).

FACILITIES

The Flood Control District's spreading facihties vary in size. Their primary function has been to conserve local storm runoff. Since the mid-1950's more than a miUion acre-feet of imported Colorado River water has been purchased for replenishing the ground-water basins. In addition, since 1962 the District has spread reclaimed water from a standard-rate activated-sludge treatment plant at a continuous rate of 20 cubic feet per second.

The gross area used ranges in size from 5 acres to 570 acres, and the average wetted area is 85 percent ofthat. Most of these facihties are located in or near high- density urban and industrial areas. They have been acquired over the years, initially to augment the recharge capacity of the natural streambeds. Diversion systems used range from sand dikes, which wash out during high flows, to large channel radial gates and automatic diversion facilities such as a collapsible rubber dam.

The spreading areas themselves are primarily shallow basins ranging in area from less than one acre to 20 acres, with the distribution of water controlled by

58

timber or concrete interbasin structures. Recent economic studies showed that concrete is by far the better alternative since the Hfe expectancy of timber is only 10 years and the possibility of failure much greater.

OPERATIONAL PROBLEMS

The District reports, as do nearly all other recharge projects, that the major operational problem is to maintain reasonable infiltration rates. Figure 30 shows the infiltration rate decreases on several spreading systems during periods of continual wetting. In most systems alternate wetting and drying periods solved the problem, but additional spreading faciHties were required to rotate the spreading from basin to basin to allow adequate drying time. Where silty storm water entered the basins, silt deposits had to be removed to

re-establish initial infiltration rates. Disking is favored by the District rather than scraping an inch or two from the basin surfaces.

Other operational problems are insects, algae growth and subsequent odors when areas are drying, and infestations of aquatic weeds.

Table 4 contains data pertaining to selected Los Angeles County Flood Control District spreading grounds. The costs are based upon amortization over a period of 50 years for the original land costs, a period of 10 years for temporary timber structures, and 30 years for concrete structures. The unit costs developed are based on the amount of storm water spread. Seasonal fluctuation may be extremely large as the amount of water available varies. In years of low flow the unit costs would be high because of fixed charges.

TABLE 4.-Data pertaining to selected spreading grounds in the Los Angeles County Flood Control District (19)

Facility Area Soil type Capacities Water Conserved Costs'

Pacoima . .

Hansen

Santa Fe

San Gabriel. spreading grounds.

San Gabriel- • River channel.

Rio Hondo .

Gross Wetted Diversion

Acres Acres Cu.ft. ¡se

175 122 Sand and 400 gravel with some silt.

156 110 Sand and 450 gravel.

193 133 Sandy gravel. 500

132 101 Silty medium 200 to coarse sand.

133 Medium to coarse sand.

Nominal To Maximum Storm water Storm tration i 3/31/66 season only imported

H.jsec Acre-feet Acre-feet Doll Acre-ft. Doll Acre-ft.

165 80,500 10,900 (57-58)

13.51 -

210 89,300 19,600 (65-66)

10.41 -

220 58,600 23,700 (65-66)

3.67 -

80 28,800 5,500 (57-58)

17.05 -

83,000

80-150 136,000

330,000

570 455 Silty to find 900 400 157,600 to medium sand.

^ 455,000

13,000 (61-62)

37,400 (57-58)

64,300 (61-62)

30,400 (57-58)

91,300 (61-62)

5.24

20.58

1.56

5.23

Cumulative through 1963-64 only. Imported Colorado River water.

Whittier Narrows Dam to Florence Avenue. Quite variable.

Grounds and channel.

59

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LITERATURE CITED

(1) American Society of Civil Engineers. 1961. Ground-water basin management. Man-

ual Engin. Prac. No. 40, 160 pp.

(2) Baver, L.D. 1938. Soil permeability in relation to non-

capillary porosity. Soil Sei. Soc. Amer. Proc. 3: 52-56.

(3) Bianchi, W. C, and Haskell, E. E. Jr. 1968. Field observations compared to Dupuit-

Forchheimer theory for mound heights under a recharge basin. Water Resources Res. 4 (5): 1049-1057.

(4) Dutt, G. R, and Tanji, K. K. 1962. Predicting concentrations of solutes in

water percolating through a colume of soil. Jour. Geophys. Res. 67: 3437-3439.

(5) Eckis, R. 1934. South Coastal Basin investigation, geol-

ogy and ground-water storage capacity of valley fill. Calif. Div. Water Resources, Sacramento, Bui. 45, 279 pp.

(6) Federal Water Pollution Control Administration 1968. Water quality criteria. Rpt. of the

National Technical Advisory Committee to the Secretary of the Interior. 234 pp.

(7) Ferris, J. G., Knowles, D. B., Brown, R. H., and Stallman,R. W.

1962. Theory of aquifer tests. U.S. Geol. Sur- vey Water-Supply Paper 1536-E, 174 pp.

(8) Glover, R. E. 1964. Ground-water movement. U.S. Bur.

Reclamation Engin. Monog. 31, 67 pp.

(9) Haskell, E. E. Jr., and Bianchi, W. C. 1964. Fixed-position device for sampUng soil

solution in depth. Jour. Amer. Water Works Assoc. 56 (5): 664-666.

(10) - and Bianchi, W. C. 1967. The hydrologie and geologic aspects of a

perching layer-San Joaquin Valley,West- ern Fresno County, Cahfornia.Ground- Water 5 (4): 12-17.

(11) Johnson, Edward E., Inc. 1966. Ground-water and wells. 440 pp. Edward

E. Johnson, Inc., St. Paul, Minn.

(12) Linsley, R. K. Jr., Köhler, M. A., and Paulhus, J. L. 1949. Applied hydrology. McGraw-Hill Co.,

New York.

(13) Meinzer,0. E. 1923. The occurrence of ground-water in the

United States. U.S. Geol. Survey Water Supply Paper 489, 321 pp.

(14) Mitchelson, A. T., and Muckel, D. C. 1937. Spreading water for storage under-

ground. U.S. Dept. Agr. Tech. Bui. 578, 60 pp.

(15) Muckel, Dean C. 1959. Replenishment of ground-water supplies

by artificial means. U.S. Dept. Agr. Tech. Bui. No. 1195,51 pp.

(16) Nightingale, Harry I. 1967. Salt balance in ground-water recharge.

Sixth Biennial Conf. on Ground Water Recharge, Development and Management Proc, Univ. Calif., Berkeley, Sept. 13-14. pp. 120-126.

(17) Rebhun, M., Wachs, A. M., Narkis, N., and Sperber, H.

1968. Removal of suspended matter and tur- bidity from water by flocculation with polyelectrolytes. Technion Research and Development Foundation Sanitary Engin. Lab. Proj. No. AlO-SWC-25 Final Rpt. 187 pp.

(18) Richter, R.C., and Chun, R.Y.D. 1959. Artificial recharge of ground-water leser-

voirs in Cahfornia. Amer. Soc. Civ. Engin. 85 (IR 4): 1-27.

(19) Scares, Frederick D. 1966. Disposal of flood and agricultural waste

waters by spreading. In Agricultural Waste Waters. Water Resources Center, Univ. Calif. Rpt. No. 10: 215-222.

(20) Soil Conservation Service, Soil Survey Staff 1951. Soil Survey manual. U.S. Dept. Agr.

Handb. No. 18. 503 pp., illus. Washing- ton.

(21) Tanji, K. K., Doñeen, L. D., and Paul, J. L. 1967. Quality of percolating waters. III. The

quality of waters percolating through stratified substrata as predicted by computer analysis. Hilgardia 38 (9).

(22) Theis,C. V. 1938. The significance and nature of the cone

of depression in ground-water bodies. Econ. Geol. 33: 889-902.

(23) Todd, David K. 1959. Ground-water hydrology. 336 pp. John

Wiley and Sons, New York.

61

(24) U.S. Department of Health, Education and Welfare (26) Wenzel, L. K. 1^62. Drinking water standards. Public Health 1942. Methods for determining permeability of

Service Pub. No. 956.61 pp. water-bearing materials. U.S. Geol. Sur- (25) United States Salinity Laboratory Staff vey Water-Supply Paper 887, 192 pp.

1954. Diagnosis and improvement of saline and (27) Wolman, Abel alkali soils. U.S. Dept. Agr. Handb. 60, 1962. Water resources. Nati. Acad. Sei. Pub. 160 pp. 1000-B.

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GROUNDJWATER RECHARGE HYDROLOGY