Subsurface Water Presentation

7/28/2019 Subsurface Water Presentation

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SUBSURFACEWATER

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Subsurface water, or ground water, is the

water that lies beneath the groundsurface, filling the pore space betweengrains in bodies of sediment and clasticsedimentary rock, and filling cracks and

crevices in all types of rock.

-It is also the water that is flowingwithin the aquifers below the water table.

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Source of ground water is rain and snow that falls to theground a portion of which percolates down into the

ground to become ground water, where it eventuallymakes it way back to surface streams, lakes, or rivers.

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Groundwater makes up about 1% ofthe water on the Earth (most water isin ocean).

But, groundwater makes up about 35times the amount of water in lakes

and rivers.

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Distribution of Water

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Groundwater occurs everywhere beneaththe Earths surface but is usuallyrestricted to depths less that about 750meters.

The volume of groundwater is an

equivalent to 55 meter thick layer spreadout over the entire surface of the Earth.

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Groundwater is recharged from, and eventuallyflows to, the surface naturally; natural discharge

often occurs at springs and seeps, and canform oases or wetlands.

Groundwater is also often withdrawn foragricultural, municipal and industrial use byconstructing and operating extraction wells.

The study of the distribution and movement ofgroundwater is hydrogeology, also calledgroundwater hydrology.

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Occurrence of Ground Water

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Ground water occurswhen water recharges

the subsurface throughcracks and pores in soiland rock

Shallow water level

is called the watertable
http://capp.water.usgs.gov/GIP/gw_gip/how_occurs.html


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Subsurface Zones

Zone of Aeration, or theVadose Zone

-a subsurface zone in whichrock openings are generallyunsaturated and filled partlywith air and partly with water.

Zone of Saturation, or thePhreatic Zone

- the subsurface zone inwhich all rock openings arefilled with water.

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Water Table-the surface separating the vadose zone from the

saturated zone.

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perched water table: the top of a body ofground water separated from the main

water table beneath it by a zone that isnot saturated

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o Water table follows the topography butmore gently

o Intersection of water table and groundsurface produces lakes, streams, spring,

wetlands

o Ground water flows from higher elevation

to lower, from areas of lower use tohigher use, from wet areas to dry areas.

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Moisture in the Vadose Zone

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Above the water table,moisture is raised by capillarityinto the capillary fringe. If the

water table is close to theground surface, the capillaryfringe and the soil moistureregion may overlap, but where

the water table is deep, anintermediate region existswhere moisture levels remainconstant at the field capacity ofthe soil and rock of the region.


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Capillary action, or capillarity, is the abilityof a liquid to flow in a narrow space

without the assistance of, and inopposition to external forces like gravity.

The capillary fringe is the subsurface layerin which groundwater seeps up from awater table by capillary action to fill

pores.

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Soil Water Relationship

Soil is a valuable resource that supports plantlife, and water is an essential component of

this system. By understanding some physicalcharacteristics of the soil, you can betterdefine the strengths and weaknesses of

different soil types.

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Equilibrium Points

Visualizing several states of water in soil, early scientiststried to define limits of these states by equilibriumpoints.

Field Capacity- is defined as the moisture content ofthe soil after gravity drainage is complete.

Wilting Point- represents the soil-moisture level whenplants cannot extract water from soil. It is themoisture held at a tension equivalent to the osmoticpressure in the plant roots.

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Measurement of Soil Moisture

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Tensiometer-an instrument used to measure surface tension of

liquids.

-it can indicate soil-moisture tension from saturationto a tension about 100kPa

The standard determination of soil moisture is theloss in weight when a soil sample is oven-dried.


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Tensiometer consist of a porous ceramiccup which is inserted in the soil, filled with

water, an connected to a manometer.

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Neutron Probe (NP)

The technique is based on themeasurement of fast moving neutrons(generated from an Americium241/Beryllium source) that are slowed

(thermalised) in the soil by an elasticcollision with existing Hydrogen particles inthe soil.

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Water is the only form of H+ that will changefrom measurement to measurement.

Therefore any change in the countsrecorded by the NP is due to a change inthe moisture with an increase in counts

relating to an increase in moisturecontent.

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Satellite remote sensing method

-used to estimate soil moisture based on the

dielectric properties of wet and dry soil. The

data from microwave remote sensing

satellite are used to estimate surface soil

moisture.

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Movement of Soil Moisture

Infiltration is the movement of waterthrough the soil surface into the soil as

distinguished from percolation, themovement of water through the soil.When water is first applied to the soil

surface, gravity water moves downthrough the larger soil openings while thesmaller surface pores take in water bycapillarity pores.

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Moisture in the Phreatic Zone

Within the phreatic zone all porespaces are filled with water, and the

different states of moisture, moisturetension, etc are of little concern.

Interest is centered on the amount of

water present, the amount which can beremoved and the movement of thiswater.

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Aquifers

-a geological unit which can store andsupply significant quantities of water.

-a body of saturated rock or sedimentthrough which water can move easily

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unconfined aquifer: a partially filed aquifer exposed tothe land surface and marked by a rising and falling watertable

-does not have confining layer between it and the

surface.

confined aquifer (artesian aquifer): an aquifercompletely filled with pressurized water and separated

from the land surface by a relatively impermeableconfining bed, such as shale

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An aquiclude is a solid,impermeable area

underlying or overlyingan aquifer.

-it is a formation which

contains water butcannot transmit it rapidlyenough to furnish asignificant supply to awell or spring.

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An aquitard is rock material that is low inporosity/permeability. Fluid flow is not good and the unitmay often be termed a "cap rock", not allowing

underlying water to flow upward.

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Porosity is a measure of the void spaces in a material,and is the ratio of the pore volume to the totalvolume of the formation.

Less porosity

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Primary porosity (% pore space) is the

initial void space present (intergranular)when the rock formed.

Secondary porosity (% added byopenings) develops later. It is the resultof fracturing, faulting, or dissolution.

Grain shape and cementation also affectporosity

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The specific yield of an acquifer is the ratioof the water which will drain freely from

the material to the total volume of theformation and is always less than theporosity.

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The Movement of GroundWater

most ground water moves relativelyslowly through rock underground

because it moves in response todifferences in water pressure andelevation, water within the upper part of

the saturated zone tends to movedownward following the slope of thewater table

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First, the groundwater moves downwarddue to the pull of gravity. But it can also

move upward because it will flow fromhigher pressure areas to lower pressureareas.

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Factors affecting the flow of ground water:

the slope of the water table - the steeperthe water table, the faster ground watermoves

permeability - if rock pores are small andpoorly connected, water moves slowly;

when openings are large and wellconnected, the flow of water is morerapid

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Porosity In sediments or sedimentary rocksthe porosity depends on grain size, the shape of

the grains, and the degree of sorting, and thedegree of cementation.

- in igneous rock and metamorphicrocks, porosity is usually low because theminerals tend to be inter grown, leaving littlefree space. Highly fractured igneous and

metamorphic rocks, however, could have highporosity.

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PERMEABILITY is the capability of a rock toallow the passage of fluids. Permeability is

dependent on the size of pore spaces andto what degree the pore spaces areconnected. Grain shape, grain packing,

and cementation affect permeability.

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Darcys Law

The velocity of groundwater is based on hydraulicconductivity (K), as well as the hydraulic head (I).

The equation to describe the relations between

subsurface materials and the movement of waterthrough them is

Q = KIA

Where: Q = Discharge

A = Area

K=Hydraulic conductivity

I= hydraulic head

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Determination of Permeability

A permeameter is an

instrument that is

capable of measuringthe ability of a porous

medium to permit

flow of fluid.

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Sources of Groundwater

Ground water has gotten into the earth fromone of the following sources:

1. Meteoric water.

2. Condensational water.

3. Connate water.

4. Juvenile water.

5. Mixed source water.

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Meteoric Water

It includes waters formed by infiltration

of atmospheric precipitation like rain, sleet,snow, hail etc. as well as by the infiltration

of water of rivers and lakes.

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Condensational Water

This water is mainly the source of

replenishment of ground waters particularly

in deserts and semi deserts, whereprecipitation is scanty and there is rapid

evaporation. In such regions, there is

ground water at certain depth below thesurface.

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Connate Water

This is also known as fossil water and

includes water entrapped in sediments at the

time of their deposition on lake or seabottom. They are classified in to two types

as syngenetic and epigenetic connate water.

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The syngenetic connate water was

trapped in the sediments containing it,

whereas the epigenetic connate water arethose which entered from the basins into the

rocks that had formed earlier. Connate

water often occurs in rock units with oil.

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Juvenile Water

It is also known as magmatic water as it

is associated with the magmatic activities

within the crust. With the cooling of

magma, its gaseous contents and water

vapour etc. separate out from it.

The water vapour then gets condensed

into superheated water and move upwards

from a region of high temperatures andpressures to that of low temperature and

pressure. This is also called virgin water.

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Mixed Source Water

It is quite natural to expect that along

their complex-migration routes the

aforesaid waters get mixed up and thus

constitute ground water of a mixed type.

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Phreatophytes- are plants deriving theirwater from underground.

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Equilibrium Hydraulics of Wells

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Wells well: a deep hole, generally cylindrical, that is dug of

drilled into the ground to penetrate an aquifer within the

saturated zone

recharge: the addition of new water to the saturated zon

Wet season: water table and rivers are high;

springs and wells flow readilyDry season: water table and rivers are low;

some springs and wells dry up

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Pumping wells

Accelerate flow

Reverse flow Cause water table

drawdown

Form cones of

depressionLow river

GainingStream

Gaining

Stream

Pumping well

Low well

Low well

Cone of

Depression

Water Table

Drawdown

Dry Spring

Effects of Pumping

Wells

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Dry river

Dry well

Effects of

Pumping Wells

Dry well

Dry well

Losing

Stream Continued water-

table drawdown

May dry up

springs and wells

May reverse flow

of rivers (and

may contaminateaquifer)

May dry up rivers

and wetlands


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cone of depression: a

depression of the watertable formed around awell when water ispumped out; it is shaped

like an inverted cone

drawdown: the loweringof the water table near a

pumped well

Non equilibrium Hydraulics of


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Non-equilibrium Hydraulics of

Wells

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Boundary Effects

Boundary conditions and the aquifer domainsize have significant influences on simulatingflow and concentration fields and estimatingmaximum pumping rates.

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Aquifer Analysis

An aquifer test (or a pumping test) isconducted to evaluate an aquifer by"stimulating" the aquifer throughconstant pumping, and observing the aquifer's"response" (drawdown) in observation wells.Aquifer testing is a common tool

that hydrogeologist use to characterize asystem of aquifers, aquitards and flow systemboundaries.

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Potential of Groundwater

Reservoir Reservoirs accumulate water from surface-water

inflows and from precipitation on the

reservoir surface; available storage is

reduced by evaporation, releases to meet

instream-flow requirements, and drinking-

water withdrawals

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The amount of water

that is available for

withdrawal from a

drinking-water supply

reservoir depends on

the water balance of

the reservoir.

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to prevent reservoir failure, a

withdrawal rate is chosen that

maximizes the amount of wateravailable for supply.

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Safe Yield

Safe yield is defined as the amount of water that

a drainage basin/reservoir can provide during

both wet and dry periods. Estimating both

existing and future safe yield is a major concernwith all State and/or municipal authorities

charged with managing water supply systems.

Two measurable and predictable variables

affecting a system's safe yield are populationgrowth and loss of reservoir capacity due to

siltation.

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Seawater Intrusion

Saltwater intrusion is the movement of saline water intofreshwater aquifers.

Most often, it is caused by ground-water pumpingfrom coastal wells, or from construction of navigationchannels or oil field canals. Salt water intrusion can alsooccur as the result of a natural process like a storm

surge from a hurricane. Saltwater intrusion occurs invirtually all coastal aquifers, where they are in hydrauliccontinuity with seawater.

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Artificial Recharge

Groundwater recharge, a hydrologic process where

water moves to groundwater

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Potential sources of ground water


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Potential sources of ground water

contamination are numerous.

Injection wells, which carry andpermanently place fluids

underground, are a potentialground water contamination

source if not properly sited,

constructed and maintained.

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Artificial Recharge

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Artesian Aquifers

An artesian aquifer is a confinedaquifer containing groundwater

under positive pressure.

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Example Layered Aquifer System


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Ground-Water Contamination

Leaking Gasoline Floats on water

table

Dissolves in ground

water

Transported by

ground water

Contaminatesshallow aquifers


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Ground-Water Contamination

Dense solvents E.g., dry cleaning

fluid (TCE)

Sinks past water

table Flows down the

slope of animpermeable layer

Contaminatesdeeper portions ofaquifers


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Ground Water Action

Ground water chemically

weathers bedrock

E.g., slightly acidic ground

water dissolves limestone Caves are formed

Permeability is increased

Caves drain Speleothems form

Water moves along fractures and bedding planes ilimestone, dissolving the limestone to form caves

below the water table

Falling water table allows cave system, now greatly

enlarged, to fill with air. Calcite precipitation forms

stalactites, stalagmites, and columns above the water table

Effects of Ground Water Action


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Effects of Ground-Water Action caves (or caverns): naturally formed

underground chamber most caves develop when slightly acidic ground

water dissolves limestone along joints andbedding planes, opening up cavern systems ascalcite is carried away in solution

most caves probably are formed by groundwater circulating below the water table

H2O + CO2 + CaCO3 Ca++ + 2HCO3-water carbon

dioxide

calcite in

limestone

calcium

ion

bicarbonate

ion

development of caves (solution)

development of flowstone and dripstone (precipitation)

Effects of Ground-Water Action


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Effects of Ground Water Action

stalactites: icicle-like pendants of dripstone

hanging from cave ceilings, generally slenderand are commonly aligned along cracks in theceiling, which act as conduits for ground water

stalagmites: cone-shaped masses of drip-stone

formed on cave floors, generally directly belowstalactites


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Eff t f G d W t A ti


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karst topography: an area with manysinkholes and with cave systems beneaththe land surface

Karst topography is marked by underground caves and numerous surface sinkholes. A major river may

cross the region, but small surface streams generally disappear down sinkholes

ff f G A i


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petrified wood: develops when porous buried wood iseither filled in or replaced by inorganic silica carried inby ground water

Petrified log


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concretion: a hard, round mass that develops when aconsiderable amount of cementing material precipitateslocally in a rock, often around an organic nucleus

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geodes: partly hollow, globe-shaped bodies found in somelimestones and locally in other rocks

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Hot Water Underground


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Hot Water Underground

hot springs: springs in which the water iswarmer than human body temperature

water can gain heat in two ways whileunderground:

1. ground water may circulate near a magmachamber or a body of cooling igneous rock

2. ground water may circulate unusually deepin the earth

Hot Water Undergroundf h i h i di ll


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geyser: a type of hot spring that periodicallyerupts hot water and stream; the water is

generally near boiling (100oC)

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Subsurface Water Presentation