Hydrogeology in the Service of Man, Mémoires of the 18th Congress of the International Association of Hydrogeologists, Cambridge, 1985.

SALINE INTRUSION

E. CUSTODIO International Groundwater Hydrology Course, Polytechnic University of Catalonia, Barcelona, Spain.

ABSTRACT Coastal aquifers contain much-needed freshwater resources.

Their study, conservation and exploitation must take into account the presence of sea water and the mixture of fresh and sea waters. Although the general principles are well known, the presence of a fluid of variable density and the fact that a small percentage of salt water can make water unusable, dramatically complicates the problem. It is aggravated by the great influence of local aquifer characteristics and heterogeneities. A review of present knowledge is presented and discussed briefly, including the basic principles, methods of calculation and geochemical properties. Detailed descriptions and mathematical developments are avoided, but some relevant references are included. A lot of applied research remains to be done, especially when a wide transition zone is present. The detailed nature of the phenomenon associated with saline intrusion can only be dealt with if surveys, data and tests have a similar degree of detail. However some general principles can be applied to derive reasonable answers to practical problems.

INTRODUCTION Coastal aquifers and the rational use and management of water

resources in coastal areas and islands are of particular importance because of:

- the increasingly high concentration of population in coastal areas and islands all around the world, especially in deltas and around major seaports and tourist resorts,

the limited availability of water resources of acceptable quality in many regions due to the high salinity of surface and groundwaters and also to man-made contamination,

the limited extent of many river basins and coastal aquifers, especially where mountain ranges are close to the sea, and in small islands.

Many small islands face or are about to face serious fresh water supply problems. They must rely entirely on their own limited resources and apply sophisticated management techniques, or intro­duce sea-water desalination or water transport by ship as an extreme and very expensive solution.

It has been widely recognised that existing technology is inadequate for the exploitation and management of fresh water aquifers in a saline environment. Also, these resources may experience

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disastrous and almost irreversible impacts if improperly managed. Fresh water may become scarce and increasingly brackish. At the same time, a considerable amount of fresh water may be lost to the sea through diffuse or concentrated outlets such as large springs.

Only sea water intrusion into coastal aquifers will be dealt with in this paper, but it is recognised that a broader consideration is needed for the rational use and management of water resources to avoid the errors introduced by taking too narrow a perspective of the problem. The different aspects involved are presented and commented on briefly. Details, discussion and quantitative treatment can be found elsewhere. Some references are included, but they are not necessarily the best nor do they cover the full spectrum of the subject. References to important old papers can be found in text-books, and are therefore not included. The present review is largely an abstract of an extensive report prepared on behalf of UNESCO (Custodio et al, 1985). An excellent review was previously presented by Kashef (1972)..

Sea water intrusion has been a common topic in most meetings on groundwater. Those organised by the International Association of Scientific Hydrology (Hydrological Sciences) and the International Association of Hydrogeologists are good examples, but meetings specifically devoted to this topic are rare. One main exception is the regular meetings organised by a group of European scientists, engineers and hydrogeologists, without any formal organisation. Under the name SWIM (Sea Water Intrusion Meeting), they started in 1968 in Hannover, and have been convened every two years since in different places, the eighth and last in Bari, Italy, in 1983. About 110 papers have been presented. De Breuck (1983) has reviewed those presented in the period 1968-1981.

UNESCO is especially active in promoting studies on water problems in coastal areas. Project 5.6, dealing with estimating changes in salt water-fresh water balance in coastal areas due to hyraulic works and groundwater exploitation, was set up under the guidelines laid down in the 1975-1980 (phase I) of the International Hydro-logical Programme (IHP). A working group was set up in 1977 and a report on groundwater aspects has been prepared and will be issued shortly, as mentioned above.

In the third phase of the IHP (1984-1989), new cooperative projects are included to take account of the needs of people working on coastal water problems, especially hydraulic and development engineers - those most in need of additional information - and policy and decision-makers, who need popularized descriptions of problems and managerial methodologies.

The recommendation is that it should focus on groundwater manage­ment in coastal areas, recognising the following priorities:

- Evaluation of the severity of the saline intrusion problem in groundwater and the status of implementing control measures,

- determination of the nature and effectiveness of existing management tools available for controlling saline intrusion, including data collection, methods of analysis, management approaches and administrative arrangements,

- determination of the nature of new projects and research needed to improve the state-of-the-art in the prevention of saline intrusion.

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BASIC PRINCIPLES The basic principles of sea water/fresh water relationships in

coastal aquifers have been widely presented and discussed in text­books on Hydrogeology and Groundwater Hydrology. Under ideal conditions, in which a sharp interface is assumed between fresh and salt water, one must consider:

the (Badon) Ghyben-Herzberg formula (GH), based on the hydrostatic equilibrium of immiscible liquids of different densities. It relates the fresh water head, hf, above mean sea level, to the interface depth, z. It states that z =ah_ in which a = Yf/(YS ~~ Yf) is a function of the specific weight contrast between fresh water, yf, and salt water, Ys; generally a s 40. Salt water is assumed to be static, its head being zero.

the Dupuit-Forchheimer (DF) assumption of horizontal hydro­static flow above the interface.

When vertical groundwater flow components are present, hf must be that existing at the interface, as measured or deduced from the head distribution. When salt water, either stagnant or moving, has a head hg - measured at the interface to allow for the possibility of vertical flow - hydrostatic equilibrium gives:

z = ahf - (1 + a) hs

The GH and DF principles, when correctly applied, yield results that fit observed field situations. When they do not agree with field observations it can be shown that after corrections to take into account the fact that mean sea altitude is not zero, non-steady salt water conditions in the aquifer, or vertical flow velocity components, they are as predicted. In real situations the necessary corrections can be made only if the flow system is known and well understood, which is not usually the case. Even "effective" mean sea level is difficult to determine in very flat coastal areas with big tidal fluctuations, along intricate coasts and in islands around which sea-currents are strong. An error of

0.5 m in sea level makes a 20 m error in the interface depth and a much larger error in the penetration of the salt water wedge.

In many instances the sharp interface assumption is too crude to be meaningful. A wide transition or mixing zone separates the fresh water zone from the salt water, sometimes occupying a very significant part of even most of the aquifer thickness. In such a situation the GH principle cannot be quantitatively applied, although qualitatively it may still yield some reasonable results that apply to the inner part of the mixing zone when due corrections are considered (Fig.1). More field experience is needed before fore­casts can be made.

Some authors define the top of the mixing zone as the surface where the salinity corresponds to the maximum total dissolved solids or chloride contents and try to define an a value from which the depth of this surface from the fresh water head may be calculated, in the same way that the GH principle calculates the interface depth. Even though some a value that fits the observations can be found experimentally in a given aquifer in a given situation, it is a

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Distance to the shore tine j in metres

500 400 300 200 100 0 100

-^ 10

4>

o a/ 0 e> c o m

£ -10

:* o

-Q a - 2 0

w>

«* - 30 5:

F-i . 2. Salinity distribution in Bisoayne Bay, Miami, Florida, in a very permeable non-homogeneous carbonate aquifer. After Kohout, in Cooper et al (1964).

Fig. 2. Schematic representation of the flow in a water-table coastal aquifer.

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local value that cannot be used for other aquifers or even the same aquifer when conditions change (for example, seasonal variations, effect of dry or wet periods, influence of water abstraction). A single a value cannot describe the different phenomena involved in the behaviour of the mixing zone. Thus reported values for a (of 10 to 20) are probably meaningless on general grounds.

It can easily be demonstrated, and confirmed by field observations, that water flows in the mixing zone as shown in Fig. 2. There is a net transport of saline water towards the sea. Diffusion and hydrodynamic dispersion are the factors that cause the mixing of fresh and salt water. On the other hand, the flow along the mixing zone keeps the width of the zone and flushes out the salt introduced, thus maintaining a balance. A slow net landward movement of the salt water is needed to maintain the saline balance. The situation is very close to that observed in stratified estuaries, but with much steeper slopes.

The thickness of the mixing zone depends on the saline flow within it, the aquifer's permeability and dispersivity, and the fresh water flow. Perturbations due to external influences such as tides, aquifer recharge and changes in abstraction rates, tend to increase the thickness.

The mixing zone expands and contracts (with increased flushing) following external influences. Only a few published reports deal with detailed field observations, and only a few qualitative descriptions are available. Since pumping, tides and recharge events greatly affect the behaviour, some statements apparently contradict some others. This is a field in which more research is needed.

In the semi-confined basaltic aquifers of Hawaii (Todd and Meyer, 1971), an expanding mixing zone has been observed when freshwater flow was decreasing as a consequence of continued abstraction, but with a fixed base and an ascending top. In the limestone aquifer of the Salentine Peninsula, Southern Italy (Cotecchia, 1977), the mixing zone contracts and expands under natural conditions, with the central part remaining at about a constant depth; after a recharge event an expansion is observed, sometimes followed by a contraction. all these and other observations are not adequate for the derivation of general guidelines.

No simple method exists to describe the characteristics of the mixing zone quantitatively. In homogeneous formations, a sigma-shaped distribution of vertical salinity can be assumed.

The potential flow must be described in terms of pressure, P, instead of water heads. Pressures can be obtained by means of point piezometers filled with water of known density. If the water's kinetic energy is negligible, Bernoulli's energy conservation principle states that the energy per unit mass at a given depth, z, below a datum is E = g(P/y-z) where g is the acceleration due to gravity and y is the water's specific weight.

Given two points 1 and 2 at the same depth, separated by Ax, the horizontal mean velocity directed from 1 towards 2, is:

„ - v Pi/Yi ~ P2/Y2 qh " \ Ax

Given two points a and b in the same vertical, separated by Az

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(with b below a), the vertical mean velocity directed from a towards b, is :

a 'a b 'b q = k nv v Az

in which k and k are respectively the horizontal and vertical hydraulic conductivity (permeability) at mean water conditions in the interval. When the vertical distribution of specific weight is known, it is possible to derive water-motion equations.

Assuming that the fluid and the rock are incompressible, the conservation of mass equation can be written as:

k - V-(yq) + TQ = 0 ; q = — (Vp - yVz)

in which q is the Darcy velocity vector, Q is a source (sink when negative) term of specific weight r, k is the medium intrinsic permeability and u the fluid viscosity, the other symbols being described earlier.

The mass transport equation can be written as:

V-(DVc) - 1 V'(qc) = 3c/3t - CQ m

in which m is the medium kinematic porosity (assumed constant), D is the dispersion tensor, c is the fluid salinity, C is the source salinity and T is time. A non-reactive solid medium has been assumed.

To relate specific weight to salinity an equation such as y = y + 0.7c can be used, in which y0 refers to fresh water; c is given in g/1 and the Y is expressed in kg/m3 . The viscosity u must be expressed also as a function of c.

HYDROGEOLOGICAL ASPECTS In water table aquifers, the salt water wedge penetrates landward

up to the position where the interface intersects the base of the aquifer at the "toe" (Fig. 3). When a thick mixing zone is present, it blurs the picture, but the essential features remain.

Real situations may not be so simple as indicated by actual observations and surveys. The classic picture applies well under steady-state situations, but under transient conditions aquifer heterogeneities may play a dominant role, allowing preferential paths for the penetration or withdrawal of the salt water (Fig. 4). Salinity inversions can even be produced by such preferential move­ments. They also appear when very high tides occur and a wide portion of the shore is periodically flooded by the sea; the same situation may be attained by salt water spray (Fig. 5).

No essential differences exist between water-table and confined coastal aquifers, and the same principles apply. In practice it must be noted that in a water-table coastal aquifer, fresh water is free to discharge into the sea, but in a confined aquifer a minimum fresh water head is needed to compensate for the heavier sea water column at the submarine outlet (Fig. 6); otherwise no fresh water flow could be established and sea water would act as a plug. In this situation pre-existing salt water would remain in the aquifer or, due to the lack of flow, the mixing zone would be free to expand by diffusion and dispersion induced by sea-tide and seasonal fluctuations; a long thick mixing zone would be found (Fig. 6). This is a common situation in many deep deltaic aquifers.

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Fig. 3. Different situations in continental and island aquifers. The fresh, salt and mixed water bodies are indicated.

B A R C E L O N A

0 500 1000 m

SCALE

V a l u e s in g/l CI"

1977

M E o I T £ R D e e p a q u i f e r

Fig. 4. Sea water intrusion in the confined aquifer of the Llobregat delta (Barcelona, Spain). Sea water follows preferential paths which coincide with the buried Wurmian valley and the lateral alluvial fans (Custodio, 1981b). In the Eastern sector vertical leakage of salt water occurs, and probably also in the central areas offshore.

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Fig. 5. Some examples of salinity inversions in coastal cross-seations. A. Along the south coast of Long Island (New York), in Queens and Nassau Countries, in an area of sandy beaches, sand bars and salt water sounds (after Lusczynski and Swarzenski, 1962). Groundwater abstraction lowers the hydraulic head and salt water penetrates by vertical leakage. B. Perpendicular to the coast, North of Amsterdam (The Nether­lands) , in a polder and canal area (after Pomper, 1981). C. Perpendicular to a flat coast subject to wide tidal fluctuations in Southern Belgium (after Lebbe, 1981).

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Fig. 6. Sketches of coastal aquifer behaviour.

A. Confined aquifer discharging into the sea.

B. Confined aquifer with insufficient head to discharge into the sea.

C. System of coastal aquifers or sub-aquifers representing discharge from a large basin.

In a confined coastal aquifer with fresh water, under undisturbed natural conditions, the piezometric head must be greater than the depth at the sea outlet divided by a, because outflowing fresh water must overcome the heavier sea water column at the outlet. In flat areas this means flowing water wells exist and this is a common situation in the coastal discharge area of regional aquifers. In many instances it has been pointed out that the GH principle

does not agree with observations, and negative head values are even found whilst the aquifer continues to yield fresh water. In such cases, a careful survey will show that a transient situation has

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been observed instead of that of a steady-state; in the aquifer, the salt-water head is negative and the sea-water wedge is either slowly moving landwards or no connection exists between the aquifer and the sea.

When dealing with confined aquifers, one of the main unknowns is whether there is a connection between the aquifer and the sea. In some cases it is clear that a connection exists and that in others a connection is not possible, but in many instances nothing can be said with certainty. Offshore surveys are costly and difficult. Usually they cannot be justified on economic grounds, except if new methods are developed. The presence of low permeability sea-bottom sediments or a progressive facies change may greatly hamper a connection. Conversely, faults and joints in overlying sediments may facilitate a connection. Hydrodynamic models, chemical and isotopic studies, together with a good understanding of the sedi-mentology and structure may give indirect clues to the proper characterization of the problem. A good deal of good judgement is needed.

When semi-permeable layers occur, vertical leakage may play a dominant role in water-head distribution and, accordingly, in salinity distribution. A good knowledge of the flow system is needed to correctly interpret saline water occurrence and the movement of saline water from the sea into the aquifers or from one aquifer into another. Salinization can proceed vertically without a true salt water wedge (Figs. 4 and 5a). Though some good studies have been carried out, little experience has been gained in studying and dealing with such situations.

In hard rocks the situation is more complicated because of the greater heterogeneity in the permeability and variable thickness of the flow system. Thus, the flow is concentrated in densely fractured parts and some conspicuous fresh water outlets can be found, but conversely a shallow, deeply penetrating salt and brackish water wedge forms. This hampers attempts to exploit fresh water. In such situations, studies must consider in detail the hydrogeological features. Routine surveys yield results that apparently cannot be interpreted, since groundwater levels and salinity distributions seem chaotic. Exploitation increases the difficulties.

All this applies to limestone, calcarenite, volcanic, granitic and other hard rock coasts. In the volcanic Canary Islands, several attempts to characterise saline intrusion have not yielded good results since it has not been possible to adequately describe the medium. In deep shafts, fresh and brackish water inlets are found. The picture is complicated by the fact that besides modern sea water, old sea water, marine spray and climatic aridity contribute saline water. However, in regional terms, the salinity distribution and fluctuations follow expected patterns.

Many coastal areas were subjected to wide variations in sea-level and important seidmentary changes throughout the Quaternary and even since the Pliocene, with changing histories of salt water intrusion, entrapment and flushing out. Also notable changes in the coastal topography have been produced, such as the development of deltas, coastal accretion and erosion, and the formation of mounds by processes such as dune formation or sediment accumulations caused by

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Top s i l t and ctay

D i s t a n c e to the shore, km

Fig. 7. Salinity distribution inside the semi-pervious intermediate lens of the Llobregat delta (Barcelona, Spain). Salt water is being displaced and diluted by diffusion of fresh water in the confined aquifer, which is able to discharge into the sea. The ionic profile (in one borehole) shows ion-exchange effects. (After Custodio et al, 1971; Custodio, 1981b).

ice advances during the last glacial period (eg. in the Netherlands). The present distribution of salinity is, therefore, complex and transient (Fig. 7). Thus, the application of basic principles seems to fail if unsteady situations are not adequately considered. Palaeohydrogeological studies, together with chemical (Fig. 8) and isotopic water studies are needed to characterise the situation. Such studies are rare, but some good examples have been published (eg. in the SWIM papers; van Dam, 1976).

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© F l u s h i n g

10O(- f r eshwa te r

200--

®

300-1- Saline deep groundwater m) 10.000 y. BP

E n c roa ch ment

100-

200 :_

300-m)

Sea water in t rus ion

>l " ' I ' I I I , I I . I . ) ,

6000 - 7000 y. B P

6

Q

© if 2 500-3 500

Exp l a n a t i o n :

3 000 years before present

''//>';/;, old salt water

1m) P a r t i a l flushing (present) salt water

Fig. 8. Cross-section of the Wittmund region (Western Germany) showing different successive phases of flushing and sea water encroachment during the Eolocene (after Hahn, 19 75). The sea is to the right. Years before present are indicated. A complicated distribution of salt and brackish water is found.

EFFECTS OF EXPLOITATION Groundwater abstraction has the primary effect of reducing fresh'

water discharge into the sea. Accordingly, the salt-water wedge moves landwards and the flushing of the mixing zone decreases, thus increasing its thickness. If some fresh-water flow is conserved at the outlet, a new equilibrium of the salt-water wedge and the mixing zone is eventually achieved. The transient state evolves

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slowly and the fresh-water displaced by the intruding salt-water is used to make up the difference in fresh-water flow between that arriving at the salt-water toe and that actually discharged into the sea.

This theoretical picture, which can easily be seen in physical hydraulic models, corresponds closely to that produced in homo­geneous aquifers. In heterogeneous and stratified aquifers the situation is complicated by the existence of macroscopic fingering and preferential paths. Some hydrogeological features may play a dominant role, but these have been as yet insufficiently studied. The saline wedge may differ from the typical picture (Fig. 4) and dispersion is greatly enhanced Some wells may begin to produce brackish water earlier than anticipated, whilst other areas seem to be temporarily protected. It is not uncommon for apparently well based predictions not to agree with observed reality.

Groundwater exploitation above the interface produces salt water mounding below drains and trenches (Fig. 9). According to aquifer characteristics, well or drain penetration, and water discharge, there is a vertical elevation of the interface below which a stable situation can be maintained, and salt water does not reach the abstraction well or drain. When the critical situation is exceeded, salt water will eventually mix with abstracted water. Vertical permeability plays a dominant role in the pace of the transient situation. Minor low permeability horizons will effectively delay coning-up problems. Their presence explains the sometimes enormous differences in behaviour of apparently similar wells. The detection and investigation of such layers is difficult when using conventional drilling techniques. In areas where they are quite extensive, drilling through them, without carefully cementing and isolating appropriately selected intervals may endanger the whole fresh water body of the aquifer.

Since the flow above the interface is generally towards the abstraction well or drain from all sides, the only possible dis­charge for the brackish water formed by flushing the interface is the well or drain. Thus, the abstracted water is polluted by salt water to some extent, even when the bottom of the well or drain is above sea level, since the specific weight of brackish water is less than that of sea water.

Although not enough experience exists to draw firm conclusions, the intermittent exploitation of small isolated wells might reduce saline pollution if the fresh-water flow is strong enough.

SPECIAL ASPECTS OF SUBMARINE SPRINGS Large submarine springs are found in many coastal areas,

especially where carbonate rocks are present, as along the Mediterranean coast or in the Caribbean. Discharged water always shows some degree of salt water pollution, but in many instances this is tolerable, especially when other water resources are scarce. Tapping such water has been a continuous challenge since times of antiquity and still defies present-day science and technology. A long history of failures has been the result, including very recent well-engineered attempts. Only small, insignificant quantities of water have been extracted whilst avoiding intolerable increases in salinity.

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Fig. 9. Sketch of saline upconing below abstraction wells. When there is an adequate fresh water flow, the well is shallow and its discharge is small; a water divide forms that does not intersect the interface, thus preventing salt water reaching the well. If there is no stagnation point, flushing of the mixing^ zone continues and prevents an excessive thickening of the mixing zone. The figure to the right shows a similar situation below a polder area in the Netherlands.

50 100 150 200

Distance from the o u t l e t , m

Fig. 10. Schematic representation of water salinity along La Falconera submarine spring (Garraf limestone massif, near Barcelona, Spain). About 150 m inland a main fissure zone yields salt water to the main stream. (After Custodio, 1981),

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Many such springs flow through underground conduits which have developed along geological discontinuities, when sea level was lower than at present. Discharge points frequently use upward ducts developed as a result of present sea levels. Since the outlet is at sea level, fresh water inside the deep part of the conduit is at a lower pressure than that of the heavier sea water at the same depth, thus permitting a continuous flow of sea water through fissures and joints connecting the main conduits to the sea floor, seaward of the main outlets (Fig. 10). A significant salt-water flow explains brackish springs at the shore-line; a small salt-water flow, when only small fissures are present, produces only a slight increase in salinity. Any attempt to catch a significant part of the spring flow will increase sea water penetration through these fissures or activate others by desilting. Perhaps the only possibility of putting this water to beneficial use is to intercept it before it reaches the main conduits.

Theoretical and applied research is needed before useful results can be achieved, if at all. Up to now most efforts have lacked scientific support, or have not possessed the minimum required observation framework.

CALCULATION METHODS Most methods for calculating the consequences or extent of sea-

water intrusion into coastal aquifers apply the GH-DF assumptions and yield useful approximate formulae for describing the interface depth, saline wedge penetration under steady-state conditions and simple geometrical configurations; they are reproduced in textbooks. Further extensions, including leaky aquifers and other configurations, have been developed later (Bear, 19 79; van Dam and Sikhema, 1982; Sikhema and van Dam, 1982). Corrections to take into account the width of the fresh-water outlet or the sea floor configuration can be introduced (see Cooper et al, 1964). They can be extended to transient situations (van der Veer, 1977). Generally, the inaccuracies involved can be accepted for practical purposes, although errors may be important in areas close to the coast or the sea-outlet. These assumptions are an "allowable evil" when compared to the mathematical simplifications they permit.

Salt-water wedge penetration in the aquifer under steady-state conditions is proportional to S i kb2/q in which k is the horizontal permeability, b the aquifer thickness (depth below sea level in a water table aquifer) and q the fresh water flow at the sea outlet. The mixing zone thickness increases with N, the aquifer dispersivity and the hydraulic diffusivity.

Although the interface movement can be mathematically described (Bruggeman, 1975; Bruggeman in Custodio et ai, 1985) no satisfactory simple formulae are available to calculate the salt-water wedge motion when the value of q is changed, although some approximate methods have been published (see Bear, 1979; Bear and Kapuler, 1981; Bear et al, 1985), but they rely on the sharp interface assumption. The existence of vertical flow velocity components, both in fresh and salt water, invalidate to some extent the direct application of the GH-DF approach. Even when applying it, some problems are difficult to solve.

More effort is required to get meaningful simplified calculation methods, which incorporate relationships between the different

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variables involved, especially those that are significant for management purposes. No simple calculation methods are available for situations which involve a thick mixing zone, or for hetero­geneous and hard rock aquifers.

Other types of problem for which more research is required are upconing and formation of salt water mounds below fresh water abstraction works. Most simple solutions were developed in the sixties and consider a sharp interface (see Custodio, 1976; Bear, 1972; 1979; Streltsova and Kashef, 1974). Simple solutions for the salt-water wedge movement toward coastal wells have been presented by Strack (1976).

Simulation models of the Hele-Shaw type, using two immiscible fluids were used some years ago to understand more adequately the shape of the salt-water wedge. Different formulae have been compared against the results of such models but their engineering value is limited. The principles of such models have been des­cribed by Christensen and Evans (1974) but they remain principally a research tool.

The formulation of mathematical models does not present special difficulties, especially if the mixing zone is thin and the GH-DF assumptions are used. However, the solution is difficult, not only because of the non-linearity of the two-flow equations (the interface depth is not known beforehand and affects the thickness of the fresh and salt-water flow in the same way as the water-table) but also because, in a transient situation, vertical flow components cannot be neglected, and affect the results of a horizontal two-dimensional model. Changes in the water-table or piezometric level produce a much larger change in the interface depth, which implies the displacement of large volumes of fresh and salt water. Vertical flow components develop and the actual interface position lags behind the expected depth. Apparently the GH principle does not hold good at all.

It has been proposed that in the calculations the interface depth at some earlier time-intervals should be used (Andersen, 1976). Although this time-lag must be calibrated, it is not a fixed value and cannot be used throughout the calculation or extrapolated to other problems.

When dealing with boundary conditions the simplest solutions are those where no salt water wedge toe is present, such as salt-fresh water mathematical simulations at oceanic islands or those in which fresh water forms a bubble floating everywhere on salt water. Published results seem acceptable, especially in near-to-steady-state conditions.

The presence of the toe of the salt-water wedge - and the wedge tip in a confined or semi-confined aquifer - present further problems. They represent the change from a two-fluid to a one-fluid domain. They are boundaries which form part of the solution. In non-steady state situations they change their position, thus adding to the complexity of the problem.

Different approaches have been used to deal with this problem (Sa da Costa and Wilson, 1979; Ferrer Polo and Ramos Ramis, 1983). The effectiveness has been compared to some analytical solutions, but no validation has been carried out against well known field studies. Instabilities in these two-fluid models can occur when applied discretization methods use coarse intervals (Aziz and

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Settari, 1979; Andrews, 1981). Only a few attempts have been made to deal with the mixing zone.

Not only are the methods of analysis difficult and time-consuming, but enough field data for calibration are not generally available. Furthermore, numerical instabilities are not easily overcome. The numerical solution of convective flow must be very accurate so that the numerical dispersion does not mask the effect of physical dis­persion, or results which are accurate only for a small grid-spacing and oscillate for coarse grids. Furthermore, the dis-persivity value is poorly known and appears to be time-dependent, at least at small scales. Even existing mass-transport codes have seldom been applied to real problems, and when applied, are usually restricted to the examination of vertical sections, assuming that no cross-flows exist. Major breakthroughs were developed some years ago (eg. Sa da Costa and Wilson, 1979; Custodio et al, 1985), but proposed models have not been applied extensively, and their use­fulness is not fully established.

Pinder and Gray (1977) presented the application of a dispersive finite element model to the Cuttler area (see Fig. 1). The steady-state solution was obtained and the effect of a rainfall infiltration pulse simulated and compared with observed results. Horizontal and vertical permeability values (390 and 0.8 m/day) are not homogeneous and the presence fo a more permeable area in the centre of the cross-serction influenced the results. Longitudinal and transverse dispersivity values (6.7 and 0.67 m) had to be obtained by a trial and error procedure. Fig. 11 shows some results.

A finite element dispersive model has been used to forecast the evolution of the aquifer of the Costa de Hermosillo (in north-west Mexico) to help in the planning of a desert area that is heavily dependent on groundwater for survival (Andrews, 1981).

PHYSICO-CHEMICAL ASPECTS OF SEA-WATER INTRUSION Water in the mixing zone does not necessarily correspond to a

simple mixture of local fresh water and sea water. Since the chloride ion is conservative, the percentage of each water can easily be calculated using the chloride-ion content. The same is generally true for bromide and sulphate ions in aerobic environments. Other major constituents can be subject to cation-exchange. When cation-exchange processes are correctly interpreted, they can indicate the dynamics of the processes involved. Invariably water in the landward advancing mixing zone is depleted in Na+ and enriched in Ca++. This effect can even be noticed in limestones and sedi­ments with a rather low clay content. A seaward retreating mixing zone or a marine formation being flushed out by fresh water produces the opposite effect, that is, water in the mixing zone is enriched in Na+ and depleted in Ca++.

In the thick mixing zone of stagnant or slowly moving groundwater in coastal and submarine confined aquifers, in which organic matter is present, ion exchange phenomena may be accompanied by redox processes such as sulphate reduction (Lawrence et al, 1976; Custodio et al, 1971 ; 1975).

When calcite is an environmental mineral, the mixture of generally slightly oversaturated sea water with close-to saturated fresh water produces an aggressive water which is able to dissolve carbonates (Fig. 12). This process can be maintained by the

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DISTANCE FROM SHORELINE , metres

480 400 300 200 100 0 100

Fig. 11. Finite-element numerical simulation of the mixing zone Bisaayne Bay, Miami, Florida, compared with observed data (Kohout and Klein, 196 7). A shows the quasi-steady state and B the change 180 hours after an intense rainfall pulse. 480 m inland the water table height rose from 0.4 m to 1.4 m and in 50 hours returned to the initial value (after Finder and Gray, 1977).

u • - I

to

0) "CI

c

c o

a 3

a 10

a U

0 10 20 30 40 50 60 70 80 90 100

Percent sea water in mixture

Fig. 12. Calcite saturation index as a function of percent sea water mixture with saturated fresh water at a given C02 pressure. At high Pco pressures mixed water may become undersaturated.

2 (After Plummer, in Back and Soelt, 1975).

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continuous flow of water along the mixing zone. Over geological time it causes progressive chemical weathering and "karstification", which, in coastal carbonate formations, leads to an increase in permeability, the formation of preferential flow paths and cavities, and a landward advancing salt-water wedge. This phenomenon has been studied in detail in Florida and Yucatan (Hanshaw and Back, 1979; Back et al, 1984) and is well recognised in many other coastal areas, such as the coasts of Catalonia and the Balearic Islands in Spain (Custodio, 1981). It adds to the difficulty of tapping fresh water in carbonate coastal areas. Water in the mixing zone shows an increased HC03~ and C03~

2 content. Cation-exchange and redox processes affect the precipitation and dissolution of calcite.

Other processes in the mixing zone explain the slow dolomiti-sation that occurs in this zone (Margaritz et al, 1980). Therefore, dolomite horizons may indicate possible positions of palaeo-mixing zones. This continuous process of dolomite precipitation in the mixing zone can explain the reported reduction of the Mg/Ca ratio in pore water. It might also tend to reduce the permeability in the rock matrix.

Fresh water and salt water rich in the alkaline earths maintain the aquifer clay fraction in a aggregated form, and water can flow through the partially-filled pore spaces. When brackish water forms, the Z-potential changes and clay particles deflocculate and swell; they may then move and clog the narrow parts of the pore-ducts, thus dramatically reducing the formation permeability when sensitive clay minerals are present, such as montmorillonite (Golden-berg et al 1984). This has been reported several times when fresh water is injected into a salt water aquifer, and has led to the failure of recharge wells (see Custodio, 1985). Under natural con­ditions it may also occur when marine formations are being flushed out by continental fresh water. The low vertical permeability of deltaic clay and silt formations is perhaps related to some degree to this fact. This is perhaps an important point for research, since it is something that has seldom been considered in any detail.

In multi-aquifer systems, with alternating fresh and salt water units, osmotic pressure gradients may develop (Greenberg and Mitchell in Witherspoon and Mitchell, 1971; Marine and Fritz, 1981). Osmotic pressure between fresh and sea water is about 24 atm, enough to fully change the hydraulic potential distribution, but slightly permeable aquitards have a very low osmotic efficiency and the phenomenon is probably not significant. Osmotic pressure gradients fully develop across very low permeability clay layers, but water and ion movements are so sluggish that only thin layers might be of any practical interest.

Environmental isotopes are playing an increasing role in coastal aquifer studies. Not only are analytical facilities and methods now commonly available, but the basis for interpretation is also well established. Even though most texts on environmental isotopes do not deal specifically with coastal aquifers, comments and references can be found in papers (see, for example, the proceedings of meetings on Isotopes in Hydrology organised by the International Atomic Energy Agency) and textbooks, and in the references included in them (Fritz and Fontes, 1980). Generally, adequate differences in isotopic com­position can be found between continental and sea waters and their mixtures.

0xygen-18 and deuterium in water are useful for studing mixing

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problems and the origin of brackish waters. Carbon-13 is also valuable for the same purposes and for studies of the behaviour of water in carbonate environments. As the ocean is in close equilibrium with modern organic matter, carbon-14 is useful for dating purposes, when the geochemistry of the system is known and the mixing of waters can be characterized. Sulphur isotopes (3 t+S/32S) are also helpful for studying the origin of sulphate. In ocean-water and evaporites, sulphates are enriched in 4S, while in rainfall they are not so enriched and those derived from sulphide are close to the standard content or are slightly depleted. The isotopic composition of oxygen in sulphate may also be helpful for solving problems.

The usefulness of tritium in coastal aquifer studies is not so clear, but it can be used to show the presence of recent continental water. The tritium content of present sea water is small but variable, depending on the depth, littoral processes and climate.

It is not certain whether other isotopes, radioactive (chlorine-36, uranium and its decay elements) or stable (strontium, nitrogen) or even noble gases (helium, argon) are a generally usable tool, although research continues. Sometimes the geochemistry is complicated and this prevents reliable quantitative, or even qualitative, results from being obtained until further research efforts possibly make a significant breakthrough. Even studies using the carbon isotopes (1 C and 1 C) are continuously subject to discussion as to the accuracy and meaning of the results.

SURVEYS AND MONITORING Surveying coastal aquifers is more difficult and costly than

other aquifers, under normal conditions. Not only is greater accuracy needed, paying special attention to the area affected by sea water, but vertical changes must also be carefully considered. Geological, potentiometric, chemical and isotopic surveys are needed bearing in mind this perspective. Common piezometric studies can lead to meaningless or even confusing results in aquifers con­taining waters of variable density. As commented on above, in order to make proper interpretations, the vertical distribution of salinity must be known, both inside the observation wells and in the aquifer. Some good studies have been published (Cooper et al, 1964; Lusczynski and Swarsenski, 1966; Cotecchia, 1977; TNO, 1980), but they are rare.

Drilling is expensive since a large number of boreholes is needed. Only in very shallow unconfined aquifers in flat coastal areas can fast, cheap drilling methods be applied. Long-screened piezometers allow for the measurement and monitoring of the interface inside the borehole by repeated salinity profiling, but when vertical flows in the borehole occur the results obtained are meaningless. Large changes in the interface depth are sometimes observed, more frequently in fissured rocks; they do not represent the true changes inside the aquifer at all, but only the adjustment of small head differences along the piezometer. Point or short-screened piezometers yield much better information, but they are more expensive, since several tubes must be installed at each site, in different boreholes or inside a large-diameter well with each carefully isolated; the interface depth or the extent of the mixing zone cannot be measured directly and must be deduced from point data.

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Monitoring must be adapted to the expected changes and the kind of control needed. In general, more frequent surveys are necessary than in studies of continental aquifers. In the past, the con­tinuous monitoring of the interface was fashionable because it was considered to be a more significant and sensitive element than the water level. It was carried out using conductivity probes linked to an electronic device and a recorder, but the approach is expensive and complicated to maintain in remote areas. It is almost a waste of time if the interface depth inside the borehole does not represent the true situation inside the aquifer. Realistic recommendations and specially developed commercial instruments are needed.

Only a few case histories of coastal aquifer monitoring have been published, and they mostly represent research situations. This is a field that needs to be examined in greater detail.

Indirect methods for surveying coastal aquifers and detecting saline water rely on geo-electrical prospecting; in spite of being well known and widely used, when carried out in a conventional form, they generally do not produce sufficiently accurate results, although some quite good studies have been reported from the Netherlands and elsewhere.

When using conventional geoelectric methods, the measurement of the induced electrical field potential appears to be a very promising tool in coastal aquifers, though expensive new measuring instruments must be added to the conventional ones.

Electro-magnetic methods, especially those of low frequency, seem to offer better prospects for accuracy and reproducibility, but their usefulness is probably limited to shallow coastal aquifers. Low altitude, airborne electro-magnetic surveys look promising for surveying large areas .

A comprehensive report on the real usefulness of geoelectric methods and the characteristics and applicability of new methods would be of enormous help to hydrogeologists and groundwater engineers. This is a field that should be covered and illustrated with examples in which geophysical methods are compared and validated against a good knowledge of the aquifer and its evolution. Unfor­tunately most published surveys lack this validation.

Infra-red thermal surveys using airborne line-scanning devices were very popular in the last decade for detecting fresh-water outlets along coast lines, especially in carbonate, volcanic and fissured rocks. In practice only large springs under special conditions can be detected, but the approach may be useful for surveying little known areas. In populated areas these outlets are commonly already well known to fishermen. A comprehensive report on existing experience and the benefits of this technique is needed, for though in many instances it may be a valuable tool, in others it may be no more than a scientific toy in the hands of developed countries, to be paid for by developing communities.

Geophysical logging of boreholes to study coastal aquifers uses conventional methods and tools, jointly with water salinity (electrical conductivity) logs. The interpretation of logs does not present special difficulties from a qualitative point of view; quantification is more difficult, but can be done if different electrical and nuclear logs are combined.

Attempts have been made to measure salinity through casing by

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neutron logging. The technique measures the chlorine content by neutron-gamma logging. Poor accuracy, together with an expensive, complicated and large-diameter probe have prevented commercial use. Other attempts focus on the high capture cross-section of chlorine for thermal neutrons, but further research is required to perfect this technique.

MANAGEMENT OF FRESHWATER RESOURCES IN COASTAL AQUIFERS As discussed in previous paragraphs, the basic principles of

fresh-water resources management in coastal aquifers are well known, although frequently not realised by managers, decision-makers and water users. In order to limit sea water penetration, some fresh water must flow above the salt water wedge and be discharged into the sea. Perennial usable resources are less than aquifer recharge, and the fraction of aquifer recharge that can be extracted depends on the extent to which sea water is allowed to penetrate. This acceptable sea water penetration is a complex function of management decisions that involve early abandonment of existing wells close to the coast, the drilling of new wells, the maintenance of higher pumping water heads and new and frequently longer water distribution mains. All these aspects can be integrated into management schemes (Morel and Birtles, 1983; Shamir and Bear, 1984). But in most common situations, historical, social, administrative and legal constraints are the decisive factors with which management must comply.

In such situations, decision-making models, using economic or maximum yield objective functions, are reduced to "academic" exercises if the other, not so easily quantified, variables and constraints are not duly considered.

Coastal aquifer management includes aspects which modify the aquifer balance such as artificial recharge. Los Angeles and other areas of California have long experience of this, but the solutions adopted are strongly influenced by conditions specific to the site. They cannot be extrapolated to other areas without careful study if costly, and sometimes irreversible errors, are to be avoided.

In areas with very limited fresh water resources, planned over­draft of coastal aquifers is a possible managerial decision in which the slow penetration of the salt water wedge may be allowed to enable existing fresh water reserves to be put to beneficial use. The progressive salting-up of the aquifer involves the early abandonment of near-coastal wells. New fresh water resources must be sought for the future, or changes in the use of water or even in social behaviour must be considered.

Fresh water that continues to discharge into the sea, after the interface has been stabilised is wasted. Attempts have been made in Israel to tap this water close to the coast, but works are required which are long, expensive and difficult to operate, and can be justified only in special situations. Furthermore, only a fraction of the fresh water flow can be obtained and some saline pollution is unavoidable.

In many instances, effective fresh water management in coastal areas needs storage reservoirs so that availability can be adapted to meet demands This is particularly necessary to store water that would otherwise be lost to the sea. Surface storage is sometimes difficult and subsurface storage in salt water aquifers (salaquifers)

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is one alternative being considered and tested, in which advances are continually being produced. Almost stagnant aquifers are the best suited, if the clay fraction does not produce drastic permeability reductions, as commented on previously. Such storage is not possible in salaquifers with significant flow, unless injected water is an appreciable part of the total aquifer volume.

Islands present very specific fresh water management problems, which must be solved in a limited space. Small islands sometimes pose difficult problems of survival in which the economic develop­ment of water resources vegetation, ecology and social conditions are deeply interrelated (Dijon, 1982). Solutions must be adopted in a broader context of land-use planning.

CASE HISTORIES Case histories are usually important for the proper understanding

of hydrological problems. They are helpful for engineers and managers, and allow politicians, administrators, and the public at large to digest the scientific and technical difficulties involved more easily. However, there are some dangers of misinterpretation or misapplication when a given situation is compared to a known one with a view to looking for solutions or forecasting future trends. Conditions must be comparable, not only geologically and hydro-geologically, but also from the economic, social and legal stand­points. This means that case histories must be well defined with clear boundaries, and an adequate understanding of the case under consideration is needed prior to making comparisons.

Unfortunately, only a few well known cases of coastal aquifer behaviour and management are available, and they do not adequately cover the wide spectrum of situations most frequently encountered. Sea water intrusion into coastal aquifers being one detailed aspect in a regional framework, need special, very costly observation networks and specifically designed studies, which have been under­taken only in highly developed areas or under extreme conditions. These are not the best conditions for presenting a case history as an example for other less wellknown areas or for developing countries.

ACKNOWLEDGEMENTS The author is indebted to Professor M.R. Llamas, Professor

A. Sahuquillo and Dr. R.A. Downing for their comments on this paper. The Curso Internacional de Hidrologia Subterrânea and the Comisaria de Aguas del Pirineo Oriental have provided typing and drawing facilities, and the Polytechnic University of Catalonia has helped in correcting the English text. Ms. F. Surbano was in charge of typing the draft.

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