Playas, sabkhas, and saline lakes

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Hydrogeologic processes in saline systems: Playas, sabkhas, and saline lakes Article in Earth-Science Reviews · October 2002 DOI: 10.1016/S0012-8252(02)00067-3

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Earth-Science Reviews 58 (2002) 343 – 365 www.elsevier.com/locate/earscirev

Hydrogeologic processes in saline systems: playas, sabkhas, and saline lakes Yoseph Yechieli a,*, Warren W. Wood b a

Geological Survey of Israel, 30 Malkhei Israel Street, Jerusalem 95501, Israel b U.S. Geological Survey, MS 430 National Center, Reston, VA 20192, USA Received 15 February 2001; accepted 19 November 2001

Abstract Pans, playas, sabkhas, salinas, saline lakes, and salt flats are hydrologically similar, varying only in their boundary conditions. Thus, in evaluating geochemical processes in these systems, a generic water and solute mass-balance approach can be utilized. A conceptual model of a coastal sabkha near the Arabian Gulf is used as an example to illustrate the various water and solute fluxes. Analysis of this model suggests that upward flux of ground water from underlying formations could be a major source of solutes in the sabkha, but contribute only a small volume of the water. Local rainfall is the main source of water in the modeled sabkha system with a surprisingly large recharge-to-rainfall ratio of more than 50%. The contribution of seawater to the solute budget depends on the ratio of the width of the supratidal zone to the total width and is generally confined to a narrow zone near the shoreline of a typical coastal sabkha. Because of a short residence time of water, steady-state flow is expected within a short time ( < 100 years), while steady state for solutes may take much longer (>50,000 years). The solute composition of the brine in a closed saline system depends largely on the original composition of the input water. The high total ion content in the brine limits the efficiency of water – rock interaction and absorption. Because most natural systems are hydrologically open, the chemistry of the brines and the associated evaporite deposits may be significantly different than that predicted for hydrologically closed systems. Seasonal changes in temperature of the unsaturated zone cause precipitation of minerals in saline systems undergoing evaporation. Thus, during the hot dry season months, minerals exhibit retrograde solubility so that gypsum, anhydrite and calcite precipitate. Evaporation near the surface is also a major process that causes mineral precipitation in the upper portion of the unsaturated zone (e.g. halite and carnallite), provided that the relative humidity of the atmosphere is less than the activity of water. The slope of the fresh/brine-water interface in saline lake systems is shallower than in fresh/seawater interface because of the greater density difference between the fresh/brine-water bodies. The interface between sabkha brines and seawater slopes seaward, unlike normal marine – fresh water systems that slope landward. Moreover, the brine/seawater interface does not achieve steady state because it is pushed toward the sea by the sabkha’s brine. D 2002 Elsevier Science B.V. All rights reserved. Keywords: brines; sabkhas; playa; fresh – saline water interface; hydrogeology; groundwater chemistry

1. Introduction *

Corresponding author. Fax: +972-2-5380688. E-mail address: [email protected] (Y. Yechieli).

Salt flats or bodies of saline water are ubiquitous features in arid and semi-arid areas of the earth (Fig. 1).

0012-8252/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 ( 0 2 ) 0 0 0 6 7 - 3

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Y. Yechieli, W.W. Wood / Earth-Science Reviews 58 (2002) 343–365

Fig. 1. Location of arid and semiarid areas (saline systems) in the world (after Strahler and Strahler, 1994).

The main requirement for the occurrence of such a system is that evaporation exceeds precipitation (Hardie et al., 1978; Langbein, 1961). Geologically, they provide information about sedimentary history, depositional environment, paleoclimate indicators, and economically valuable evaporite minerals and brines. They are also important in water resource assessments because they represent the discharge point or base level of local and regional ground-water and surface-water flow systems and can affect the ground-water quality in their vicinity. While saline water bodies are widespread in these environments, they usually represent transient conditions. It is, therefore, frequently necessary to take into account past hydrological conditions, including changes in sea level. These geomorphic features have different names, depending upon the culture that dominates the region of their occurrence. Thus, we find sabkhas (inland and coastal) and its variant spellings in the Arabic-speaking world; salinas, salares, saladas, salars, and playas in many Spanish-speaking cultures; pans, saline lakes, alkali flats, salt plains, dry lakes, and salt flats in English-speaking cultures. Rosen (1994) provides useful information regarding the terminology of these saline environments in different areas of the earth. Perusal of the Glossary of Geology (1987) provides nearly 40 names for these features. To hydrogeologists, however, they represent a single hydrogeologic process with varying elevation of ground water rela-

tive to the surface, and different boundary and initial conditions. The objective of this publication is to examine the hydrologic and chemical processes that control the solute and water budgets of these saline features. It is our belief that much of the confusion or misunderstanding in the literature regarding sabkhas results from a lack of understanding about the hydrologic and chemical fluxes active in their development. It is the water that is largely responsible for the introduction and removal of solutes from the features. Thus, hydrology controls the sediment and solute chemistry that make these systems geologically interesting. Traditionally, the major academic interest in sabkhas has been by carbonate petrographers and sedimentologists because of the presence of dolomite in geologically young sediments (e.g. Evans et al., 1964; Rosen et al., 1989; Warren, 2000). The abundance of sabkha sediments associated with petroleum production in the geologic record has also prompted significant interest (Schenk, 1969; Jacka and Franco, 1974; McGowen, 1979; Montanez and Read, 1992). Topographically, these features grade from essentially planar surfaces with no discernible depressions or permanent standing water (like the sabkhas of the Arabian Gulf), through closed, shallow basins with seasonal standing water (like the saline playas of Western Texas), to well developed, closed basins containing permanent water bodies (like the Dead Sea or Great Salt


Lake). They may be hydrologically active, or they may represent past hydrogeologic conditions. The sedimentary records of these systems was utilized in many studies for paleoclimatic research (e.g. Smith and Bischoff, 1997; Chivas and De-Deckker, 1991). There is often a dynamic equilibrium between the surface of the ground-water table and eolian processes (Stokes, 1964, 1968). That is, as the water table rises and intersects the surface, that surface will accumulate sediments and rise commensurate with the sediment supply and other factors. Conversely, as the water table drops, the land surface is eroded by eolian processes to the level of the capillary fringe (the elevation to which groundwater ascends by capillary force), since eolian process cannot remove material below the water table region. The process of deflation keeps the balance between the ground-water table (and capillary rise) and surface elevation (Rosen, 1994). The steady-state equilibrium in a sabkha between the surface elevation and ground water levels does not exclude a simultaneous change in both levels. A climate change could affect these levels and so can sea level change, which will affect the ground-water level.

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2. Hydrogeologic background A schematic view of the saline systems reviewed in this paper is given in Fig. 2. It is clear that the physical, hydrological processes are similar in all of these systems. The main difference between the playa environment (inland sabkha, saline pan, saline lakes, etc.) and coastal sabkhas is the contribution of seawater in the latter. There are, however, several variations within each system. 2.1. Coastal sabkha In the coastal environment, a hydraulic connection exists between the sea and the sabkha (Fig. 2, A1; e.g. in Abu Dhabi, Patterson and Kinsman, 1981). A modification of this system occurs when the sabkha is prevented from receiving ocean water directly by the presence of a shore ridge (Fig. 2, A2; e.g. in Kuwait—Robinson and Gunatilaka, 1991; Sinai, Egypt—Gat and Levy, 1978, Northern Egypt—Ali and West, 1983). Generally, the hydrological regime of the sabkhas in coastal areas was divided into two main parts: coastal

Fig. 2. Schematic cross-sections of saline environments (sabkha, salina, playa, saline lake).

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(marine) sabkhas and continental sabkhas (Robinson and Gunatilaka, 1991; Patterson and Kinsman, 1981). The division between the two parts is at the area where no marine influence is observed, above the highest sea level of the Holocene (Fig. 3). It should be mentioned that this level could vary in different seasons and climates. The continental sabkha is similar to the inland sabkha described below. 2.2. Playa (inland sabkha) The main difference between the different types of inland sabkhas or playas is the elevation of ground water in relation to the elevation of the surface of the playa (Rosen, 1994). A discharge playa occurs when the level of the capillary fringe is coincident with the playa floor (Fig. 2, B). The water level usually varies seasonally or annually; however, on the average the water level depth is less than the thickness of the capillary fringe (depth of not more than several meters, depending on interstitial pore size, Fig. 2, B2). Rosen (1994) concludes that the most important factor in obtaining accumulation of evaporites in playas is ground-water level close to ground surface. Extensive accumulation of evaporites usually occur in saline lake, which is a specific case of playa system. Playas are found in many parts of the world, including Western US (Rosen, 1991), Australia (Bowler, 1986), Iran (Krinsley, 1972) and others. Accumulation of thick layers of salts in this system is possible provided there

is subsidence of the basin (Rosen, 1991). A recharge playa occurs when the recharge to the ground water is greater than discharge of the ground water (Fig. 2, B3). In this case, evaporites do not accumulate on the surface (Rosen, 1994). Some recharge playas can be explained as remnants of time in which water levels were higher, then decreased due to climate change and/ or tectonic activity or anthropogenic effects. In recharge playas some evaporites can be found due to evaporation of surface water but only for relatively short periods of time. 2.3. Saline lake The terminal saline lake or sea is an important end member of a saline system. Closed saline lakes can be described as playas in which the open saline water reservoir is maintained throughout its history. In the case of a true closed terminal lake, such as the Dead Sea, all ground water and surface water flows converging toward the lake, and there is no surface or ground-water outlet (Fig. 2, B1). There are some saline lakes which have outlets, usually of ground water and not of surface water. The existence of overflow in past saline lake was studied by sedimentological indicators in surface exposure or subsurface cores (e.g. Brown and Rosen, 1995). Terminal lakes do not remain in a long-term steady state. They are usually kept active by tectonic activity, which is responsible for continuous subsidence at a rate that

Fig. 3. Components of mass balance in coastal sabkha.


exceed the rate of sediment accumulation of both clastic and chemical deposits. Examples of saline lakes in such tectonically active places are the Dead Sea (Neev and Emery, 1967) and Lake Asal in Africa (Fontes et al., 1979). 2.4. Salinas Like the closed lake system, salinas are not expected to remain in a steady state since they are filled by sediments (chemical, alluvial and/or eolian). The Australian salinas have been active only since 6000 –7000 years ago (Warren and Kendall, 1985). Salinas usually refer to evaporite basins in which wet conditions prevail most of the time. These basins usually have a perennial lake for most of the year. An interesting type of salina is reported from the coastal area of South Australia. These salinas are saline lakes adjacent to the ocean but without surface connection as they are sealed from the sea by high shore ridges. According to Warren and Kendall (1985), groundwater and subsurface ocean water feed the salinas.

3. Water and solute balance The general hydrogeologic principles of water and solute transport are given in numerous textbooks (e.g. Bear, 1972; Freeze and Cherry, 1979; Domenico and Schwartz, 1990; Fetter, 1994; Ingebritsen and Sanford, 1998). It is important to differentiate between the sources of both water and solutes in order to understand the various systems. Fig. 3 illustrates schematically the components of a coastal sabkha, although it is also applicable, with some changes, to other saline systems. The conceptual equilibrium model (Fig. 3) of coastal sabkhas is as follows: shallow ground water flow down gradient to the sabkha. Because coastal sabkhas usually represent the base level in the hydrologic system, they become regional discharge areas also for fluids from the underlying aquifers. In such case, deep basin brines can discharge to the base of the sabkha. Flooding of storm surges from the adjacent sea can add water at the proximal edge of the sabkha. Fresh water is added by direct rain and overland runoff from adjacent areas. Removal of water occurs by discharge to the sea and by evapotranspiration. In the case of most coastal sabkhas, they are

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usually too saline to support vegetation and thus there is no significant loss from transpiration. The explicit water mass balance for a steady state situation associated with this conceptual model can be formulated as follows, where fundamental dimensions are mass (M), length (L), and time (T): Qm ¼ Qr þ Qf þ Qsw þ Qgs þ Qgd Qe ;

ð1Þ

where Qm is average outflow from the sabkha to the ocean (L3/T), Qr is average rainfall (L3/T), Qf is average overland flooding on to the sabkha (L3/T), Qsw is average recharge due to marine flooding (L3/T), Qgs is average shallow ground-water inflow to the sabkha (L3/T), Qgd is average deep ground-water inflow to the sabkha (L3/T), and Qe is net evapotranspiration (L3/T), which is the amount of evaporated water minus the humidity captured from the atmosphere. Solutes in the coastal sabkha are derived from rain, flooding by seawater, overland flooding, shallow ground-water discharge, and deep ground-water discharge. Solutes are lost from solution by mineral precipitation, discharge to the sea, conversion of liquid to gas phase and radioactive decay. They are lost or gained by eolian transport and water –rock interaction processes. The explicit solute mass balance for steady state is: Qm Cm ¼ Qr Cr þ Qsw Csw þ Qf Cf þ Qgs Cgs þ Qgd Cgd P GFE;

ð2Þ

where Cm is average solute concentration in outflow to the sea (M/L3); Cr is average solute concentration in the rain (M/L3); Csw is average solute concentration of the marine flooding (M/L3); Cf is average solute concentration of overland flooding (M/L3); Cgs is average solute concentration of the shallow groundwater inflow (M/L3); Cgd is average regional solute concentration of the deep ground-water inflow (M/L3); P is mineral precipitation rate and water – rock interaction processes (M/T), G is rate of conversion of solute to gas and radioactive decay (M/T), and E is eolian and other erosion processes (M/T). All other parameters ( Q terms) were defined in the previous paragraph. The concentration units here are in terms of mass/volume, although in very saline brine unit of mass/mass would be more appropriate. This

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was done in order to allow comparison with most values in the literature (in g/l) and since the following calculations were done only for an order of magnitude. If the system is not in steady state, the change in volume and concentration of the water should be considered. In many cases, the water balance may be in equilibrium (constant volume) but the solute balance is not. Thus, for the solute mass balance, the added term should describe only the change in concentration. Similarly, mass balance equation can be applied for inland sabkhas, but without the direct contribution from the sea (no marine flooding). The outflow would be flow down gradient not to the sea ( Qout instead of Qm). The same balance applies as well for saline lakes that are hydrologically open (with underground output). In a closed terminal basin, the output term would be zero.

4. Sources of solutes In the following section, we discuss the relative contribution of solutes from the various sources iden-

tified in Eq. (2). Values for the hydraulic properties are taken mainly from the sabkha of Abu Dhabi (Sanford and Wood, 2001). Some of the hydraulic and chemical parameters are not very well constrained, and therefore the following estimation should be regarded as an example of a typical sabkha for illustration only. These estimations can serve as an order of magnitude and allow comparison among the different sources. We compared the solute contribution of each component to a specific control volume (taken here as an area of 1 1 m2 and the saturated thickness of the aquifer) in the central area of the sabkha (Fig. 4). 4.1. Initial solute source—original seawater Seawater, which originally filled the interstitial pores of coastal sabkhas, is the first source of solutes. The contribution of this initial seawater in the central area of the sabkha was calculated: Ssw ¼ V hCsw ;

ð3Þ

where Ssw is amount of solute contributed by seawater, V is volume of 1 m2 column in the middle of the sabkha (L3), h is porosity.

Fig. 4. Schematic illustration of the zones in coastal sabkhas. Note that the mass balance in this work was conducted in an area in the central part of the sabkha.


Porosity of sediments in the sabkhas, which varies according to the sediment’s grain size and uniformity (h = 0.1– 0.4), was taken to be 0.3. For a volume of 10 m3 and concentration (Csw) of 40 g/l, the solute contribution is 120 kg. 4.2. Rainfall Rainfall contains similar elements to those found in brines but in lower concentrations. Nevertheless, evaporation is responsible for removal of water leaving the solutes in the solution. The solute contribution from direct rain can be evaluated, using the following equation: Sr ¼ Qr Cr t;

ð4Þ

where Sr is total solute mass contribution of rain (M/ L3); t is total time since the formation of the sabkha (T ). In our example of the Arabian Gulf’s sabkha, the peak of the sea level rise was about 6000 years ago, about 2 m above the present sea level (Lambeck, 1996). For the purpose of our calculation, this value is taken to

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be the total time (t). The amount of rainfall for this arid area is assumed to be 100 mm/year, and the solute concentration (TDS) in rainfall is assumed to be 10 mg/ l. Solving Eq. (5) with these values yields a total of 6 kg/m2. The actual importance of this source depends largely on the thickness of the aquifer (Fig. 5), i.e. the actual volume into which the solutes from rain mix. 4.3. Shallow ground water The contribution from lateral flow of shallow ground water is an additional source of solutes to the coastal sabkhas (Eq. (2)). This solute flux can be evaluated by calculating the horizontal water flux and multiplying it by the solute concentration in this water. The water flux can be estimated using Darcy’s Law: q ¼ kðdh=dlÞ;

ð5Þ

where q is water flux (L3/L2) (T), k is hydraulic conductivity (L)/(T ), and dh/dl is gradient (dimensionless).

Fig. 5. The contribution of the main sources of salts in sabkha (schematic). The relative contribution of each component is shown. It should be noted that the maximal sabkha salinity increase with time from that of seawater salinity to that of the evaporated sabkha brine. Also note the effect of the thickness of the sabkha (1 – 10 m) on the different sources, and that when the lateral flowing ground water reaches the central part of the sabkha, it replaces the original seawater component.

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The sediments in sabkhas are variable; but for the sake of the present calculation, we assumed homogenous, fine sand with a hydraulic conductivity of 1 m/ day (Sanford and Wood, 2001). In most sabkhas, the ground-water gradient is very low (dh/dl = 1/4000; Patterson and Kinsman, 1981). Therefore, horizontal water flux ( q) is 0.00025 m/day. Viewing this contribution in the perspective of travel distance of the shallow ground-water solutes is given by the following equation: Dt ¼ ðq=hÞt;

ð6Þ

where Dt is distance traveled (L), t is time (T ). Assuming, for the sake of simplicity, plug flow (negligible hydrodynamic dispersion), in 6000 years the solute travel distance is approximately 1.8 km. Thus, in coastal sabkhas with a width of 10 km, the lateral ground water entering the sabkha has only displaced 18% of the total width of the sabkha. Thus, the shallow ground-water component is important at the distal or continental edge of the sabkha for this time frame (Fig. 3). The mass of a solute in the shallow groundwater can be calculated by the following equation: Sgs ¼ Cgs V h;

ð7Þ

where Sgs is mass of solute (M ). Assuming lateral ground water has a concentration of 10 g/l, porosity of 0.3, and volume of 1 or 10 m3, the mass contributed by lateral ground-water flow is 3 or 30 kg, respectively. As mentioned above, this contribution affects only the proximal edge.

reach them. However, it is an important factor in playa and other inland features that receive runoff from surrounding topography (e.g. in some Chinese lake systems, Lowenstein et al., 1994). 4.5. Deep ground water Evaluation of deep ground water component requires knowledge of both the vertical hydraulic conductivity and the vertical gradient of the underlying, less permeable sediments and knowledge of the solute chemistry of the water. While these data are clearly site-specific, it is possible to gain a sense of this component by making some reasonable approximations. Taking a vertical gradient of 0.12 and a hydraulic conductivity of 10 4 m/day, as found in Abu Dhabi (Sanford and Wood, 2001), then a water flux of 1.2 10 5 m/day, or approximately 4 mm/ year, results when Darcy’s Law is applied. The solute contribution of this component can be calculated according to the following equation: Sgd ¼ ðQgd ÞðCgd ÞðtÞ;

ð8Þ

where Sgd is solute mass contribution of deep ground water (M). Assuming Cgd to be approximately that of seawater (40 g/l), which is slightly higher than ocean water, the contribution of solute across a square meter is 960 kg over 6000 years. Thus, although the water flux from this component is small, the solute flux is the largest. The possible increase in concentration from this deep ground water component, using a volume of 10 m3 and porosity of 0.3, is 300 g/l. 4.6. Seawater

4.4. Overland runoff Fluxes enumerated in Eq. (2) include a component for overland runoff or flooding by sheet wash derived from lands further from the coast. This input depends on the topographic relief, storm intensity, and geology. Some of the coastal sabkhas have flood contributions (e.g. in south Australia; Bye and Harbison, 1991 and Baja California, Castens-Seidell, 1984), while others are at such a great distance from the alluvial fans (e.g. in Abu Dhabi Sabkha) that only exceptional flooding events can

Fluxes enumerated in Eq. (2) include a component for seawater flooding. The coastal sabkhas can be divided into three areas (Fig. 4): (1) the proximal area, which is occasionally flooded by seawater (supratidal). The extent of this flooding is not very well defined and was reported to be as much as 5 km in cases of extremely strong wind (Patterson and Kinsman, 1981); (2) the distal area near the maximum extension of past sea level high stand, where the original seawater has already been flushed by lateral flow of ground water; and (3) the area between the two (in many case, this


may be the largest area). The seawater component is, therefore, important mainly in the proximal area, where sea floods occasionally. In this area, the water is expected to have seawater composition and probably somewhat higher salinity due to evaporation and contribution of solutes from deep ground water. 4.7. Sinks of solutes (outputs) A significant removal of solutes is done by groundwater discharge to the sea ( QmCm). This is larger than the solute contribution from horizontal ground-water inflow because of the contribution of the deep ground water and the concentration effect of evaporation. Other outputs of solutes are precipitation of salts, loss by conversion to gas, and radioactive decay of solutes within the sediments. Flooding of the surface by intense rainfall events causes runoff of dissolved surface salts to the sea. The concentration of solutes in the sabkha will increase until a dynamic equilibrium among solute source, mineral precipitation, and discharge to the sea is reached. 4.8. Eolian transport The contribution of eolian process should be regarded with caution. The effect of this process can be local, where solutes are moved from one area of the sabkha to another, eroding salt at one point and accumulating it at another point. It is, therefore, shown in Fig. 3 as arrows to all directions, and the net eolian effect is generally difficult to determine. It may be less important in large sabkhas compared to smaller ones. The sources of salt could be sea spray, carbonates from the proximal edge toward the distal edge, or dust from the desert toward the sea. In some playas, eolian salts can be transported distances of 35 km from the source (Wood and Sanford, 1995). However, mobility is also dependent on the mineralogy: as sodium carbonates and sulfates form fine grained, fluffy deposits on the surface that are easily eroded by wind, while sodium chloride may form an ‘‘ice’’ like, well-cemented surface that is not easily eroded. 4.9. Summary of the contribution of the various sources The relative contribution of seawater becomes smaller with time at a rate that depends on the other

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sources (Fig. 5), because the seawater contribution is only at the initial state. It is clear that in the sabkha system we described, the most important solute component is that of deep ground water. This component is basically cumulative (except small portion that is taken away by horizontal groundwater flow), and therefore, its significance is observed sometime after the beginning of the sabkha’s evolution. The rainwater contribution could be significant for some ions that are not abundant in the original seawater or in the ground water (e.g. NO3). The seawater-flooding hypothesis (Kinsman, 1969 and its modifications—Paterson and Kinsman, 1981) suggests that storm surges push seawater onto the shallow, low-lying sabkhas near the Arabian Gulf. This seawater would then recharge to the aquifer and be refluxed to the ocean. This model, in its various modification, is a widely referenced in textbooks (Berner and Berner, 1997; Drever, 1998). But, there are several physical, hydrologic problems with this model. First, the hydraulic conductivity and gradient are so low that there is insufficient flux (i.e. much less than one pore volume) to move solutes in the 6000 to 7000 years since the start of the system. Second, the ocean water is less dense than sabkha water and would float on the surface of the water table, not mixing with the rest of the water. There is, certainly, a component of seawater, but the amount is a relatively small percentage of the total and is confined to the proximal portion of the aquifer. Because of rapid progradation the aquifer is moving faster than groundwater. The observed seawater results from entrapment rather than flooding. 4.10. Chemical constraints for estimation of the contribution of the various solute sources The sources of solutes can be differentiated utilizing their chemical and isotopic composition. Using these identification methods, it was shown that the deep ground-water component could be much larger than the seawater component. Robinson and Gunatilaka (1991) used the oxygen and deuterium isotopic composition of water in the sabkhas in Kuwait to show that deep, old ground water is a major source of water. The isotopic composition of sulfur and oxygen in the dissolved sulfate also indicates that the marine contribution is minor in that system. Mu¨ller et al. (1990) showed that strontium isotopes can be used to separate

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ground water and ocean water in the sabkhas of Abu Dhabi. Chemical analyses in the coastal sabkhas of Abu Dhabi (Butler, 1969; McKenzie et al., 1980; McKenzie, 1981; Patterson and Kinsman, 1981, 1982) are consistent with the presence of significant masses of solute derived from ground water. Patterson and Kinsman (1977) used the K/Br ratio to differentiate between marine and ground water sources. Another chemical method, which can be applied in certain circumstances, evaluates the difference between the equivalents of sulfate and calcium in different solute sources. In seawater, the equivalents of sulfate are always greater than those of calcium, and the water usually evolves to a low Ca, high SO4, high Mg, and high pH brine (Hardie and Eugster, 1970). In many continental and deep basin brines, the reverse is true, and the brine evolves to a low pH, high Ca and Mg, and low SO4 brine. The comparison of equivalents assumes no significant chemical modification due to water – rock interaction and that significant amounts of sulfate are not removed from the sabkha system by some other process, such as sulfate reduction to hydrogen sulfide gas (H2S), precipitation of sulfides such as iron (FeS2), or precipitation of sulfate with other cations such as sodium, potassium, or magnesium. Using chemical composition, then, it is often possible to differentiate between brines of recently evaporated marine origin (similar ionic ratios to that of seawater) and older brines of a more complicated genesis, whose origin could be from deep ground water systems. In general, the solute evolution in coastal sabkhas operates as follows: water and solutes are transported from the shallow water table to the surface by capillary forces where water is evaporated, leaving the efflorescent salts near the surface. Rainfall dissolves the efflorescent salts on the surface, and they are recharged to the aquifer. The addition of new solutes, plus the return of solutes derived from the dissolution of efflorescent salt, causes the solute concentration in the sabkha to increase over time. The recharge of rainwater that is saturated with salt from the surface causes instability due to differences in density, and mixing in the aquifer. This increase in solute concentration is constrained by mineral precipitation and solute discharge to the ocean. Thus, the ultimate steady-state solute concentration depends upon transport rates of the solutes to the ocean, evaporation rates, thermodynamic equilibrium with the minerals, and water and

solute flux of solutes to the sabkha. In many present systems, steady state has not yet been achieved, and the solute concentration is increasing with time. 4.11. Water mass balance Our interest to this point has been the solute contribution to the sabkha from various sources; but water balance is also important. The fact that no significant changes in water levels is observed in many sabkhas indicates that these systems are in steady-state conditions and therefore input equals output. Thus, the residence time can be estimated using the evaporation flux ( Qe) as the value for influx, according to the following equation: Tr ¼ bh=Qe ;

ð9Þ

where Tr is residence time (T), b is saturated thickness (L). Assuming that the saturate thickness is 10 m, porosity is 0.3, and evaporation rate is 5 cm/year (which is reasonable for the Gulf Sabkhas according to Sanford and Wood, 2001), the average residence time of water in an aquifer is 60 years. The relatively short residence time is supported by the fact that ground water in the sabkhas of Abu Dhabi contain tritium (MacKenzie, 1981), indicating residence times of a young component of less than 40 –50 years. In general, the recharge of precipitation in arid areas is very low (2– 4%, e.g. Wood and Sanford, 1995; Bazuhair and Wood, 1996). There are, however, several reasons to assume that in sabkhas and playas, the situation could be different: (1) rain dissolves the soluble salts of the crust at the surface, elevating salinity quickly, which lowers water activity and, therefore, lowers the evaporation rate (Sanford and Wood, 2001); (2) in many locations, the water table is very shallow. The capillary zone reaches the surface, and, therefore, the whole sediment column is always almost saturated. In such a system, a continual water film exists around the grains so that water can move with low friction through the interstices. It should be noted that a shallow water table also has the opposite effect of higher evaporation (discussed later) but this effect is smaller than that described here; (3) the relatively high soil moisture in most saline system means only small amount of water is required to saturate the soil (e.g. in Abu Dhabi; Sanford and Wood, 2001). The high


moisture is mainly due to the high salt content (including hygroscopic salt such as Mg salts) and probably also due to clay content; (4) in many areas, there are polygonal desiccation cracks that can provide access through the low permeability, cemented crust; and (5) the lack of topographic relief prevents significant runoff (Sanford and Wood, 2001). Low topography is generally considered a factor that would limit recharge, because recharge in arid areas usually takes place where water is focused in stream beds or depressions. But in these saline environments, it can play a positive role, allowing the water to penetrate downward without loss to runoff. Because the water level in sabkhas remains relatively constant, the input must balance the output. Thus, if rainfall is about 100 mm/year, discharge by evaporation can only exceed this value by a small amount equivalent to the added ground-water flux, regardless of potential evaporation. In fact, the potential evaporation could be 1 –2 orders of magnitude higher than the actual measured evaporation, which should be the actual value used as an output component. The rate of evaporation depends on various factors related to the climatic conditions and on water activity and saturation vapor pressure (Harbeck, 1955; Langmuir, 1997). Basically, the rate of evaporation decreases with increasing salinity, leading to low evaporation rates in very saline brines (1.2 m/year in the Dead Sea brine; Stanhill, 1994, down to zero evaporation in extremely saline brines of evaporated Dead Sea water; Yechieli et al., 1998). A large difference exists between evaporation from open reservoirs, such as saline lakes, and from shallow ground water. The difference can be of 1 –2 orders of magnitude (e.g. 1 –3 m/year vs. 3– 10 cm/year; Allison and Barnes, 1985; Tyler et al., 1997). Even in the case where ground water is very near the surface and capillary force keeps the section almost saturated, the rate of evaporation is much lower than in open water. Generally, the rate of evaporation from such sediments depends on the depth of ground water, with higher rates for shallower ground water (Thorburn et al., 1992). As an example, the rate of evaporation was found to be a function of the depth to water table raised to the fifth power (Ripple et al., 1972). Low permeability sediment and/or salt crust will lower the evaporation rate (Ripple et al., 1972; Thorburn et al., 1992).

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In saline systems, there are several different time scales that should be considered. The above water mass balance relates to a short time scale. For longer periods (scale of solute mass balance), one should consider the effects of sea level changes and climate change on different parts of the equation. The detailed sea level history is a crucial point in sabkha formation (Patterson and Kinsman, 1981). This is especially true because of the low hydraulic conductivity in most sabkha environments. 4.12. Playa brine formation—estimation of evolution time Playas are very common features in arid and semiarid environments. One important question regarding these playas has to do with the evolution time of the brine in these systems. In the following section, we estimate the time that it takes brine to evolve to a salinity of 200 g/l in a playa system. There are several options in such calculations. Most age calculations were done for saline lakes (open water reservoirs) assuming a known constant solute input (Langbein, 1961). The following estimation was done under the assumption of steady state conditions, in which the amount of water input equal to that of output by evaporation. In this case, it is not necessary to assume known water inputs, which are very difficult parameters to determine in many playa systems. The water input is rain that falls either directly on the playa or elsewhere in the watershed area and flow to the playa either on the surface as floods or in the subsurface as groundwater. For such a calculation, a system with water input salinity of 0.1 g/l was selected. The present calculation was done for a playa in which there is either a shallow pond or none at all and thus the brine evolves within the sediments. The main factors that affect this evolution are the thickness of the playa and the evaporation rate. The thickness of the playa to the depth of the lower confining layer was taken to be 10 or 100 m (Fig. 6A). The evaporation rate, which varies significantly, was chosen to be 2 m/year for an open water body of 1 m depth. In the case of no open water body, the evaporation rate was taken to be 0.1 m/year. In the case of a steady state, water input (mainly ground water) compensates for the loss of water by evaporation. We can, therefore, calculate the amount

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Fig. 6. Evolution of a brine in a playa system: (A) Schematic illustration of the configuration used for the calculation of the brine evolution time. (B) Evolution of brines for various conditions—(1) Vp = 1, Ev = 2, Thickness (Vs) = 10; (2) Vp = 0, Ev = 0.1, Thickness = 10; (3) Vp = 1, Ev = 2, Thickness = 100; (4) Vp = 0, Ev = 0.1, Thickness = 100. (C) Schematic cross-sections showing the flow regime and the recycling of the brines.

of salt that is added each year by ground water according to the following equation: Si ¼ Ev Cg ; ð10Þ where Si is salt addition each year (M/T), Ev is evaporated water (L3), and Cg is salinity of ground water (M/L3).

Then, the estimation the evolution time is according to the following equation: t ¼ Cbf ðVp þ Vs hÞ=Si ¼ Cbf ðVp þ Vs hÞ=Ev Cg ;

ð11Þ

where t is evolution time (T), Cbf is final salinity of the brine (M/L3), Vp is volume of open water pond


(L3), Vs is volume of the playa sediment (L3) and u is porosity. The resulting evolution time to reach final salinity (Cbf = 200 g/l) was approximately 4000 years for the case of a playa, 10 m thick and 2 m/year evaporation rate (Fig. 6B). At the other extreme, for a playa with no open water body (evaporation rate of 0.1 m/year) and thickness of 100 m, the evolution time would be 600,000 years (Fig. 6B). It should be noted that the evaporation rate is expected to decrease significantly as the salinity increase above 200 g/l; therefore, the extrapolation of these calculations should be done with caution. The evolution times in the calculation above should be regarded as a minimal value, since we use constant evaporation rates. It should be noted that in many playas the salinity exceed the degree of saturation for several (e.g. gypsum and halite) and therefore, the volume of accumulated salt should also be taken into account. One example is that of Bristol Dry Lake, where the age was found to be about 4 million years, using a Cl mass balance (Rosen, 1991). There are several other factors that can affect the chemistry and mineralogy in playas: (1) recycling of brines (Fig. 6C) where they evaporate and sink at the center of the playa and ascend at the playa’s margins (Smoot and Lowenstein, 1991). While recycling of brines does not affect significantly the general water and solute balance of the playa, it changes the salt composition due to the precipitation of various salts and changes in chemistry of the original brine; (2) water –rock interaction could change the chemistry but will not change the salinity significantly; and (3) geothermal fluids which may ascend from great depth. The contribution of salts from this source depends on its salinity, which is usually higher than that of shallow groundwater. 4.13. Other source of salinity in playas There are two other possible salinity sources in playas, which were not mentioned before. (1) Past seawater intrusion—this is the effect of seawater intrusion of ancient times at past high sea levels. In arid areas where flushing by fresh ground water is limited, remnants of residual brines from ancient eras (e.g. Neogene or even older) should also be regarded as possible. Such is the case in the Dead Sea area (Zak, 1967; Starinsky, 1974) and in the

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Fig. 7. Seawater intrusion inland in two cases: (1) Gavish sabkha in Sinai, Egypt (Gavish et al., 1985). (2) Lake Asal in Africa (after Kafri, 1984). Note the different scale in the two graphs.

Southern Arava Valley in Israel (Bein et al., 1995; Starinsky, 1974). (2) Current seawater intrusion—in some systems, where the base level is below sea level, subsurface seawater intrusion toward the lower base level is evident (Fig. 7; Kafri, 1984). A good example is Lake Asal (Kafri, 1984), which is 12 km from the Red Sea and 155 m below it. Such an intrusion can occur in an arid environment where there is a small amount of precipitation, and, therefore, no significant input of fresh water (Fig. 7).

5. Factors controlling the chemical evolution of brines Evaporation is the main process forcing solute evolution of brines in most sabkhas, playas, lakes, and other open-water systems in arid areas. Removal of water by evaporation increases the concentration of the solutes, to the point when precipitation of minerals occurs. Transpiration could also be important in some systems. Topographically closed basins can be hydrologically closed, with no output, or open, where brine leaks out of the system (Wood and Sanford, 1990). Mineral precipitation and solute evolution for a chemically closed system is controlled by the chemical divide concept (Hardie and Eugster, 1970). As solute concentrations increase in the brine, mineral preci-

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pitation occurs and removes some of each of the reacting solutes. The solute of the lowest concentration in this precipitation reaction will decrease in concentration, while the remaining solutes increase in concentration as evaporation proceeds. For example, if water contained greater equivalents of chloride than sodium, the sodium would ultimately become limited because the precipitation of halite removes both chloride and sodium. Sodium concentration would approach zero as the chloride concentration continued to increase due to evaporation. In this model, the initial solute ratio in the source water entering the evaporation process defines the chemical divides and the unique path along which the brine will evolve. That is, for a closed system, the milliequivalents per liter of the cation and anions in input water and the equilibrium constants of the evaporite minerals are the only factors that control the solute evolution of the brine. Wood and Sanford (1990) and Sanford and Wood (1991) modified this chemical divide concept for the more common hydrologically open system. By considering solute removal from the basins through ground water leakage, they were able to show that solute evolution of the brine and the type and amount of minerals deposited depended on the degree of leakage (Fig. 8). Applying this leakage concept, Wood and Sanford (1990) could account for many observed evaporite assemblages that could not be explained by closed-system evaluation. Two constraints are applicable to the open and closed systems: one of constant water volume and the other of declining water volume. In most geomorphologically stable environments, the constant volume model is probably the more appropriate choice for simulating the solute or mineral evolution. While there may be annual changes in water levels, one can usually assume that the net long-term water level over hundreds of years does not change significantly, unless there is a climate change. In the case of complete desiccation of a lake, the declining volume model would be the appropriate choice. The choice of a constant or declining model is not trivial, because the mass of minerals precipitated is different, given the same starting solute chemistry. In the constant volume model for

a closed system, both the concentration and total mass of solutes increase because solutes and water are continuously introduced into the system. In a declining volume model for a closed system, the concentration of the solutes increases with evaporation, while the total solute mass of solutes and mineral remains constant. A limit, or constraint, to the evaporative mechanism of concentration is imposed by the thermodynamic activity of water (Langmuir, 1997). As the amount of dissolved salts increases, the activity coefficient decreases. The activity of pure water is 1.0; 0.98 in seawater; in saturated solution of NaCl it is reduced to 0.78; and for highly evaporated brines, it approaches 0.5. One can see that for any given pressure and temperature, increasing the dissolved ions reduces the ability of the water to evaporate, because there is less free, unbound water available for evaporation in a unit of time. There are a number of ways to calculate the approximate activity of water (Langmuir, 1997). All the proposed methods have some limitations. For concentrated brines, it is generally agreed that the Pitzer equations (Pitzer, 1987) currently provide the most satisfactory method of calculating the thermodynamic activity of water. These equations are incorporated into geochemical speciation programs, such as PHRQPITZ (Plummer et al., 1988). Thermodynamic activity is also expressed as a ratio of the vapor pressure of the solution of interest, to that of pure water at the same temperature and pressure. Vapor pressure of pure water in the atmosphere is equal to the relative humidity. Thus, comparing the activity of water in a brine with the atmospheric relative humidity will determine if evaporation will occur or if the water from the atmosphere will be transferred to the brine. Water in a pond or a sabkha cannot evaporate if the activity is less than the relative humidity in the atmosphere. For example, if the relative humidity of the atmosphere is 70%, then water cannot evaporate if the thermodynamic activity of the brine is less than 70%. In fact, if the activity of water in the brine were less than 70% in this example, water would be transported from the atmosphere to the brine. Fig. 9 illustrates the weight of two pans of fresh and saline water

Fig. 8. Variation in mineral assemblage and the chemistry of the remaining brine, depending on the degree of openness of the system (Sanford and Wood, 1991).


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358


Fig. 9. Daily variations in humidity and evaporation/condensation in brine and fresh water in a sabkha system. Note the difference between the fresh water, which continue to evaporate throughout the night, and the brine that during the night does not evaporate but instead collect condensing water.

undergoing evaporation under identical conditions of changing relative humidity. Two important observations can be made from this data. First, it can be seen that the rate of evaporation of the brine is less than that of the fresh water, due to the smaller gradient between the activity of water vapor in the air and brine than that in the air and fresh water. Secondly, it can be seen that when the relative humidity increases, the weight of the brine increases, because water is transferred from the atmosphere to the pan of brine. The fresh water, with its activity coefficient of approximately 1.0, continues to lose weight throughout the period of measurement because the relative humidity of the atmosphere is always below 100%. There are several mechanisms by which solutes in the saline environments are concentrated and may cause precipitation of salts. Some of these occur in the saturated zone while others in the unsaturated zone. Addition of solutes by dissolution of a previously deposited evaporite bed or eolian input can raise the concentration beyond saturation of a specific mineral. Mixing of two solutions can also cause similar results. An example for the later could be mixture between Ca rich brine with SO4 rich solution, neither of which is saturated with respect to any mineral, but when mixed results in the precipitation of gypsum or anhydrite. The processes in the unsaturated zone can be of two types (Fig. 10): (1) removal of water due to evaporation; (2) rise in temperature, which lowers the solubility

for some minerals. In the first of the proposed mechanisms, water is removed by vapor flux from above the capillary fringe more rapidly than it is replaced by recharge. Thus, there is an increase in concentrations of the remaining solutes, permitting salts to precipitate. In this case, evaporation results in the lack of accumulation of salts on the surface because only water in vapor phase is transported, not solutes, which must be transmitted in the liquid phase. Evaporation can also trigger a capillary rise from ground water table, in which solution ascend while the concentration increase. In such a case, the increase in ion concentration causes precipitation of a sequence of salts (Yechieli and Ronen, 1997), according to the different degree of saturation of each mineral (Fig. 10). In the second conceptual model, solutes are transported by capillary rise from the water table to the surface, without significant loss of water as vapor (providing constant moisture content, Fig. 10). On the path to the surface, they experience a seasonal thermal change imposed by the ambient temperature at the surface. It is expected that during summer when temperature is highest, gypsum, anhydrite, and calcite will precipitate in the capillary zone because these minerals exhibit retrograde solubility (decreasing solubility with increasing temperature). Conversely, when the temperature is cooler during the winter, salts of Na and Mg chlorides can precipitate as they become supersaturated at cooler temperatures. In this temperature rise model, the concentration of conservative ions in the solution remains constant. Even in the latter mechanism, there is usually evaporation from the top of the capillary fringe where it intersects the land surface. In this case, water and solutes are delivered to the surface by capillary rise, where the water is lost to evaporation and the solutes are concentrated as evaporative salts on the surface. The surface is able to achieve dryness, in spite of decreasing activity of water brought about by increased dissolved solids, because the temperature of the land surface in many arid, hot climates may exceed 80 jC. Thus, even as the activity of water declines with increasing evaporation, the solar heat contributes sufficient energy to remove water by evaporation. In nature, it is expected that both mechanisms will take place. Both can occur simultaneously or at different periods (different seasons). Another factor that should be mentioned is the kinetic effect. In such a


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Fig. 10. Mechanisms of salt precipitation near the surface in a sabkha system: (A) Loss of water vapor due to evaporation; (B) salt precipitation due to decrease of solubility as a result of temperature rise. p1, p2, p3, p4, and p5 are the points at which saturation is reached for a mineral containing this specific ion. Above this point, the concentration of that ion decreases.

system, thermodynamic equilibrium is not always reached, and many processes occur according to the specific kinetic conditions. Therefore, it is possible that certain minerals will not precipitate or be dissolved because of kinetic reasons, even though the brine reaches its thermodynamic saturation level. 5.1. Steady state of sabkhas and playas The final situation of a closed playa or sabkha is not simple to predict. While the water balance usually is rapidly reached, the solute balance is not. The ion concentration increases with time until a point when it has reached steady state (at least for most ions), due to the lowering of water activity (decreasing evaporation) and mineral precipitation (limit the increase in concentration). However, due to accumulation of salts, the total salt content of these systems (which include ions in both liquid and solid phases) is expected to increase. Farther input of solutes leads to farther precipitation of salts, mainly near the surface. In a hydrologically closed system, these salts will form salt

layers that will reduce the permeability for ascending evaporating water and also decrease the recharge from above. The blocking is expected to increase the confinement condition (increased storativity) of ground water in the sabkha, which will tend to fracture the surface seal. In a sabkha, part of the additional salts will be transported with groundwater discharging to the sea at a rate that depends on the hydraulic characteristics of the sediments. Similarly, in a hydraulically open playa system, the extremely saline brine will partly leak out. In these systems, the final situation is expected to yield deposition of relatively thick layers of evaporites, and partial separation of the brine from the land surface. 5.2. Factors controlling chemical composition As described in the previous section on the mass balance equation, there are many possible sources that contribute solutes in saline environments. These include seawater, dissolution of rocks, ground water, geothermal water (mainly in rift sequences), surface

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water (floods), rain, and eolian processes (dust). Once the water arrives at the discharge zone (e.g. sabkha, playa) or the saline water reservoir, its salinity increases by the mechanisms of evaporation and/or freezing. Both processes are responsible for loss of water, decrease in volume, and, therefore, increase in salinity. The evaporite mineral assemblage, which precipitates from these brines will be entirely different in either of the two processes (Sonnenfeld, 1984). Playas and saline lakes can be found in both cold and warm climates. Examples for playas and saline lakes in cold climate are those in Canada (Last and Slezak, 1988) and Antarctica (Lyons et al., 1998). The chemistry of the residual brine will also depend, therefore, on the mechanism of water loss. In the present review, we deal only with the mechanism of evaporation. Throughout the history of the brine, including the stages of accumulating salts from the above-mentioned sources and the salinity increase, there are several processes that can change the chemistry of the water. These processes are the following. Precipitation of salts—this is the major sink for solutes in many evaporite systems. Sequential precipitation according to the degree of saturation for various salts following evaporation (Eugster and Hardie, 1978) will change the residual brine, depending on the original chemical composition and specific minerals that precipitate. For example, in the Dead Sea area, Starinsky (1974) suggested that precipitation of halite (NaCl) from the original seawater is the reason for the present composition of the brine, whose Na/Cl ratio is much lower (0.27) in Dead Sea brine compared to seawater (0.86). Dissolution – precipitation processes—throughout the flow path, water can interact with the surrounding rocks and dissolve and precipitate, depending on the degree of saturation for the particular mineral. One of the most commonly described process is dolomitization, whereby calcite is dissolved and dolomite precipitates. Dolomitization will increase the calcium concentration in the brines and may cause precipitation of calcium sulfate. This process can change seawater to a Ca-chloride brine, both in saline lake systems (Starinsky, 1974) and sabkha systems (Levy, 1980). Ion exchange—suggested by many scientists to be important in controlling solute ratios in water (e.g. Appelo and Postma, 1993). It should be noted that this process is probably less effective in highly saline

brine because the specific ion capacity of the clay is small relative to the ions in the brine. Therefore, it can be assumed that ion exchange is a limited process in these environments. One of the best-known cation exchange processes takes place in clay, where sodium from the solution replaces calcium from the clay. The significance of this process was shown by a quantitative approach which modeled the amount of ion exchange in a specific site in Texas, USA (Sanford et al., 1992). Absorption on clay is another process that affects the chemistry of water, especially for some specific ions (B, Li, K; Vengosh et al., 1991; Jones et al., 1999). This process is also expected to be limited for the same reasons stated above for cation exchange. Redox reaction—associated with organic matter, can be responsible for changes in chemistry (Jones et al., 1999), especially at the interface between fresh and saline water bodies (Custudio et al., 1987). One of the best-described redox processes in such a system is reduction of sulfate to sulfide. The mechanism is mainly bacterial reduction of sulfur, as deduced from their sulfur isotopic composition (e.g. Nissenbaum and Kaplan, 1975). Reduction of other ions, such as nitrate (or organic nitrogen), can also take place in such an environment, producing ammonium or N2 gas. Obviously, in such brines, the concentration of dissolved oxygen is expected to be zero. Biological activity in saline environments are caused by specific biological communities which are adapted to extremely saline condition (e.g. Oren, 1997).

6. Interface between saline water bodies and groundwater in their vicinity The zone of density contrast between fresh and saline water bodies is defined as the fresh – saline water interface. The location of this interface is of extreme importance because many chemical and mineralogical processes take place in the mixing zone among the different water bodies. In general, the interface between fresh ground water and seawater has been studied in coastal aquifers by many researchers (e.g. Bear, 1972; Souza and Voss, 1987), because seawater intrusion is a major source of ground-water contamination in many coastal areas. The Ghyben (1888) and Herzberg (1901) approximation defined the basic relationships at the beginning of the 20th


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Fig. 11. Interface between water bodies of different densities in sabkha and saline lake systems—normal seawater, Dead Sea water, sabkhas. Note the difference in the depth of the interface between normal seawater and extremely saline lake (Dead Sea; A). In sabkha system, two interfaces are expected—between the seawater and the sabkha brine and between the brine and the relatively fresh ground water (B).

century. Hubbert (1940) gave a more realistic description, including the dynamic nature of the interface due to the flow of fresh ground water. However, the fresh – saline water interface in the vicinity of sabkhas, salinas, and saline lakes has had little investigation. The contact plane between the two water bodies of contrasting density in the vicinity of saline lakes can be approximately defined by the Ghyben –Herzberg equation: Hs ¼ ðqf =ðqs qf ÞÞHf ;

ð12Þ

where Hs is depth to the interface (Fig. 11A), Hf is elevation of the water table, qf is density of the ground water, and qs is density of seawater. For the ocean, where qs = 1.025, Hs = 40Hf. In the case of a lake whose salinity and density are greater than that of the ocean, the location of the interface (Hs) is expected to be shallower. For example, in the Dead Sea, where qs = 1.23, the location of the interface will be found at Hs = 4.35Hf (Yechieli, 1993; 2000). A relatively shallow interface was found in a playa in the vicinity of the Great Salt Lake (Fan et al., 1997; Duffy and Al-Hassan, 1988) in a study that utilized

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both field data and numerical modeling. The simulations were done with the SUTRA code (Voss, 1984), which is widely used for variable density problems. Numerical simulations of playa systems in Australia showed the effect of mixing processes between water bodies of different salinity that occur due to instability created by density differences (Barnes et al., 1991; Simmons and Narayan, 1997). Another USGS code capable of simulations of variable density problems is the HST3D (Kipp, 1998), which was used in the Bonneville Salt Flats (Mason and Kipp, 1998). In a typical coastal sabkha, two interfaces are expected. These are the interface between the sabkha’s brine and the seawater, and the interface between the sabkha’s brine and the local fresher ground water (Fig. 11B). As described previously, the slope of this interface depends on the density differences between the water bodies and should be shallower than the normal interface in the vicinity of the ocean. Another expected phenomenon is that unlike the interface in the normal seawater case which penetrate inland, the interface of the brine of the sabkhas, which is generally much more dense, penetrates into the sediments beneath the sea (Fig. 11). Moreover, this brine/seawater interface does not achieve steady state because it is pushed toward the sea by the sabkha’s brine. The factor that determines the flow direction in these systems is the fluid pressure difference. In these environments, where several bodies of water of different densities exist, the density should be taken into account on top of head differences. In some places, the difference in water level was large enough to allow seawater to intrude inland into a sabkha system even though the sabkha brine is much denser than seawater (Gavish et al., 1985; Fig. 7). An important difference between coastal aquifers and some of the aquifers near inland saline lakes (e.g. the coastal aquifer of the Dead Sea) is the relatively rapid changes that occur in the latter dynamic system. In areas where the water level and density of the water are rapidly changing, the simple model of the fresh – saline water interface needs to be modified to account for the changes that occurred in the history of the lake (Yechieli, 2000). The effect of water level and salinity changes over time have been recognized by Rogers and Dreiss (1995) and Fan et al. (1997).

6.1. Contribution of sabkha brine to the oceans and seas As shown above, the sabkha brine is expected to flow beneath the seawater (Fig. 11B) and contribute solutes to the sea. These density-driven brines are expected to be seen within the sediments at the bottom of the sea and possibly in flow-restricted areas in the lower water mass of the sea. Such brines are reported by De Lange et al. (1990) and others and were attributed by Vengosh et al. (1994) to originate from evaporative stages of the Mediterranean Sea during the Neogene. Part of these brines could, therefore, originate from more recent sabkha systems that exist in many locations along the southern Mediterranean Sea.

7. Summary The main processes that control the water and solute transport in continental saline systems have been identified and presented in the form of simple mass-balance equations and illustrated using a sabkha environment as an example. This approach provides a quantitative framework for physical and chemical investigations of pans, playas, sabkhas, salinas, saline lakes, and salt flats in many arid and semi-arid regions of the earth. A large difference was identified between the resident time of water and that of solutes. It was shown that discharge of deep-basin brine to the sabkha could be a major long-term source of solutes. It is expected that hydrologically open systems will produce an entirely different suite of mineral and relative abundance from that of a closed system. Mineral precipitation in the capillary and unsaturated zone illustrates the role of both evaporation and annual temperature changes in causing thermodynamic supersaturation. The role of the thermodynamic activity of water was shown to be important in controlling evaporation and condensation to and from the atmosphere. Simulation of the location of the fresh – saline water interface indicate that the slope of the ground water/ brine-water interface in saline systems is shallower than in groundwater/seawater interface because of the greater density difference between the ground water/ brine. The interface between sabkha brines and seawater slopes seaward, the reverse of the normal ma-


rine – fresh water systems that slope landward. Moreover, this interface can never achieve steady state. Acknowledgements We wish to thank W. Sanford, C. Voss and B. Jones of the U.S. Geological Survey, for many discussions during the preparation of the manuscript and U. Kafri from the Geological Survey of Israel and M. Rosen from Wairekei Research Centre in New Zealand, for critical reading. The review of Scot Tyler was very helpful. Most of this work was conducted while the senior author (Y.Y.) was on a sabbatical leave at the USGS in Reston. The Directors of the United States and Israeli Geological Surveys have approved the publication. References Ali, Y.A., West, I., 1983. Relationships of modern gypsum nodules in sabkhas of loess to compositions of brines and sediments in northern Egypt. Journal of Sedimentary Petrology 53 (4), 1151 – 1168. Allison, G.B., Barnes, C.J., 1985. Estimation of evaporation from the normally ‘‘dry’’ Lake Frome in South Australia. Journal of Hydrology 78, 229 – 242. Appelo, C.A.J., Postma, D., 1993. Geochemistry, Groundwater and Pollution. Bakema, Rotterdam, 536 pp. Barnes, C.J., Chambers, L.A., Herczeg, A.L., Jacobson, G., Williams, D.G., Wooding, R.A., 1991. Mixing processes between saline groundwater and evaporation brines in groundwater discharge zones. International Conference on Groundwater in Large Sedimentary Basins, Perth, 1990. Bates, R.L., Jackson, J.A., 1987. Glossary of Geology. American Geological Institute, USA. Bazuhair, A.S., Wood, W.W., 1996. Chloride mass-balance method for estimating groundwater recharge in arid areas: example from western Saudi Arabia. Journal of Hydrology 186, 153 – 159. Bear, J., 1972. Dynamics of Fluids in Porous Media. Elsevier, New York. Bein, A., Yechieli, Y., Halicz, L., 1995. The geochemistry of groundwater in the southern Arava: Geological Survey Report GSI/43/95 (in Hebrew). Berner, E.K., Berner, R.A., 1997. Global Environment: Water, Air, and Geochemical Cycles. Prentice-Hall, New Jersey, 376 pp. Bowler, J.M., 1986. Spatial variability and hydrologic evolution of Australian lake basins: analogue for Pleistocene hydrologic change and evaporite formation. Palaeogeography, Palaeoclimatology, Palaeoecology 54, 21 – 41. Brown, W.J., Rosen, M.R., 1995. Was there a Pliocene – Pleistocene fluvial – lacustrine connection between Death Valley and the Colorado River. Quaternary Research 43, 286 – 296.

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