Seasonal dynamics of groundwater-lake interactions at Doñana National Park, Spain

Journal of Hydrology, 136 (1992) 123-154 Elsevier Science Publishers B.V., Amsterdam

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Seasonal dynamics of groundwater-lake interactions at Dofiana National Park, Spain L a u r a A. S a c k s a'l, J a n e t S. H e r m a n a, L e o n a r d F. K o n i k o w b a n d A n t o n i o L. Vela c'2

aDepartment of Environmental Sciences, University of Virginia, Charlottesville,, VA 22903, USA bUS Geological Survey, 431 National Center, Reston, VA 22092, USA cCatedra Geodinamica, Facultad de Ciencias Geologicas, Universidad Complutense, Madrid 28040, Spain (Received 25 July 1991; revision accepted 17 November 1991)

ABSTRACT Sacks, L.A., Herman, J.S., Konikow, L.F. and Vela, A.L.. 1992. Seasonal dynamics of groundwater-lake interactions at Dofiana National Park, Spain. J. Hydrol., 136: 123-154. The hydrologic and solute budgets of a lake can be strongly influenced by transient groundwater flow. Several shallow interdunal lakes in southwest Spain are in close hydraulic connection with the shallow ground water. Two permanent lakes and one intermittent lake have chloride concentrations that differ by almost an order of magnitude. A two-dimensional solute-transport model, modified to simulate transient groundwater-lake interaction, suggests that the rising water table during the wet season leads to local flow reversals toward the lakes. Response of the individual lakes, however, varies depending on the lake's position in the regional flow system. The most dilute lake is a flow-through lake during the entire year; the through flow is driven by regional groundwater flow. The other permanent lake, which has a higher solute concentration, undergoes seasonal groundwater flow reversals at its downgradient end, resulting in complex seepage patterns and higher solute concentrations in the ground water near the lake. The solute concentration of the intermittent lake is influenced more strongly by the seasonal wetting and drying cycle than by the regional flow system. Although evaporation is the major process affecting the concentration of conservative solutes in the lakes, geochemical and biochemical reactions influence the concentration of nonconservative solutes. Probable reactions in the lakes include biological uptake of solutes and calcite precipitation; probable reactions as lake water seeps into the aquifer are sulfate reduction and calcite dissolution. Seepage reversals can result in water composition that appears inconsistent with predictions based on head measurements because, under transient flow conditions, the flow direction at any instant may not satisfactorily depict the source of the water. Understanding the dynamic nature of groundwaterlake interaction aids in the interpretation of hydrologic and chemical relations between the lakes and the ground water.

Correspondence to: L.A. Sacks, US Geological Survey, 4710 Eisenhower Blvd., Suite B-5, Tampa, F L 33634, USA. l Present address: US Geological Survey, 47 l0 Eisenhower Blvd., Suite B-5, Tampa, FL 33634, USA. 2Present address: Consejo de Seguridad Nuclear, c/sor Angela de ia Cruz, 3, Madrid 28020, Spain.

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INTRODUCTION

Ground water plays an important role in the hydrologic budget and chemical composition of many lakes (Winter, 1076, Born pt al., lo-mx Similarly, chemical and physicaiprocesses occurring in a lake can affect the solute composition of downgradient ground water (LaBaugh, 1986). Seasonal groundwater flow reversals can result in significant changes in the location, magnitude, and direction of groundwater seepage to a lake (Meyboom, 1967; Anderson and Munter, 1981; Winter, 1986; Cherkauer and Zaker, 1989). Local flow systems can dominate groundwater flow directions in part of the year, and regional flow may control seepage directions during other times of the year. These chafiges in the flow regime can result in highly variable groundwater flow patterns around a lake. The effects of transient groundwater flow on solute exchange between ground water and lakes are not well understood. The magnitude and chemical composition of the groundwater flux may vary with time, thus influencing a lake's solute budget. Several studies have alluded to the importance of transient solute exchange with lakes (Anderson and Munter, 1981; Winter, 1983; LaBaugh, 1986), but a quantitative evaluation has not been made. The purposes of this study include examining seasonal hydrologic and geochemical controls on a shallow lake system that experiences distinct wet and dry seasons and evaluating whether differences in chemical composition between nearby lakes are influenced by the groundwater system. Hydrologic and geochemical modeling were used to help quantify seasonal groundwater and solute fluxes around three shallow lakes located in southwest Spain. The lakes have. variable solute concentrations that generally increase in the direction of groundwater flow. To assess further the significance of transient groundwater flow, the potential effects of extreme climatic conditions on groundwater and solute exchange were also analyzed. Factors controlling both conservative and nonconservative constituents in the lakes and shallow ground water were evaluated considering transient flow conditions. .

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The three lakes that ace the focus of this study are located in Dofiana National Park, on the southwest coast of Spain at the mouth of the Guadalquivir River (Fig. 1). The park is an important wetlands preserve for migratory waterfowl. The three lakes, Lakes Dulce, Santa Olalla, and Las Pajas, are situated in the western part of the park in an area of partially vegetated, stabilized dunes, and are closed to surface drainage. A ridge of unvegetated active dunes that are approximately 10-15m above sea-level is

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located to the west, parallel to the ocean. To the east of the lakes lie seasonally inundated wetlands called the Marismas. Surficial deposits near the lakes consist of 10-25 rn of very permeable eolian sands of Quaternary age, known locally as the duna! aquifer ~'ela, 1984). These permeable sands overlie the primary part of the Almonte-Marismas aquifer, which extends laterally over 3400 km 2. This more heterogeneous unit consists of approximately 140 m of coastal-barrier sands, interfingered with

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semipezmeable estuarine silts and clays and coarse fluvial deposits. The base of the aquifer is delineated by a low-permeability unit of marine marls. A regional groundwater divide is present at the topographic high of the active dune ridge (Instituto Geoiogico y Mmero de Espafia (IGME), 1983). On a regional scale, ground water flows eastward from the stabilized dunes to the Marismas. These regional flow patterns are disrupted by local flow systems driven toward the lakes in the stabilized dunes by evaporative discharge (Vela, 1984). Lakes Dulce, Santa Olalla, and Las Pajas are water-table lakes in depressions in the stabilized dunes (Fig. 1). Because evaporation is an important flux from the lakes, groundwater inflow can be crucial in determining whether a lake is perennial or seasonally dry. The lakes are very shallow (average depth is approximately 1 m) and have highly variable surface areas depending on the water-table elevation. Although the lakes receive no surfacewater drainage, they periodically connect during extremely wet periods. The three lakes are located sequentially along the regional groundwater flow path. Dulce is the furthest upgradient, followed by Santa Olalla, the largest lake, with a level approximately 0.5m lower than that of Dulce. Las Pajas is furthest downgradient and becomes seasonally dry when the water table drops below the lake bottom. The regional climate is Mediterranean sub-humid (IGME, 1983; Lulla, 1987) and is characterized by distinct wet and dry seasons. The dry season lasts from April until September and is distinguished from the wet season by a water deficit (i.e. potential evapotranspiration exceeds precipitation). Average rainfall is approximately 600mmyear -~, of which approximately 40% occurs between the months of December and February. HYDROLOGIC AND GEOCHEMICAL MEASUREMENTS

Hydrologic and geochemical data were collected in July and December 1985, from Lakes Dulce, Santa OlaUa, and Las Pajas, and from the shallow ground water around the perimeter of the lakes. Seepage directions below the lakes were established from head differences between the ground water and the lake measured with a minipiezometer (i.e. hydraulic potentiomanometer; Lee and Cherry, 1978; Winter et al., 1988). The minipiezometer was handdriven approximately 0.5 m deep into the lake sediments at points within 5 m of the shore. Nineteen measurements were made in July, and eight measurements were made in December. Locations of seepage measurements and corresponding head gradients relative to the lakes are shown in Fig. 2. Water samples for chemical analysis were collected from selected sites. In July, nine groundwater samples and three lake-water samples were collected

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(Fig. 2A); in December, six groundwater samples and four lake-water samples (two from Lake Santa Olalla) were collected (Fig. 2B). Four of the nine minipiezometer sites had a measured hydraulic head higher than the lakesurface elevation in July, indicating groundwater inflow; one sample site had a measured head equal to the lake-surface elevation; and four sample sites had groundwater heads lower than the lake-surface elevation, indicating outflow from the lake. In December, six groundwater inflow samples and one outflow sample were; collected. Temperature, specific conductance, pH, and alkalinity (in July) were determined in the field. Samples were collected according to US Geological Survey water-quality sampling protocol. Samples were analyzed in the laboratory for concentrations of major ions, deuterium, and oxygen-18. A complete listing of hydrologic, chemical, and isotopic measurements has been given by Sacks (1989). Na ÷ and CI- were the dominant ions in all the lakes, but concentrations of these ions were different among the three lakes (Table 1). Lake Dulce had

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the most dilute water (490mgl -! chloride in July 1985), and Lakes Santa Olalla and Las Pajas were considerably more concentrated (2080mg 1-~ and 3260mg 1-~ chloride in July, respectively). The pH values of the lakes were alkaline, ranging from 8.5 to 9.9. All lake-water samples were enriched in oxygen-18 and deuterium, but the Jt~l.y samples were more enriched than the December samples (Table 1). '~ The shallow ground water vaned from Na-CI to Na-HCO3 type waters (Table 1). Chloride concentrations ranged from 25 to 3870 mg 1-~, whereas total alkalinity (assumed to equal HCO~- + COl-) ranged from 50 to 520mgl -~. Groundwater pH ranged from 5.8 to 7.5. Groundwater inflow samples generally were isotopicaUy lighter and more dilute than groundwater outflow samples (Table 1). HYDROLOGIC MODELING

Description of model Groundwater and solute interactions with the lakes were examined using the method of characteristics (MOC) groundwater flow and solute-transport computer model (Konikow and Bredehoeft, 1978). This mathematical model combines a finite-difference approximation to the transient groundwater-flow equation with the MOC solution to the solute-transport equation. The model simulates horizontal, two-dimensional, transient groundwater flow through a heterogeneous and anisotropic aquifer. Water and solutes can entei: or leave the aquifer by direct recharge and withdrawal (e.g. meteoric recharge and evapotranspiration), and by leakage through a confining layer (e.g. a lake bed). Solute concentrations in the aquifer are affected by advective transport, hydrodynamic dispersion, and flux of solutes through the aquifer boundaries.

Lake budget To improve the accuracy of the aquifer-lake system simulation, an algorithm was added to the documented model, which recalculates lake volume and solute concentration after each time-step (Sacks, 1989). This routine was designed for use with transient, two-dimensional areal simulations. A lake is linked to the aquifer through leakage (e.g. ground water enters the lake when the calculated head in the aquifer is greater than the head specified in the lake). The constant-head boundary condition for lakes is achieved through a head-dependent leakage condition specifying a high leakance coefficient. This implies that the entire lake bottom is in good hydraulic conr.ection with the aquifer and that the sediments offer negligible resistance to flow. Data on sediment permeability are not available, but

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sediments below the lakes are generally thin (usually less than I m) and sandy (but contain some silt and organic matter), and minipiezometer measurements indicate good hydraulic connection. Lake solute concentration, volume, surface area, and head vary over time, although in the model they are constant during each time-step. Lake volume changes as a function of the model-calculated groundwater flux to and from the lake and specified lake evaporation and precipitation. Because of the permeable nature of the soil near the lakes, overland flow is assumed to be negligible. The solute concentration of a lake changes as a function of the dissolved-solids content of these fluxes. The model assumes that the lake fully mixes between time-steps, that density gradients in the lake are negligible, and that chloride, the modeled solute, is conservative in both the lake and the ground water. An innovative modeling approach allows a more accurate representation in the model of the transient lake hydrology. The lake surface area is allowed to change by assigning bottom topographical elevations to all potential lake nodes. A node (or finite-difference cell) is considered part of the lake if the head (or stage) of the lake is higher than the land-surface elevation of that node; those nodes that are not part of the lake itself are designated as meteoric-recharge nodes. The lake head is recalculated after each time-step by 'filling the lake' with its new volume of water from its lowest elevation upward. This allows the specified-head elevation for the lake to change in a gradual step-wise manner with time in response to the changing lake volume. The new lake head, in turn, 'feeds back' to the groundwater flux to and from the lake. A lake will dry up completely if the flux out of the lake exceeds the previous volume plus the influx. When this occurs, the mass of solutes in the lake during the previous time-step is stored in the model to represent precipitated salts or solutes stored within the lake sediments, and the dry lake nodes become meteoric-recharge nodes. The rewetting of a lake is initiated when the calculated groundwater head exceeds the lowest topographic elevation of the lake bottom, and these nodes are designated as constant-head boundaries for the following time-step. The new solute concentration of the lake is calculated by dividing the stored mass of the solute by the new lake volume, which assumes that all the salts are redissolved within that time-step. The flow chart in Fig. 3 summarizes the procedures used to calculate the water and solute budgets of the lakes; corresponding program changes were incorporated into the MOC solute-transport model of Konikow and Bredehoeft (1978).

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Model use Grid dimensions and boundmy conditions The areal grid used in model simulations represented 10.5 km 2 (Fig. 4). Grid spacing was expanded in the x-direction to allow efficient representation of the physical boundaries of the system. The solute-transport subgrid had a uniform node-spacing of 50 m on a side and covered 2 km 2, including the lakes and the areas immediately surrounding them. The water-table divide in the active dune ridge was represented by a no-flow boundary, whereas the fiat, low-elevation wetlands were represented by a constant-head boundary. No flow was assumed across the grid boundaries parallel to the regional flow direction. Active lake nodes were constant-head boundaries during a given time-step, but the constant-head elevation was allowed to change between time-steps. The remaining nodes in the; grid were designated as meteoric recharge nodes with a specified recharge or evapotranspiration (ET) flux. Recharge rates Recharge, precipitation, and lake evaporation rates were specified as average values for four 3-month periods. These recharge periods represent the major seasonal changes in rainfall and recharge, and correspond to the early wet season (October-December), the late wet season (January-March), the early dry season (April-June), and the late dry season (July-September). An average rainfall year was simulated, and average monthly rainfall and

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calculated potential evapotranspiration (PET) rates using the Thornthwaite (1948) method were obtained from the meteorological record at a station 20 km from the site (Instituto Nacional de Investigaciones Agrarias (INIA), 1977, unpublished data, 1952-1980). Because estimates of recharge were not available and meteorological data were scarce, assumptions were necessary to estimate recharge rates. Aquifer recharge was calculated as the difference between seasonally averaged precipitation and seasonally averaged PET, assuming losses to interception and runoff were negligible and assuming a finite amount of storage in the unsaturated zone. Two different rates of recharge were specified for meteoric-recharge nodes, depending upon water-table depth and season. Areas where the water table is less than I m below the surface (e.g. lake margins, seasonally inundated areas, and vegetated interdunal areas) receive more recharge than areas where the water table is deeper (e.g. sparsely vegetated dune crests and dune fronts). When PET was greater than precipitation, an ET flux (calculated as a percentage of PET minus precipitation) was specified for shallow water-table nodes; no recharge was assigned to the deeper water-table nodes when PET was greater than precipitation. Lake evaporation rates used in the model were seasonally averaged PET rates (INIA, unpublished data, 1952-1980). Because the lakes are very shallow, it was assumed that the influence of stored heat on evaporation rates was negligible and that lake evaporation approximated PET. Initial conditions The initial conditions for the transient model corresponded to the end of the dry season (i.e. early October). The initial chloride concentrations in the lakes were set so that modeled chloride concentrations over the course of the year fell within the range of observed concentrations (Table 1 and Vela (1984)). The initial chloride concentration in the aquifer corresponded to the observed concentration in the shallow ground water upgradient of the lakes; the chloride concentration of rainwater used in the model was the average value of several samples collected near the site in 1986. To establish lake area, volume, and stage relations, topographic elevations for all potential lake nodes were assigned. Because no bathymetric map exists for the lakes, these values were interpolated from a topographic map completed during the dry season, when lakes were at a low stage (Cartografia General, 1985), and from information on maximum lake depth. The lake levels established for the initial conditions corresponded to the approximate lake sizes at the end of the dry season. The initial head distribution in the aquifer was set to a zero-recharge steady-state condition. Because of this sim[dification and to minimize the effect of possible changes in response to the initial conditions themselves, the

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first year of simulation was a stabilization period, during which head distribution and lake sizes adjusted and stabilized under long-term average meteorological conditions. Results presented in this paper are for the year after this stabilization period. Calibration The model was calibrated to maintain stable annual lake volumes and chloride concentrations for the average rainfall simulation. Slight dilutions in lake chloride concentrations were considered acceptable because of the simplification of the dilute ~,quifer initial condition. Modeled values were consistent vtith observed data for a particular time of year (Table 1; Vela, 1984; Sacks, 1989). The simulated water-table elevations were calibrated against head data from several permanent piezometers near the lakes (Vela, 1984). Long-term chloride concentrations in the lakes exhibited a satisfactory dynamic equilibrium level when solute transport was modeled in only the upper 10 m, which is the thickness of the dunal aquifer that Vela (1984) found played a dominant role in the recharge and discharge mechanisms of the aquifer. This approximation assumes that groundwater flow below 10m is essentially horizontal and fluid and solute exchange between the lakes and this deeper ground water is negligible. Horizontal hydraulic conductivity, aquifer transmissivity, and porosity were estimated from values reported by Vela (1984) and IGME (1983) for the area. The time-step in the model was kept small (i.e. less than 10 days) so that large changes in lake stage did not occur between time-steps. Large changes in lake stage tended to cause instabilities in Las Pajas when its volume was very small. Parameters that were used in final modeling runs are reported in Table 2. RESULTS

Modeled groundwater flow patterns near the lakes varied seasonally during the average rainfall year (Fig. 5). In the middle of the wet season, groundwater flow near the lakes followed the regional flow path toward the Marismas and hydraulic gradients were low (Fig. 5A). By the end of the wet season, the rising water table resulted in increased head gradients toward the lakes. Ground water flowed toward the lakes on all sides, except for flow away from Dulce toward Santa Olalla (Fig. 5B). These flow patterns affected the locations of seepage into and out of the lake. The perimeters of the lakes were the dominant areas for groundwater inflow, whereas the centers were the dominant location for seepage out of the lakes. However, outflow also occurred from the downgradient end of Lake Dulce. In the middle of the dry season, groundwater flow was still directed toward

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INTERACTIONS

TABLE 2 Parameters used in model simulations Hydraulic conductivity Porosity Storativity Longitudinal dispersivity Transverse/longitudinal dispersivity Aquifer thickness Transmissivity Grid spacing Flow model grid dimensions Transport subgrid Initial time step Time increment multiplier Length of recharge period

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the permanent lakes, but hydraulic gradients decreased. Flow was directed away from Las •ajas toward Santa Olalla (Fig. 5C). By the end of the dry season, groundwater flow near the lakes again followed the regional flow path (Fig. 5D), and Las Pajas dried Upo The upgradient ends and centers of the lakes were groundwater inflow atea~ during the dry season, but mest of the inflow occurred at the nodes adjacent to the lake perimeters. The downgradient ends of both Dulce and Santa Ollala lost water to the groundwater system during the dry season. Precipitation was the major water input to the lakes during the wet season, and groundwater seepage was the dominant water input during the dry season (Fig. 6A). Groundwater inflow to Du!ce and Santa Olalla was low during the early wet season but increased by a factor of two during the remainder of the year. In contrast, modeled groundwater inflow to Las Pajas was highest during the wet season and progressively decreased throughout the dry season. Evaporation was, by far, the dominant water loss component in the hydrologic budgets of the lakes (Fig. 6A). Groundwater outflow from Lake Dulce remained relatively high and constant throughout the year. In contrast, groundwater outflow from Santa OlaUa declined during the dry season to less than 25% of the value for outflow during the wet season. Groundwater outflow from Las Pajas was very low during the wet season, but it increased by a factor of five during the dry season. The model-calculated chloride concentration and volume of the lakes varied seasonally. The modeled chloride concentration of Lake Dulce ranged from 275 to 620 mg l-~, and the chloride concentration of Santa Olalla ranged from 1340 to 3250mg l -~. Because of evaporative concentration, the highest chloride concentrations and smallest lake volumes occurred at the end of the dry season. Modeling results for the intermittent lake, Las Pajas, showed a more extreme range in chloride concentrations - - from 150 mg l-~ at the end of the wet season to over 12000mgl -~ immediately before the lake dried. Las Pajas remained dry from early August until early January. Ground water played an even greater role in the chloride budget of the lakes than it did in the water budget (Fig. 6B). Ground water accounted for about 60% of the annual chloride flux to the lakes, but it accounted for only 30-40% of the water inputs. Although groundwater outflow is a small component of the hydrologic budget, it is the mechanism by which solutes are transported away from the lakes. Seasonal differences in the amount of chloride leaving the lakes are the result of variations in groundwater outflow rates and the seasonal evaporative concentration of chloride in the lakes. For example, about 40% more chloride leaves Santa O!alla during the early wet season than in the late wet season, even though groundwater outflow remains approximately constant, because the mean chloride concentration of the lake

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decreases from about 2200 to 1500mgl -t during this period. The model calculates more chloride leaving the lakes than entering them during the !-year simulation period. Thi:~ difference is balanced by a decrease in the mass of chloride stored in the lakes at the end of this same time period. However, this is mostly an artifact of the specification of a low uniform unitial chloride concentration in the aquifer based on the regional background chloride concentration. The calculated increases in chloride concentration in the

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-60

-80

~

I 1

I I 2 3 Dulce

I 4

I

Groundwater inflow

~

[ ~ ] Direct rainfall

I 1

I I 1 2 3 4 Santa Olalla

Groundwater outflow

I

I 1

I I 2 3 Las Pajas

I 4

1 2 3 4

Oct- Dec"-~ Wet Jan - Mar/Season A p r - J u n ' ~ Dry Jul- Sep/Seadon

Fig. 6. Continued.

aquifer near the lakes during the simulated year indicate that the specified initial concentrations were too low in those areas and that the actual chloride concentration of groundwater inflow to the lakes is higher than initially computed by the model, particularly at Santa Olalla. In addition to modeling the average rainfall year, a high- and a low-rainfall year were modeled to examine controls on lake concentration, lake size, and groundwater seepage. During the high-rainfall year, the lakes surficially connected into one large body, and groundwater flow was toward this large

SEASONAL DYNAMICS OF GROUNDWATER-LAKE INTERACTIONS

139

A. End of Wet S e a s o n

iiiil!i:i

;

i J

p

End of Dry ooso,1

/ [~

G r o u n d w a t e r inflow

~

G,-otindwater oulflow

~ D ~ - D i r e c l i o n of gl'O tlli(tw ~a|~1' flow

Fig. 7. Model results from a high-rainfall year showing locations of groundwater seepage into and out of the lakes and the direction of groundwater flow near the lakes at the end of the wet season (A) and the end of the dry season (B).

lake on all sides (Fig. 7). The lake volumes were high and the chloride concentrations were very low throughout the year. During the low-rainfall year, the water table did not rise sufficiently to cause groundwater flow reversals toward the lakes, and ground water followed the regional flow path (Fig. 8). In addition, Las Pajas remained dry throughout the entire year.

140

L.A. SACKSET AL.

A. End of Wet Season

1/4

B. End of Dry Season

D O

5

I~

-~"

[ ~ Groundwater inflow [_~ Groundwater outflow

Direction of groundwater flow

Fig. 8. Model results from a low-rainfall year showing locations of groundwater seepage into and out of the lakes anO the direction of groundwater flow near the lakes at the end of the wet season (A) and the end of the dry season (B).

Ground water entered the upgradient ends of Dulce and Santa Olalla, and lake water recharged the ground water of the downgradient ends of these lakes. Both lakes had very low volumes by the end of the dry season and had considerably higher chloride concentrations at that time than during the average rainfall year.

SEASONAL DYNAMICS OF GROUNDWATER-LAKE INTERACTIONS

141

DISCUSSION Seasonal changes in groundwater-lake interaction During the average rainfall simulation, the three lakes display unique seasonal behavior that depends on the lake's relation to the hydrogeologic system. Because of the relatively large hydraulic gradient between Lakes Dulce and Santa Olalla, seasonal flow reversals do not occur at the downgradient end of Dulce, and groundwater outflow remains relatively high throughout the year (Fig. 5). Dulce is a flow-through lake, and dilute ground water flows into the lake at its upgradient end, is concentrated by evaporation, and recharges the ground water at its downgradient end. Thus, a 'plume' of water with a chloride concentration higher than that of the background concentration in the aquifer moves toward Santa Olalla. Ground water in several permanent piezometers between the two lakes has a chloride concentration of approximately 500 mg 1-~, indicating solute transport between Dulce and Santa Olalla (Vela, 1984). In contrast, Lake Santa Olalla is affected by the adjacent development of local flow systems during the wet season, when the entire perimeter of Santa Olalla receives groundwater inflow (Fig. 5B). The lowered water table in the dry season, however, results in outflow from the lake's downgradient end (Fig. 5D). Field measurements fi'om July 1985 indicate additional groundwater outflow from several locations at the lake's upgradient end (Fig. 2). Hydrophyllic vegetation grows in the low-lying areas between Lakes Dulce and Santa Olalla, and discharges by ET may depress the shallow water table and reverse the expected seepage into Santa Olalla (T.C. Winter, persona! communication, 1986). These depressions are not seen in the model, probably because daytime ET rates are higher than the seasonal averages used in the model. Flow reversals near Lake Santa Olalla affect the seasonal locations of solute exchange between the lake and the aquifer. For example, during the dry season, concentrated lake water seeps into the ground water on the lake's downgradient end. During the subsequent wet season, modeled groundwater inflow at this end of the lake has a higher chloride concentration than dilute upgradient ground water that had never been in contact with the lake. Solutes transported in the ground water from Lake Dulce also contribute to the solute content of Santa Olalla. The lake loses significant amounts of chloride during the course of the year (Fig. 6B). Some of this chloride re-enters the lakes after seepage reversals. Significant amounts of chloride, however, remain in the aquifer beneath the lake. Vertical leakage from the lake was investigated in several simplified modeling scenarios (Sacks, 1989), and results indicated that vertical hydraulic gradients below Santa Olalla could be responsible for transporting selutes to deeper parts of the aquifer.

142

L.A. SACKS ET AL.

The hydrologic and solute budgets of Las Pajas are very different from those of the permanent lakes (Fig. 6). During the wet season, the ratio of groundwater inflow to outflow is much greater for Las Pajas than for the other lakes. As the water table drops during the dry season, groundwater inflow to the lake decreases rapidly and groundwater outflow increases, in contrast to the trends seen in the permanent lakes. Because the lake bottom is topographically higher than the other lakes, the water-table altitude eventually falls below the bottom of the lake, and the lake recharges the aquifer, exporting significant amounts of chloride to the aquifer. Thus, the decreased groundwater inflow, increased groundwater outflow, and increased evaporation rates lead to the drying out of the lake in the middle to late summer. The large fluctuations in the modeled chloride concentration of Las Pajas over the course of the year (e.g. 300mgl -~ in March and 12000mgl -~ in August) illustrate the effects of the wetting and drying cycle on the concentration of conservative solutes in the lake.

Effects of extreme rainfall on groundwater-lake interaction High-rainfall year During a high-rainfall year, the water table rises substantially, and strong hydraulic gradients drive flow toward all three lakes (Fig. 7). Increases in groundwater inflow and the higher rates of direct rainfall cause the lakes to rise to very high levels, although the rises in lake levels are less than the rise in the water table adjacent to the lakes. Groundwater outflow from Santa Olalla and Las Pajas can be very small at times, and so it is possible that the actual water-table rise during or after high-recharge periods would be great enough to make these lakes complete sinks for the groundwater .~ystcm. Toward the end of the wet season, low-lying land between the lakes becomes flooded, and the lakes connect into one large system (Fig. 7A) that remains connected throughout the dry season (Fig. 7B). Similar behavior was observed in the field during a high-rainfall period in 1987 and 1988 (A.L. Veia, personal communication, 1989). The interconnection of the three lakes has important implications in the solute budgets of the lakes. Although the present model does not account for the mixing of lakes because of surficial connections, the chloride concentration of the new, larger lake was calculated by summing the mass of chloride in the separate lakes and mixing it in the combined lake volume. The chloride concentration in the large lake system would be 450mgl -t, a concentration considerably higher than that in Lakes Dulce and Las Pajas if they had remained unmixed (130 m~z1-~ and 52 mg 1-~, respectively). If the lake volumes were reduced to initial volumes, the chloride concentrations of Dulce and Las

SEASONAL DYNAMICS OF GROUNDWATER-LAKE INTERACTIONS

143

Pajas would be much higher than they were initially and the chloride concentration of Santa Olalla would be much lower. To investigate what happens to lake solutes after a subsequent year of normal recharge, the average rainfall year was modeled using as the initial condition the mass of chloride in the large mixed lake, reduced in size to initial lake volumes. The chloride concentration of Lake Dulce at the end of the year was reduced by 26%, as concentrated lake water seeps out of the downgradient end of the lake. Thus, solutes that originally came from Santa Olalla when the lakes mixed will eventually reach that lake again. This movement through the ground water is considerably slower than through the surface connection, and so Santa Olalla remained relatively dilute by the end of the second year. Over the period of the model year, the ?tored mass of chloride in Las Pajas did not change significantly because o( the low rate of outflow from the lake. In contrast to the solutes in Lake Oulce, the solutes gained by Las Pajas when the lakes were connected are not transported back to Santa Olalla. This modeling exercise clearly illustrates that alternative pathways exist for solute movement when the lakes surficially connect, and these pathways dramatically affect the solute budgets of the lakes.

Low-rainfall year During the simulation of a low-rainfall year, the lakes continuously decrease in size. The below-normal rainfall during the wet season results in groundwater flow patterns and lake sizes at the end of the wet season (Fig. 8A) that are very similar to those at the end of the dry season in th~ average rainfall simulation (Fig. 5D). With the onset of the dry season, the surface areas of the lakes continue to decrease. Lakes Dulce and Santa Olalla, however, do not completely dry (Fig. 8B) because groundwater inflow during the summer months is relatively high, constituting 100% of the inflows to the lakes. Las Pajas, on the other hand., is dry during the entire year because the water table remains below the lake bed. A strong regional flow component exists throughout the year, and groundwater outflow from the d~w~'~gradie,t end of both lakes remains high. Local flow systems do not develop, and therefore flow reversals toward Santa Olalla do not occur as they do during an average rai~fall simulation. Santa Olalla is a flow-through lake during the entire year, and solutes move away from the lakes along the regional flow path. Because of the large evaporative flux, the chloride concentrations of the lakes are much higher than during the average rainfall year. Thus, if several relatively dry years occurred successively, concentrated ground water derived from the lakes would be continuously transported downgradiant in the aquifer away from Santa OlaUa. A very low rainfall year produces seepage patterns very different from those

144

L.A. SACKS ET AL.

that would occur during the average rainfall simulation. These changes would be apparent not only in the drought year but also in subsequent years, because of reduced lake volumes and increased chloride concentrations in the lake. In addition, it may take several years of higher-than-normal rainfall to dilute the lakes and increase their volumes to predrought conditions. Based on simulations of years with extreme climatic conditions, it is apparent that the groundwater-lake system is sensitive to annual variations in local rainfall. The amount of rainfall during a particular year affects not only the solute concentration and volume of the lakes, but also the development and persistence of seasonal groundwater flow reversals around the lakes. These changes in groundwater flow patterns, in turn, affect the solute exchange between the aquifer and the lakes. GEOCHEMICAL INTERPRETATION

Nonconservative chemical reactions

The hydrologic modeling has aided in understanding the transport of a conservative solute, chloride, in the groundwater-lake system. The results illustrate that evaporative concentration is the dominant process affecting the concentration of conservative solutes in this system. Seasonal changes in flow patterns near the lakes change the dominant areas of solute exchange with the lakes. This leads to a complex spatial distribution of chloride concentrations in the shallow ground water near the lakes. The behavior of other ions can be assessed relative to that of chloride. Sodium behaves conservatively in the groundwater-lake system (Fig. 9A),.but most other ions do not. Calcium, magnesium, and the carbonate species are depleted relative to evaporative concentration (Fig. 9B-9D). All of the samples are strongly depleted in sulfate in July, whereas they are much less depleted in December (Fig. 9E). Changes in solute concentrations as water moves between two points along a flow path were evaluated on the basis of the relative changes in ion concentrations normalized to chloride, removing the effects of evaporative concentration. Flow paths were assumed from the seepage direction measured when the groundwater sample was collected. The percent gain or loss of a dissolved constituent was calculated according to

q

% change = [(Ionr). -- (Clr/Cli)(Ioni (I-~f) d x 100

(l)

where Clf and Cli are the chloride concelltrations and Ionf and Ion~ are the concentrations of the given ion in the final and initial watels, respectively. The changes in nonconservative ion concentrations depend cn the seepage

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direction (Table 3). Generally, Ca 2+, Mg 2+, SOl-, and alkalinity are depleted in the lake waters relative to groundwater inflow samples. As water seeps out of the lakes, Ca 2+, Mg 2+, and alkalinity tend to increase relative to the lake-water composition, whereas SOl- is depleted further. Carbon dioxide and the carbonate system The dominant geochemical control on the amount of Ca2+, Mg 2+, and

146

L.A. SACKS ET AL.

TABLE

3

Per cent change in major ion concentrations, normalized to chloride, between groundwater inflow and lake waters and between Jake and groundwater outflow waters; flowlines estimated from head gradients measured in the field were used to determine seepage directions Initial water

Final ,~ ater

Sampling month

Cl~ina I Clinitia I

Ca a

Mg

Na

SO4

Alkalinity

(1985)

Ground water-lake 9-DUL

Dulce

July

9.6

- 80

- 43

- 10

- 83

2-OLA

Olalla

July

1.6

- 24

- 20

4

823

- 77 - 70

5-OLA

Olalla

July

13.4

- 52

- 60

23

- 43

- 66

10-OLA

Olalla

July

2.5

- 28

- 18

-

!

- 39

- 72

19-OLA

Olalla

Dec.

16.5

- 43

- 54

- 2

- 85

- 84

3A-OLA

Olalla

Dec.

4.9

- 49

- 53

- 20

- 89

- 92

2-OLA

Oialla

Dec.

13.5

- 71

- 67

- 34

- 96

- 94

10-OLA

Olalla

Dec.

57.4

- 86

- 90

-

14

- 98

- 97

18-PAJ

Pajas

Dec.

0.45

51

16

12

1!7

162

Lake-ground water Dulce I-DUL Dulce I-DUL

July

1.19

38

- 14

- I

-81

Dec.

2.16

-3

-

Olalla

3A-OLA

July

0.80

95

55

- 14

- 85

174

Pajas

7-PAJ

July

0.90

67

68

- 25

72

- 80

Pajas

8-PAJ

July

0.25

189

42

- 7

- 72

313

'~ Per

cent change determined from eqn

-22

-21

13

18 -67

(!).

HCO; + CO~- in solution is the amount of dissolved CO2 in the water, which affects the pH and equilibrium state with respect to calcite. The partial pressure of carbon dioxide (Pco2) of the ground water is elevated relative to its atmospheric value (10-3'Satin); the/'co, of lake water ranges from near the atmospheric value to severe depletion, particularly in July (Table 4). The elevated Pco2 of the ground water results from generation of CO2 in the subsurthce by root respiration and microbial activity. As ground water seeps into the lake, CO2 outgasses, with a subsequent rise in pH and a decrease in total dissolved inorganic carbon. Photosynthetic activity from algal blooms in the lakes further depletes the dissolved CO2 (Table 4), driving pH values even higher (Table 1). Lakes can remain depleted in CO2 relative to atmospheric levels because the diffusion of gaseous CO2 from the atmosphere into a lake is slow relative to the ionization equilibria of dissolved carbon (Stumm and Morgan, 1981). The ground water, with its elevated/'co2, is undersaturated with respect to

147

SEASONAL DYNAMICS OF GROUNDWATER-LAKE INTERACTIONS

TABLE

4

T h e s a t u r a t i o n i n d i c e s a o f c a l c i t e (Slcal) , s e p i o l i t e (Slsep) , a n d g y p s u m (Slgyp), a n d t h e l o g Pco, o f g r o u n d w a t e r a n d l a k e - w a t e r s a m p l e s c o l l e c t e d i n J u l y a n d D e c e m b e r 1985 Station

Sampling

Sl~t

Slsep

Slgyp

l o g Pco2

month (1985)

Ground water 9-DUL

July

- 2.5

- 9.7

- 3.4

- 0.75

I-DUL

July

- 0.77

- 4.9

- 3.7

- 1.1

- 1.6

- 1.4

I-DUL

Dec.

- 1.2

-

3A-OLA

July

- 0.31

- 5.8

3A-OLA

Dec.

- 1.1

-2.9

- 0.83

-

- 1.9

- 0.80

19-OLA

Dec.

- 1.9

-

- 2.4

- 2.0

2~OLA 2-OLA

July Dec.

- 0.73 - 1.3

- 7.0 -

- 3.3 - 1.8

- 0.68 - 1.4

I0-OLA 10-OLA

July Dec.

- 0.88 - 0.24

- 8. i -

- 2.4 - 2.4

- 0.95 - 2.0

I I-OLA

July

-0.59

5-OLA

July

- 2.7

- 6.4 - 11

- 2.3

- 1.1

- 3.3

- 1.1

7-PAJ

July

- !.6

-6.8

- 1.0

- 1.4

8-PAJ

July

- 0.93

- 5.9

- 2.9

- 0.87

18-PAJ

Dec.

- 0.84

-

- 0.84

- 1.1

Lakes Dulce

July

0.94

3.4

- 3.2

- 3.4

Dulce

Dec.

0.82

-

- 1.9

,- 3.4

Olalla

July

1.7

4.8

- 2.2

- 4.4

Olalla

Dec.

1.2

-

- 1.6

- 3.9

Ollala

Dec.

1.8

-

- 2.0

- 5.4

Las Pajas

July

1.7

6.0

- 2.0

- 4.6

Las Pajas

Dec.

1.3

-

- 0.83

- 3.3

a S a t u r a t i o n i n d e x is d e f i n e d a s t h e l o g a r i t h m o f t h e r a t i o o f t h e i o n a c t i v i t y p r o d u l : t t o t h e equilibrium constant, both corrected to sample temperature. The solutions were modeled with WATEQF,

a c o m p u t e r i z e d e , ~ u i l i b r i u m m o d e l f o r s p e c i a t i o n in a q u e o u s s o l u t i o n ( P i u m r a e r et ai.,

1976). W A T I N ,

a pre-processor iL: WATEQF

( M o s e s a n d H e r m a n , 1986) w a s u s e d t o p r e p a r e

t h e i n p u t d a t a file.

calcite. The CO2 depletion and elevated pH of the lake water cause supersatura0on (Table 4). The precipitation of calcite may contribute to the depletion of C a 2+ and HCO~- in the lake waters (Table 3). C~lcite precipitation is a well-documented process occurring in lakes in a wide range of environmental settings (Pentecost, 1978; Kelts and Hsu, 1978; Emeis et al., 1987), although

148

L.A. SACKS ET AL.

kinetic constraints often control precipitation (Wetzel, 1983). Calcite precipitation in lakes can occur seasonally because of depletion of CO2 as a result of photosynthetic activity and seasonal temperature increases (Brunskill, 1969; Otsuki and Wetzel, 1974). Although solid-phase samples from the lake sediments have not been collected to date, calcite precipitation is possible within the time constraints of high biological activity in the lakes (i.e. seasonally) and definitely within the residence time of the permanent lakes (i.e. years). Mg 2÷ also is depleted in the lakes relative to groundwater inflow (Table 3). It is reasonable to suspect that some Mg is present in the CaCO3 lattice, whether precipitation occurs biogenically or by purely chemical processes. Another possible removal process for Mg 2÷ is the precipitation of magnesiumrich clays. The lake waters are highly supersaturated with respect to sepiolite (MgSi306(OH)2; Table 4). The slow rate of sepiolite precipitation, however, could inhibit direct Mg-clay precipitation (B.F. Jones, personal communication, 1988). Until lake sediments are analyzed, the processes by which Mg 2÷ is lost from solution remain unresolved. As lake water seeps into the ground water, the Pco2 of the water rises because of biological activity in the sediments (Table 4). As a result, the pH decreases, the amount of dissolved inorganic carbon in solution increases, and the water becomes undersaturated with respect to calcite. Calcite dissolution can occur in a short time period m of the order of hours (Plummer et al., 1978). Based on the measured hydraulic gradients between the lakes and the groundwater outflow, it would take weeks to months for lake water to travel to the depth of the groundwater samples. Calcite dissolution can occur on that time scale and is a likely process by which Ca 2+, Mg 2+, and carbonate species are added to solution as the water seeps from the lakes to the shallow aquifer.

Sulfate dynamics Sulfate is depleted relative to rain-water and upgradient ground water in all groundwater and lake-water samples collected in July (Fig. 9E). Thus, a process removes SO 2- from the ground water before it enters the lake sediments. The groundwater and lake-water samples are undersaturated with respect to gypsum (Table 4), and so mineral precipitation is not a likely sink for SO 2- . The most likely mechanism for SO 2- loss i~ microbially mediated sulfate reduction in the shallow ground water, which would also add HCO~to solution: 2CH20(s) ÷ S O 2 - ~ - H 2 S ( g ) + 2HCO~-

(2)

The activity of microorganisms is very temperature dependent (Atlas and Bartha, 1987; King, 1988), which could account for the greater depletion of

SEASONAL DYNAMICS OF GROUNDWATER-LAKE INTERACTIONS

149

SO~- in July (typical groundwater temperature of 25°C; Table i) than in December (typical groundwater temperature of 13°C; Table 1). In addition, the reduced sulfur can react with metals such as Fe 2÷ to form sulfide minerals (Berner, 1984). Besides the general SO~- loss observed in the July samples, the lake waters are depleted in 8042- relative to the inflowing ground water (Table 3), probab!y because of the reduction of additional sulfate as water seeps through organic-rich lake sediments. At Lake Santa Olalla, the upper 15cm of sediments contain abundant organic matter (Vela, 1984), which serves as a carbon source that is readily available for biological sulfate reduction. Further SO~"~ depletion occurs as water seeps out of the lakes (Table 3). Transient seepage conditions can explain an anomalous increase in SO42between a groundwater inflow sample (sample 2-OLA in July; Table 3) and Lake Santa Ohdla. This groundwater sample appears to have been in contact with the lake at an earlier time, as indicated by its relatively high C1- concentration (1280 mg 1- ~) and the enrichment of deuterium and oxygen- 18, (Table 1, Fig. 1[0). This type of behavior was predicted by the groundwater model for some locations of Santa Olalla, but not at the exact location of sample 2-OLA. A likely explanation for the low SO~- concentration in the groundwater sample is that the water had previously been in the lake and then moved through the lake sediments out of the lake, during which time SO4z- was removed from solution by sulfate reduction. Subsequent seepage reversals caused this water to move back toward the lake with an extremely low SO~- concentration. This example illustrates how instantaneous hydrologic measurements do not accurately depict the origin of a water in a transient setting. Chemical and isotopic evidence can illustrate the water's more complex history by integrating previous reactions and flow paths.

Hydrologic and geochemical processes at Las Pajas The isotopic composition of all of the lakes is lighter in December than in July (Table 1), as a result of the input of ~isotopically light raiafall and lower evaporation rates during the wet season. Tt~edifference in isotopic composi6on of the lake waters from July to December, however, is considerably greater in Las Pajas than in the permanent lakes (Fig. !0). Although the isotopically light water in Las Pajas in December implies a local meteoric origin for the water, the high chloride concentration suggc~;t.~ a solute source other than rain-water. The sampled ground water flowing toward Las Pajas in December (sample 18-PAJ, Table 1; also plotted in Fig. | 0) has a similarly light isotopic composition, but it has the highest chloride concentration of all the samples collected (7600mgl-'). The high C1- concentration of the groundwater

150

L.A. SACKS ET A L

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10

6 Oxygen-18 (o/oo)

Fig. 10. Plot of deuterium as a function of oxygen-18. Dashed line is the meteoric water line from Gibraltar (International Atomic Energy Agency, 1981). Lake-water samples (D - - Dulce, SO - - Santa Olalla, LP - - Las Pajas) from July (open symbols) and December (closed symbols) are all isotopically heavier than meteoric water. Groundwater samples are further subdivided into inflow (circles) and outflow (squares) on the basis of measured head gradients at the time of sample collection. Groundwater samples are identified by numbers as listed in Tables I, 3, and 4.

sample suggests that this water Dad been associated with the lake during its final stages of drying. In contrast to the trends seen in the other lakes, the relative amounts of Ca 2+, Mg 2+, and SO42- (normalized to chloride) in Las Pajas in December is greater than in this groundwater inflow sample (Table 3).

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Differences in the chemical and isotopic composition of Las Pajas compared with the permanent lakes can be explained by the wetting and drying cycle of the lake. During the dry season, Las Pajas becomes enriched in heavy isotopes (e.g. July lake water; T~'" ~.e 1) and depleted in Ca 2+, Mg 2+, and SO~- because of high rates of biological activity, calcite precipitation, and sulfate reduction. Before the lake dries, groundwater outflow increases (Fig. 6A). When the lake is dry, residual moisture that has a high chloride concentration is retained in the unsaturated zone. During the wet season, solutes that had been stored in the unsaturated zone mix with recent recharge and seep toward the lake (e.g. groundwater sample 18-PAJ). The lake water from the early wet season (i.e. December) has a high concentration of conservative solutes and a light isotopic composition. Dissolution of minerals, such as calcite, which may have precipitated in the lake earlier in the year, add Ca 2+, Mg 2+, and HCOj- to solution. When the lake is dry, atmospheric exposure of the surficia! lake sediments allows oxidation of previously reduced sulfur (e.g. sulfide minerals). Thus, as meteoric water comes in contact with these lake sediments, SO~- is redissolved into the new lake water. As the wet season progresses, the chloride concentration of Las Pajas declines as dilute groundwater inflow and rainfall become increasingly large components in the lake's hydrologic budget. CONCLUSIONS

Adjacent lakes i~ the same hydrogeologic setting can sometimes have very different chemical compt~sitions, which cannot be e~:plained on the basis of differences in the amount or quality of surface inflows. An example is the system of three snailow lakes in Dofiana National Park, Spain, where an integrated hydrologic and ~eochemical approach led to new insights into the processes controlling solute exchange between lakes and ground water. The groundwater flow and solute-transport model incorporated a dynamic representation of the lakes that y'~elded a realistic simulation of lransi~,,t changes in lake hydrology and chemical composition. Water aod s,Aute budgets for the lakes were updated at the end of each time-~'~ep~b,~scd on lake stage-area-volume relations and on the calculated net ~rou~d'~ater inflow or outflow. Model boundary conditions of the lakes, represe:~ted by the locations, heads, and concentration of the lakes, were the~eby allowed '.o adjust over time. In this manne~, the model adequately reproduced the observed permanent nature of two of the lakes and the i~terrnittent nature of the third lake. The three lakes respond differently to the changing hydrologic conditions of ti~e wet and dry seasons, illustrating how the chemical characteristics of

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similar water-table lakes, located within a small area, can vary depending upon their relation to the hydrogeologic system. Lake Dulce is a flow-through lake, strongly linked with the regional flow system, and exhibits the least seasonal variability in groundwater seepage locations. The comparatively low chloride concentration in the lake is tcontrolled by groundwater outflow transporting solutes downgradient in the aquifer toward Santa Oia!la. A more dynamic groundwater flow system around Santa Olalla causes local flow systems to develop during the wet season and to dissipate during ~he dry season, when regional flow prevails. Seepage directions at the downgradient end of the lake reverse from inflow during the wet season to outflow during the dry season, resulting in concentrated groundwater inflow to the lake at this end. In addition, flow reversals driven by evapotranspiration also occur around the lake margin. During dry years, solutes from Santa Olalla may travel downgradient in the aquifer along the regional flow path, whereas during wet years, the connection and mixing of the lakes causes solutes to be lost from Santa Olalla to the other lakes. Because Las Pajas is an intermittent lake, the hydrologic and chemical characteristics of this lake are more seasonally variable than those of the permanent lakes. The great variability in lake volume associated with the wetti~lg and drying of the lake produces an extreme range in chloride concentration and isotopic composition compared with the permanent lakes. Evaporation controls the concentration of conservative solutes in the groundwater-lake system, but concentrations of nonconservative solutes also are controlled by geochemical and biochemical reactions occurring in the lake and its sediments. The depletion of nonconservative ions in the lake waters relative to groundwater inflow can be explained by calcite precipitation and biological uptake of solutes in the h'~kes and sulfate reduction in the lake sediments. Reactions probably occurring in the lake sediments as water seeps out of the lake include calcite dissolution and further sulfate reduction. Seasonal seepage reversals can produce groundwater compositions that vary spatially and temporally over the course of the year and between years, depending on the amount of rainfall. Evaporation and reactions occurring in a lake can affect the composition of ground water downgradient from the lake by outseepage from the lake to the aquifer. If seasonal groundwater flow reversals occur, groundwater inflow to the lake can have a very different composition from that of the regional ground water. A transient flow system can influence surface-water and groundwater compositions by transient solute exchange between the lake and aquifer. If a steady,state groundwater flow model had been used for the Dofiana area, as is commonly done in other groundwater studies, the nature of the solute-transport patterns and concentration distributions could not

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have been explained satisfactorily. This study illustrates the advantages of integrating the consideration of hydrologic and geochemical processes, because interpretation of the chemical and isotopic analyses led to an improved understanding of the hydrology, and the hydrologic analyses helped explain the geochemistry. ACKNOWLEDGMENTS

Partial funding for this project was provided by the US-Spain Joint Committee on Scientific and Technological Cooperation (grant 83-007). Ramon Llamas, Emilio Custodio, and William Back provided thoughtful comments throughout this study. Thomas Winter provided critical assistance in the hydrologic study, collection of samples, and preparation of this manuscript. We thank Jesfis Tenajas, Javier Rodriguez Ar6valo, and Isabel Herrfiez for their help, with data collection and far insightful discussions. We thank the administration of E1 Parque de Dofiana for allowing access to the sampling sites, and .~os6 Maria Perez de Ayala for his assistance in the field. Shirley Rettig and Joe Chemerys peformed the major ion analyses, and Carol Kendall and Tyler Coplen performed the isotope anslyses at the US Geological Survey. Discussions with Blair Jones, George Hornberger, and William Nuttle, and review of the manuscript by Joe Donovan and Shirley Dreiss have been very helpful. REFERENCES Anderson, M.P. and Munter, J.A., 1981. Seasonal reversals of groundwater flow around lakes and the relevance to stagnation points and lake budgets. Water Resour. Res., 17: 1139-1150. Atlas, R.M. and Bartha, R., 1987. Microbial Ecology: Fundamentals and Applications, 2nd edn. Benjamin/Cummings, Menlo Park, CA, 533 pp. Berner, R.A., 1984. Sedimentary pyrite formation: an update. Geochim. Cosmochim. Acta, 48: 605-615. Born, S.M., Smith, S.A. and Stephenson, D.A., 1979. Hydrogeo!ogy of glacial-terrain lakes, with management and planning applications. J. Hydrol., 43: 7-43. Brunskill, G.J., 1969. Fa,-et~eville Green Lake, New York. II: Precipitation and sedimentation , , . ~in a meromictic lake with laminated sediments. Limnol. Oceanogr., 14: 830-847. of ~--'-'" Cartografia General, S.A., 1985. Piano Fotogrametrico del Parque Nacional del Dorian:,. Ministerio de Obras Publicas y Urbanis~o, Madrid Cherkauer, D.S. and Zaker, J.P., 1989. Groundwater interaction with a kettle-hole lake: relation of observations to digital simulatic,ns. J. Hydrol., 109: 167-184. Emeis, E.C., Richow, H.H. and Kempe, S., t987. Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control. Sedimentology, 34: 595-609. lnstituto Gco!ogico y Minero de Espafia (IGME), 1983. Hidrogeologia del Parque National de Dofiana y su Entorno. Ministria de Industria y Energia, Madrid, 120 pp. Instituto Nacional de Investigac~ones Agrarias (~NiA), 1977. In: F.E. Castillo and L.R. Beltram (Editors), Cuaderno INIA No. 7. Ministerio de Agricuii~ara, Madrid. ii~ternational Atomic Energy Agency, 198~. Statistical treatment ofervironmental isotope dal.a

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