Oxygen isotope study of evaporating brines in Sambhar Lake, Rajasthan (India)

,NCL”zhvG

EISEVIER

ISOTOPE GEOSCIENCE

Chemical Geology 138 (1997) 109-118

Oxygen isotope study of evaporating brines in Sambhar Lake, Rajasthan ( India) D.N. Yadav Physical Research Laboratory, Ahmedabad, 380009, India

Received 20 February 1996; accepted 8 October 19%

Abstract Sambhar Lake in the Thar desert of India behaves as a “terminal lake” in which inflow is balanced by evaporation. At present the source of the water to the lake is mainly through atmospheric precipitation and through surface runoff via two major seasonal streams (Roopangarh and Mendha). The meteoric contribution of water in the lake during the recharge period is suggested by the oxygen isotope data of lake waters and precipitation samples which were collected during the lake’s recharge period. In subsequent months, the lake undergoes evaporation resulting in formation of hyper saline brine. The 6”O results for groundwaters adjacent to the lake suggest that they are not influenced by recharge from the lake, whereas the shallow sub-surfac:e brines occurring below the lake bed show mixing with meteoric and/or lake waters during the monsoon season and with lake waters during post-monsoon seasons. The isotopic evolution of oxygen in the Sambhar Lake waters during the annual evaporation cycle is explained through

the Craig-Gordon model taking into account the fluctuation in lake water volumes due to evaporation, equilibrium and kinetic fractionation factors, and the effect of changing seasonal temperature and humidity. The model calculations suggest that back-condensation of isotopically light atmospheric water vapor (6’*0, -20%0) dominantly controls the isotopic evolution of oxygen in, the lake during the annual evaporation cycle. Keywords: Salt lake; Evaporation

cycle; Brine: Surface runoff; Meteoric waters

1. Introduction Sambhar Lake (26’52’-27’2’N, 74’53’-75”13’E) is a Na-Cl type alk:aline salt lake in a closed sedimentary basin, situated in the Thar desert, Rajasthan state of India. The lake is located at an elevation of 360 m above mean sea level with an area of N 225 km’ and average depth of _ 1 m. The lake receives an average annual rainfall of 50 cm (Swain et al., 1983; Yadav, 1995) over its catchment area of w 5600 km2 (Biswas et al., 1982). The lake is fed by small and large streams, namely Roopangarh, 0009-2541/97/$17.00 Copyright PII SOOOS-2541(96)00154-4

Mendha, Kharian and Khandel (Aggarwal, 1951). However, surface runoff to the lake is limited by the formation of sand dunes in the drainage basin and man-made dams constructed for irrigation purposes. In the present-day context, runoff via the two principal streams, the Roopangarh and Mendha, is highly seasonal and occurs only for a few days during the SW monsoon (July-September). The field observations of the water table of adjacent groundwaters indicate them to be generally N 5-10 m above the lake bed. However, at places within the river basin (e.g., near Roopangarh town), groundwaters are

0 1997 Elsevier Science B.V. All rights reserved.

D.N. Yadav/Chemical

110

available at 20-30 m above the lake bed. This suggests a significant hydraulic gradient that would favor the groundwaters to flow towards the lake basin in addition to the surface runoff during the monsoon season. The characteristic feature of the lake is such that the annual evaporation (OctoberJune) almost balances the inflow, leaving behind a negligible residual lake water during peak summer. The lake is subsequently recharged during the SW monsoon (July-September). Stable isotopic compositions of oxygen and deu-

Geology 138 (1997) 109-118

terium for Sambhar Lake and adjacent groundwaters have been reported by Ramesh et al. (1993); however, their data lack seasonal variability over the annual cycle of evaporation for the lake water. This paper reports the evolution of the oxygen isotopic composition in Sambhar Lake during the annual wetting and evaporation cycles. Furthermore, an important aspect of this study is to characterize and identify various water bodies in and around the lake, which have direct influence over the brines evolution and their role in the sustained supply of sub-surface

27°10’h 4 --

flKucha$_na

Khoria

ssifk&dn,h 5’

It Lake

N

27O

fh__&

m

SAMBHAR

LAKE

bGudha

hal!arl

8

26045’N

Area of Study

7 Fig. 1. Map showing the sample locations in and around Sambhar Lake. The groundwater wells are indicated by encircled points and sub-surface brine wells are shown by asrerisks. The shallow groundwater samples from the river bed are taken to be representative of river waters. In the figure, major village sites for location of the groundwaters and some of the salt producing sites for sub-surface brine wells are also shown; however, additional samples were also collected at a distance of 50-100 m from those marked locations. The map also describes important geomorphic features of the lake area.

D.N. Yadav/Chemical

brines to the current: salt production. Water samples were collected periodically from Sambhar Lake as it evolved from a reasonably fresh to a dry salt lake. Groundwaters and sub-surface brines were also sampled to study their seasonal variation in oxygen isotope ratios. The changes in oxygen isotope ratios as a function of evaporation observed in the lake during 1992-1993 have been explained using the Craig and Gordon (1965) model. The results obtained are presented below. 2. Sampling and experimental methods Fig. 1 shows the location map of Sambhar Lake and various sampling points for groundwaters, subsurface brines and river waters. Since rivers draining into Sambhar Lake flow only for a few days during the monsoon season, it was difficult to collect flowing river waters for chemical and isotopic analysis. Therefore, near-surface waters at a depth of 2-4 m from the river bed have been taken to be representative of river waters The groundwaters, lake waters and sub-surface brines were sampled during different seasons (February 1992 and October 1993) whereas water samples from the river basin were collected once during October 1993. Two rainwater samples at two different instances were also collected during July 1993 from the Sambhar Lake town. The chemical analyses of samples from various water reservoirs in and around the lake were carried out using conventional atomic absorption spectrophotometry, titration and gravimetric methods (Yadav, 1995). The salt content (TDS, total dissolved solids) of water samples was estimated by summing all the dissolved major ions, e.g. Na, K, Ca, Mg, Cl, SO, and alkalinity. In this study, TDS and concentration a’f uranium (to be reported elsewhere) are considered to represent the extent of evaporation in lake water since they behave conservatively over the annual evaporation cycle. The measurement of the “O/ 160 ratio, expressed as 6 “0 for water samples, was made following the CO, equilibration method of Epstein and Mayeda (1953). In brief, 2 ml of wa.ter sample (acidified with a few drops of 100% pure orthophosphoric acid) were equilibrated with tank CO, at 25°C for 48 hr, the CO, was extracted :md analyzed using a VG Micromass 903 mass spectrometer (Yadav, 1995).

111

Geology 138 (1997) 109-118

The 6 180 is defined as:

(180/160)sampIe

6180 = [

xlo3

(180/160)stmdard - l1

(1)

and expressed in permil (%o). The standard refers to Standard Mean Ocean Water @MOW) (Craig, 1961) supplied by IAEA, Vienna, having an “O/ 160 isotopic ratio of N 2000 X 10e6. The 6 I80 measured in freshwater replicate samples showed a precision of rt 0.1%0 whereas for brines, the reproducibility was found to be +0.3%0 (1 a>. Since the brine samples, analyzed for 6 “0, are NaCl type with trace concentrations of Ca and Mg ions, the salt-effect on the 180 fractionation is assumed to be negligible (Gat, 19811, and hence no salt-effect correction has been made.

3. Results and discussion The results of salt content (TDS) and 6 ‘*0 for various water samples collected during February 1992-October 1993 from the lake and adjacent groundwaters and river waters are shown in Table 1. 3.1. Oxygen isotopes in atmospheric groundwaters and river waters

precipitation,

6 ‘*O of the groundwaters collected from open wells (adjacent to the lake) ranges from -6.5 to -3.0%0 and the TDS content is < 12.3 g/l (Table 1). Similar to the groundwaters, the river water samples from the Mendha and Roopangarh (with TDS < 1.8 g/l) show a 6 “0 variation from - 5.4 to - 3.6%0. These results of 6 la0 overlap with the values of two rainwater samples ( - 6.4 and - 3.2%& collected during July 1993. This observation, though based on a limited number of rainwater samples, suggests that the S 180 of the groundwaters along the periphery of lake is dominated by atmospheric precipitation. In Fig. 2 the 6 ‘*0 values of the groundwaters are plotted against TDS for the samples collected during the October (recharge period) and February (winter) months. This plot has been made to discern any relation between S l80 and TDS which may provide information on the mixing trend among the ground-

112 Table 1 Isotopic composition Water reservoir/location

D.N. Yadav / Chemical Geology 138 (1997) 109-l 18

of oxygen in waters from Sambhar Lake and its adjacent areas Collection period

TDS range

6180 range (%o vs. V-SMOW)

(g/l) Rainwaters:

Sambhar city

Jul. 1993

(0.8-2.0)

x 1O-3

- 6.4 to - 3.2

River waters:

Roopangarh Mendha

Oct. 1993 Oct. 1993

0.6-1.2 1.5-1.9

-5.4 to -4.4 - 4.2 to - 3.6

Feb. and Oct. 1992, Oct. 1993 Feb. and Oct. 1992, Feb. and Oct. 1993 Feb., Jul. and Oct. 1992, Feb. and Oct. 1993 Feb., Jul. and Oct. 1992, Feb. and Oct. 1993 Feb. 1992, Oct. 1993

0.6-1.8

-6.5

to -5.4

S-9.6

-4.3

to -3.0

Feb. 1992 Feb. 1993

0.9-1.8 3.2-12.3

Groundwaters:

Rly Station Gudha Sambhar Town (Madarsa School) Sambhar Town Sambhar Town (Pipe Factory) Sambhar Town (Guest House) Jhapok, Korsina Jhag, Jajota

1.5-1.9

- 5.6 to -4.9

1.5-1.8

-5.8

to -4.1

0.6-0.9

-5.9

to -4.5

-5 to -4 -3.6 to -3

Lake water:

Sambhar

Feb. 1992-May

1993

8.8-376

-5.5

to +24

109-235 39-242 240-269

+3.5 to +5.1 - 1.1 to +5.1 +4 to +7.4

77-187 221

-1.1

Sub-surface brines:

Gudha Deodani Main Line Pan works Khakarki Nawa

Feb. 1992 Feb. and Oct. 1992, Feb. 1993 Oct. 1992, Feb., Apr. and Jun. 1993 Feb. 1993 Apr. 1993

waters. The results for the October and February months show that the S ‘*0 of groundwaters around Sambhar Lake are within the range observed for rainwater from the adjacent region. The differences in salt content among the various groundwaters could be attributed to varying amounts of salt derived from weathering of their local recharge zones. An alternate explanation for the trend in Fig. 2 is mixing of

to +1.5 +6.5

two end-members, one end-member with low 6 180 and TDS and the other with high 6 “0 and TDS. Such mixing, however, is not envisaged from the saline lake and sub-surface brines as they occur below groundwater levels (see Section 3.2). Therefore, the two end-member mixing stands for “within the groundwater system”. The influence of evaporation on the groundwaters can be assessed through the

D.N. Yadav/ Chemical Geology I38 (1997) 109-l 18

‘3

4 KS

6

12

(g/l)

Fig. 2. Scatter diagram cP 6”O vs. TDS for the Sambhar Lake groundwaters collected during October and February. The distribution of sample points essentially show two distinct groups of groundwaters in terms of their salt contents. The 6’sO’s in these groundwaters are within the range of atmospheric precipitation.

6D-S180 relationship. The evaporation generally causes a shift in the slope of the 6D-6 IgO meteoric water line (MWL) (Dansgaard, 1964). The MWL line (Craig, 1961; ‘Yurstsever and Gat, 1981) has been defined as: 6D = (8.17 f 0.08) al80 + (10.56 f 0.64) In the present study, 6D has not been measured but results of SD and al80 in groundwaters from the nearby region (Sambhar, Didwana and Kuchaman) show the following relation (Ramesh et al., 1993): 6D=3.156’80-22.4

(n=26)

(2) A significant deviation in the slope of the 6 D6 IgO line with respect to the MWL indicates the influence of evaporation occurring in the groundwaters of this region. 3.2. Oxygen isotopes in lake waters and sub-sueace brines

The lowest S180 value of -5.5%0 was measured during July 1992 when the lake was recharged by precipitation. This value is within the range measured for rain- and river waters (Table 11. The effect of evaporation is observed in subsequent months with the enrichment in both S IgO and salt content. The 6 180 shows a linear increasing trend up to a value of +20%0 with the salt content @IX) (Fig. 3a). The 6 I80 values and salt content during July-

113

April show a strong positive correlation ( r = 0.90, both of them steadily increasing due to evaporation. At salinities exceeding 100 g/l (during May 1993) there is a change in the slope of the 6 “0-TDS trend, the increase in S I80 with TDS is much less in the salinity range 100-400 g/l compared to that at lower salinities (9-100 g/l). With an exception of one sample, though salinity increases, the S IgO of lake water saturates at a mean value of +21%0 during the late stage of evaporation in May (Fig. 3a). In contrast to the evaporating lake waters, the seasonal variations in the 6 l80 are less pronounced in sub-surface brines. The temporal evolution of S IgO in lake brines (in fact, these brines are subsurface brines; R. Ramesh, pers. commun., 1995) has been explained in an earlier study through the evaporation pattern of a “terminal lake” in which evaporation is more or less balanced by inputs from precipitation and surface runoff during the monsoon season (Ramesh et al., 1993). Using a Rayleigh isotopic fractionation model for the typical temperature and humidity conditions (temperature 291 K, humidity 55%) found in this region, they have shown that the saturation in S180 is about +4.8%0, which is quite similar to that observed in most of the samples measured in this study during seasons other than the monsoon. With the onset of the monsoon (July-September), the lake and the open dug wells are recharged by meteoric water. As mentioned earlier, a number of open dug wells have been constructed on the lake bed for pumping out the subsurface brines for salt production. These open wells get filled with rainwater during the monsoon and thus promotes recharge of sub-surface aquifers with atmospheric precipitation. However, the mixing of meteoric water with higher-density sub-surface brines may be constrained, whereas the pumping operations carried out for salt production after monsoon season may aid this mixing process. In the sub-surface brines, the mean S180 for October 1992 was lowest (+ l%o) and by June the value increased to + 7%0 (based on a sample from one of the wells). The 6 l80 in sub-surface brines sampled between October 1992 to June 1993 show a significant positive correlation (r = 0.92) with the salt content (Fig. 3b). Such a linear trend for sub-surface brines could be explained due to mixing with meteoric and/or lake waters during the monsoon season and with lake waters

114

D.N. Yadav/Chemical

Geology 138 (1997) 109-118

25

Sub-surface 100

200

300

400

0

100

brine 200

300

TD%dI)

Fig. 3. Scatter diagram of oxygen isotopes in lake and sub-surface brines observed during annual wetting-and-drying cycles (1992-1993). (a) Shows a strong positive correlation (r = 0.91) between 6 “0 and salt content for the Sambhar Lake waters during July 1992-April 1993. Such variation in 6”O and TDS is explained through the process of evaporation. The G”O-TDS relationship during the late stage of evaporation (May 1993) has a much gentler slope resulting from back-condensation of ‘so-depleted atmospheric water vapor (see Section 3.2 for detailed discussion). (b) Illustrates strong positive correlation ( r = 0.92) between S Is 0 and salt content for the sub-surface brines. The linear trend between them is mainly attributed to two-component mixing of waters, i.e. lake/atmospheric precipitation and sub-surface brines.

during post-monsoon seasons. The range in 6 l8O, - 1 to +4%0, of different wells in a single month (October 1992), shows that the extent of dilution of sub-surface brines with relatively depleted 6 180 end-member (direct precipitation or lake water) is not uniform. A factor contributing to the different extents of mixing of two water bodies (rain- or lake water and sub-surface brines) is the variable drawdown of the sub-surface aquifer by pumping during salt production. In the above consideration, the influence of evaporation on controlling the salinity of sub-surface brines has not been taken into account as these brines occur below the lake bed where evaporation could be restricted and the 6 igO evolution trend could be unlike of the evaporating lake water. It is clear that recharge to sub-surface brines occurs mainly during the monsoon season and thereafter it is augmented by the pumping process carried out for salt production. The two end-member mixing calculation using 6 “0 values for lake and sub-surface brines indicates _ 12% of mixing from evaporating lake waters assuming a resultant water 8 l80 value of +7%0 and lake water and sub-surface brines having al80 of +21 and +5%0, respectively. Such an extent of water mixing from the lake during the summer, when the lake has virtually no water, can-

not account for the salt production from the subsurface brines. An inference could be drawn that the salt production from sub-surface brines is sustainable, most likely, due to their recharge from direct meteoric precipitation and/or lake waters during the monsoon and post-monsoon seasons. The highly enriched 6 180 results of the lake waters and sub-surface brines relative to that of groundwaters during most of the seasons have an important implication to the latter’s recharge conditions. The results indicate that groundwaters adjacent to the lake are isolated water bodies and are not recharged by either lake waters or sub-surface brines. This conclusion is further substantiated by the physical setting of the lake since most of the groundwaters are at higher elevations than those of the lake bed and water tables for the sub-surface brines. 3.3. Model calculations for the evolution of oxygen isotope in Sambhar Lake during 1992-1993 The saturation in 6 “0 during the late stage of evaporation (Fig. 3a) in the annual evaporation cycle has been studied by applying the Craig and Gordon (1965) model. This model takes into account processes such as evaporation and isotopic exchange

115

D.N. Yadau/Chemical Geology 138 (1997) 109-118

between liquid and atmospheric water vapor. The saturation in S is 0 during the late stages of evaporation (TDS > 100 g/l) could be due to a decrease in the isotopic fractionation factor arising from the increase in solute molality (Safer and Gat, 1975). In addition, reduction of the fractionation factor could be due to the change in chemical activity of water, which gets influenced by precipitation of hydrated mineral salts from evaporating water. For example, Friedman et al. (1976) reported a decrease in the fractionation factor for the deuterium isotope and related it to the salinity increase in Owens Lake, California, USA, during the period 1969- 197 1. Later a model calculation was performed for the isotopic evolution of Owens Lake by Phillips et al. (1986) using input variables of fluctuation in lake water volumes, surface-water inflow volumes, amount of precipitation and water-surface temperature. They concluded a dominant role of isotopically light atmospheric water vapor over the isotopic evolution of Owens Lake. Following a similar line, the temporal evolution of oxygen isotope in Sambhar Lake evaporating water has been studied considering the role of both equilibrium and kinetic fractionation factors. In addition, effects of changing lake water volumes, seasonal temperature and humidity (data collected from the meteorological station Ajmer, courtesy India Meteorological IDepartment) were also included in the model calculation. Meteorological data used for studying the temporal evolution of 6 180 in the Sambhar Lake waters are presented in Table 2. Another input parameter for model calculation, fractional residual lake water volumes, is given in Table 3 along with its corresponding observed al80 values. The residual fraction of lake water is estimated Table 2 Meteorological parameters a used in the calculation of the CraigGordon model for the oxygen isotope evolution in Sambhar Lake Time period

20 Sep. 1992-26 Sep. 19!92 31 Jan. 1993-6 Feb.-1995; 4 Apr. 1993-10 Apr. 1993 25 Apr. 1993-1 May 1993

Average temperature (“Cl 28.2 18.8 28.0 35.2

Average relative humidity (8) 47.2 42 18.9 21.9

* Data are taken from the records of India Meteorological Department, Jaipur, for the climatic station, Ajmer.

Table 3 Residual fraction of lake water and observed 6”O values in Sambhar Lake Date.

Residual fraction of lake water,

6”O (observed)

f

(%ovs. V-SMOW)

20 Oct. 1992 6 Feb. 1993 6 Apr. 1993 21 Apr. 1993 3 May 1993 I May 1993 15 May 1993

-1.0 +4.50 +9.60 + 19.1 + 19.7 +21.4 +24.0

1.0 0.589 0.330 0.125 0.079 0.059 0.035

from the concentration of uranium in the water at each evaporation step relative to that of the October value, as U is known to be a conservative tracer in an oxidizing environment (Osmond and Cowart, 1976). During the annual evaporation cycle of 19921993, the fractional residual lake water volume became as low as 0.035, which qualifies the lake to be a “terminal lake” since evaporation almost balances for inflow water inputs. Here, an assumption is made that loss of water from the lake basin is mainly due to evaporation, though water is pumped out for the salt production. The evolution of oxygen isotope as a function of time is calculated from the following relationship (Gonfiantini, 1986): s=s,+

da -Alnf dlnf

where -=da

-+S,)-(S+l)(Ar+g) w

dlnf

l-k+A~ a,

ff = exp( 1137/T* - 0.4156/T-

0.00207)

(Majoube, 19711, a, = -0.000543/f*

-0.018521/f+

0.99931

S, is the S180 of atmospheric water vapor; h is the

116

D.N. Yadau/Chemical

Geology 138 (1997) 109-118

fractional relative humidity; T is the temperature in K; and a, is the thermodynamic activity of water. The Craig-Gordon model calculation was carried out until the residual fraction of lake water was 10%. Further calculations could not be made as the expression for thermodynamic activity of NaCl solution (a,> is valid only up to the residual fraction f = 0.1 (Gonfiantini, 1986). The evolution of al80 as a function of residual amount of water in the lake was calculated for: (1) different values of 6 180 in the atmospheric water vapor (Fig. 4) undergoing exchange with lake water; and (2) different initial values for 6 l80 in the lake water. These results are presented in Table 4 and Fig. 4, which indicate two important findings. First, the most sensitive variable affecting the S”O evolution of lake water is the isotopic composition of the atmospheric water vapor and secondly, the al80 evolution in the lake with evaporation is not critically dependent on the initial isotopic composition of either - 5.5 or - 1.0%~~. Reasonably good fit with the experimental data (Fig. 4) is obtained for the value of -20%0 for the atmospheric water vapor. These results suggest that back-condensation of isotopically light atmospheric vapor having 6 l80 as low as - 20%0 plays a dominant role in the isotopic evolution of the lake water during the late stage of evaporation. The S “0 of

30

30

2o

20

L% ;5 10

10

G E

t

Table 4 Comparison of the results between 6”O observed and aI80 calculated following the Craig-Gordon model for different initial oxygen isotopic composition of lake water and atmospheric water vapour acal. (%0) at f 1.0

0.59

0.33

0.125

6, = - l%o, S, = -4%o 6, = - l%o, s, = - lo%0 s,=-l%o, 6,=-20%0 a,=-l%o, 6,=-30%0 a,=-1%0,6,=-40%0

-1 - 1 -1 -1 -1

+11 + 8.7 +4.7 +0.8 -3

+22 + 16.7 + 10.5 +4.3 -2

+ 36.5 + 32.2 +25 + 17.8 + 10.6

so 6, 6, 6, so

- 5.5 -5.5 -5.5 - 5.5 -5.5

f8.2 +5.8 +1.9 - 1.9 -5.8

+ 18.6 + 14.9 +8.7 + 2.5 -3.7

+35.1 + 30.8 + 23.6 + 16.4 + 9.2

-1

+4.50

+9.60

+19.1

= = = = =

-

5.5%0, 5.5%0, 5.5%0, 5.5%0, 5.5%0,

s, s, s, S, 8,

= = = = =

-

4%* lo%0 20%0 30%0 40%0

s ohs. a a

‘ohs.

indicates

observed

Si80

in the lake at different

values of

f.

atmospheric water vapor over Sambhar Lake has not been measured. However, a value (- 20%0) for atmospheric water vapor oxygen isotope composition seems to be reasonable during the pre- and postmonsoon seasons when the Western disturbance brings precipitation from far distance to the regions north of Sambhar Lake. Although this exercise does not prove that saturation in S “0 during late stages of evaporation is due to back-condensation of isotopically light atmospheric water vapor, however, it does strongly suggest the above case.

4. Summary and conclusions

‘0 0

-10

0

1.0

0.8

0.6

0.4

0.2

f. the

0.0 residual

-10

1.0

fraction

0.8 of

0.6

0.4

lake

water

0.2

0.0

4. The Craig and Gordon (1965) model calculations applied to annual isotopic evolution of oxygen (1992-1993) in Sambhar Lake waters with initial S’s0 (a) - 5.5%0 and (b) - 1%~ and different isotopic composition for atmospheric water vapor (S,). Comparison of Fig. 3a and b indicates that the at80 evolution trend does not depend critically on the initial value. The model curve (3) approximates the observed data for S, = - 20%0. Meteorological parameters used in the model calculations are given in Table 2. Fig.

The S l80 composition of shallow groundwaters and river waters occurring in the lake basin is similar to that observed in atmospheric precipitation, indicating that the recharge in these water bodies is mainly influenced by local precipitation. The relationship between TDS and 6 “0 relation for the groundwaters during October and February shows the influence of different weathering regimes. Also, groundwaters are isotopically as well as hydrologically isolated water bodies with respect to evaporating lake waters and sub-surface brines as their 6 180 content is distinctly different during most of the

D.N. Yadav / Chemical Geology 138 (1997) 109-118

seasons. Extending this conclusion to the geological past, it may be inferred that the major pool of brines occurring in the lake basin may not have any relationship with the shallow groundwaters. The isotopic composition of oxygen in lake water during July 1993 (-- 5.5%0) overlaps that of atmospheric precipitation and many of the river and groundwater samples. This suggests that the lake water is completely replenished by atmospheric precipitation and surface runoff. During the initial stages of evaporation, the 6 180 of the lake waters varies linearly with salt content (IDS range, 9-100 g/l), which indicates the dominating influence of evaporation. However, the 6 180 evolution trend in the Sambhar Lake waters during the late stage of evaporation is explained through the Craig and Gordon (1965) model. The results of the Craig-Gordon model suggest that back- condensation of atmospheric moisture (S ‘*O = - 20%0) during the late stages of evaporation controls the observed pattern of 6 “0 in the lake brines. In contrast to the evaporating lake waters, the seasonal variations in the al80 are less pronounced in the sub-surface brines. A strong positive correlation (r = 0.92) between S 180 and salt content for the sub-surface brines collected during different seasons is attributed to a two-component mixing of waters, i.e. lake/meteoric waters and sub-surface brines. Variation in 6lsO in sub-surface brines collected from different wells during a single season (e.g., October 1992) indicated differential extent of mixing of sub-surface brines with meteoric precipitation (rain- or lake waters). Such variable drawdown of the sub-surface aquifer is probably aided by the pumping process carried out for the salt production in the lake basin. As the major salt production is carried out from the sub-surface brines, the oxygen isotope study of these brines indicate that their commercial exploitation is mainly due to the dissolution of embedded salt in the lake basin followed by the recharge to the sub-isurface aquifers by the rain and surface lake waters during the monsoon and immediately after the monsoon seasons. Acknowledgements

Staff members of Sambhar Salt Limited, Sambhar Lake, Rajastban, are thanked for providing logistic

117

help during water sampling. Encouragement and discussions with Professor S. Krishnaswami and Dr. M.M. Sarin during the study period have greatly improved the interpretation of the data. The measurement of oxygen isotope and help in modeling the observed data by Dr. R. Ramesh is gratefully acknowledged. The study was supported in part by the Department of Ocean Development (New Delhi) and ISRO/DOS Global Change Programme (I/DGBP).

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