Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. Baskaran ¹, S. Krishnaswami ¹, N. Bhandari ¹

Affiliations
1 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, IN

Abstract

The concentrations of ²³⁸U, ²³⁴Th and (²³⁴U²³⁸U) activity ratios and major ion chemistry of several water samples from the salt lakes and their adjacent ground waters in Rajasthan have been measured. ²³⁴Th concentration in all the samples, except in one high alkalinity water from the Sambhar Lake, are quite deficient relative to its parent ²³⁸U the average (²³⁴Th/²³⁸U)activity ratio being 0.12. This deficiency corresponds to a mean residence time of - 5 days for ²³⁴Th in these waters. In the high alkalinity water, ²³⁴Th concentration is at near equilibrium with ²³⁸U, yielding a residence time much longer than radioactive mean life. The long residence time of Th in high alkalinity waters may result from the complexing of ²³⁴Th by (CO₃)^-2 ion. ²³⁸U concentration in some of the ground waters near the salt Iakes are high, the highest being in a Didwana sample, 565 dpm/l, The Didwana samples also have the highest (²³⁴U²³⁸U) activity ratios observed for Indian waters. The high uranium concentration in the Didwana ground water is suggestive of uranium rich aquifer solids.

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D. N. Yadav ¹, M. M. Sarin ², S. Krishnaswami ²

Affiliations
1 Bhabha Atomic Research Centre, Trombay, Mumbai - 400085, IN
2 Physical Research Laboratory, Earth Science Division, Ahmedabad - 380 009, IN

Abstract

Geochemical analysis of groundwater samples and brines from the Sambhar Salt lake shows a predominance of NaCl with respect to total salt content (TDS), which varies from 0.5 to -400 g/L. All brine (lake + shallow subsurface) samples indicate a Na-Cl type alkaline solutions with pH varying from 8.5 to 10. All this suggests a general provenance for halite or dissolution of desiccated salt minerals in the catchment and drainage areas of the lake. Adjacent groundwaters are mostly neutral Na-Cl type with TDS ranging from 0.5 to 13 g/L. In contrast to groundwaters, the river waters from Roopangarh and Mendha reflect that out of the TDS, the major ions constitute Na, Alk and Cl with an average equivalent Na/Cl ratio of 2.7, indicating dissolution of Na from silicate weathering of rocks. However, the lake water composition overwhelms the river water upon mixing during the recharge period. A complete annual wetting and drying cycles of evolution of lake brine shows generation of Na-Cl type of brine with steady increase in major ions except Ca and Mg ions which decrease in successive stages of evaporation. Relative change in Na and Cl ions is also observed during the late stage of evaporation at or beyond halite saturation stage. Observation of such chemical change in brine composition is in line with the identification of salt minerals in lake sediments and salt encrustation by XRD technique.

In present-day hydrologic set-up, the estimated salt inventory in lake is _10⁶ tons as compared to riverine input of 6x10⁴ tons and an upper limit of aeolian input of about 5xl0⁴ tons. Such an estimate indicates that majority of salt is derived from the recycling and re-solution of salt encrustation from the lake bed during recharge period.

Keywords

Hydrogeochemistry, TDS, Nacl, Brines, Annual Wetting and Drying Cycles, Evaporation, Brine Evolution, Salt Encrustation, Aeolian Salt Input, Recharge Period, Sambhar Salt Lake, Rajasthan.

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