Geo-chemical behavior of uranium in the sambhar salt lake, Rajasthan (India): implications to "source" of salt and uranium "sink

Abstract

Among several salt lakes in the Thar Desert of western India, the Sambhar is the largest lake producing about 2 × 10⁵ tons of salt (NaCl) annually. The "lake system"(lake waters, inflowing river waters, and sub-surface brines) provides a unique setting to study the geo-chemical behavior of uranium isotopes (²³⁸U, ²³⁴U) in conjunction with the evolution of brines over the annual wetting and evaporation cycles. The concentration of ²³⁸U and the total dissolved solids (TDS) in lake water increase from ~8 μg L^-1 and ~8 g L^-1 in monsoon to ~1,400 μg L^-1 and 370 g L^-1, respectively, during summer time. The U/TDS ratio (~1 μg g^-1 salt) and the ²³⁴U/²³⁸U activity ratio (1.65 ± 0.05), however, remain almost unchanged throughout the year, except when U/TDS ratio approaches to 3.8 at/or beyond halite crystallization. These observations suggest that uranium behaves conservatively in the lake waters during the annual cycle of evaporation. Also, uranium and salt content (TDS) are intimately coupled, which has been used to infer the origin and source of salt in the lake basin. Furthermore, near uniform ratios in evaporating lake waters, when compared to the ratio in seawater (~0.1μg g^-1 salt and 1.14 ± 0.02, respectively), imply that aeolian transport of marine salts is unlikely to be significant source of salt to the lake in the present-day hydrologic conditions. This inference is further consistent with the chemical composition of wet-precipitation occurring in and around the Sambhar lake. The seasonal streams feeding the lake and groundwaters (within the lake's periphery) have distinctly different ratios of U/TDS (2-69 μg g^-1 salt) and ²³⁴U/²³⁸U (1.15-2.26) compared to those in the lake. The average U/TDS ratio of ~1 μg g^-1 salt in lake waters and ~19 μg g^-1 salt in river waters suggest dilution of the uranium content by the recycled salt and/or removal processes presently operating in the lake during the extraction of salt for commercial use. Based on mass-balance calculations, a conservative estimate of "uranium sink" (in the form of bittern crust) accounts for ~5 tons year^-1 from the lake basin, an estimate similar to its input flux from rivers, i.e., 4.4 tons year^-1.