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

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 δ 18O results for groundwaters adjacent to the lake suggest that they are not influenced by recharge from the lake, whereas the shallow sub-surface 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 ( δ 18O, −20%.) dominantly controls the isotopic evolution of oxygen in the lake during the annual evaporation cycle.