Sources of sulfur in Deccan Trap rivers: A reconnaissance isotope study

Sulfur isotopic composition (δ34S) of dissolved SO42- was measured in waters of rivers draining the Deccan Trap basalts in order to elucidate the source(s) of elevated SO42- content in some of these rivers, observed in an earlier study. The δ34S values range from 8.7‰ to 19.5‰, with an average of 14.5 ± 2.8‰, values generally enriched in 34S compared to data available from other Indian rivers. The Bhima River (a tributary of the Krishna) and most of its tributaries have δ34S within the 14 ± 2‰ range and high dissolved SO42- concentrations (256–1556 μM). Measured δ34S of 7 of the saline/alkaline soils suggest that the high dissolved SO42- in the Bhima river system is likely derived from this source and/or gypsum concretions in the soils. The δ34S–SO4 relationship for the Krishna River and its smaller tributaries follows a mixing pattern of two end-members, rainwater with low SO42- and low δ34S and an unknown, likely a pollution, source with intermediate SO42- content and heavy δ34S. The most important finding that this study brings out is the minimal role that the weathering of basalt-sulfides play in contributing to the dissolved SO42- in the rivers. This rules out the role of H2SO4-, and supports H2CO3-mediated weathering as the dominant process in the Deccan settings.

Research highlights► Origin of sulfate in rivers draining Deccan Traps was studied by using S isotopes. ► Highly enriched 34S was found in most of the rivers, especially the Bhima River system ► Attest to the role of H2CO3 as major proton supplying agent for weathering in Deccan Trap region. ► Minimal role of basalt-sulfides in contributing to sulphate in Deccan Rivers.