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

Sulfur isotopic composition (δ34S) of dissolved SO 4 2 - was measured in waters of rivers draining the Deccan Trap basalts in order to elucidate the source(s) of elevated SO 4 2 - 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 SO 4 2 - concentrations (256–1556 μM). Measured δ34S of 7 of the saline/alkaline soils suggest that the high dissolved SO 4 2 - 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 SO 4 2 - and low δ34S and an unknown, likely a pollution, source with intermediate SO 4 2 - 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 SO 4 2 - in the rivers. This rules out the role of H2SO4-, and supports H2CO3-mediated weathering as the dominant process in the Deccan settings.