Oxygen and hydrogen isotopic composition of major Indian rivers: A first global assessment

Nine major rivers have been sampled around the Indian subcontinent to give an overview of the surface water characteristics. Both ¹⁸O and deuterium have been measured to determine the origin of the water and the possible evapotranspiration process. The major ions have also been analysed to obtain complementary information. Although some basins have been studied previously (mainly in the north), this is the first attempt at a wider investigation of major Indian rivers. The results are discussed from the perspective of the hydroclimatological, geographical and geological specificity of the river basins. δ ¹⁸O values vary from light-isotope-enriched Himalayan rivers to heavy-isotope-enriched peninsular Indian rivers in a northwest-southeast gradient across the subcontinent. There is more evapotranspiration, leading to heavy isotope enrichment, in the peninsular (southern Deccan) rivers compared with the light-isotope-enriched snow- and glacier-melt-derived waters of the Himalayan rivers. The δ¹⁸O values of Indian rivers correspond roughly to the δ¹⁸O values of the rains falling over the subcontinent. However, the influence of tributaries is dominant over rainfall in rivers like the Narmada and Tapti. The Cauvery and Krishna rivers show maximum evapotranspiration and sodium pollution, as indicated by the δ¹⁸O values, deuterium excess and major ion data.