Distribution of oxygen and hydrogen isotopes in shallow groundwaters from Southern India: influence of a dual monsoon system

Oxygen and hydrogen isotopic investigation of groundwater and river water samples from the southern Indian peninsula was undertaken to characterise the isotopic nature of the near surface water sources and provide basic framework for future hydrological studies. It is assumed that the shallow groundwater retains the isotopic signature of the local precipitation averaged over a few tens of years except in a few cases where the same gets modified by post precipitation evaporation and/or recharge contribution from surface water bodies. Therefore, isotopic character of the groundwater can, in principle, be used to determine the relative influence of the different vapour sources contributing to the local/regional precipitation and to characterise the modifications before groundwater recharge. In the present context, vapour sources for two rainy seasons, namely, southwest (SW) and northeast (NE) monsoon, are respectively, the Indian Ocean/Arabian Sea and the Asian continental sources together with the Bay of Bengal. This study shows that: (i) The regions dominated by NE monsoon have distinctly depleted isotopic composition compared to those dominated by SW monsoon. (ii) The delta18O–deltaD regression line slope of ~6 in the east coast region is lower than that expected for local precipitation suggesting secondary evaporation. (iii) The orography of the Western Ghats hill ranges plays a significant role in controlling the isotopic distribution along the west coast region. (iv) The low `d-excessʹ values in most part of study area indicate secondary evaporation. (v) The high `d-excessʹ values over the Deccan Plateau region in the NW part of the study area suggest admixture of recycled moisture with the inflowing oceanic vapour.