Regional-scale stable isotopic signatures of recharge and deep groundwater in the arsenic affected areas of West Bengal, India

Samples of deep groundwater, river water and rainwater were collected for δ 18O and δ 2H analyses from an area of ∼22,000 km 2 in the arsenic-affected districts of West Bengal, India, in order to examine groundwater recharge. A plot of δ 18O versus δ 2H of groundwater falls subparallel to the constructed local meteoric water line (δ 2H = 7.2 δ 18O + 7.7), suggesting a predominance of meteoric recharge with some evaporation. The stable isotopic signature of groundwater from the deeper part of the semiconfined main aquifer is similar to that of shallower groundwater, which suggests that deeper groundwater has been recharged in the present-day climatic regime, mostly from monsoonal rainfall. Groundwater in deeper isolated aquifers falls within the isotopic range observed for the main aquifer. A trend of isotopic depletion of groundwater that extends northward and westward from the Bay of Bengal (the monsoonal moisture source area) indicates a rain-out process following Rayleigh-type distillation. Some recharge may have taken place from the rivers Ganges and Bhagirathi-Hoogly, with minimal inflow from other rivers (Jalangi and Ichamati).