MECHANISMS OF ARSENIC CONTAMINATION OF DEEP GROUNDWATER OF THE WESTERN BENGAL BASIN, INDIA

Deep groundwater in the Bengal basin has been hypothesized to be a safe alternate drinking water source in areas where elevated As concentrations in shallow groundwater have endangered the lives of more than 50 million people. However, a multi-year study within a ~21,000 km² area in West Bengal state, India, has shown that >60% of the sampled wells (n = 112, constituting about 1/3 of all the rural water-supply systems in the study area) have As ³ 10 ppb (WHO MCL). Although profiles from part of the study area indicate that As concentrations in groundwater decrease with depth, the exceedance of the MCL for the majority of sampled wells (>70% in the northern parts of the study area) warrants detailed investigation on mechanisms of contamination. Hydrostratigraphic modeling shows an absence of major confining layers (except in southern coastal areas) between shallow and deep sediments, thereby leading to the possibility of natural mixing between contaminated and as-yet-uncontaminated waters along intermediate and regional-scale flow paths. However, groundwater modeling suggests that larger-scale flow paths may have been distorted or destroyed by extensive irrigational pumping, leading to multiple local flow cells with enhanced recharge (from surface water bodies and agricultural return flow) and increased vertical flow. Such increased flow may have homogenized any pre-existing differences in groundwater chemistry between shallow and deep parts of the aquifer, as indicated by similar stable isotopic compositions of oxygen and hydrogen, and thereby drawn shallow contaminated water to greater depths. Hydrochemical analyses and geochemical modeling indicate that groundwater is undersaturated with respect to As and Mn phases and oversaturated with respect to Fe-oxides. Groundwater is in equilibrium with kaolinite and Na-smectite, and its major-ion composition appears to reflect both carbonate dissolution and silicate weathering. The co-occurrence of ammonium, Fe, sulfate, and methane in groundwater indicates the mobilization of As within overlapping redox zones.