Hydrochemistry of Arsenic in Nadia District, West Bengal

In Nadia District, West Bengal, arsenic-polluted groundwater occurs in shallow unconfined aquifer. Both As(II1) and As(V) occur in groundwater at depths greater than 14 metres below ground level (mbgl). Iron, bicarbonate, calcium, magnesium, sodium and chloride are the other important dissolved ions in groundwater. The saturation index (S.I.) of groundwater for calcite ranges between 0.28 and 1.07, and estimated pCO₂ exceeds the atmospheric value of 10^-3.5Atm. Groundwater in the study area occurs in shallow unconfined aquifer in which reducing agent viz. organic matter is abundant. Concentrations of iron, bicarbonate, trivalent arsenic is high. Thus, the aquifer bears the signature of reducing condition. As a consequence, ferric-iron oxides are reduced and their sorbed load of arsenic is released to groundwater as they dissolve. Ferric-iron oxide reduction and release of arsenic from its surface may be due to emergence of reducing condition through oxidation of natural organic matter or waste waste organic matter and consequent attenuation (or depletion) of oxygen from the ambit of the aquifer sediment and/or water.