AbstractIn many areas of south and south-eastern Asia, concentrations of As in ground water have been found to exceed the WHO maximum concentration limit of 10 μg/l. This is adversely affecting the health of millions of people and has grave current and future health implications. It has recently been suggested that extensive abstraction of ground water in these areas may accelerate the release of As to ground water. This study uses geochemical and isotopic data to assess this hypothesis. The area investigated in this study is in the Chakdaha block of the Nadia District, West Bengal. The ground water is predominantly of the Ca-Mg-HCO3 type, although some samples were found to contain elevated concentrations of Na, Cl and SO4. This is thought to reflect a greater degree of water-rock interaction at the locations of these particular samples. Arsenic concentrations exceeded the national limit of 50 μg/l in 13 of the 22 samples collected. Four of the 13 samples with high As were recovered from tubewells with depths of 60 m or more. Shallow ground water samples were found to have a stable isotopic composition which falls subparallel to the Global Meteoric Water Line. This probably represents a contribution of evaporated surface water to the ground water, possibly from surface ponds or re-infiltrating irrigation water. Deep ground water, conversely, was shown to have a composition that closely reflects that of meteoric water. The data presented in this study suggest that, whilst the drawdown of surface waters may drive As release in shallow ground waters, it is not responsible for driving As release in deep ground water. However, local abstraction may have resulted in changes in the ground water flow regime of the area, with contaminated shallow ground waters being drawn into previously uncontaminated deep aquifers.
