Elevated Groundwater Arsenic in Deeper Aquifers of the Brahmaputra Floodplains in Assam, India

The deeper aquifers are considered safe for human consumption in various parts of South Asian countries, including the Ganges floodplains of India and Bangladesh. The present study explored the distribution and controls of groundwater arsenic (As) in deeper aquifers (depth >80 m) as well as shallow aquifers (depth <30 m) that are considered a major source for drinking and irrigation purposes in Brahmaputra floodplains, India. We collected groundwater samples from the two most As-contaminated districts in Assam, India. About 87% of sampled wells in deeper aquifers had As concentration above the WHO permissible limit of 10 µg/l, with a mean concentration of 61 µg/l and 82 µg/l during pre- and post-monsoon seasons, respectively. In addition, about 55% of sampled wells in shallow aquifers had As concentration >10 µg/l, with a mean concentration of 47 µg/l and 56 µg/l during pre- and post-monsoon seasons, respectively. Various physico-chemical parameters, viz; pH, EC, TDS, Ca2+, Na+, K+, Mg2+, Cl–, HCO3–, NO3–, and SO42– were analyzed to elucidate the groundwater hydrochemistry of the region. The Piper plot indicated Ca-HCO3 and Na-HCO3 as the major water types in the study area in both deeper and shallow aquifers. The Gibbs plot depicted the rock-water interaction as a major source that governs the groundwater chemistry in the region. The regional scale study indicated that the vertical movement of elevated As from shallow aquifers to deeper depths, coupled with extensive irrigation pumping, could be a probable cause of As in the deeper aquifers.