GROUNDWATER ARSENIC IN THE LOWER GANGES DELTA PLAIN IN WEST BENGAL, INDIA AND BANGLADESH: A HYDROGEOCHEMICAL COMPARISON

Arsenic contaminations in groundwater have been widely reported. The serious arsenic contamination of groundwater of Lower Ganga delta Plain (LGDP) in West Bengal, India and Bangladesh has emerged as a global natural environmental health disaster. The Bengal Delta Plain (BDP) is one of the largest deltas in the world, drained by the Ganges, Brahmaputra and Meghna river (GBM) systems. Groundwater samples were collected from 67 different sites located in the districts of 24-Parganas (S), 24-Parganas (N) and Nadia in West Bengal, India along the western margin (Bhagirathi sub-basin), and 40 different sites located in the districts of Comilla, Laxmipur, Munshiganj, Faridpur and Jhenaida districts of Bangladesh along the eastern part of the Bengal Basin (Padma-Meghna sub-basin).

Groundwater in the Nadia, West Bengal is mostly of Ca–HCO₃ type while in the lower part of the delta, the groundwater is of Ca-Mg-SO₄ type. The concentrations of major solutes (Na⁺, Mg²⁺, Ca²⁺, K⁺, HCO₃^-, SO₄^2-, NO₃^- and PO₄^3- in groundwater of Meghna sub-basin is more variable than Bhagirathi sub-basin that indicating different hydrological setting in the parts of the Bengal basin. The trace element concentrations such as As, Fe and Mn also show considerable variability in the two distinct parts of the Bengal basin. Most groundwaters of the LGDP contain arsenic above the WHO and the BIS standard of 0.01 mg/L as well as in many case above the Bangladesh drinking water standard (0.05 mg/L). Both sites have moderately reducing environment, with high concentrations of dissolved organic carbon, indicating dominantly metal-reducing processes and nearly similar mechanism in As mobilization. The occurrence of elevated arsenic in groundwater is generally associated with natural biogeochemical reactions (such as reductive dissolution of iron oxides/hydroxides) by altering groundwater redox state and releasing arsenic from sediment to aqueous phase. The various redox-sensitive solutes indicate overlapping redox zones, leading to partial redox equilibrium conditions where As, once liberated from minerals of sediments, would tend to remain in groundwater because of the complex interplay among the electron acceptors. Also, microbes in organic matter environment are acting as the major electron acceptor, in the Lower Ganges Delta Plain.