Hydrological and geochemical constraints on the mechanism of formation of arsenic contaminated groundwater in Sonargaon, Bangladesh

The geochemical characteristics and hydrological constraints of high As groundwater in Sonargaon, in mid-eastern Bangladesh were investigated in order to ascertain the mechanism of As release into the groundwaters from host sediments in the Ganges–Brahmaputra delta. Samples of groundwater were collected from ca. 230 tube wells in both the rainy and dry seasons. Similar to previous studies, high As groundwater was found in the Holocene unconfined aquifer but not in the Pleistocene aquifer. Groundwaters in the Holocene aquifer were of the Ca–Mg–HCO3 type with major solutes derived from chemical weathering of detrital minerals such as plagioclase and biotite. Groundwater with high As was generally characterized by high image, possibly derived from the agricultural application of fertilizer as suggested by the small variation of δ15NNH4δ15NNH4 (mostly 2–4‰). Concentrations of Fe changed between the rainy and dry seasons by precipitation/dissolution of Fe oxyhydroxide and siderite, whilst there was not an apparent concomitant change in As. Inhomogeneous spatial distribution of δ18O in the Holocene unconfined aquifer indicates poor mixing of groundwater in the horizontal direction. Spatial variation of redox conditions is associated with localized variations in subsurface permeability and the recharge/discharge cycle of groundwater. Hydrogeochemical data presented in this paper suggest that reduction of Fe oxyhydroxide is not the only mechanism of As mobilization, and chemical weathering of biotite and/or other basic minerals in the Holocene aquifer could also be important as a primary cause of As mobilization.