Hydrogeochemical evolution of shallow and deeper aquifers in central Bangladesh: arsenic mobilization process and health risk implications from the potable use of groundwater

Protection of groundwater quality from various natural and anthropogenic forces is a prime concern in Bangladesh. In this study, we utilized groundwater geochemistry of shallow and deeper aquifers to investigate the hydrogeochemical processes controlling water quality, and the sources and mechanism of Arsenic (As) release to water and associated human health risks in the Faridpur district, Bangladesh. Analysis of hydrochemical facies indicated that groundwaters were Ca-Mg-HCO(3)type and that water-rock interactions were the dominant factors controlling their major-ion chemical composition. The dissolution of calcite, dolomite, and silicates, as well as cation exchange processes regulated the major ions chemistry in the groundwater. Dissolved fluoride (F-) concentrations (0.02-0.4 mg/L) were lower than the drinking water standard of 1.5 mg/L set by the World Health Organization (WHO). Arsenic contamination of groundwater is among the biggest health threats in Bangladesh. The measured As concentration (0.01-1.46 mg/L with a mean of 0.12 mg/L) exceeded the maximum permissible limit of Bangladesh and WHO for drinking water. The estimated carcinogenic risk of As exceeded the upper benchmark of 1 x 10(-4)for both adult and children, and health threats from shallow groundwater were more severe than the deeper water. The vertical distribution of As resembled Fe and Mn with their higher concentrations in shallow Holocene aquifers and lower in deeper Pleistocene aquifers. Speciation calculation indicated the majority of groundwater samples were oversaturated with respect to siderite, calcite, and dolomite, while undersaturated with respect to rhodochrosite. The saturation state of the minerals along with other processes may exert kinetic control on As, Fe, and Mn distribution in groundwater and lead to their lack of statistically significant correlations. Microbially mediated reductive dissolution of Fe and Mn oxyhydroxides is envisaged as the primary controlling mechanism of As mobilization in Faridpur groundwater. Pyrite oxidation was not postulated as a plausible explanation of As pollution.