Groundwater Geochemistry, Geology, and Microbiology of Arsenic-contaminated Holocene Alluvial Aquifers, Manikganj, Bangladesh

Abstract

A multi-disciplinary study, using integrated aspects of geology, groundwater geochemistry including carbon isotope geochemistry, scanning electron microscopy, energy dispersive spectroscopy, and geochemical modeling, reveals linked geochemical and microbial processes that cause natural arsenic-contamination of Holocene alluvial aquifers in Manikganj, Bangladesh. Natural arsenic contamination in south Asia is considered to be the worst environmental crisis in the world. It is estimated that more than 25 million people in Bangladesh and West Bengal, India are consuming drinking water containing dangerous concentrations of arsenic. Existing and new data indicate that groundwaters in Manikganj aquifers contain elevated concentrations of As, Fe, Mn, Ba, Sr, and HCO3-. Elevated dissolved H2 gas contents (1.3 to 2.4 nM), predominantly reducing ORP values (-125 to +103 mV), carbon isotopic signatures [-3.32‰ to +4.07‰ DIC, -22.8‰ to -25.0‰ DOC (VPDB)], presence of authigenic siderite concretions in aquifer sediments, and high organic carbon contents (up to 4%) of sediments indicate that bacterial Fe(III)-reduction is responsible for elevated As and Fe concentrations. Strong correlations between elevated As and Fe2+ concentrations support the hypothesis that bacterial reduction of (insoluble) hydrous-ferric-hydroxide minerals causes release of associated sorbed arsenic back into solution. Sulfate concentrations are very low where As levels are greater than 0.03 mg/L, indicating biogenic sulfate reduction is limited by low levels of dissolved sulfate. This implies that only an insignificant fraction of As were removed by iron-sulfide solids. Geochemical modeling indicates that precipitation and dissolution of Fe- and Mn-minerals controls the mobility of trace metals in groundwater. Geochemical speciation models show that highly toxic As(III) is the dominant As species in Manikganj groundwater geochemical conditions. Moreover, adsorption-desorption geochemical models show that desorption is not the sole mechanism for As release.