Vertical distribution and mobilization of arsenic in shallow alluvial aquifers of Chapai-Nawabganj, Northwestern Bangladesh

Core sediments from two undisturbed boreholes and groundwater from fifty four arsenic (As)-contaminated well waters in the Chapai-Nawabganj area of northwestern Bangladesh were collected for geochemical analyses. Groundwater arsenic concentrations in the uppermost aquifer (10 to 40 m depth) range from 2.76-315 |xg/L (mean 48 |xg/L) whereas arsenic concentration in sediments ranges from 3.26-10 mg/kg. In January 2008, 2009 and 2010, maximum As concentration occurred at the same depth. Arsenic and Fe speciation in sediments were determined using XANES and the results imply that As(V) and Fe(III) are the dominant species in most sediments. X-ray absorption fine structure (EXAFS) analysis shows that FeOOH is the main carrier of As in the sediments. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. The dominant groundwater type is calcium (Ca)-bicarbonate (HCO ₃ ^-) with high concentrations of As and iron (Fe) but low levels of nitrate (NO ₃ ^-) and sulfate (SO ₄ ^-2). Statistical analysis clearly shows that As is closely associated with Fe (R ² = 0.64) and manganese (Mn) (R ² = 0.91) in sediments while As is not correlated with Fe and Mn in groundwater. Comparatively low Fe and Mn concentrations in some groundwaters, suggest that probably siderite and/or rhodochrosite precipitated as secondary mineral on the surface of the sediment particles. The correlations along with results of sequential leaching experiments suggest that reductive dissolution of FeOOH and MnOOH mediated by anaerobic bacteria represents mechanism for releasing arsenic into the groundwater.