A transect of groundwater and sediment properties in Araihazar, Bangladesh: Further evidence of decoupling between As and Fe mobilization

A new sampling device was used to obtain 8 detailed profiles of groundwater and associated sediment properties to ∼30 m depth in a 4 km2 area of Bangladesh that is characterized by high spatial variability in groundwater As. Concentrations of dissolved As, Fe, and Mn ranged from < 0.1 to 600 μg/L, < 0.1 to 18 mg/L, and < 0.1 to 4 mg/L, respectively. Voltammetric measurements conducted in the field indicate that > 95% of the dissolved arsenic was As(III). The proportion of Fe(II) in the hot-leachable iron fraction of the sediment ranged from 0.2 to 0.9. The concentration of phosphate-extractable As in aquifer sands ranged from < 0.1 to 1.1 mg/kg. Although the highest dissolved As concentration was measured at the site where the highest groundwater Fe and Mn concentrations were also observed, the dissolved As maximum was located several meters below the dissolved Fe and Mn maxima. Fe(II) / Fe ratios and P-extractable As concentrations in the solid phase were particularly elevated at the same site, though again the profiles of these properties did not consistently match that of groundwater As concentrations. A 500-m section composed of 5 vertical profiles confirms that whereas low As concentrations are broadly associated with low dissolved Fe and Mn concentrations, as well as low Fe(II) / Fe ratios in the solid phase, the spatial distributions of these properties in the subsurface do not follow a simple relationship. In combination with other observations from the area, the results suggest that elevated local recharge in areas where the permeability of surface soils is high prevents As from accumulating in groundwater. Conversely, dissolved As concentrations tend to be high in areas where local recharge is restricted by a surface cover of low permeability. The data demonstrate that further investigation of the mechanisms underlying As mobilization is likely to require sampling at lateral scales < 1 km.