Mobility of arsenic in the sub-surface environment: An integrated hydrogeochemical study and sorption model of the sandy aquifer materials

SummaryGroundwater and aquifer materials have been characterized geochemically at a field site located in the Chakdaha municipality of West Bengal, India. Sorption experiments were also carried out on a sandy aquifer material to understand the mobility of arsenic (As) in the sub-surface environments. The result shows that the areas associated with high groundwater As (mean: 1.8 μM) is typically associated with low Eh (mean: −129 mV), and high Fe (mean: 0.11 mM), where Fe2+/Fe(OH)3 couple is controlling groundwater redox potential. Analysis of the aquifer material total concentrations showed the dominance of As (range: 8.9–22 mg kg−1), Fe (range: 3.0–9.7% as Fe2O3) and Mn (range: 0.05–0.18% as MnO) in the silt-/clay-rich sediments; whereas fine-/medium-sand rich sediment contains considerably lower amount of As (<8.1 mg kg−1), Fe (range: 1.6–3.9% as Fe2O3) and Mn (range: 0.02–0.08% as MnO). The acid extractable As do not correlate with ascorbate extractable Fe-oxyhydroxide, however Fe-oxyhydroxide is generally high in the sediments from low groundwater As areas. Chemical speciation computations indicated Fe(II), Ca(II), Mg(II) and Mn(II) to be at equilibrium (with respect to calcite, dolomite and rhodochrosite) or slightly over-saturated (with respect to siderite). These carbonate minerals may therefore participate to the As immobilization. The measured total organic carbon (∼1%) and groundwater temperature (26–32 °C) coupled with sorption studies strongly favors microbially mediated Fe(III)-oxyhydroxide reduction as the dominant mechanism for the release of As in the groundwater. Oscillations of As, Mn and Fe concentrations with depth reflected pCO2 oscillations consecutive to microbial respiration intensity.