INSIGHTS INTO THE SORPTION BEHAVIOUR OF ARSENIC IN THE OXIDIZED AQUIFERS IN MATLAB UPAZILA, BANGLADESH

Elevated concentrations of arsenic (As) in the groundwater of the shallow Holocene alluvial aquifers in the Bengal Delta Plain (BDP) is a major health concern for millions of people in Bangladesh and West Bengal who rely on tubewells as their primary source of drinking water. Reductive dissolution of oxide minerals and subsequent release of absorbed As is widely recognized as the main mechanism leading to the mobilization of As in the BDP. While the geological conditions and distribution of As is extremely heterogeneous, recent studies have indicated that using sediment color to target shallow low-As aquifers may be a viable mitigation solution in some regions for supplying safe drinking water to rural communities. The sustainability of this option however still needs to be evaluated. This study provides insight into the risks of contamination of the As safe aquifer layer, and in particular the adsorption behaviour of the oxidized sediments and their capacity to immobilize As if flow cross-contamination from the upper As unsafe aquifer occurs. While chemical analysis reveal a low amount of As in the oxidized sediments, below 2.5 mg/kg, batch and column tests suggest that these sediments have a high capacity to adsorb As(V) if cross-contamination occurs. Replicate column experiments reveal that the adsorption capacity of the oxidized sediments however will likely be significantly reduced by high levels of reactive DOC in groundwater infiltrating from the reduced layer. Finally, sensitivity analyses performed in the laboratory and extended by PHREEQC simulations indicate that the adsorption site density and concentration of solutes that compete for these sites strongly influence the As adsorption characteristics. These parameters need to be better understood to more accurately predict the attenuation and potential risks of As(V) if cross-contamination between aquifer layers occurs.