GEOCHEMICAL INDICATORS OF GROUNDWATER ARSENIC MOBILIZATION MECHANISMS IN THE GANGES FLOODPLAIN OF NEPAL

High concentrations of groundwater arsenic are found throughout the Ganges floodplain, extending upstream from Bangladesh to headwater regions in Nepal. Studies of arsenic mobilization in Bangladesh indicate that either oxidation of aquifer matrix pyrite or reduction of arsenic adsorbed to ferric hydroxides is the principal mobilization mechanism. Considerable spatial heterogeneity in arsenic concentrations suggests strongly localized controls on mobilization, largely dependent on aquifer heterogeneity and perhaps microbial diversity. These heterogeneity effects can be expected to be most prominent in headwater regions near the source of aquifer materials.

This study focuses on the Nawalparasi District, the most arsenic-affected area in the Ganges headwater region of Nepal. 50 tubewells were sampled and analyzed by ICP for major and trace elements including arsenic. Field measurements of pH and oxidation state were made (one or more of Eh, Fe²⁺/Fe_total, As^III/As_total). Arsenic values ranged from 2.5-400 ppb, positively correlated with total Fe (with Spearman Rank 0.610, essentially all Fe as Fe²⁺). All samples exhibited HCO^-₃ as the primary anion, SO^-₄ is typically present in very low concentrations (all but 7 samples < 10 mg/l). These low SO^-₄ concentrations, high total iron and Fe²⁺/Fe_total indicate that pyrite oxidation is not the mobilization mechanism for arsenic in these wells. The strongly reducing conditions in all samples are consistent with a reduction of Fe-oxyhydroxide as the mechanism (presumably microbially mediated) for arsenic mobilization. Planned speciation and geochemical modelling studies of these samples are expected to further elucidate arsenic mobilization processes and the impact of aquifer heterogeneity in this region.