Unique Geochemistry of Arsenic-Contaminated Groundwater and Corresponding Mitigation Efforts in Southern Nepal

In the lowlands of Nepal, as well as in several other South Asian countries, the level of arsenic in groundwater extracted from Quaternary alluvial sediments frequently exceeds the World Health Organization (WHO) drinking water guideline of 10 μg/L. The widely accepted explanation refers to the reductive dissolution of Fe-bearing minerals releasing As-oxyanions from the soil minerals. However, this hypothesis is only to some extent applicable. Given that arsenic and iron in the groundwater are weakly correlated or decoupled and iron­(III) (hydr)­oxides are scarcely present, a substantial portion of As and Fe has to be retained in clay minerals. The low concentration of iron in groundwater can be explained by the origin of As and Fe in the rocks (leucogranites); in the High Himalayas, they are low in Fe but considerably enriched in Li, B, As, Se, Br, Sr, Mo, Cd, P, and U. This unique geochemical setting has a major impact on the performance of the Kanchan Arsenic Filters (KAFs) used to eliminate As from groundwater in Nepal. Lack of sufficient Fe in groundwater, combined with limited release of iron by corrosion of iron nails, call for an adapted version of these filters. The first results from 20 modified filters show a clear increase in As removal efficiency, achieved mainly by the replacement of old nails, an added upper sand layer over the nails, and elongation of the outlet tube to avoid wet–dry cycles and to keep the nail bed immersed. Proper instructions to the users on the operation and maintenance of filters, by replacing the reactive materials (iron nails) and the lower sand bed regularly, are imperative.