NEPALESE GROUNDWATER ARSENIC CONTAMINATION IS RELATED TO SIWALIK SOURCE ROCK

Statistical GIS analysis of 37 fan-watershed pairs, covering all of Nepal, indicates correlation of both average and maximum arsenic concentration in shallow ground water from alluvial fans to the proportion of range-front Plio-Pleistocene Siwalik source rocks in the erosional part of watersheds, where fan sediment is produced. Areas of greatest arsenic contamination occur on fans where Siwalik rocks are the only sediment source, although areas free of arsenic contamination also occur on the same fans. On fans lacking Siwalik sediment, arsenic contamination is rare, despite the presence of secondary ferric oxyhydroxides and detrital pyrite containing arsenic. One hypothesis to explain this relationship postulates that much of the arsenic in ground water comes from oxidation of sulfides in the alluvial aquifer, and authigenic sulfides found in Siwalik debris break down more readily than primary hydrothermal sulfide minerals found in debris of veins eroded from metamorphic rocks of the middle and high Himalaya. Fans dominated by Siwalik sediment are generally smaller than 2000 km2 and restricted to the alluvial belt just south of the Himalayan front, so these observations do not explain distribution of arsenic contamination in Bangladesh or in most of northern India. Three coverages were necessary for the analysis. Point data on groundwater contamination was generated from a database of 18,000+ water samples from shallow tube wells distributed across the southern Nepalese plains. These data, collected by many organizations, were compiled by the Environment and Public Health Organization for the National Arsenic Steering Committee of Nepal. Geologic polygons were digitized by the International Centre for Integrated Mountain Development from geologic maps compiled by Nepal Department of Mines and Geology. Polygons of the fan-watershed systems were interpreted and digitized from scanned topographic maps and a coverage of stream networks.