INVESTIGATIONS ON THE RELATION OF ARSENIC-BEARING SEDIMENTS TO ARSENIC CONTAMINATED GROUNDWATER BENEATH THE PLAINS OF NEPAL

Millions of people on alluvial plains in southern Nepal drink well water containing arsenic at concentrations much greater than international standards. The USGS South Asia Arsenic Project and local cooperators are investigating surficial geology, stratigraphy, sediment chemistry, groundwater chemistry, and distribution of arsenic contamination to understand the origin of the problem. A database, containing 18,000 water analyses from across all of the southern plains, was compiled through collaboration of USGS, Nepal National Arsenic Steering Committee, and Environment and Public Health Organization. Fourteen out of twenty districts have wells above the 50 µg/L standard, and three districts are particularly vulnerable. Alluvium derived from soft Plio-Pleistocene sedimentary rocks of the foothills appears to produce more groundwater arsenic than first cycle alluvium from crystalline rock higher in the Himalaya. The highest values affecting the most people are concentrated around Parasi Bazaar, so USGS drilled 4 boreholes near that village to sample alluvium from areas producing water with a range of arsenic contents (>500 to <10µg/L). Ninety-nine samples from two core holes drilled to depths of 40 and 23 m were preserved in original oxidation states in argon-filled vials. Fifty-two additional samples were collected from two other bore holes drilled to 30 m. The 4 boreholes represent an area of 100 km². Lithology and stratigraphy of alluvium among the boreholes are similar despite strong contrast in arsenic content of the water. The depth interval from 7 and 20 m is gray in contrast to the more oxidized tan and olive-brown appearance of the shallower and deeper alluvium. A micaceous gray sand aquifer at the bottom of the gray sequence tends to produce water with the highest arsenic content. Analyses of sediment detected arsenic concentrations from <2 to 19 ppm with similar contents in gray and brown alluvium. Gray alluvium has higher sulfur present as biogenic pyrite and higher organic carbon. Arsenic in alluvium is redistributed in response to changing redox conditions in weathering, transport and depositional environments. Whether the arsenic release to water results from reduction of ferric oxyhydroxides as commonly proposed or from oxidation of reduced forms of arsenic in the gray sediment remains uncertain.