Geological and geochemical examination of arsenic contamination in groundwater in the Holocene Terai Basin, Nepal

Arsenic contamination of groundwater is a problem worldwide, and the Terai Basin in the southern plains of Nepal is no exception. The Department of Water Supply and Sanitation of Nepal, with assistance from the World Health Organization (WHO), surveyed three districts of eastern Terai for the first time in 1999, and found that arsenic concentration exceeded the 10 ppb guideline recommended by WHO. Other agencies working with drinking water supply such as the Nepal Red Cross Society, the Rural Water Supply and Sanitation Support Project, Nepal Water for Health, UNICEF, and local NGOs then started testing for arsenic in groundwater, and found positive results in most places. Most of the work carried out so far has concentrated on quantitative tests of arsenic, short-term mitigation activities, and health impacts. Of the 18,635 tube wells tested, 23.7% have arsenic contents above the WHO limits of 10 ppb, and 7.4% were above the Nepal interim standard of 50 ppb (Shrestha et al. 2004). The aim of this study was to evaluate geological and geochemical conditions of the Terai basin relevant to arsenic contamination. Core sediments samples from a western Terai district of Nepal (Fig. 1) were collected by boring and analyzed for major oxides and trace elements by X-ray fluorescence (XRF). Physical and chemical properties of water samples were determined with field kits as well as laboratory tests. The chemistry of sediments and water from a contaminated area in Nawalparasi was compared with data from an uncontaminated area in Bhairahawa. The results show that the chemistry of sediments is similar, but water chemistry varies depending upon the grain size of sediments and the organic matter concentration.