ABSTRACT
ABSTRACT: Natural arsenic (As) was detected in groundwaters in the Terai Alluvial Plain (TAP) in southern Nepal in the year 1999. By the end of March 2004, about 245,000 wells have been tested for As, out of which about 3% samples are found to have exceeded Interim Nepalese Standard of 50g/L. From the detail study conducted in hotspot district Nawalparasi, natural rocks are thought to be the sources of As that are leached mainly due to the weathering of As bearing minerals from the Himalayas towards the northern Nepal. In this paper, the chemistry of groundwater from highly arsenic affected Nawalparasi district in the central part of the TAP in southern Nepal has been presented. TAP groundwaters are found to be predominantly of reducing character with low SO4
concentrations. Total arsenic (Astot) concentration in groundwater varied from 1.7g/L to as high as 404g/L. As(III) species is found to be predominant along with elevated levels of dissolved Fe and Mn. The correlation between DOC, HCO3
, Fetot and Astot strongly supports the hypothesis of reductive dissolution of Fe-oxyhydroxides as the main mechanism of mobilization of As in groundwater in TAP. Blanket testing by As-field test kits is the easiest way to find out As free sources nearby for tubewell switching. In the absence of As-free source, the only available option is the treatment of water either at the point of entry or at the point of use to meet the drinking water standard. DWSS in collaboration with UNICEF and WHO is conducting blanket testing of As in 10 Terai districts. Based on the blanket test result, As treatment methods such as 3-gagri filter, arsenic biosand filter etc., which are simple, effective, affordable and socially acceptable will be provided as a short-term option to the affected communities in hotspot areas.
