The origin and mobility of arsenic (As) in the groundwater environment has received serious attention in recent years. Recent studies have reported naturally occurring As in groundwaters of the Terai Alluvial Plains (TAP) in southern Nepal, where groundwater exploitation has increased since the 1960s. The source of As in TAP is geogenic and leached primarily due to weathering of As bearing rocks and sediments in the Himalayas. In our present study, we have investigated the groundwater chemistry in the central part of the TAP in Nawalparasi district. TAP groundwaters are near-neutral to alkaline, with predominantly reducing character and high HCO_3~-, low SO_4~(2-) and NO_3~- concentrations. Elevated HCO_3 levels possibly result due to the oxidation of organic matter, low SO_4~(2-) levels reflect sulfate reduction. Elevated NH_4~+ concentrations in these groundwaters suggest dissimilatory nitrate reduction in the aquifers. Total arsenic (As_(tot)) levels in groundwater varied from 1.7 μg/L to as high as 404 μg/L with dominance of As(III) species and elevated levels of dissolved Fe and Mn. Arsenic is mobilized in groundwaters as a result of desorption of As-oxyanions adsorbed onto Fe- and Mn-oxides as well as reductive dissolution of these surface reactive phases from the sediments along with release of As in anoxic groundwaters.
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