ARSENIC AND OTHER HEAVY METALS IN SURFACE WATER AND GROUNDWATER OF POKHARA VALLEY, NEPAL HIMALAYA

Elevated levels of As in groundwater in the floodplain of the Ganges River in Bangladesh, West Bengal (India) and southern Nepal have been well-documented over the past 15 years. The dominant paradigm for the occurrence of elevated As in South Asia is the reductive dissolution model in which the strongly reducing conditions in the thick sedimentary package cause the dissolution of Fe oxyhydroxides and the release of the adsorbed As into groundwater. The implication of the reductive dissolution model is that elevated As does not result even indirectly from any human activity. Seven recent studies, including a study by the authors, have shown that elevated As in groundwater occurs in Kathmandu Valley in Nepal, a tectonic valley well upstream of the floodplain of the Ganges River. Our study showed groundwater As to be uncorrelated with Fe and with any combination of the transition elements, which is inconsistent with both the reductive dissolution model and the competing sulfide oxidation model. Moreover, our study showed surface water As to be statistically indistinguishable from groundwater As, which led us to propose the fluvial recharge model in which elevated groundwater As results from losing streams with elevated As, which is a consequence of rapid erosion caused by a combination of monsoon climate, tectonic uplift and deforestation. The objective of this study was to further test the fluvial recharge model and other existing models in Pokhara Valley, another tectonic valley in the Nepal Himalaya far upstream from the floodplain of the Ganges River. In November 2010 water samples were collected from 20 hand-dug wells (depths 2 – 18 m), 12 machine-drilled wells (depths 34 – 220 m), six springs, 15 streams, two lakes and two caves. Electrical conductivity, pH and water temperature were measured on-site. Samples have analyzed for sulfate and the transition elements Fe, Cu, Ni, Co, Mn, Zn and Cr with the Hach DR-2700 Spectrophotometer and are currently being analyzed for As. Results will be reported at the meeting.