TESTING THE ACCELERATED EROSION MODEL FOR ARSENIC CONTAMINATION OF GROUNDWATER IN KATHMANDU VALLEY, NEPAL HIMALAYA

Recent studies have shown that elevated groundwater As occurs in Kathmandu, Mustang and Pokhara Valleys in Nepal Himalaya and that surface water As is statistically indistinguishable from groundwater As. The above observations led to the accelerated erosion 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, deforestation and overgrazing. The important question is whether the anthropogenic activities of deforestation and overgrazing are significant factors in As contamination of groundwater, especially of the deep (> 200 m) confined aquifer of Kathmandu Valley. One study has used Cl-36 and an assumption regarding the pathway of groundwater recharge to argue that the groundwater of the confined aquifer is 200,000 – 400,000 years old so that present-day land use is irrelevant to water quality. However, another study has shown changes in As from deep wells between the pre-monsoon and the monsoon period, implying that the confined aquifer is recharged over a seasonal time scale. The objective of this study is to test the accelerated erosion model in Kathmandu Valley by (1) comparing chemistry of surface water and deep groundwater (2) comparing chemistry of surface water with bedrock chemistry (3) comparing stable isotopes of surface water and deep groundwater. Water samples were collected from 19 deep (> 200 m) wells and from 19 surface water sites from the rim of the valley upstream from any anthropogenic pollution. Rock samples were collected from 44 sites, representing the eight mapped bedrock units of Kathmandu Valley. Stable isotopes of hydrogen and oxygen in water have been measured using the Picarro Cavity Ringdown Spectrometer. Aqueous concentrations of nitrate, phosphate and sulfate have been measured using the Hach DR-2700 Spectrophotometer. The Optima 8000 ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometer) is being used to measure aqueous concentrations of As and the associated sulfide-forming elements Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti and Zn. The rock samples are being sequentially leached to extract As and heavy metals from five phases, which is being followed by analysis for As and heavy metals using the ICP-OES. Results will be reported at the meeting.