ARSENIC AND OTHER HEAVY METALS IN THE RIVERS OF NEPAL

Elevated levels of groundwater As have been found in the Terai region (Indo-Gangetic plain) of Nepal with 24% of wells showing As concentrations above the WHO standard (10 µg/L). The objective of this study was to determine the source of As by measuring concentrations of As, Co, Cu, Fe, Ni and Zn in rivers that drain into the Terai. Concentrations were measured using portable spectrophotometers and the Hach As test kit in 303 samples collected from 120 locations over a three-year period. In samples collected where rivers dissected alluvial sediments of the Terai, sedimentary rocks of the Siwalik Group, or low-grade metamorphic rocks and carbonates of the Lesser Himalaya, 55% of samples had As concentrations ≥ 10 µg/L. For these samples, elevated levels of Co, Cu, Fe and Ni, a low level of Zn, and lack of correlation between As and any of the above elements indicates that there are multiple sources of As associated with mineralization of Co, Cu, Fe and Ni, but not Pb-Zn. Outside of the Kathmandu Valley, fluvial As decreased when pH decreased, due to the increase in the number of positively-charged sorption sites on river bed sediment. Inside of the heavily polluted Kathmandu Valley, fluvial As increased when pH decreased, due to the organic complexation of As and the negative correlation between organic matter and pH. In samples collected in the Khumbu (Everest) region where rivers dissected the high-grade metamorphic rocks of the Higher Himalaya, <1% of samples had As concentrations ≥ 10 µg/L. It is unlikely that As was present in fluvial sediment in the Khumbu region because (1) As was undetectable even when Co, Cu, Fe, Ni or Zn was high indicating that As was not present in crystalline form, (2) pH did not correlate with any of the measured elements indicating that sediments were poor in sorption sites. The result that Higher Himalayan rocks are not a source of As is consistent with the general low level of As in schists and gneisses compared with phyllites, slates, carbonates and clastic rocks. Comparison of elemental concentrations inside the Kathmandu Valley with the rest of central Nepal indicates that elevated levels of As, Co, Cu and Ni inside the Kathmandu Valley are naturally occurring, but elevated levels of Fe are not naturally occurring.