Arsenic and other heavy metals in the rivers of central Nepal

Abstract

The objective of this study was to measure fluvial As, Co, Cu, Fe, and Zn in order to determine whether central Nepal has a geographically-limited source of As. Seventeen rivers in central Nepal outside and eight rivers inside the heavily polluted Kathmandu valley were sampled monthly for six months. Outside the Kathmandu valley, fluvial As (11 ± 1 mg/l), Co (110 ± 30 mg/l), Cu (93 ± 4 mg/l), Fe (550 ± 80 mg/l), and Ni (50 ± 3 mg/l) were over 5, 550, 13, and 13 times the global averages for each respective element, while Zn (27 ± 4 mg/l) was very close to the global average. The only statistically significant differences between inside and outside the Kathmandu valley were pH (inside: 7.13 ± 0.06, outside: 8.13 ± 0.06), Fe (inside: 1060 ± 90 mg/l) and Cu (inside: 70 ± 10 mg/l) so that Co, Cu, Ni, and Zn levels inside the Kathmandu valley were all naturally occurring. Fluvial As was correlated both in space and time with pH. Outside 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 the 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. Central Nepal has multiple sources of As associated with mineralisation of Co, Cu, Fe, and Ni, but not Pb-Zn.