ARSENIC AND OTHER HEAVY METALS IN THE SUN KOSHI AND SAPTA KOSHI RIVERS, EASTERN NEPAL

Elevated levels of groundwater As have been well-documented in the flood plain of the Ganges River in Bangladesh, West Bengal (India) and the Terai (Indo-Gangetic Plain of southern Nepal). The dominant paradigm for the presence of elevated As is that the reducing conditions in the thick sedimentary package cause release of As from sediment into groundwater. This paradigm has no role for recharge of groundwater by arsenic-enriched rivers and, in fact, fluvial arsenic has been found to be very low in Bangladesh and West Bengal. However, previous studies by the authors have found elevated fluvial arsenic in many tributaries of the Ganges River system in Nepal, upstream of West Bengal and Bangladesh. The objective of this study was to determine where in the Ganges River system arsenic precipitates or becomes adsorbed onto sediments in order to gain insight into the arsenic cycle in South Asia. The objective was accomplished by rafting the Sun Koshi River to the Sapta Koshi River across eastern Nepal from Dolalghat to the Sapta Koshi Barrage at the Indian border (total length of 219 km), and collecting water samples twice daily or whenever a major tributary was crossed, and collecting sediment samples daily for a total of 27 water samples and 11 sediment samples. Electrical conductivity, pH, and temperature were measured on site. The Hach DR-2700 Spectrophotometer was used to measure As and the transition elements that are normally associated with arsenic (Fe, Cu, Mn, Zn, Ni, Co, Cr). Of water samples collected upstream of the confluence of the Sun Koshi with the Sapta Koshi (upstream of the Indo-Gangetic Plain), 45% showed As levels exceeding the WHO standard (As = 0.01 mg/L), which is consistent with previous work that showed 53% of fluvial samples from Nepal outside of the Higher Himalayan Zone exceeding the WHO standard. Water samples collected from the Sapta Koshi River within the Indo-Gangetic Plain uniformly showed no detectable As, suggesting that As is precipitated or adsorbed onto sediment in Nepal at the boundary between the Siwalik Hills and the Indo-Gangetic Plain. Sediment samples will be analyzed for mobile (adsorbed) As by extracting As with 1 M HCl. Further results will be reported at the meeting.