TEMPORAL VARIABILITY OF GROUNDWATER ARSENIC AND ITS LINK TO AQUIFER PROPERTIES, NAWALPARASHI, NEPAL

The Nawalparashi district of Nepal lies in the headwaters of the South Asian Arsenic Crisis region, and exhibits strong temporal variability in groundwater arsenic concentrations. Such variability may severely complicate arsenic mitigation strategies pursued in Nepal and elsewhere in South Asia. This study characterizes temporal arsenic variability and other hydrologic parameters at 10 monitoring points in the Ganges floodplain of Nepal, evaluating the role of localized hydrologic differences in generating that variability.

Six clustered piezometer stations were established to monitor water levels, major cations and anions, arsenic, iron and phosphate on a monthly basis. Three river points were also monitored for hydrochemical parameters and two were also monitored for discharge. Similar to other South Asia sites, water levels in shallow wells (< 30m) in Nawalparashi respond quickly and with larger magnitude to monsoon recharge compared to delayed and muted response in deeper wells (30-50m depth). Each multi-level site exhibits a downward hydraulic gradient. Arsenic in the observation wells exhibits a regular annual variation, with ~100ppb concentrations at all levels during the monsoon period, followed by rising concentrations in shallower wells to ~200 ppb during the dry season.

At the study sites temporal variability of arsenic is inversely correlated with hydraulic diffusivity. Diffusivity in other regions, e.g. Chaudanga and Jhenaidah, Bangladesh (Japan International Cooperation Agency) is about 4 times higher, with little temporal variability in arsenic. Similarly, slug tests indicate lower hydraulic conductivity in Nawalparashi by 2-3 orders of magnitude, e.g. compared to Araihazar wells in Bangladesh. The temporal variation appears to reflect monsoonal dilution, followed by rapid return to dry-season high arsenic conditions in most shallow wells in Nawalparashi.