Preliminary Hydrological and Hydrochemical Assessment of Arsenic Contaminated Groundwater of Nawalparashi District of Nepal

The Nawalparashi district of Nepal lies in the headwaters of the South Asian arsenic crisis region. Strong heterogeneity in groundwater arsenic distribution is characteristic of this area for uncertain reasons. This study investigates the role of localized hydrologic differences in generating that heterogeneity by affecting the hydrochemical environment and arsenic mobility.

Six nested piezometer stations were established to monitor water levels, major cations and anions, arsenic, and iron on a monthly basis. Four of these stations monitor both shallow and deeper aquifers. Monitoring in three existing wells brings the total to ten observation points in three dimensions. Three river points were also monitored for hydrochemical parameters and two were also monitored for discharge. Based on water level variations and monsoon response these observation wells can be categorized into two groups. One group (Group A) has the greatest and quickest monsoon response, representing wells at depths less than 40 m. Other group (Group B, consisting of wells greater than 40m depth) has a delayed and subdued response to monsoon. Each multi-level site exhibits a downward hydraulic gradient.

All wells have bicarbonate dominated water. Group B has sodium dominated bicarbonate whereas most of Group A has calcium dominated bicarbonate water. The highest sodium to calcium ratio is observed in Group B, strongly suggesting a cation exchange process is active in intervening clays. Seven out of eight wells in Group A and one out of two wells in Group B have arsenic greater than WHO guideline value of 10 ppb. Arsenic was below detection limits in the river points. The relationships between major cations/anions, arsenic and water levels remain uncertain. Observation for a full hydrologic year should resolve these correlations with arsenic concentration, and planned geophysical surveys between observation points should resolve hydrostratigraphic correlations between observation points.