TEMPORAL VARIABILITY OF GROUNDWATER HYDROCHEMISTRY IN AQUIFERS OF NAWALPARASHI, 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 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. 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 30 m. Other group (Group B, consisting of wells having around 50m 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. Six 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 in all time series data. Arsenic was below detection limits in the river points. Total dissolved solids (TDS) in all wells declined to an almost uniform value of 650 ppm by monsoon end, indicating thorough flushing. TDS then increases steadily in shallow zones during the dry season up to about 1000 ppm which is probably due to poor sanitation. Considerable variability is observed in major cations/anions, arsenic and water levels and their relationships remain uncertain. Lack of correlation between Arsenic and Sulphate indicates that Pyrite oxidation is not the cause for Arsenic release. Good correlation between Arsenic and Phosphate shows that Phosphate is not exchanged for Arsenate. Arsenic exhibits good correlation with bicarbonate and total iron indicating that reductive dissolution of ferric oxyhydroxides is the cause for Arsenic release in the wells having Arsenic greater than WHO guideline values.