HYDROGEOLOGY OF A GROUNDWATER ARSENIC HOTSPOT: THULOKUNUWAR VILLAGE, NAWALPARASHI , NEPAL

Thulokunuwar village of Nawalparashi district lies in the most severely arsenic-affected area of Nepal, and 76 % of its handpump tubewells have arsenic concentration greater than 10 µg/l (WHO Guideline Value). As a short term mitigation measure to address this problem, villagers are encouraged to share safe tube wells for drinking and cooking purposes and are provided with arsenic removal filters coupled with education about the risk of arsenic. A permanent solution requires better understanding of local hydrology and hydrochemistry.

To evaluate the hydrology of arsenic mobilization at Thulokunuwar village, observation wells were installed and the piezometric surface was observed for 10 months. Two aquifers were identified at 14.5-16.5 m and 44.5-46.5 m depth with recharge areas 4.5 km and 9.5 km respectively to the north in coarse sediments at the base of the Himalayan foothills. The maximum average linear horizontal velocities for the shallow and deep aquifers of the Thulokunuwar area have been estimated as 2.7*10^-3 m/d (~1 m/y; range 0.175-1.001 m/y) and 1.8*10^-3 m/d (0.66 m/y; range 0.117-0.664 m/y). This difference arises from the presence of coarser grained sediments in the shallow aquifer.

A preliminary ground water volume balance indicates the existence of vertical ground water movement due to infiltration. The vertical flow component is approximately 3.5*10^-6 m/d and 2.9*10^-6 m/d (1.3-1.1 mm/y) in the shallow and deep aquifers respectively. The presence of numerous clay interbeds is responsible for this reduction in vertical velocities relative to horizontal.

Weak (R²=0.2839) to very weak (R²=0.028) correlation was observed between arsenic concentration and seasonal potentiometric water level variation for the shallow and deep aquifers respectively. Relatively stronger correlation in the coarser shallow aquifer suggest greater impact of seasonal (monsoon) flushing of pore waters in that aquifer.

Additional monitoring is planned using wells arranged in two orthogonal 10-15 kilometer profiles to better characterize the local hydrology and geochemistry of this arsenic hotspot. Pump testing will be used to quantify aquifer properties.