ELECTRICAL RESISTIVITY PROFILING TO DELINEATE LOW GROUNDWATER ARSENIC TARGET ZONES IN THE TERAI, NAWALPARASI, NEPAL

The pandemic Asian Arsenic Crisis (widespread groundwater arsenic poisoning in South Asia) continues to expand, and a new source of safe drinking water is required for over 100 million people in the Ganges floodplain alone. The groundwater arsenic distribution is extremely heterogeneous, and in Bangladesh at the mouth of the Ganges drainage, a correlation has been noted between fine-textured surficial sediments and high arsenic content in shallow wells. Our study in the Ganges headwater areas of Nepal reveals a similar correlation in the vertical direction between groundwater arsenic and clay content at well screen depth, independent of surficial sediment texture. This sediment texture-arsenic content correlation offers the potential for a proactive, long-term solution to the Asian Arsenic Crisis. Arsenic avoidance strategies based on non-invasive surveys of aquifer texture could be used to delineate target zones of probable low groundwater arsenic prior to well drilling.

The sediment texture-arsenic content correlation depends on redox control of arsenic mobility, and such control has been demonstrated to be the dominant mechanism of arsenic release in South Asia. Presumed rapid infiltration of oxidizing waters in coarser sediments limits reductive desorption of arsenic, resulting in low groundwater arsenic concentrations. In this preliminary study a 2-km 2-D electrical resistivity profile was made across an arsenic hot-spot in Nawalparasi, the highest arsenic district of Nepal. Resistivity was calibrated to aquifer lithology by anchoring the profile at the USGS Kasiya lithologic core hole. Groundwater arsenic measurements from a previous blanket testing program (ENPHO) were used to determine arsenic at well total-depth (TD). Out of 240 wells, 75% with resistivity < 100 ohm-m (significant clay content) at TD exhibited high arsenic (> 150 ppb), while 75% of zones with higher resistivity contained arsenic < 150 ppb. Arsenic below 150 ppb can be remediated with extremely low-cost solar oxidation (SORAS) using household materials, and may be the most realistic target threshold for resistivity surveys. Presumably at least in headwater regions coarser sediments at depth are hydraulically well-connected to the surface, and therefore support relatively oxidizing conditions.