Journal - Correlation of electrical resistivity and groundwater arsenic concentration, Nawalparasi, Nepal

Correlation of electrical resistivity and groundwater arsenic concentration, Nawalparasi, Nepal

Tom H. Brikowski, Linda S. Smith and Tai-Chyi Shei

Geosciences Department FO-21, University of Texas at Dallas

P.O. Box 830688, Richardson, TX 75083-0688, USA

Suresh Das Shrestha

Central Department of Geology, Tribhuvan University, Kathmandu, Nepal

Jour. Nep. Geol. Soc., Vol. 30, 2004, 99-107


Abstract

The Asian Arsenic Crisis has expanded into the headwaters of the Ganges River, now including the plains (Terai) of Nepal.This study seeks a non-invasive predictive tool to estimate groundwater arsenic concentration prior to drilling, enabling “arsenic avoidance” in contaminated areas. Detailed chemical studies indicate that in Himalayan-sourced aquifers arsenic is released by microbially-mediated redox reactions. Likely hydrogeological settings for oxidising chemical conditions (immobile arsenic) should be more porous (higher infiltration rate for oxygenated waters) and contain fewer fine organic sediments (oxygen-consuming material). Both conditions should yield higher electrical resistivity, and such aquifer heterogeneity effects should be most prominent in headwater regions such as Nepal. To test this approach, a series of vertical electrical resistivity soundings were made near Parasi, Nepal, constituting a profile extending 2 km across a known high-arsenic area. Correlation of the horizontal and vertical distribution of measured resistivity and ENPHO groundwater arsenic measurements demonstrated a distinct inverse relationship between these variables. Out of 240 arsenic sample points, 75% of those extracted from high resistivity zones (>100 ohm-m, inferred lower clay content) exhibited arsenic <150 ppb. Conversely, 75% of samples from low resistivity zones exhibited arsenic >150 ppb. Given these preliminary results, the resistivity technique appears to hold great promise as a predictive tool for finding low-arsenic groundwater zones within contaminated areas, thereby allowing “well-switching” from highly toxic to new safe or more readily treatable wells. The method should be applicable in most circum-Himalayan high-arsenic areas.

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