DELINEATION OF THE TARGETED ARSENIC-SAFE AQUIFERS USING VERTICAL ELECTRICAL SOUNDINGS IN MATLAB, BANGLADESH

Matlab, comprises an area of 400 sq. km with a population of more than 0.5 million is recognized as one of the highly arsenic contaminated areas in Bangladesh. Based on local indigenous knowledge, a shallow oxidized aquifer has been targeted as a suitable mitigation option for safe drinking water supply. In order to delineate the geometry of the targeted oxidized aquifers and identify the risks of cross contamination, vertical electrical soundings (VES) has been carried out at 54 locations covering the study area using Schlumberger configuration with an array of 800 m that provided measurements up to a depth of 400 m.

Based on the interpreted from the geoelectrical sounding curves we have identified five different geoelectric units with resistivity values ranging between 3 and 160 Ω-m. The surface sediments show extreme variability of the resistivity values between 9 to 160 Ω-m caused by variations in the moisture and variable characteristics of the surface sediments. Upper aquifers are dominated by fine sand and gives a value up to 50 Ω-m. Lower aquifers comprising fine to medium sand, possess resistivity values of 20 to 130 Ω-m. These geoelectric units correspond to two-aquifer systems over a major part of the study area and separated by a clayey aquitard. The aquitard is missing in a number of locations and gradually thickens towards north. A lateral variation in the thickness of aquifers and aquitard is also evident. No saline water occurrence has been encountered within the depths explored.