Application of electrical resistivity techniques to identify areas of groundwater contamination

Abstract

ABSTRACT : The electrical resistivity method has been used to investigate possible groundwater contamination from two different sources at two sites. The first in a coastal aquifer is due to the intrusion of seawater and the second in an aquifer underlying a landfill site is due to possible migration of leachate. The effectiveness of several different electrode configurations for carrying out Vertical Electrical Sounding (VES) has been investigated in relation to the detection of a saline interface. The Offset Wenner technique was found to be the best option in terms of ease of use, recognising lateral problems and providing error estimates. The Schlumberger array was considered as the next best alternative if the switching box and muliticore cable required for the offset Wenner technique was unavailable. Investigations on the coastal aquifer site using VES, Wenner pseudosection and Sclumberger AB profiling techniques enabled the lateral variations of the resistivity to be mapped. The VES data provide reasonable vertical resolution at a relatively few sounding locations. The pseudosection data provided good horizontal resolution with a limited depth of penetration (6 to 8 metres). The Schlumberger AB profile showed the lateral changes in apparent resistivity over a small rectangular area and averaged over a depth of about 50m. High resistivity areas were correlated to elevated areas of the site which had a surficial layer of dry sandy gravel overlying alluvium. The lowest resistivity areas occured in the lower regions of the site and were correlated with the alluvium, Repeated VES and pseudosection surveys were conducted to investigate any temporal changes in groundwater conductivity due to the effects of tidal pumping. No significant systematic changes with time were observed. On the landfill site VES and pseudosection techniques were used and proved capable of detecting the presence of leachate within the waste tip. Leachate contamination away from the base of the waste tip was observed. Some of the plumes of leachate could be correlated with leaks from the side of the tip which had spread out on the surface adjacent to the tip. The use of these different techniques has given the auther some confidence in their application for delineating and mapping interfaces between fresh and saline water in aquifers in Bahrain.