DC Resistivity Studies for Mapping Groundwater Contamination in and around Ash-Disposal Site of Kolaghat Thermal Power Plant, West Bengal

Coal combustion in thermal power plants contributes to more than 70% of the e1ectricity generated in India. Combustion of coal generates huge amounts of wastes which are disposed off in nearby ponds. The ash generated from the coal combustion process contains a lot of toxic elements which are leached out by the percolating rain water and subsurface runoff from the ash ponds. These toxics cause sufficient amount of groundwater contamination.

DC resistivity studies are very helpful in determining the subsurface lithology and in delineating the zones of groundwater contamination. The present study was conducted in and around the ash disposal sites of Kolaghat Thermal Power Plant in West Bengal. Six Schlumberger soundings were conducted in the four ash ponds of the thermal power plant. The quantitative interpretation of the VES curves has been done by a very fast simulated annealing computer program developed for this purpose. The subsurface lithology as determined from the VES curves shows alternate layers of clay and sand upto a depth of 6Om. All the sounding locations show a general thin top layer of ash of high resistivity mixed with coarse sand or clay followed by a 29-50m thick layer of wet clay of very low resistivity. The lowermost layer is sandy clay to clayey sand in all the cases. Groundwater samples have also been collected from the tube wells located near the ash ponds and analysed for pH, TDS and trace elements. Results of the chemical analysis show high values of TDS and high concentration of the toxics. Earlier studies on the geochemistry of the pond ash have shown that the ashes are characterized by hlgh concentration of As, Pb, Cu, Ni, Fe, Zn, Cr, CO, Mn all of which exceed the crustal abundance by a factor of 3-5. The high concentration of the toxics (As, Al, Li, As, Zn, Ag, Sb, Si, Mo, Ba, Rb, Se, Pb) in the water samples implies significant input from the ash pile due to leaching. This increases the TDS values of the water. The low resistivity values of the intermediate clayey layer along with the high values of TDS and high concentration of the toxics imply that the contaminant zone lies at a shallow depth of 29-50m thick. It is formed due to leaching of the toxics from the top sandy layer and adsorption in the clayey layer.