35.-Delineation of contaminated zone through electrical imaging technique- A case study near Dindigul, Tamilnadu

Abstract

Electrical imaging technique was found to be a powerful tool to delineate sub-surface contaminated zone, when there is sufficient resistivity contrast. Electrical tomography (imaging) involves measuring a series of constant separation traverses with the electrode spacing being increased with each successive traverse. Since increasing separation leads to information from greater depths, the measured apparent resistivities will be used to construct a vertical continued section (pseudo section) displaying the variation of resistivities both laterally and vertically over the section. Normally pseudo section contains geometrical effects, geological noise and the distorting effects of near surface lateral changes in resistivities, which occur close to the electrodes (electrode effects). In order to remove geometrical effects as well as to produce an image of true depths and true formation resistivities, inversion technique is used. The inversion technique described by Loke and Barker (1995) was used in the present study for processing pseudo section data to provide contoured image of true depths and true formation resistivity.

Groundwater in and around Dindigul town (Tamilnadu) was contaminated due to untreated tannery effluents. Five profiles were used to measure the apparent resistivity of sub-surface soil. Imaging produced in that condition is difficult to interpret as the resistivities of the weathered overburden may vary due to degree of weathering, saturation and presence of contaminated water. In general, surveys in the shallow basement areas suggest that the regolith (weathered overburden) is characterized by resistivities in the range of 50 to 200 Q m, whereas in the neighbourhood of tanneries, it is expected thick layer of contaminant zone. In that case, the resistivity of shallow regolith might be expected to have weathered to a greater degree and, therefore. exhibit a resistivity of less than 50 S2 m. Since the water table is shallow and the regolith is largely saturated, any resistivity much below 20 S2 m will indicate groundwater contamination, in the absence of clay. Out of 5 profiles carried out in the polluted area, 4 profiles have shown that the top 10-15 meters of regolith has resistivity of less than 10 SI m with top 5 m having a resistivity of less than 4 Q m. This has clearly indicated that the soil is strongly contaminated. One profile has shown low resistivity at low topography of profile and resistivity of above 100 S2 m in the elevated area is not contaminated. Thus this technique provides useful and interesting information about the polluted zone.