Geo-electrical approach to determine the lithological contact and groundwater quality along the KT boundary of Tamilnadu, India

A geophysical survey involving electrical resistivity method has been carried out at Ariyalur region to delineate the geo-electric characteristics of the subsurface formation and evaluate the groundwater quality variation along the lithological contact. A total of 15 vertical electrical sounding (VES) stations were established along the various geological formation such as 3 in Archean, 7 in Cretaceous, 2 in Tertiary and 3 in Quaternary formation. The data acquisition was carried out by adopting Schlumberger configuration. The layer parameters like apparent resistivity (ρa) and thickness (h) of different layers were arrived. The results of interpretation revealed three distinct geoelectric layers, which is comprised of weathered, fractured and massive layer in hard-rock region and topsoil, sandy layer and clayey layer in sedimentary region. Sounding curves obtained in the area are mostly of A, H, K type. The 1st layer thickness shows a decreasing trend in the following order Tertiary > Alluvium > Achaean. However, for the 2nd layer, the trend is Achaean > Tertiary > Alluvium. The higher 1st layer thickness along the southeastern part of the study area is due to the overexploitation of the groundwater represented by alluvium. Resistivity values and their corresponding depth for some selected VES locations were used in producing three resistivity cross sections (A–A′, B–B′ and C–C′), which shows the geoelectric distribution of the subsurface. The observed high resistivity in a shallow depth in the western part is due to the variations in lithology. The relationship between the resistivity and conductivity are also taken into consideration to identify the shallow contaminated zones of the study area.