0016-7622/2016-87-2-169/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.87, February 2016, pp.169-178 1D Geoelectrical Resistivity Survey for Groundwater Studies in Coastal Area: A Case Study from Pearl City, Tamil Nadu S. SELVAM Department of Geology, V.O. Chidambaram College, Tuticorin – 628 008, India Email: geoselvam10@gmail.com Abstract: Geoelectrical resistivity method involving vertical electrical sounding (VES) was carried out in a sedimentary environment to determine the suitability of the method for sub-surface groundwater investigations. The EC and TDS hydrochemical data in the study area clearly showed the influence of seawater intrusion. The abundance of the major cations and anions are in the following order, Na + > Ca 2+ > Mg 2+ > K + = Cl - > HCO 3 - > SO 4 2- > CO 3 > NO 3 > PO 4 . Results suggest that the groundwater in this study area is very hard and alkaline in nature. As indicated by Piper trilinear diagram, NaCl and Ca 2+ - Mg 2+ – Cl - - SO 4 2- facies are the dominant hydrochemical facies in the groundwater of Pearl city. The VES method by Schlamberger electrode array was applied in 12 locations, which is expected to represent the whole area. The resistivity meter (aquameter CRM 5OO) was used to collect the VES data by employing a Schlumberger electrode configuration, with half current electrode spacing (AB/2) ranging from 2 to 180 m and the potential electrode (MN) from 1 to 50 m. The resistivity data is then interpreted by WINSEV 1-D inversion program geoelectric software to entirely describe the aquifer system as well as the occurrence of groundwater. The outputs of sub-surface layers with resistivities and thickness presented in contour maps and 2-D views by using SURFER software were created. Accordingly, three zones with different resistivity values were detected, corresponding to three different formations: (1) a transition zone of sandy soil (aeolian deposits) thick formation, (2) strata’s saturated with fresh groundwater in the east disturbed by the presence of sandy shell limestone horizons, (3) a water-bearing formation in the west containing low saltwater horizons. The bedrock is encountered at an average depth of 95m. This study indicates that the groundwater reservoirs are mainly confined to the alluvial aquifer. Keywords: VES, Schlumberger electrode, WINSEV, Sandy Shell limestone, Water-bearing formation, Tamil Nadu. INTRODUCTION Due to rapid urban development in Pearl city, Tamil Nadu, India and the adjoining places, there is an increase in demand for the utilization of groundwater. Because of the over exploitation of groundwater, the groundwater level has declined in recent times which warrants groundwater assessment for sustainable utilization within the study area. An adequate supply of drinking water is one of the pre- requisites for every type of developmental programme. For this reason the efforts connected with the location, development and conservation of groundwater supplies are of a fundamental economic importance for any country. Groundwater supplies like any other kind of buried natural resources of the earth are becoming progressively more difficult to locate (Selvam et al. 2010). Accordingly any new technique which will assist in the location of borehole site and eliminate the sinking of unproductive wells in large numbers is of a great value. Complementing the more traditional water finding methods, modern geophysical investigation techniques are more capable for locating water bearing formations than the older conventional methods. (Chidambaram et al. 2009; Devi et al. 2001; Muthuraj et al. 2010). The most commonly employed geophysical method in groundwater exploration are electrical resistivity and seismic (mostly refraction) techniques. Other methods such as gravity and magnetic are used for mapping the concealed features of the bed rock which is an indirect means of locating favourable areas for groundwater. Geophysical electrical resistivity methods due to simplicity of its technique has been extensively used for groundwater investigation by many workers (Majumdar et al. 2000; Bhattacharya and Patra 1969; Oluronfemi and Fasuyi 1993; Kumar and Riyazuddin, 2008; Srinivasamoorty et al. 2011) and considered to be the most suitable method for groundwater investigation in most geological environment. The Vertical Electrical Sounding (VES) method has been chosen for this study because the interpretation, field works are simple and analysis of data is