An attempt has been made to identify the chemical processes that control the hydrochemistry of groundwater in the coastal aquifers of Thiruvallur coastal village of Thiruvallur district, Tamil Nadu, south India. The parameters such as pH, EC, TDS and major ion concentrations of Na, K, Ca, Mg, Cl, HCO 3 , SO 4 and NO 3 of the groundwater were analyzed. Abundances of these ions are in the following order Na [Ca[Mg [K and HCO 3 [ Cl[SO 4 [NO 3 . The dominant water types are in the order of NaCl[ mixed CaMgCl [ CaHCO 3 [ CaNaHCO 3 . Water types (mixed CaHCO 3, mixed CaMgCl and NaCl) suggest that the mixing of high salinity water caused from surface contamination sources such as irrigation return flow, domestic wastewater and septic tank effluents with existing water followed by ion exchange reaction processes, silicate weathering and evaporation are responsible for the groundwater chemistry of the study area. The above statement is further supported by Gibbs plot where most of the samples fall within the evaporation zone.

The groundwater is the main resource of water for irrigation activity in river lacking area. The freshwater-seawater interface in the study region that has existed 3 km away from the coast in the year 1969 has been found to be migrated to distance of 13 km from the coast during the year 2007 noticed by the Central Ground Water Board (Central ground water board's district groundwater brochure, Thiruvallur district, Tamil Nadu, 2007). Integrated geochemical and geophysical techniques were carried out in the study area to decode subsurface geologic pattern and delineate the seawater-freshwater zones. Total numbers of 50 samples were collected from the entire study area and analyzed for major ions. The considerable samples are brackish scenery of groundwater water at low depth. Chadha and Piper's plots categorize the coastal groundwater into Na-HCO 3 , Ca-Na-HCO 3 , Ca-HCO 3 , and Na-Cl water facies, with Ca-HCO 3 as the dominant. Cl/CO 3 + HCO 3 ratio, Cl/ HCO 3 , and ionic strength, Mg/Ca and Cl/HCO 3 ratios show that most of the samples in the study area are affected by seawater intrusion, which is also confirmed by the geophysical method. The results of vertical electrical sounding carried out in the study area reveal the low transverse resistance and high longitudinal conductance. It suggests the brackish nature of the groundwater in the eastern part of the study area may be due to the seawater intrusion. The final map using GIS platforms productively delineates the location that is really undergoing seawater and freshwater zone is migrated toward the inland. The article suggested further studies to arrest the migration of sea/freshwater interface into the land and avoid overexploitation of groundwater to further development.

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