Use of geophysical and hydrochemical tools to investigate seawater intrusion in coastal alluvial aquifer, Andhra Pradesh, India

India has a very long coastline and 25 % of the country’s population live in the coastal zone. Urban centers are located along the coast and three out of four metro cities are located on the coast. The high population density along the banks of major rives and coast Increasing population and demand for water putting the coastal aquifers under stress and causing sea water inrush and salinity upcoming in the coastal aquifers. Apart from sea water contamination, urban waste releases and agriculture inputs threatening the coastal groundwater aquifer systems. Generally coastal areas receive more pollutant loads from different sources including geogenic and anthropogenic sources. Central Godavari delta is located adjacent to the Bay of Bengal Coast, Andhra Pradesh, India and is drained by Pikaleru, Kunavaram and Vasalatippa drains. The area is occupied by recent Quaternary alluvium and gone through a series of marine transgression and regression. The entire study area comes under Godavari central canal command area, water is available throughout year except first week of June and last week of April in the canals. Water requirements for irrigation met from surface water in the delta. There is no groundwater pumping for agriculture as wells as for domestic purpose due to brackish nature of the groundwater at shallow depths. The groundwater depths varying from 0.8 to 3.4 m dug wells and in bore wells located near the coast 4.5–13.3 m. The established groundwater flow direction is to be towards Bay of Bengal from Amalapuram. Geophysical and hydrochemical tools were applied to identify the source of the salinity and to assess the saline water intrusion in the Godavari delta. Electrical Resistivity Tomography (ERT) surveys were carried out at several locations in the deltaic region to delineate the aquifer geometry and to identify saline water aquifer zones. The results inferred from ERT indicate 12–15 m thick loamy sands were existed from surface to subsurface and it is followed by 18–25 m thick clay layers. The thickness of clay is being increased toward Sea from inland. The low resistivity values in the delta are attributed to existence of the thick marine clays in the subsurface and relative high resistivities are attributed to existence of fresh water. The resistivity values similar to saline water <0.01 Om is attributed to the mixing of the saline water along surface water drains. In the Ravva Onshore Terminal low resistivity values indicated up coning of brines and mixing of saline water from Pikaleru drain. Groundwater samples were collected and analyzed for major ions (pH, EC, Ca2+, Mg2+, Na+, F-, HCO3 2-, Cl-, SO4 2-, NO3 -). The elevated TDS, Na+ and Cl is due to dilution of clay minerals upstream and in the downstream mixing of sea water along the drains in the pre monsoon. The quality is being increase in the post monsoon season. The molar ratios of Na+2/Cl (>0.86) and SO4 -2/Cl- (<0.05) in the pre monsoon indicated strong influence of sea water and in the post monsoon increased Na+2/Cl- and SO4 -2/Cl- (>0.05) indicated marine palaeo salinity, dilution of marine clays and dissolution of evaporites. The high SO4 -2/Cl in the post monsoon is attributed to dilution groundwater salinity due to rainfall infiltration and irrigation return flows in the delta. The low Na+2/Cl- ratios in upstream of the delta are due to sand exposures and isolated fresh water lances in the perched aquifers.