Abstract: The lower reaches of the Sabarmati River Basin in Gujarat have intense agricultural and industrial activities and this part is affected by problems of groundwater salinity. Here we attempt to assess the processes governing the causes of groundwater salinity in the coastal alluvial aquifer, employing δ18O and δD isotopes in integration with ionic ratio. The different hydrochemical facies such Na–Mg–HCO3–Cl, Na–Cl–SO4, Na–Mg–Cl–HCO3–SO4 and Na–Cl of groundwater show the occurrence of complex geochemical phenomenon in the study area. Ionic ratio (such as Mg2+/Ca2+, Na+/Cl−, SO24/Cl-, K+/Cl−) and isotopic composition (δ18O and δD) of groundwater indicate that while in coastal areas seawater intrusion is taking place, in inland areas various anthropogenic activities and overexploitation have induced salinity in groundwater. Over-pumping of groundwater has also induced lateral intermixing of highly saline water in the vicinity of coastal areas with relatively fresh/low saline groundwater along specific flow pathways.