In Chennai (India) public water supply and agriculture depend on groundwater to various extents, but the valuable resource shows increasing salinity over the past decades due to seawater intrusion. This study aims at identifying major hydrogeological processes which lead to salinity ingress in the main aquifer and investigates the effect of MAR structures such as check dams. Regional hydrochemistry is discussed by a combination of stiff diagrams, Cl/Br ratios, ion exchange diagram and stable isotopes (δ18O, δD). The identified hydrogeochemical processes were high saline evolution due to intensive seawater evaporation for commercial salt production and typical ion displacement under refreshening and salinization conditions. Stable isotopes give new insights on (1) mixing processes of different end members (2) occurrence and degree of evaporation in ground- and surface water and (3) isotopical characterisation of groundwater recharge of the region. The identified processes were summarized in a conceptual model of the region. © 2014, Springer-Verlag Berlin Heidelberg.