Water chemical and isotopic composition (deuterium content and oxygen-18), as well as the activity of tritium in groundwater of Riphean aquifer and Vendian complex in the Karelian Isthmus were studied. Estimation of recharge rate for groundwater of the Vendian complex by tritium yielded groundwater ages up to 100 years for this area, which is located not far from Central Karelian watershed. For the area where the Vendian complex is overlapped by the Kotlin aquiclude, water age varied from several hundreds to several thousands years as determined by the radium/radon method. Isotopic data (δ²H and δ¹⁸O) indicate that the Vendian complex has no recharge there, and the complex is therefore protected from contamination from the surface, and at the same time its resources are mainly supplied by lateral inflow. The Ladoga graben is apparently dominated by slow water exchange regime, given that at a depth of 250 m relict waters of the Baltic Ice Lake were identified by isotope and hydrochemical criteria.