The groundwater age in the Middle-Upper Devonian aquifer system, Lithuania

(3)H, delta(13)C and hydrochemical data were used to estimate the corrected groundwater age derived from conventional (14)C age of dissolved inorganic carbon (DIC). The Middle-Upper Devonian aquifer system from the Baltic upland recharge area in eastern Lithuania towards the discharge area on the Baltic Sea coast in the west was considered. The concentration of total dissolved solids (TDS) in groundwater changes from 300 to 24,000 mg/L and increases downgradient towards the coast. The other major constituents have the same trend as the TDS. The hydrochemical facies of groundwater vary from an alkali-earth carbonates facies at the eastern upland area to an alkali-earth carbonate-sulfate and chloride facies at transit and discharge areas. Meteoric water percolating through the Quaternary and Devonian aquifers regulate the initial (14)C activities of groundwater involving two main members of DIC: soil CO(2) with modern (14)C activity uptake and dissolution of (14)C-free aquifer carbonates. Other sources of DIC are less common. (14)C activity of DIC in the groundwater ranged from 60 to 108 pMC at the shallow depths. With an increase of the aquifers depth the dolomitization of aqueous solution and leakage of the "old" groundwater from lower aquifers take place, traced by lower activities (7-30 pMC).