Groundwater samples from the Ledo-Paniselian aquifer have been interpreted for chemical reaction patterns, 14C age, and recharge conditions. This confined Tertiary aquifer dips NNE from its outcrop in Belgium toward the North Sea over a length of ~50 km. Conventional 14C ages of the water samples range from 3 to over 40 ka. Inverse chemical modeling was done to correct the 14C ages for the chemical reactions in the aquifer, while accounting for changes in the recharge water quality during the Holocene and late Pleistocene. The aquifer shows a zonal pattern with (going upstream) Na-, K-, NH4-, Mg-, and Ca-HCO3 water types. The pattern is a result of freshening: Ca displaces the saline cations Na, K, NH4, and Mg from the aquifer's cation exchange complex in a chromatographic sequence. The loss of Ca2+ from solution by cation exchange is by far the most important reaction for dissolution of calcite, which increases the apparent 14C age of the water samples. The 14C age furthermore depends on open/closed conditions of calcite dissolution and CO2 gas exchange and CO2 pressure in the recharge area. It is shown that Δ13C and CO2 pressure in a soil are interrelated and that the changes in CO2 pressure can be included in an inverse model which considers variations in infiltration water quality. The overall correction for 14C age is obtained by inverse modeling of water quality and Δ13C, with optimization on CO2 pressure in recharge water using PHREEQC <Parkhurst, 1995>. The optimized CO2 pressure for the recharge area varies with age and is generally lower in the water samples with an age above 13 ka. The lower CO2 pressure is corroborated by lower Δ18O values of the water. ¿ 2000 American Geophysical Union