Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: The Tertiary Ledo‐Paniselian Aquifer in Flanders, Belgium

Abstract

Groundwater samples from the Ledo‐Paniselian aquifer have been interpreted for chemical reaction patterns, 14 C 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 14 C ages of the water samples range from 3 to over 40 ka. Inverse chemical modeling was done to correct the 14 C 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‐, NH 4 ‐, Mg‐, and Ca‐HCO 3 water types. The pattern is a result of freshening: Ca displaces the saline cations Na, K, NH 4 , and Mg from the aquifer's cation exchange complex in a Chromatographic sequence. The loss of Ca 2+ from solution by cation exchange is by far the most important reaction for dissolution of calcite, which increases the apparent 14 C age of the water samples. The 14 C age furthermore depends on open/closed conditions of calcite dissolution and CO 2 gas exchange and CO 2 pressure in the recharge area. It is shown that δ 13 C and CO 2 pressure in a soil are interrelated and that the changes in CO 2 pressure can be included in an inverse model which considers variations in infiltration water quality. The overall correction for 14 C age is obtained by inverse modeling of water quality and δ 13 C, with optimization on CO 2 pressure in recharge water using PHREEQC [ Parkhurst , 1995]. The optimized CO 2 pressure for the recharge area varies with age and is generally lower in the water samples with an age above 13 ka. The lower CO 2 pressure is corroborated by lower δ 18 O values of the water.