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

Groundwater samples from the Ledo-Paniselian aquifer have been interpreted for chemical reaction patterns,C-14 age, and recharge conditions. This conf ined Tertiary aquifer dips NNE from its outcrop in Belgium toward the North Sea over a length of similar to 50 km, Conventional C-14 ages of the water samples range from 3 to over 40 ka, Inverse chemical modeling was done to correct the C ages for the chemical reactions in the aquifer, while account ing 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 freshenin g: Ca displaces the saline cations Na, K, NH4, and Mg from the aquifer's ca tion exchange complex in a chromatographic sequence. The loss of Ca2+ from solution by cation exchange is by far the most important reaction for disso lution of calcite, which increases the apparent C-14 age of the water sampl es. The C-14 age furthermore depends on open/closed conditions of calcite d issolution and CO2 gas exchange and CO2 pressure in the recharge area. It i s shown that delta(13)C and CO2 pressure in a soil are interrelated and tha t the changes in CO2 pressure can be included in an inverse model which con siders variations in infiltration water quality The overall correction for 14C age is obtained by inverse modeling of water quality and delta(13)C, wi th optimization on CO2 pressure in recharge water using PHREEQC [Parkhurst, 1995]. The optimized CO2 pressure for the recharge area varies with age an d is generally lower in the water samples with an age above 13 ka, The lowe r CO2 pressure is corroborated by lower delta(18)O values of the water.