Wjm. Van Der Kemp et al., Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: The Tertiary Ledo-Paniselian aquifer in Flanders, Belgium, WATER RES R, 36(5), 2000, pp. 1277-1287
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.