A new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution: The Ledo-Paniselian Aquifer, Belgium

Blaser, P.C.; Coetsiers, M.; Aeschbach-Hertig, W.; Kipfer, R.; Van Camp, M.; Loosli, H.H.; Walraevens, K. (2010). A new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution: The Ledo-Paniselian Aquifer, Belgium. Applied geochemistry, 25(3), pp. 437-455. Pergamon 10.1016/j.apgeochem.2009.12.011

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The particular objective of the present work is the development of a new radiocarbon correction approach accounting for palaeoclimate conditions at recharge and hydrochemical evolution. Relevant climate conditions at recharge are atmospheric pCO2 and infiltration temperatures, influencing C isotope concentrations in recharge waters. The new method is applied to the Ledo-Paniselian Aquifer in Belgium. This is a typical freshening aquifer where recharge takes place through the semi-confining cover of the Bartonian Clay. Besides cation exchange which is the major influencing process for the evolution of groundwater chemistry (particularly in the Bartonian Clay), also mixing with the original porewater solution (fossil seawater) occurs in the aquifer. Recharge temperatures were based on noble gas measurements. Potential infiltration water compositions, for a range of possible pCO2, temperature and calcite dissolution system conditions, were calculated by means of PHREEQC. Then the sampled groundwaters were modelled starting from these infiltration waters, using the computer code NETPATH and considering a wide range of geochemical processes. Fitting models were selected on the basis of correspondence of calculated δ13C with measured δ13C. The 14C modelling resulted in residence times ranging from Holocene to Pleistocene (few hundred years to over 40 ka) and yielded consistent results within the uncertainty estimation. Comparison was made with the δ13C and Fontes and Garnier correction models, that do not take climate conditions at recharge into account. To date these are considered as the most representative process-oriented existing models, yet differences in calculated residence times of mostly several thousands of years (up to 19 ka) are revealed with the newly calculated ages being mostly (though not always) younger. Not accounting for climate conditions at recharge (pCO2 and temperature) is thus producing substantial error on deduced residence times. The derived 14C model ages are correlated with He concentrations measured in the groundwater of the aquifer. The obtained residence times show a gap between about 14 and 21 ka indicating possible permafrost conditions which inhibited any groundwater recharge.

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