Investigating Flow in a Complex Aquifer Using Isotopic Geochemistry: the Example of the Geothermal Reservoir of Hainaut (Belgium, NW Europe)

Abstract

The geothermal reservoir of Hainaut is a complex aquifer in terms of lithology, tectonic history and hydrogeological behavior. It is composed by Mississipian carbonates, with thick layers of massive anhydrite laterally replaced by collapse breccias. The opening of the aquifer to meteoric water is supposed to have taken place during Cretaceous extensional phase, inducing carbonate karstification and anhydrite leaching. The heterogeneous removal of anhydrites had several influences, first on reservoir permeability (i.e. local increase), and second on subsidence rate during post-Palaeozoic times.
In spite of more than twenty years exploitation experience, the hydrogeological behavior of the aquifer is still obscure. Recharge zone is thought to extend over outcropping reservoir rocks, a few km North from the exploited geothermal wells. The only supposed natural exsurgences are a few springs, located in the same area, that show anomalies in temperature and/or in chemical composition. Isotopic analyses have been carried out on spring and geothermal waters. D and 18O data are consistent with meteoric origin. Deep waters depleting in heavy H and O isotopes confirms 14C dating of 15000 years of average residence time, and infiltration under glacial climate. Deep reservoir waters are at isotopic equilibrium with carbonate matrix following 13C results, whereas shallow waters are mainly influenced by atmospheric CO2. Sulfate isotopic ratios confirm mixing of deep and shallow waters and allow the differentiation between anhydrite dissolution and sulfide oxidation origin. The mixing rate can be estimated, and natural recharge flow of the deep reservoir can further be assessed