Regional fluid characterisation and modelling of water–rock equilibria in the Boom clay Formation and in the Rupelian aquifer at Mol, Belgium

Interstitial water from the Boom clay Formation around the HADES (high activity disposal experimental site, an underground research facility belonging to the Nuclear research centre, SCK·CEN, at Mol-Dessel, Belgium.) underground research facility at Mol, Belgium, and waters sampled at different locations in the Rupelian aquifer, underlying the clay formation, have been studied. The Boom clay is a Tertiary mudrock containing about 55 % clay and the Rupelian aquifer is located in a silty layer. Special care was taken to adapt or improve measurement, sampling, conservation and analysis methods to get a reliable regional and local database, for the purpose of testing a general model describing the regulation and acquisition of the composition of these fluids. The isotopic and chemical composition of the waters allows them to be assigned to a common origin, namely mixing between a marine endmember and the clay interstitial water, followed by reequilibration with the host rock through dissolution–precipitation reactions involving identified secondary minerals. Oxidation–reduction state and trace element behavior are also discussed and the limits of the model are outlined.