Modelling of water - rock interactions in deep groundwaters: equilibrium and perturbations.
H. Pitsch and C. Beaucaire
Eurosafe 2000 Cologne - 6/7 novembre
A predictive model for the interpretation of water-rock equilibrium and perturbations of different origins is presented. The geochemical control on elemental concentrations by dissolution-precipitation reactions and the role of ion exchange on clays are explained. Application to the Boom clay interstitial water and aquifer fluids from the belgian province Kempen is shown, limits of the model and open questions are presented.
The model presented here was first developed to explain the chemical composition of low temperature granitic waters at Stripa (1). It related the concentration of certain elements in equilibrated groundwaters to a geochemical control through dissolution – precipitation equilibria involving secondary minerals. In the early nineties Beaucaire and Toulhoat made the hypothesis that the same modelling approach could be applied in a sedimentary context, namely to interpret the groundwater composition at the Mol site.
The results presented here were obtained from different programmes (see for example 2,3) carried out at the Service d’étude du stockage et de l’entreposage de déchets, Commissariat à l’énergie atomique (CEA), France, in collaboration with the Waste and disposal unit of the Belgian nuclear research centre (CEN·SCK) at Mol and other partners. Financial support was from the European commission, CEA, CEN·SCK, Agence pour la gestion des déchets radioactifs (ANDRA) and the Research division of Electricité de France (EDF).
The site of investigation consists of an area of ca 10 000 km2 in the northern province of Kempen in Belgium. The Boom clay belongs to a system of successive layers of tertiary clays and clayey sands or silts. Interstitial water from the Boom clay and from two aquifers are considered here: the Ruisbroek-Berg (Rupelian) aquifer, underlying the Boom clay and the deeper Lede-Brussel (Lutetian) aquifer, separated from the Ruisbroek-Berg sands by the Asse clay.