QUANTITATIVE STUDY OF GEOCHEMICAL PROCESSES IN THE DOGGER AQUIFER, PARIS BASIN, FRANCE

The quantification of geochemical reactions in hydrothermal aquifers requires an exhaustive approach because of their interdependence. All chemical elements likely to have a quantitative influence on dissolution or precipitation reactions have to be taken into account. Geochemical constraints, the number of which equals the number of the chemical elements, were determined with the help of chemical analyses of the solution. In the case study of the Dogger aquifer (Paris Basin, France), 12 elements are taken into account (Al, C, Cl, Ca, F, H, K, Mg, Na, 0, S, Si). Three constraints apply to water-activity, neutrality and conservation of chloride in aqueous solutions. The determination of the remaining constraints was based on saturation indices, stability diagrams of minerals and partial pressure of carbon dioxide (pCO2). The present-day solutions are at equilibrium with respect to nine minerals (albite, anhydrite, chalcedony, calcite, dolomite, fluorite, kaolinite, K-feldspar and illite or chlorite). Thus, it was demonstrated that the composition of these solutions (including computed pCO2) is only a function of temperature and chloride content. Moreover, it was possible to test the validity of the geochemical system by computing its speciation and comparing these theoretical results with actual chemical analyses (pH and concentrations). Finally, geochemical simulations were used in predicting what quantities would be dissolved or precipitated, as either the temperature or the chlorinity varied. Although the rock is predominantly calcareous, these quantities could not be determined if the influence of the aluminosilicates were neglected. This chemical component approach with which one can pose and solve rigorously the chemical equilibrium problem constitutes a prerequisite for the quantitative study of geochemical processes related to fluid flow.