Groundwater characterisation and modelling of water–rock interaction in an argillaceous formation (Tournemire, France)

Chemical (Na, K, Mg, Ca, Cl, SO4, HCO3, Si, Al, Fe, Mn, TOC) and isotopic (18O[H2O], 2H[H2O], 3H[H2O], 14C[DIC], 13C[DIC], 34S[SO4], 18O[SO4]) data were obtained on groundwater collected from fractures inside the consolidated argillaceous formation of Tournemire (Aveyron) and from the aquifers surrounding this formation. Because of the very low transmissivity of such fractures (10−10 m2 s−1), specific devices were developed, for limiting out-gassing and air-contamination of groundwater during the sampling. Two modelling approaches are proposed to account for the chemical evolution of fracture groundwater from Na–Cl–HCO3 to Na–SO4 type. The first one is based on mineral equilibria only and the second one also takes into account the ion-exchange reactions. The two modelling approaches are able to reproduce K+ concentrations and they led to an almost constant pCO2 (10−2.6 atm), which only depends on equilibrium with the calcite–dolomite–kaolinite–quartz assemblage. The modelling of Na+, Ca2+ and image behaviours is improved by taking the ion-exchange reactions into account.