CHARACTERIZATION AND TRANSPORT MODELING OF NATURAL TRACERS THROUGH ARGILLITES (TOURNEMIRE, FRANCE)

In order to support the counter-expertise of the industrial projects concerning the feasibility of deep radioactive waste repositories in clayey massifs, the French Institute for Protection and Nuclear Safety (IPSN) is investigating the argillaceous units of the Lias formation from its underground experimental facility at Tournemire (Aveyron, France). Because of the very low water content and hydraulic conductivity values of argillites, migration through this medium is also expected to be extremely slow. Fluid flow regimes of interstitial water and transport of tracer elements have been studied investigating stable isotopes and chloride contents.

Deuterium and chloride were chosen because of their conservative behavior. After the development of an analytical protocol for quantitative extraction of chloride from interstitial water and evaluation of a vacuum distillation method for water extraction from the rock, data were acquired from the argillaceous sequence of the massif, with particular focus on areas associated with fracturing. Both chloride and deuterium profiles suggest that transfers are mainly diffusive at the massif scale; however, such profiles show enrichment in dD and d¹⁸O in fractured areas as compared to pore water of more distal samples. Therefore, these observations suggest that a second process could generate localized transfers in the vicinity of faults.

The hypothesis of molecular/ionic diffusion as a dominant process for transport was successfully tested using a transport model, covering a period of several tens of millions years, taking into account geodynamical features of the region (such as tectonic and induced faults) and assuming that some variations of the tracer concentrations at the system boundaries occurred during major climate-change periods. Even if the transfer of tracers is mainly diffusive at the massif scale, a second process that caused heterogeneities in the vicinity of faults must have affected their concentrations. This process likely involves either intrusion of brines or internal transfers due to local overpressures of the formation.