Effect of microbial activity on the chemical composition of groundwater in the argillaceous formation of Tournemire (France)

For many years, IRSN has been conducting research programs at the Tournemire experimental site, with the aim of assessing the mode of fluid migration in an argillaceous formation and the regulation mechanisms of the groundwater chemistry. The experimental Tournemire URL is located in a Mesozoic marine sedimentary basin on the Southern border of the French Central Massif. The studied formation corresponds to a 250 m thick argillaceous layer (Toarcian and Domerian) bounded by two aquifer carbonate layers (300 and 500 m thick). The argillaceous formation is composed of about 40% of clay minerals (kaolinite, illite-montmorillonite) and mica, quartz (∼20%), calcite (∼20%), pyrite and siderite. The Tournemire site was installed in a sub-horizontal monoclinal structure affected by regional E-W striking major Cernon fault (80 km length) in the northern part. The massif is also affected by some secondary faults of hecto-metric scale, which are associated with fractured zones. Lastly, fissures of metric to decametric scale were observed. The structural analysis indicated that most of this fracturing is related to the compressional N-S to NW-SE trending Pyrenean orogeny (40-50 Ma ago). All these fractures are generally filled with calcite with occurrence of cubic pyrite. However, this compressional phase created some apertures, with geodic cavities, mainly located in the relay zones. Discharges of water occur in these geodic systems. Transmissivities of these systems are about 10-10 m2.s-1, four to five orders of magnitude higher than in the argillaceous matrix. Fluids in these fractures were regularly sampled from several vertical and sub-horizontal boreholes. Their chemical (T, pH, cations, anions, TIC and TOC) and isotopic (34S, 13C, 14C) characteristics were monitored during more than one year, since the drilling. Results show that the chemical composition of fracture water evolved with time, mainly in the deepest drill-hole, which intercepts the producing fracture in an organic-matter rich level at 150 m depth. (authors)