Comparison of surface-, aquifer- and pore-waters from a Mesozoic basin and its underlying Palaeozoic basement, southeast France: chemical evolution of waters and relationships between aquifers

As part of the Deep Geology of France (GPF) `Ardècheʹ project, water samples were collected along the Ardèche margin of the Massif Central crystalline shield. Water is contained within various rocks, including Early Jurassic carbonate, Triassic sandstone and evaporite, Permian continental deposits, and the crystalline basement. Three sample types were studied: surface water from streams and springs, formation water sampled from fractured zones (aquifers) at various depths, and pore water from rocks cored in the `Morte-Mérieʹ scientific borehole, drilled to a depth of 980 m in 1993 as part of the `Ardècheʹ project. main water-chemistry signatures are associated with the three major geological groups: (1) HCO3–Ca, Mg water occurs in Early Jurassic carbonate and Late Triassic sandstone with dolomitic cement; (2) Ca–SO4 water occurs in Triassic sulphated shale and sandstone; (3) Na–Cl water occurs in Permian-sandstone and crystalline-basement. mples enable a comparison between waters that flow on the surface, those that have been in the underlying geological formations for a short time, and those that have resided in closed pores of the host rock for a long time. Saturation indices computed for surface and formation waters show that dissolution reactions progressively increase the concentration of elements until saturation is reached. Surface waters were saturated for quartz and carbonate, and formation waters for quartz, carbonate, gypsum, muscovite and K-feldspar. Saline water (12 g/l TDS) from the Permian rocks is controlled by a mineralogical assemblage of quartz, calcite, anhydrite, muscovite, K-feldspar and albite, at temperatures 80–90°C. Although equilibrium was reached for the formation waters, concentrations in the pore waters reflect a general enrichment as compared to free formation waters from the fractured zones of the corresponding formation, which suggests introduction of mobile elements in the solution. Only in Permian rocks was a stable equilibrium observed since pore waters show no enrichment as compared to formation waters. er mixing was observed, except along faults that form pathways for the fluid between formations. Each geological group generally behaves as a closed system, except the Permian rocks that seem to be (or were) exposed to circulation from the crystalline basement.