Assessing Initial Conditions for Chloride Transport Across Low- permeability Argillaceous Rocks, Wellenberg, Switzerland

Information about fluid evolution and solute transport in a low-permeability metamorphic rock sequence has been obtained by comparing chloride concentrations and chlorine isotope ratios of pore water, groundwater, and fluid inclusions. The similarity of d37Cl values in fluid inclusions and groundwater suggests a closed-system evolution during the metamorphic overprint, and signatures established at this time appear to form the initial conditions for chloride transport after exhumation of the rock sequence

The Isotope Geochemistry of Carbon in Groundwater at Stripa.

The carbon isotopic composition of the total dissolved inorganic carbon in groundwater associated with a granitic pluton at Stripa (Sweden) reflects both inorganic and organic carbon sources. Following the uptake of soil carbon-dioxide, calcite dissolution dominates the geochemical evolution of shallow groundwater. Calcite saturation is reached at a depth of about 100 m. In deeper waters geochemical release of Ca and increasing pH cause calcite precipitation. Radiocarbon contents suggest carbon (and water ?) ages in excess of 20 000 years for waters at 300–400 m depth. In deep groundwaters with enhanced salinities organic carbon is added to the dissolved inorganic carbon either through bacterial activity (e.g, sulphate reducing bacteria) or the oxidation of organic compounds such as methane. The lowest radiocarbon contents were measured at the 300–400 meter levels and not in the deepest fluids. The distribution of 13C in the deep groundwaters suggests the existence of well-defined flowsystems with limited active hydraulic interaction. Isotope analyses on fracture calcites substantiate the complex geochemical history of the pluton

Modeling Cl- concentration and 37 Cl profiles in porewater across a 250m-thick inducated argillite at the Tournemire URL (France)

International audienceDissolved chloride in argillite porewater has been studied as a natural analogue for radionuclides potentially released from radioactive waste disposal. The Tournemire URL intersects impervious and compacted argillite. A previously obtained chloride concentration profile of intact rock is symmetric with a maximum concentration of 0.6±0.1 g/L, compared to 19 g/L for the original connate seawater. Dissolved chloride shows high δ37Cl values, ranging between +6 and +8‰ vs. SMOC. The modeled profile considers diffusive exchange between connate seawater and meteoric freshwater. Transport parameters were obtained by radial diffusion experiments. Numerical modeling was performed with the coupled reactive-transport code Hytec. Simulations suggest a diffusive-exchange time of 85±10 Ma for Cl, which correlates with a major erosional period. Simulated δ37Cl values between 1.002 and agree with observed porewater δ37Cl. This study strongly suggests that the dissolved chloride profile in the argillites results from diffusive exchange and indicates that unfractured argillites can provide good confinement