Geochemistry and palaeo-hydrogeology of the weathered zone in the Opalinus Clay

The effects of weathering and decompaction on the Opalinus Clay in northern Switzerland were studied basis a 146 m long drillcore profile. Below 6 Quaternary sediments, rock down to 16 depth is affected by partial or complete dissolution siderite, calcite, pyrite organic matter, with goethite as only identified product. reduction level Fe (Fe2+/Fetot) increases from <20% >80% unweathered below 24 m. Porosity reaches 0.39 shallowest samples before dropping normal values around 0.13 30 Pore-water composition obtained squeezing evolves fresh water Ca-SO4-HCO3 type 10–12 g/L Na-SO4-Cl 40 Ground-water taken weathered zone indicate reducing conditions at present therefore reflect remaining redox buffering capacity even strongly oxidised rock. interpretation properties, conjunction chemical isotopic signatures ground pore waters, suggests that substantial occurred primarily during stages lower ground-water table Pleistocene, providing access oxygen via gas diffusion across partially unsaturated zone. Three evolutionary can be distinguished: 1) Long-term oxidation Pleistocene surface exposure under dry climate diffusive transport SO42− into formation. This explains higher pore-water concentrations when compared regional data deep boreholes. 2) Build-up curved Cl− water-isotope profiles since ∼50–60 ka penetration 40–80 3) Holocene rise table, leading self sealing fractures establishment regime Under saturated conditions, quite limited, which illustrates against external geochemical disturbances.