[en] The Effingen Member is a low-permeability rock unit ca. 160 Ma old that occurs across northern Switzerland. It comprises sandy calcareous marls and argillaceous limestones. This report describes its hydro geochemistry, mineralogy and physical properties in three boreholes. During drilling ground-water flows were identified at the top, but not at the base, of the rock volume, and in a limestone layer. The marls and limestones have carbonate contents of 46-91 wt.-% and clay-mineral contents of 5-37 wt.-%. Pyrite contents are up to 1.6 wt.-%, but no sulfate minerals were detected. Clay minerals are predominantly mixed-layer illite-smectite, illite and kaolinite, with sporadic traces of chlorite and smectite. Water contents are 0.7-4.2 wt.-%, corresponding to a water-loss porosity range of 1.9-10.8 vol.-%. Water activity has yielded surprisingly low values down to 0.8. Cation exchange capacity and exchangeable cation populations have been studied by the Ni-en method. Cation concentrations are in the order Na"+ ≥ Ca"2"+ > Mg"2"+ > K"+ > Sr"2"+. The analytical results from the Ni-en extractions have additional contributions from cations originating from pore water and from mineral dissolution reactions that occurred during extraction. Three different methods have been used for characterising the chemical compositions of pore waters: a) advective displacement produces results that approach complete hydrochemical compositions; b) aqueous extraction and; c) out-diffusion give data for Cl"- and Br"- only. For both aqueous extraction and out-diffusion, reaction of water with rock affected concentrations of cations, SO_4"2"- and alkalinity. Centrifugation failed to extract pore water. Stable isotope ratios (δ"1"8O and δ"2H) of pore waters were analysed by the diffusive exchange method; helium contents of pore water samples were extracted for mass spectrometric analysis. Comparisons were done on water-loss porosities with physical porosities, water-activity measurements, and high contents of dissolved gas as inferred from ground-water samples. The largest uncertainty in the calculated pore-water Cl"- concentrations is the magnitude of the anion-accessible fraction of water-loss porosity. Experience of clay-mineral rich formations suggests that the anion-accessible porosity fraction is in a range of 0.3 to 0.6 and tends to be inversely correlated with clay-mineral contents. Comparisons of the Cl"- concentration in pore water with that calculated from aqueous extraction of an adjacent core sample suggests a fraction of 0.27 for a sample, and a value of 0.64 for another one. The difference may be due to the local mineralogical heterogeneity. The value of 0.5 is proposed as a typical average for rocks of the Effingen Member, bearing in mind that it varies on a local scale. Using an anion-accessible porosity fraction of 0.5, Cl"- concentrations reach a maximum of about 14 g/L in the centre of the rock volume; it decreases upwards and downwards from that. Calculated Cl"- concentrations from out-diffusion experiments are similar to those from aqueous extraction. Advective displacement of pore water allows obtaining complete pore-water compositions. The pore waters have Na-(Ca)-Cl compositions and SO_4"2"- concentrations of about 1.1 g/L. The Br/Cl ratio is similar or lower to that of sea water. An upper limit of ca. 20 g/L on Cl"- concentration seems reasonable for pore water in the Effingen Member. There are major discrepancies between pore-water SO_4"2"- concentrations inferred from aqueous extraction or out-diffusion experiments and those obtained from advective displacement. Part of them are attributable to perturbation of the sulfur system and enhancement of SO_4"2"- by sulfate mineral dissolution and minor pyrite oxidation during aqueous extraction and out-diffusion. Stable isotope compositions, δ"1"8O and δ"2H, of pore waters plot to the right of the meteoric water line, suggesting that "1"8O has been enriched by water-rock exchange, which indicates that the pore waters have a long residence time. This is supported by the level of dissolved "4He that has accumulated in pore water. The geochemical system evolved slowly over geological periods of time. The irregularity of Cl"- and δ"1"8O profiles and spatial variability of advective ground-water flows suggests that responses to changing tectonic and surface environmental conditions were complex