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Arbeitsbericht NAB 09-15

Reference pore water for the Helvetic marls for the provisional safety analysis in the framework of sectoral plan – interim results (SGT-ZE)

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Four variants of a Na-Cl reference pore water (Table 6-2) are suggested for the marl stack (Palfris Formation, Vitznau Mergel) at the Wellenberg siting region. Low-pH and high-pH variants gauge the uncertainty with respect to alkalinity and PCO2. The redox conditions are reducing and modelled at pyrite equilibrium, but with negligible methane for the "base-case". A more reducing variant accounts for the observed high partial pressures of methane, and it is also modelled at pyrite equilibrium (Table 6-2). A "High-Na-Cl" variant with a high salinity (Table 6-3) is also modelled to cover the uncertainty in salinity arising from poorly constrained porosity data.

A Na-HCO3-type pore water in analogy to the Na-HCO3 fracture water of Nagra (1997b) was not included because a planned repository will be located well within the rock volume containing Na-Cl-type pore water. This region is expected to extend upwards as matrix rock into the zone with Na-HCO3-type fracture water.

The Na-Cl reference pore water proposed here has a somewhat higher pH and lower PCO2 compared to that of Nagra (1997b), but one of the different variants that bracket uncertainty is comparable. Redox conditions are modelled by mineral saturation constraints for all reference pore water compositions, or at a prescribed partial pressure of methane for the low-pε variant. All waters except the low-pε variant are sulphate-dominant compared to sulphide. The Na-Cl "base-case" reference pore water is similar somewhat more saline) to that derived by Baeyens and Bradbury (1994) used to derive a sorption data base.

A detailed historical and factual account of the relevant geochemical issues is provided and discussed in detail. Uncertainties and open issues are also addressed.

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