Palaeohydrogeology: A methodology based on fracture mineral studies

Two sites on the east coast of Sweden (Forsmark and Laxemar/Simpevarp) are currently being investigated as potential geologic hosts for a deep repository isolating high-level nuclear waste. In this paper, a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh/saline water interface and location of the redox front in the bedrock. The most commonly precipitated fracture minerals in crystalline rocks are chlorite, calcite, quartz, K-feldspar, Ca–Al-silicates like epidote, prehnite and laumontite, sulphides and Fe-oxides. Of these, calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low-temperature conditions during the Pleistocene and Holocene epochs. Sulphides and Fe-oxides/hydroxides provide information on the position of the redox front. In order to carry out palaeohydrological studies, a number of prerequisites are required such as; high quality drill core material, geological knowledge of the sequence of fracture mineralizations; the post-glacial (Holocene) evolution in the area; high quality groundwater chemistry, including stable isotopes; and a conceptual model of the hydrogeochemistry that is to be tested. The choice of methods used here is based on the fact that both the Forsmark and Laxemar/Simpevarp sites are situated in Palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1.5 Ga, and they have been exposed to glaciations/deglaciations and transgressions/regressions of the Baltic Sea during the Quaternary. This has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition. The suggested scheme for solving the variation in depth of the fresh/saline water interface focuses on fracture calcite. It includes a step-by-step procedure with;(1)Initial δ18O and δ13C, analyses and complementary petrographic studies of thin sections and crystal morphology followed by(2)87Sr/86Sr and trace element analyses, and finally(3)Fluid inclusion analyses and cathodoluminescence (CL) studies of zoning.For redox front investigations, the following methods are suggested:(1)Study of the distribution of redox sensitive minerals;(2)U-series measurements;(3)Trace element analyses of fracture fillings;(4)Fe(III)/Fe(II) analyses +/− Fe isotope analyses.It is concluded that fracture mineral studies can reveal a palaeohydrological record in crystalline rock that is essential to understand the stability or evolution of the groundwater system over a time scale that is relevant to performance assessment for a spent nuclear fuel repository. It is also concluded that the suggested methodology for palaeohydrogeological studies is site-specific.