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Florian Eichinger, University of Bern (Switzerland)
Niklaus, H. Waber, University of Bern (Switzerland)
John A.T. Smellie, Conterra AB (Sweden)
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The amount of pore water that resides in the connected pore space of the low permeable bedrock mass of crystalline rocks is considerable compared to that of water circulating in the fracture networks. In this low permeable rock matrix, solute transport appears to be dominated by diffusion and the pore water cannot be sampled by conventional sampling methods. Therefore, indirect sampling methods need to be applied. As a consequence of the low permeability of the rock matrix the pore water might have preserved information about its palaeo-hydrogeological origin and evolution in the range of tens of thousands to millions of years. Such knowledge is of particular importance for long-term safety assessment of the deep disposal of radioactive waste, not only because of possible radionuclide retardation in the rock matrix (matrix diffusion), but also with time the pore water will react with the technical barriers (canister, bentonite).
Chloride concentrations in the pore water from deep drillcore samples vary between 90 and 1570 mg/L. The water stable isotope composition of the pore water samples range from -6.84 to -11.19 ‰-VSMOW for δ18O and from -65.14 to -90.21 ‰-VSMOW for δ2H. In the δ18O-δ2H diagram they plot to the right of the GMWL with increasing sample depth. In the bedrock mass with a high fracture frequency (distance to the next fracture <5 m), equal Cl concentrations and stable isotope signatures are established in pore water and fracture water indicating steady state conditions. In contrast, at greater depth where the fracture frequency drastically decreases, the Cl concentrations in the pore water are lower and stable isotope signatures enriched compared to the fracture water. This indicates that over a long time period there existed dilute/fresh fracture groundwater that must have had resided there longer than the ingress of more brackish water into the system in the more recent past. The enrichment of the pore water in 18O and 2H suggests that this dilute water originated during warm-climate periods.
The preservation of such a warm-climate meteoric water signature in the rock matrix pore water might seem surprising considering subsequent cold climate and glacial periods and the submergence of the Olkiluoto area under the Littorina and Baltic Sea in the more recent past, and thereby facilitating infiltration of brackish water into the fracture system. The preservation of the dilute warm-climate meteoric water signatures in the deeper part of the rocks is thus attributed to the low fracture frequency and low-permeability of the rock at these depths, combined with a low regional hydraulic gradient which prevented flushing of the dilute warm-climate meteoric water from the fracture system over a long time period. The ingress of glacial water to deeper levels appears to have been limited further by permafrost or was present for too short in time to leave its signature in the pore waters at equivalent depths.
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