OVERVIEW OF THE FRACTURE-ZONE PROJECT AT FINNSJON, SWEDEN

At the Finnsjon site, central Sweden, a 100-m-wide, low-angle fracture zone in granodiorite, Zone 2, has been studied by means of borehole investigations. This has included core and geophysical logging as well as hydraulic tests and groundwater sampling. Zone 2 was developed at 1.7-1.6 Ga as a ductile shear zone at a depth of approximately 10-15 km and repeated reactivation has occurred during Precambrian time and later. Today, the upper boundary of the zone is defined in nine boreholes at depths ranging from 100 to 295 m. Pumping tests have demonstrated that the upper part of the zone is highly water conductive with a transmissivity in excess of 10(-3) m2 s-1. From tracer tests the equivalent single fracture hydraulic conductivity was calculated to be 0.03-0.2 m s-1. This extremely high transmissivity and conductivity over such a large areal extent can be speculated to be the result of recent opening of Zone 2 during the last inland ice period. In situ measurements of the natural groundwater flow show that the flow is high (67-90 m3 m-2 year-1) at the upper boundary of Zone 2, whereas near-stagnant conditions occur at the lower boundary, and in the relict saline water beneath the zone. Isotope data suggest a complex origin for the saline water. The hydraulic head measurements as well as the groundwater chemistry strongly suggest that the low-angle Zone 2 acts as a hydraulic barrier preventing ground water from percolating downward through the zone. These studies have helped to focus on repository safety assessment problems which hitherto had been oversimplified for the benefit of performance models. Furthermore, the results have shown that these low-angle large-scale fracture zones are likely to be even more important controls than steep zones, when considering bedrock stability, rock stress regimes, and lateral groundwater flow patterns. Quantification of such phenomena should provide an invaluable input into the various safety assessment performance codes for model validation and siting criteria for a repository.