THE ATTEMPT TO CHARACTERISE THE HYDROGEOLOGICAL PROPERTIES OF A FRACTURED SHEAR ZONE BY MODELLING OF SEVERAL TRACER DIPOLE TESTS

摘要

For more than 10 years, tracer migration experiments have been performed at the Grimsel Test Site to improve the understanding of radionuclide transport in fractured rocks and to develop and test models that describe these experiments. Models are needed for performance assess-ment modelling of nuclear waste repositories. Most experiments have been performed in the Migration (MI) shear zone AU96 (Frick, et al., 1992). A new type of reactive transport experi-ment is foreseen in a parallel, neighbour shear zone, AU126, to investigate the interaction of a high pH solution with the fractured rock and its influence on radionuclide migration within a long-term experiment. In advance of the planned long-term reactive transport experiment, it is necessary to characterise its hydrogeological and transport properties as well as to identify the relevant transport processes. The Grimsel granodiorite is characterised by the presence of ductile shear zones that include mylonite bands. In the shear zones, brittle fractures developed, mainly in the mylonite bands, and including a highly porous fault gauge material (crushed mylonite). A detailed description can be found in (Bossart and Mazurek, 1991). Ten boreholes (7 m to 17 m long) have been drilled from the access tunnel through the shear zone in order to get information on its hydro-geological properties. Their distance from each other in the shear zone is from 0.77 m to 16.26 m. Six boreholes intersect the shear zone within a 2 m by 2 m area (Fig. 1). At a time, two of them have been used to install the dipole flow field. A step-pulse injection of different tracers was performed (Table 1) by an injection loop parallel to a pumping loop that preconditioned the stationary flow conditions during the dipole ex-peri-ments. The tracer is monitored by a fluorescence signal at the injection and extraction side of the dipole. Four independent tracer dipole experiments between different boreholes have been conducted under different dipole geometries, pumping rates and orientation to the back-ground water flow to deduce transport properties of the shear zone and test the model applicability.