Stochastic continuum hydrological model of Aespoe for the SITE-94 performance assessment project

As part of the SKI SITE-94 deep repository performance assessment project, a stochastic continuum hydrological model conditioned on the point injection data from boreholes KAS02-08 is developed for the southern Aespoe Island. Both the more conductive fractures and the less permeable matrix are generated within the framework of a single continuum stochastic model based on nonparametric indicator geostatistics. In the stochastic model, the fracture zones are made distinct from the matrix by imposing a long range correlation structure for a small fraction (∼ the highest 11%) of the hydraulic conductivity in the preferred planes of fracture zones. The issue of spatial variability, giving rise to uncertainty in transport results in the site characterization of a heterogeneous medium, was addressed. Simulations were carried out for 3D transport from point tracer sources at hundreds of locations in the medium. The breakthrough curves from each 'point' source release is characterized by two parameters: the mean transport velocity and the dispersion coefficient. The results were presented as a statistical distribution of the transport parameters, thus quantifying the spatial variability in site characterization by flow and transport measurements. Results of flow and transport simulation in 3D were also used to illustrate the large spatial variability of 'point' measurements, but that for spatially integrated quantities the uncertainty is reduced and results become less sensitive to correlation structure. Therefore it was suggested that spatially integrated quantities may be a more appropriate alternative choice of predictive quantities for performance assessment, in that they are more commensurate with the level of our ignorance of the strongly heterogeneous character of the medium. 14 refs, 37 figs