Three-dimensional discrete fracture network simulations of flow and tracer migration based on Laxemar site data (Sweden)

We study particle transport in a 3D DFN scenario based on Laxemar site characterisation data in Sweden, which is a candidate repository site for high level radioactive waste in the Swedish nuclear waste management program. The site characterisation data has revealed several interesting geometric and hydraulic fracture properties, such as power-law distributed fracture sizes and transmissivities.
A fundamental aspect towards understanding tracer migration in subsurface sparsely fractured rock formations is the relationship between the Eulerian flow field at a sub-fracture scale with the Lagrangian flow field at a characteristic (model domain) scale.
In this work we present results from a new technique for upscaling particle transitions obtained from Eulerian flow statistics to predictions of tracer discharge at a characteristic transport scale, based on previously developed methods used for 2D DFN’s. This includes a mapping algorithm for transforming Eulerian into Lagrangian flow statistics without a priori knowledge of network connectivity, and by retaining the correlation between the water residence time τ and the hydrodynamic control of retention β we present accurate tracer discharge predictions. These results are illustrated using the unlimited diffusion model, and for some hypothetical tracers with properties designed to capture the behaviour of many common radionuclides. Finally we emphasise the importance of capturing the early arrival and peak of tracer breakthrough curves, i.e. to capture the bulk of the tracer mass arrival, in order to make accurate and conservative predictions.