Final report of the TRUE Block Scale project. 3. Modelling of flow and transport

A series of tracer experiments were performed as part of the TRUE Block Scale experiment over length scales ranging from 10 to 100 m. The in situ experimentation was preceded by a comprehensive iterative characterisation campaign - the results from one borehole was used to update descriptive models and provide the basis for continued characterisation. Apart from core drilling, various types of laboratory investigations, core logging, borehole TV imaging and various types of hydraulic tests (single hole and cross-hole) were performed. Based on the characterisation data a hydro structural model of the investigated rock volume was constructed including deterministic structures and a stochastic background fracture population, and their material properties. In addition, a generic microstructure conceptual model of the investigated structures was developed. Tracer tests with radioactive sorbing tracers performed in three flow paths were preceded by various pre-tests including tracer dilution tests, which were used to select suitable configurations of tracer injection and pumping in the established borehole array. The in situ experimentation was preceded by formulation of basic questions and associated hypotheses to be addressed by the tracer tests and the subsequent evaluation. The hypotheses included address of the validity of the hydro structural model, the effects of heterogeneity and block scale retention. Model predictions and subsequent evaluation modelling was performed using a wide variety of model concepts. These included stochastic continuum, discrete feature network and channel network models formulated in 3D, which also solved the flow problem. In addition, two 'single channel' approaches (Posiva Streamtube and LaSAR extended to the block scale) were employed. A common basis for transport was formulated. The difference between the approaches was found in how heterogeneity is accounted for, both in terms of number of different types of immobile zones included, and if and how heterogeneity in retention parameters was accounted for. The integration of the modelling performed emphasised assessment the relative roles of advection, diffusion and sorption. The evaluation results showed similar retention characteristics for the three tested flow paths, which in turn were found to be similar to those seen in the flow paths tested by the TRUE-1 experiment. The evaluation modelling showed that the most consistent interpretation of the performed tests is obtained when diffusional mass transfer and sorption in immobile zones is included. It was also noted that no additional phenomena/processes (apart from surface sorption) were required to explain the observed in situ retention. It was further found not possible to fully discriminate the relative importance of potentially available immobile zones along the flow paths using available data. It was finally observed that heterogeneity in retention properties calls for additional analysis of 'effective' retention parameters in order to improve understanding and interpretation of in situ retention parameters