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Technical Report NTB 91-31

Grimsel Test Site A simple transport model for the Grimsel migration experiments

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To understand the results of the field migration experiments of Nagra at the Grimsel Test Site (Central Switzerland), it is necessary to develop models (hydraulic as well as transport models). In this paper a simple approach with stream tubes is described which allows to model the hydraulic situation for asymmetric dipole arrangements and transport along streamlines, including sorption and diffusion into fracture infill material. The model is confronted with two experiments, one with a non-sorbing tracer (uranine) and the other with sorbing (24Na) and non-sorbing tracer material. The fit-by-eye parameter values are compatible with those from batch experiments. The sensitivity of breakthrough curves on the chosen parameters was checked with a parameter variation study. In the range of considered variations the breakthrough curves of non-sorbing tracers are not sensitive to matrix properties because saturation is achieved in all the cases considered. For sorbing tracers the situation is more complicated as saturation may be achieved or not within the experimental timescale; due to this fact, it is emphasized, that step input experiments should be performed instead of pulse test experiments.

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