[1] Transport and retention of sorbing tracers in a single, altered crystalline rock fracture on a 5 m scale is investigated. We evaluate the results of a comprehensive field study (referred to as Tracer Retention Understanding Experiments, first phase (TRUE-1)), at a 400 m depth of the Ä spö Hard Rock Laboratory (Sweden). A total of 16 breakthrough curves are analyzed, from three test configurations using six radioactive tracers with a broad range of sorption properties. A transport-retention model is proposed, and its applicability is assessed based on available data. We find that the conventional model with an asymptotic power law slope of À3/2 (one-dimensional diffusion into an unlimited rock matrix) is a reasonable approximation for the conditions of the TRUE-1 tests. Retention in the altered rock of the rim zone appears to be significantly stronger than implied by retention properties inferred from generic (unaltered) rock samples. The effective physical parameters which control retention (matrix porosity and retention aperture) are comparable for all three test configurations. The most plausible in situ (rim zone) porosity is in the range 1%-2%, which constrains the effective retention aperture to the range 0.2-0.7 mm. For all sorbing tracers the estimated in situ sorption coefficient appears to be larger by at least a factor of 10, compared to the value inferred from through-diffusion tests using unaltered rock samples.

[1] Mineralogical and retardation properties of rock materials responsible for water-rock interaction in in situ migration experiments with sorbing radioactive tracers were studied in laboratory experiments. The porosity was studied by water saturation measurements and the PMMA method was used for detailed porosity characterization of heterogeneity distributions and porosity profiles toward the fracture surface. Mylonite and altered diorite sampled in the rim zone of the fracture and representative bulk rock types were investigated by batch sorption measurements with crushed materials and through-diffusion and in-diffusion experiments in intact rock pieces. Autoradiography was used for visualization of in-diffusion profiles of sorbing tracers. The use of detailed porosity information and quantitative data on heterogeneity in porosity is shown to significantly improve the interpretation and evaluation of laboratory-scale diffusion experiments. We show through the combined approach of detailed porosity characterization and laboratory sorption and diffusion investigations that we can distinguish retention properties of bulk rock and altered rock and provide qualitative and quantitative data of heterogeneous rock properties that expand the possibility for including relevant processes in the interpretation of the results of in situ tracer tests.

[1] We evaluate the breakthrough curves obtained within a comprehensive experimental program for investigating the retention properties of crystalline rock, referred to as Tracer Retention Understanding Experiments (TRUE). The tracer tests were conducted at the Äspö Hard Rock Laboratory (Sweden) in two phases jointly referred to as TRUE Block Scale (TBS); the TBS tests comprise a total of 17 breakthrough curves with nonsorbing and a range of sorbing tracers. The Euclidian length scales are between 10 and 30 m, compared to 5 m for the earlier tests TRUE-1. The unlimited diffusion model is consistent with measured breakthrough curves and is adopted here for evaluation. The model has four independent parameters, two of which are related to advection and dispersion, one which is related to diffusion-sorption, and one which is related to surface sorption; the individual retention parameters or properties cannot be inferred from breakthrough curves alone and require additional constraints. The mean water residence times for the TBS tests are in the range 15-250 h, whereas the coefficient of variation of the water residence times is in the range 0.4-0.6. A consistent trend is found in the calibrated retention parameters with the sorption affinities of the tracers involved. Using Bode sensitivity functions, it is shown that sensitivity increases for the retention parameter with increasing sorption affinity; for nonsorbing tracers, diffusion and hydrodynamic dispersion are shown to "compete," exhibiting similar effects; hence, their estimates are uncertain. The analysis presented here exposes a few fundamental limitations and sensitivities when evaluating diffusioncontrolled retention in the subsurface; it is general and applicable to any site with comparable tracer test data. In part 2, it will be shown how discrete fracture network simulations based on the hydrostructural information available can be used for further constraining individual retention parameters, in particular, the active specific surface area (s f ) and the rock matrix porosity ().

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