Evaluation report of Task 9B based on comparisons and analyses of modelling results for the Äspö HRL LTDE-SD experiments. Task 9 of SKB Task Force GWFTS – Increasing the realism in solute transport modelling based on the field experiments REPRO and LTDE-SD

Task 9B of the SKB Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS) was the second subtask within Task 9 and focused on the modelling of experimental results from the LTDE-SD in situ tracer test. The test had been performed at a depth of about 410 m in the Äspö Hard Rock Laboratory. Synthetic groundwater containing a cocktail of radionuclide tracers was circulated for 198 days on the natural surface of a fracture and in a narrow slim hole drilled in unaltered rock matrix. Overcoring of the rock after the end of the test allowed for the measurement of tracer distribution profiles in the rock from the fracture surface (A cores) and also from the slim hole (D cores). The measured tracer activities in the rock samples showed long profiles (several cm) for non- or weakly-sorbing tracers (Cl-36, Na-22), but also for many of the more strongly-sorbing radionuclides. The understanding of this unexpected feature was one of the main motivations for this modelling exercise. However, re-evaluation and revision of the data during the course of Task 9B provided evidence that the anomalous long tails at low activities for strongly sorbing tracers were an artefact due to cross-contamination during rock sample preparation. A few data points remained for Cs-137, Ba-133, Ni-63 and Cd-109, but most measurements at long distances from the tracer source (> 10 mm) were now below the reported detection limits.Ten different modelling teams provided results for this exercise, using different concepts and codes. One additional team provided results related to conceptual development. The tracers that were finally considered were Na-22, Cl-36, Co-57, Ni-63, Ba-133, Cs-137, Cd-109, Ra-226 and Np-237. Three main types of models were used: (1) analytical solutions to the transport-retention equations, (2) continuum-porous-medium numerical models, and (3) microstructure-based models accounting for small-scale heterogeneity (i.e. mineral grains, porosities and/or microfracture distributions) and potential centimetre-scale fractures. The modelling by the different teams led to some important conclusions summarised below.Concerning Na-22 and Cl-36, which showed long penetration profiles, tracer profiles within ca 30 mm from the tracer source could be interpreted with transport and retention parameters consistent with those obtained from laboratory-scale experiments. A disturbed zone, with a thickness of about 5 mm, could also be identified. This disturbed zone was especially evident in the Cl-36 data. However, some of the measured cores showed rather flat end tails for Na-22, which could not be reproduced by the homogeneous (i.e. constant transport and retention properties) or the continuum-porous-medium models using parameters consistent with those derived from laboratory-scale experiments. Reproduction of those tails by some of the microstructure-based models was performed by implementing fast transport along microfractures and cm-scale fractures.For the rest of the tracers, which were more strongly sorbing, the profiles did not in general extend beyond 10 mm from the tracer source, with only some data points showing measurable activities further into the rock. Overall, the best fits to the measured profiles within a few mm from the tracer source were achieved by the homogeneous models (constant transport and retention parameters with distance), with apparent diffusion coefficients consistent with laboratory-derived experimental results. Good fits were also achieved by models assuming the presence of a disturbed zone with gradually changing parameters. The fact that most data points above detection limits fell within 5 to 8 mm from the tracer source, and the observations from the Cl-36 data, suggest the existence of a disturbed zone with a thickness of a few mm and characterised by rather constant transport and retention parameters. The longer profiles for Cl-36 and Na-22 advocate for an undisturbed rock matrix (unaffected by borehole drilling or alteration zones next to fractures) beyond these 5 to 8 mm from the tracer source. Additionally, the flat profile tails observed for some of the Na-22 profiles may point to the effect of microfractures and cm-scale fractures on radionuclide transport.These conclusions could be reached after (1) the re-evaluation and revision of the experimental data (tracer profiles in the rock), and (2) the analysis of the different sets of model results performed by the different teams.(1) The revision of the experimental data led to the dismissal of most of the measurements showing anomalously high activities far from the tracer source (long flat profile tail ends) for the strongly sorbing tracers, as possible contamination mechanisms during sample preparation were identified. As an additional consequence, this discovery highlighted the importance of using blank samples in future tracer transport experiments. Using blank samples together with samples from the test rock sections during preparation and analysis will aid in the detection of potential contamination or background effects.(2) The work performed by the different modelling teams allowed the comparison of many different model concepts, especially in terms of potential zonations of rock properties, such as the presence of a disturbed zone close to the tracer source, the potential effects of micro- and cm-scale fractures, or the implementation of microstructure-based models. An added value was the motivation provided by these exercises to advance in conceptual and numerical model development, which is a key goal of Task 9