Search

The REPRO-TDE test was performed at a depth of about 400 m in the ONKALO underground research facility in Finland. Synthetic groundwater containing radionuclide tracers [tritiated water tracer (HTO), 36Cl, 22Na, 133Ba, and 134Cs] was circulated for about 4 years in a packed-off interval of the injection borehole. Tracer activities were additionally monitored in two observation boreholes. The test was the subject of a modeling exercise by the SKB GroundWater Flow and Transport of Solutes Task Force. Eleven teams participated in the exercise, using different model concepts and approaches. Predictive model calculations were based on laboratory-based information concerning porosities, diffusion coefficients, and sorption partition coefficients. After the experimental results were made available, the teams were able to revise their models to reproduce the observations. General conclusions from these back-analysis calculations include the need for reduced effective diffusion coefficients for 36Cl compared to those applicable to HTO (anion exclusion), the need to implement weaker sorption for 22Na compared to results from laboratory batch sorption experiments, and the observation of large differences between the theoretical initial concentrations for the strongly sorbing 133Ba and 134Cs, and the first measured values a few hours after tracer injection. Different teams applied different concepts, concerning mainly the implementation of isotropic versus anisotropic diffusion, or the possible existence of borehole disturbed zones around the different boreholes. The role of microstructure was also addressed in two of the models. [ABSTRACT FROM AUTHOR]

We use a realistic fracture along with pore‐scale reactive transport simulations to investigate the impact of fracture filling minerals and water‐mineral reactions on in‐plane groundwater flow patterns and on contaminant uptake by matrix diffusion. The results of the simulations show that sparsely distributed fracture filling calcite patches provide a significant buffering capacity against the infiltration of acidic water. The advance of the geochemical perturbation is characterized by channeling, which is partly smeared out when low Peclet numbers are used. Mineral patches have a significant impact on groundwater flow patterns as shown by the computed distributions of groundwater travel time, which are characterized by long tails. The ability of the fracture to retain contaminants by uptake due to matrix diffusion is investigated by means of two metrics: the Eulerian quantity denoted as F‐factor and the Lagrangian variable known as transport resistance. F‐factor is shown to provide an average single estimate of the bulk retention capacity of the fracture whereas transport resistance is characterized by a distribution, whose very long tail is related to streamlines forced to travel through very narrow throats. A reduction of up to ∼70% in the mean travel time and ∼85% in the mean transport resistance is observed as a consequence of geochemical dissolution reactions, which also lead to an overall reduction of their variability. Key Points: Fracture internal variability and infilling minerals lead to channelingSparsely distributed calcite mineral patches provide a significant buffering capacity against the infiltration of acidic waterMineral dissolution leads to a general decrease of contaminant retention by matrix diffusion [ABSTRACT FROM AUTHOR]