Solute transport and retention in three‐dimensional fracture networks

Resolving the hydrodynamic control of retention is an important step in predictive modeling of transport of sorbing tracers in fractured rock. The statistics of the transport resistance parameter β [T/L] and the related effective active specific surface area s f [1/L] are studied in a crystalline rock volume on a 100 m scale. Groundwater flow and advective transport are based on generic boundary conditions and realistic discrete fracture networks inferred from the Laxemar site, southeast Sweden. The overall statistics of β are consistent with statistics of the water residence time τ ; the moments of β vary linearly with distance, at least up to 100 m. The correlation between log τ and log β is predominantly linear, however, there is significant dispersion; the parameter s f strongly depends on the assumed hydraulic law (theoretical cubic or empirical quadratic). Fast and slow trajectories/segments in the network determine the shape of the β distribution that cannot be reproduced by infinitely divisible model over the entire range; the low value range and median can be reproduced reasonably well with the tempered one‐sided stable density using the exponent in the range 0.35–0.7. The low percentiles of the β distribution seems to converge to a Fickian type of behavior from a 50 to 100 m scale.