Water‐Mineral Reactions in a Translated Single Realistic Fracture: Consequences for Contaminant Uptake by Matrix Diffusion

Abstract

AbstractWe 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.