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

Abstract

Open fractures are primary conduits for groundwater flow and thus constitute preferential transport pathways for dissolved contaminants. Fracture internal variability, caused, for example, by shearing, leads to significant variability in the in-plane groundwater velocity field (Egert et al., 2021;Zou et al., 2017), which in turn affects mass exchange processes with the stagnant water in the bordering porous rock matrix . Fracture filling minerals, that might have precipitated during past hydrothermal events, add more pieces to this already complex puzzle. In fact they might alter the local velocity field and react with some of the dissolved species. A thorough characterization of in-plane groundwater flow and reactive transport processes occurring at the scale of a single fracture has a direct impact on applications such as the remediation of a polluted site or the safety assessment study of a deep geological repository for nuclear waste.Groundwater channeling occurring at the scale of a single fracture was assessed by different experimental and numerical works. Brown et al. (1998) assessed in-plane channeling by injecting dye into a water-saturated precise replica of a natural fracture. The visual analysis of dye concentration shows the dye entering the fracture preferentially through a single large channel. The authors found the velocity contrast (i.e., the ratio between maximum and average velocity) to be within a factor of 5. Watanabe et al. (2009) performed experimental and numerical analyses on flow through shear (Mode II) fractures, which were generated by direct shear on granite under different constant normal load. Two different shear displacements (1 and 5 mm) were considered. The authors found that contact areas occupy around 30%-70% of the fracture plane, depending on the confining pressure. Numerical simulations, performed using the depth-average Reynolds equation, showed that only around 10%-30% of the non-contact areas contribute to fluid flow. More recently, Zou et al. ( 2017) used a laser-scanned real rock sample to assess the effect of roughness and variability in fracture aperture on radionuclide transport. The numerical models, which were based on the Navier-Stokes

Buoyancy-Driven Dissolution Instability in a Horizontal Hele-Shaw Cell

“…Mineral dissolution occurring during reactive fluid flow in fractures produces distinct dissolution patterns, significantly affecting the permeability and the hydrodynamic behavior of the large-scale fractured rock. Understanding the dissolution dynamics is of particular interest in many geophysical processes, including the cavity enlargement, − genesis of karst systems, − groundwater contaminant migration, , geological CO 2 storage and leakage, − acid injection for stimulating petroleum wells, − and dam stability in soluble rocks. − The fracture dissolution without buoyancy-driven convection induced by density difference has been extensively studied in previous works. − In these circumstances, the fracture dissolution processes are mainly controlled by the interplay among the forced convection of fluid, the diffusion of solute, and the kinetics of reactions. Note that the forced convection of fluid refers to externally imposed flow.…”

Reactive Transport Modeling in Porous Fractured Media: Application to Weathering Processes

“…These models mainly focus on the topographic impact on dissolution processes by considering lateral flows (Lebedeva & Brantley, 2017; Myagkiy et al., 2019). Some occasionally study the occurrence of a single fracture on the weathering patterns (Deng & Spycher, 2019; Myagkiy et al., 2019; Trinchero et al., 2021; Zhang et al., 2022). Yet, these models remain rare and do not address the issues of fluid mixing and flow channelization driven by complex discrete fracture network (DFN) geometries because of the challenges of explicitly modeling fracture‐matrix interactions (Berre et al., 2019).…”

Significance of Low‐Velocity Zones on Solute Retention in Rough Fractures

“…A review listing several observations of flow channeling effects can be found in Tsang and Neretnieks (1998). This channeling behavior is dependent on external drivers such as mechanical stress (Zou et al., 2020) or water‐mineral reactions (Trinchero et al., 2021).…”