Authors: Megan Sebbena and Adrian Werner
Discrete flow features (DFFs) such as fractures, faults, sand lenses and clay layers are common geologic features in groundwater systems. DFFs can provide preferential pathways (i.e. ‘preferential flow features’; PFFs) or act as barriers (i.e. ‘barrier flow features’, BFFs) to fluid flow and solute transport. Considerably less research attention has been paid to the role of DFFs in modifying groundwater flow and solute transport where the host rock is permeable, compared to low-permeability rocks. A numerical investigation was conducted within this study to explore how the distributions of solute plumes in permeable aquifers are influenced by a DFF.
HydroGeoSphere was used to evaluate the impact of 2D flow effects within DFFs, which were treated as thin bands of porous media. We show the changes to solute plumes that occur where both BFFs and PFFs are encountered in otherwise permeable rock aquifers (e.g. sandstone and limestone). Additionally, the potential role of ‘back dispersion’ (i.e. the anomalous movement of solutes from the PFF back into the matrix against the direction of groundwater flow) on predictions of PFF effects are explored. The numerical simulations were used to quantify the displacement and widening (or narrowing) of a steady-state solute plume as it crosses a DFF in idealised, 1 × 1 m aquifers. A simple analytical expression for the advective displacement of a solute plume encountering a DFF is provided.
The outcomes of this study suggest that PFFs typically have a more significant influence on plume distributions than BFFs, and the impact of DFFs on solute plumes generally increases with increasing aperture. Plumes crossing a PFF are less symmetrical, and peak solute concentrations beneath PFFs are considerably lower than plumes in BFF cases.