Solute transport in permeable porous media containing a preferential flow feature: Investigation of non-Darcian flow effects

Fractures, faults, karstic features and/or interbedded layers of coarse sediment often manifest in the form of preferential flow features (PFFs). Previous investigations into the effects of PFFs on solute plume distributions within otherwise permeable host rocks have treated flow in PFFs as Darcian. However, flow through faults and fractures is known to exhibit non-Darcian flow behavior. The current study extends previous investigations of the effects of PFFs (e.g., a fault or fracture) on solute plumes in permeable aquifers that assume Darcian flow in the fault/fracture. The finite-element model COMSOL Multiphysics was applied to examine how non-Darcian flow in a single PFF influences flow fields and steady-state solute plume distributions in permeable aquifers, compared with the Darcian flow assumption. The Forchheimer equation was applied within numerical models to represent flow in the PFF for a variety of PFF-permeability contrasts, PFF apertures and matrix flow directions. The refraction of flow lines at matrix-PFF interfaces and the specific discharge in the PFF were found to be smaller in non-Darcian models, leading to larger peak solute concentrations (by up to 160%) and narrower plumes compared to Darcian models. In addition, the impact of non-Darcian flow on flow fields and solute plumes was greater with smaller matrix hydraulic conductivity, larger PFF aperture and larger incidence angle at the matrix-PFF interface. Upstream dispersion at the matrix-PFF interface, observed in previous matrix-PFF studies of solute transport as the result of steep solute concentration gradients opposing head gradients, was found to be reduced when non-Darcian flow was considered in PFFs. This study highlights that non-Darcian flow effects may be significant in the simulation of flow and solute transport within high-permeability PFFs embedded in otherwise porous aquifers.