The impact of sedimentary anisotropy on solute mixing in stacked scour-pool structures

The spatial variability of hydraulic conductivity is known to have a strong impact on solute spreading and mixing. In most investigations, its local anisotropy has been neglected. Recent studies have shown that spatially varying orientation in sedimentary anisotropy can lead to twisting flow enhancing transverse mixing, but most of these studies used geologically implausible geometries. We use an object-based approach to generate stacked scour-pool structures with either isotropic or anisotropic filling which are typically reported in glacial outwash deposits. We analyze how spatially variable isotropic conductivity and variation of internal anisotropy in these features impacts transverse plume deformation and both longitudinal and transverse spreading and mixing. In five test cases, either the scalar values of conductivity or the spatial orientation of its anisotropy is varied between the scour-pool structures. Based on 100 random configurations, we compare the variability of velocity components, stretching and folding metrics, advective travel-time distributions, one and two-particle statistics in advective-dispersive transport, and the flux-related dilution indices for steady state advective-dispersive transport among the five test cases. Variation in the orientation of internal anisotropy causes strong variability in the lateral velocity components, which leads to deformation in transverse directions and enhances transverse mixing, whereas it hardly affects the variability of the longitudinal velocity component and thus longitudinal spreading and mixing. The latter is controlled by the spatial variability in the scalar values of hydraulic conductivity. Our results demonstrate that sedimentary anisotropy is important for transverse mixing, whereas it may be neglected when considering longitudinal spreading and mixing. Plain Language Summary When sediments are deposited in stream channels they retain the "imprint'' of the stream flow that deposited them. Groundwater flows more easily along the path of this streamflow imprint than against it-this is called anisotropy. Many groundwater systems are made up of deposits from many different streams and so will have many different imprints, even when the deposits are close to each other. We found that this can cause groundwater to flow along complicated and tangled paths. These tangled groundwater paths can change the way that compounds move through the system, especially at right angles to the main groundwater flow direction. This is important because groundwater scientists often do not think about the imprint, or anisotropy, of the sediments in their studies, and perhaps they should.