Upscaling of regional scale transport under transient conditions: evaluation of the Multi‐rate Mass Transfer model

Abstract: Regional scale transport models are needed to support the long‐term evaluation of groundwater quality and to develop management strategies aiming to prevent serious groundwater degradation. The purpose of this study is to evaluate the capacity of a previously developed upscaling approach to adequately describe the main solute transport processes, including the capture of late‐time tails under changing boundary conditions. Potential factors that impact the performance of upscaling methods, including temporal variations in mass transfer rates and mass distributions, were investigated. Advective‐dispersive contaminant transport in a 3D heterogeneous domain was simulated and used as a reference solution. The equivalent transport under homogeneous flow conditions was then evaluated by applying the Multi‐Rate Mass Transfer (MRMT) model. The random walk particle tracking (RWPT) method was used to solve the solute transport for heterogeneous and homogeneous MRMT scenarios under steady‐state and transient conditions. The results indicate that the MRMT model can capture the tails satisfactorily for plumes transported with ambient steady‐state flow fields at all studied scales using the same parameters. However, when the boundary conditions change in either local, plume or regional scale, the mass transfer model calibrated for transport under steady‐state conditions cannot accurately reproduce the tailings observed for the heterogeneous scenario. The deteriorating impacts of transient boundary conditions on the upscaled model are more significant for regions where the flow fields are dramatically affected, which highlights the poor applicability of the MRMT approach for complex field settings. This finding also has implications for the suitability of other potential upscaling approaches.