Lab Investigation Using a Box Model and Image Analysis of a Contaminant Back-Diffusion Process from Low-Permeability Layers

Contaminants stored in low-permeability soils can continue to threaten the adjacent groundwater system even after the aquifer is considered remediated. The redistribution of contaminants from low-to-high-permeability aquifer zones (Back-Diffusion) can generate a long-term plume tail, commonly considered one of the main obstacles to effective groundwater remediation. In this paper, a laboratory test was performed to reproduce the redistribution process from low-permeability silt lenses (k ≈ 1 × 10−7 m/s) to high-permeability sand aquifers (k ≈ 1 × 10−3 m/s). The target of the experimental and numerical approach was finalized to verify what influence the shape and position of the lenses could have, with respect to the bulk flow, on the time necessary to complete the depletion of the dissolved substances present in the lenses. For this purpose, an image analysis procedure was used to estimate the diffusive flux of contaminants released by these low-permeability zones in different boundary conditions. The results obtained in the laboratory test were used to calibrate a numerical model implemented to simulate the Back-Diffusion process. Once calibrated, the numerical model was used to simulate further scenarios to evaluate the influence of the location and shape of the low-permeability lenses on the time necessary to diminish its contaminant content when subjected to a steady-state flow. The numerical model was also used to investigate the effect of different groundwater velocities on the depletion time of the process. The results show that the shape and position of the lens have an important impact on the time necessary to empty the lens, and an increase in the velocity field in the bulk medium (flow rate rising from 1.6 l/h to 2.5 l/h) does not correspond to diminishing total depletion times, as the process is mainly governed by diffusive transport inside the lens. This appears to be significant when the remediation approach relies on pumping technology. Future research will verify the behavior of the released plume in a strongly heterogeneous porous medium.