Modeling Chlorinated Solvent Persistence Due to Back Diffusion from Low-permeability Strata in Heterogeneous Aquifers

Back diffusion of chlorinated contaminants sequestrated in low-permeability strata is emerging as a critical impediment to site management, limiting remediation performance and sustaining plume longevity. Although increasingly important, modeling investigations to explore this phenomenon to date have generally focused on simplified geologic settings and hypothetical source zone architecture configurations in reduced dimensionality (1- and 2-D) domains. Furthermore, sorption has typically been neglected or treated as a linear equilibrium process. Serving as screening-level assessments, such simulations are not able to capture the full complexity of real field conditions. In this study, three-dimensional simulations are conducted to investigate the distribution and persistence of tetrachloroethene (PCE) contamination in highly heterogeneous media. In particular, the effects of back diffusion on plume persistence are compared to the effects of dissolution from a non-aqueous phase. An adapted version of the modular three-dimensional transport simulator MT3D is used to solve for the aqueous phase transport. A modification to the program enables the calculation of non-equilibrium PCE dissolution, using a lumped mass transfer coefficient. The flow field is adjusted to account for water saturation changes arising from PCE dissolution. The initial PCE distributions are chosen from a suite of PCE release scenarios in high variance formations with persistent layering. A statistical analysis for different spill scenarios improves our understanding of the effect of source zone architecture on the sequestration of dissolved and adsorbed PCE mass. The influence of different sorption models on mass sequestration and back diffusion is also explored. Complementary simulations in two dimensions provide insight into the importance of model dimensionality in predictions of plume persistence.