Influence of Mass Transfer Via Diffusion on Plume Persistence in Heterogeneous Aquifers

Heterogeneities in aquifer soils play a significant role in determining remediation timeframes for groundwater contamination.  Previous investigations have shown that diffusion into and out of low-permeability sediments will delay remediation.  Considering an aquifer system comprised of high-permeability channels interbedded within a low-permeability porous medium (or  the converse), contaminant concentrations within these low-permeability sediments may remain above cleanup goals for a prolonged time period while at the same time diffusing contaminant mass back into the more permeable (i.e., mobile) portions of the aquifer.  This diffusion-based transfer of contaminants into and out of low-permeability media, therefore, could be a significant impediment to reaching conventional remediation goals at all compliance points within a plume.  Using a multi-layered three-dimensional numerical model, successive simulations were performed that incorporated increasing amounts of heterogeneity for a hypothetical aquifer system.  A single-domain model was utilized whereby aquifer heterogeneities were explicitly modeled on a cell-by-cell basis by varying both the soil pore structure and hydraulic conductivity field.  Contaminants were permitted to adsorb on the solid matrix in the high-permeability material, diffuse into and sorb onto the low-permeability sediments, as well as back diffuse into the more permeable aquifer soils.  Following source reduction, simulated transient concentration responses at a down-gradient monitoring point screened across a range of vertical intervals were evaluated by considering both volume-based concentrations and flux-based concentrations.  Modeling results not only indicate that the diffusive flux of contaminant into and within the low-permeability zones may significantly lengthen remedial timeframes but that the potential for regulatory non-compliance (or plume persistence) can be attributed to the ratio of fine-grained sediments contained within the screened interval of the compliance wells.