The 14-compartment framework highlights characterization challenges that significantly influence optimization of remedial actions and the transition to long-term management, including the source/plume distinction, spatial heterogeneity in hydraulic conductivity, and the potential role of the vapor pathway when volatile organic compounds (VOCs) are present. A more comprehensive application of the framework that fully accounts for the relative magnitudes of contaminant mass in each of the compartments and the rates of mass transfer between compartments will require further development to better understand (1) the potential roles of desorption and of back-diffusion from low-permeability compartments to advective zones, (2) the variety of aquifer materials and conditions that comprise the “less transmissive” compartments, (3) the reactive characteristics of the aquifer that control the potential success of long-term strategies such as monitored natural attenuation, and (4) the complex factors that control the fate of volatile contaminants, which can exhibit markedly different behavior at seemingly similar sites because of variability in subsurface conditions, building characteristics at the soil interface, and climate conditions. Each of these issues is further explored below.
Back-Diffusion and Desorption
For many complex sites that have been subject to partial or complete source removal, the transition to long-term management is largely controlled by volatilization into the vapor phase (if applicable) and transport into the aqueous phase plume, as these two phases are the primary media for both off-site contaminant migration and the biotic and abiotic transformation processes associated with natural attenuation. Current conceptualizations of the plume have focused on three potential sources of contaminant mass influx in the groundwater: (1) discharge from undetected mass remaining in the upgradient source zone, (2) aqueous back-diffusion from aquifer materials to the pore water within low-permeability plume material and subsequent diffusive transport to advective zones, and (3) mass transfer (desorption) from aquifer sediments within both transmissive and low-permeability plume materials. For successful transition to long-term management, the contaminant influx from these three processes must be balanced by natural attenuation processes or controlled by physical/hydraulic containment.
The potential loading of dissolved mass from the source zone to the plume has received considerable attention and is straightforward to assess because the mass discharge occurs at the boundary of, rather than within, the plume compartment. However, back-diffusion and desorption of contaminants from materials within the plume are much more difficult to analyze because they are spatially nonuniform, dependent on the history