Back-Diffusion from Low-Permeability Layers

Abstract:

There has been increasing recognition of the importance of interaction between aquifers and aquitards on contaminant fate and transport. Aquitards can serve as long-term contaminant sources to aquifers because contaminant mass diffuses out of the aquitards when source mass is depleted by aquifer remediation. This study describes analytical and experimental approaches to understand reactive and non-reactive solute transport in a thin aquitard bounded by an aquifer system. We solved a one-dimensional solute diffusion and transport equation for an aquifer with a finite aquitard using the method of images, and we demonstrated the application in a series of well-controlled laboratory experiments. Solute concentrations in the aquitard were measured in situ using dyes and a light-reflection visualization technique. The analytical solutions with image sources showed very good agreement with aquitard concentration distributions measured in situ in laboratory experiments (mean goodness of fit coefficient of efficiency, E = 0.97). Additionally, reactive and non-reactive solute concentration breakthrough curves (BTCs) were measured from effluent samples and compared to analytical solutions. These results enabled quantitative assessment of the effect of back diffusion on plume persistence. The measured BTCs were predicted very well by solutions that included image sources (E = 0.98) compared to E = 0.82 without image sources. Low-retardation solutes accumulated more stored mass with greater penetration distance in the aquitard, and then generated relatively higher back diffusion flux compared to high-retardation solutes that stored less mass with lower penetration depth in the aquitard. However, because the duration of aquitard mass release was much longer, high-retardation solutes have a great effect on long-term back diffusion risk.