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Back diffusion evaluation and remediation: A summary of ongoing research activities
Citation:
Brooks, M. Back diffusion evaluation and remediation: A summary of ongoing research activities. Groundwater Forum, Ada, OK, March 06, 2025.
Impact/Purpose:
The purpose of this presentation is to provide a summary of ongoing research activities related to plume persistence due to back diffusion to the EPA's Groundwater Forum.
Description:
Back diffusion refers to the contamination of a high permeability layer or zone (HPZ) by the diffusive transport of contaminant out of an adjacent low permeability layer or zone (LPZ). It may become a significant factor at older sites once the primary contaminant source is exhausted, isolated, or removed. Research is being conducted on several back diffusion topics, and two will be highlighted in this presentation. First, to help evaluate the potential significance of plume persistence due to back diffusion at groundwater contaminated sites, an evaluation framework based on five lines-of-evidence was developed. This framework was then used to construct a Bayesian network model that provides a quantitative prediction of the likelihood for back diffusion. The prediction relies on an evaluation of the strength or weakness of each line-of-evidence, which are assessed using site characterization data. The second research area concerns the forward diffusion of remedial amendment to degrade contaminant in the LPZ. While amendment injection has not traditionally been considered efficient when LPZs are present due to flushing limitations, it is important to recognize that flushing limitations may be minimized, or at least predicted, with design considerations that explicitly account for LPZ diffusive transport. In particular, the amendment will undergo back diffusion just like the contaminant, and it is therefore important to ensure the residence time of the amendment in the LPZ is comparable to that of the contaminant under diffusive transport. These concepts are demonstrated using one-dimensional computer modeling, and a screening-level evaluation is being explored to aid in remedial designs.