Back-diffusion from low permeability materials, such as clays, often determines natural attenuation timeframes for chemicals and the overall effectiveness of remedial strategies. Surprisingly, despite its high solubility, loading of 1,4-dioxane (1,4-D) into a clay matrix from a source area (e.g., site where 1,1,1-trichloroethane (TCA) with 1,4-D was released) can sustain a 1,4-D plume at concentrations orders-of-magnitude higher than relevant regulatory considerations for several decades in the absence of appreciable biotic and/or abiotic transformation. Recent studies provide clear evidence that 1,4-D is attenuating in a high percentage of impacted aquifers, but rates are widely variable, and mechanisms are speculative. The project team hypothesizes that both biotic and abiotic processes are contributing to the natural attenuation of 1,4-D and that one of the key locations for these processes is the clay-sand interface where 1,4-D is slowly back-diffusing from clay reservoirs.

The core objective of this project is to develop an improved understanding of the key biotic and abiotic degradation mechanisms of 1,4-D near interfaces of low and high permeability materials. Key studies will be conducted at environmentally relevant chemical concentrations (i.e., ≤ 100 µg/L), with consideration of the effects of common co-occurring chemicals (e.g., TCE and 1,1-dichloroethene comingled with 1,4-D). The goal will be to provide a clearer understanding of the role of both abiotic processes (e.g., oxidation via iron-derived hydroxyl radicals) and biotic processes (e.g., cometabolism) on the attenuation of low concentrations of 1,4-D diffusing from a low permeability clay matrix. Studies will be conducted in the laboratory with both natural aquifer materials and artificial samples.

This project is composed of a series of laboratory studies designed to improve the understanding of key attenuation mechanisms impacting 1,4-D as it diffuses from low permeability materials. Specific tasks evaluate the following:

• the generation of reactive oxygen species (ROS) by pure ferrous minerals and natural iron-containing clays and the kinetics of 1,4-D transformation by these ROS;
• the potential for microbial cometabolism to contribute to the transformation of 1,4-D via non-traditional substrates that are natural (e.g., humic and fulvic acids, root exudate compounds) or anthropogenic (e.g. co-occurring chemicals, such as toluene or mixed organics in landfill leachate); and
• the effects of combined abiotic and biotic mechanisms on 1,4-D transformation during diffusive flux from natural clays.

A series of state-of-the art techniques will be utilized during these tasks including batch experiments with ¹⁴C-1,4-D, allowing sensitive determination of 1,4-D half-lives from biotic/abiotic processes at field-relevant concentrations, and packed clay bed interfacial studies designed to provide measurement of both diffusion of 1,4-D (and other species, such as O₂) through natural clays and reactions at the clay-sand interface. The effort will culminate with a modeling effort designed to simulate the extent to which the relevant processes and their reaction kinetics potentially impact 1,4-D plume longevity and the timeframes for back-diffusion in impacted aquifers. This will play an important role in future monitored natural attenuation assessments for 1,4-D. A series of technology transfer activities are planned to ensure that the information gained during this project reaches the Department of Defense (DoD) community.

The key benefit of this effort to the DoD is the development of a new understanding of the critical biotic and abiotic reaction processes that contribute to the natural attenuation of 1,4-D in groundwater aquifers. Further, the project data will provide a new ability to estimate and model the longevity of 1,4-D plumes that are created by back diffusion from clay layers imbedded in the aquifer matrix. (Anticipated Project Completion - 2027)