Coupled Diffusion and Reaction Processes in Rock Matrices: Impact on Dilute Groundwater Plumes
ER-1685
Objective
Dissolved contaminants present in rock matrices can serve as a long-term source in fractured bedrock aquifers, sustaining dilute groundwater plumes for extended periods of time, even after sources in conductive fractures have been treated. The intensity and longevity of the dilute groundwater plume will be impacted by the diffusional flux between the rock matrix and adjacent conductive fractures, as well as the fate of contaminants residing within the rock matrix itself. However, processes controlling diffusive flux from rock matrices are not well understood. Orientation of mineral bedding planes, naturally occurring weathering or redox fronts emanating from fracture interfaces, and coatings formed at fracture-rock interfaces are all factors that make estimating effective diffusivities through the rock difficult. Impacts of oxidants used for in situ remediation also can impact rock structure and diffusive flux through rock matrices, further impeding the ability to predict long-term contaminant diffusive flux through the rock matrix. In addition, mineralogy can impact dissolved contaminant fate, as ferrous minerals can facilitate the reductive dechlorination of contaminants within the rock matrix, thereby serving as a slow natural attenuation mechanism that can mitigate impacts to the groundwater plume.
The overall objective of this project is to measure and evaluate the impacts of bedrock structure and mineralogy on the persistence and diffusive flux of trichloroethene (TCE) from rock matrices to groundwater. The impact of oxidation fronts (generated naturally or via in situ chemical oxidation) between the fracture and matrix on these processes will be investigated, and TCE dechlorination kinetics via ferrous iron minerals within the rock also will be evaluated.
Technical Approach
These objectives will be attained through a series of laboratory experiments coupled with development of a conceptual and mathematical model. A series of diffusion experiments will be performed on rock cores collected from multiple Department of Defense (DoD) facilities. All rocks will be thoroughly analyzed with respect to mineralogy, porosity, and pore structure. Diffusive flux of TCE and chemical oxidants will be measured for each rock type, with a focus on determining the relationship among rock mineralogy, weathering, and diffusional transport. Rates of reductive dechlorination via ferrous minerals also will be measured. Evaluation of mineralogy, diffusive flux, and reductive dechlorination for each rock will be repeated after exposing the rock to chemical oxidants or dissolved iron species, as such exposure is often encountered as a result of in situ remedial activities (i.e., chemical oxidation or biostimulation).
Benefits
Results of this study are expected to enhance DoD’s ability to predict the long-term rate of dissolved contaminant flux from rock matrices, thereby improving estimates of groundwater plume contaminant concentrations and natural attenuation time frames. Specifically, identifying processes that would limit the flux or longevity of contaminant mass in bedrock matrices (e.g., reduced porosity and diffusional flux due to mineralogical changes following permanganate injections, or reductive dechlorination of TCE via ferrous minerals) could prove useful in demonstrating the efficacy on monitored natural attenuation (MNA) remedies. (Anticipated Project Completion - 2012)
Points of Contact
Principal Investigator
Dr. Charles Schaefer
Shaw Environmental, Inc.
Phone: 609-895-5372
Fax: 609-895-1858
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