The overall objective of this research was 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, and to verify that abiotic dechlorination reactions capable of significantly reducing monitored natural attenuation (MNA) time frames actually occur in the field within bedrock matrices. Rates of abiotic chlorinated ethene degradation due to reaction with naturally occurring ferrous minerals within the rock matrices were compared to the rate of diffusive flux through the rock matrix. The impact of this degradation on attenuation timeframes subsequently was assessed.
Specific objectives of this project were as follows:
- Measure and evaluate the effective diffusivity of TCE in the rock matrix as a function of rock type, mineralogy, and orientation of mineral bedding planes.
- Measure and evaluate the rate and extent of abiotic dechlorination in TCE-impacted rock matrices and how rock mineralogical and physical properties affect these processes.
- Measure and evaluate the coupled diffusion and reaction of chemical oxidants into rock matrices.
- Determine the impact of chemical oxidation fronts emanating from the fracture plane on rock structure and mineralogy, and ultimately on the diffusion and reaction of TCE from the rock matrix.
- Identify the rate of TCE dechlorination due to the presence of ferrous iron in the rock matrix.
- Measure and evaluate the rate and extent of abiotic dechlorination of additional volatile organic compounds (e.g., chlorinated ethanes) in rock matrices and how rock mineralogical and physical properties affect these processes.
- Develop and validate a conceptual and mathematical model to describe the above processes for the purpose of providing improved estimates of timeframes for which matrix diffusion can sustain a contaminant groundwater plume.
The approach for measuring and evaluating the impacts of bedrock structure and mineralogy on the persistence and diffusive flux of TCE from rock matrices to groundwater consisted of a series of diffusion and reaction experiments performed on intact rock cores collected from Department of Defense (DoD) facilities. Rocks were first assessed for mineralogy, porosity, and pore structure. Diffusion experiments were performed using a modified diaphragm cell technique. Diffusive flux of tracers and TCE, coupled with the abiotic reaction of TCE in rock matrices, were measured for multiple rock types. Experiments were performed on both pristine (i.e., no historical TCE impacts) and TCE-impacted (i.e., exposed to TCE for decades) rock to assess the extent to which the measured abiotic dechlorination reactions are occurring in situ at TCE-contaminated bedrock sites. The relative importance of these reactions in rock matrices at the field scale were assessed by comparing reaction and diffusion timescales.
To verify the extent to which application of in situ chemical oxidation (ISCO) can be effective for treating TCE mass within the rock matrix, additional experiments were performed with permanganate and persulfate. In these oxidant experiments, the migration of oxidant into the rock matrix was measured as a function of rock type, and the impact of oxidant exposure on the effective diffusion coefficient was subsequently determined. Both the extent of oxidant migration into the rock, and the impact of oxidant exposure on the effective diffusion coefficient, will play a large role in the overall effectiveness of ISCO in fractured bedrock where contaminant uptake into the rock matrix is substantial.
Results from the diffusion experiments showed that the orientation of mineral bedding relative to the direction of diffusion could have a substantial impact on the diffusive flux. By attaining a measurement of the rock matrix porosity in the orientation of the diffusion gradient, a reasonable prediction of the effective diffusion coefficient was attained.
Experiments examining the coupled diffusion and reaction of TCE in rock matrices showed that, for all rock types examined, measureable abiotic dechlorination occurred within the rock matrices. Abiotic reactions, which generated ethene, ethane, acetylene, and/or propane, were well described by a first-order rate constant. The observed first-order rate constants for the various rock types were related to the ferrous mineral content of the rock that was in contact with the rock matrix porosity. The observed rate constants in rock that been exposed to TCE for decades were on the same order of magnitude as rate constants observed in rocks that had no known prior exposure to TCE, suggesting that these reactions likely are occurring in situ in bedrock plumes. Results show that the measured rates of reaction are significant when considering the time and length scales of matrix diffusion in the field.
Oxidant penetration into the rock matrix over a two-month period was on the order of 100 microns. This diffusional migration was well predicted based on the measured effective diffusion coefficient and oxidant demand for each rock type examined. Furthermore, manganese oxide precipitate coating of the rock matrix did not provide a significant diffusion barrier. Thus, the use of chemical oxidants is unlikely to be effective for treating chlorinated ethane contaminants within rock matrices, or in mitigating matrix back diffusion.
This research has resulted in improved approaches, both experimental and conceptual, for estimating contaminant flux and longevity in rock matrices. Identifying and quantifying the abiotic dechlorination reactions in rock matrices have highlighted an important attenuation mechanism in fractured bedrock aquifers that are impacted by chlorinated solvents. Results of this study suggest that, at least for some bedrock sites where ferrous iron minerals are present within the rock matrices, abiotic reaction in rock matrices may serve as an important mechanism for mitigating the adverse impacts of matrix back diffusion on plume intensity and longevity. This research highlights the importance of understanding these diffusion and reaction processes for improved insight into managing chlorinated solvent plumes in fractured bedrock aquifers.