Mass transfer from low-permeability zones into more active flow paths is controlled by diffusion, sorption, and degradation in the low-permeability zones. The effects of these natural attenuation processes must be included in defensible predictions of long-term chlorinated volatile organic compound (CVOC) fate. Importantly, even very slow transformation reactions in the low-permeability zones can significantly reduce back diffusion. The rates of these processes depend on site-specific physical and biogeochemical subsurface properties. The objective of this project was to develop a method, including tools and protocols, for determining the site-specific transport properties of CVOC in low permeability zones in fractured sedimentary rock aquifers.
In an uncased portion of a well, a dual packer system isolates a low permeability test interval that has been exposed to CVOCs. The monitoring data supplies the record of historical exposure of the matrix rock to contamination in the borehole. The project team initiated a field test by replacing the contaminated groundwater in the test interval with groundwater with the contaminants removed and tracers added. The concentrations of the CVOCs, including degradation products (DP), and tracers are monitored over time. Simulations are used to estimate the diffusion and sorption coefficients in the matrix, and the biodegradation rate coefficients in the borehole for the contaminants and DPs.
In order to achieve the project objective, the project team developed and tested: 1) a packer tool, 2) a specialized sampling apparatus, 3) a dosing tool (for tracer introduction), 4) a practicable test protocol, and 5) a numerical model capable of simulating the field data. The project team completed laboratory experiments on individual processes (e.g. sorption, biodegradation) against which to evaluate the parameters estimated from the field test.
The results show there were two successful tracer tests in adjacent vertical intervals of the same borehole; the similarity of the parameters estimated indicates good reproducibility for the field conditions of the tests. The parameters estimated from the field tests are also reasonably consistent with supporting laboratory measurements and literature data, verifying the test protocol.
The test simultaneously determines CVOC transport parameters needed to model back diffusion from low permeability sedimentary rock in situ. The information gained is suitable for both enhancing site conceptual models and conducting simulation analysis of field conditions. A single test provides information equivalent to at least three sets of research-quality independent laboratory tests of the matrix properties (a diffusion test, a sorption study, a biodegradation study). Further development of the test procedure is needed to broaden its applicability; this is best achieved through application to additional field site(s) with contrasting field properties to the test site.
Brotsch, J. 2017. Trichloroethylene (TCE) Sorption to Organic Matter in Sedimentary Rocks of the Newark Basin (Master’s Thesis). University at Buffalo, State University of New York.
Kiekhaefer, R. 2018. Evaluation of a Field Method for Monitoring the Diffusion of Trichloroethene (TCE) and its Degradation Products in Fractured Sedimentary Rock (Master’s Thesis). University at Buffalo, State University of New York.
Pugnetti, M. 2018. Trichloroethene and Trichlorofluoroethene Equilibrium Competitive Sorption to Sedimentary Rock from the Newark Basin, New Jersey (Master’s Thesis). University at Buffalo, State University of New York.