Evaluation of a Field Method for Monitoring the Diffusion of Trichloroethene (TCE) and its Degradation Products in Fractured Sedimentary Rock
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Low-permeability rock strata at contaminated fractured rock sites frequently are the predominant long-term source of chlorinated volatile organic compounds (CVOCs) to the high-transmissivity fractures. The fate of dissolved trichloroethylene (TCE) in the low-permeability matrix of a fractured sedimentary rock is controlled by degradation, sorption, and diffusion. The goal of this work was to develop, evaluate, and increase the effectiveness of a field method capable of producing data to parameterize these processes for TCE in the low-permeability matrix within fractured sedimentary bedrock. The evaluation of the field method included a qualitative assessment of the behavior of TCE and its degradation products for two field diffusion tests. In collaboration with U.S. Geological Survey colleagues, tests were conducted in 0.5 meter intervals of existing TCE contaminated bedrock wells isolated using a straddle packer system at the former Naval Air Warfare Center in West Trenton, New Jersey. The CVOCs were removed from the borehole fluid using a method and apparatus developed for this work. A reactive tracer and a non-reactive tracer were added to the test interval. The concentrations of TCE, its degradation products, and tracers were monitored for at least three months. Samples were collected over time using a closed-loop sampling apparatus designed to prevent the introduction of oxygen, the volatilization of CVOCs, and to limit pressure changes and the potential for advective transport during the sampling process. The results of this work indicate that oxygen introduction was improved during test initiation between the two field tests. The concentration results obtained from the method were sensible and appeared to be representative of the processes expected in the test interval. The evaluation of the field method showed that minor procedural changes would improve the method, but by monitoring CVOC concentrations over time, it was possible to interpret the relative rates of processes affecting CVOC behavior.