FIELD METHOD TO QUANTIFY FATE OF TRICHLOROETHYLENE (TCE) AND DEGRADATION PRODUCTS IN FRACTURED ROCK LOW-PERMEABILITY ZONES

The fate of dissolved trichloroethylene (TCE) in low-permeability matrix in a fractured sedimentary rock is controlled by degradation, sorption, and diffusion. The goal of this research is to develop and test a field method capable of concurrently quantifying site-specific TCE and degradation product (DP) diffusion and degradation rates, and sorption coefficients in low-permeability zones within fractured sedimentary bedrock.

We conducted tests in 0.5 meter intervals of existing TCE contaminated bedrock wells at the field site in West Trenton, New Jersey. Intervals selected were low-permeability mudstone strata isolated using two packers. We prepared the tracer solution using groundwater from the borehole with the TCE, DPs, and other volatile organic compounds (VOCs) removed. We replaced the fluid in the packed-off test interval with the tracer amended VOC-free groundwater, and we monitored the concentrations of TCE, DPs, and tracers over time. We use the conservative tracer (bromide) to deduce the formation tortuosity and the reactive tracer (trichlorofluoroethene, TCFE) to evaluate fate parameters. TCFE was selected as the reactive tracer because it degrades and sorbs similarly to TCE. Concentrations of TCE, TCFE, and organic DPs were obtained by gas chromatography. Numerical modeling will be used to estimate the tortuosity, degradation rate coefficients, and sorption distribution coefficient and their uncertainties. The initial tests using our prototype system produced interpretable concentration patterns within approximately one month of deployment. Initial modelling indicates that the VOC sorption has a significant effect on the rate of VOC diffusion from the rock to the borehole.

Here, we present the results of initial tests that will be used to refine the physical apparatus, test and interpretive procedures in future tests. Ultimately, the field method developed will enable site-specific determinations of the values of diffusion and sorption coefficients for TCE and its DPs, and the abiotic degradation rate of TCE, in low-permeability zones. These site-specific values can be used within predictive models of site-scale contaminant fate that are an important element in evaluating natural attenuation and other long-term management strategies.