FIELD MEASUREMENTS TO QUANTIFY TRICHLOROETHENE DIFFUSION, DEGRADATION, AND SORPTION IN FRACTURED SEDIMENTARY ROCK

In chlorinated solvent contaminated fractured sedimentary rock aquifers, low-permeability strata typically act as a long-term or secondary source of contamination to mobile groundwater in the high-permeability fractures. The fate of dissolved trichloroethylene (TCE) in the low-permeability matrix is controlled by abiotic degradation, sorption, and diffusion in the matrix, and biodegradation reactions that occur principally in the fractures. The goal of this work is to develop 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.

In collaboration with U.S. Geological Survey colleagues, we conducted tests in 0.5 meter intervals of existing TCE contaminated bedrock boreholes at a field site in West Trenton, NJ. Low-permeability mudstone strata were isolated using two packers. We replaced the water in the packed-off test interval with tracer amended groundwater from which we had removed the TCE and DPs. We monitored the concentrations of TCE, DPs, and tracers over time. Trichlorofluoroethene(TCFE) was selected as the reactive tracer because its biodegradation rate and DP distribution are similar to those of TCE. Supporting laboratory experiments demonstrated that TCE and TCFE sorption isotherms for mudstones from the field site have a similar shapes; that their sorption coefficients are inversely proportional to their water solubilities.

The in situ results showed increases in TCE and DP concentrations, and decreases in the added tracer concentrations, with time that were quantifiable. Through inverse modeling, we determined the formation factor (or tortuosity) from the bromide data and we used the TCFE and its DP data to constrain the TCE biodegradation rate and sorption coefficients. Our pilot tests show potential for this method to quantify the process affecting TCE in low permeability rock matrices that will improve predictions of TCE longevity from legacy contamination.