In Situ Characterization of Processes Controlling Long-Term Release of CVOCs from Low-Permeability Zones

The U.S. Geological Survey and the University at Buffalo, in cooperation with the U.S. Navy and the Strategic Environmental Research and Development Program, are developing a field method for characterization of diffusion, sorption, and reactions of chlorinated volatile organic compounds (CVOCs) in low-hydraulic-conductivity (low-K) strata. Although CVOC migration in groundwater is attenuated by diffusion and sorption in low-K strata, such strata can also act as long-term secondary sources. The method involves monitoring tracer concentrations in packer–isolated, 2-foot-thick, low-K intervals of open boreholes, in which advective transport is minimal, and estimating reaction rates and diffusion and sorption coefficients by inverse modeling. Proof-of-concept field testing has been conducted in monitoring wells open to mudstone rocks that underlie the former Naval Air Warfare Center (NAWC) in West Trenton, New Jersey. High concentrations of trichloroethene (TCE) and its degradation products (DPs) have persisted in groundwater at NAWC, despite decades of natural attenuation and remediation pumping. A test using conservative and reactive tracers was conducted by gas-stripping volatile components, including TCE and its DPs, from the packer-isolated-interval water and adding the tracers to the isolated zone. The reactive tracer trichlorofluoroethene (TCFE) is considered an analog to TCE for estimating sorption coefficients and reaction rates, for both biotic (biodegradation) and abiotic reactions. Low-volume water samples were collected using peristaltic pumping, closed-loop tubing, and syringes to prevent exposure to air. The same volume of preserved borehole ‘make-up’ water was injected into the test interval to maintain a constant water volume in the system. Gradual, but substantial, changes in tracer concentrations were measured during field tests lasting less than 3 months. Use of the zero-net-volume water-sampling method minimized artifacts associated with advection into and out of the isolated interval, but some fluctuations in the measured concentrations, possibly due to dilution and incomplete mixing, were observed.