FIELD TESTS YIELD DIFFUSION COEFFICIENTS AND TRICHLOROETHENE (TCE) DEGRADATION RATES FOR A FRACTURED SEDIMENTARY ROCK AQUIFER

We compare results from two approximately three month long field diffusion tests executed in trichloroethene (TCE) contaminated low permeability strata within fractured sedimentary rocks of the Newark Basin. We developed a straddle packer system and method capable of simultaneously quantifying the effective diffusion coefficients of TCE and its chlorinated degradation products (DP) in low permeability strata, and the TCE degradation rate coefficient in the borehole groundwater within the test interval. Each test was initiated by flushing the historically contaminated test interval with groundwater without volatiles and adding tracers. During the test, the TCE back-diffused out of the matrix into the test interval groundwater and the tracers diffused into the matrix. Water samples from the test interval over time were analyzed. Diffusive exchange between the groundwater and matrix, and degradation reactions in borehole groundwater were modeled to determine diffusion coefficients, degradation rates and form of the degradation reaction.

Different initial conditions led to different outcomes between the two tests. In borehole 71BR, TCE that back diffused into the borehole rapidly biodegraded to cis-1,2-dichloroethene (cDCE) and a zeroth order model provided a better fit to the data than a first order model. In borehole 07BR, the historical groundwater chemistry indicated more reduced conditions (i.e. greater Fe(II) and methane concentrations). The results from this test are consistent with a rapid Fenton-like reaction oxidizing TCE during the initial ~50 days of the test. The unintended introduction of a low concentration of dissolved oxygen during test initiation created conditions that would support this type of rapid degradation reaction. An increase in the methane concentration after about 50 days coincided with a shift to a slower TCE biodegradation reaction that continued through the end of the test. The porosity and tortuosity factors for this test were lower than in the other test. These experiments illustrate how the diffusion testing and interpretation method can be used to simultaneously determine site specific transport parameters in low permeability sedimentary rock with implications for improved remedy selection.