Comparison of Near Replicate In Situ Trichloroethene Diffusion Tests in Fractured Sedimentary Rock Matrix

We developed a method to simultaneously measure sorption and diffusion coefficients for TCE and degradation products (DPs) in the low permeability matrix of fractured sedimentary rock in situ. We compare the results of two tests conducted in adjacent low permeability intervals of a single borehole in successive field seasons (2017 or 2018). The concentration patterns over time were qualitatively consistent for these two ‘near replicate’ tests building confidence in the method. Transport parameters in the matrix and biodegradation rate in the borehole were estimated using a radial diffusion model. The best-fit TCE sorption coefficients were greater than expected for low organic carbon content rocks, but consistent with independent laboratory measurements. The TCE biodegradation rates were very fast, greater than previous reports for this field site, and also consistent with laboratory experiments that used field samples. The porosities and tortuosity factors estimated are also reasonable. The best fit parameters estimated for each test differ modestly and differences may be attributable to lithologic variability and/or pre-test TCE concentration differences. Field and simulation sensitivity analysis combined suggest that abiotic TCE degradation had little effect on the TCE back diffusion rate.