Field Measurement of Sorption Coefficients and Rates of Diffusion, Biodegradation, and Abiotic Degradation in the Rock Matrix
Low-transmissivity rock strata at contaminated fractured rock sites frequently remain the predominant long-term source of chlorinated volatile organic compounds (VOCs) to the high-transmissivity fractures even after years of engineered or natural remediation. The US Geological Survey and the University at Buffalo, in cooperation with the US Navy and the Strategic Environmental Research Development Program, are developing and testing a downhole packer tool to estimate sorption coefficients and diffusion, biodegradation, and abiotic degradation rates of chlorinated VOCs in these low-transmissivity rock strata. The tool is used to isolate a 2-foot-long section of the open interval of a borehole that does not contain high-transmissivity fractures. A closed-loop system is used to conduct tracer tests by first stripping VOCs from the native water and then re-injecting this water, along with organic and inorganic tracers, back into the test section. The closed-loop system is used to periodically collect low-volume water samples to monitor for the reappearance of VOCs, the disappearance of tracers, and the appearance of degradation products.
Several in situ experiments were conducted in boreholes in different low-transmissivity rock strata with different levels of contamination by trichloroethene (TCE) and cis-1,2-dichloroethene (cisDCE). In general the results showed increases in TCE and cisDCE concentrations in the test section with time that were used to estimate bulk diffusion rates and sorption coefficients. Concentrations of bromide, the inorganic tracer diffusing into the rock matrix from the test section, decreased slowly with time and were used to estimate the rock matrix diffusion rate not affected by sorption, biodegradation, or abiotic degradation. The rate of production of degradation products of TCE and trichlorofluoroethene (the organic tracer not present at the site and a TCE analog) provided estimates of the biodegradation rates. Inverse solute transport modeling was used to estimate the best-fit sorption coefficients, diffusion parameters, and degradation rates.