Bioremediation as a Full-Scale Remedy for a Trichlorethene-Contaminated Source Area and Diffuse Plume

The US EPA is completing a full-scale remedial action for a trichloroethene (TCE) source area and diffuse plume using bioremediation at the Bountiful/Woods Cross Operable Unit 1 Site in Utah. The site includes a high-concentration source area, with solvent concentrations as high as approximately 80 parts per million, and a diffuse plume that stretches approximately one-half mile downgradient. The contamination is present within the shallow aquifer, consisting of interbedded sands, silts, clays, and gravels. Initially, a pilot study was completed at the site to select the most appropriate amendment for stimulating reductive dechlorination. The selected remedy includes injection of emulsified oil into the subsurface to stimulate bioremediation; use of emulsified oil provides a long-term source of carbon (up to two years), while also enhancing mass transfer of contaminants from the non-aqueous phase into the groundwater. The source area remedy includes flooding the subsurface through injection into a grid-pattern of permanent injection wells, while the downgradient plume remedy includes injection into rows of injection wells (biobarriers). Advanced characterization using a membrane-interface probe was completed to identify areas requiring treatment.

Initial injections in the source area and first biobarrier were completed in late 2008, with a second injection completed in 2010. Reductive dechlorination was established within the source area within nine months after initial injections were completed, and complete destruction of groundwater contaminants was achieved within approximately three years after initial injections, with the exception of two hot spots. Targeted treatment of the hot spots has continued, with periodic injections of a sodium lactate solution to increase degradation rates of TCE and its daughter products. The downgradient biobarriers were installed in 2011, with injections completed during the summers of 2011 and 2013. Results indicate that substantial progress has been made in remediating contaminants in the diffuse plume using the biobarriers.