Field evaluation of biological enhanced reductive dechlorination of chloroethenes in clayey till

The performance of enhanced reductive dechlorination (ERD) for in situ remediation of cis-1,2-dichloroethene (cDCE) and vinyl chloride in clayey till was investigated in a pilot test. A dilute groundwater solution containing emulsified soybean oil and Dehalococcoides bacteria was injected into a sand-filled hydraulic fracture. Fermentation of the ERD solution caused the establishment of a dechlorinating bioactive zone in the fracture within 1 month of injection. By 148 days, all the cDCE in the fracture was dechlorinated to ethene. Analysis of a clay core from Day 150 indicated that electron donor and fermentation products diffused from the fracture at least 10 cm into clay and that stimulated dechlorination occurred in the clay in the presence of Dehalococcoides (7.9·10⁴ cells g⁻¹). Comparison of chloroethene profiles in the Day 150 core to modeled diffusion profiles indicated degradation occurred in a bioactive zone extending approximately 5 to 6 cm into the clay matrix. These data suggest that a bioactive zone established in a sand-filled fracture can expand into the adjacent clayey till matrix and facilitate mass transfer from the matrix to the bioactive zone. These findings offer promise for ERD and support further development of methods for deploying ERD in clayey till and other low-permeability deposits.