Pilot-test evaluation for enhanced anaerobic biodegradation of chlorinated ethanes
In March 2004, an organic substrate mixture was injected through four permanent injection wells at an industrial site to enhance the anaerobic biodegradation of chlorinated ethanes in groundwater. Chemicals of concern (COCs) at the site are the solvent 1,1,1-trichloroethane (1,1,1-TCA), and degradation products 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethene (1,1-DCE), and chloroethane (CA), all of which are classified as chlorinated aliphatic hydrocarbons (CAHs). The technical approach utilized a combination of a readily degradable, soluble substrate (sodium lactate) to quickly establish anaerobic conditions, with a slow-release substrate (vegetable oil) to sustain the reaction zone. Together, sulfate reduction and methanogenesis were estimated to exert over 94 percent of the native electron acceptor demand at the site. Chlorinated solvents in groundwater and sorbed to the soil matrix were estimated to account for less than 2 percent of the total substrate demand. Sufficient substrate mass was applied to meet the estimated electron acceptor demand for a design life of 18 to 24 months. At 20 months post-injection, 1,1,1-TCA, 1,1-DCA, and 1,1-DCE were no longer detected in the treatment zone. Concentrations of CA peaked at 6 to 9 months post-injection before beginning to decline, but remained above the groundwater action level at 20 months. Therefore, the reaction zone may need to be sustained beyond the initial pilot test design life to achieve target concentrations for CA. Concentrations of total organic carbon (TOC), an indicator of substrate availability, have steadily declined since injection but remain above 20 milligrams per liter (mg/L), the level of TOC considered to be the minimum threshold concentration necessary to support anaerobic dechlorination at this site. The decline in TOC, coupled with increasing sulfate concentrations, indicated additional injection of substrate was necessary to achieve final cleanup goals within the pilot test area. A full-scale application was designed based on results of the pilot test, and was implemented from November 2005 to January 2006 to treat the entire contaminant plume.