(281d) Enhanced Anaerobic Bioremediation Shuts Down a Tce Plume Groundwater Treatment System

Shallow groundwater beneath a manufacturing facility in Connecticut is contaminated with chloroethenes. The affected aquifer starts approximately four feet (1.2 m) below ground surface (bgs), and is composed of sand underlain by a clay aquitard at approximately 14 feet (4.3 m) bgs. The plume is approximately 500 ft (150 m) long by 140 feet (43 m) wide. In 1997, a groundwater pump and treat system was installed. The system maintained hydraulic control preventing further contaminants from leaving the site, however, volatile organic mass removal rate from the groundwater collection system was limited to less than three pounds (1.4 kg) per year.

In 2004, an enhanced anaerobic bioremediation (EAB) program was implemented in phases, with the injection of sodium lactate solution into the most concentrated part of the source area where maximum trichloroethene (TCE) concentrations ranged from 5,000 to 20,000 µg/L. The carbon source was gravity fed through rows of two-inch (5.1 cm) diameter injection wells, with the existing downgradient groundwater extraction horizontal wells used to speed its distribution throughout the treatment zone. When elevated dissolved organic carbon was detected in the extraction wells, the pumps were turned off to minimize carbon source removal from the system. A single downgradient groundwater extraction well, located near the property boundary, remained in operation until early 2008. After two additions of lactate into the source area, maximum TCE groundwater concentrations fell by more than 70%, while cis-1,2-dichloroethene and vinyl chloride concentrations temporarily increased, and ethene concentrations rose to as high as 700 µg/L, indicating significant dechlorination of the parent compounds was occurring. Furthermore, DNA analysis of the treated groundwater showed significant cells of Dehalococcoides spp. per milliliter, and a similar concentration of vinyl chloride reductase functional genes, indicating that bioaugmentation was not necessary to promote complete dechlorination.

Ultimate groundwater cleanup goals for the aquifer were set by the state regulatory agency to drinking water criteria. Based on the successful initial stages of bioremediation and a site conceptual model, the Connecticut Department of Environmental Protection (CT DEP) agreed to alternative cleanup levels (ACL) for ending active groundwater remediation. The point of compliance became the nearest downgradient, off-site residences, where groundwater to indoor air volatilization criteria must be met at all times. The alternative cleanup levels were established for suspending P&T operations: PCE = 340 µg/L, TCE = 50 µg/L, DCE = 100 µg/L, and VC = 2.9 µg/L at selected wells along the down gradient portion of the site.

Based upon several 2007 groundwater sampling rounds, chloroethene concentrations observed near the downgradient site boundary were below the ACL criteria established by CT DEP. Consequently, the existing groundwater collection and treatment system was shut down. The agreement requires the system to be maintained in standby mode for up to one year of additional quarterly sampling. Beyond active groundwater pumping, once chloroethene concentrations observed in the source area meet similar ACL criteria, active bioremediation will end and the site will be granted No Further Action status.