IN SITU BIOREMEDIATION OF A DNAPL SOURCE AND PLUME - 8 YEARS OF PERFORMANCE DATA

Background/Objectives: Full-scale treatment of a trichloroethene (TCE) source zone and plume began in 2005 beneath an active manufacturing building near Portland, OR using food-grade vegetable oil (VO). Remediation targets the source zone beneath a former vapor degreaser and a 12-acre downgradient plume. Contamination occurs within a shallow alluvial aquifer from 10-30 ft deep, underlain by a competent aquitard. Dense non-aqueous phase liquid (DNAPL) was observed and the maximum baseline TCE concentration in groundwater (1,170,000 µg/L) was at the TCE solubility limit. Baseline monitoring documented TCE and cis-1,2-dichloroethene (cDCE) predominance in the source and plume, respectively, with low levels of vinyl chloride (VC) and end products ethene/ethane (E/E); total organic carbon (TOC) < 10 mg/L was inadequate for robust reductive dechlorination (RD).

Approach/Activities: Effective treatment has been achieved at a low cost and with minimal disruption to manufacturing operations using a 2-year injection frequency. A coarse emulsion of VO and potable water was injected to 18 source wells on 10- to 20-ft centers. Approximately 20,000 gallons of 23 percent VO emulsion was injected during each event. Source injection of high-concentration VO emulsion is designed to sequester and treat contaminants through three primary mechanisms: 1) A reduction in source zone hydraulic conductivity and groundwater flux; 2) Partitioning of dissolved TCE from groundwater and dissolution of contacted DNAPL to injected VO; 3) enhanced RD within the source zone and downgradient resulting from VO fermentation.

Results: Source zone VO injection has resulted in active bioremediation in the source and up to 800 ft downgradient, and in decreased total chlorinated ethene (CE) molar mass in the source and throughout the downgradient plume. Increased TOC and methanogenic aquifer redox conditions characterize the active bioremediation zone. Source wells have transitioned from TCE to cDCE predominance with substantial VC and some E/E also detected. Abiotic degradation product acetylene is detected in source zone groundwater, indicating concurrent abiotic mass destruction. Downgradient wells have transitioned from cDCE to E/E predominance and total CEs have decreased by 99 percent throughout the downgradient plume.