A three-tiered remediation approach is being used for successful field treatment of a 1,1- dichloroethene (1,1-DCE) groundwater plume. The plume is characterized by a chemical source area subject to anaerobic conditions (<1.0 mg/L dissolved oxygen, DO) with high levels of dissolved phase 1,1-DCE (>8.0 mg/L); an interior plume area of moderate aerobic (<4.0 mg/L DO) and chemical concentrations (<1.0 mg/L DCE); and a 300-foot perimeter plume area of high aerobic (>4.0 mg/L DO) and low chemical concentrations (<0.01 mg/L DCE).
Previous studies from 1998 and early 1999 indicated that downgradient of the source area, elevated DO levels and intrinsic aerobically-degrading microbes appeared to promote natural attenuation processes of chemical degradation of 1,1-DCE to non-toxic endpoints such as carbon dioxide in the absence of vinyl chloride. Low, but persistent, concentrations of 1,1-DCE along the plume perimeter indicated the rate of plume movement exceeded the rate of natural attenuation. In late 1999 and mid 2000, two groundwater circulation wells were installed near the source area, in part, to facilitate aerobic transformation of 1,1-DCE to its primary daughter product vinyl chloride. Also in late 1999, perimeter areas of the plume with high levels of DO and low 1,1-DCE were stimulated using a nutrient mixture of Oxygen Release Compound (ORC) and simple table sugar. Pilot studies conducted in 1998 indicated the nutrient mixture, when injected at perimeter wells, provided a carbon substrate (sugar) for microbial activity to degrade the 1,1-DCE while the ORC offset the reductive dechlorination transformation process of 1,1-DCE to vinyl chloride. Since late 1999 the in-situ bioremediation injection technique at the plume perimeter has stimulated biological activity without the creation of vinyl chloride. Groundwater concentrations at the plume source have diminished with a corresponding increase of the daughter product vinyl chloride. Monitoring of plume geochemistry and geometry indicate vinyl chloride is naturally degraded by the groundwater system aerobic microbes downgradient from the source area. Closure of the site subject to state risk assessment requirements is expected in 2002.
Historical releases of chlorinated solvents at a manufacturing facility in Houston, Texas impacted shallow groundwater, creating a plume that extends approximately 500 feet in length and 300 feet in width (Figure 1). Groundwater sampling since 1994 indicated that chlorinated hydrocarbons, predominantly 1,1-dichloroethene (DCE), tetrachloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC) exceeded Texas-regulated groundwater cleanup criteria. Remedial implementation of groundwater restoration to prescribed Texas Natural Resource Conservation Commission (TNRCC) Voluntary Cleanup Program (VCP) cleanup criteria commenced in 1999 (Phase I remediation) after a pilot study of 1998 indicated enhanced biodegradation at the site was feasible by stimulating indigeneous microbes in the groundwater.