Evaluation of natural attenuation in a chlorinated ethene plume undergoing groundwater extraction

Monitored natural attenuation (MNA) is being considered as a replacement for groundwater extraction and treatment as the long-term remedy for a chlorinated ethene plume at the Western Processing Superfund site in Kent, Washington. The protocol for evaluating natural attenuation of chlorinated solvents suggests collecting at least two lines of evidence: 1) loss of contaminant mass in groundwater over time, and 2) geochemical and biochemical evidence of transformation and degradation. Historical groundwater data were reviewed for evidence of contaminant mass loss over and above that attributable to groundwater extraction. The absence of trichloroethylene (TCE) and the presence of the dechlorination daughter products cis-1,2-dichloroethylene (cis-DCE) and vinyl chloride indicated TCE was undergoing significant natural biodegradation. A subsequent field investigation provided compelling evidence of natural attenuation. Conditions conducive to reductive dechlorination and anaerobic oxidation were observed in most wells: low dissolved oxygen (DO) and redox, and high concentrations of methane and dissolved iron. Complete dechlorination products (ethene, ethane) were found at levels up to 89 times higher than vinyl chloride on a molar basis. Modeling plume migration using BIOCHLOR(R) required estimates of half-lives for cis-DCE and vinyl chloride. A half-life for cis-DCE of 1.1 years was estimated using data from wells outside the influence of the groundwater extraction system. A half-life for vinyl chloride of 0.55 year was estimated by the same method and two other methods. Using conservative input parameters, modeling indicated vinyl chloride would not migrate beyond the current monitoring well network if groundwater extraction ceased. A monitoring plan that included use of the groundwater extraction system as the contingency remedy was presented to EPA. Tentative approval has been given to shut down the groundwater extraction system in May 2000.