Natural attenuation: better than groundwater extraction?
Monitored natural attenuation (MNA) was evaluated and implemented in May 2000 as a replacement for groundwater extraction and treatment as the long-term remedy for a chlorinated ethene plume at a Superfund site in Washington (Lehmicke et al. 2000). MNA was approved because the plume met the first two lines of evidence in EPA’s protocol on natural attenuation for chlorinated solvents (U.S. EPA 1998): 1) loss of contaminant mass in groundwater over time, and 2) geochemical and biochemical evidence of trans¬formation and degradation. Modeling of the potential plume migration using BIOCHLOR® required inputs for half-lives of cis-DCE and vinyl chloride. Half-lives for cis-DCE and vinyl chloride were estimated using data from wells outside the influence of the groundwater extrac¬tion system, and by two other methods (Lehmicke et al. 2000). Using conservative input parameters, modeling indicated that vinyl chloride would not migrate beyond the monitoring well network if groundwater extraction ceased. Using historical data collected during the active groundwater extraction period, it was predicted in 2000 that by late 2004 cis-DCE would drop below the concentration that could produce vinyl chloride above its maximum contaminant level (MCL) (2 µg/l) in the well with the highest concentration. Monitoring data over the past four years, including the spring of 2004, indicate that cis-DCE and vinyl chloride are no longer present above the MCL in any other wells, and vinyl chloride is close to the MCL in the well with the highest concentration. It appears that modeling was effective in predicting groundwater cleanup. After groundwater extraction ceased, concentrations continued to decline as predicted. Therefore, natural attenuation was at least as effective as groundwater extraction, and biodegradation must have been the largest component of mass reduction even while the groundwater extraction system was active. This work was done for the Western Processing Trust, Seattle, WA.