Scientific Investigations Report 2006-5056
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5056
Continued Biodegradation of Chloroethene Compounds in Ground Water at Operable
Unit 1, Naval Undersea Warfare Center, Division Keyport, Washington
The U.S. Geological Survey evaluated the biodegradation of chloroethene compounds in ground water beneath the former landfill at Operable Unit 1 (OU-1) of the Naval Undersea Warfare Center (NUWC), Division Keyport. The predominant contaminants in ground water are the chloroethene compounds trichloroethene (TCE), cis 1,2 dichloroethene (cis-DCE), and vinyl chloride (VC). The remedy selected for ground-water contamination at OU-1 includes phytoremediation and natural attenuation. In 1999, the Navy planted two hybrid poplar plantations, referred to as the northern and southern plantations, over the most contaminated parts of the landfill. The Navy monitors tree health, ground-water levels, and contaminant concentrations to assess the effectiveness of phytoremediation. The U.S. Geological Survey began a cooperative effort with the Navy in 1995 to monitor the effectiveness of natural attenuation processes for removing and controlling the migration of chloroethenes and chloroethanes. Field and laboratory studies from 1996 through 2000 demonstrated that natural attenuation, particularly biodegradation, of chloroethenes and chloroethanes in shallow ground water at OU-1 was substantial. The U.S. Geological Survey monitored geochemical and contaminant concentrations in ground water annually from 2001 through 2004. This report presents ground water geochemical and contaminant data collected by the U.S. Geological Survey during June and July 2004, and evaluates evidence for continued biodegradation of chloroethenes in ground water through 2004.
Geochemical and contaminant concentration data through 2004 indicate that substantial biodegradation of chloroethenes in ground water continued beneath the landfill at OU-1. Contaminant concentrations in ground water decreased beneath most of the 9-acre landfill between 1999 and 2004. The evidence indicating biodegradation was a primary cause for the decreased concentrations includes decreasing ratios of more highly chlorinated compounds to less chlorinated compounds over time, and widespread detection on non chlorinated biodegradation end-products ethane and ethene. No widespread changes in ground-water reduction oxidation (redox) conditions were observed since 1996 that could result in either more- or less-efficient biodegradation. Anaerobic redox conditions prevailed at the site and, despite interannual variation in the predominant electron-accepting process, no consistent trend was determined indicating either more strongly or more mildly reducing conditions.
Even with continued biodegradation, dissolved-phase contaminant concentrations in the tens of milligrams per liter have persisted beneath the southern part of the 0.7 acre southern plantation. The magnitude and persistence of those concentrations indicate that non-aqueous phase liquid chloroethenes likely are present beneath the southern plantation and are not substantially affected by biodegradation. During 2004, chloroethenes continued to discharge to the adjacent marsh at concentrations exceeding quantitative remediation goals for the site. Chloroethene concentrations in ground water discharging to a 100-foot long reach in the southern part of the marsh were at least 300 times greater than concentrations in ground water discharging elsewhere in the marsh.
Flux calculations based on 2004 data indicate that 87 percent of dissolved-phase chloroethenes in the upper aquifer beneath the landfill were degraded before they discharged to surface water. The flux calculations based on 1999–2000 data indicate a larger dissolved contaminant flux from the landfill than in 2004, along with a larger percentage (93 percent) that was biodegraded and a smaller contaminant flux to surface water. Flux calculations were extremely sensitive to a few measured chloroethene concentrations at highly contaminated wells and passive-diffusion samplers near the southern plantation where the interannual variability was expected to be high. Comparison of calculated fluxes to measured surface-water concentrations indicated that not all TCE flux from the landfill was represented by measured concentrations from the passive-diffusion sampler sites. VC concentrations continued to attenuate in the upper 2 feet of organic-rich marsh sediments, likely due to microbial degradation in the sediments and volatilization from the overlying surface water.
In the intermediate aquifer near the downgradient margin of the landfill, chloroethene concentrations decreased inconsistently between 1999 and 2004. At the farthest downgradient monitoring points in the intermediate aquifer, TCE remained undetected and cis-DCE and VC were consistently measured at low but detectable concentrations. Biodegradation of chloroethenes in the most contaminated part of the intermediate aquifer was uncertain based on the 1995–2000 data, but the data through 2004 reliably indicate that some reductive dechlorination in the intermediate aquifer occurred.
Overall, biodegradation of chloroethenes in ground water throughout OU-1 continued through 2004 and is responsible for preventing most of the mass of dissolved-phase chloroethenes in the upper aquifer beneath the landfill from discharging to surface water.
For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page.
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