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U.S. GEOLOGICAL SURVEY
Open-File Report 2007-1430

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Summary

Previous investigations have shown that natural attenuation and biodegradation of chlorinated VOCs are substantial in shallow ground water beneath the 9-acre former landfill at Operable Unit 1 (OU 1), Naval Undersea Warfare Center (NUWC), Division Keyport, Washington. The U.S. Geological Survey (USGS) has continued to monitor ground-water geochemistry to assure that conditions remain favorable for contaminant biodegradation. This report presents the ground-water geochemical and selected VOC data collected at OU 1 by the USGS during June 12–14, 2006, in support of long-term monitoring for natural attenuation. Data include concentrations of redox-sensitive geochemical constituents and dissolved gases at 13 wells and 9 piezometers, and concentrations of VOCs at 9 piezometers.

For June 2006, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of VOCs were inferred for 5 of 15 upper-aquifer sites in the northern and southern plantations. Predominant redox conditions in ground water from the intermediate aquifer just downgradient of the landfill remained mildly reducing and somewhat favorable for reductive dechlorination. Since about 2003, measured H₂ concentrations in the upper aquifer generally have been lower than those previously measured, although methane and sulfide have continued to be detected throughout the upper aquifer beneath the landfill. Overall, no widespread changes in ground-water redox conditions were measured that should result in either more or less efficient biodegradation of chlorinated VOCs.

For the northern plantation in 2006, chlorinated VOC concentrations at piezometers P1-3 and P1-4 were lower than previously measured, and TCE, cis-DCE, or VC were not detected at piezometers P1-1 and P1-5. The steady decrease in contaminant concentrations has been consistent in the upper aquifer beneath the northern plantation. The reductive dechlorination end-products ethene and ethane were measured at concentrations as high as 61 micrograms per liter in the upper aquifer beneath the northern plantation, which is reliable evidence that reductive dechlorination of VOCs is ongoing.

For the southern plantation in 2006, changes in chlorinated VOC concentrations at the piezometers were highly variable. At piezometer P1-9, total chlorinated VOCs, as well as cis-DCE and VC in 2006, were measured at their highest concentrations to date; contaminant concentrations between June 2004 and June 2005 substantially decreased at piezometer P1-9 for reasons unknown. At piezometer P1-10, the consistent temporal trend of decreasing chlorinated VOC concentrations measured since 1999 ended, and the total chlorinated VOC concentration in 2006 substantially increased. The reductive dechlorination end-products ethene and ethane were measured at concentrations as high as 1,300 µg/L in the upper aquifer beneath the southern plantation, which is reliable evidence that reductive dechlorination of VOCs is ongoing.

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