Open-File Report 2006-1374
U.S. GEOLOGICAL SURVEY
Open-File Report 2006-1374
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 21-24, 2005, in support of long-term monitoring for natural attenuation.
The June 2005 data include concentrations of redox-sensitive geochemical constituents at 13 wells and 9 piezometers, and concentrations of VOCs at 9 piezometers, 3 intermediate aquifer wells, 10 passive-diffusion sampler sites beneath the marsh stream adjacent to the southern plantation, and 2 surface-water sites in the marsh stream adjacent to the southern plantation.
For June 2005, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of chlorinated VOCs were found in fewer upper-aquifer wells than were found during 2004. Redox conditions in ground water from the intermediate aquifer just down-gradient of the landfill remained somewhat favorable for reductive dechlorination. Measured dissolved H2 concentrations have generally decreased at OU-1 over the past 5 years, suggesting a trend from strongly- to mildly-reducing predominant redox conditions. However, sulfide and methane concentrations were consistent over that period, indicating persistent sulfate reduction and methanogenesis. Overall, the changes in redox conditions measured at individual wells have not been consistent or substantial throughout either the upper or the intermediate aquifers.
In the northern plantation, chlorinated VOC concentrations at P1-3 and P1-4 were lower than previously measured. The decrease in contaminant concentrations beneath the northern plantation, and the positive detection of reductive dechlorination end-products are consistent with 2000-04 results.
In the southern plantation, changes in chlorinated VOC concentrations at the piezometers were variable. Most notable was a substantial decrease in the total chlorinated VOC concentration at P1-9 from 75,000 to 1,000 µg/L between 2004 and 2005. The total chlorinated VOC concentration at the most highly contaminated piezometer at OU-1 (P1-7) in 2005 was 92,000 µg/L and was consistent with previous levels. Chlorinated VOC concentrations at P1-8 decreased substantially after 2002 and remained relatively low during 2005. The high concentrations of the reductive dechlorination end-products ethane and ethene measured at the most contaminated sites (P1-6 and P1-7), as well as measurable concentrations at sites P1-9 and P1-10, are reliable evidence that reductive dechlorination of chlorinated VOCs is ongoing within the southern plantation.
In the 10 passive-diffusion samplers that were deployed beneath the marsh stream, the highest chlorinated VOC concentrations were measured at a site about mid-way along the sampled stream reach (S-4). In 2005, the total chlorinated VOC concentration increased nearly two fold in comparison to 2004. It is not certain that the apparent increase in concentrations is representative of site conditions. However, the chlorinated VOC concentrations have increased each time at the two most contaminated passive-diffusion sampler sites sampled over multiple years. In the marsh stream, chlorinated VOC concentrations in surface water at site SW-S6 near the upgradient margin of the former landfill were low. Concentrations in the stream increased substantially after flowing past the southern phytoremediation plantation to the downstream site (MA-12).
Overall, the 2005 data were consistent with previous findings of continued biodegradation of chlorinated VOCs in ground water, along with continued discharge of some chlorinated VOCs to surface water in the marsh stream.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: http://pubs.water.usgs.gov/ofr20061374
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