Water-Resources Investigations Report 02-4119
Natural Attenuation of Chlorinated Volatile Organic Compounds in Ground Water at Operable Unit 1, Naval Undersea Warfare Center, Division Keyport, Washington
By R.S. Dinicola, S.E. Cox, J.E. Landmeyer, and P.M. Bradley
The U.S. Geological Survey (USGS) evaluated the natural attenuation of chlorinated
volatile organic compounds (CVOCs) in ground water beneath the former landfill
at Operable Unit 1 (OU 1), Naval Undersea Warfare Center, Division Keyport,
Washington. The predominant contaminants in ground water are trichloroethene
(TCE) and its degradation byproducts cis-1,2-dichloroethene (cisDCE) and vinyl
chloride (VC). The Navy planted two hybrid poplar plantations on the landfill
in spring of 1999 to remove and control the migration of CVOCs in shallow ground
water. Previous studies provided evidence that microbial degradation processes
also reduce CVOC concentrations in ground water at OU 1, so monitored natural
attenuation is a potential alternative remedy if phytoremediation is ineffective.
This report describes the current (2000) understanding of natural attenuation
of CVOCs in ground water at OU 1 and the impacts that phytoremediation activities
to date have had on attenuation processes. The evaluation is based on ground-water
and surface-water chemistry data and hydrogeologic data collected at the site
by the USGS and Navy contractors between 1991 and 2000. Previously unpublished
data collected by the USGS during 1996-2000 are presented.
Natural attenuation of CVOCs in shallow ground water at OU 1 is substantial.
For 1999-2000 conditions, approximately 70 percent of the mass of dissolved
chlorinated ethenes that was available to migrate from the landfill was completely
degraded in shallow ground water before it could migrate to the intermediate
aquifer or discharge to surface water. Attenuation of CVOC concentrations appears
also to be substantial in the intermediate aquifer, but biodegradation appears
to be less significant; those conclusions are less certain because of the paucity
of data downgradient of the landfill beneath the tide flats. Attenuation of
CVOC concentrations is also substantial in surface water as it flows through
the adjacent marsh and out to the tide flats. Attenuation processes other than
dilution reduce the CVOC flux in marsh surface water by about 40 percent by
the time the water discharges to the tide flats. Despite the importance of natural
attenuation processes at reducing both the contaminant concentrations and the
contaminant mass at OU 1, natural attenuation alone was not effective enough
in the year 2000 to meet current numerical remediation goals for the site. That
was in part due to the relatively short distance between the landfill and the
adjacent marsh, and in part due to the extremely high CVOC concentrations directly
beneath the landfill.
Phytoremediation activities had some apparent effect on contaminant concentrations
in ground water and surface water, but ground-water redox conditions to date
(2000) were not affected by the February 1999 asphalt removal for tree planting.
The poplar trees in the phytoremediation plantations were not yet mature in
2000, so the lack of discernible changes to date is understandable. Concentration
changes of some redox-sensitive compounds suggest that increased recharge following
asphalt removal diluted ambient landfill ground water. CVOC concentrations increased
in some downgradient wells in both the northern and southern plantations after
asphalt removal, whereas CVOC concentrations decreased in some upgradient wells
in the southern plantation. A clear increase in CVOC concentrations in marsh
surface water followed asphalt removal, apparently from increased contaminant
discharge in ground water beneath the southern plantation.
The results of the natural attenuation evaluation suggest than minor modifications to the current sampling plan may be beneficial to understanding the future impacts of phytoremediation and natural attenuation on the fate and distribution of CVOCs at OU 1.
Redox Conditions in Contaminated Ground Water
Natural Attenuation in Ground Water
Methods of Data Analysis
Hydrogeologic and Geochemical Characterization for Natural Attenuation
Evaluation of Natural Attenuation
Appendix A. Quality Assurance and Control of U.S. Geological Survey Geochemical Sampling
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Send questions or comments about this report to the author, Richard Dinicola (email@example.com) 253.428.3600 ext. 2603.
For more information about USGS activities in Washington, visit the USGS Washington District home page.
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