Abstract

Previous investigations indicate that natural attenuation and biodegradation of chlorinated volatile organic compounds (VOCs) are substantial in groundwater beneath the 9-acre former landfill at Operable Unit 1 (OU 1), Naval Undersea Warfare Center, Division Keyport, Washington. Phytoremediation combined with ongoing natural attenuation processes was the preferred remedy selected by the U.S. Navy, as specified in the Record of Decision for the site. The U.S. Navy planted two hybrid poplar plantations on the landfill in spring 1999 to remove and to control the migration of chlorinated VOCs in shallow groundwater. The U.S. Geological Survey (USGS) has continued to monitor groundwater geochemistry to ensure that conditions remain favorable for contaminant biodegradation as specified in the Record of Decision. This report presents groundwater geochemical and selected VOC data collected at OU 1 by the USGS during June 15–17, 2009, in support of long-term monitoring for natural attenuation.

For 2009, groundwater samples were collected from 13 wells and 9 piezometers. Samples from all wells and piezometers were analyzed for redox sensitive constituents, and samples from 10 of 18 upper-aquifer wells and piezometers and 3 of 4 intermediate-aquifer wells also were analyzed for chlorinated VOCs. Concentrations of redox sensitive constituents measured in 2009 were consistent with previous years, with dissolved hydrogen (H₂) concentrations ranging from less than 0.1 to 1.8 nanomolar (nM), dissolved oxygen concentrations all at 0.6 milligram per liter or less; little to no detectable nitrate; abundant dissolved manganese, iron, and methane; and commonly detected sulfide.

The reductive declorination byproducts—methane, ethane, and ethene—were not detected in samples collected from the upgradient wells in the landfill or the upper aquifer beneath the northern phytoremediation plantation. Chlorinated VOC concentrations in 2009 at most piezometers were similar to or slightly less than chlorinated VOC concentrations measured in previous years. In 2009, concentrations of reductive dechlorination byproducts ethane and ethene were less than those measured in 2008 at most northern plantation wells and piezometers.

For the upper aquifer beneath the southern phytoremediation plantation, chlorinated VOC concentrations in 2009 at the piezometers were extremely high and continued to vary considerably over space and between years. At piezometer P1-9, the total chlorinated VOC concentration increased from 25,000 micrograms per liter in 2008 to more than 172,000 micrograms per liter in 2009. At piezometer P1-7 in 2009, the concentrations of trichloroethene and cis-1,2-dichloroethene (cis-DCE) were the highest to date. The reductive dechlorination byproducts ethane and ethene were detected at all wells and piezometers in the southern plantation with the exception of piezometer P1-8, although the measured concentrations were not consistently high.

For the intermediate aquifer, concentrations of redox sensitive constituents and VOCs in 2009 at wells MW1-25, MW1-28, and MW1-39 were consistent with concentrations measured in previous years. Concentrations of the reductive dechlorination byproducts ethane and ethene at wells MW1-25 and MW1-28 were equal to or greater than previously measured concentrations.

First posted December 5, 2011