CASE STUDY: Chlorinated Solvents
Project Name: Travis Air Force Base (Klein, 2011) (Doucette et al 2013) (Parsons, 2010)
Caption: Gaseous TCE releases from trees and soil surface into the atmosphere were measured on this site to determine the significance of volatilization in removing TCE from the subsurface
Location: Fairfield, CA
Institutions: (a) Utah Water Research Laboratory, Utah State University, 8200 Old Main Hill, Logan, Utah 84322-8200, United States (b) Parsons, 1700 Broadway, Suite 900, Denver, Colorado 80290, United States
Date installed: 1998-2000
Number of trees/ species installed: 480 Eucalyptus sideroxylon ‘Rosea’ (Red Ironbark Eucalyptus)
Target Media & Depth: Groundwater at 6−12 meters (20−40 feet). Concentrations of TCE in groundwater varied between <500 micrograms/liter and >9000 micrograms/liter
Travis Air Force Base is located near the city of Fairfield, CA. The base contains several contaminated areas, including Building 755, which was once used to test liquid‐fueled rocket engines and later used as a battery and electric shop. The shop regularly discharged battery acids and chlorinated solvents into the groundwater until 1978. Since then, the building structure has been removed and the source of the solvent plume excavated. Beginning in 1998, 100 Red Ironbark Eucalyptus trees (Eucalyptus sideroxylon ‘Rosea’) were installed on a 2.24 acre plot southeast of the former building to hydraulically control the solvent plume. Two years later in 2000, another 380 Eucalyptus trees were installed. By 2009 almost 100 trees had died, leaving 388 trees remaining, however this was still enough to create an effective system.
A phytotechnology system was chosen for this site because of its likely effectiveness and low cost for reducing the amount of TCE in the groundwater. In addition, dry climates with low summer precipitation, as found at this site, help ensure that the trees depend solely on the targeted contaminated groundwater for their water needs. The Travis AFB phytotechnology site is part of a larger network of demonstration projects at six US Air Force bases across the country that were installed in the late 1990s. Phytotechnologies are of significant interest to the Department of Defense because of frequent past use of trichloroethylene (TCE) as a degreasing solvent for aircraft maintenance, resulting in widespread groundwater contamination at Air Force bases around the world.
In addition to the tree planting down gradient of the source, a bioreactor consisting of a mixture of iron pyrite, gravel and woodchips mixed with vegetable oil was installed near the source area in 2008. Since installation of the bioreactor, TCE contamination directly around the source area has decreased by as much as 94%. Phytovolatilization (add icon) of TCE from the trees and soil surface into the atmosphere were the primary mechanisms of TCE removal from the groundwater that were evaluated in this study. Using groundwater data collected in 2004 and 2009, calculations based on the field measurements indicate that volatilization from leaves and soil accounts for almost half the 3.75 lbs. of TCE removed from the phytoremediation site each year. Transfer of TCE into the atmosphere is not considered a concern at this site since TCE has a relatively short half‐life in the atmosphere (Atkinson 1989) and historical sampling at the site has indicated no short‐term risk to worker health.” (Parson, 2010) The tree stand has been successful enough that the engineers have recommended expansion onto other areas of the base impacted with TCE groundwater plumes.