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Toxics Program Remediation Activities

Can Trees Clean Up Ground Water? Phytoremediation of Trichloroethene-Contaminated Ground Water at Air Force Plant 4, Fort Worth, Texas

Type
  • Testing of Remediation Technologies
  • Performance Monitoring
  • Site Characterization
Location

Air Force Plant 4, Fort Worth, Texas, which is adjacent to the Naval Air Station Fort Worth (formerly Carswell Air Force Base), Fort Worth, Texas

Partners
Technology Phytoremediation
Contaminants Chlorinated Solvents (trichloroethene [TCE])
Description

The U.S. Air Force and its partners conducted a project to test the concept of phytoremediation. The test consisted of planting eastern cottonwoods (Populus deltoides) above shallow ground water contaminated with low levels of trichloroethene (TCE) near Air Force Plant 4, Fort Worth, Texas. The objective of the test was to test the hypothesis that:

  • Planting trees can hydraulicly contain and remove TCE contaminated water from an aquifer by transpiration and volatilization of TCE into the atmosphere, and
  • The trees will promote the natural attenuation of dissolved trichloroethene within the aquifer.

USGS scientists and their partners designed, implemented, and monitored the performance of the cottonwood trees. The scientists constructed a ground-water flow model based on monitoring data, and coupled it to a model of tree transpiration developed by the U.S. Forest Service. With the assistance of the coupled model the scientists were able to show that after the third growing season containment was not achieved, but the volume of ground water moving past the trees was reduced by 12 percent. A solute transport model also showed that the mass of TCE moving across the downgradient end of the site decreased by 11 percent. This demonstrated that after only three years the trees were removing TCE from the aquifer. Additional monitoring is being conducted to assess the long-term effectiveness of the trees as they continue to grow and mature.

The scientists also tracked the plume through the use of tree cores, and investigated the effect that rain events (recharge events) had on the concentrations of TCE in the trees. More information on the tree core study will be available soon.

A major focus of the work was to evaluate if the trees enhanced the natural attenuation of the TCE plume. USGS scientists and their partners conducted geochemical and microbiological studies to determine the geochemical conditions of the ground water in and around the cottonwood trees and to determine the role that microorganisms associated with the cottonwoods' roots played in the biodegradation of TCE at the site. After two years the cottonwood trees were beginning to affect the microbiology and geochemistry of the ground water near the trees. Indicators that the system was becoming anaerobic (without oxygen) were increasing. The increase in the anaerobic bacteria in general and the appearance of methanogenic microorganisms in the area suggested that the areas under the cottonwood plantings were evolving towards an anaerobic ecosystem; however, the constant concentrations of TCE and the daughter products suggested that volatilization and transpiration accounted for the majority of the loss of TCE mass at the time. Continued monitoring and analysis showed that after six years the system was becoming increasingly anaerobic, biodegradation rates had increased by two orders of magnitude, and the extent of the plume had decreased by an order of magnitude. If the ecosystem continues to evolve towards a mature methanogenic ecosystem, the natural attenuation (biodegradation) stimulated by the trees could exceed transpiration and volatilization as the process most responsible for the loss of TCE from the contaminant plume.

More Information
Contact
  • Project Design and Ground Water Modeling -- Sandra M. Eberts, USGS, Ohio Water Science Center, Columbus, OH,
  • Microbiology -- Edward M. Godsy, USGS, National Research Program, Menlo Park, CA,
  • Plume Geochemistry -- Sonya A. Jones, USGS, Texas Water Science Center, Austin, TX,
  • Tree Core Investigation -- Don A. Vroblesky, USGS, South Carolina Water Science Center, Columbia, SC,
Publications
Eberts, S.M., Schalk, C.W., Vose, James, and Harvey, G.J., 1999,
Hydrologic effects of cottonwood trees on a shallow aquifer containing trichloroethene: Hydrological Science and Technology, v. 15, no. 1-4, p. 115-121.
Godsy, E.M., Warren, Ean, and Paganelli, V.V., 2003,
The role of microbial reductive dechlorination of TCE at the phytoremediation site at the Naval Air Station, Fort Worth, Texas: International Journal of Phytoremediation, v. 5, no. 1, p. 73-87.
Jones, S.A., Lee, R.W., and Kuniansky, E.L., 1999,
Phytoremediation of trichloroethene (TCE) using cottonwood trees, in Leeson, Andrea, and Alleman, B.C., eds., Phytoremediation and innovative strategies for specialized remedial applications, in The Fifth International In Situ and On-Site Bioremediation Symposium, San Diego, Calif., April 19-22, 1999: Battelle Press, v. 6, p. 101-108.
Lee, R.W., Jones, S.A., and Kuniansky, E.L., 1999,
Geochemical effects of cottonwood trees on a shallow aquifer containing trichloroethene, in American Institute of Hydrology, Fourth USA/CIS Joint Conference on Hydrology and Hydrogeology, San Francisco, Calif., November 1999, Proceedings: Hydrological Science and Technology Special Issue.
Lee, R.W., Jones, S.A., Kuniansky, E.L., Harvey, G.J., and Eberts, S.M., 1998,
Phreatophyte influence on reductive dechlorination in a shallow aquifer containing TCE, in The First International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, Calif., May 18-21, 1998, Bioremediation and phytoremediation: Battelle Press, p. 263-268.
Lee, R.W., Jones, S.A., Kuniansky, E.L., Harvey, Gregory, Sherwood Lollar, B., and Slater, G.F., 2000,
Phreatophyte influence on reductive dechlorination in a shallow aquifer contaminated with trichloroethene (TCE): International Journal of Phytoremediation, v. 2, no. 3, p. 193-211.
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