Abstract

A demonstration of a phytoremediation ground water treatment system was conducted at the Carswell Naval Air Station (NAS) Golf Club in Fort Worth, Texas, to investigate the ability of purposely planted eastern cottonwood trees, Populus deltoides, to help remediate shallow trichloroethene (TCE)-contaminated ground water in a subhumid climate. Specifically, the study was undertaken to:

• Determine the potential for a planted system to hydraulically control the migration of contaminated ground water
• Biologically enhance the subsurface environment to optimize in situ reductive dechlorination of chlorinated ethenes present (TCE and cis-1,2-dichloroethene) in the shallow aquifer system beneath a portion of the golf course

Populus deltoides, like other phreatophytes, have long been recognized as having the ability to tap into the saturated zone to extract water for metabolic processes. Based on this characteristic, the species was considered well suited for applications where shallow aquifers are contaminated with biodegradable organic contaminants.

A planted system of cottonwood trees is believed to effectuate two processes that aid and accelerate contaminant attenuation. First, transpiration of ground water through the trees is believed to modify and control the hydraulic ground water gradient. This can minimize the rate and magnitude of migrating contaminants downgradient of the tree plantation. Second, the establishment of the root biomass, or rhizosphere, promotes microbial activity and may enhance biodegradative processes in the subsurface.

To assess the performance of the system, hydrologic and geochemical data were collected over a three-year period (August 1996 through September 1998). In addition to investigating changes in ground water hydrology and chemistry, project personnel studied the trees to determine important physiological processes, such as rates of water usage, translocation and volatilization of these volatile organic compounds, and biological transformations of chlorinated ethenes within the plant organs. The demonstration site is situated about one mile from the southern area of the main assembly building at Air Force Plant 4 at the Carswell NAS. The assembly building is suspected to be the primary source of TCE at the demonstration site.

The evaluation of this technology application was a joint effort among the U.S. Air Force, the U.S. Geological Survey, the U.S. Forest Service, the Department of Defense’s Environmental Security Technology Certification Program, and EPA's Superfund Innovative Technology Evaluation Program.

See Also

Superfund Innovative Technology Evaluation

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