Phytoremediation of Groundwater at Air Force Plant 4 Carswell, Texas - Innovative Technology Evaluation Report
A demonstration of a Phytoremediation Groundwater 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 TCE-contaminated groundwater in a subhumid climate. Specifically, the study was undertaken to determine the potential for a planted system to hydraulically control the migration of contaminated groundwater, as well as biologically enhance the subsurface environment to optimize in-situ reductive dechlorination of chlorinated ethenes present (trichloroethene 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 upon 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 air and accelerate contaminant attenuation. First, transpiration of groundwater through the trees is believed to be able to to modify and hopefully control the hydraulic groundwater gradient. This can minimize the rate and magnitude of migrating contaminants downgradient of the tree plantation. Secondly, 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 groundwater hydrology and chemistry, the trees were studied 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 (Plant 4) at the Carswell NAS. The assembly building is the primary suspected source of TCE at the demonstration site. The evaluation of this technology application was a joint effort between the U.S. Air Force (USAF), the U.S. Geological Survey, the U.S. Forest Service, the Department of Defense's (DoD's) Environmental Security Technology Certification Program (ESTCP), and the U.S. EPA's SITE program.