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A demonstration of a Phytoremediation Groundwater
Treatment system was conducted at the Carswell Naval Air Sation
(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 aid and accelerate contaminant attenuation.
First, transpiration of groundwater through the trees is believed
to be able 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.