Environmental Engineering Reference
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a cross-sectional area located downgradient from a planted
site relative to that groundwater flux that enters the planted
area in upgradient areas. The groundwater flux is
not
com-
pared to
ET
P
, but to determine the change in groundwater
flux caused by the plants. The expected decrease in ground-
water flux would be observed as a decrease in groundwater
level in monitoring wells located in the planted area. In this
manner, the shortcomings of using
ET
P
are eliminated and
are replaced by direct observations of changes in the ground-
water system caused by trees. More on this method can be
found in Eberts et al. (1999) and Landmeyer (2001).
8.4.2.1 Case Study: Air Force Plant 4, Texas
A phytoremediation project was initiated in 1996 at a site
near Air Force Plant 4, located on the Naval Air Station west
of Fort Worth, TX. The site has been used to manufacture
aircraft since 1942, even before the United States entered
into World War II. As part of aircraft construction,
chlorinated solvents such as TCE were used and disposed
of in landfills or fire training pits. As a result, TCE is present
in the shallow aquifer downgradient from the aircraft
manufacturing buildings. The facility is still used to produce
aircraft such as the F-16 fighter jet. The plume of TCE-
contaminated groundwater was first detected in 1982.
Phytoremediation was initiated to (1) reduce the mass
flux of contaminated groundwater leaving the planted area,
(2) alter the redox condition of the aquifer to one more
favorable for the destruction of TCE, as is discussed in
Chap. 13, and (3) remove groundwater and alter the ground-
water-flow path from the site. The project was initiated by
the U.S. Air Force, the Department of Defense Environmen-
tal Security Technology and Certification Program, and was
a charter site for the U.S. Environmental Protection Agency
SITE Program. Other researchers associated with the follow-
ing agencies also have been involved at the site: Science
Applications International Corporation; University of
Georgia; U.S. Forest Service; USGS.
The contaminated shallow aquifer consists of alluvial
sediments composed of particles sizes ranging from clays
to sands and gravels with some limestone, and a porosity of
about 25%. It is located in the Osage Plains of the Central
Lowland Physiographic Province (Vose et al. 2000). Due to
its alluvial progeny, the aquifer is thin, between 1.6 and
4.92 ft (0.5 and 1.5 m) thick, and depth to groundwater
ranges from 2.5 to 13 ft (0.7-4 m) below grade. The hydrau-
lic conductivity ranges from 3.2 to 98 ft/day (1-30 m/day),
with an average groundwater flow velocity of 1.6 ft/day
(0.5 m/day). The contaminated groundwater flows
downgradient and discharges to a local creek. In this part
of Texas, the subhumid climate provides precipitation of
about 31 in. (78 cm) per year, with recharge of about 3 in.
(7.6 cm) per year. Most precipitation occurs from May to
October (Vose et al. 2000).
Fig. 8.7
Average mechanical pumping rate during winter, 19 gpm
(gal/min) (71.8 L/min), for the pump-and-treat system with no ground-
water removed by trees (Modified from Ferro et al. (2000)).
Fig. 8.8
Average mechanical pumping rate, in gpm (gal/min) could be
cut back by almost 50% during the summer, when trees were removing
groundwater (Modified from Ferro et al. (2000)).
complicating factor and decrease individual plant removal of
groundwater.
8.4.2 Groundwater Flux Framework
This approach to determine the interaction between plants
and groundwater is based on the water-balance equation.
Instead of calculating the various components of the water
balance and comparing them to each other, a flux approach is
based on observed decreases in the groundwater flux through
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