Environmental Engineering Reference
In-Depth Information
ground in long roots of small diameter. Whereas TCE was
not detected in leaf material of poplars planted over a TCE-
contaminated aquifer at the Carswell AFB site in Texas,
some TCE vapor was detected emanating from the leaves
of planted poplar trees at the APG site in Maryland.
Any dissolved-phase TCE in the transpiration stream that
reaches the leaves may interact with the atmosphere in the
stomata. There, the dissolved-phase TCE may transform into
TCE vapor, similar to the evaporation of liquid water. Sto-
mata that control the rate of transpiration also to some extent
control the release of TCE; this does not include, however,
the diffusive flux of contaminants such as TCE from the
xylem out through the bark (Ma and Burken 2003) as was
discussed above.
The concept of a unique
TSCF
for TCE, or other contam-
inant compounds for that matter, has not yet been proven.
For example, the
TSCF
for TCE derived by theoretical
methods is 0.62 using the method of Briggs et al. (1982),
measured is 0.75 (Burken and Schnoor 1998), and in a
laboratory study with
14
C-TCE was found to range from
0.02 to 0.22, and was a function of concentration (Orchard
et al. 2000). These differences in
TSCF
are most likely due
to differences in experimental setup, plant type, and use of a
soil versus hydroponic solution.
The fate of TCE in groundwater that discharged to a
wetland containing cattails and cottonwood trees was
investigated by Bankston et al. (2002). Cattails can allow
the diffusive transport of atmospheric oxygen to the root
zone, and its diffusion into pore water that contains TCE
could promote its co-metabolism by MMO-producing
methanotrophic bacteria. Microcosms were built in the lab
that contained site sediments and plants, sandy soil for the
cottonwoods and organic soil for the cattails. To these
microcosms was added
14
C-TCE and its fate monitored.
Most of the recovered
14
C-TCE was in the volatilized
form, being greater than 50%. In the cottonwoods and
cattails, up to 33% and 39%, respectively, of the label was
present in the plant tissues. Low amounts of
14
CO
2
were
present in the control and treatment microcosm, suggesting a
minimum amount of oxidation by soil microbes.
Researchers at the University of Washington extended
the investigation into the fate of TCE from the laboratory
into the field (Newman et al. 1999b). Their test site was
located near Fife, Washington. The test was not performed
in a contaminated aquifer; rather, the test was performed in
“cells” composed of 1.5-m by 3-m wide by 5.7-m long of 60
mil polyethylene filled with a sand layer on the bottom
covered by a thicker layer of clay loam collected on site.
This type of study is actually an intermediate level between
laboratory testing and field application at a contaminated
site. Fifteen hybrid poplar clone cuttings of
Populus
trichocarpa
at the depth of the sand layer to which plain water or water
laced with TCE mixed in 40% ethanol was added. TCE was
added to three treatment cells over time, with one cell not
receiving TCE acting as the control. The TCE concentration
of the influent averaged about 0.38 millimolar (mM)
between 1995 and 1997. The total amount of water added
over the 3-year test varied with variations in plant transpira-
tion. Various aspects of the fate of TCE were monitored, as
briefly summarized below.
As was stated in Newman et al. (1997), the overall growth
of the trees after exposure to TCE was not significantly
different than the growth of the trees in the control cell.
Mean tree height was 3 m after 1995, 7 m by 1996, and
11 m by 1997. Although a water budget was presented, the
lack of a distinction between water lost by transpiration or
evaporation from the cells precludes an accurate picture of
the effect of the trees themselves. The amount of TCE and
reductive dechlorination intermediates measured in the
effluent of planted cells increased after leaf drop each year,
indicating the effect of transpiration while in full leaf
accounts for a large percentage of TCE loss, which is then
decreased after leaf drop. Over the 3-year study, more of the
original TCE added to the influent was collected as effluent
in the control cell with no trees (67%) than in the cells with
trees (1-2%).
The fate of TCE taken up by the trees was examined—
TCE, and both dichloro- and trichloroacetic acid
intermediates of TCE transformation, were detected in
plant tissues, such as leaves, branches, roots, and the trunk.
Some of the TCE removed from the influent was observed to
be transpired by leaves, as shown by the measurement of
2
10
8
mol TCE/leaf/h. Scaling this individual leaf tran-
spiration rate to the entire plant for the entire season leads to
an estimate of about 0.30 mol TCE transpired per cell per
year, or about 9% of the total amount of TCE lost per season.
By the end of the experiment, 95% of the added TCE was
removed in the planted cells.
A study of the interaction between TCE at a contaminated
site with existing vegetation was performed by Doucette
et al. (1998). The site was located on the east coast of Florida
at Cape Canaveral. It had been exposed to releases of TCE
from metal-cleaning activities for at least three decades. The
shallow sediments that comprised the vadose and saturated
zones to depths near 35 ft (10.5 m) from ground surface
included coarse to fine-grained sands and shells. The average
depth to the water table was between 4 and 7 ft across the
site, and varied due to recharge and the level of adjacent
surface-water bodies to which the groundwater discharged.
Above a plume of TCE delineated by monitoring wells grew
indigenous plants that included castor bean (
Ricinus
communis
), live oak (
Quercus virginiana
), and saw palmetto
(
Serenoa repens
). Samples of plant tissues, such as leaf,
stem, and root, were collected from all three types of plants.
P. deltoides
were planted every 1 m in each
cell in early 1995. To each cell was added a perforated pipe
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