Environmental Engineering Reference
In-Depth Information
This occurred at the same time that groundwater, transpira-
tion gas, and soil-gas samples were collected. The stem
samples were cores from the live oak but whole stems for
the smaller saw palmetto and castor bean. Plant samples
were preserved in methanol in vials or jars sealed with
Teflon-lined caps. The groundwater samples were collected
from temporary well points installed with a hand auger, and
with the well screen set at the water-table surface. Soil
samples were obtained from the unsaturated zone above
the water table during removal of material for well installa-
tion. Soil-gas samples were collected at 3 ft below the
ground. All nonplant samples were analyzed for TCE using
purge-and-trap gas chromatography with electron-capture
detection, according to EPA method 8010B. Plant samples
were analyzed using a similar method, with the addition of
methanol exposure and shaking for 1 day, and the removal of
a 250-
PCB compounds also are common chlorinated hydro-
carbons but are associated more with soil contamination
rather than with groundwater contamination. This is because
of the physical properties of PCB, and low solubility, which
render them absorbed to soils. These factors vary, of course,
with the amount of chlorine substitution. Because PCBs
share similar properties with both the PAHs and chlorinated
solvents, their fate in plants warrants discussion. For exam-
ple, the white rot fungi, often present in the rhizosphere,
have demonstrated the capability to degrade PCBs. Schnabel
and White (2001) investigated the interaction of willow
(
Salix alaxensis
) and balsam poplar (
Populus balsamifera
)
that were exposed to a representative PCB in soils under
laboratory conditions. The PCB was 3,3
0
,4,4
0
-tetrachloro-
biphenyl (TCB), and was used in uniformly
14
C-radiolabeled
form to trace the fate of the
14
C. In contrast to the fate of
14
C-
TCE discussed above, for which the label was primarily in
the shoots and leaves, up to 88.9% of the added PCB label
remained in the soils in the root zone, and less than 1% in the
roots themselves. This can be explained by the low solubility
(near 0.175 mg/L) and high sorption of TCB, and its low
bioavailability. Even so, Schnabel and White (2001) suggest
that the presence of
14
C-TCB in the root tissue indicates that
these compounds can enter the root cells. Moreover, the root
cells had up to nine times higher concentrations of total
radiolabel relative to the soil concentrations, indication of
bioconcentration. TCB was not detected in any shoot mate-
rial, so it can apparently enter the root hair cells but not pass
the Casparian strip and, therefore, gain entry into the xylem.
PCB-degrading bacteria associated with the root zone of
trees were found to degrade PCB-contaminated soil (Leigh
et al. 2006). Higher concentrations of PCB-degrading bacte-
ria, such as
Rhodococcus
, were associated with plant roots,
at values between 2.7 and 56.7 times higher than in
unplanted contaminated soils. The highest numbers of
PCB-degrading bacteria were associated with the root
zones of Austrian pine (
P. nigra
) and willow (
S. caprea
).
These plants can produce in the root zone such compounds
as terpenoids, tannins, phenols, and salicylic acid, which can
serve as substrate for or
m
L sample that was injected into a purge-and-trap
tube.
Soil-gas samples contained TCE, but the soil samples did
not, indicating that TCE vapors were emanating from the
contaminated water table. The highest soil gas was detected
under saw palmetto, 3,400 parts per billion TCE by volume
(ppbv), and ranged from 85 to 603 ppbv for the unsaturated
zone beneath the live oak and saw palmetto, respectively. Up
to 90% of all root mass was detected in soils no deeper than
2 ft, above the water table and source of contamination. The
roots of the saw palmetto and castor bean contained higher
concentrations, or 16.2 and 2.88
m
g/kg plant fresh weight,
respectively, than the other tissues from these plants, with no
more than 2.8 and 2.14
g/kg, in the stem and leaf, respec-
tively. Conversely, the stems of the live oak contained the
highest concentrations, near 3.68
m
g/kg relative to the roots
and leaves, that contained no more than 2.07
m
g/kg. TCE in
groundwater beneath the saw palmetto, where both root
concentration and soil-gas concentrations were highest,
was also the highest, at 64,500
m
m
g/L. TCE in the groundwa-
m
ter beneath the live oak was 527
g/L and beneath the castor
bean plants was 3,741
g/L.
The implication of these and other results where soil-gas
contamination is being degraded by plants is that plants at
phytoremediation sites can help decrease groundwater
contaminants even when not taking up groundwater. The
extent to which this occurs is a function of the vapor pressure
of the contaminant, the resistance of gas flow in the unsatu-
rated zone, and the type of plant-root system in place.
Clinton et al. (2004) reported that TCE concentrations
decreased in tree cores collected at increasing heights above
ground following artificial irrigation of the soil around the
base of the tree with water that did not contain TCE. The
concentration of TCE went down, as would be expected if
the plant was now taking up a more dilute solution of water.
It is possible, however, that TCE in the vadose zone was
entrained in the water during irrigation.
m
induction by PCB-degrading
bacteria.
Liu and Schnoor (2008) report that exposure of lesser-
chlorinated PCB congeners to poplar plants resulted in the
congeners being sorbed to the roots. Translocation of the
PCBs beyond root to stems was for those congeners with
lower sorption potential. Very little PCB was lost through
the leaves and it was not clear if the congeners were
metabolized. In Liu et al. (2009), poplars and switchgrass
(
Panicum vigratum
, Alamo) were exposed to solutions that
contained 3,3
0
,4,4
0
-tetrachlorobiphenyl and they observed
plant-mediated metabolism of the PCB.
In field sites where phytoremediation is being considered,
TCE often is not
the sole component of groundwater
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