Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 326

response to gravity rather than groundwater flow. This is true
only if the PCE and TCE are present in a free phase of pure
product, however. Once these compounds are dissolved in
groundwater at concentrations less than their solubility, for
example at concentrations of TCE less than 1,100 mg/L, the
dissolved compound will move with the prevailing flow of
groundwater.
Perhaps some of the early resistance to using
phytoremediation at chlorinated-solvent-contaminated sites
resulted from statements made in Cunningham et al. (1996)
that PCE and TCE would be hard to remediate using
phytoremediation because they formed dense pools near
the bottom of aquifers. While this statement is true when
these compounds are released as pure products, in fact, these
compounds can be routinely detected in tree-tissue samples
where PCE and TCE have been released. The application of
phytoremediation at sites characterized by chlorinated-sol-
vent contaminated groundwater has become more common
(U.S. Environmental Protection Agency 2006).

water uptake are passive processes. These workers also
looked at the effect of 14 C-TCE on a plant that had not
been previously exposed to TCE. The soybean plant tested
( Glycine max ) did mineralize slightly more TCE than the
controls, but growth was not inhibited.
The direct uptake of groundwater contaminants such as
chlorinated solvents by poplar trees has been studied previ-
ously, and can occur by the aqueous pathway (McFarlane
et al. 1990) or by the gaseous pathway (Bromilow and
Chamberlain 1995; Neitch et al. 1999). Regardless of the
physical state of the contaminant being taken up, however,
the fate of the contaminant in the transpiration stream can be
assessed using tree-core collection and analysis and is
discussed in Chap. 15.
Fortunately, TCE has other chemical properties that make
it amenable to remediation by phytoremediation. It has a
relatively high vapor pressure of 80 mmHg at 20 C, a
dimensionless Henry's Law constant of 0.38, and a log K ow
of 2.29. These properties indicate that TCE has the potential
to be taken up in both the vapor and dissolved phases by
roots (Schnoor et al. 1995). The process is more compli-
cated, however, in subsurface environments that have con-
siderable amounts of organic matter, which leads to more
absorption of TCE onto the sediment surfaces. A report by
Doucette et al. (1998) indicates that uptake of TCE vapors
by plants was a major pathway of subsurface remediation of
a chlorinated solvent plume located in Florida. This was
an important pathway, because the majority of root mass
was above the water table. This pathway of attenuation is
often ignored in phytoremediation studies relative to the fate
of the aqueous state and its importance should not be
overlooked.
Struckhoff et al. (2005) confirmed the observation by
Doucette et al. (1998) that the vapor phase of chlorinated
solvents is an important avenue between their accumulation
in the unsaturated zone and uptake by plant roots. They
reported that the concentration of PCE in trees, from core
material, more greatly reflected the concentration of PCE in
the soil-gas concentration relative to the groundwater PCE
concentration. The data were originally collected from a
PCE-contaminated aquifer at New Haven, Missouri, adja-
cent to the Missouri River. Tree cores were collected as well
as soil samples from above the water table, both for VOC
analysis in the headspace. The correlation between tree-core
PCE concentration and soil-gas PCE was higher than the
correlation between tree-core PCE concentration and
groundwater PCE concentration (Fig. 13.15 ).
To further examine this relation, the authors determined
partition coefficients for PCE between plant-air and plant-
water of 8.1 and 49 L/kg, respectively. The partition
coefficients were determined by adding a known amount of
PCE to cores collected from uncontaminated trees, and then
sampled after 1 week.

13.5.1 Plant Interaction and Uptake Pathways

A study by Brigmon et al. (1998) demonstrated that the
presence of the rhizosphere in soils at a TCE-contaminated
site in South Carolina increased the potential for natural
attenuation even though very little attenuation was caused
by the rhizospheric effect. Rather, the reduction in TCE
concentrations observed in laboratory microcosms was
caused by sorption to sediments. For example, up to 90%
of the TCE added to laboratory microcosms was removed
from solution within 7 days. Donnelly and Fletcher (1994)
reported that some PCBs could be degraded in the root zone
under similar conditions.
A study by Anderson and Walton (1995) demonstrated
that TCE in the presence of planted soils had increased
degradation relative to the unplanted soils. Their studies
confirmed what other researchers have reported, that in
soils planted with a legume ( Lespedeza cuneata ), loblolly
pine ( Pinus taeda ), and goldenrod ( Solidago ), that minerali-
zation of the 14 C-TCE to 14 CO 2 accounted for greater than
26% of the total radiolabel added, relative to 15% in the
unplanted soils. However, they report that the TCE taken up
into the plant was between 1% and 21% of that added. In was
detected in the leaves (needles for Pinus ), stems, and roots.
Moreover, the novel part of their study was the selection of
plants used, because the root types ranged from fibrous to
leguminous to tap. In addition, although the raw data they
collected indicated a difference in uptake rate for 14 C-TCE
between plant types, after correction for water uptake and
use, the differences were no longer apparent, suggesting a
linear relation between water use and TCE uptake; this result
is not surprising in light of the fact that contaminant and