Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 327

For trees growing with most of their root mass above the
water table or capillary fringe, which both contain dissolved
contaminants such as TCE, the main interaction with the
TCE would be the vapor phase. This was noted because soil-
gas concentrations in clean fill were low even though the
groundwater concentration was high.
Neitch et al. (1999) also compared the relative impact of
TCE uptake into wetland trees such as baldcypress
( Taxodium distichum (L) Rich) as a dissolved phase or
gaseous phase. They observed that the uptake of TCE from
the dissolved phase was controlled by transpiration by the
physical process of osmosis, the TCE concentration, and the
TSCF for TCE, all passive processes. The uptake of gaseous
TCE was controlled by the Henry's Law partition coefficient
for TCE, the TCE concentration, and the diffusive flux of
TCE to the roots, as well as the percent of total root mass that
consisted of air space, or aerenchymal cortex tissue, as
would be expected for the investigated woody plants,
which have to aerate anaerobic flooded soils.
Neitch et al. (1999) also reported their results of TCE
uptake in baldcypress seedlings grown in the laboratory in
containers (Fig. 13.16 ). Transpiration by the seedlings was
measured as water loss from Marriotte bottles placed at a
higher elevation than the water covering the seedlings in the
bottles. TCE flux was determined using a static chamber
technique (Neitch et al. 1999). The TCE flux through the
seedlings after uptake from the dissolved phase was found to
be higher during the day relative to night, or 80 and 50 m L
TCE per hour, respectively, when transpiration was higher,
and both values decreased from highs in August to lows in
December. Interestingly, dead seedlings also showed the
removal of TCE during the summer. Because water loss
was still occurring due to capillary wicking effects through
the dead plants, TCE was being removed although at lower
rates. Therefore, during the growing season, the TCE con-
centration multiplied by the transpiration or ET rate can
provide an estimate of TCE loss. A diffusive model can be
an approximate model for TCE losses during the dormant
period, where the total mass loss based on diffusion relative
to transpiration will be small or no greater than 1% of total
losses.
TCE loss flux was inversely correlated during the day
with CO 2 plant uptake, but during the night, the production
of CO 2 by plant respiration was not correlated with TCE loss
flux (Neitch et al. 1999). This suggests that a controlling
factor of TCE loss occurs through plant control of stomatal
aperture.
Another mechanism that may be responsible for the
phytoremediation of TCE is the plant-root formation and
release into the rhizosphere of dehalogenase enzymes.
These enzymes can be used to oxidize the strongly
halogenated compounds present in the root zone. This pro-
duction of such enzymes useful for chlorinated solvent

Fig. 13.15 PCE in headspace of cores collected from trees growing
over PCE-contaminated soil and groundwater (Modified from

Struckhoff et al. 2005).