Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 329

Fig. 13.18 Gas exchange rates for TCE (Modified from Ma and
Burken 2003). One centimeter is equivalent to 0.39 in.

for the cuttings grown in soil. For the cuttings grown in
hydroponic solution, the total mass of TCE in the diffusion
traps was higher for the lower traps and lower for the higher
traps. Moreover, TCE was detected in leaf tissue

(Fig. 13.18 ).
TCE diffusion rates in the stems were directly related to
transpiration rates (Ma and Burken 2003). The higher the
uptake rates of water the higher the diffusion of TCE. In the
tree center, where the xylem contained the TCE with
decreasing concentrations up the tree (diffusion in vertical
flow direction), TCE concentrations also decreased radially
laterally from the center to the atmosphere, as a result of
diffusion. On account of this vertical and radial diffusion to
the atmosphere, it is not surprising that less than 0.05% of
the TCE added to the soils was detected in the plants at the
end of the experiment.
The lack of TCE detection in leaves may be a result of
stem TCE diffusion rates being higher than the transport rate
by diffusion of xylem TCE to the leaves. This is especially
true for ring-porous trees that have most solution transport
near the bark-atmosphere interface, resulting in a much
shorter diffusion path than for diffusively porous trees (Ma
and Burken 2003). These processes also happen below

Fig. 13.17 Simulated diffusion of TCE from test trees as a function of
porosity, water content, and air content (Modified from Narayanan et al.
1999). One centimeter is equivalent to 0.39 in.

are taking up groundwater contaminants at lower, gaseous
concentrations and are not being exposed to the higher
aqueous concentrations. In doing so, they may be avoiding
potential toxicity effects of the contaminants.
Ma and Burken (2003) used laboratory tests of poplar
cuttings (DN-34) and diffusion traps placed around stems to
determine the fate of TCE-laced irrigation water added to the
plants. They also collected tree cores from the poplar trees
that have been growing above a TCE-contaminated aquifer
since 1996. The diffusion traps placed lower on the cuttings
contained more TCE than the traps placed higher up the stem