Economic and Regulatory Factors That Affect the Phytoremediation of Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 369

can be addressed using procedures such as the TCLP. These
issues may need to be addressed in the near future, as many
phytoremediation projects in the United States started in the
1990s and are about 15 years old.

A greenhouse study investigated the translocation of a
widely used chlorinated solvent to different parts of a plant
including the fruit (Chard et al. 2006). These investigators
were concerned with the question of whether or not fruit
could be a sink for chlorinated solvents taken up in the root
zone, because it raises a concern as to the risk exposure of
phytoremediation to wildlife and humans. Unlike entry into
leaves, where there is a loss mechanism by diffusion to the
atmosphere through stomata and cell metabolism, only cell
metabolism is present as a loss mechanism for fruits. This
fact underscores the previous path of using sterile trees at
phytoremediation sites rather than those that produce seeds,
nuts, or berries. In this case, Chard et al. (2006) noticed that
trees growing above a plume of TCE-contaminated ground-
water contained TCE in concentrations proportional to the
groundwater TCE concentration. Moreover, groundwater
flow placed the plumes in residential areas where fruit
trees were grown. Preliminary studies did indicate the pres-
ence of TCE in fruit from trees above the TCE plume.
To further address this potential incorporation of TCE
into fruit, representative fruit trees were grown in a green-
house setting. Two types of fruit trees were used: a
dwarf apple ( Malus domestica Borkh cv. 'Golden Deli-
cious') and a 5-year-old peach tree ( Prunus persica Batsch
cv. 'Redhaven'). Irrigation water that contained no TCE was
used as the control, and the treatments consisted of TCE
concentrations of 5 or 500

16.1.5 Contaminant Fate in Food Crops

Even though fruit trees are not used for the phytoremediation
of contaminated groundwater, the potential exists for
contaminants to be transported to offsite areas where the
contaminated groundwater may be unknowingly either
taken up or applied through irrigation to crop plants.
Although most fruits contain high amounts of water, the
source of this water is predominantly that routed from
the phloem rather than the transpiration stream, since
this sap first must receive sugar from the leaves. For this
reason, many studies have concluded that for hydrophobic
contaminants such as PAHs, loading and movement of
PAHs in the phloem to fruits is negligible (Trapp et al.
2007).
For xenobiotics that have the characteristics of a weak
acid, however, these compounds may enter the phloem.
Because groundwater contaminants primarily will be present
in the xylem, to address this issue, samples from fruit trees
and various vegetable plants grown in a residential area
downgradient of a TCE-release from Hill AFB in Utah
were collected to determine if TCE could be transferred
from the TCE-contaminated groundwater to the fruits and
vegetables. Depth to groundwater ranged from less than 10 ft
(3 m) below ground surface to almost 20 ft (6 m) below land.
Because this study site is in an arid area where ET is greater
than precipitation at 45 in./year (114 cm/year) relative to
19.8 in./year (50 cm/year), respectively, groundwater is a
potential source of water for plants. TCE was not detected in
the headspace of samples of the fruits at levels above
the detection limit, but cores collected from the tree
trunk did have TCE (Doucette et al. 2007). Groundwater
samples collected near sampled trees did not have TCE
concentrations

g/L 14 C-TCE and unlabeled
TCE. Control trees were grown either separately or singly
in each TCE treatment area to determine if atmospheric TCE
uptake occurred. After 2 years of investigation, it was deter-
mined that even at the 500

m

g/L TCE, no toxicity was
observed. Fruit was produced both years by the peach trees
but not the second year of the apple trees exposed to 5

m

g/L
TCE. Radiolabeled 14 C-TCE was found in all tissue samples
analyzed in the treatment trees, but not in the control; this
may indicate that the route of 14 C-TCE entry was through
the roots and translocation, rather than foliar uptake of
volatilized 14 C-TCE (Chard et al. 2006). Radiolabel was
detected in amounts that decreased from leaves to branches
to fruit. The radiolabel detected probably represented the
byproducts of TCE degradation. Hence, although TCE was
taken up by the trees, both in the field and laboratory study,
no TCE was detected in the fruit.
This study expands on work done previously to investi-
gate the fate of contaminants in sewage sludge spread for use
in agricultural settings, a very common practice and highly
regulated due to the use of organic-rich wastes as soil
enhancements, as discussed in Chap. 11. Witte et al.
(1988) studied the fate of PAHs in a greenhouse study
where sewage sludge that contained milligram per kilogram
concentrations of benzo[k]fluoroanthene and fluoroanthene
was exposed to a variety of cereal and vegetable crops such
as wheat, rye, carrots, and sugarbeet. At the end of the study,

m

that could be related to plant tissue

concentration.
Because of the variability in the data from the field, a
greenhouse study was performed where a controlled amount
of 14 C-TCE was added to representative apple and pear trees
(Doucette et al. 2007). The level of 14 C in various tree
tissues, including in some cases the fruit, was proportional
to the level of TCE exposure, confirming other reports that
suggest a diffusive uptake pathway as a function of contam-
inant concentration and transpiration. The highest concen-
tration of 14 C was in the leaves. Because TCE was detected
only in the roots and trunk, the 14 C in the other tissues such
as the fruit was assumed to be TCE-transformation products
(Doucette et al. 2007).