Monitoring for Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 356

The effect of different tree tissue samples on contaminant
concentration was investigated by Vroblesky et al. (2004).
At the Ft. Worth, Texas site introduced in Chap. 8, tree-core
samples contained higher TCE concentrations than samples
of stems from the same tree. Collection of stems would
be easier than collection of core material, but the rate of
equilibrium for gas exchange to take place in a VOC vial is
different for stem material than that of a tree core with
no bark.
At a site in Colorado, higher TCE concentrations were
detected in cores from trees that had shallower depths to the
groundwater. In one case, TCE was detected in a cottonwood
tree even though the depth to groundwater was 26 ft (8 m).
This site was semiarid, with about 17 in. (44 cm) of precipi-
tation per year, most of which was as snow. The trees present
to sample were eastern cottonwood ( Populus deltoides
Bartr.) and were found growing along an adjacent stream.
At a site in Charleston, South Carolina, a TCE-
contaminated aquifer was present beneath a 3-m thick clay
unit. All trees sampled were growing above groundwater
that contained TCE (Vroblesky et al. 2004). The cored
trees consisted of loblolly pine ( Pinus taeda L.) and oak
( Quercus spp.). The TCE dissolved in groundwater was
flowing underneath three sampled trees in which one had
been growing above the plume since 2000 and two were
growing at the leading edge of the downgradient part of the
TCE plume. The TCE concentration in the tree cores col-
lected from the tree growing over the plume in 2000 was a
slightly more than 100 ppbv of headspace, whereas cores
collected from the two downgradient trees were at 10 ppbv.
From 2001 to 2003, however, these downgradient trees
(trees SC2 and SC32) had increasing detections of TCE, as
well as the upgradient tree (Fig. 15.4 ). This indicates the
possibility that the two downgradient trees were acting as
sentry wells and were indicating plume transport. These data

also could suggest, however, that plants were accumulating
TCE over time from a common fixed source. It also is not
clear from the data if the TCE entered the plant tissues from
the dissolved phase or as a vapor. In any case, these data
indicate that tree-core collection and analysis does work if the
goal is to detect the interaction between tree vascular systems
and groundwater contaminants such as chlorinated solvents.
Tree-core collection and analysis to detect contaminants
in groundwater has also revealed the usefulness for this
approach to investigate VOC contamination present above
the water table. Schumacher et al. (2004) found a stronger
relation between tree-core results for PCE and the PCE
present in subsurface soils than for the relation between
tree-core PCE results and PCE in groundwater. In fact, the
authors report a poor relation between groundwater PCE and
tree-core PCE concentrations, which suggests that tree-core
collection should not be the sole tool used to assess and
delineate groundwater contamination at a site. The lack of
relation between tree-core and groundwater contaminant
level is more likely to occur when the depth to water table
is near the maximum depth of root penetration, around 30 ft
(9 m). As was stated in Chap. 12, volatile organic
compounds can passively enter root hairs by diffusion in
the vapor or dissolved phases. In either case, tree-core col-
lection and analysis provides a relatively inexpensive way to
delineate both saturated- and unsaturated-zone contamina-
tion by VOCs.
As was discussed previously, early research done to
examine the interaction of plants and groundwater geochem-
istry was performed by Hem (1967), who used leaves and
branches to investigate inorganics, and the various work
done more recently by others looking at organics in tree
cores. A combination of approaches was reported by
Gopalakrishnan et al. (2007) that looked at organics in
branch material at a site characterized by chlorinated-sol-
vent-contaminated groundwater (see Chap. 7 for more
details about the site). The authors recognized the inherent
limitations to the collection of tree cores as proxies for
groundwater contamination evidence, such as damage to
trees and lack of suitability for smaller trees or younger
plantings. They collected branches from willows and poplars
that were planted to remediate chlorinated-solvent-
contaminated soils and groundwater, respectively. The
trees were 4 years old, and the diameters were about 1.9 in.
(7.5 cm). In order to quantify the predictive ability of the tree
branch approach, soil and groundwater samples near the
sampled trees were collected and analyzed for PCE, TCE,
and CCl 4 .
The plant samples were collected by Gopalakrishnan
et al. (2007) using pruning shears to remove the branch
and leaves closest to the ground surface. Branch samples
were cut to fit into 20-mL volatile organic analysis (VOA)
vials. Leaves without petioles were placed in 20-mL VOA

Fig. 15.4 The appearance of TCE in tree cores collected over time as
TCE-contaminated groundwater moved beneath the tree (Modified
from Vroblesky et al. 2004).