Monitoring for Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 355

those wishing to collect the sap to make syrup. Also, the
greatest amount of sap flow is derived from that part of the
trunk beneath which is growing the largest branches above
or roots below, which need more food to expand and
lengthen. Schumacher et al. (2004) provided data for PCE
in tree cores that indicated higher PCE concentrations for
core collected from the southern side of the same tree rela-
tive to cores collected from the northern side.
It is good practice not to collect cores from the same
general area over time, because the core hole created from
a previous collection will be rendered into non-conducting
(heartwood) tissue and, therefore, yield little, if any, water.
As the tree expands in girth, this heartwood will become
covered by newly developed sapwood. By extension, cau-
tion should be exercised when applying any plant-tissue
approach to monitor the performance of a phytoremediation
planting in which a large number of trees or cores from the
same trees may need to be taken. A user guide prepared
by the USGS (Vroblesky 2008) and is useful for
phytoremediation of contaminated groundwater can be
consulted prior to core collection.
Because the use of tree-core collection and analysis for
phytoremediation projects is an application of a method
historically used in forestry, there has been ample discussion
about the effect of core collection on trees, especially over
time in one tree (Grissino-Mayer 2003). One of the loudest
and clearest voices in this discussion had been Dr. Alex
Shigo, known as the Father of Arboriculture, who excelled
at challenging the accepted practice of tree investigations
during his life time. One of these was the practice of coring
trees, which he unilaterally did not support. He cited that
core collection exposed the tree to the entrance of fungal
spores, bacteria, viruses, and insects.
Perhaps the best example for the apparent lack of a
negative effect of tree coring on tree health, however, and
perhaps actually an example of a stimulant to plant growth,
can be taken from the maple sugar industry. Some maples
have been tapped close to 100 years and still survive and
produce sap the next season. Although boring does cause
injury to trees, the injury is compartmentalized (sealed off to
limit the spread of any infection) by the living tissue
(Grissino-Mayer 2003).
One of the interesting data trends reported in Vroblesky
et al. (1999a) was the difference in TCE concentration
between different genera of trees growing over TCE-
contaminated groundwater that had a uniform concentration.
Tree cores collected from baldcypress and tupelo had similar
TCE concentrations, but oak had lower TCE concentrations
even though the groundwater concentration of TCE was the
same. The difference between uptake by the different genera
is probably due to the difference between xylem conduc-
tance of water in diffusely porous trees, such as baldcypress
and tupelo versus ring porous trees, such as oak.

Fig. 15.3 The concentrations of TCE in tree-core samples decrease
up a tree (Modified from Vroblesky et al. 1999a, b). One meter is
equivalent to 3.2 ft.

concentrations of TCE in tree cores differed by 44-92% and
for cis -DCE between 6% and 90% based on the core location
around the tree at a given height (Vroblesky et al. 1999a)
(Fig. 15.3 ). These results may be due to passive losses of
VOCs from the xylem to the atmosphere through the bark;
plant transformation of the parent compound; sorption; in-
tree dilution as the sectoral ascent spreads out the root water
to a more dilute ring ascent up the trunk or stem; or other
processes.
A similar pattern of parent-to-daughter contaminant
profiles was shown in trees located along a groundwater
flowpath and downgradient from a PCE spill (Larsen et al.
2008). Cores collected from trees sampled at increasing
distances from the spill had more daughter products, such
as TCE and cis -DCE than PCE, reflecting the possible
natural attenuation of PCE to these compounds in the con-
taminated aquifer. However, a full assessment of the use of
tree-core samples in lieu of groundwater samples to assess
natural attenuation was precluded by the lack of information
provided on the redox status of the contaminated aquifer.
In most cases, core samples that should be representative
of the flow of water through the plant can be collected on the
south side of the tree. This is because the southern side of
most plants is most often exposed to sunlight. Sap flow
measured on the northern side of trees can be less than sap
flow on the southern side (Steinberg et al. 1990b). For
example, the southern side of maple trees is tapped by