Environmental Engineering Reference
In-Depth Information
particle movement, such that mean concentrations reached
3,500 mg/kg, rather than dissolved-phase contamination.
PAHs also can be derived from natural sources. PAHs can
be produced by fires and volcanic eruptions that occur at
temperatures near 200
C, and by fungi, bacteria, and some
plants, especially those that grow in coniferous forests and
peat bogs. For example, the PAHs naphthoquinine and qui-
nine are produced naturally in some plants, as was
introduced in Chap. 11. Under anoxic subsurface conditions,
these quinines are reduced to hydroquinones and then ulti-
mately PAHs.
It is evident that the detection of PAHs in the environ-
ment could implicate many potential sources, both natural
and industrial. Differing industrial sources have characteris-
tic release patterns and, therefore, contaminant distributions.
Spills tend to be characterized by higher concentrations in a
smaller area or volume of environment, due to the physical
chemical characteristics of many PAHs of low water solu-
bility and high affinity for absorption onto soil organic
matter.
absorption onto roots and were not taken up in the transpira-
tion stream by plants. On the other hand, PAHs that had from
two to four rings could be taken up by plants, because of their
increased, although still relatively low, solubility. These
compounds include naphthalene, anthracene, benzo[
a
]pyrene,
acenaphthene, fluorine, phenanthrene, anthracene, fluoran-
thene, pyrene, benzo[a]anthracene, and chrysene. It is not
surprising, therefore, that most of our knowledge about the
interaction of plants with PAHs is derived from research at
former MGP (Anderson et al. 1997).
The PAH of concern at many contaminated groundwater
sites is naphthalene, because it, like other lower ring PAHs,
tends to be found in coal-tar-derived products as well as
gasoline, and is the PAH with the highest solubility in
water, being near 30 mg/L. High concentrations of PAHs
are exposed to plants in soil with relatively lower SOM
concentrations.
The
TSCF
of various PAHs was measured in a laboratory
study where three types of plants were exposed to soils that
contained phenanthrene, anthracene, fluoranthene, and
pyrene (Mattina et al. 2006). The plants were all vegetable
plants, and included zucchini, summer squash, and cucum-
ber, all members of the
Cucurbitaceae
. They were grown in
rhizotrons. The
TSCF
for the three-ringed PAHs of phenan-
threne ranged from 2.8 to 11.6, and for anthracene ranged
from 3.5 to 26.5. The
TSCF
for the four-ringed PAHs for
fluoranthrene ranged from 1.1 to 5.17, and for pyrene ranged
from 0.72 to 4.0.
The uptake of phenanthrene and chlorobenzene by black
willow (
Salix nigra
) was investigated by Gomez-Hermosillo
et al. (2006). Total uptake of radiolabeled contaminants in
the laboratory was between 3.8% and 5.7% of the initial
concentration of desorption-resistant contamination. This
experiment was performed to test the assumption behind
the conceptual models of
TSCF
and
RCF
that the water in
the soil pores near the roots will contain contaminants that
can be reversibly bound to the soil; that is, they are all
bioavailable. In other words, does reversibly bound contam-
ination enter plants? Most of the contaminant mass remained
in the roots and was not translocated, such that translocation
was between 0.38% and 0.47%. These results suggest that
highly sorptive contaminants can be taken up by plants but at
levels less than that predicted by log
K
ow
. This information
should be useful for designing monitoring strategies at PAH-
contaminated sites.
Groundwater beneath the former MGP site near
Charleston, SC, described previously is characterized by
dissolved-phase concentrations of monoaromatic petroleum
hydrocarbons such as BTEX, and PAHs such as naphtha-
lene, that are associated with the raw materials and wastes
that were produced during the operation of the former MGP
(Landmeyer et al. 1998a). The highest concentrations of the
more soluble benzene and toluene compounds, up to 5 mg/L,
13.3.1 Plant Interaction and Uptake Pathways
That portion of organic contaminants that are bioavailable
will be affected by plant-based processes (Cunningham et al.
1995). As was discussed in Chap. 12, bioavailability is
related to the physical and chemical properties of a contami-
nant such as log
K
ow
, pH, soil type, and degree of contami-
nant weathering. Plant interaction with PAHs can become
lethal if the imbibed PAHs are exposed to UV radiation,
because the energy the PAHs absorb can be transferred to
very reactive singlet oxygen. The toxicity of a group of
PAHs to willow trees was investigated by Thygesen and
Trapp (2002).
A literature review of plant and PAH interaction was
published in 1992 by the Electric Power Research Institute
(Electric Power Research Institute 2002). The review
presented information on the interaction of vegetation
often found at former MGPs and was concerned not with
the use of vegetation to remediate MGP contaminants but as
a potential route of contaminant exposure; this mindset was
common at a time prior to much knowledge about
phytoremediation. The review concluded that the potential
for uptake into plants was, as can be expected, related to the
physical and chemical properties of the PAH. The log
K
ow
of
most PAHs is high, on account of their low solubility and
tendency to partition into organic matter. Because of the
high log
K
ow
, plant interactions with PAHs would tend to
favor initial absorption to root material rather than uptake by
root hair cells, with uptake into the transpiration stream
being restricted to those PAHs with lower solubilities. For
example, PAHs that had five or more rings tended to undergo
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