Environmental Engineering Reference
In-Depth Information
16.1.2 Plant Transfer of Subsurface
Contaminants to the Air
Even deciduous trees emitted isoprene at levels expected
from the pines. Organic acids, such as the oxygenated
aldehydes and ketones such as formaldehyde, were detected
as an emission from predominantly deciduous trees. The
gaseous emissions were detected in Tedlar bags placed
over branches and sealed with tape or rubber bands. As
discussed in Chap. 13, the amount of VOCs released by
plants into the air will differ, in general, as a function of
plant species and contaminant chemical and physical
properties. Some volatile organic chemicals after transloca-
tion will diffuse to the atmosphere relatively unchanged.
On the other hand, release to the atmosphere of subsur-
face contaminants through plants actually will often enhance
contaminant remediation. For example, for the fuel oxygen-
ate MTBE, the half-life of this compound in groundwater
where oxygen has been depleted is on the order of months to
years, whereas if volatilized to the atmosphere after translo-
cation through a plant, the half life can be on the order of
minutes to hours, after attack by hydroxyl radicals in the
atmosphere. Similarly, the chlorinated solvent PCE has a
half life of near 2 years if not longer in oxic groundwater,
but in the atmosphere it is lowered to between 1 h and 100 d.
Benzene has been shown to volatilize from plants to the
atmosphere (Collins et al. 2000) as well as TCE (Ma and
Burken 2003) and become rapidly degraded. These
scenarios may not be the case for every groundwater con-
taminant, however.
The installation of many above-ground treatment tech-
nologies for groundwater contamination characterized by
volatile organic compounds, such as BTEX, MTBE, or
TCE, also requires that these chemicals be monitored for
release to the atmosphere. If an air-strip, air-vapor extraction
(AS/AVE) system is installed, for example, the ambient air
near the extraction equipment is monitored for the potential
for contaminant release to the atmosphere. In fact, such
monitoring is required as part of corrective action plans
(CAPs) or RODs to meet State or Federal air-quality
mandates.
For phytoremediation of contaminated groundwater,
there also is the concern for air-quality degradation from
the release of unattenuated contaminants from the plants.
This concern is well founded, given that chemicals such as
MTBE and TCE have been documented to move through
plants in the transpiration stream from groundwater to the
air, as was discussed previously. This potential release to the
atmosphere should be viewed, however, in the context of (1)
the release of natural VOCs by plants in uncontaminated
areas and (2) the ultimate fate of the VOCs released by
plants at contaminated sites.
In the 1980s, former U.S. President Ronald Reagan was
chastised about his comment that natural plants were respon-
sible for 80% of measured atmospheric pollution in certain
areas. In fact, his comment was meant to explain only one
type of airborne pollutant, from the organic species called
olefins, such as isoprenes and monoterpenes. Isoprenes are
5-carbon units that comprise 10-carbon units called terpenes,
such as the commonly known turpentine. These turpenes are
found in the sap of many coniferous trees. These olefins are
naturally released in volatile form by many tree species. This
synthesis and release is the cause of the haze that resides
over most of the Appalachian Mountains, also called the
Blue Ridge Mountains, because this haze of organics
appears blue from a distance.
This phenomenon of the naturally occurring plant-release
of VOCs is not limited to the eastern United States.
Martin et al. (1999) reported that trees such as aspens
(
Populus tremuloides
), cottonwoods (
Populus fremontii
),
oaks (
Quercus gambelii
), fir (
Pseudotsuga menziesii
), spruce
(
Picea engelmannii
), juniper (
Juniperus scopulorum
), and
pine (
Pinus edulis
and
Pinus ponderosa
) growing in
New Mexico all released to the atmosphere nonmethane
hydrocarbons, monocarboxylic acids, and low-molecular-
weight aldehyde and ketones. The pines emitted predomi-
nantly
16.1.3 Fate of Contaminants in Leaf Litter
Some essential plant minerals or nutrients can have a gas-
eous phase and, therefore, if removed from the plant can
readily be reassimilated. Other plants minerals such as phos-
phorus and iron, however, do not have a volatile phase, and
as such, can be removed readily from plant access by
leaching. Therefore, plants have evolved to retain certain
easily leached minerals.
In a classic study, the loss of leachable versus sequestered
minerals, such as calcium, was investigated using
134
Cs as a
surrogate (Witherspoon 1964). This element was injected
into the base of trees, and its fate monitored over time in
the various compartments of a tree's environment. Up to
40% of the
134
Cs added ended up in the leaves, but the
leaves returned more than half back to the trunk. The
remainder, however, stayed with the leaves until leaf drop.
Overall, no more than 20% of the
134
Cs left the tree by
leaching. This study indicates that plants can store such
mobile phases by sequestration into wood. The red heart-
wood of pine trees and oaks is a testament to the removal of
these minerals to areas deep in the tree. This radial flow
of nutrients from the phloem toward the center occurs by
the rays.
-pinene, up to 100-10,000 nanograms per gram of
dry weight per h (ng/g/h). Additional hydrocarbons such as
isoprene, camphene,
d
-limonene, and
a
b
-pinene also were
measured.
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