Environmental Engineering Reference
In-Depth Information
tissues), phytostimulation or plant-assisted bio-
remediation (stimulation of microbial and fungal
degradation by release of exudates/enzymes into
the root zone), and phytostabilization (absorption
and precipitation of contaminants, principally
metals, by plants). A wide range of organic and
inorganic contaminants; most appropriate for
sites where large volumes of groundwater with
relatively low concentrations of contaminants
must be remediate to strict standards. Most ef-
fective where ground-water is within 10 ft of the
ground surface, and soil contamination is within
3 ft of the ground surface.
Use of native plants in phytoremediation pro-
vides advantages over other species and helps
bring back the heritage of flora lost through
human activity. In addition to restoring biodiver-
sity in areas that have been disturbed, remediat-
ing superfund sites using native species provides
for wildlife habitat enhancement and conserva-
tion and saves money over alternative cleanup
methods. Unlike many introduced species, once
established, native plants do not require fertiliz-
ers, pesticides, or watering. As encouraged by the
Superfund Redevelopment Initiative, use of na-
tive plants in site restoration may serve to restore
wetlands and other habitats and create nature
parks, sanctuaries, and other green areas.
Phytoremediation is the use of specialized
plants to clean up polluted soil. While most of
the plants exposed to high levels of soil toxins
will get injured or die, scientists have discovered
that certain plants are resistant and even a smaller
group actually thrive. Both groups of plants are
of interest to researchers, but the thriving plants
show a particular potential for remediation be-
cause it has been shown that some of them ac-
tually transport and accumulate extremely high
levels of soil pollutants within their bodies. They
are therefore aptly named hyperaccumulators.
Hyperaccumulators already are being used
throughout the country to help clean up heavy
metal-polluted soil. Heavy metals are some of
the most stubborn soil pollutants. They can bond
very tightly to soil particles, and they cannot
be broken down by microbial processes. Most
heavy metals are also essential plant nutrients, so
plants have the ability to take up the metals and
transport them throughout their bodies. However,
on polluted soil, the levels of heavy metals are
often hundreds of times greater than normal, and
this overexposure is toxic to the vast majority of
plants. Hyperaccumulators, on the other hand,
actually prefer these high concentrations. Essen-
tially, hyperaccumulators are acting as natural
vacuum cleaners, sucking pollutants out of the
soil and depositing them in their above-ground
leaves and shoots. Removing the metals is as sim-
ple as pruning or cutting the hyperaccumulators'
above-ground mass, not excavating tons of soil.
Resistant, but not hyperaccumulating, plants also
have a role in phytoremediation. Organic toxins,
those that contain carbon such as the hydrocar-
bons found in gasoline and other fuels, can be
broken down by microbial processes. Plants play
a key role in determining the size and health of
soil microbial populations. All plant roots secrete
organic materials that can be used as food for mi-
crobes, and this creates a healthier, larger, more
diverse, and active microbial population, which
in turn causes a faster breakdown of pollutants.
Resistant plants can thrive on sites that are often
too toxic for other plants to grow. They in turn
give the microbial processes the boost they need
to remove organic pollution more quickly from
the soil.
Both forms of phytoremediation have the
added benefit of not disturbing the soil. While
excavation is an effective way to get rid of pol-
lution, it removes the organic matter rich topsoil
and, because of the use of heavy machinery, com-
pact the soil that is left behind. Phytoremediation
does not degrade the physical or chemical health
of the soil. Actually, it creates a more fertile soil.
Soil organic matter is increased as a result of
root secretions and falling stems and leaves, and
the roots create pores through which water and
oxygen can flow. Additionally, few would argue
that a dusty excavation site is more aesthetically
pleasing than a nicely planted field.
However, there are many limitations to phy-
toremediation. It is a slow process that may
take many growing seasons before an adequate
reduction of pollution is seen, whereas soil ex-
cavation and treatment clean up the site quick-
ly. Also, hyperaccumulators can be a pollution
Search WWH ::
Custom Search