 Using Plants to Clean Contaminated Sites
There are an estimated 30,000 contaminated sites in Canada.
These include properties like former gas stations, factories, or
rail yards that are contaminated by heavy metals, organic
compounds, or other toxins. Redevelopment of these sites,
which are often found in prime downtown areas, is important to
provincial and local governments.
Traditional remediation includes removal of contaminated soil or
treatment with chemicals. These options have been
criticized for several reasons, such as their high cost and
destructiveness to sites. Plant-based remediation
(phytoremediation) is a relatively inexpensive, low-tech
approach that may lead to at least partial decontamination and
restoration of contaminated sites. It takes advantage of
the natural abilities of some plants to remove contaminants from
the environment.
Plant roots absorb contaminants into their leaves, branches
and/or stems, which are then harvested and removed, for
destruction (usually incineration). The roots may also
capture and prevent migration of soil contaminants. Plants
may break down organic contaminants like petroleum products into
less toxic compounds.
Phytoremediation works best on large sites where contaminants
are located at a relatively shallow depth and at low
concentrations. Several growing seasons may be needed to
clean up a site. The technology is not suitable for some
sites, such as those that are highly contaminated with metals,
which may take decades to clean up.
Among the most critical considerations in selecting a plant
species to use in phytoremediation are the following:
Does it work? Plants that are suited to remediation take
up toxins into their leaves, stems and roots. A recent study of
poplar and willow species that had been irrigated with leachate
from landfill showed that selecting the right plant for a site
is not a simple process. Poplars had better update of
phosphorus, potassium, sulfur, copper and chloride, while
willows absorbed more zinc, boron, iron and aluminum.
Willow leaves had higher calcium and magnesium concentrations,
as did poplar stems and roots. Poplar leaves and willow
roots had higher manganese and sodium levels.
Typically, plant species are first tested at the remediation
site. For example, an ongoing Chicago-area
phytoremediation project began with testing of native trees,
including black willow, to determine effectiveness in taking up
metals and organic compounds from soil and groundwater.
Can it survive? Plants must be able to accumulate and
tolerate contaminants, to adapt to the climate at the site, and
be easy to maintain. They also have to tolerate stressors like
pests and diseases, road salt or vehicle exhaust.
Effect on the ecosystem. Non-native plant species could
thrive and ultimately threaten the local ecosystem if they
escape from the remediation site.
Willows, which grow quickly and are known for their deep roots
and ability to absorb large volumes of water, are among the most
common tree species used. They remove a variety of organic
and inorganic contaminants, as well as herbicides, pesticides
and radionuclides. In Sweden, large-scale willow plantings
are used to treat municipal wastewater, landfill leachate, and
sewage sludge. Willow roots prevent spread of contaminated
water, and the willow can be pruned back hard, yielding a
significant amount of (contaminated) plant matter for disposal.
There are several drawbacks to phytoremediation. Some
plants accumulate only certain elements, so the technology may
not be applicable to sites where there are mixed contaminants.
As well, with many plant species, there is simply not enough
information available to conduct appropriate risk assessments.
There are many unknowns about the technology. For example, what
is the risk to animals that eat contaminated plant materials?
In some cases, plants may concentrate toxins, or convert
contaminants into more toxic by-products. Are there health
consequences when plants give off vapours that contain high
concentrations of volatile substances? What about toxins
in wood and leaves that are used for firewood or mulch?
These concerns and other practical considerations, such as
whether the technology can achieve adequate site cleanup, how to
monitor it, and even how to categorize the plant waste resulting
from cleanup in order that it can be properly disposed of, make
phytoremediation complex to regulate. As yet, standards
have not been developed, and it is not clear how
phytoremediation fits into the environmental regulatory
framework.
On the other hand, phytoremediation may produce plant residues
rich in metals that can be recycled. The vegetation
reduces erosion by wind and water, and as ground cover, may
decrease community exposure to certain contaminants, like lead.
The plant residue that results from the process is much lighter
and takes up significantly less space in landfills than does
waste from other remediation methods, like excavated soils. The
presence of trees and plants in an otherwise desolate landscape
makes the sites more appealing and public acceptance has been
high.
We are only just starting to understand how plants work to bring
toxic sites back to life, and return the environment to its
natural state.
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