Environmental Engineering Reference
In-Depth Information
water to such plants typically is precipitation, which is taken
up by their shallow fibrous root systems. Soon, slower grow-
ing plants become established, either herbaceous or woody.
The deeper root systems of these plants permit them to grow
taller and shade the annual grasses. This provides the oppor-
tunity for pine species to become established. Hardwood
trees then become established, and outcompete the slower
growing pines for resources, including water and groundwa-
ter. Ecologists call these plants k-specialists, and their arrival
proclaims the climax community, which is the end of plant
succession, at least until the site is disturbed again.
What does plant succession have to do with phyto-
remediation of contaminated groundwater? Many sites
characterized by groundwater contamination are located in
abandoned lands, often cleared of surficial evidence of past
waste-generating structures or virgin forests. These sites
when visited often are characterized by annual grasses and
are in the first stages of community succession. If a
phytoremediation plan is implemented at such a site, the
natural succession of plant communities, as described
above, is short circuited, with a goal of establishing a climax
community as quickly as possible. If cuttings or whips of
woody plants are used to establish the phytoremediation
system, forest conditions can take 3-5 years to be realized.
Therefore, the presence of a phytoremediation site can
benefit local ecology by providing a climax community in
a much shorter time frame than if such abused lands were not
planted and plant succession proceeded at natural and slower
rates. This benefit often will make phytoremediation a
more acceptable remediation strategy.
discrete redox zones must be taken into account, because
oxic and anoxic groundwater will mix during pumping and
may result in clogged filters that require routine maintenance
to stay open. Alternatively, if a thermal heating design were
to be used to degrade contaminants, engineers would have to
calculate the amount of heat needed to volatilize a specific
contaminant mass over a particular area and match this with
the appropriate network of vapor-extraction wells to collect
the vapor. Essentially, the desired data drive the design and
engineering of such remediation strategies.
The design of a phytoremediation planting should be
approached in a manner similar to the design of remedial
actions that involve mechanical and civil engineering, with
the additional consideration that the technology is based on
living organisms. The rooting depth of plants is a key factor
in determining the potential for plant and groundwater inter-
action at phytoremediation sites. Many other factors also
need to be considered before, during, and after planting.
For example, the parameters of temperature, light intensity,
water availability, and gas exchange are needed as input to
determine the most appropriate design.
Perhaps an appropriate analogy of this approach in terms
of maximizing plant health at a phytoremediation site is a
greenhouse. A greenhouse permits the regulation of the
parameters essential to the stewardship of plant growth.
Trying to design a phytoremediation site similar to
conditions found at a greenhouse, however, would be
prohibitively expensive even at small sites less than an
acre in size. However, a given set of parameters at each
site can be controlled by the phytoremediation designer.
Part of the design of a phytoremediation site in the future
will be the increased use or application of molecular biology
techniques. For example, some fruit and vegetable crops are
genetically modified to enable them to grow in climates far
from their original habitats. It may also be possible to use
such genetically controlled plants for phytoremediation
applications, such as those that transpire larger amounts of
water than even the current best hybrids. This should be an
area of exciting research.
7.2
Site Preparation, Design, and Plant
Installation
A civilization flourishes when people plant trees under whose
shade they will never sit.
Greek Proverb
Although phytoremediation and conventional remediation
engineering represent different approaches to the remedia-
tion of contaminated groundwater, in many cases, similar
data are used and both require monitoring of performance to
ensure the efficient hydrologic control of the site. In the
case of a classical mechanical engineering approach to the
remediation of groundwater contamination, such as a pump-
and-treat system, the number and size of wells to be pumped
need to be rated to match the specific yield of the
contaminated aquifer. This needs to be done to ensure that
the wells do not continually pump dry and that the time
needed to remove the required aquifer pore volume is met.
Also, in placing well screens for each well the spanning of
7.2.1 Site Preparation
The addition of plants at a site for phytoremediation to
achieve hydrologic goals is preceded, in most cases, by
site-preparation activities. This is true of most endeavors
that involve or have involved plants. For example, early
North American Indians burned forests to create open spaces
for planting. Colonial farmers in New England had to pre-
pare their fields by removing stones and large boulders. In
contrast, farmers in the wooded mid-Atlantic to Ohio Valley
felled trees to prepare their land. Site-preparation activities
also occur in other areas of the world to remove existing
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