Environmental Engineering Reference
In-Depth Information
information to help guide plant selection for use at
phytoremediation sites where groundwater is contaminated.
The relation of the roots to the water table is the deter-
mining factor in deciding whether or not a particular plant
will be useful in achieving hydrologic control through a
barrier design. Most phreatophytes establish roots to a
depth that represents the capillary fringe directly above the
water because oxygen is available for root respiration. On
the other hand, the depth to water table, and by definition,
capillary fringe, is not a fixed location but can change over
time. Even for phreatophytes, if the water table and capillary
fringe rise because of infiltration, there will be a period when
previously exposed roots will be submerged. There are, of
course, some phreatophytes, such as cottonwood, alfalfa,
and mesquite, which can have at least some roots below
the water table continually. This can occur if the osmotic
pressure is higher in the roots (i.e., a lower water content in
the roots caused by concentrated salts) than that in the water
table. Moreover, this occurs even if salts are not present
because the roots can remove groundwater more easily at
pressures equal to or greater than 1 atm than water under
tension in the capillary fringe.
Perhaps the second most important aspect of successful
phytoremediation as a hydrologic barrier is the correct selec-
tion of plants to interact with the water table. The selection
of the appropriate plants must meet a few criteria, such
as whether (1) the plants will grow under the climatic
conditions of the site, (2) the plants will interact with the
water table, and (3) the plant interaction with the groundwa-
ter system will be to the extent that remedial goals can be
met in a reasonable amount of time in a cost-effective
manner.
A first step in plant selection for a hydrologic-barrier
design is to determine what types of native plants are
doing well at or near the site under ambient conditions.
The presence of grasses may indicate that adequate
conditions of soil nutrients and moisture are present. The
presence of larger shrubs and trees may confirm what
the grasses indicate but also that deeper sources of water
are available. An advantage to such observation of native
plants is the knowledge that they already are adapted to
the climate of the area. The disadvantage, however, is that
the presence of vegetation at a site does not directly indicate
that the plants are using groundwater through uptake or
decreasing recharge. These questions must be answered by
field studies and extensive monitoring.
A second step is to look at the use of genetically
engineered plants that have been specifically designed to
perform a particular function. All characteristics are part of
specific genes. More recently, specific genes linked to a
desirable trait, such as the ability to detoxify a contaminant,
have been identified in certain plants, isolated, and added to
other plants to confer this trait; this topic is discussed in Part
III. Other plants that have genes that result in longer or faster
root growth also could be used to facilitate hydrologic con-
trol through the barrier design. In either case, the regulatory
approval of the use of such genetically modified plants
remains the biggest hurdle to their potential use at
phytoremediation sites.
Plants have been added to affect groundwater flow at
some sites where the results are published. For example,
at a Superfund site in the southeastern United States
characterized by the blending of pesticides between 1936
and 1987, pesticide-contaminated soil was removed, treated,
and replaced. Pesticides, such as toxaphene and benzene
hexachloride (lindane), were detected in groundwater
(Leavitt et al. 2001). A pump-and-treat system was proposed
to contain and treat hot spots of contamination that could not
be excavated. Because the concentrations of pesticides were
less than 50
m
g/L and various metals were present, which
would complicate treatment, it was determined through pilot
tests that such a system would not be efficient. Therefore, a
phytoremediation-based treatment of the contaminated
groundwater using a barrier design was investigated.
The hydrogeology of the contaminated site consists of
sands and clays. Groundwater flows in the surficial aquifer,
which is about 20-ft (6 m) thick, from the about 28 acre
(113,316 m
2
) site, to an adjacent lake. In 1998, about 2,500
hybrid poplar trees were installed within a 2.2-acre
(8,903 m
2
) area. The trees were planted to depths between
2 and 12 ft (0.6 and 3.6 m), and extensive monitoring was
initiated to determine the amount of groundwater being used
by the trees. Over time following plant installation, the
removal of water by the stand of trees went from less than
1 gal/min to more than 8 gal/min as the leaf area of the
phytoremediation system increased from less than 21,527 ft
2
(2,000 m
2
) to between 64,583 and 193,750 ft
2
(6,000 and
18,000 m
2
). Leavitt et al. (2001) reported that the
phytoremediation system removed 7.7 Mgal (29 million
liters) of water between early 1998 and the summer of
2000. However, it is important to note that the authors did
not discriminate the source of water being removed by the
trees.
7.3
Environmental Factors That Affect Plant
Growth and Groundwater Use
Even casual observation reveals that plants can grow in
some of the most unexpected places. These unlikely
environments range from large hardwood trees growing in
cracks in igneous rocks to tiny herbaceous weeds growing in
the cracks in sidewalks or parking lots. These plants are
growing, but are they thriving? If plants are to be used as
part of a phytoremediation system, simply getting them to
grow is a first step, but it is far from the ideal scenario, where
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