Environmental Engineering Reference
In-Depth Information
wetlands, swamps, springs, rivers, and creeks. Site ground-
water was characterized by a wide range of contaminants,
from chlorinated solvents to petroleum hydrocarbons.
Groundwater flow was from the source area a relatively
short distance of less than 200 ft (61 m) to a wetland
swamp area that bordered the site. The groundwater-flow
rate was estimated to be about 55 ft/year (16.7 m/year). The
wetland is actually more characteristic of a swamp, because
the flooded conditions support hardwood tree species such as
tulip poplars, sweet gum, white oaks, and water oaks.
Because the groundwater contaminant plume did not extend
beyond the swamp, and because the hydraulic gradient from
the source area to the swamp increased, the authors
concluded that uptake of groundwater by the native swamp
plants was partially responsible for hydrologic control of the
groundwater contamination (Bankston et al. 2001).
This hydrologic control of the contaminant plume may
not simply be due to the uptake of groundwater by the
swamp plants, however. The presence of a wetland or
swamp indicates, in most instances, that the area is a location
of groundwater discharge for local groundwater flow or a
place where surface runoff collects after precipitation
events. As such, groundwater-flow paths would tend to ter-
minate at such low wet spots even if trees were not present.
This flow-path termination still represents hydrologic con-
trol, but it is not necessarily solely due to phytoremediation.
It would have been interesting had Bankston et al. (2001)
assessed the
source
of water in the transpiration stream of
the plants to determine whether or not the plants were taking
up groundwater and dissolved contaminants or if they were
taking up the surface water in the swamp, which could have
been a mixture of recent groundwater discharge that had not
yet evaporated or transpired, and surface-water runoff. It is
important in these types of investigations in which
contaminated groundwater will interact with natural ground-
water discharge areas to thoroughly document the influence
of plants on groundwater.
Native plants may have an advantage over introduced
phreatophytes when it comes to increased disease resistance.
With clones, disease susceptibility can be a limiting factor
relative to superior disease resistance in native plants. This is
because under natural conditions of predominately sexual
reproduction in native plants, there is a selective battle
between plant health and insect infestation, and only the
strongest survive to reproduce. With the clones that can be
added to phytoremediation sites, plants are at a disadvantage
in this battle, because each clone is an exact genetic copy of
the parent only. The parent may have been selected for an
original advantage, but without sexual reproduction and
natural selection, clones are more susceptible to the ravages
of new diseases or cyclical insect infestation. Moreover,
such a scenario sets up the need for increased pesticide
usage.
Another benefit of using native vegetation rather than
clones is increased lifespan. The cause of reduced lifespan
in clones could be a result of changes that occur in the
chromosomes, particularly the teleomeric sequences, but
insufficient data exist to support or refute this idea.
10.1.2 Hydrologic Control: Phytoremediation
Compared to Pump-and-Treat and
Trenching
In order to reach the specific remedial goal of hydrologic
control at sites characterized by contaminated groundwater,
the water table needs to be affected. Control typically has
been accomplished using mechanical approaches, such as
pump-and-treat systems or trenching. For a conventional
pump-and-treat system, wells, pumps, electrical lines,
pipes, and treatment all need to be installed before any
groundwater is pumped. Trenching requires equipment, the
installation of drainage materials in the trench, and disposal
of the excavated sediments. The installation of trees is rela-
tively less expensive in terms of capital investment and less
mechanically intensive, assuming that an irrigation system,
significant soil removal, or amendment, are not required.
Strand et al. (1995), for example, showed that
phytoremediation systems remove groundwater at 20%
the cost of pump-and-treat systems. Once the pump-and-
treat system is installed, annual costs of power and water
treatment will continue, and water disposal also is needed
for trenched projects. These concerns do not exist at
phytoremediation sites, other than performance monitoring
of the plant tissues and groundwater.
In reality, however, the perceived cost savings of
phytoremediation, either during installation or operation
and maintenance over the life of the project, may not neces-
sarily be borne out. For example, if site assessment and
characterization activities, which themselves represent a
cost, indicate that the depth to the water table is greater
than about 25 ft (4.5 m), more resource-intensive and pro-
prietary deep-planting methods may need to be used—at a
minimum, more expensive longer cuttings or poles will be
required. If over time monitoring suggests that hydrologic
control will require more acreage to be planted, the addition
of more trees and expense of additional land, if available,
also will increase costs. Losses of plants due to mortality
will require new plants to be installed, with additional mobi-
lization costs to replant. Moreover, because the use of
phytoremediation to control groundwater hydrology is still
a relatively new technology to many regulators, they may
require the collection of performance data such as ground-
water levels and samples on a more frequent basis than if a
conventional alternative technology were chosen. Some
regulators may even require that a backup containment
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