Environmental Engineering Reference
In-Depth Information
vegetation to be part of a phytoremediation system can be
called intrinsic phytoremediation. As we saw in Chap. 9,
tissue samples of existing vegetation can be used to delineate
the location of subsurface contamination, prior to the
installation of initial or additional monitoring wells. If
existing vegetation is being considered as part of the
phytoremediation system, however, various factors that
will contribute to the overall efficiency in groundwater con-
tainment or control should be considered. For example, the
existing plants may not be phreatophytes and, therefore,
will not directly take up groundwater. However, the water
table may still be affected by the presence of native plants
through recharge reduction, as was discussed in Chap. 5.
Even so, this benefit on contaminated groundwater may be
limited in areas where native plant density is sparse and,
therefore plant-water uptake is less than the site recharge
rate. Even if the plant density is thick, the existing plants,
such as evergreens, grasses, or other drought tolerant plants,
may not transpire much water.
Based on the limitations that affect the interaction of
existing vegetation and groundwater, it often is prudent to
artificially introduce plants to augment the background level
of intrinsic phytoremediation at such sites—simply to
increase plant density. Increased plant density brings the
removal of groundwater by transpiration in line with the
flux of groundwater at the site, as described in Chap. 8.
Also, plants that are introduced can be those species that
have a higher potential to degrade groundwater
contaminants at rates faster than rates of native plants. We
have seen for example that the readily available hybrid
poplar trees have some of the highest transpiration rates
(Pallardy and Kozlowski 1981; Interstate Technology and
Regulatory Council 2009).
An advantage to using native vegetation at a site is a
reduction in initial costs relative to the installation of
phreatophytes. The cost associated with site preparation,
installation, amendments, and labor, as introduced in Chap.
7, would not be necessary if existing vegetation were used.
And as was discussed in Chap. 8, most sites have the very
real need to minimize the size of the planting to maximize
use of the land for other purposes, such as found at active
gasoline stations or other industrial areas. If more land is
needed to capture a plume of dissolved-phase groundwater
contamination, for example, then additional lands may be
needed. This acquisition may be cost prohibitive, even if
native vegetation can be used.
A case study of the use of native existing vegetation to
meet remedial goals was presented by Bankston et al. (2001).
The site in question is a Superfund site in the Coastal Plain, or
low country, of South Carolina. In this physiographic prov-
ince, the topography is flat, the aquifers comprise fine silts
and sands of alluvial deposition, and the depth to water table
is shallow and in some areas is above land surface to support
10.1
Economic Factors that Affect the
Implementation of Phytoremediation
Many different economic drivers can determine which par-
ticular remedial strategy is selected for implementation at a
site characterized by contaminated groundwater. Whereas a
range of strategies may work to meet regulatory
requirements, the technologies that often are preferred are
those that also are cost effective.
The factors of economics cannot be ruled out of the
selection of any remedial decision. For example, there may
be a cost savings up front in the implementation of bioreme-
diation or phytoremediation as a remedial strategy compared
to a more engineering-intensive alternative such as pump-
and-recovery. A lower initial cost may, however, be
overshadowed by the costs associated with the long-term
needs of phytoremediation, such as monitoring and opera-
tion and maintenance. As such, the total cost of these
technologies may be equal to or exceed the costs of the
alternative technologies.
Most sites under investigation for environmental contam-
ination under RCRA or CERCLA are required to undergo
a series of remedial investigation and feasibility studies
(RI/FS). At each site, a range of remedial alternatives is
evaluated to determine which strategy will provide the
most environmental and human health protection and be
cost effective enough so that it can be implemented as
soon as possible. The alternatives are selected during nego-
tiation between the responsible party and regulatory
agencies. For the goal of hydrologic control of groundwater,
alternatives to phytoremediation range from capping to
reduce recharge, to digging trenches to intercept the water
table, to full-scale pump-and-treat systems to remove
groundwater from contaminated aquifers.
10.1.1 The Native Vegetation Versus Planted
Vegetation Dilemma
Most groundwater contaminated sites tend to have at least
some type of native vegetation present. The presence of
native vegetation can be beneficial, as was discussed in
Chap. 6 during site assessment and characterization
activities, because the presence of such native plants
indicates that, at some minimum level, conditions necessary
to support plants exist and suggest a phytoremediation plant-
ing may be viable. In addition, if the plants are interacting
with any contaminants at the site, then their growth indicates
that concentrations of groundwater contaminants may be
below toxic levels.
Such native vegetation often can be useful in planning
a phytoremediation system. The application of existing
Search WWH ::
Custom Search