Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 136

groundwater-flow lines can be assumed to terminate at a
surface-water feature.

the time when roots have not yet interacted with groundwa-
ter. Conversely, at sites where long-running pump-and-treat
systems have become ineffective or cost intensive due to
biofouling or low specific yields, phytoremediation can be
an alternative that is not affected by these obstacles. More-
over, real or perceived concerns regarding a potential lapse
in water-removal processes during plant dormancy can be
alleviated by using a strategy that integrates source-removal
activities, such as the occasional operation of a pump-and-
treat or AS/AVE system, with phytoremediation.
One particular scenario in which phytoremediation can be
used successfully in conjunction with other remedial
strategies is as an additional component of monitored natural
attenuation (MNA) strategies. At sites where MNA has been
implemented as a corrective action, a principal concern is
the movement of the plume of contamination at rates faster
than the rate of prevailing natural attenuation processes.
A phytoremediation system applied as a downgradient
hydrologic barrier could alleviate these concerns.

6.2.3 Reduction in Groundwater Recharge
at Source Areas

In general, localized source areas of potential contamination
may seem as the last place to implement phytoremediation to
achieve hydrologic control because these source areas often
contain separate-phase contamination at levels toxic to
plants. Contamination at source areas often extends
completely from land surface to the water table. Moreover,
source areas tend to be inaccessible, often being covered by
buildings or paved surfaces.
Phytoremediation, however, can be used successfully to
remediate such source areas. For example, installing plants in
source areas can decrease recharge because of plant uptake of
infiltration prior to flow through contaminated soil and sedi-
ment. This process of plant-facilitated reduction in recharge
decreases both gross and net annualized recharge.
As discussed in Chap. 5, a decrease in the formation
of leachate from source areas is a legitimate goal of
phytoremediation. In fact, source areas such as landfills can
be covered with plants rather than remain unplanted as is
often the case (see Rock 2003 for a comprehensive review).
Conventional landfill covers act to decrease soil permeabil-
ity, and act as a water-percolation barrier, to achieve the goal
of leachate-formation reduction, and are designed to comply
with RCRA regulations promulgated with their creation in
1976. The goal of this approach is one of waste isolation.
A reduction in soil permeability is achieved with mechanical
soil compaction or with geotextiles. Such decreased soil
permeability often is not sustainable, however, as a result
of animal burrowing activities, erosion, earthquakes, etc.,
and often leads to excessive runoff and Horton overland
flows. Conversely, vegetated covers, essentially a special
case of phytoremediation where plants are installed on the
landfill cover, rely on plants to reduce infiltration by first
removing the water by evapotranspiration before the water
can leach contaminants to the water table.

6.2.5 Contingency Plans

Contingency plans are required for all forms of site remedi-
ation in case the specified mechanisms do not work to the
full satisfaction of the responsible party or regulator.
Because of the uncertainty inherent in all decisions regard-
ing site assessment, characterization, and phytoremediation,
it is important to establish a contingency plan. Even if plants
are shown to meet the goals of reducing recharge or off-site
contaminant migration, the health of the phytoremediation
system can be negatively affected by perils not related to
groundwater contamination. This can include infestation by
leaf-eating insects, molds, voles, shrews, beavers, and large
quadrupeds such as deer; lighting strikes; fires; early frosts;
snow; hail; drought conditions; inundation; and high winds,
just to name a few. If the planting area is compromised by
one or more of these threats, the contingency plan can be
implemented.

6.2.6 Regulatory Approval

6.2.4 Supplemental Use with Other Hydrologic
Strategies

Planting trees at a contaminated site generally does not
require regulatory approval. However, the purposeful appli-
cation of plants to affect a reduction in contamination level,
to decrease recharge, to alter the site hydrology, or even
transpiration of volatile organic compounds generally does
require regulatory contact and approval. Regulatory officials
at the local, state, and federal levels involved with a specific
site should be included during preliminary discussions of the
potential implementation of a phytoremediation strategy,
such as during remedial investigations and feasibility studies

The use of phytoremediation to achieve any of the three
noted hydrologic goals also can be evaluated at sites where
engineered hydrologic-control systems are in place, such as
pump-and-treat or air stripping/air vacuum extraction (AS/
AVE). In fact, a strong case can be made to integrate
phytoremediation with conventional water and air control
technologies, in order to accelerate site remediation during