Environmental Engineering Reference
In-Depth Information
free-phase contaminant that enters the borehole. The occur-
rence and distribution of root material from existing vegeta-
tion also can be assessed during borehole drilling. These
boreholes then can be used as locations to install temporary
or permanent monitoring wells.
With respect to phytoremediation, one of the goals of site
assessment and characterization is to map the extent of the
contamination in context to the surrounding above- and
below-ground features. Identification of these features can
allow the general direction of groundwater flow to be deter-
mined, even before wells are installed, and guide the instal-
lation of monitoring wells. Because samples will be
collected from below ground as part of site assessment and
characterization, the locations of buried utilities, such as
electric, gas, fuel, water, fiber optics, etc., must be identified
as soon as possible. Even if these underground utilities have
been previously located on a map, many local and state laws
require that a private location service be contacted prior to
subsurface investigation; otherwise, the party conducting the
subsurface investigation may be held liable for damage.
Moreover, sufficient space must be allowed on either side
of located underground utilities to account for uncertainties
inherent to the accuracy of these locations.
The source area or suspected contaminant-release must
be delineated as accurately as possible within given site
constraints. Source areas usually are the first part of a
contaminated site that is delineated; a burst pipeline that
spews raw product or the presence of objectionable odors
are hard to attribute to natural causes. Conversely, if
dissolved-phase contaminants have been detected in wells
but the source area is unknown, the area of contaminant
release often can be delineated by plotting the contaminated
wells on a map and searching in upgradient areas—this
approach is discussed further in Chap. 15.
The vertical extent of contamination at a source area from
land surface through the contaminated zone(s) and to uncon-
taminated media also must be identified. This identification
is facilitated where contamination tends to be closer to land
surface and the contaminant is more likely to be con-
centrated over a relatively smaller volume of the subsurface.
In areas downgradient from the source area, however, verti-
cal delineation of the dissolved-phase contaminant is less
likely to be accurate.
This decrease in assessment accuracy in downgradient
areas is not a result of a lack of approach or technology;
rather, it is an artifact of the conventional approach used to
assess contaminated sites, in which monitoring wells are
installed with screens located only across the water table.
This conventional approach has led to at least two problems.
First, the water-table surface is not a constant, fixed surface,
but fluctuates sometimes substantially over time. Thus, the
location of the water table during site-assessment activities
may not be representative of the seasonal mean water-table
surface. Wells installed with screens across the water table
can go dry during droughts, periods of low recharge, or by
unanticipated seasonal fluctuations. Second, groundwater in
areas away from the source area may be recharged by uncon-
taminated water derived locally from above, which tends to
push contaminated groundwater deeper below the surface of
the water table. In this specific scenario, downgradient moni-
toring wells installed using the conventional approach of
screening wells across the water table can produce uncontam-
inated samples while deeper, contaminated groundwater
flows underneath the well and remains undetected. Examples
of this scenario, effect on contaminant plume migration and
detection, and solutions on how to avoid it are provided in
Landmeyer et al. (1998b) and Wilson et al. (2005).
Within the context of contaminated sites that have a
known history of release and some initial contaminant delin-
eation, the following sections describe a potential approach
that can be used to evaluate existing site- assessment infor-
mation for the purposes of phytoremediation to achieve the
three hydrologic goals. States may require, however, addi-
tional information be collected to comply with local
regulations not covered here.
6.2
Site-Specific Hydrologic Goals
Prior to any discussion of the application of phytore-
mediation at a site characterized by contaminated ground-
water, the question must be asked and answered, “Can the
site be hydrologically controlled to decrease contaminant
movement in groundwater?” Although a seemingly simple
question, the answer is deceptively complex, because the
answer often is site specific.
As described at the beginning of this chapter, phytore-
mediation can be implemented at sites characterized by
groundwater contamination to achieve one or all of the
following goals:
• Prevention of contaminated-groundwater flow to cleaner
offsite areas—a reduction in groundwater flow across
property boundaries;
• Prevention of contaminated-groundwater flow from
reaching regulated receptors, such as surface-water
bodies, that may be located on or off site—a reduction
in groundwater flow to potential receptors, and;
• Reduction in leachate formation and subsequent ground-
water contamination near source areas—a reduction in
groundwater recharge at source areas.
The three hydrologic goals share a similar approach that
can be used as part of comprehensive site-assessment and
characterization. The following discussion will focus on the
effect of plants on groundwater and how their effectiveness
can be evaluated in terms of the three hydrologic goals.
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