Environmental Engineering Reference
In-Depth Information
2. Restoration of aqueous or dissolved-phase plumes, or
zones of contaminant mass, to pre-contaminant or site-
specific permissible levels.
activities can occur voluntarily by the responsible party or
in response to regulations that typically require any spill
greater than the federal reporting limit of 25 gal (95 L) to
be assessed for potential corrective action. As part of this
formal response to a contamination event, the site assess-
ment and history also can be supplemented with anecdotal
evidence from former or current employees.
Achievement of these goals is the restoration objective at
most sites. At each site, specific contaminant-reduction goals
must be met, and the restoration goal may be different for the
same contaminant released in different areas. For example,
in cases of small dissolved-phase plumes located in isolated
areas with little potential for human or ecological impact,
restoration goals often are implemented according to a risk-
reduction standard, where permissible levels of contamina-
tion can be higher than if the contaminant was released in a
more populated area. Overall, however, these two goals are
the most common reasons for taking remedial actions at a
site and,
6.1.2 Contamination Assessment
and Characterization
An important part of the initial determination of the history
of contamination at a site is the assessment and characteri-
zation of the extent of contamination in the subsurface. This
includes delineation of the vertical and horizontal
dimensions of contamination to a point where uncontami-
nated, or background conditions, are known, and to identify
potential human or ecological receptors. For example, a site
with a known point-source release, such as a leak from an
underground storage tank (UST) located within a populated
area, will have to be surveyed for the presence of domestic or
public drinking-water wells. If such a receptor already has
been affected by contamination, such as water from a well
that starts to taste or smell different, the process of site
assessment is reversed; the source(s) of the contaminant
release will need to be identified.
A contaminant release to the subsurface may affect the
physical components that comprise the subsurface, such as
the air, soil, water, and microbial ecology. The extent of a
contaminant release on each component must be determined.
For example, determination must be made about whether
or not free-phase contamination exists, such as gasoline
floating on the water table, because such free product is a
long-term source of contaminant release to the water table.
Moreover, the removal of free-phase contamination should
be the initial goal of site-remediation plans that include
phytoremediation because free-phase contamination typi-
cally will be toxic to plants. If the free-phase contamination
is located some depth below the root zone, however, plants
can be installed directly over the area to decrease groundwa-
ter recharge, increase subsurface oxygenation, and to
decrease additional dissolved-phase contamination.
To assess the affect of contamination on the different
components in the subsurface, the appropriate samples
need to be collected and analyzed. One of the most efficient
methods of assessing and characterizing the distribution of
contaminants below the surface at a site is to drill boreholes.
Drilling a borehole permits the collection of samples, both
contaminated and uncontaminated, to determine soil physi-
cal and chemical properties, the collection of groundwater
samples, the collection of soil-gas samples from the unsatu-
rated zone, and the measurement of the thickness of any
therefore, are important
to an evaluation of
phytoremediation.
6.1.1 Contaminant-Release History
All contaminated sites have unique histories of contaminant
release and environmental effect. Contaminant releases can
be characterized in terms of differences in space and time.
Contaminant releases can be acute, such as a spill or accident
that releases a substantial volume of contaminant over a
larger area, or chronic, such as a slow low-volume release,
as when a pipe joint or fitting leaks underground. Either type
of release may have occurred in the past, over time, or more
recently, but the contaminant remains in the ground. To
complicate matters, the party responsible for the contami-
nant release may be the current owner of the property, a
newer owner who assumes the liability of releases by previ-
ous owners, an owner of property adjacent to or
downgradient from a contaminant source, or an unknown
or disputed owner, as when contaminant sources are located
in adjoining properties. Moreover, although zones of
contaminated media can be delineated in most cases, the
volume of
the material(s)
released is rarely,
if ever,
completely known.
Thanks, in part, to newer reporting regulations and more
effective enforcement actions, contaminant releases that
may have gone undetected 20 years ago now can be more
rapidly detected. As a consequence, more recent releases
generally have more specific information regarding the
times and volumes of contaminant released as part of the
site history. The importance of rapid-release detection
systems required at most gasoline stations in the United
States, for example, cannot be overstated.
Following most contaminant releases and detection, local
or federal regulations stipulate the commencement of a
formal site assessment and characterization of the extent of
the environmental effects,
if any. These site-assessment
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