United States
Environmental Protection
Agency
Office of
Solid Waste and
Emergency Response
EPA-540-F-98-051 ""
OSWER 9320.8-01 FS
PB98-963326
September 1998
SEPA Evaluating Ground Water Plumes
Under the Hazard Ranking System
Office of Emergency and Remedial Response
State, Tribal, and Site Identification Center (5204G)
Quick Reference Fact Sheet
This fact sheet provides information on potential National Priorities List (NPL) sites evaluated as contaminated ground water
plumes with no identified source of contamination and how such sites are evaluated under the Hazard Ranking System (HRS).
It defines steps that should be taken before a ground water plume can be evaluated as a source of contamination and summarizes
scoring considerations for sites that consist solely of a ground water plume. Responses to commonly asked questions about
evaluating contaminated ground water plumes at potential NPL sites are also presented. This fact sheet provides guidance only
and may be amended by EPA on a site-specific basis.
BACKGROUND
The HRS is the primary tool that EPA uses to place
hazardous waste sites on the NPL under the
Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA). NPL sites are those sites at
which releases or potential releases of hazardous
substances pose a threat to human health or the
environment. For HRS purposes, "site" is defined as:
Area(s) where a hazardous substance has been
deposited, stored, disposed or placed, or has
otherwise come to be located. (55 FR 51587,
December 14,1990)
In some instances, ground water has become contaminated,
and the source of the contamination has not been clearly
identified. These sites are referred to as ground water
plume sites. While the HRS specifically excludes from the
definition of "source" those volumes of air, ground water,
surface water, or surface water sediments that have become
contaminated through migration, it does make an
exception:
"... in the case of either a ground water plume
with no identified source or contaminated
surface water sediments with no identified
source, the plume or contaminated sediments
may be considered the source."
A contaminated ground water plume is generated when
hazardous substances, pollutants, or contaminants enter an
aquifer system. Contaminants can enter and spread
horizontally and vertically through the aquifer system by
infiltration, migration, interaquifer exchange, and
interaction with surface water. This movement of
contaminants throughout an aquifer is usually in the
direction of ground water flow but can spread against the
predominant flow direction. In the case of dense non-
aqueous phase liquids (DNAPLs), for example,
contaminants may follow a localized geological contour at
the bottom of a formation which may not conform to the
hydrologic gradient
The Extent of a Ground Water Plume Site
Since ground water plume boundaries change over time
with the introduction of additional contaminants from an
unknown source and the dispersion of contaminants
through the aquifer, the extent of a plume is difficult to
define. For HRS scoring purposes, ground water samples
which meet HRS observed release criteria may be used to
delineate plume boundaries.
The purpose of listing sites on the NPL is to identify
releases and potential releases that are priorities for further
evaluation, not to identify the extent of the site. Defining
the boundaries of a contaminated ground water plume in
precise geographical terms would require more information
than is routinely available at the listing stage. The same
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
-------
considerations that impact the definition of all sites come
into play in defining ground water plume sites.
An NPL site listing encompasses all areas found to be
contaminated. Therefore, in evaluating ground water
plumes with no identified source, the extent of the plume
site is not limited by actual property boundaries. The site,
however, is also limited to those areas that are
contaminated. Owners of property above aquifers
contaminated by migration of hazardous substances from
a source outside the property may not be responsible for
cleanup if the property owner did not cause, contribute to,
or exacerbate the release ("Announcement and Publication
of Final Policy Toward Owners of Property Containing
Contaminated Aquifers," 60 FR 34790, July 3, 1995).
Liability will be assigned to those parties contributing to
the plume when those parties are identified.
Naming Ground Water Plume Sites
To avoid confusion, sites consisting of ground water
plumes with no identified source generally should be
named according to the geographical area in which the
plume is located (i.e., XYZ Ground Water Plume), instead
of including potentially responsible parties in the site name
(i.e., Smith Co. Plant). Regardless, the site as defined by
the HRS is neither equal to nor confined by the boundaries
of any specific property from which the site name is
derived.
EVALUATING A GROUND WATER PLUME AS A
SOURCE
The first step in evaluating a plume site is to show the
contamination of an aquifer. Generally, contaminant
plumes are identified as a result of routine monitoring of
drinking water supplies. Additional ground water sampling
to confirm the elevated concentrations and establish
appropriate background levels will verify that a
contaminated ground water plume is present While the
exact extent of the plume may not be defined, all samples
used to characterize the ground water plume should meet
the criteria for establishing an observed release. Areas
lying between such samples should be considered a part of
the plume, unless information indicates contamination
should not be inferred between these points.
Once ground water contamination is documented, an
attempt should be made to identify a source in the area
which has caused or contributed to the plume. If hazardous
substances in the plume can be attributed to a specific
source in the vicinity, the actual plume is not considered the
source. In this case, the identified source of the plume is
evaluated as the source and the plume is considered an
observed release attributable to that source.
The source of ground water contamination can be
designated as unidentified if, within the context of an
expanded site investigation (ESI) or equivalent effort which
involves sampling, the original source of the ground water
contamination has not been documented. The ESI should
at least include a site reconnaissance in the vicinity of the
observed release samples, record searches, employee
interviews, and sampling for information on possible
origins of the ground water contamination. The attempt to
identify a source should be discussed in the HRS
documentation record and potential sources and potentially
responsible parties should be identified.
EXHIBIT 1
UNIDENTIFIED VS. UNALLOCATED
SOURCES
Under the HRS, the terms unidentified and
unallocated are not equivalent.
• An unidentified source is an example of
the HRS source type "other." A source
may be classified as unidentified when a
contaminated ground water plume or
surface water sediment contamination
exists, but the original source of
contamination is unknown and cannot be
identified through the appropriate means.
• An unallocated source is not an HRS
source type. Unallocated sources are
used to account for the hazardous waste
quantity at a site where the hazardous
substances cannot be associated with a
specific source. A ground water plume
should not be considered an unallocated
source.
Source: The Hazard Ranking System guidance
Manual, Interim Final, OSWER Publication
9345.1-07,1992.
After the source of contamination is designated as
unidentified, the source for HRS scoring is classified as
"ground water plume with no identified source." The
ground water plume should be evaluated as the source with
an HRS source type of "other." Once the ground water
plume has been established as a source, the extent of the
plume is generally considered as the extent of the site. An
-------
unidentified source should not be confused with an
unallocated source (refer to Exhibit 1).
In summary, make note of the following points before
evaluating a ground water plume as a source:
• The area of known ground water contamination has
been established by sampling or inference using the
observed release criteria outlined in Section 2.3 of the
HRS.
• A reasonable search for potential sources of ground
water contamination indicates that no definite source
of ground water contamination has been identified
(the level of effort to identify the original source
should be equivalent to an ESI).
HRS SCORING CONSIDERATIONS OF GROUND
WATER PLUMES AS SOURCES
As for any site, an HRS score is calculated for the ground
water pathway for each aquifer at or below the source at the
site or, in this case, the contaminated ground water plume.
Assign the highest ground water migration score for an
aquifer as the ground water migration pathway score for the
site. In general, the aquifer in which the ground water
plume has been identified will score higher than an
underlying aquifer evaluated for potential release, except
when the underlying aquifer is associated with a
significantly larger target population. Then the target
factor category value must be sufficient to compensate for
the lower likelihood of release factor category value.
Evaluating Likelihood of Release
The likelihood of release factor category reflects either an
observed release to an aquifer or the potential to release to
an aquifer. The presence of a ground water plume with
contaminant levels significantly above background in an
aquifer constitutes an observed release to that aquifer.
Documenting the presence of a ground water plume
involves establishing an observed release by chemical
analysis. Establishing an observed release by chemical
analysis requires documenting that the concentration of at
least one hazardous substance in the ground water plume is
significantly increased above its background level.
Background samples should be selected to demonstrate that
the release cannot reasonably be attributed to any known
source. As stated in the HRS, to establish an observed
release by chemical analysis at a ground water plume site,
no separate attribution is required. As discussed below,
only those substances identified as being in the plume are
used in selecting toxicity and mobility values.
Data presented in the HRS documentation record to
characterize a contaminated ground water plume should be
of known and documented quality. Well siting,
installation, completion, and development should be
performed according to protocols specified in EPA ground
water monitoring guidance, in particular, The Hazard
Ranking System Guidance Manual, Interim Final,
OSWER Publication 9345.1-07 (1992). In addition, all
guidelines specified in the guidance manual regarding
similarity of background and observed release samples
should be met. For example, Section 5.2 of the HRS
Guidance Manual, Selecting Appropriate Background
Samples, states that background and release samples
should be representative of the same portion of the aquifer
that is being evaluated (see HRS Guidance Manual, page
67). Once an observed release has been established by
chemical analysis, the analytical data used to associate the
hazardous substances with the plume may also be used to
define the extent of the source and the site for HRS scoring.
As discussed in Section 7.1 of the HRS Guidance Manual,
Determining Aquifer Boundaries and Number of Aquifers,
with multiple aquifers, determining the potential for the
release to migrate to an underlying aquifer may also be
necessary. In this case, the contaminated ground water
plume is evaluated for each of the four potential to release
factors (containment, net precipitation, depth to aquifer,
and travel time), as for any other source.
Evaluating Waste Characteristics
At a contaminated ground water plume site with no
identified source, hazardous substances eligible for
consideration in determining the pathway waste
characteristics factor category value are those that meet the
observed release criteria. Mobility and toxicity should be
evaluated for all observed release hazardous substances in
the ground water plume.
In evaluating ground water mobility, any hazardous
substance meeting the criteria of an observed release by
chemical analysis to an aquifer underlying a source is
assigned a mobility factor value of one (1), regardless of
the aquifer being evaluated. Therefore, a mobility factor
value of one (1) is assigned to any hazardous substance in
the ground water plume that is used to characterize the
contaminated plume (i.e., all substances that meet observed
release criteria).
As the extent of ground water plumes are not completely
characterized during the site investigation process, a
definitive hazardous waste quantity cannot be adequately
determined at the listing stage. When evaluating the
hazardous waste quantity at sites consisting solely of a
-------
ground water plume, only three of the four hazardous waste
quantity tiers apply to the source: hazardous constituent
quantity (tier A); hazardous wastestream quantity (tier B);
and volume (tier C). The area measure (tier D) cannot be
evaluated because the hazardous waste quantity table (HRS
Table 2-5) does not provide a divisor for source type
"other" in this tier.
Further considerations may restrict the application of tiers
A and B. Tier B wastestream data, for example, is unlikely
because its availability would imply some knowledge of an
originating source of contaminants in the plume. If such
information is available, the originating source should be
evaluated rather than the plume. Tier A is to be used only
when the mass of CERCLA hazardous substances "is
known or is estimated with reasonable confidence." Given
the inherent uncertainty in establishing the extent of ground
water plume sites, it is unlikely that sufficient analytical
data will be available to characterize most such sites to this
degree.
While data for tiers A and B may be available, it is more
likely that the volume tier will be the best available means
of evaluating the source hazardous waste quantity. For
HRS purposes, the area! and vertical extent of the plume as
delineated by ground water samples that meet observed
release criteria should be used in evaluating the volume
measure. Therefore, the hazardous waste quantity for a
plume site with no identified source can be determined by
measuring the area within all observed release samples with
the vertical extent of contamination, to arrive at an estimate
of the plume volume.
If the volume of the plume cannot be determined or if it is
not cost effective to do so, a volume of greater than zero
cubic yards can be used if it is explained that the presence
of contaminants in the plume demonstrates the presence of
some unknown quantity. In this case, minimum hazardous
waste quantity factor values may be assigned. The HRS
specifies that if the hazardous constituent quantity is not
adequately determined, and if any target is subject to Level
I or Level n concentrations, assign a value of 100 as the
minimum pathway hazardous waste quantity factor value.
If none of the targets are subject to Level I or Level II
concentrations, assign a minimum value of 10 as the
pathway hazardous waste quantity factor value, if the
hazardous constituent quantity cannot be adequately
determined (55 FR 51587, December 14, 1990). If the
plume is established exclusively with contaminated wells
that are not associated with target populations, additional
sampling may be advisable to determine whether the extent
of contamination can be expanded to encompass target
wells. The objective is to characterize the threat posed by
the plume as thoroughly as possible within the limited
context of an expanded site inspection. The need for
additional sampling will depend on other waste
characteristics factor values (i.e., toxicity) and the number
of targets involved.
Evaluating Targets
As with all sites, evaluate targets for ground water plumes
based on nearest well, actual and potential contamination
of the population, resources, and wellhead protection areas.
The following are specific factors that should be
considered at sites when a ground water plume with no
identified source is evaluated:
Ground Water Target Distance Limit
The 4-mile target distance limit and associated
distance rings should be measured from the
geometric center of the area of observed ground
water contamination. The area of
contamination should be based on samples that
meet observed release criteria.
Establishing Actual Contamination
Actual contamination is established for drinking
water wells that meet observed release criteria.
Evaluate other drinking water wells within the 4-mile
target limit, including those within the area of the
plume that do not meet the observed release criteria,
under potential contamination. If a drinking water
well has been closed due to contamination from the
plume being evaluated, classify the pre-closure
population associated with the well as subject to
actual contamination. Actual contamination cannot
be inferred for drinking water wells that are screened
within the dimensions of the plume. If none of the
drinking water wells are subject to actual
contamination, select a nearest well factor value
based on the shortest distance to any drinking water
well, as measured from the geometric center of the
ground water plume (55 FR 51587, December 14,
1990).
QUESTIONS AND ANSWERS
Q: Can a contaminated ground water plume formed by
the drainage of leachate from a landfill be evaluated
as the source at a site?
A: If the contaminants in the plume can be attributed to
the landfill, the plume should not be scored as the
source. In this case, the source at the site should be
classified as a landfill.
-------
Q: If we have data on one or two likely sources, but
information shows there are other likely sources as
well, can we list the site as a ground water plume?
A: Yes. The issue is attribution. If data are available
that allow definite attribution of contaminants to a
particular source, that source should be evaluated
independently. If attribution is not possible, the site
can be scored as a plume.
Q: If a ground water plume is conclusively determined
to be coming from one large facility that has a single
owner, but the exact source of the contamination is
not known, should the plume be considered the
source?
A: If the proper effort to identify a source is
unsuccessful at the site, then the scorer should
consider evaluating the facility as the source under
HRS source type "other." This may be done when
contamination is attributed to operations or
processes at the facility but not to a specific source.
Q: If two distinct ground water plumes with no
identified source commingle and one of the plumes
consists of petroleum products and the other contains
CERCLA eligible hazardous substances, can the
release be listed on the NPL?
A: The release of any CERCLA eligible hazardous
substance may be listed on the NPL Petroleum
products may not be used in scoring under the HRS.
Q: If a ground water plume with no apparent identified
source is documented, but soil-gas samples show
contamination at a nearby area with the same
substances, should the contaminated soil or the
plume be evaluated as the source?
A: Since soil-gas data do not establish actual soil
contamination or ground water contamination at a
site, the contaminated ground water plume can be
considered a source if observed release samples are
available and there is no other information to link
the plume with the soil-gas samples. The soil-gas
samples, however, should be further investigated to
determine if such a linkage exists.
Q. Do site boundaries correspond to property
boundaries at ground water plume sites?
A. No. Ground water plume sites are defined by the
locations of wells showing elevated concentrations
of CERCLA hazardous substances. All substances
considered to be present in the plume must meet
HRS observed release criteria.
Q. Does site name imply the extent of the site or identify
responsible parties?
A. Not for ground water plume sites. In most cases, the
name assigned to such sites will be that of a
geographic location. Depending on what is known
about the plume, the name selected should be
specific enough to indicate the general area of the
plume but broad enough so as not to appear to
suggest any particular source.
-------
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Float
Chicago, tl 60504-3590
------- |