This fact sheet addresses the use of analytical data to establish areas of observed contamination at hazardous waste sites
when evaluating the soil exposure pathway under the Hazard Ranking System (HRS) (40 CFR Part 300). The data may
also be used to evaluate hazardous waste quantity for some HRS source types. The soil exposure pathway is evaluated only
if observed contamination is established based on analytical data. Factors that are critical to determine observed
contamination include the concentration levels of contaminants, and the area of contamination. An integrated sampling
approach under the Superfund Accelerated Cleanup Model (SACM) should be considered when planning the sampling
strategy to establish observed contamination.
-&EPA
United States	Office of	Directive 9285.7-18FS
Environmental Protection Solid Waste and
Agency Emergency Response EPA/540/F-94/029
	September 1995
Establishing Areas of Observed
Contamination
Office of Emergency and Remedial Response	Quick Reference Fact Sheet
Abstract
Introduction
The Hazard Ranking System (HRS) Final Rule (40 CFR
Part 300, App. A) establishes general criteria to
document an observed release of hazardous substances to
media (e.g., ground water, surface water, air) and to
document observed contamination in the soil exposure
pathway. An observed release is based on evidence that
contaminants have migrated away from a site through a
medium. In contrast, observed contamination is based
on evidence that targets (e.g., human populations,
resources, and sensitive environments) have come into
direct contact with the contaminants.
Unlike other pathways, the soil exposure pathway can be
evaluated based only on current site conditions, with only
limited exceptions. (See The Revised Hazard Ranking
System: Evaluating Sites After Waste Removals, October
1991, OSWER Directive 9345.1-03FS, for information on
exceptions.)
The HRS criteria for documenting an observed release
and observed contamination are: evidence of a
hazardous substance in the medium of concern at a
concentration significantly above the background level
and at or above the appropriate detection limit; and at
least partial attribution of the hazardous substance to a
release from the site (see Figure 1). (For more
information on observed releases, refer to the fact sheet
Establishing an Observed Release, September 1995,
OSWER Directive 9285.7-20FS.)
Resource Considerations
An integrated sampling approach under the Superfund
Accelerated Cleanup Model (SACM) should be
considered when planning the sampling strategy to
establish observed contamination. The data quality
objective (DQO) process provides a logical framework
for planning multiple field investigations, thereby fulfilling
the integrated site assessment goal of cross-program
response planning and allowing optimal cross-program
data usability. (See Data Quality Objectives Process for
Superfund, September 1993, OERR Directive 9355.9-01
for further details on the DQO process.)
The data gathered from the Site Inspection (SI) may be
useful later in the overall site strategy, especially where
it appears that a response action may be required. In
such cases, site managers may consider a broader
sampling strategy. For instance, such efforts might

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Figure 1: Flowchart for Establishing Observed Contamination
START
Is the concentration of a
hazardous substance in
the source sample
significantly above
YES
ilgnlficantly
oackgrou
NO
NO
YES
NO
YES
* Direct observation does not apply to the soil
exposure pathway
Figure adapted from OSWER Directive 9345.1-07 (p. 56)
Is the hazardous
substance
attributable to ,
\ the site?
Is a portion of
the significant
increase
attributable
\to the site?/
Observed
contamination
established
No observed
contamination
at the site
include collection of the necessary site information for
development and use of Soil Screening Levels (SSLs) for
use during the Remedial Investigation/Feasibility Study
(RI/FS). It is appropriate to use data gathered during
the SI for the RI, especially to develop the Conceptual
Site Model.
SSLs are not appropriate for use at the SI stage because
the objectives of the SI and SSLs are different. The
objective of the SI is to obtain information on "worst
case" or "hot spot" contamination. It is not intended to
be a detailed analysis of the extent of contamination, nor
a risk assessment. Based on the results of the SI, EPA
decides whether the site qualifies for possible inclusion
on the National Priorities List or elimination from
further Superfund consideration. SSLs are used in the
RI to screen out potential contaminants and exposure
areas for remedial action under the Comprehensive
Environmental Response, Compensation, and Liability
Act (CERCLA) (Draft Soil Screening Guidance,
December 1994, OSWER Directive 9355.4-14FS).
Establishing Observed Contamination
The soil exposure pathway can be evaluated only if there
are documented areas of observed contamination. The
source samples are compared to a background level.
Most samples consist of soil, but leachate, waste,
sediment, and other surficial samples also may be used
{Guidance for Performing Site Inspections Under
CERCLA, September 1992, OSWER Directive 9345.1-
05).
The following criteria must be met in order to document
observed contamination by chemical analysis:
•	The release of the hazardous substance must be at
least partially attributable to a source at the site.
•	The source sample concentration must be greater
than or equal to the appropriate and properly
determined detection limit.
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•	If the hazardous substance of concern is not
detected in the background samples (or its
concentration is less than the detection limit), the
source sample concentration must be greater than or
equal to its detection limit.
•	If background levels are greater than or equal to the
detection limit, the source sample concentration
must be at least three times the background
concentration.
•	The hazardous substance is present at the surface or
is covered by no more than two feet of penetrable
material (except for gamma radiation emitters,
which have no depth restriction) {Hazard Ranking
System Guidance Manual, November 1992, OSWER
Directive 9345.1-07).
Sampling to Meet the HRS Sampling Objective
Unlike the other HRS pathways, contamination in the
soil exposure pathway must be based on actual exposure.
The potential for contamination is not evaluated in the
soil exposure pathway. Thus, strategic sampling is
critical. Consider both the types and locations of targets
when selecting sampling locations. Establish an area of
observed contamination as close to targets as possible.
Evaluate targets under the HRS "resident population
threat" when an area of observed contamination lies
within 200 feet of a residence, school, day care center, or
workplace, and also lies on the property. Evaluate
sensitive environments and resources under the HRS
"resident population threat" only if the area of observed
contamination lies within the boundaries of a terrestrial
sensitive environment or resource. Evaluate targets
beyond 200 feet but within one mile travel distance of the
area of observed contamination under the HRS "nearby
population threat" (40 CFR Part 300, App. A). Collect
samples no deeper than two feet below the surface.
Document the absence of a maintained, essentially
impenetrable cover material (e.g., asphalt, concrete) over
any portion of an area of observed contamination
(OSWER Directive 9345.1-07). Since surficial
contamination is not limited to soil, sampling of other
surface media, such as leachate or waste, should be
considered.
Contamination may be attributed to a site by collecting
appropriate background samples outside the influence of
sources. Obtain source samples from locations where the
substances are suspected to have been deposited (e.g.,
contaminated soil along the flood plain of a contaminated
surface water body) (OSWER Directive 9345.1-07).
Exhibit 1 suggests appropriate locations for background
samples by source type.
Estimating Waste Quantity by Defining Areas of
Observed Contamination
The following criteria are important to consider when
evaluating the soil exposure pathway under the HRS:
•	The soil exposure pathway can be evaluated only if
there are areas of observed contamination.
•	Target values are assigned based on the distance of
targets from the area of observed contamination.
•	Waste quantity can be calculated based on the area
of observed contamination.
A site may have more than one area of observed
contamination. Each area of observed contamination
may have its own targets.
Areas of observed contamination can be established with
sampling locations and analytical data that meet the HRS
criteria for observed contamination, including
determination of background levels (OSWER Directive
9345.1-07). A minimum of three samples showing site
contamination is sufficient to establish an area of
observed contamination for soil. The area of observed
contamination includes the three sampling points and the
area within them (OSWER Directives 9345.1-05 and
9345.1-07). However, the following sub-areas are
excluded:
•	Areas covered by permanent or otherwise
maintained and essentially impenetrable material
(e.g., asphalt, concrete);
•	Areas of higher ground not influenced by runoff
from the site, if contamination results from runoff;
•	Areas where the types of operations at a facility
preclude the presence of hazardous substances (e.g.,
contamination at loading docks but not elsewhere on
site);
•	Contaminated areas covered by more than two feet
of fill or other material (Refer to specific examples
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Exhibit 1: Possible Locations of Background Samples for Areas of Observed Contamination
Source
Background Sample
Contaminated soil
Soil in vicinity of the site1
Tanks/Drums filled with contaminated soil
Same as for the soil at the site
Tanks/Drums containing liquid or solid wastes
Background is zero
Landfill8
Soil in vicinity of the site
Piles"
Soil in vicinity of the site
Surface impoundment (liquid)8
Aqueous samples from vicinity of the site;
background may be zero
Surface impoundment (sludges or backfilled)8
Soil in the vicinity of the site
Other sources
Review on a site-specific basis
1See sections 5.1 and 5.2 of OSWER Directive 9345.1-07 for additional considerations.
"For these source types, the indicated sample is likely to be the most appropriate background.
Note: Do not evaluate intact containers.
Figure adapted from Highlight 9-1 of OSWER Directive 9345.1-07 (p. 344)
in Highlights 9-3 through 9-6 in OSWER Directive
9345.1-07).
Points and linear strips of observed contamination may
be evaluated as areas of observed contamination for the
soil exposure pathway, even though it may not be
possible to delineate an actual "area." For soils, one
contaminated sample denotes a point of observed
contamination. Two contaminated soil samples denote a
linear strip of observed contamination. Either a point or
a linear strip can be used to identify other targets and to
demonstrate a hazardous waste quantity value greater
than zero. When possible, however, establishing an area
of observed contamination is preferred.
For non-soil sources, such as waste piles, observed
contamination at a single point generally is sufficient to
establish the entire source as an area of observed
contamination.
Inferring an Area of Observed Contamination
For contaminated soil, an area of observed contamination
may be inferred within sampling locations that meet the
observed contamination criteria and have been properly
documented. Select sampling locations that will allow
efficient use of inferred areas of observed contamination.
It is likely that with this strategy, more targets may be
identified with fewer samples. Consider the following
when inferring an area of observed soil contamination:
•	Density of sampling points
•	Physiography
•	Topography and drainage patterns
•	Operational history
•	Transport and deposition of hazardous substances,
such as wind dispersion
•	Contamination in the downgradient portion of a
well-defined migration route
•	Data derived from other investigations (e.g.,
geophysical surveys)
•	Soil staining
•	Stressed vegetation patterns
•	Aerial and ground photography
•	Infrared satellite imagery indicating soil anomalies
•	Use of composite samples—samples within one grid
cell may be combined; vertical samples from a single
point within a zero to two-foot depth may be
combined. In general, avoid using non-grid
horizontal composite samples to infer areas of
observed contamination (OSWER Directives 9345.1-
05 and 9345.1-07).
4

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Additionally, consider the modes of contaminant
transportation and deposition when inferring an area of
observed contamination. Contaminants dispersed by air
would be distributed differently than those transported by
water. Avoid inferring an area of observed
contamination between soils in the floodplain of a
contaminated surface water body and those soils
contaminated from other modes of transportation and
deposition.
Determining Levels of Actual Contamination
Finding positive evidence of observed contamination is a
prerequisite for evaluating actual contamination at
targets. Actual contamination at targets indicates a high
likelihood of exposure to hazardous substances. Note
that the presence of contamination at targets is not in
itself sufficient to establish observed contamination or
actual contamination. The level of actual contamination
is determined by comparing the release sample
concentration to substance-specific benchmark values,
where applicable (OSWER Directive 9345.1-07).
Samples taken to fmd observed contamination can be
strategically located to establish an area of contamination
and to include one or more targets (dual purpose
sampling). Analytical data with appropriate and
adequate quality assurance/quality control (QA/QC) are
needed since benchmarks are expressed in concentration
units. Analytical data should provide definitive
identification of the hazardous substances (OERR
Directive 9355.9-01).
Use of Grid Samples
Grid samples may consist of grab samples (from a single
point) or composite samples (from multiple points).
Either grab or composite grid samples may be used to
evaluate the area of observed contamination if all the
following conditions apply:
analyses by definitive methods; refer to OERR
Directive 9355.9-01.)
•	The verified analytical data meet the HRS definition
of observed contamination as defined in section 2.3
of the Hazard Ranking System, Final Rule (40 CFR
Part 300, App. A).
Contaminated grid cells are those with identified
hazardous substances that meet HRS criteria for depth,
attribution to the site, and significance above background
level. For SI sampling purposes, the grid size need not
be standardized. The grid size can be site-specific
depending upon contaminant locations. However, the
grid size should be standardized when statistical sampling
may be necessary, especially where it appears that a
response action may be required. The area within these
grid cells may be used to define an area of observed
contamination when the appropriate sampling criteria for
observed contamination are met.
Contamination can be inferred at grid cells that have not
been sampled if they lie between contaminated grid cells.
Areas lying within inferred contaminated cells are
themselves inferred to be contaminated. The area within
inferred contaminated grid cells may be included as part
of an area of observed contamination. (Refer to
Highlight 9-4 of OSWER Directive 9345.1-07)
The following guidelines should be used when
considering grid sampling data:
•	Uncontaminated grid cells, and unsampled grid cells
that do not he between contaminated or inferred
contaminated ones, should be excluded from the
area of observed contamination.
•	Any grid cells or sub-areas which are covered with
impervious materials, or meet other criteria for
exclusion, should be subtracted from the defined
area of observed contamination (OSWER Directive
9345.1-07).
•	The same methods to define both the excluded sub-
areas and areas of observed contamination should
be used. All samples should be of the same quality,
and analyzed by similar procedures. Sub-areas from
the inferred area of observed contamination should
be eliminated on a case-by-case basis.
•	Samples are obtained from a depth of two feet or
less from the source or soil surface, and the source
is not covered by impervious material.
•	The available analytical data verify analyte identity
and quantitation with adequate QA/QC. (This may
consist of confirming 10 percent of screening
5

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Determining an Area of Observed Contamination for
Sources Other Than Soil
Section 5.0.1 of the HRS states "...for all sources except
contaminated soil, if observed contamination from the
site is present at any sampling location within the source,
consider that entire source to be an area of observed
contamination." For example, a dry, buried, or backfilled
surface impoundment should be evaluated as an area of
observed contamination. An area of observed
contamination is determined as follows:
•	For dry surface impoundments, landfills, and land
treatment units—the surface area of the source is
used;
•	For piles—the land surface area under the pile is
used;
•	For tanks, drums, and other containers—the volume
of the container is used.
Determining the Hazardous Waste Quantity Factor
Value for the Soil Exposure Pathway
Assign a source hazardous waste quantity value for each
area of observed contamination. Sum the source
hazardous waste quantity values assigned to each area of
observed contamination to determine the hazardous
waste quantity factor value. Table 5-2 in the HRS Final
Rule provides equations for assigning hazardous waste
quantity values for all types of sources in the soil
exposure pathway (40 CFR Part 300, App. A).
Example Site
The following example from an Expanded Site Inspection
(ESI) illustrates some of the challenges encountered in
the field, and EPA's approach to resolve the issues.
For a number of years, reclamation of automotive
batteries occurred at a scrap metal yard, which was
surrounded by a residential area. A prior removal action
mitigated severe soil contamination and secured the site
from public access, but did not generate enough data to
allow HRS evaluation. As a result of the removal action,
EPA discovered extensive lead contamination within the
property boundaries of the scrap yard. However, EPA
had not sampled the residential area. Eleven residences
were situated on a tract adjacent to the site; six
residences abutted the scrap yard boundary. The
proximity of the residential area raised the possibility that
inhabitants could be exposed to lead from sources at the
scrap yard. A study of the area revealed that lead could
be deposited on the residential tract from surface runoff,
dispersion of particulates from wind, and vehicular
movement. EPA hypothesized that these modes of soil
transport created an area of observed contamination in
the residential tract.
EPA sampled the soil at each residence and at border
areas to demonstrate attribution of lead contamination
and contiguity of the contaminated area. Background
samples were collected at nearby areas that were outside
the influence of sources at the scrap yard. In an
industrial area, it is always possible that background
concentration is elevated from various sources. To
account for this possibility, seven spatially divergent
sample locations were selected within the background
area to ensure provision of at least one representative
background level. Soils in all sample locations were
classified so that release samples could be compared to
background samples of similar soil composition. All
samples were collected within six inches of the ground
surface. Analytical results from the area of suspected
lead contamination revealed lead concentrations ranging
from 740 to 12,600 mg/kg (see Figure 2). Lead
concentrations from the background area ranged from
448 to 1,410 mg/kg. Observed and actual contamination
were clearly established since three residences had lead
concentrations greater than or equal to three times the
highest background level, and the lead was attributable to
the scrap yard.
Data from local and regional health agencies indicated
that the highest background level, which is usually the
one selected for HRS evaluation, was elevated. Because
this was the case, the number of residences with actual
contamination might have been underestimated. EPA
decided to examine the background data more closely.
According to data from the health agencies, background
levels of lead in area soils ranged from 500 to 1,000
mg/kg. Statistical analysis of the background levels
showed that the highest value, 1,410 mg/kg, was not an
outlier, but did lie well above the upper quartile of the
data distribution. EPA suspected that the highest
background value was not a representative level, and
considered using a statistically derived concentration.
The use of the mean concentration was immediately
rejected because it was subject to inflation from the
6

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Figure 2: Lead Concentrations in Residential Soils Related to Various Background Levels
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