EPA uses the Hazard Ranking System (HRS) (40 CFR Part 300, App. A) to evaluate Superfund sites to be proposed on
the National Priorities List (NPL). Based on the HRS, an observed release or observed contamination is established when
contaminants that are significantly above background levels have migrated away from a site through environmental media.
This fact sheet describes how to determine background levels for each migration pathway, and emphasizes the necessity
of strategic, efficient sampling, which is particularly important in light of conducting integrated assessments under the
Superfund Accelerated Cleanup Model (SACM).
United States	Office of	Directive 9285.7-19FS
Environmental Protection	Solid Waste and
Agency	Emergency Response EPA/540/F-94/030
September 1995
Establishing Background Levels
Office of Emergency and Remedial Response	Quick Reference Fact Sheet
Abstract
Introduction
The determination of background levels under the
Hazard Ranking System (HRS), usually by chemical
analysis, is necessary to evaluate an observed release
{Hazard Ranking System, Final Rule, 40 CFR Part 300,
App. A). Background levels are key in establishing
attribution of contaminants where multiple sources or
contaminant contributors exist. Additionally, an
integrated sampling strategy should be considered when
determining background levels, as appropriate.
A background level is "the concentration of a hazardous
substance that provides a defensible reference point that
can be used to evaluate whether or not a release from
the site has occurred. The background level should
reflect the concentration of the hazardous substance in
the medium of concern for the environmental setting on
or near a site. Background level does not necessarily
represent pre-release conditions, nor conditions in the
absence of influence from source(s) at the site" {Hazard
Ranking System Guidance Manual, November 1992,
OSWER Directive 9345.1-07). Background levels do not
have to reflect pristine conditions.
Obtaining suitable background samples can be
challenging because.of varying media compositions and
potentially false assumptions regarding ambient
conditions. Consult the Guidance for Performing Site
Inspections Under CERCLA, September 1992, OSWER
Directive 9345.1-05, for information on establishing
background levels.
Resource Considerations
A sampling strategy for an integrated assessment under
the Superfund Accelerated Cleanup Model (SACM)
should be considered, and planned if appropriate. The
elements deemed necessary for an integrated assessment
depend on the particular needs of a specific site and
could involve similar, additional, or slightly different
activities compared to traditional removal or remedial
site assessments. (For more information, see Integrated
Removal and Remedial Site Assessment Investigations,
September 1993, OSWER Directive 9345.1-16FS.)
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

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sampling strategy. For instance, such efforts might
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).
The Agency for Toxic Substances and Disease Registry
(ATSDR) should be consulted when there is the
potential for human exposure to toxic substances. The
sampling approach for these sites should include data
collection for the human exposure information that
ATSDR uses to determine if a health advisory is needed.
ATSDR also uses this data in the preparation of a public
health assessment, which is required within 12 months of
proposal to the National Priorities List.
Usually a few background samples are necessary to
evaluate an observed release and attribution for HRS
purposes. The selection of strategic sampling locations
is critical to the success of the SI, which is a limited-
scope biased sampling event. When feasible, evaluate the
benefits of sampling at specific locations and assess the
validity of available data to meet SI or other integrated
assessment objectives. 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.)
Background Level Determination Without Sampling
Establishing a background level requires determining the
concentration level of a hazardous substance. Sampling
is not always necessary to make this determination.
Some man-made hazardous substances (e.g., pesticides,
chlorinated organic solvents, except drinking water
chlorination) can be attributed only to a contaminant
source. The presence of these substances in the release
is sufficient to show contamination; a background sample
is not needed (OSWER Directive 9345.1-05).
Sampling may not be necessary for certain sample
locations, such as wells, which may serve as their own
background (OSWER Directive 9345.1-05). A release
can be demonstrated when historical data from a
contaminated well or intake show that it was previously
uncontaminated or less contaminated. Detailed historical
data are useful to define encroachment of a contaminant
plume. Often, historical data are available for wells and
surface water intakes at industrial sites or municipal
water facilities which have a regular monitoring program
(OSWER Directive 9345.1-07). For example, a ground-
water well or surface water intake may have associated
historical analytical data.
In some cases, published background levels may exist
that can be applied to a specific site. The following
published data sources may be consulted:
•	Background sample results from other nearby
CERCLA site investigations
•	Local surveys by other Federal or State agencies
(e.g., U.S. Geological Survey (USGS), Soil
Conservation Service (SCS))
•	University studies
•	Tables or databases with natural concentration
ranges and averages in local or regional soils
(OSWER Directive 9345.1-05)
Published data may be useful when selecting background
sampling locations. If published data are used, multiple
sources of information help to support a comparison
determination. The use of background level data without
sampling (e.g., published data) may be acceptable for SI
or HRS scoring activities. The analytical package for the
published data should be obtained whenever possible
(OSWER Directive 9345.1-05).
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Published information may not be appropriate to use in
many cases. Published data may not account for regional
variations or unique site-specific characteristics (OSWER
Directive 9345.1-05). Background levels may vary with
regional and local geology (e.g., ore veins, soils with
naturally high metals content). It may be difficult to
demonstrate comparability using published data because
of the difficulty of duplicating sample method and
analysis. For comparison, published or existing data
should be generated under quality assurance/quality
control (QA/QC) measures equivalent to EPA
requirements.
Considerations for Selecting Background Samples
Under the HRS, the collection of background samples
usually is necessary for the following reasons:
•	A release cannot be determined by direct
observation;
•	The source consists of contaminated soil;
•	Historical data are unavailable or insufficient;
•	The substance of interest is ubiquitous.
Under the HRS, the highest background sample
concentration generally can be used as a background
level. In a non-industrial area, average background
concentrations may be used when sufficient background
samples are collected in a relatively homogeneous
environment, and there are no alternative sources of
contamination nearby. Qualified analytical data may also
be used for background level determination (refer to the
interim fact sheet Using Qualified Data to Document an
Observed Release, July 1994, OSWER Directive 9285.7-
14FS, for more information). At least one background
sample per pathway or medium of concern should be
collected. Preferably the samples should be taken
outside the area believed to be influenced by the site.
However, it is often necessary to collect more than one
background sample.
Determining the location and number of background
samples depends upon the following factors:
•	Hazardous substances present at the site and
expected concentrations the following factors:
•	Availability and quality of existing information and
analytical data;
•	Objectives of the investigation;
•	Site hypotheses to be tested;
•	Media variability;
•	Size of the site, number, and type of sources;
•	Pathway-specific considerations (e.g^ geologic
formations, types of surface water bodies); and
•	Other potential sources of contamination in the
vicinity of the site (OSWER Directive 9345.1-05)
The number of background samples collected may also
depend upon the type of investigation performed. At
times, a contaminated background sample can be
compared with a release to demonstrate that the site
under investigation contributes at least part of the
contamination in the release (OSWER Directive 9345.1-
05).
In all evaluations, release and background samples must
be similar for comparison. Factors which determine
sample similarity include location, type, depth, medium,
sampling method, preservation, handling timing, and
weather conditions during sampling.
In some situations, collection of a comparable
background sample is not possible (e.g., when there is no
surface water sample similar to an isolated pond, or
when a surface water body originates from a spring)
(OSWER Directive 9345.1-05). If background sampling
is not possible, substitute published data, as available.
When collecting samples at a site, the activities of the
investigation team should not introduce any
non-attributable contaminants to samples. Sampling
methodology can minimize these potential errors.
Variability introduced by sampling methods can be much
greater than that introduced by the analytical laboratory.
When feasible, consider variability factors for each HRS
pathway under investigation. The following are specific
considerations when selecting background samples for
each HRS pathway.
Ground-water Pathway
A direct observation of a release to ground water can be
documented if it is observed or known that a hazardous
substance has been deposited, or the source lies below
the water table of the aquifer of concern (e.g., injection
well, buried waste). Therefore, a direct observation of a
release to ground water does not require establishing a
background level, but the presence of a hazardous
substance in the release should be documented by
manifest data or chemical analysis (OSWER Directives
9345.1-05 and 9345.1-07).
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When establishing an observed release to ground water
by chemical analysis, background samples generally are
needed. Background samples should be collected from
nearby wells that are not expected to be influenced by
the source of contamination or by other sites. If there
are other sites or potential local sources of ground-water
contamination, additional background samples should be
collected where possible to differentiate their
contribution from that of the site under investigation
(OSWER Directive 9345.1-05).
Similarity of Aquifers
Aqueous release and background samples must be
collected from comparable zones (e.g., saturated zone) in
the same aquifer and, where possible, should be collected
during the same sampling event. Interconnected aquifers
should not be considered as one aquifer when comparing
samples for an observed release. When collecting
background samples, it is preferable not to use samples
from a well screened in two or more aquifers (OSWER
Directive 9345.1-05).
When feasible, evaluate aquifer characteristics before
selecting wells for sampling, especially in areas of
complex or variable geology. Be aware of the existence
of mines, faults, or other aquifer intrusions which may
affect sample representativeness. (Note: Section 7.1 of
OSWER Directive 9345.1-07 provides detailed guidance
on determining aquifers and aquifer boundaries.)
Note information on ground-water flow direction if it is
known or can be easily determined. This information
may also be useful in selecting monitoring well
installation locations for Expanded Site Inspection (ESI)
and Remedial Investigation (RI) work. Information on
flow direction should be obtained by using piezometers,
by comparing static water levels in existing wells in the
same aquifer, and by using data from published reports.
The well used for background sampling should be beyond
the influence of the site (OSWER Directive 9345.1-05).
Comparability of Well Samples
Samples from any two wells can be considered
comparable if both are collected from the same aquifer.
Filtered ground-water samples should be compared to
filtered background samples; unfiltered ground-water
samples should be compared to unfiltered background
samples. Ideally, well completion techniques and usage
of background wells should be similar to those of the
well under investigation. Sampled wells generally should
be screened at similar zones within the same aquifer,
depending on the site hydrogeologic setting, because
different depths may have different contaminant levels
and water chemistry. Depth should be measured as
elevation relative to a reference (e.g., mean sea level)
instead of below ground surface for data consistency.
Where possible, duplicate purge parameters and method,
sampling method, and sampling equipment for all well
samples. If possible, sample release and background
samples on the same day, but preferably not more than
three days apart (OSWER Directive 9345.1-05).
In cases where a background well is not available,
consider the possibility of sampling a spring before it
reaches the surface by inserting a pipe or well point near
the location where ground water discharges at the spring
(OSWER Directive 9345.1-05). Sampling data may be
supplemented with applicable published data. Springs
may be used for background sampling of surficial
aquifers only.
Surface Water Pathway
Direct observation of a release to surface water may be
documented if material containing a hazardous substance
is seen entering surface water; is known to have entered
surface water through direct deposition; or is present in
a source which is in contact with surface water through
flooding. Direct observation of a release to surface water
eliminates the need for background sampling, but the
presence of a hazardous substance in the release should
be documented analytically. A background sample is not
required when sampling an effluent discharge from the
site into surface water, because the effluent is considered
a direct observation.
In non-tidal surface water bodies, it is preferable to
sample downstream to upstream. Background sediment
samples should be from a location comparable to that of
the release (e.g., fine sediments from quiescent zones)
(OSWER Directive 9345.1-05). Generally, sediment
samples are preferred over aqueous samples for
evaluating the surface water pathway because sediments
are more likely to retain contaminants. In general,
aqueous samples might represent current release
conditions, whereas sediment samples might exhibit
historical release conditions.
When it is necessary to collect biological samples,
background samples from essentially sessile, benthic
organisms (e.g., sponges, oysters) can be compared to
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similar (same species) tissue samples. Organisms
selected for background tissue sampling should be the
same gender and approximate age, wherever possible, of
those selected for release tissue sampling (OSWER
Directive 9345.1-05).
Special Considerations for Tidal Water Bodies
Where appropriate, determine the need to collect
aqueous and sediment samples when the surface water
body is tidally influenced. One approach for background
sampling is to collect outside of the zone of tidal
influence (this can be gauged by the level of the highest
tide). It is possible that tidal flow could pick up
additional sources upstream. The effect of the tides on
contaminant concentration should be considered.
(Upstream concentrations would be highest during the
rising tide and lowest at falling tide.) Consider collecting
release and background samples at the same tidal level
(OSWER Directive 9345.1-07).
Comparability of Water Bodies
Consider collecting release and background samples from
the same type of water body. Flow characteristics can be
used to determine similar water bodies. For example, a
background sample from a small tributary should not be
compared to a sample from a river. Physical and
chemical properties of the surface water (e.g., lack of
mixing in large, slow-flow segments of rivers, physical
transport mechanisms, and biological influences) are
other ways of determining whether water bodies are
similar. Where possible, collect release and background
samples during the same time period, since thermal
stratification and salt/freshwater stratification vary with
the time of year. Consider the thermoclines of a pond or
lake or measure them in the field prior to sampling
(OSWER Directives 9345.1-05 and 9345.1-07).
Simple surface water pathway sampling generally consists
of taking a minimum of one Probable Point of Entry
(PPE) sample and one upstream background sample. If
the surface water pathway has multiple PPEs, multiple
background samples may be needed. The number of
background samples collected depends on the complexity
of the path of the surface water body. The presence of
multiple tributaries upstream with multiple potential
sources would require collecting multiple background
samples in each tributary to differentiate the potential
contribution of contamination from off-site sources
(OSWER Directives 9345.1-05 and 9345.1-07).
For ponds and lakes, background samples may be
collected near the inflow to the water body if it is not
influenced by the source. A pond near the site may be
selected for background sampling if it exhibits similar
physical characteristics to the pond on site. For large
ponds and lakes, background samples may be collected
from the water body itself, but as far away as possible
from the influence of the PPE and other potential
sources (OSWER Directive 9345.1-07).
Air Pathway
Background levels need not be established for the air
pathway when an observed release by direct observation
is documented. Direct observation of release to the air
pathway can be documented in two ways: a release
containing hazardous substances is seen entering the
atmosphere directly (e.g., observing dust blowing off a
pile known to contain hazardous substances), or an
adverse effect is demonstrated (e.g., a documented health
effect from a reaction of incompatible substances).
Generally, it is necessary to determine background levels
for the air pathway because weather conditions can
greatly affect them. Throughout the sampling period, it
may be necessary to determine the predominant wind
direction and speed, effects of low temperatures,
existence of flat, open terrain, and any atmospheric
instability or lack of air movement. Background
sampling should be collected upwind of site sources,
although cross-wind samples may be acceptable. It is
preferable to collect multiple samples for this pathway,
from the same height, and at the same time. Samples
from great heights such as rooftops generally are not
useful because they do not represent target conditions;
very low heights are subject to potential interference
from particulates introduced by field activities. Dust,
wipe, soil, and soil gas samples are not acceptable for
background sampling in the air pathway. Even though
these types of samples may be used, along with field air
monitoring equipment, to select release and background
sample locations, it is recommended that samples and
background be collected concurrently. A minimum 12-
hour monitoring period is recommended for sampling the
air pathway, particularly during hot and dry weather
conditions (OSWER Directives 9345.1-05 and 9345.1-07).
Wind roses may be used to determine predominant wind
direction, or to document changes in it. Wind direction
is important when selecting sample stations (OSWER
Directive 9345.1-07). The "rose" diagrams consist of bars
on a compass face indicating the frequency of each wind
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direction during the selected time period, as well as the
average high wind speed for the period. If wind roses
are utilized, try to determine the elevation for which the
wind rose was calculated; this elevation should be
representative of target exposure. Weather stations and
airports may provide information on local wind direction
at ground level and at various elevations.
Soil Exposure Pathway
Because it is not possible to directly observe
contamination in the soil exposure pathway, soil must be
sampled to determine background levels. As previously
mentioned, there will be sites that appear to require
response actions (e.g., early actions). In such cases, site
managers may consider a broader sampling strategy.
Such efforts might include collection of the necessary
information for development of soil screening levels
(SSLs).
Establishing background levels in the soil pathway can be
difficult, particularly if the hazardous substances
attributed to the site are naturally occurring. Where
possible, collect on-site background soil samples from
surficial soils not likely to be affected by the source. Off-
site background soil samples should be collected from
shallow soils which ideally should not be affected by
other sources and sites in the area. However, if there
are alternative sources of contamination in the area,
background levels should be measured to determine
contributions from them. When possible, sample release
and background samples on the same day or within three
days. (See Highlight 9-1 of OSWER 9345.1-07 for
information on background samples for non-soil sources
in the soil exposure pathway.)
Carefully document location, depth, and appearance of
all soil samples. If depths and thicknesses of soil strata
vary with location, ensure that release and background
samples are from a similar stratum and soil type.
Samples should have similar texture, color, and grain size
(OSWER Directive 9345.1-05). Grab samples (as
opposed to composite) are preferred for determining soil
contamination in the SI. Preferably obtain the
background sample from an undisturbed, unfilled area,
because fill may have contaminants which are not
representative of background conditions. If a site is
located on fill, it may be necessary to obtain the
background samples from a similarly filled area (where
the fill is not considered one of the areas of observed
contamination at the site) (OSWER Directive 9345.1-07).
It is a recommended strategy to select more than one
background sample and location for the soil exposure
pathway. Avoid collecting background soil samples from
a drainage channel which receives water from off site
(OSWER Directive 9345.1-07). Where possible, collect
background samples from a higher elevation than the
sources to avoid the effect of potential surface drainage.
Avoid background sample locations that are subject to
airborne contamination from the site or other sources
(OSWER Directives 9345.1-05 and 9345.1-07).
Determining Background Levels in Industrial, Mining,
and Radioactive Areas
Industrial areas can pose a special challenge to
determining background levels. Ambient conditions may
include elevated concentrations of common contaminants
from sources not associated with the site. Some common
contaminants in background samples in industrial and
urban areas include:
•	Metals in soils (e.g., lead)
•	Trichloroethene (TCE) and perchloroethene (PCE)
in urban aquifers
•	Organic substances in harbor sediments (OSWER
Directive 9345.1-05)
In industrial areas, the investigator often needs to
document that a sample is above background sample
variability. One approach is to determine where
potential alternative sources exist and where they could
possibly interfere with release or background samples.
Because industrial areas are affected by increased levels
of contaminants and greater local variability, additional
background samples may be required to establish off-site
conditions. Be sure to collect a sufficient number of
samples between the site and all other potential sources
of contamination in order to attribute the increase to the
site (OSWER Directive 9345.1-05). In general, it is
inappropriate to average background samples in an
industrial area where more than one type of industry
existed because doing so could lead to unacceptable
levels of local variability (see Establishing Areas of
Observed Contamination, September 1995, OSWER
Directive 9285.7-18FS, for more detailed information).
Mining areas, like industrial areas, pose a challenge to
determining background levels. Often the contaminants
associated with the mine are naturally occurring
elements. Surface water may originate from the mine,
presenting no upstream location for background
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sampling. Surface water may pass through the mined
watershed. Consequently, the nearest upstream location
away from the influence of the site may be in a different
geologic formation, with different water chemistry,
producing uncertainty about comparability.
When surface water in mining areas originates in the
source or when no similar upstream location exists, a
water body with similar physical characteristics (e.g., a
similar stream on the other side of a mined hill) should
be selected for background sampling. The similar water
body should not be directly affected by the site.
Concentrations in the vicinity of mining sites may be so
significantly elevated that published data may provide a
more reasonable background level.
Mines are often located in areas with aquifers that are
highly fractured or influenced by mine drainage tunnels.
In mining areas, it may be difficult to find undisturbed
areas in which to locate ground-water wells and
therefore, difficult to determine ground-water
background levels. In such cases, it is preferable to
determine an observed release by direct observation.
Mine tailings generally contain minerals and are
considered waste. Collecting background samples is not
necessary if tailings are analyzed and the mineral
concentrations are shown to be elevated well beyond
what might be expected under natural conditions.
To sample sites with radioactive wastes, follow sampling
strategies similar to those for other hazardous substances.
Criteria to establish an observed release through
chemical analysis for radioactive substances exist for the
following three groups:
•	Radionuclides that occur naturally, or ubiquitous
man-made radionuclides;
•	Non-ubiquitous man-made radionuclides; and
•	External gamma radiation (soil exposure pathway
only).
Some portion of the release must be attributable to the
site. For each group, compare release concentrations
against known background radionuclide concentrations or
against detection limits for a sample medium. Section
4.9.4 of OSWER Directive 9345.1-05 provides details on
establishing an observed release for each group.
Summary
Thorough documentation of the locations of the
background samples and potential alternative sources is
necessary to assess the adequacy of the background levels
and to evaluate release and attribution. The benefits of
sampling at specific locations should be evaluated and the
validity of existing analytical data should be assessed.
Meet SI objectives while conserving Superfund resources.
Direct observation of a release does not require
background sampling if detectable concentrations of
hazardous substances are documented to be present in
the source. Background samples may not be necessary
for certain man-made compounds. If demonstrating a
release or establishing actual contamination is critical to
evaluating a site, background or QA/QC samples should
not be limited unduly because of budgetary
considerations—collecting these samples may prevent
having to return to the site.
To establish background levels by chemical analysis, on-
site and off-site sources and their locations should be
thoroughly reviewed. Release and background samples
should be collected from similar locations and media.
Ground-water samples are similar when they come from
the same zone within an aquifer and undergo similar
sample preparation. Background samples for surface
water should be collected upstream of the PPE.
Additional site reconnaissance and review are often
needed to select sampling locations in industrial and
mining areas and at complex sites.
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