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HRS Guidance Manual, Section 5.2, Page 67

A background level for a site provides a reference point by which to evaluate whether or not a release of a hazardous substance from the site has occurred. Determining background level is necessary to establish an observed release (or observed contamination) by chemical analysis. This section provides guidance on selecting appropriate samples for determining background level for a site. The application of background levels in establishing an observed release (or observed contamination) by chemical analysis is discussed in Section 5.1 of this document.

When chemical analysis is used to determine background levels, the background and release samples must be from the same medium (e.g., soil, water, tissue) and should be as similar as possible except for potential influence from the site. Similar sampling methods should be used to obtain background and release samples. Ideally, background samples should be outside the influence of contamination from the site, but background levels may be determined from samples that contain measurable levels of contamination. Background levels also do not need to represent pre-release conditions at the site.

  Section 2.3   Likelihood of release
  Section 3.1.1   Observed release (ground water)
  Section   Observed release (surface water)
  Section 5.0.1   General considerations (soil exposure)
  Section 6.1.1   Observed release (air)


Background Level: 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. A background level may or may not be less than the DL, but if it is greater than the DL, it should account for variability in local concentrations. A background level need not be established by chemical analysis.

Background Sample: A sample used in establishing a background level.

Release Sample: A sample taken to determine whether the concentration of a hazardous substance is significantly above Its background level in order to determine whether an observed release (or observed contamination) has occurred.

Similar Samples: Samples from the same environmental medium that are identical or similar in every way (e.g., field collection procedure, analytical technique) except the degree to which they are affected by a site.


The minimum data requirements for establishing background levels by chemical analysis include the actual analytical data from the background sample(s) and sufficient other information to establish similarity between background and release samples. Analytical data may be obtained from one or more background sample(s).

Number of Samples for Establishing Background Levels by Chemical Analysis

— Where background is established by chemical analysis, a single sample may provide a defensible background level. However, when the hazardous substances being considered are widespread in the environment (e.g., pesticides in an agricultural area, naturally occurring trace metals) and/or may have come from other nearby sites, one sample generally will not be sufficient. At such sites, attribution also may be difficult (see Section 5.1). Factors influencing the number of samples used to establish background levels by chemical analysis include:

  • Physical complexity of the site (e.g., size, number of source types);

  • Physical complexity of migration routes (e.g., number of watersheds, number of overland segments in each hazardous substance migration path);

  • Temporal complexity of site data (e.g., time periods over which sampling and other data were collected);

  • Meteorological conditions under which samples were collected;

  • Number of hazardous substances present at the site, their expected concentrations in sources and releases, and the degree to which they are widespread in the vicinity of the site;

  • Number and physical/chemical complexity of environmental media being sampled (e.g., number and interconnection of aquifers, heterogeneity of soils and sediments, number and type of water bodies within watershed);

  • Type of samples (e.g., filtered or unfiltered); and

  • Other potential sources in the vicinity of the site.

At some sites, multiple background samples appropriate for a particular environmental medium will exhibit different concentrations for the same hazardous substance. In this situation, using the sample with the highest concentration is always defensible in a legal sense (i.e., the background level based on available samples could not be higher than the value selected), but it may not always be appropriate. Generally, it is best to decide on a case-by-case basis whether to use the highest, lowest, or a measure of central tendency of the samples to establish background.

Establishing Similarity Between Background and Release Samples

Analytical data from background samples is necessary but may not be sufficient to establish background levels by chemical analysis. Additional information related to the site and sampling procedures is often desirable to establish similarity between the background and release samples. Examples of things to consider in establishing similarity may include:

  • Type of samples (e.g., soil, sediment, air);

  • Time and location at which samples were collected;

  • Physical conditions under which samples were collected (e.g., meteorological conditions, season);

  • Sampling, handling, and analytical chemistry procedures used; and

  • Environmental setting for each sample (e.g., topography, land use in the vicinity of the sampling locations, streamflow).


Temporal and spatial variations in measured concentrations often make it difficult to define background. Large differences in analytical results may result from differences that are independent of site-related contamination (e.g., differences in the manner in which samples were collected, differences in the physical or chemical conditions under which the samples were collected). This section provides guidance for selecting background samples that will yield the most defensible background levels. General considerations are followed by pathway-specific considerations.

General Considerations

  • In most cases, samples will be designated as background at the time of an SI. In some cases, however, it may be necessary to re-evaluate which samples are background and release after the data have been collected (e.g., when analytical data or additional site information suggest a different pattern of contamination than originally expected).

  • Sampling and analysis methods should be the same for background and release samples.

  • Background samples do not have to be completely outside the influence of the site. This may be particularly applicable in areas where the presence of other potential sources and/or the complexity of the nearby environment make it difficult to select a background sampling location that is not influenced by the site.

Ground Water Pathway

Data evaluation guidelines for the ground water pathway are presented below. General guidelines are

presented first, followed by guidelines specific to the following situations: the background well and release well are in the same aquifer; there is no background well in the aquifer in which the release well is located; and the release well serves as its own background well.

General Guidelines

  • An understanding and description of aquifers and their boundaries are necessary for identifying background samples. Information must be sufficient to identify the types and boundaries of geologic materials within the TDL for the site. Minimum information includes types of bedrock, soil, or other non-consolidated material, and their lateral and vertical boundaries; types of surficial deposits and their boundaries (i.e., thicknesses and lateral extents); and locations and screened depths of release and background wells. Guidance on determining aquifers and aquifer boundaries is presented in Section 7.1.

  • When a connection has been established between two individual aquifers, the background sample must be taken from the same aquifer as the release sample (e.g., a background sample taken from a bedrock aquifer cannot be compared to a release sample taken from an overlying alluvial aquifer, even if a hydrologic connection has been documented between the two aquifers and they are being considered a single hydrologic unit for purposes of HRS scoring). Different aquifers may have very different background levels as well as other important differences in water chemistry.

  • Information on ground water flow gradients in the area is not required and may not be known completely at the time of the SI. Depending on site conditions, background wells may be upgradient, side-gradient, or downgradient from sources. In complex situations, with multiple sources and aquifers, selecting or installing wells for background samples will require considerable knowledge of aquifers, aquifer boundaries, and aquifer interconnections.

Background Well and Release Well in Same Aquifer

At some sites, one or more potential background wells already exist in the aquifer(s) of concern (i.e., these wells did not need to be installed during the SI). Such a situation generally will make it easier to obtain background samples. However, existing wells may not be suitable for background samples, even if they are not influenced by sources at the site. Highlights 5-5, 5-6 , and 5-7 provide illustrations of appropriate background wells for the ground water pathway. Note that these illustrations are highly idealized and are not meant to reflect expected site-specific conditions.

  • In general, background and release samples should be from approximately the same depths in an aquifer, although different depths may be appropriate under certain circumstances. Factors to consider include aquifer structure, the nature of the hazardous substances, and other possible sources, including natural sources. Ground water tends not to be well mixed, and water quality can vary significantly in the vertical plane within an aquifer. This is particularly true when substances that have a tendency to sink or float in the aquifer are present (i.e., dense non-aqueous phase liquids (DNAPLs) and light non-aqueous phase liquids (LNAPLs). Depth should be determined relative to a fixed reference point (e.g., mean sea level) rather than the ground surface to eliminate apparent differences caused by surface topography.

  • If the background sample well is screened, the well screen interval must be in the same aquifer as the release sample well.

  • A well screened over two or more distinct aquifers cannot be used to establish background or release levels of hazardous substances.

  • Take particular care in areas that are hydrogeologically complex. In glaciated terrain, for example, water may occur within sand lenses of limited areal extent, and surrounding soil with a substantial clay component could serve to isolate these lenses. Thus, each sand lens may be, in effect, a small, independent aquifer, making it difficult to establish background. In such areas, geologic cross sections may be necessary to understand the underlying aquifer system.

No Background Well in Release Well Aquifer

At some sites, no potential background wells exist in the aquifer(s) of concern prior to the SI. At these sites, background levels may be determined only in two ways: by installing monitoring wells at appropriate background locations, or based on appropriate published concentration data. Data from monitoring wells generally are preferred over data from the literature. Guidance for using published concentration data to establish background levels is presented in Section 5.1.

Release Well Serves as its Own Background

Under some circumstances a single well, over time, may provide both background and release samples. For example, where a regular water quality monitoring program is in effect (e.g., at municipal wells), a time series of monitoring data may document encroachment of a hazardous substance plume. Data must be available from a sufficient period of record, so that a trend in increasing concentrations can be demonstrated clearly.

Surface Water Pathway

Data evaluation guidelines for the surface water pathway are presented below. General guidelines are presented first, followed by guidelines specific to particular types of surface water bodies (i.e., streams and rivers; lakes, Great Lakes, and other large water bodies; and tidally influenced areas) and particular types of samples (i.e., aqueous, sediment, and tissue). Highlight 5-8 provides an idealized illustration of appropriate background sampling locations for the surface water pathway.

General Guidelines

  • An observed release by chemical analysis can be established in the surface water pathway using aqueous samples, sediment samples, and/or tissue samples from essentially sessile benthic organisms. Background and release samples must be of the same type (e.g., aqueous samples must be compared to aqueous samples, sediment to sediment).

  • Chemical and physical properties of surface water and sediments may vary substantially within a small area. Stratification of lakes, lack of mixing in slowly moving rivers, and mixing effects induced by tributaries may affect the appropriateness of a given sampling location for establishing background levels. Environmental conditions at both the background and release sample locations should be similar.

Non-tidal Streams and Rivers

  • Background samples should be collected upstream from the potentially contaminated area. In the simplest case (i.e., one PPE and one main channel), one background sample may be sufficient. In cases where there is significant branching or tributary input upstream of the PPE, more than one background sample may be appropriate.

  • If there are multiple PPEs, background samples may be appropriate for each PPE, particularly if the hazardous substances for each PPE are different and significant branching or tributary input occurs between PPEs.

  • Where possible, background and release samples should be collected from the same general part of the surface water body (e.g., a background sample taken near one bank generally should not be compared with a release sample taken from the center of the main channel).

Ponds and Other Small, Isolated Water Bodies

In ponds and other small, isolated water bodies, it may not be possible to collect background and release samples from the same water body (e.g., the entire pond may be influenced by the site). In that case, background can be established as follows.

  • Samples of water flowing into the pond may provide background levels if there is a clear inflow and this is not influenced by the site.

  • Samples from an analogous water body outside of the area influenced by the site (e.g., a nearby pond of similar size and type) may provide background levels.

  • Background levels may be established based on literature values without having to take samples (see Section 5.1).

Lakes, Great Lakes, and Other Large Water Bodies

  • In smaller lakes, samples at the point where surface water enters the lake generally will provide appropriate background levels. If there is no obvious point of entry, it generally is best to use samples as far as possible from the PPE(s) to establish background levels. However, the presence of springs, other potential sources, and points of flow out of the lake may influence selection of background locations.

  • If other potential sources are near the site, background samples should be collected between the PPE for the site and the PPE for other potential sources. Ideally, background samples should also be out of the zone of influence of the other potential sources.

  • In large water bodies, background samples should be collected as far from the PPE as possible, except when other potential sources, points of flow into the lake, or points of flow out of the lake are present in between.

Tidal Areas

  • In tidal water bodies, background samples ideally should be collected beyond the farthest upstream point at which substances from the site might be transported by the tide. If it is difficult to determine exactly how far upstream substances might be transported, it may be appropriate to collect background samples above the "head of the tide" (i.e., the most upstream point at which tidal cycles are present), as long as it isn't too far upstream to be unrepresentative of background. In some cases, a series of samples successively farther upstream may be required.

  • In tidally influenced areas, it is especially important to be aware of attribution problems that might be presented by non-site related sources of contamination either upstream or downstream from the PPE. In general, attribution will be more difficult as distance from the PPE increases.

  • For aqueous samples, sample collection times in relation to tidal cycles should be considered. Hazardous substance transport upstream will be greatest during a rising tide and lowest during a falling tide. Background aqueous samples are most likely to have the least site-related contamination toward the end of the falling tide, when downstream flow is expected to exert maximum flushing effect. Site-related concentrations in the background sample are likely to be higher toward the end of the rising tide, when contaminated water is carried upstream to the maximum extent.

Sediment Samples

  • Sediment type should be similar in background and release samples. Fine clay particles are more likely to adsorb hazardous substances such as metals and hydrophobic organic compounds than are larger particles or particles with a predominately sandy matrix. Different sediment types tend to accumulate in different areas of a stream or lake. Fine sediments will predominate in quiescent zones, whereas sandy sediments, with fewer fine particles, will be found in more turbulent areas. Visual documentation of sediment type similarity is generally sufficient.

Tissue Samples

  • The only tissue samples that may be used to establish an observed release are those from essentially sessile, benthic organisms. Such organisms do not need to be human food chain species. This ensures that any contamination found in the tissue can be attributed to the immediate area in which the organism was collected. Benthic organisms are generally those which spend most of their lives on the bottom of a water body, and sessile organisms are those which are relatively immobile. Examples of essentially sessile, benthic organisms include sponges, oysters, and mussels.

  • Concentrations of hazardous substances in tissue samples may vary among different species, different individuals within a species, and different organs and tissues within an individual organism. At a minimum, background and release tissue samples must be of the same species. Ideally, background and release samples should be from organisms of similar age, if age can be determined. If variability among individuals is high, multiple background and release samples may be appropriate.

Soil Exposure Pathway

  • Soil is a heterogeneous material that may vary substantially in texture and other physical and chemical properties. Background and release samples should be collected in areas with similar soil characteristics.

  • Site setting and operational history should be considered in selecting background samples. Information about site operations may indicate which areas were subject to a particular type of contamination and which areas may serve as background for the contaminated areas. Land features might prevent the migration of liquids to certain portions of the site. Other contaminated sites nearby may affect the appropriateness of a particular location for background samples.

  • Some sites may be located in or near areas that have been filled, and the fill soils may have come from different locations. If possible, background samples should be from undisturbed areas (e.g., those with mature vegetation).

  • Soil within a dry drainage ditch or swale is subject to many outside influences and generally should not be used for determining background levels. An exception might be if the contaminated soil source is in the same swale or drainage ditch.

Air Pathway

Wind direction is of paramount importance in determining background levels for air samples. A background air sample will ideally be collected upwind from the area of contamination. However, cross-wind samples may also be acceptable for background conditions and should be used if potential sources of similar contamination are located cross-wind. Consideration must be given to the entire time period over which a sample was collected. Data on the predominant wind direction in an area are insufficient to determine background; wind direction must be established during the sampling period.

  • During any sampling event it is likely that changes in wind speed and direction will occur. A wind rose, based upon continuous data collected during the entire period of site sampling, may be helpful for selecting background.

  • Background and release samples should be from approximately the same heights above the ground surface. Samples do not need to be collected from the "breathing zone." Samples from very low heights should be evaluated carefully because field activities, particularly soil disturbance, may introduce contamination.

  • Background and release samples generally should be collected simultaneously.

  • Indoor air samples cannot be used to establish background levels (or to establish an observed release).


  • Large differences in the physical or chemical characteristics of background and release samples may indicate artifacts introduced during the sampling process. For example, a high concentration of suspended solids in a ground water sample may indicate insufficient purging of the well prior to sampling and/or substantial disturbance to the well during sampling.

  • Ground water wells from which background and release samples are obtained must be completed in the same aquifer and should generally be at approximately the same relative depth in the aquifer.

  • Background and release samples should be collected within the same time frame, as appropriate for the pathway.

  • Background and release sediment (or soil) samples should be of similar type.

  • Tidal effects should be considered when establishing background sampling locations in surface water.

  • Knowledge of site operations can often provide clues to appropriate locations for background soil samples.


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