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Section 4: Source Characterization

Although the Hazard Ranking System (HRS) is designed to score releases and sites, sources and areas of observed contamination play a crucial role in determining HRS pathway scores. Sources and areas of observed contamination are central to the evaluation of the likelihood of release factor categories and the hazardous waste quantity factors. Source and area of observed contamination information is also frequently used in identifying the hazardous substances associated with a site and in determining where targets are located relative to the site. Thus, complete and accurate characterization and documentation of both sources and areas of observed contamination are essential in producing defensible site scores. Both "source" and "area of observed contamination" are specifically defined concepts in the HRS whose definitions do not exactly correspond to their common usage. Areas of observed contamination will be addressed in the soil exposure pathway sections.

4.1 What is a Source?

Examples of sources include:

  • Contaminated Soil
  • Landfills
  • Surface Impoundments
  • Drums
  • Piles
  • Tanks

The following emphasize certain aspects of the HRS definition of source. The following are NOT sources for purposes of HRS scoring.

  • Ground water plumes originating from known sources (such as a landfill);
  • Surface water plumes originating from known sources (such as discharge pipes or overland runoff discharge areas);
  • Areas of contaminated surface water sediments arising from discharges from known sources; and
  • Volumes of contaminated ambient air.

The following ARE considered sources for purposes of HRS scoring:

  • Ground water and surface water plumes of unknown origin;
  • Areas of contaminated surface water sediments arising from direct placement (other than discharge) of waste materials into surface water bodies when the origin is unknown;
  • Cylinders containing confined, gaseous hazardous substances; and
  • Soils contaminated as a result of overland runoff, volatilization of ground water contaminants, or atmospheric deposition (from non-vehicular sources).

Areas of observed soil contamination are sources. Sources need not contain waste materials. Materials that might not be considered a source if undisturbed, may become a source if excavated and moved (e.g., contaminated dredge disposal materials).

4.2 Source Characterization

A complete characterization of a source should include:

  • Source identification;
  • Delineation of source boundaries;
  • Estimation of source dimensions;
  • Identification of hazardous substances associated with the source;
  • Pathway-specific evaluations of source containment characteristics;
  • Determinations of the pathways to which the source might release; and
  • Waste characteristics (hazardous waste quantity and substance-specific characteristics).

The HRS provides a detailed procedure for characterizing sources (see HRS Rule Section 2.2).

4.3 Source Identification Types

Any area meeting the HRS definition of "source" is a source and can be used in determining an HRS score. The only issue is whether the source is associated with the site (or release) under evaluation or whether it is associated with a different site. If the site consists solely of one source (and possibly plumes migrating from the source) the association of the source with the site is immediate. Otherwise, the issue is less straight-forward.

If other sources at the site have been identified, three considerations apply in determining whether a source is associated with a site:

  • Commonality of origin with the site, e.g., the source came to contain hazardous substances as a result of the same activities that created the site or as a result of migration from other sources at the site (except as specifically excluded in the definition of source);
  • Commonality of location with the site, e.g., the source is located on the same property as other sources at the site and within a reasonable distance or the source physically overlaps with another source at the site (e.g., a contaminated soil source extending over a landfill); and
  • Commonality of risk with the site (e.g., the source together with the other sources at the site would pose a common risk to some individual or ecological receptor).

Unless the relationship is immediate, a rationale for associating a source with a site should be included in the HRS documentation record.

Once sources have been identified, they should be classified as one of the HRS source type categories.

Definitions for specific source types evaluated in the HRS can be found in the HRS Guidance Manual, Section 4.1.

Proper identification of Source Type is important because it has a significant impact on other HRS factors calculations (e.g., waste quantity).

Source types are not always easily defined. Professional judgement may be required. Documentation of the rationale underlying such judgement should be included in the HRS documentation record.

Sources are evaluated in the three migration pathways. In the Soil Exposure Pathway, areas of observed contamination are evaluated.

4.4 Source Boundaries and Dimensions

Accurate, documented determinations of source boundaries are very important in evaluating many HRS pathway factors due to their dependence on distances from sources to features of concern. Examples of factors whose evaluation depends on measuring distances from sources include:

  • Ground water target factors: In the absence of an observed release, only those targets located within 4 miles of sources at the site are included in the target factor category evaluation.
  • Ground water depth to aquifer and travel time factors: In the absence of an observed release, determine depth to aquifer and travel time based on locations within 2 miles of sources at the site.
  • Surface water pathway overland flow distance to surface water factor: Evaluated based on distance from nearest source over the overland flow segment; if distance is greater than 2 miles then overland flow potential to release is 0.
  • Air pathway target factors: In the absence of an observed release, the nearest individual, potential contamination populations, and sensitive environments factors are evaluated based on those targets present within 4 miles of sources at the site.
  • Air pathway target factors: Targets are subject to actual contamination if they are closer to sources at the site than is the observed release sampling location.
  • Air pathway resource target factor: Evaluated based on resources located within 1/2 mile of sources at the site.

Sources and source boundaries are usually identified using:

  • Visual observations during the site visit;
  • Historical records;
  • Sampling results;
  • Aerial photographs; and/or
  • Interviews with individuals having knowledge of the site.

Site maps showing accurate source boundaries should be included in the HRS documentation record. Site maps can also be used to estimate the area of sources using a planimeter.

Estimates of source volumes can frequently be derived from survey data, visual observation, and sampling data. If the survey data is available.

4.5 Association of Hazardous Substances with Sources

Associating hazardous substances with individual sources is important in several respects. The association of a hazardous substance with sources at a site can form the basis of attributing a significant increase in pathway contaminant concentrations to the site, fulfilling the requirements for an observed release. Any hazardous substances associated with sources at a site are, by inference, associated with the site and could potentially form the basis for evaluating the numerous substance-specific HRS factors (e.g. toxicity, mobility, persistence, and bioaccumulation potential). However, in general, only those hazardous substances that are associated with sources with non-zero containment values for the pathway in question can be used in the pathway scoring.

  • Hazardous substances, pollutants, and contaminants associated with a source can be identified using:
    • Sampling data;
    • Potentially Resposible Party records, labels, and manifests;
    • State or Federal records and permits;
    • Information on site operations; and/or
    • Historical records.
  • If a hazardous substance can be documented as being present at a site (for example, by labels, manifests, oral or written statements), but the specific source(s) containing that hazardous substance cannot be documented, the hazardous substance can be considered present in every source at the site (in the absence of definitive evidence to the contrary).
  • Because individual samples of contaminants may be successfully refuted during public comment, it is important that all hazardous substances associated with every source (with non-zero containment) at the site be identified at the time of proposal.

4.6 Containment

The containment factor reflects the presence of physical barriers to contaminant migration associated with a source. The containment factors are used in determining likelihood of release, as well as many of the factors included in the waste characteristics factor category.

Containment factor values are specific to the pathway being evaluated.

Hazardous substances associated solely with sources with containment factor values of zero have no material impact on a pathway score unless they are otherwise associated with an observed release to that pathway.

  • The characteristics necessary to warrant assigning a containment factor value of zero are rarely encountered.

There may be more than one type of containment factor associated with a pathway:

  • surface water pathway containment
  • surface water pathway flood containment
  • air pathway gas containment
  • air pathway particulate containment

Note that Source Identification Type must be established before Containment can be evaluated.

Specific containment features must be observed and reported during the site visit if the HRS is to be properly applied. Highlight 7-22 Data Needs For Evaluating Source Containment.

Observations required to document ground water and surface water containment are quite similar. For the air pathway, they are different (e.g., windbreaks).

A detailed log of what is observed about containment at each source should be kept. These field observations will become a reference in the HRS documentation record and will support the assignment of the containment factor value.

4.7 Questions and Answers

What information do you need to document about each source?

Location, source type, containment features, hazardous substances, source boundaries and rationale for inclusion.

When writing an HRS documentation package, what are your best sources of "source characterization" information?

Site records (especially business records, manifests, and permits), sampling data, drum labels, oral or written statements concerning site activities, and site investigation log books are all good sources for this information.

If you have questions about how to evaluate a source, where can you find information?

The HRS Rule Section 2.2 provides instructions and a worksheet (HRS Rule Table 2–2) to assist in characterizing sources. Subsection are provided on selected topics as follows:

  • identifying sources — Section 2.2.1

  • associating hazardous substances with sources — Section 2.2.2

  • identifying hazardous substances available to a pathway — Section 2.2.3

The source types applicable to HRS scoring are listed in HRS Rule Table 2–5, the various containment factor tables (e.g., Table 3–2), and Table 6–4. Guidance in interpreting these source types is provided in Section 4.1 of the HRS Guidance Manual (pp. 41–43).

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