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Calculating Substance - Specific Factors

Substance-specific factors are the chemical characteristics of the hazardous substances found at a site that are available to migrate from the site sources. These chemical characteristics include toxicity (human or environmental), mobility, persistence, and bioaccumulation potential. The product of these factors is referred to as the "combined factor value," which will allow the scorer to select the single most potentially hazardous substance at the site, which will in turn be used in scoring WC. The following formulas show how the product of substance-specific factors are calculated in each pathway:

Ground Water

  • Toxicity  x  Ground Water Mobility

Surface Water1 Toxicity x Persistence

  • Drinking Water:    Toxicity  x  Persistence
  • Human Food Chain:    Toxicity  x  Persistence  x   Bioaccumulation (FC)
  • Environmental:    EcoToxicity  x  Persistence  x  Bioaccumulation (EN)
  • Ground Water to Surface Water:    Multiply Ground Water Mobility to the above

Soil Exposure

  • Toxicity


  • Toxicity  x  Air Mobility

1 If surface water is threatened, check the possibility of polyaromatic hydrocarbons, heavy metals, and pesticides. They tend to have high bioaccumulation values.

The HRS Rule, Section 2.4.1, page 51589, contains a rather complex method for determining the toxicity factor value for a substance. Pre-calculated values for toxicity, as well as mobility, persistence, and bioaccumulation factors, can be obtained from the Superfund Chemical Data Matrix (SCDM). SCDM has calculated factors for roughly 300 of the most commonly evaluated substances, significantly reducing the time it takes to evaluate hazardous substance characteristics. Even so, additional, non-substance-specific information may be needed to identify the appropriate value from SCDM. For example, ground water mobility values are presented for "karst" and "non-karst" aquifers for substances in a "liquid" or "non-liquid" state. As each pathway is reviewed in this training material, obtaining the correct factor values from SCDM will be discussed. If a chemical value is not in SCDM, contact EPA. Do not try to work out the value yourself from the detailed instructions in the HRS unless EPA first approves.

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