Appendix A to Part 300 -- The Hazard Ranking System

List of Figures

List of Tables

1.0.      Introduction.

1.1      Definitions.

2.0      Evaluations Common to Multiple Pathways.

2.1      Overview.

2.1.1      Calculation of HRS site score.

2.1.2      Calculation of pathway score.

2.1.3      Common evaluations.

2.2      Characterize sources.

2.2.1      Identify sources.

2.2.2      Identify hazardous substances associated with a source.

2.2.3      Identify hazardous substances available to a pathway.

2.3      Likelihood of release.

2.4      Waste characteristics.

2.4.1      Selection of substance potentially posing greatest hazard.
    2.4.1.1      Toxicity factor.

    2.4.1.2      Hazardous substance selection.

2.4.2      Hazardous waste quantity.
    2.4.2.1      Source hazardous waste quantity.

    2.4.2.1.1      Hazardous constituent quantity.

    2.4.2.1.2      Hazardous wastestream quantity.

    2.4.2.1.3      Volume.

    2.4.2.1.4      Area.

    2.4.2.1.5      Calculation of source hazardous waste quantity value.

    2.4.2.2      Calculation of hazardous waste quantity factor value.

2.4.3      Waste characteristics factor category value.
    2.4.3.1      Factor category value.

    2.4.3.2      Factor category value, considering bioaccumulation potential.

2.5      Targets.

2.5.1      Determination of level of actual contamination at a sampling location.

2.5.2      Comparison to benchmarks.

3.0      Ground Water Migration Pathway.

3.0.1      General considerations.
    3.0.1.1      Ground water target distance limit.

    3.0.1.2      Aquifer boundaries.

    3.0.1.2.1      Aquifer interconnections.

    3.0.1.2.2      Aquifer discontinuities.

    3.0.1.3      Karst aquifer.

3.1      Likelihood of release.

3.1.1      Observed release.

3.1.2      Potential to release.
    3.1.2.1      Containment.

    3.1.2.2      Net precipitation.

    3.1.2.3      Depth to aquifer.

    3.1.2.4      Travel time.

    3.1.2.5      Calculation of potential to release factor value.

3.1.3      Calculation of likelihood of release factor category value.

3.2      Waste characteristics.

3.2.1      Toxicity/mobility.
    3.2.1.1      Toxicity.

    3.2.1.2      Mobility.

    3.2.1.3      Calculation of toxicity/mobility factor value.

3.2.2      Hazardous waste quantity.

3.2.3      Calculation of waste characteristics factor category value.

3.3      Targets.

3.3.1      Nearest well.

3.3.2      Population.
    3.3.2.1      Level of contamination.

    3.3.2.2      Level I concentrations.

    3.3.2.3      Level II concentrations.

    3.3.2.4      Potential contamination.

    3.3.2.5      Calculation of population factor value.

3.3.3      Resources.

3.3.4      Wellhead Protection Area.

3.3.5      Calculation of targets factor category value.

3.4      Ground water migration score for an aquifer.

3.5      Calculation of ground water migration pathway score.

4.0      Surface Water Migration Pathway.

4.0.1      Migration components.

4.0.2      Surface water categories.

4.1      Overland/flood migration component.

4.1.1      General considerations.
    4.1.1.1      Definition of hazardous substance migration path for overland/flood migration component.

    4.1.1.2      Target distance limit.

    4.1.1.3      Evaluation of overland/flood migration component.

4.1.2      Drinking water threat.
    4.1.2.1      Drinking water threat-likelihood of release.

    4.1.2.1.1      Observed release.

    4.1.2.1.2      Potential to release.

    4.1.2.1.2.1      Potential to release by overland flow.

    4.1.2.1.2.1.1      Containment.

    4.1.2.1.2.1.2      Runoff.

    4.1.2.1.2.1.3      Distance to surface water.

    4.1.2.1.2.1.4      Calculation of factor value for potential to release by overland flow.

    4.1.2.1.2.2      Potential to release by flood.

    4.1.2.1.2.2.1      Containment (flood).

    4.1.2.1.2.2.2      Flood frequency.

    4.1.2.1.2.2.3      Calculation of factor value for potential to release by flood.

    4.1.2.1.2.3      Calculation of potential to release factor value.

    4.1.2.1.3      Calculation of drinking water threat-likelihood of release factor category value.

    4.1.2.2      Drinking water threat-waste characteristics.

    4.1.2.2.1      Toxicity/persistence.

    4.1.2.2.1.1      Toxicity.

    4.1.2.2.1.2      Persistence.

    4.1.2.2.1.3      Calculation of toxicity/persistence factor value.

    4.1.2.2.2      Hazardous waste quantity.

    4.1.2.2.3      Calculation of drinking water threat-waste characteristics factor category value.

    4.1.2.3      Drinking water threat-targets.

    4.1.2.3.1      Nearest intake.

    4.1.2.3.2      Population.

    4.1.2.3.2.1      Level of contamination.

    4.1.2.3.2.2      Level I concentrations.

    4.1.2.3.2.3      Level II concentrations.

    4.1.2.3.2.4      Potential contamination.

    4.1.2.3.2.5      Calculation of population factor value.

    4.1.2.3.3      Resources.

    4.1.2.3.4      Calculation of drinking water threat-targets factor category value.

    4.1.2.4      Calculation of the drinking water threat score for a watershed.

4.1.3      Human food chain threat.
    4.1.3.1      Human food chain threat-likelihood of release.

    4.1.3.2      Human food chain threat-waste characteristics.

    4.1.3.2.1      Toxicity/persistence/bioaccumulation.

    4.1.3.2.1.1      Toxicity.

    4.1.3.2.1.2      Persistence.

    4.1.3.2.1.3      Bioaccumulation potential.

    4.1.3.2.1.4      Calculation of toxicity/persistence/bioaccumulation factor value.

    4.1.3.2.2      Hazardous waste quantity.

    4.1.3.2.3      Calculation of human food chain threat-waste characteristics factor category value.

    4.1.3.3      Human food chain threat-targets.

    4.1.3.3.1      Food chain individual.

    4.1.3.3.2      Population.

    4.1.3.3.2.1      Level I concentrations.

    4.1.3.3.2.2      Level II concentrations.

    4.1.3.3.2.3      Potential human food chain contamination.

    4.1.3.3.2.4      Calculation of population factor value.

    4.1.3.3.3      Calculation of human food chain threat-targets factor category value.

    4.1.3.4      Calculation of human food chain threat score for a watershed.

4.1.4      Environmental threat.
    4.1.4.1      Environmental threat-likelihood of release.

    4.1.4.2      Environmental threat-waste characteristics.

    4.1.4.2.1      Ecosystem toxicity/persistence/bioaccumulation.

    4.1.4.2.1.1      Ecosystem toxicity.

    4.1.4.2.1.2      Persistence.

    4.1.4.2.1.3      Ecosystem bioaccumulation potential.

    4.1.4.2.1.4      Calculation of ecosystem toxicity/persistence/bioaccumulation factor value.

    4.1.4.2.2      Hazardous waste quantity.

    4.1.4.2.3      Calculation of environmental threat-waste characteristics factor category value.

    4.1.4.3      Environmental threat-targets.

    4.1.4.3.1      Sensitive environments.

    4.1.4.3.1.1      Level I concentrations.

    4.1.4.3.1.2      Level II concentrations.

    4.1.4.3.1.3      Potential contamination.

    4.1.4.3.1.4      Calculation of environmental threat-targets factor category value.

    4.1.4.4      Calculation of environmental threat score for a watershed.

4.1.5      Calculation of overland/flood migration component score for a watershed.

4.1.6      Calculation of overland/flood migration component score.

4.2      Ground water to surface water migration component.

4.2.1      General Considerations.
    4.2.1.1      Eligible surface waters.

    4.2.1.2      Definition of hazardous substance migration path for ground water to surface water migration component.

    4.2.1.3      Observed release of a specific hazardous substance to surface water in-water segment.

    4.2.1.4      Target distance limit.

    4.2.1.5      Evaluation of ground water to surface water migration component.

4.2.2      Drinking water threat.
    4.2.2.1      Drinking water threat-likelihood of release.

    4.2.2.1.1      Observed release.

    4.2.2.1.2      Potential to release.

    4.2.2.1.3      Calculation of drinking water threat-likelihood of release factor category value.

    4.2.2.2      Drinking water threat-waste characteristics.

    4.2.2.2.1      Toxicity/mobility/persistence.

    4.2.2.2.1.1      Toxicity.

    4.2.2.2.1.2      Mobility.

    4.2.2.2.1.3      Persistence.

    4.2.2.2.1.4      Calculation of toxicity/mobility/persistence factor value.

    4.2.2.2.2      Hazardous waste quantity.

    4.2.2.2.3      Calculation of drinking water threat-waste characteristics factor category value.

    4.2.2.3      Drinking water threat-targets.

    4.2.2.3.1      Nearest intake.

    4.2.2.3.2      Population.

    4.2.2.3.2.1      Level I concentrations.

    4.2.2.3.2.2      Level II concentrations.

    4.2.2.3.2.3      Potential contamination.

    4.2.2.3.2.4      Calculation of population factor value.

    4.2.2.3.3      Resources.

    4.2.2.3.4      Calculation of drinking water threat-targets factor category value.

    4.2.2.4      Calculation of drinking water threat score for a watershed.

4.2.3      Human food chain threat.
    4.2.3.1      Human food chain threat-likelihood of release.

    4.2.3.2      Human food chain threat-waste characteristics.

    4.2.3.2.1      Toxicity/mobility/persistence/bioaccumulation.

    4.2.3.2.1.1      Toxicity.

    4.2.3.2.1.2      Mobility.

    4.2.3.2.1.3      Persistence.

    4.2.3.2.1.4      Bioaccumulation potential.

    4.2.3.2.1.5      Calculation of toxicity/mobility/persistence/bioaccumulation factor value.

    4.2.3.2.2      Hazardous waste quantity.

    4.2.3.2.3      Calculation of human food chain threat-waste characteristics factor category value.

    4.2.3.3      Human food chain threat-targets.

    4.2.3.3.1      Food chain individual.

    4.2.3.3.2      Population.

    4.2.3.3.2.1      Level I concentrations.

    4.2.3.3.2.2      Level II concentrations.

    4.2.3.3.2.3      Potential human food chain contamination.

    4.2.3.3.2.4      Calculation of population factor value.

    4.2.3.3.3      Calculation of human food chain threat-targets factor category value.

    4.2.3.4      Calculation of human food chain threat score for a watershed.

4.2.4      Environmental threat.
    4.2.4.1      Environmental threat-likelihood of release.

    4.2.4.2      Environmental threat-waste characteristics.

    4.2.4.2.1      Ecosystem toxicity/mobility/persistence/bioaccumulation.

    4.2.4.2.1.1      Ecosystem toxicity.

    4.2.4.2.1.2      Mobility.

    4.2.4.2.1.3      Persistence.

    4.2.4.2.1.4      Ecosystem bioaccumulation potential.

    4.2.4.2.1.5      Calculation of ecosystem toxicity/mobility/persistence/bioaccumulation factor value.

    4.2.4.2.2      Hazardous waste quantity.

    4.2.4.2.3      Calculation of environmental threat-waste characteristics factor category value.

    4.2.4.3      Environmental threat-targets.

    4.2.4.3.1      Sensitive environments.

    4.2.4.3.1.1      Level I concentrations.

    4.2.4.3.1.2      Level II concentrations.

    4.2.4.3.1.3      Potential contamination.

    4.2.4.3.1.4      Calculation of environmental threat-targets factor category value.

    4.2.4.4      Calculation of environmental threat score for a watershed.

4.2.5      Calculation of ground water to surface water migration component score for a watershed.

4.2.6      Calculation of ground water to surface water migration component score.

4.3      Calculation of surface water migration pathway score.

5.0      Soil Exposure Pathway.

5.0.1      General considerations.

5.1      Resident population threat.

5.1.1      Likelihood of exposure.

5.1.2      Waste characteristics.
    5.1.2.1      Toxicity.

    5.1.2.2      Hazardous waste quantity.

    5.1.2.3      Calculation of waste characteristics factor category value.

5.1.3      Targets.
    5.1.3.1      Resident individual.

    5.1.3.2      Resident population.

    5.1.3.2.1      Level I concentrations.

    5.1.3.2.2      Level II concentrations.

    5.1.3.2.3      Calculation of resident population factor value.

    5.1.3.3      Workers.

    5.1.3.4      Resources.

    5.1.3.5      Terrestrial sensitive environments.

    5.1.3.6      Calculation of resident population targets factor category value.

5.1.4      Calculation of resident population threat score.

5.2      Nearby population threat.

5.2.1      Likelihood of exposure.
    5.2.1.1      Attractiveness/accessibility.

    5.2.1.2      Area of contamination.

    5.2.1.3      Likelihood of exposure factor category value.

5.2.2      Waste characteristics.
    5.2.2.1      Toxicity.

    5.2.2.2      Hazardous waste quantity.

    5.2.2.3      Calculation of waste characteristics factor category value.

5.2.3      Targets.
    5.2.3.1      Nearby individual.

    5.2.3.2      Population within 1 mile.

    5.2.3.3      Calculation of nearby population targets factor category value.

5.2.4      Calculation of nearby population threat score.

5.3      Calculation of soil exposure pathway score.

6.0      Air Migration Pathway.

6.1      Likelihood of release.

6.1.1      Observed release.

6.1.2      Potential to release.
    6.1.2.1      Gas potential to release.

    6.1.2.1.1      Gas containment.

    6.1.2.1.2      Gas source type.

    6.1.2.1.3      Gas migration potential.

    6.1.2.1.4      Calculation of gas potential to release value.

    6.1.2.2      Particulate potential to release.

    6.1.2.2.1      Particulate containment.

    6.1.2.2.2      Pariculate source type.

    6.1.2.2.3      Particulate migration potential.

    6.1.2.2.4      Calculation of particulate potential to release value.

    6.1.2.3      Calculation of potential to release factor value for the site.

6.1.3      Calculation of likelihood of release factor category value.

6.2      Waste characteristics.

6.2.1      Toxicity/mobility.
    6.2.1.1      Toxicity.

    6.2.1.2      Mobility.

    6.2.1.3      Calculation of toxicity/mobility factor value.

6.2.2      Hazardous waste quantity.

6.2.3      Calculation of waste characteristics factor category value.

6.3      Targets.

6.3.1      Nearest individual.

6.3.2      Population.
    6.3.2.1      Level of contamination.

    6.3.2.2      Level I concentrations.

    6.3.2.3      Level II concentrations.

    6.3.2.4      Potential contamination.

    6.3.2.5      Calculation of population factor value.

6.3.3      Resources.

6.3.4      Sensitive environments.
    6.3.4.1      Actual contamination.

    6.3.4.2      Potential contamination.

    6.3.4.3      Calculation of sensitive environments factor value.

6.3.5      Calculation of targets factor category value.

6.4      Calculation of air migration pathway score.

7.0      Sites Containing Radioactive Substances.

7.1      Likelihood of release/likelihood of exposure.

7.1.1      Observed release/observed contamination.

7.1.2      Potential to release.

7.2      Waste characteristics.

7.2.1      Human toxicity.

7.2.2      Ecosystem toxicity.

7.2.3      Persistence.

7.2.4      Selection of substance potentially posing greatest hazard.

7.2.5      Hazardous waste quantity.
    7.2.5.1      Source hazardous waste quantity for radionuclides.

    7.2.5.1.1      Radionuclide constituent quantity (Tier A).

    7.2.5.1.2      Radionuclide wastestream quantity (Tier B).

    7.2.5.1.3      Calculation of source hazardous waste quantity value for radionuclides.

    7.2.5.2      Calculation of hazardous waste quantity factor value for radionuclides.

    7.2.5.3      Calculation of hazardous waste quantity factor value for sites containing mixed radioactive and other hazardous substances.

7.3      Targets.

7.3.1      Level of contamination at a sampling location.

7.3.2      Comparison to benchmarks.

3-1      Overview of ground water migration pathway.

3-2      Net precipitation factor values.

4-1      Overview of surface water overland/flood migration component.

4-2      Overview of ground water to surface water migration component.

4-3      Sample determination of ground water to surface water angle.

5-1      Overview of soil exposure pathway.

6-1      Overview of air migration pathway.

6-2      Particulate migration potential factor values.

6-3      Particulate mobility factor values.

2-1      Sample pathway scoresheet.

2-2      Sample source characterization worksheet.

2-3      Observed release criteria for chemical analysis.

2-4      Toxicity factor evaluation.

2-5      Hazardous waste quantity evaluation equations.

2-6      Hazardous waste quantity factor values.

2-7      Waste characteristics factor category values.

3-1      Ground water migration pathway scoresheet.

3-2      Containment factor values for ground water migration pathway.

3-3      Monthly latitude adjusting values.

3-4      Net precipitation factor values.

3-5      Depth to aquifer factor values.

3-6      Hydraulic conductivity of geologic materials.

3-7      Travel time factor values.

3-8      Ground water mobility factor values.

3-9      Toxicity/mobility factor values.

3-10      Health-based benchmarks for hazardous substances in drinking water.

3-11      Nearest well factor values.

3-12      Distance-weighted population values for potential contamination factor for ground water migration pathway.

4-1      Surface water overland/flood migration component scoresheet.

4-2      Containment factor values for surface water migration pathway.

4-3      Drainage area values.

4-4      Soil group designations.

4-5      Rainfall/runoff values.

4-6      Runoff factor values.

4-7      Distance to surface water factor values.

4-8      Containment (flood) factor values.

4-9      Flood frequency factor values.

4-10      Persistence factor values -- half-life.

4-11      Persistence factor values -- log Kow

4-12      Toxicity/persistence factor values.

4-13      Surface water dilution weights.

4-14      Dilution-weighted population values for potential contamination factor for surface water migration pathway.

4-15      Bioaccumulation potential factor values.

4-16      Toxicity/persistence/bioaccumulation factor values.

4-17      Health-based benchmarks for hazardous substances in human food chain.

4-18      Human food chain population values.

4-19      Ecosystem toxicity factor values.

4-20      Ecosystem toxicity/persistence factor values.

4-21      Ecosystem toxicity/persistence/bioaccumulation factor values.

4-22      Ecological-based benchmarks for hazardous substances in surface water.

4-23      Sensitive environments rating values.

4-24      Wetlands rating values for surface water migration pathway.

4-25      Ground water to surface water migration component scoresheet.

4-26      Toxicity/mobility/persistence factor values.

4-27      Dilution weight adjustments.

4-28      Toxicity/mobility/persistence/bioaccumulation factor values.

4-29      Ecosystem toxicity/mobility/persistence factor values.

4-30      Ecosystem toxicity/mobility/persistence/bioaccumulation factor values.

5-1      Soil exposure pathway scoresheet.

5-2      Hazardous waste quantity evaluation equations for soil exposure pathway.

5-3      Health-based benchmarks for hazardous substances in soils.

5-4      Factor values for workers.

5-5      Terrestrial sensitive environments rating values.

5-6      Attractiveness/accessibility values.

5-7      Area of contamination factor values.

5-8      Nearby population likelihood of exposure factor values.

5-9      Nearby individual factor values.

5-10      Distance-weighted population values for nearby population threat.

6-1      Air migration pathway scoresheet.

6-2      Gas potential to release evaluation.

6-3      Gas containment factor values.

6-4      Source type factor values.

6-5      Values for vapor pressure and Henry's constant.

6-6      Gas migration potential values for a hazardous substance.

6-7      Gas migration potential values for the source.

6-8      Particulate potential to release evaluation.

6-9      Particulate containment factor values.

6-10      Particulate migration potential values.

6-11      Gas mobility factor values.

6-12      Particulate mobility factor values.

6-13      Toxicity/mobility factor values.

6-14      Health-based benchmarks for hazardous substances in air.

6-15      Air migration pathway distance weights.

6-16      Nearest individual factor values.

6-17      Distance-weighted population values for potential contamination factor for air pathway.

6-18      Wetlands rating values for air migration pathway.

7-1      HRS factors evaluated differently for radionuclides.

7-2      Toxicity factor values for radionuclides.

The Hazard Ranking System (HRS) is the principal mechanism the U.S. Environmental Protection Agency (EPA) uses to place sites on the National Priorities List (NPL). The HRS serves as a screening device to evaluate the potential for releases of uncontrolled hazardous substances to cause human health or environmental damage. The HRS provides a measure of relative rather than absolute risk. It is designed so that it can be consistently applied to a wide variety of sites.

Acute toxicity: Measure of toxicological responses that result from a single exposure to a substance or from multiple exposures within a short period of time (typically several days or less). Specific measures of acute toxicity used within the HRS include lethal dose50 (LD50) and lethal concentration50 (LC50), typically measured within a 24-hour to 96-hour period.

Ambient Aquatic Life Advisory Concentrations (AALACs): EPA's advisory concentration limit for acute or chronic toxicity to aquatic organisms as established under section 304(a)(1) of the Clean Water Act, as amended.

Ambient Water Quality Criteria (AWQC): EPA's maximum acute or chronic toxicity concentrations for protection of aquatic life and its uses as established under section 304(a)(1) of the Clean Water Act, as amended.

Bioconcentration factor (BCF): Measure of the tendency for a substance to accumulate in the tissue of an aquatic organism. BCF is determined by the extent of partitioning of a substance, at equilibrium, between the tissue of an aquatic organism and water. As the ratio of concentration of a substance in the organism divided by the concentration in water, higher BCF values reflect a tendency for substances to accumulate in the tissue of aquatic organisms. [unitless].

Biodegradation: Chemical reaction of a substance induced by enzymatic activity of microorganisms.

CERCLA: Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended (Pub. L. 96-510, as amended).

Chronic toxicity: Measure of toxicological responses that result from repeated exposure to a substance over an extended period of time (typically 3 months or longer). Such responses may persist beyond the exposure or may not appear until much later in time than the exposure. HRS measures of chronic toxicity include Reference Dose (RfD) values.

Contract Laboratory Program (CLP): Analytical program developed for CERCLA waste site samples to fill the need for legally defensible analytical results supported by a high level of quality assurance and documentation.

Contract-Required Detection Limit (CRDL): Term equivalent to contract-required quantitation limit, but used primarily for inorganic substances.

Contract-Required Quantitation Limit (CRQL): Substance-specific level that a CLP laboratory must be able to routinely and reliably detect in specific sample matrices. It is not the lowest detectable level achievable, but rather the level that a CLP laboratory should reasonably quantify. The CRQL may or may not be equal to the quantitation limit of a given substance in a given sample. For HRS purposes, the term CRQL refers to both the contract-required quantitation limit and the contract-required detection limit.

Curie (Ci): Measure used to quantify the amount of radioactivity. One curie equals 37 billion nuclear transformations per second, and one picocurie (pCi) equals 10⁻¹² Ci.

Decay product: Isotope formed by the radioactive decay of some other isotope. This newly formed isotope possesses physical and chemical properties that are different from those of its parent isotope, and may also be radioactive.

Detection Limit (DL): Lowest amount that can be distinguished from the normal random "noise" of an analytical instrument or method. For HRS purposes, the detection limit used is the method detection limit (MDL) or, for real-time field instruments, the detection limit of the instrument as used in the field.

Dilution weight: Parameter in the HRS surface water migration pathway that reduces the point value assigned to targets as the flow or depth of the relevant surface water body increases. [unitless].

Distance weight: Parameter in the HRS air migration, ground water migration, and soil exposure pathways that reduces the point value assigned to targets as their distance increases from the site. [unitless].

Distribution coefficient (Kd): Measure of the extent of partitioning of a substance between geologic materials (for example, soil, sediment, rock) and water (also called partition coefficient). The distribution coefficient is used in the HRS in evaluating the mobility of a substance for the ground water migration pathway. [ml/g].

ED10 (10 percent effective dose): Estimated dose associated with a 10 percent increase in response over control groups. For HRS purposes, the response considered is cancer. [milligrams toxicant per kilogram body weight per day (mg/kg-day)].

Food and Drug Administration Action Level (FDAAL): Under section 408 of the Federal Food, Drug and Cosmetic Act, as amended, concentration of a poisonous or deleterious substance in human food or animal feed at or above which FDA will take legal action to remove adulterated products from the market. Only FDAALs established for fish and shellfish apply in the HRS.

Half-life: Length of time required for an initial concentration of a substance to be halved as a result of loss through decay. The HRS considers five decay processes: biodegradation, hydrolysis, photolysis, radioactive decay, and volatilization.

Hazardous substance: CERCLA hazardous substances, pollutants, and contaminants as defined in CERCLA sections 101(14) and 101(33), except where otherwise specifically noted in the HRS.

Hazardous wastestream: Material containing CERCLA hazardous substances (as defined in CERCLA section 101[14]) that was deposited, stored, disposed, or placed in, or that otherwise migrated to, a source.

HRS "factor": Primary rating elements internal to the HRS.

HRS "factor category": Set of HRS factors (that is, likelihood of release [or exposure], waste characteristics, targets).

HRS "migration pathways": HRS ground water, surface water, and air migration pathways.

HRS "pathway": Set of HRS factor categories combined to produce a score to measure relative risks posed by a site in one of four environmental pathways (that is, ground water, surface water, soil, and air).

HRS "site score": Composite of the four HRS pathway scores.

Henry's law constant: Measure of the volatility of a substance in a dilute solution of water at equilibrium. It is the ratio of the vapor pressure exerted by a substance in the gas phase over a dilute aqueous solution of that substance to its concentration in the solution at a given temperature. For HRS purposes, use the value reported at or near 25 °C. [atmosphere-cubic meters per mole (atm-m³/mol)].

Hydrolysis: Chemical reaction of a substance with water.

Karst: Terrain with characteristics of relief and drainage arising from a high degree of rock solubility in natural waters. The majority of karst occurs in limestones, but karst may also form in dolomite, gypsum, and salt deposits. Features associated with karst terrains typically include irregular topography, sinkholes, vertical shafts, abrupt ridges, caverns, abundant springs, and/or disappearing streams. Karst aquifers are associated with karst terrain.

LC50 (lethal concentration, 50 percent): Concentration of a substance in air [typically micrograms per cubic meter (μg/m³)] or water [typically micrograms per liter (μg/l)] that kills 50 percent of a group of exposed organisms. The LC50 is used in the HRS in assessing acute toxicity.

LD50 (lethal dose, 50 percent): Dose of a substance that kills 50 percent of a group of exposed organisms. The LD50 is used in the HRS in assessing acute toxicity [milligrams toxicant per kilogram body weight (mg/kg)].

Maximum Contaminant Level (MCL): Under section 1412 of the Safe Drinking Water Act, as amended, the maximum permissible concentration of a substance in water that is delivered to any user of a public water supply.

Maximum Contaminant Level Goal (MCLG): Under section 1412 of the Safe Drinking Water Act, as amended, a nonenforceable concentration for a substance in drinking water that is protective of adverse human health effects and allows an adequate margin of safety.

Method Detection Limit (MDL): Lowest concentration of analyte that a method can detect reliably in either a sample or blank.

Mixed radioactive and other hazardous substances: Material containing both radioactive hazardous substances and nonradioactive hazardous substances, regardless of whether these types of substances are physically separated, combined chemically, or simply mixed together.

National Ambient Air Quality Standards (NAAQS): Primary standards for air quality established under sections 108 and 109 of the Clean Air Act, as amended.

National Emission Standards for Hazardous Air Pollutants (NESHAPs): Standards established for substances listed under section 112 of the Clean Air Act, as amended. Only those NESHAPs promulgated in ambient concentration units apply in the HRS.

Octanol-water partition coefficient (Kow [or P]): Measure of the extent of partitioning of a substance between water and octanol at equilibrium. The Kow is determined by the ratio between the concentration in octanol divided by the concentration in water at equilibrium. [unitless].

Organic carbon partition coefficient (Koc): Measure of the extent of partitioning of a substance, at equilibrium, between organic carbon in geologic materials and water. The higher the Koc, the more likely a substance is to bind to geologic materials than to remain in water. [ml/g].

Photolysis: Chemical reaction of a substance caused by direct absorption of solar energy (direct photolysis) or caused by other substances that absorb solar energy (indirect photolysis).

Radiation: Particles (alpha, beta, neutrons) or photons (x- and gamma-rays) emitted by radionuclides.

Radioactive decay: Process of spontaneous nuclear transformation, whereby an isotope of one element is transformed into an isotope of another element, releasing excess energy in the form of radiation.

Radioactive half-life: Time required for one-half the atoms in a given quantity of a specific radionuclide to undergo radioactive decay.

Radioactive substance: Solid, liquid, or gas containing atoms of a single radionuclide or multiple radionuclides.

Radioactivity: Property of those isotopes of elements that exhibit radioactive decay and emit radiation.

Radionuclide/radioisotope: Isotope of an element exhibiting radioactivity. For HRS purposes, "radionuclide" and "radioisotope" are used synonymously.

Reference dose (RfD): Estimate of a daily exposure level of a substance to a human population below which adverse noncancer health effects are not anticipated. [milligrams toxicant per kilogram body weight per day (mg/kg-day)].

Removal action: Action that removes hazardous substances from the site for proper disposal or destruction in a facility permitted under the Resource Conservation and Recovery Act or the Toxic Substances Control Act or by the Nuclear Regulatory Commission.

Roentgen (R): Measure of external exposures to ionizing radiation. One roentgen equals that amount of x-ray or gamma radiation required to produce ions carrying a charge of 1 electrostatic unit (esu) in 1 cubic centimeter of dry air under standard conditions. One microroentgen (μR) equals 10⁻⁶ R.

Sample quantitation limit (SQL): Quantity of a substance that can be reasonably quantified given the limits of detection for the methods of analysis and sample characteristics that may affect quantitation (for example, dilution, concentration).

Screening concentration: Media-specific benchmark concentration for a hazardous substance that is used in the HRS for comparison with the concentration of that hazardous substance in a sample from that media. The screening concentration for a specific hazardous substance corresponds to its reference dose for inhalation exposures or for oral exposures, as appropriate, and, if the substance is a human carcinogen with a weight-of-evidence classification of A, B, or C, to that concentration that corresponds to its 10⁻⁶ individual lifetime excess cancer risk for inhalation exposures or for oral exposures, as appropriate.

Site: Area(s) where a hazardous substance has been deposited, stored, disposed, or placed, or has otherwise come to be located. Such areas may include multiple sources and may include the area between sources.

Slope factor (also referred to as cancer potency factor): Estimate of the probability of response (for example, cancer) per unit intake of a substance over a lifetime. The slope factor is typically used to estimate upper-bound probability of an individual developing cancer as a result of exposure to a particular level of a human carcinogen with a weight-of-evidence classification of A, B, or C. [(mg/kg-day)⁻¹ for non-radioactive substances and (pCi)⁻¹ for radioactive substances].

Target distance limit: Maximum distance over which targets for the site are evaluated. The target distance limit varies by HRS pathway.

Uranium Mill Tailings Radiation Control Act (UMTRCA) Standards: Standards for radionuclides established under sections 102, 104, and 108 of the Uranium Mill Tailings Radiation Control Act, as amended.

Vapor pressure: Pressure exerted by the vapor of a substance when it is in equilibrium with its solid or liquid form at a given temperature. For HRS purposes, use the value reported at or near 25 °C. [atmosphere or torr].

Volatilization: Physical transfer process through which a substance undergoes a change of state from a solid or liquid to a gas.

Water solubility: Maximum concentration of a substance in pure water at a given temperature. For HRS purposes, use the value reported at or near 25 °C. [milligrams per liter (mg/l)].

Weight-of-evidence: EPA classification system for characterizing the evidence supporting the designation of a substance as a human carcinogen. EPA weight-of-evidence groupings include:
    Group A: Human carcinogen--sufficient evidence of carcinogenicity in humans.

    Group B1: Probable human carcinogen--limited evidence of carcinogenicity in humans.

    Group B2: Probable human carcinogen--sufficient evidence of carcinogenicity in animals.

    Group C: Possible human carcinogen--limited evidence of carcinogenicity in animals.

    Group D: Not classifiable as to human carcinogenicity--applicable when there is no animal evidence, or when human or animal evidence is inadequate.

    Group E: Evidence of noncarcinogenicity for humans.

2.1      Overview. The HRS site score (S) is the result of an evaluation of four pathways:

•    Ground Water Migration (Sgw).

•    Surface Water Migration (Ssw).

•    Soil Exposure (Ss).

•    Air Migration (Sa).

The ground water and air migration pathways use single threat evaluations, while the surface water migration and soil exposure pathways use multiple threat evaluations. Three threats are evaluated for the surface water migration pathway: drinking water, human food chain, and environmental. These threats are evaluated for two separate migration components--overland/flood migration and ground water to surface water migration. Two threats are evaluated for the soil exposure pathway: resident population and nearby population.

The HRS is structured to provide a parallel evaluation for each of these pathways and threats. This section focuses on these parallel evaluations, starting with the calculation of the HRS site score and the individual pathway scores.

2.1.1      Calculation of HRS site score. Scores are first calculated for the individual pathways as specified in sections 2 through 7 and then are combined for the site using the following root-mean-square equation to determine the overall HRS site score, which ranges from 0 to 100: $$

2.1.2      Calculation of pathway score. Table 2-1, which is based on the air migration pathway, illustrates the basic parameters used to calculate a pathway score. As table 2-1 shows, each pathway (or threat) score is the product of three "factor categories": likelihood of release, waste characteristics, and targets. (The soil exposure pathway uses likelihood of exposure rather than likelihood of release.) Each of the three factor categories contains a set of factors that are assigned numerical values and combined as specified in sections 2 through 7. The factor values are rounded to the nearest integer, except where otherwise noted.

2.1.3      Common evaluations. Evaluations common to all four HRS pathways include:

•    Characterizing sources. -Identifying sources (and, for the soil exposure pathway, areas of observed contamination [see section 5.0.1]). -Identifying hazardous substances associated with each source (or area of observed contamination). -Identifying hazardous substances available to a pathway.

                   Table 2-1_Sample Pathway Scoresheet
------------------------------------------------------------------------
                                                       Maximum    Value
                   Factor category                      value   assigned
------------------------------------------------------------------------
                Likelihood of Release
 
1  1  1  1. Observed Release.........................      550
2. Potential to Release..............................      500
3. Likelihood of Release (higher of lines 1 and 2)...      550
                Waste Characteristics
 
4. Toxicity/Mobility.................................      (a)
5. Hazardous Waste Quantity..........................      (a)
6. Waste Characteristics.............................      100
                       Targets
 
7. Nearest Individual
  7a. Level I........................................       50
  7b. Level II.......................................       45
  7c. Potential Contamination........................       20
  7d. Nearest Individual (higher of lines 7a, 7b, or        50
   7c)...............................................
8. Population
  8a. Level I........................................      (b)
  8b. Level II.......................................      (b)
  8c. Potential Contamination........................      (b)
  8d. Total Population (lines
   8a+8b+8c).........................................      (b)
9. Resources.........................................        5
10. Sensitive Environments...........................      (b)
  10a. Actual Contamination..........................      (b)
  10b. Potential Contamination.......................      (b)
  10c. Sensitive Environments
   (lines 10a+10b)...................................      (b)
11. Targets (lines 7d+8d+9+10c)......................      (b)
12. Pathway Score is the product of Likelihood of Release, Waste
 Characteristics, and Targets, divided by 82,500. Pathway scores are
 limited to a maximum of 100 points.
------------------------------------------------------------------------
[SU]a[/SU] Maximum value applies to waste characteristics category. The
  product of lines 4 and 5 is used in table 2-7 to derive the value for
  the waste characteristics factor category.
[SU]b[/SU] There is no limit to the human population or sensitive
  environments factor values. However, the pathway score based solely on
  sensitive environments is limited to a maximum of 60 points.

•    Scoring likelihood of release (or likelihood of exposure) factor category. -Scoring observed release (or observed contamination). -Scoring potential to release when there is no observed release.

•    Scoring waste characteristics factor category. -Evaluating toxicity. -Combining toxicity with mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential, as appropriate to the pathway (or threat). -Evaluating hazardous waste quantity. -Combining hazardous waste quantity with the other waste characteristics factors. -Determining waste characteristics factor category value.

•    Scoring targets factor category. -Determining level of contamination for targets.

These evaluations are essentially identical for the three migration pathways (ground water, surface water, and air). However, the evaluations differ in certain respects for the soil exposure pathway.

Section 7 specifies modifications that apply to each pathway when evaluating sites containing radioactive substances.

Section 2 focuses on evaluations common at the pathway and threat levels. Note that for the ground water and surface water migration pathways, separate scores are calculated for each aquifer (see section 3.0) and each watershed (see sections 4.1.1.3 and 4.2.1.5) when determining the pathway scores for a site. Although the evaluations in section 2 do not vary when different aquifers or watersheds are scored at a site, the specific factor values (for example, observed release, hazardous waste quantity, toxicity/mobility) that result from these evaluations can vary by aquifer and by watershed at the site. This can occur through differences both in the specific sources and targets eligible to be evaluated for each aquifer and watershed and in whether observed releases can be established for each aquifer and watershed. Such differences in scoring at the aquifer and watershed level are addressed in sections 3 and 4, not section 2.

2.2      Characterize sources. Source characterization includes identification of the following:

•    Sources (and areas of observed contamination) at the site.

•    Hazardous substances associated with these sources (or areas of observed contamination).

•    Pathways potentially threatened by these hazardous substances.

Table 2-2 presents a sample worksheet for source characterization.

2.2.1      Identify sources. For the three migration pathways, identify the sources at the site that contain hazardous substances. Identify the migration pathway(s) to which each source applies. For the soil exposure pathway, identify areas of observed contamination at the site (see section 5.0.1).

           Table 2-2_Sample Source Characterization Worksheet
 
 
 
Source: __________________
A. Source dimensions and hazardous waste quantity.
 Hazardous constituent quantity: ___
 Hazardous wastestream quantity: ___
 Volume: ___
 Area: ___
 Area of observed contamination: ___
B. Hazardous substances associated with the source.
 

 
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                     Available to pathway
                                                                    ------------------------------------------------------------------------------------
                                                                                Air                         Surface water (SW)             Soil
                        Hazardous substance                         -------------------------   Ground   -----------------------------------------------
                                                                                              water (GW)   Overland/
                                                                         Gas     Particulate                 flood     GW to SW    Resident     Nearby
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
                                                                     ..........  ...........  ..........  ..........  ..........  ..........  ..........
--------------------------------------------------------------------------------------------------------------------------------------------------------

2.2.2      Identify hazardous substances associated with a source. For each of the three migration pathways, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that source when evaluating each pathway. In some instances, 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. For the three migration pathways, in those instances when the specific source(s) cannot be documented for a hazardous substance, consider the hazardous substance to be present in each source at the site, except sources for which definitive information indicates that the hazardous substance was not or could not be present.

For an area of observed contamination in the soil exposure pathway, consider only those hazardous substances that meet the criteria for observed contamination for that area (see section 5.0.1) to be associated with that area when evaluating the pathway.

2.2.3      Identify hazardous substances available to a pathway. In evaluating each migration pathway, consider the following hazardous substances available to migrate from the sources at the site to the pathway:

•    Ground water migration. -Hazardous substances that meet the criteria for an observed release (see section 2.3) to ground water. -All hazardous substances associated with a source with a ground water containment factor value greater than 0 (see section 3.1.2.1).

•    Surface water migration -- overland/flood component. -Hazardous substances that meet the criteria for an observed release to surface water in the watershed being evaluated. -All hazardous substances associated with a source with a surface water containment factor value greater than 0 for the watershed (see sections 4.1.2.1.2.1.1 and 4.1.2.1.2.2.1).

•    Surface water migration -- ground water to surface water component. -Hazardous substances that meet the criteria for an observed release to ground water. -All hazardous substances associated with a source with a ground water containment factor value greater than 0 (see sections 4.2.2.1.2 and 3.1.2.1).

•    Air migration. -Hazardous substances that meet the criteria for an observed release to the atmosphere. -All gaseous hazardous substances associated with a source with a gas containment factor value greater than 0 (see section 6.1.2.1.1). -All particulate hazardous substances associated with a source with a particulate containment factor value greater than 0 (see section 6.1.2.2.1).

•    For each migration pathway, in those instances when the specific source(s) containing the hazardous substance cannot be documented, consider that hazardous substance to be available to migrate to the pathway when it can be associated (see section 2.2.2) with at least one source having a containment factor value greater than 0 for that pathway.

In evaluating the soil exposure pathway, consider the following hazardous substances available to the pathway:

•    Soil exposure -- resident population threat. -All hazardous substances that meet the criteria for observed contamination at the site (see section 5.0.1).

•    Soil exposure -- nearby population threat. -All hazardous substances that meet the criteria for observed contamination at areas with an attractiveness/accessibility factor value greater than 0 (see section 5.2.1.1).

2.3      Likelihood of release. Likelihood of release is a measure of the likelihood that a waste has been or will be released to the environment. The likelihood of release factor category is assigned the maximum value of 550 for a migration pathway whenever the criteria for an observed release are met for that pathway. If the criteria for an observed release are met, do not evaluate potential to release for that pathway. When the criteria for an observed release are not met, evaluate potential to release for that pathway, with a maximum value of 500. The evaluation of potential to release varies by migration pathway (see sections 3, 4 and 6).

Establish an observed release either by direct observation of the release of a hazardous substance into the media being evaluated (for example, surface water) or by chemical analysis of samples appropriate to the pathway being evaluated (see sections 3, 4, and 6). The minimum standard to establish an observed release by chemical analysis is analytical evidence of a hazardous substance in the media significantly above the background level. Further, some portion of the release must be attributable to the site. Use the criteria in table 2-3 as the standard for determining analytical significance. (The criteria in table 2-3 are also used in establishing observed contamination for the soil exposure pathway, see section 5.0.1.) Separate criteria apply to radionuclides (see section 7.1.1).

        Table 2-3_Observed Release Criteria for Chemical Analysis
 
 
-------------------------------------------------------------------------
Sample Measurement < Sample Quantitation Limit [SU]a[/SU]
No observed release is established.
Sample Measurement >== Sample Quantitation Limit [SU]a[/SU]
An observed release is established as follows:
  [sbull] If the background concentration is not detected (or is less
   than the detection limit), an observed release is established when
   the sample measurement equals or exceeds the sample quantitation
   limit.[SU]a[/SU]
  [sbull] If the background concentration equals or exceeds the
   detection limit, an observed release is established when the sample
   measurement is 3 times or more above the background concentration.
------------------------------------------------------------------------
[SU]a[/SU] If the sample quantitation limit (SQL) cannot be established,
  determine if there is an observed release as follows:
 _If the sample analysis was performed under the EPA Contract Laboratory
  Program, use the EPA contract-required quantitation limit (CRQL) in
  place of the SQL.
 _If the sample analysis is not performed under the EPA Contract
  Laboratory Program, use the detection limit (DL) in place of the SQL.

2.4      Waste characteristics. The waste characteristics factor category includes the following factors: hazardous waste quantity, toxicity, and as appropriate to the pathway or threat being evaluated, mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential.

2.4.1      Selection of substance potentially posing greatest hazard. For all pathways (and threats), select the hazardous substance potentially posing the greatest hazard for the pathway (or threat) and use that substance in evaluating the waste characteristics category of the pathway (or threat). For the three migration pathways (and threats), base the selection of this hazardous substance on the toxicity factor value for the substance, combined with its mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential factor values, as applicable to the migration pathway (or threat). For the soil exposure pathway, base the selection on the toxicity factor alone.

Evaluation of the toxicity factor is specified in section 2.4.1.1. Use and evaluation of the mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential factors vary by pathway (or threat) and are specified under the appropriate pathway (or threat) section. Section 2.4.1.2 identifies the specific factors that are combined with toxicity in evaluating each pathway (or threat).

2.4.1.1      Toxicity factor. Evaluate toxicity for those hazardous substances at the site that are available to the pathway being scored. For all pathways and threats, except the surface water environmental threat, evaluate human toxicity as specified below. For the surface water environmental threat, evaluate ecosystem toxicity as specified in section 4.1.4.2.1.1.

Establish human toxicity factor values based on quantitative dose-response parameters for the following three types of toxicity:

•    Cancer- -Use slope factors (also referred to as cancer potency factors) combined with weight-of-evidence ratings for carcinogenicity. If a slope factor is not available for a substance, use its ED10 value to estimate a slope factor as follows: $$

•    Noncancer toxicological responses of chronic exposure- -use reference dose (RfD) values.

•    Noncancer toxicological responses of acute exposure- -use acute toxicity parameters, such as the LD50.

Assign human toxicity factor values to a hazardous substance using table 2-4, as follows:

•    If RfD and slope factor values are both available for the hazardous substance, assign the substance a value from table 2-4 for each. Select the higher of the two values assigned and use it as the overall toxicity factor value for the hazardous substance.

•    If either an RfD or slope factor value is available, but not both, assign the hazardous substance an overall toxicity factor value from table 2-4 based solely on the available value (RfD or slope factor).

•    If neither an RfD nor slope factor value is available, assign the hazardous substance an overall toxicity factor value from table 2-4 based solely on acute toxicity. That is, consider acute toxicity in table 2-4 only when both RfD and slope factor values are not available.

•    If neither an RfD, nor slope factor, nor acute toxicity value is available, assign the hazardous substance an overall toxicity factor value of 0 and use other hazardous substances for which information is available in evaluating the pathway.

                  Table 2-4_Toxicity Factor Evaluation
                        Chronic Toxicity (Human)
------------------------------------------------------------------------
                                                                Assigned
               Reference dose (RfD) (mg/kg-day)                  value
------------------------------------------------------------------------
RfD < 0.0005..............................................     10,000
0.0005 [lE] RfD < 0.005...................................      1,000
0.005 [lE] RfD < 0.05.....................................        100
0.05 [lE] RfD < 0.5.......................................         10
0.5 [lE] RfD.................................................          1
RfD not available............................................          0
------------------------------------------------------------------------

                                             Carcinogenicity (Human)
----------------------------------------------------------------------------------------------------------------
                   Weight-of-evidence[SU]a[/SU]/slope factor (mg/kg-day)[SU]-1[/SU]
------------------------------------------------------------------------------------------------------  Assigned
                    A                                    B                             C                 value
----------------------------------------------------------------------------------------------------------------
0.5 [lE] SF[SU]b[/SU]                      5 [lE] SF                     50 [lE] SF                       10,000
----------------------------------------------------------------------------------------------------------------
                                                                                                       1  1  1
1  0.05 [lE] SF < 0.5                   0.5 [lE] SF < 5            5 [lE] SF < 50                 1,000
----------------------------------------------------------------------------------------------------------------
 
SF < 0.05                               0.05 [lE] SF < 0.5         0.5 [lE] SF < 5                  100
----------------------------------------------------------------------------------------------------------------
 
                  _ _ _                    SF < 0.05                  SF < 0.5                          10
----------------------------------------------------------------------------------------------------------------
 
Slope factor not available...............  Slope factor not available..  Slope factor not available..          0
----------------------------------------------------------------------------------------------------------------
[SU]a[/SU] A, B, and C refer to weight-of-evidence categories. Assign substances with a weight-of-evidence
  category of D (inadequate evidence of carcinogenicity) or E (evidence of lack of carcinogenicity) a value of 0
  for carcinogenicity.
[SU]b[/SU] SF = Slope factor.

                                 Table 2-4_Toxicity Factor Evaluation_Concluded
                                             Acute Toxicity (Human)
----------------------------------------------------------------------------------------------------------------
                                                               Dust or mist          Gas or vapor
   Oral LD[INF]50[/INF] (mg/kg)       Dermal LD[INF]50[/   LC[INF]50[/INF] (mg/     LC[INF]50[/INF]     Assigned
                                         INF] (mg/kg)               l)                   (ppm)           value
----------------------------------------------------------------------------------------------------------------
               _ _ _                        _ _ _                  _ _ _                 _ _ _           _ _ _
LD [INF]50[/INF] < 5...........  LD[INF]50[/INF] <   LC[INF]50[/INF] <  LC[INF]50[/INF] <      1,000
                                     2.                     0.2.                  20.
5 [lE] LD[INF]50[/INF] < 50....  2 [lE] LD[INF]50[/     0.2 [lE] LC[INF]50[/  20 [lE] LC[INF]50[/         100
                                     INF] < 20.          INF] < 2.          INF] < 200.
50 [lE] LD[INF]50[/INF] < 500..  20 [lE] LD[INF]50[/    2 [lE] LC[INF]50[/    200 [lE] LC[INF]50[/         10
                                     INF] < 200.         INF] < 20.         INF] < 2,000.
500 [lE] LD[INF]50[/INF]..........  200 [lE] LD[INF]50[/   20 [lE] LC[INF]50[/   2,000 [lE]                    1
                                     INF].                  INF].                 LC[INF]50[/INF].
LD[INF]50[/INF] not available.....  LD[INF]50[/INF] not    LC[INF]50[/INF] not   LC[INF]50[/INF] not           0
                                     available.             available.            available.
----------------------------------------------------------------------------------------------------------------

If a toxicity factor value of 0 is assigned to all hazardous substances available to a particular pathway (that is, insufficient toxicity data are available for evaluating all the substances), use a default value of 100 as the overall human toxicity factor value for all hazardous substances available to the pathway. For hazardous substances having usable toxicity data for multiple exposure routes (for example, inhalation and ingestion), consider all exposure routes and use the highest assigned value, regardless of exposure route, as the toxicity factor value.

For HRS purposes, assign both asbestos and lead (and its compounds) a human toxicity factor value of 10,000.

Separate criteria apply for assigning factor values for human toxicity and ecosystem toxicity for radionuclides (see sections 7.2.1 and 7.2.2).

2.4.1.2      Hazardous substance selection. For each hazardous substance evaluated for a migration pathway (or threat), combine the human toxicity factor value (or ecosystem toxicity factor value) for the hazardous substance with a mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential factor value as follows:

•    Ground water migration. -Determine a combined human toxicity/mobility factor value for the hazardous substance (see section 3.2.1).

•    Surface water migration-overland/flood migration component. -Determine a combined human toxicity/persistence factor value for the hazardous substance for the drinking water threat (see section 4.1.2.2.1). -Determine a combined human toxicity/persistence/bioaccumulation factor value for the hazardous substance for the human food chain threat (see section 4.1.3.2.1). -Determine a combined ecosystem toxicity/persistence/bioaccumulation factor value for the hazardous substance for the environmental threat (see section 4.1.4.2.1).

•    Surface water migration-ground water to surface water migration component. -Determine a combined human toxicity/mobility/persistence factor value for the hazardous substance for the drinking water threat (see section 4.2.2.2.1). -Determine a combined human toxicity/mobility/persistence/bioaccumulation factor value for the hazardous substance for the human food chain threat (see section 4.2.3.2.1). -Determine a combined ecosystem toxicity/mobility/persistence/bioaccumulation factor value for the hazardous substance for the environmental threat (see section 4.2.4.2.1).

•    Air migration. -Determine a combined human toxicity/mobility factor value for the hazardous substance (see section 6.2.1).

Determine each combined factor value for a hazardous substance by multiplying the individual factor values appropriate to the pathway (or threat). For each migration pathway (or threat) being evaluated, select the hazardous substance with the highest combined factor value and use that substance in evaluating the waste characteristics factor category of the pathway (or threat).

For the soil exposure pathway, select the hazardous substance with the highest human toxicity factor value from among the substances that meet the criteria for observed contamination for the threat evaluated and use that substance in evaluating the waste characteristics factor category.

2.4.2      Hazardous waste quantity. Evaluate the hazardous waste quantity factor by first assigning each source (or area of observed contamination) a source hazardous waste quantity value as specified below. Sum these values to obtain the hazardous waste quantity factor value for the pathway being evaluated.

In evaluating the hazardous waste quantity factor for the three migration pathways, allocate hazardous substances and hazardous wastestreams to specific sources in the manner specified in section 2.2.2, except: consider hazardous substances and hazardous wastestreams that cannot be allocated to any specific source to constitute a separate "unallocated source" for purposes of evaluating only this factor for the three migration pathways. Do not, however, include a hazardous substance or hazardous wastestream in the unallocated source for a migration pathway if there is definitive information indicating that the substance or wastestream could only have been placed in sources with a containment factor value of 0 for that migration pathway.

In evaluating the hazardous waste quantity factor for the soil exposure pathway, allocate to each area of observed contamination only those hazardous substances that meet the criteria for observed contamination for that area of observed contamination and only those hazardous wastestreams that contain hazardous substances that meet the criteria for observed contamination for that area of observed contamination. Do not consider other hazardous substances or hazardous wastestreams at the site in evaluating this factor for the soil exposure pathway.

2.4.2.1      Source hazardous waste quantity. For each of the three migration pathways, assign a source hazardous waste quantity value to each source (including the unallocated source) having a containment factor value greater than 0 for the pathway being evaluated. Consider the unallocated source to have a containment factor value greater than 0 for each migration pathway.

For the soil exposure pathway, assign a source hazardous waste quantity value to each area of observed contamination, as applicable to the threat being evaluated.

For all pathways, evaluate source hazardous waste quantity using the following four measures in the following hierarchy:

•    Hazardous constituent quantity.

•    Hazardous wastestream quantity.

•    Volume.

•    Area.

For the unallocated source, use only the first two measures.

Separate criteria apply for assigning a source hazardous waste quantity value for radionuclides (see section 7.2.5).

2.4.2.1.1      Hazardous constituent quantity. Evaluate hazardous constituent quantity for the source (or area of observed contamination) based solely on the mass of CERCLA hazardous substances (as defined in CERCLA section 101(14), as amended) allocated to the source (or area of observed contamination), except:

•    For a hazardous waste listed pursuant to section 3001 of the Solid Waste Disposal Act, as amended by the Resource Conservation and Recovery Act of 1976 (RCRA), 42 U.S.C. 6901 et seq., determine its mass for the evaluation of this measure as follows: -If the hazardous waste is listed solely for Hazard Code T (toxic waste), include only the mass of constituents in the hazardous waste that are CERCLA hazardous substances and not the mass of the entire hazardous waste. -If the hazardous waste is listed for any other Hazard Code (including T plus any other Hazard Code), include the mass of the entire hazardous waste.

•    For a RCRA hazardous waste that exhibits the characteristics identified under section 3001 of RCRA, as amended, determine its mass for the evaluation of this measure as follows: -If the hazardous waste exhibits only the characteristic of toxicity (or only the characteristic of EP toxicity), include only the mass of constituents in the hazardous waste that are CERCLA hazardous substances and not the mass of the entire hazardous waste. -If the hazardous waste exhibits any other characteristic identified under section 3001 (including any other characteristic plus the characteristic of toxicity [or the characteristic of EP toxicity]), include the mass of the entire hazardous waste.

Based on this mass, designated as C, assign a value for hazardous constituent quantity as follows:

•    For the migration pathways, assign the source a value for hazardous constituent quantity using the Tier A equation of table 2-5.

•    For the soil exposure pathway, assign the area of observed contamination a value using the Tier A equation of table 5-2 (section 5.1.2.2).

If the hazardous constituent quantity for the source (or area of observed contamination) is adequately determined (that is, the total mass of all CERCLA hazardous substances in the source and releases from the source [or in the area of observed contamination] is known or is estimated with reasonable confidence), do not evaluate the other three measures discussed below. Instead assign these other three measures a value of 0 for the source (or area of observed contamination) and proceed to section 2.4.2.1.5.

If the hazardous constituent quantity is not adequately determined, assign the source (or area of observed contamination) a value for hazardous constituent quantity based on the available data and proceed to section 2.4.2.1.2.

         Table 2-5_Hazardous Waste Quantity Evaluation Equations
------------------------------------------------------------------------
                                                          Equation for
     Tier              Measure              Units        assigning value
                                                           [SU]a[/SU]
------------------------------------------------------------------------
A              Hazardous constituent   lb.............  C
                quantity (C)
B [SU]b[/SU]   Hazardous wastestream   lb.............  W/5,000
                quantity (W)
C [SU]b[/SU]   Volume (V)
                Landfill.............  yd\3\..........  V/2,500
                Surface impoundment    yd\3\..........  V/2.5
                Surface impoundment    yd\3\..........  V/2.5
                (buried/backfilled)
                Drums [SU]c[/SU].....  gallon.........  V/500
                Tanks and containers   yd\3\..........  V/2.5
                other than drums
                Contaminated soil....  yd\3\..........  V/2,500
                Pile.................  yd\3\..........  V/2.5
                Other................  yd\3\..........  V/2.5
D [SU]b[/SU]   Area (A)..............
                Landfill.............  ft\2\..........  A/3,400
                Surface impoundment    ft\2\..........  A/13
                Surface impoundment    ft\2\..........  A/13
                (buried/backfilled)
                Land treatment.......  ft\2\..........  A/270
                Pile [SU]d[/SU]......  ft\2\..........  A/13
                Contaminated soil....  ft\2\..........  A/34,000
------------------------------------------------------------------------
[SU]a[/SU] Do not round to nearest integer.
[SU]b[/SU] Convert volume to mass when necessary: 1 ton=2,000 pounds=1
  cubic yard=4 drums=200 gallons.
[SU]c[/SU] If actual volume of drums is unavailable, assume 1 drum=50
  gallons.
[SU]d[/SU] Use land surface area under pile, not surface area of pile.

2.4.2.1.2      Hazardous wastestream quantity. Evaluate hazardous wastestream quantity for the source (or area of observed contamination) based on the mass of hazardous wastestreams plus the mass of any additional CERCLA pollutants and contaminants (as defined in CERCLA section 101[33], as amended) that are allocated to the source (or area of observed contamination). For a wastestream that consists solely of a hazardous waste listed pursuant to section 3001 of RCRA, as amended or that consists solely of a RCRA hazardous waste that exhibits the characteristics identified under section 3001 of RCRA, as amended, include the mass of that entire hazardous waste in the evaluation of this measure.

Based on this mass, designated as W, assign a value for hazardous wastestream quantity as follows:

•    For the migration pathways, assign the source a value for hazardous wastestream quantity using the Tier B equation of table 2-5.

•    For the soil exposure pathway, assign the area of observed contamination a value using the Tier B equation of table 5-2 (section 5.1.2.2).

Do not evaluate the volume and area measures described below if the source is the unallocated source or if the following condition applies:

•    The hazardous wastestream quantity for the source (or area of observed contamination) is adequately determined -- that is, total mass of all hazardous wastestreams and CERCLA pollutants and contaminants for the source and releases from the source (or for the area of observed contamination) is known or is estimated with reasonable confidence.

If the source is the unallocated source or if this condition applies, assign the volume and area measures a value of 0 for the source (or area of observed contamination) and proceed to section 2.4.2.1.5. Otherwise, assign the source (or area of observed contamination) a value for hazardous wastestream quantity based on the available data and proceed to section 2.4.2.1.3.

2.4.2.1.3      Volume. Evaluate the volume measure using the volume of the source (or the volume of the area of observed contamination). For the soil exposure pathway, restrict the use of the volume measure to those areas of observed contamination specified in section 5.1.2.2.

Based on the volume, designated as V, assign a value to the volume measure as follows:

•    For the migration pathways, assign the source a value for volume using the appropriate Tier C equation of table 2-5.

•    For the soil exposure pathway, assign the area of observed contamination a value for volume using the appropriate Tier C equation of table 5-2 (section 5.1.2.2).

If the volume of the source (or volume of the area of observed contamination, if applicable) can be determined, do not evaluate the area measure. Instead, assign the area measure a value of 0 and proceed to section 2.4.2.1.5. If the volume cannot be determined (or is not applicable for the soil exposure pathway), assign the source (or area of observed contamination) a value of 0 for the volume measure and proceed to section 2.4.2.1.4.

2.4.2.1.4      Area. Evaluate the area measure using the area of the source (or the area of the area of observed contamination). Based on this area, designated as A, assign a value to the area measure as follows:

•    For the migration pathways, assign the source a value for area using the appropriate Tier D equation of table 2-5.

•    For the soil exposure pathway, assign the area of observed contamination a value for area using the appropriate Tier D equation of table 5-2 (section 5.1.2.2).

2.4.2.1.5      Calculation of source hazardous waste quantity value. Select the highest of the values assigned to the source (or area of observed contamination) for the hazardous constituent quantity, hazardous wastestream quantity, volume, and area measures. Assign this value as the source hazardous waste quantity value. Do not round to the nearest integer.

2.4.2.2      Calculation of hazardous waste quantity factor value. Sum the source hazardous waste quantity values assigned to all sources (including the unallocated source) or areas of observed contamination for the pathway being evaluated and round this sum to the nearest integer, except: if the sum is greater than 0, but less than 1, round it to 1. Based on this value, select a hazardous waste quantity factor value for the pathway from table 2-6.

            Table 2-6_Hazardous Waste Quantity Factor Values
------------------------------------------------------------------------
                                                               Assigned
               Hazardous waste quantity value                    value
------------------------------------------------------------------------
0...........................................................           0
1[SU]a[/SU] to 100..........................................   1 [SU]b[/
                                                                     SU]
Greater than 100 to 10,000..................................         100
Greater than 10,000 to 1,000,000............................      10,000
Greater than 1,000,000......................................   1,000,000
------------------------------------------------------------------------
[SU]a[/SU] If the hazardous waste quantity value is greater than 0, but
  less than 1, round it to 1 as specified in text.
[SU]b[/SU] For the pathway, if hazardous constituent quantity is not
  adequately determined, assign a value as specified in the text; do not
  assign the value of 1.

For a migration pathway, if the hazardous constituent quantity is adequately determined (see section 2.4.2.1.1) for all sources (or all portions of sources and releases remaining after a removal action), assign the value from table 2-6 as the hazardous waste quantity factor value for the pathway. If the hazardous constituent quantity is not adequately determined for one or more sources (or one or more portions of sources or releases remaining after a removal action) assign a factor value as follows:

•    If any target for that migration pathway is subject to Level I or Level II concentrations (see section 2.5), assign either the value from table 2-6 or a value of 100, whichever is greater, as the hazardous waste quantity factor value for that pathway.

•    If none of the targets for that pathway is subject to Level I or Level II concentrations, assign a factor value as follows: -If there has been no removal action, assign either the value from table 2-6 or a value of 10, whichever is greater, as the hazardous waste quantity factor value for that pathway. -If there has been a removal action:
          -Determine values from table 2-6 with and without consideration of the removal action.

          -If the value that would be assigned from table 2-6 without consideration of the removal action would be 100 or greater, assign either the value from table 2-6 with consideration of the removal action or a value of 100, whichever is greater, as the hazardous waste quantity factor value for the pathway.

          -If the value that would be assigned from table 2-6 without consideration of the removal action would be less than 100, assign a value of 10 as the hazardous waste quantity factor value for the pathway.

For the soil exposure pathway, if the hazardous constituent quantity is adequately determined for all areas of observed contamination, assign the value from table 2-6 as the hazardous waste quantity factor value. If the hazardous constituent quantity is not adequately determined for one or more areas of observed contamination, assign either the value from table 2-6 or a value of 10, whichever is greater, as the hazardous waste quantity factor value.

2.4.3      Waste characteristics factor category value. Determine the waste characteristics factor category value as specified in section 2.4.3.1 for all pathways and threats, except the surface water-human food chain threat and the surface water-environmental threat. Determine the waste characteristics factor category value for these latter two threats as specified in section 2.4.3.2.

2.4.3.1      Factor category value. For the pathway (or threat) being evaluated, multiply the toxicity or combined factor value, as appropriate, from section 2.4.1.2 and the hazardous waste quantity factor value from section 2.4.2.2, subject to a maximum product of 1×10 ⁸. Based on this waste characteristics product, assign a waste characteristics factor category value to the pathway (or threat) from table 2-7.

         Table 2-7_Waste Characteristics Factor Category Values
------------------------------------------------------------------------
                                                                Assigned
                Waste characteristics product                    value
------------------------------------------------------------------------
0............................................................          0
Greater than 0 to less than 10...............................          1
10 to less than 1x10\2\......................................          2
1x10\2\ to less than 1x10\3\.................................          3
1x10\3\ to less than 1x10\4\.................................          6
1x10\4\ to less than 1x10\5\.................................         10
1x10\5\ to less than 1x10\6\.................................         18
1x10\6\ to less than 1x10\7\.................................         32
1x10\7\ to less than 1x10\8\.................................         56
1x10\8\ to less than 1x10\9\.................................        100
1x10\9\ to less than 1x10\10\................................        180
1x10\10\ to less than 1x10\11\...............................        320
1x10\11\ to less than 1x10\12\...............................        560
1x10\12\.....................................................      1,000
------------------------------------------------------------------------

2.4.3.2      Factor category value, considering bioaccumulation potential. For the surface water-human food chain threat and the surface water-environmental threat, multiply the toxicity or combined factor value, as appropriate, from section 2.4.1.2 and the hazardous waste quantity factor value from section 2.4.2.2, subject to:

•    A maximum product of 1×10 ¹², and

•    A maximum product exclusive of the bioaccumulation (or ecosystem bioaccumulation) potential factor of 1×10 ⁸.

Based on the total waste characteristics product, assign a waste characteristics factor category value to these threats from table 2-7.

2.5      Targets.

The types of targets evaluated include the following:

•    Individual (factor name varies by pathway and threat).

•    Human population.

•    Resources (these vary by pathway and threat).

•    Sensitive environments (included for all pathways except ground water migration).

The factor values that may be assigned to each type of target have the same range for each pathway for which that type of target is evaluated. The factor value for most types of targets depends on whether the target is subject to actual or potential contamination for the pathway and whether the actual contamination is Level I or Level II:

•    Actual contamination: Target is associated either with a sampling location that meets the criteria for an observed release (or observed contamination) for the pathway or with an observed release based on direct observation for the pathway (additional criteria apply for establishing actual contamination for the human food chain threat in the surface water migration pathway, see sections 4.1.3.3 and 4.2.3.3). sections 3 through 6 specify how to determine the targets associated with a sampling location or with an observed release based on direct observation. Determine whether the actual contamination is Level I or Level II as follows: -Level I:
          -Media-specific concentrations for the target meet the criteria for an observed release (or observed contamination) for the pathway and are at or above media-specific benchmark values. These benchmark values (see section 2.5.2) include both screening concentrations and concentrations specified in regulatory limits (such as Maximum Contaminant Level (MCL) values), or

          -For the human food chain threat in the surface water migration pathway, concentrations in tissue samples from aquatic human food chain organisms are at or above benchmark values. Such tissue samples may be used in addition to media-specific concentrations only as specified in sections 4.1.3.3 and 4.2.3.3.
-Level II:
          -Media-specific concentrations for the target meet the criteria for an observed release (or observed contamination) for the pathway, but are less than media-specific benchmarks. If none of the hazardous substances eligible to be evaluated for the sampling location has an applicable benchmark, assign Level II to the actual contamination at the sampling location, or

          -For observed releases based on direct observation, assign Level II to targets as specified in sections 3, 4, and 6, or

          -For the human food chain threat in the surface water migration pathway, concentrations in tissue samples from aquatic human food chain organisms, when applicable, are below benchmark values.
-If a target is subject to both Level I and Level II concentrations for a pathway (or threat), evaluate the target using Level I concentrations for that pathway (or threat).

•    Potential contamination: Target is subject to a potential release (that is, target is not associated with actual contamination for that pathway or threat).

Assign a factor value for individual risk as follows (select the highest value that applies to the pathway or threat):

•    50 points if any individual is exposed to Level I concentrations.

•    45 points if any individual is exposed to Level II concentrations.

•    Maximum of 20 points if any individual is subject to potential contamination. The value assigned is 20 multiplied by the distance or dilution weight appropriate to the pathway.

Assign factor values for population and sensitive environments as follows:

•    Sum Level I targets and multiply by 10. (Level I is not used for sensitive environments in the soil exposure and air migration pathways.)

•    Sum Level II targets.

•    Multiply potential targets by distance or dilution weights appropriate to the pathway, sum, and divide by 10. Distance or dilution weighting accounts for diminishing exposure with increasing distance or dilution within the different pathways.

•    Sum the values for the three levels.

In addition, resource value points are assigned within all pathways for welfare-related impacts (for example, impacts to agricultural land), but do not depend on whether there is actual or potential contamination.

2.5.1      Determination of level of actual contamination at a sampling location. Determine whether Level I concentrations or Level II concentrations apply at a sampling location (and thus to the associated targets) as follows:

•    Select the benchmarks applicable to the pathway (or threat) being evaluated.

•    Compare the concentrations of hazardous substances in the sample (or comparable samples) to their benchmark concentrations for the pathway (or threat), as specified in section 2.5.2.

•    Determine which level applies based on this comparison.

•    If none of the hazardous substances eligible to be evaluated for the sampling location has an applicable benchmark, assign Level II to the actual contamination at that sampling location for the pathway (or threat).

In making the comparison, consider only those samples, and only those hazardous substances in the sample, that meet the criteria for an observed release (or observed contamination) for the pathway, except: tissue samples from aquatic human food chain organisms may also be used as specified in sections 4.1.3.3 and 4.2.3.3 of the surface water-human food chain threat. If any hazardous substance is present in more than one comparable sample for the sampling location, use the highest concentration of that hazardous substance from any of the comparable samples in making the comparisons.

Treat sets of samples that are not comparable separately and make a separate comparison for each such set.

2.5.2      Comparison to benchmarks. Use the following media-specific benchmarks for making the comparisons for the indicated pathway (or threat):

•    Maximum Contaminant Level Goals (MCLGs) -- ground water migration pathway and drinking water threat in surface water migration pathway. Use only MCLG values greater than 0.

•    Maximum Contaminant Levels (MCLs) -- ground water migration pathway and drinking water threat in surface water migration pathway.

•    Food and Drug Administration Action Level (FDAAL) for fish or shellfish -- human food chain threat in surface water migration pathway.

•    EPA Ambient Water Quality Criteria (AWQC) for protection of aquatic life -- environmental threat in surface water migration pathway.

•    EPA Ambient Aquatic Life Advisory Concentrations (AALAC) -- environmental threat in surface water migration pathway.

•    National Ambient Air Quality Standards (NAAQS) -- air migration pathway.

•    National Emission Standards for Hazardous Air Pollutants (NESHAPs) -- air migration pathway. Use only those NESHAPs promulgated in ambient concentration units.

•    Screening concentration for cancer corresponding to that concentration that corresponds to the 10⁻⁶ individual cancer risk for inhalation exposures (air migration pathway) or for oral exposures (ground water migration pathway; drinking water and human food chain threats in surface water migration pathway; and soil exposure pathway).

•    Screening concentration for noncancer toxicological responses corresponding to the RfD for inhalation exposures (air migration pathway) or for oral exposures (ground water migration pathway; drinking water and human food chain threats in surface water migration pathway; and soil exposure pathway).

Select the benchmark(s) applicable to the pathway (or threat) being evaluated as specified in sections 3 through 6. Compare the concentration of each hazardous substance from the sampling location to its benchmark concentration(s) for that pathway (or threat). Use only those samples and only those hazardous substances in the sample that meet the criteria for an observed release (or observed contamination) for the pathway, except: tissue samples from aquatic human food chain organisms may be used as specified in sections 4.1.3.3 and 4.2.3.3. If the concentration of any applicable hazardous substance from any sample equals or exceeds its benchmark concentration, consider the sampling location to be subject to Level I concentrations for that pathway (or threat). If more than one benchmark applies to the hazardous substance, assign Level I if the concentration of the hazardous substance equals or exceeds the lowest applicable benchmark concentration.

If no hazardous substance individually equals or exceeds its benchmark concentration, but more than one hazardous substance either meets the criteria for an observed release (or observed contamination) for the sample (or comparable samples) or is eligible to be evaluated for a tissue sample (see sections 4.1.3.3 and 4.2.3.3), calculate the indices I and J specified below based on these hazardous substances.

For those hazardous substances that are carcinogens (that is, those having a carcinogen weight-of-evidence classification of A, B, or C), calculate an index I for the sample location as follows: $$

where:

Ci=Concentration of hazardous substance i in sample (or highest concentration of hazardous substance i from among comparable samples).

SCi=Screening concentration for cancer corresponding to that concentration that corresponds to its 10⁻⁶ individual cancer risk for applicable exposure (inhalation or oral) for hazardous substance i.

n=Number of applicable hazardous substances in sample (or comparable samples) that are carcinogens and for which an SCi is available.

For those hazardous substances for which an RfD is available, calculate an index J for the sample location as follows: $$

where:

Cj=Concentration of hazardous substance j in sample (or highest concentration of hazardous substance j from among comparable samples).

CRj=Screening concentration for noncancer toxicological responses corresponding to RfD for applicable exposure (inhalation or oral) for hazardous substance j.

m=Number of applicable hazardous substances in sample (or comparable samples) for which a CRj is available.

If either I or J equals or exceeds 1, consider the sampling location to be subject to Level I concentrations for that pathway (or threat). If both I and J are less than 1, consider the sampling location to be subject to Level II concentrations for that pathway (or threat). If, for the sampling location, there are sets of samples that are not comparable, calculate I and J separately for each such set, and use the highest calculated values of I and J to assign Level I and Level II.

See sections 7.3.1 and 7.3.2 for criteria for determining the level of contamination for radioactive substances.

Evaluate the ground water migration pathway based on three factor categories: likelihood of release, waste characteristics, and targets. Figure 3-1 indicates the factors included within each factor category.

Determine the ground water migration pathway score (Sgw) in terms of the factor category values as follows: $$

where:

LR=Likelihood of release factor category value.

WC=Waste characteristics factor category value.

T=Targets factor category value.

SF=Scaling factor.

Table 3-1 outlines the specific calculation procedure.

Calculate a separate ground water migration pathway score for each aquifer, using the factor category values for that aquifer for likelihood of release, waste characteristics, and targets. In doing so, include both the targets using water from that aquifer and the targets using water from all overlying aquifers through which the hazardous substances would migrate to reach the aquifer being evaluated. Assign the highest ground water migration pathway score that results for any aquifer as the ground water migration pathway score for the site.

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           Table 3-1_Ground Water Migration Pathway Scoresheet
------------------------------------------------------------------------
      Factor categories and factors        Maximum value  Value assigned
------------------------------------------------------------------------
Likelihood of Release to an Aquifer:
   1. Observed Release..................             550             ___
   2. Potential to Release:
        2a. Containment.................              10             ___
        2b. Net Precipitation...........              10             ___
        2c. Depth to Aquifer............               5             ___
        2d. Travel Time.................              35             ___
        2e. Potential to Release [lines              500             ___
         2a(2b+2c+2d)]..................
   3. Likelihood of Release (higher of               550             ___
   lines 1 and 2e)......................
Waste Characteristics:
   4. Toxicity/Mobility.................             (a)             ___
   5. Hazardous Waste Quantity..........             (a)             ___
   6. Waste Characteristics.............             100             ___
Targets:
   7. Nearest Well......................              50             ___
   8. Population:
        8a. Level I Concentrations......             (b)             ___
        8b. Level II Concentrations.....             (b)             ___
        8c. Potential Contamination.....             (b)             ___
        8d. Population (lines 8a+8b+8c).             (b)             ___
   9. Resources.........................               5             ___
  10. Wellhead Protection Area..........              20             ___
  11. Targets (lines 7+8d+9+10).........             (b)             ___
Ground Water Migration Score for an
 Aquifer:
  12. Aquifer Score [(lines 3 x 6 x 11)/             100             ___
   82,500] [SU]c[/SU]...................
Ground Water Migration Pathway Score:
  13. Pathway Score (S[INF]gw[/INF]),                100            ___
   (highest value from line 12 for all
   aquifers evaluated)[SU]c[/SU]........
------------------------------------------------------------------------
[SU]a[/SU] Maximum value applies to waste characteristics category.
[SU]b[/SU] Maximum value not applicable.
[SU]c[/SU] Do not round to nearest integer.

3.0.1      General considerations

3.0.1.1      Ground water target distance limit. The target distance limit defines the maximum distance from the sources at the site over which targets are evaluated. Use a target distance limit of 4 miles for the ground water migration pathway, except when aquifer discontinuities apply (see section 3.0.1.2.2). Furthermore, consider any well with an observed release from a source at the site (see section 3.1.1) to lie within the target distance limit of the site, regardless of the well's distance from the sources at the site.

For sites that consist solely of a contaminated ground water plume with no identified source, begin measuring the 4-mile target distance limit at the center of the area of observed ground water contamination. Determine the area of observed ground water contamination based on available samples that meet the criteria for an observed release.

3.0.1.2      Aquifer boundaries. Combine multiple aquifers into a single hydrologic unit for scoring purposes if aquifer interconnections can be established for these aquifers. In contrast, restrict aquifer boundaries if aquifer discontinuities can be established.

3.0.1.2.1      Aquifer interconnections. Evaluate whether aquifer interconnections occur within 2 miles of the sources at the site. If they occur within this 2-mile distance, combine the aquifers having interconnections in scoring the site. In addition, if observed ground water contamination attributable to the sources at the site extends beyond 2 miles from the sources, use any locations within the limits of this observed ground water contamination in evaluating aquifer interconnections. If data are not adequate to establish aquifer interconnections, evaluate the aquifers as separate aquifers.

3.0.1.2.2      Aquifer discontinuities. Evaluate whether aquifer discontinuities occur within the 4-mile target distance limit. An aquifer discontinuity occurs for scoring purposes only when a geologic, topographic, or other structure or feature entirely transects an aquifer within the 4-mile target distance limit, thereby creating a continuous boundary to ground water flow within this limit. If two or more aquifers can be combined into a single hydrologic unit for scoring purposes, an aquifer discontinuity occurs only when the structure or feature entirely transects the boundaries of this single hydrologic unit.

When an aquifer discontinuity is established within the 4-mile target distance limit, exclude that portion of the aquifer beyond the discontinuity in evaluating the ground water migration pathway. However, if hazardous substances have migrated across an apparent discontinuity within the 4-mile target distance limit, do not consider this to be a discontinuity in scoring the site.

3.0.1.3       Karst aquifer. Give a karst aquifer that underlies any portion of the sources at the site special consideration in the evaluation of two potential to release factors (depth to aquifer in section 3.1.2.3 and travel time in section 3.1.2.4), one waste characteristics factor (mobility in section 3.2.1.2), and two targets factors (nearest well in section 3.3.1 and potential contamination in section 3.3.2.4).

3.1      Likelihood of release. For an aquifer, evaluate the likelihood of release factor category in terms of an observed release factor or a potential to release factor.

3.1.1      Observed release. Establish an observed release to an aquifer by demonstrating that th