Title 40

PART 300 APPENDIX A



Appendix A to Part 300 - The Hazard Ranking System

40:30.0.1.1.1.12.1.2.1 : Appendix A

Appendix A to Part 300 - The Hazard Ranking System Table of Contents 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 and Subsurface Intrusion Pathway. 5.0.1 Exposure components. 5.1 Soil exposure component. 5.1.0 General considerations. 5.1.1 Resident population threat. 5.1.1.1 Likelihood of exposure. 5.1.1.2 Waste characteristics. 5.1.1.2.1 Toxicity. 5.1.1.2.2 Hazardous waste quantity. 5.1.1.2.3 Calculation of waste characteristics factor category value. 5.1.1.3 Targets. 5.1.1.3.1 Resident individual. 5.1.1.3.2 Resident population. 5.1.1.3.2.1 Level I concentrations. 5.1.1.3.2.2 Level II concentrations. 5.1.1.3.2.3 Calculation of resident population factor value. 5.1.1.3.3 Workers. 5.1.1.3.4 Resources. 5.1.1.3.5 Terrestrial sensitive environments. 5.1.1.3.6 Calculation of resident population targets factor category value. 5.1.1.4 Calculation of resident population threat score. 5.1.2 Nearby population threat. 5.1.2.1 Likelihood of exposure. 5.1.2.1.1 Attractiveness/accessibility. 5.1.2.1.2 Area of contamination. 5.1.2.1.3 Likelihood of exposure factor category value. 5.1.2.2 Waste characteristics. 5.1.2.2.1 Toxicity. 5.1.2.2.2 Hazardous waste quantity. 5.1.2.2.3 Calculation of waste characteristics factor category value. 5.1.2.3 Targets. 5.1.2.3.1 Nearby individual. 5.1.2.3.2 Population within 1 mile. 5.1.2.3.3 Calculation of nearby population targets factor category value. 5.1.2.4 Calculation of nearby population threat score. 5.1.3 Calculation of soil exposure component score. 5.2 Subsurface intrusion component. 5.2.0 General considerations. 5.2.1 Subsurface intrusion component. 5.2.1.1 Likelihood of exposure. 5.2.1.1.1 Observed exposure. 5.2.1.1.2 Potential for exposure. 5.2.1.1.2.1 Structure containment. 5.2.1.1.2.2 Depth to contamination. 5.2.1.1.2.3 Vertical migration. 5.2.1.1.2.4 Vapor migration potential. 5.2.1.1.2.5 Calculation of potential for exposure factor value. 5.2.1.1.3 Calculation of likelihood of exposure factor category value. 5.2.1.2 Waste characteristics. 5.2.1.2.1 Toxicity/degradation. 5.2.1.2.1.1 Toxicity. 5.2.1.2.1.2 Degradation. 5.2.1.2.1.3 Calculation of toxicity/degradation factor value. 5.2.1.2.2 Hazardous waste quantity. 5.2.1.2.3 Calculation of waste characteristics factor category value. 5.2.1.3 Targets. 5.2.1.3.1 Exposed individual. 5.2.1.3.2 Population. 5.2.1.3.2.1 Level I concentrations. 5.2.1.3.2.2 Level II concentrations. 5.2.1.3.2.3 Population within area(s) of subsurface contamination. 5.2.1.3.2.4 Calculation of population factor value. 5.2.1.3.3 Resources. 5.2.1.3.4 Calculation of targets factor category value. 5.2.2 Calculation of subsurface intrusion component score. 5.3 Calculation of the soil exposure and subsurface intrusion 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/observed exposure. 7.1.2 Potential to release/potential for exposure. 7.2 Waste characteristics. 7.2.1 Human toxicity. 7.2.2 Ecosystem toxicity. 7.2.3 Persistence/degradation. 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. 7.3.3 Weighting of targets within an area of subsurface contamination. List of Figures Figure number 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 the soil exposure and subsurface intrusion pathway. 6-1 Overview of air migration pathway. 6-2 Particulate migration potential factor values. 6-3 Particulate mobility factor values. List of Tables Table number 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 component scoresheet. 5-2 Hazardous waste quantity evaluation equations for soil exposure component. 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. 5-11 Subsurface intrusion component scoresheet. 5-12 Structure containment. 5-13 Depth to contamination. 5-14 Effective porosity/permeability of geological materials. 5-15 Vertical migration factor values. 5-16 Values for vapor pressure and Henry's constant. 5-17 Vapor migration potential factor values for a hazardous substance. 5-18 Degradation factor value table. 5-19 Hazardous waste quantity evaluation equations for subsurface intrusion component. 5-20 Health-based benchmarks for hazardous substances in the subsurface intrusion component. 5-21 Weighting factor values for populations within an area of subsurface contamination. 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. 1.0 Introduction

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.

1.1 Definitions

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)/National Recommended Water Quality Criteria: EPA's maximum acute (Criteria Maximum Concentration or CMC) or chronic (Criterion Continuous Concentration or CCC) 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).

Channelized flow: Natural geological or manmade features such as karst, fractures, lava tubes, and utility conduits (e.g., sewer lines), which allow ground water and/or soil gas to move through the subsurface environment more easily.

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) and Reference Concentration (RfC) 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.

Crawl space: The enclosed or semi-enclosed area between a regularly occupied structure's foundation (e.g., pier and beam construction) and the ground surface. Crawl space samples are collected to determine the concentration of hazardous substances in the air beneath a regularly occupied structure.

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 pathway, ground water migration pathway, and the soil exposure component of the soil exposure and subsurface intrusion pathway 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 for assigning surface water persistence: Biodegradation, hydrolysis, photolysis, radioactive decay, and volatilization. The HRS considers two decay processes for assigning subsurface intrusion degradation: Biodegradation and hydrolysis.

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 exposure and subsurface intrusion, 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-m 3/mol)].

Hydrolysis: Chemical reaction of a substance with water.

Indoor air: The air present within a structure.

Inhalation Unit Risk (IUR): The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent (i.e., hazardous substance) at a concentration of 1µg/m 3 in air.

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/m 3)] 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.

Non-Aqueous Phase Liquid (NAPL): Contaminants and substances that are water-immiscible liquids composed of constituents with varying degrees of water solubility.

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).

Preferential subsurface intrusion pathways: Subsurface features such as animal burrows, cracks in walls, spaces around utility lines, or drains through which a hazardous substance moves more easily into a regularly occupied structure.

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 concentration (RfC): An estimate of a continuous inhalation exposure to the human population that is likely to be without an appreciable risk of deleterious effects during a lifetime.

Reference dose (RfD): An estimate of a daily oral exposure to the human population that is likely to be without an appreciable risk of deleterious effects during a lifetime.

Regularly occupied structures: Structures with enclosed air space, where people either reside, attend school or day care, or work on a regular basis, or that were previously occupied but vacated due to a site-related hazardous substance(s). This also includes resource structures (e.g., library, church, tribal structure).

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 concentration for inhalation exposures or reference dose for oral exposures, as appropriate, and, if the substance is a human carcinogen with either a weight-of-evidence classification of A, B, or C, or a weight-of-evidence classification of carcinogenic to humans, likely to be carcinogenic to humans or suggestive evidence of carcinogenic potential, to that concentration that corresponds to its 10−6 individual lifetime excess cancer risk for inhalation exposures or for oral exposures, as appropriate.

Shallow ground water: The uppermost saturated zone, typically unconfined.

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 either a weight-of-evidence classification of A, B, or C, or a weight-of-evidence classification of carcinogenic to humans, likely to be carcinogenic to humans or having suggestive evidence of carcinogenic potential. [(mg/kg-day)−1 for non-radioactive substances and (pCi)−1 for radioactive substances].

Soil gas: The gaseous elements and compounds in the small spaces between particles of soil.

Soil porosity: The degree to which the total volume of soil is permeated with pores or cavities through which fluids (including air or gas) can move. It is typically calculated as the ratio of the pore spaces within the soil to the overall volume of the soil.

Source: Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminated from migration of a hazardous substance. Sources do not include those volumes of air, ground water, surface water, or surface water sediments that have become contaminated by migration, except: In the case of either a ground water plume with no identified source or contaminated surface water sediments with no identified source, the plume or contaminated sediments may be considered a source.

Subslab: The area immediately beneath a regularly occupied structure with a basement foundation or a slab-on-grade foundation. Subslab samples are collected to determine the concentration of hazardous substances in the soil gas beneath a home or building.

Subsurface intrusion: The migration of hazardous substances from the unsaturated zone and/or ground water into overlying structures.

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

Unit risk: The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent (i.e., hazardous substance) at a concentration of 1 µg/L in water, or 1 µg/m 3 in air.

Unsaturated zone: The portion of subsurface between the land surface and the zone of saturation. It extends from the ground surface to the top of the shallowest ground water table (excluding localized or perched water).

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. The EPA weight-of-evidence, depending on the date EPA updated the profile, includes either the groupings:

• 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.

Or the descriptors:

• Carcinogenic to humans.

• Likely to be carcinogenic to humans.

• Suggestive evidence of carcinogenic potential.

• Inadequate information to assess carcinogenic potential.

• Not likely to be carcinogenic to humans.

2.0 Evaluations Common to Multiple Pathways

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 and Subsurface Intrusion (Ssessi).

• Air Migration (Sa).

The ground water and air migration pathways use single threat evaluations, while the surface water migration and soil exposure and subsurface intrusion 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 components are evaluated for the soil exposure and subsurface intrusion pathway: Soil exposure and subsurface intrusion. The soil exposure component evaluates two threats: Resident population and nearby population, and the subsurface intrusion component is a single threat evaluation.

The HRS is structured to provide a parallel evaluation for each of these pathways, components, 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 (component or threat) score is the product of three “factor categories”: Likelihood of release, waste characteristics, and targets. (The soil exposure and subsurface intrusion 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 and subsurface intrusion pathway, areas of observed contamination, areas of observed exposure and/or areas of subsurface contamination [see sections 5.1.0 and 5.2.0]). - Identifying hazardous substances associated with each source (or area of observed contamination, or observed exposure, or subsurface contamination). - Identifying hazardous substances available to a pathway.

Table 2-1 - Sample Pathway Scoresheet

Factor category Maximum value Value
assigned
Likelihood of Release
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 7c) 50
8. Population ( b)
8a. Level I ( b)
8b. Level II ( b)
8c. Potential Contamination ( b)
8d. Total Population (lines 8a+8b+8c)
9. Resources 5
10. Sensitive Environments ( b)
10a. Actual Contamination ( b)
10b. Potential Environments ( 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