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6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 300

[EPA-HQ-SFUND-2010-1086; FRL-_9925-69-OSWER] 

RIN 2050-AG67

Addition of a Subsurface Intrusion Component to the Hazard Ranking System

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

______________________________________________________________________

SUMMARY: The U.S. Environmental Protection Agency (EPA) is proposing to add a 

subsurface intrusion (SsI) component to the Hazard Ranking System (HRS) which is the principal 

mechanism that EPA uses to evaluate sites for placement on the National Priorities List (NPL). 

The subsurface intrusion component (this addition) would expand the number of available options 

for EPA and state and tribal organizations performing work on behalf of EPA to evaluate 

potential threats to public health from releases of hazardous substances, pollutants, or 

contaminants. This addition will allow an HRS evaluation to directly consider human exposure to 

hazardous substances, pollutants, or contaminants that enter regularly occupied structures through 

subsurface intrusion in assessing a site’s relative risk, and thus, enable sites with subsurface 

intrusion contamination to be evaluated for placement on the NPL. The agency is not considering 

changes to the remainder of the HRS except for minor updates reflecting changes in terminology. 

DATES: Comments shall be submitted (postmarked) on or before [INSERT DATE XX DAYS 

FROM DATE OF PUBLICATION IN THE FEDERAL REGISTER].
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ADDRESSES: Submit your comments, identified by the appropriate Docket number, by one of 

the following methods:

• www.regulations.gov: Follow the online instructions for submitting comments.

• Email: superfund.docket@epa.gov

• Mail: Mail comments (no facsimiles or tapes) to Docket Coordinator, Headquarters; U.S. 

Environmental Protection Agency; CERCLA Docket Office; (Mail Code 5305T); 1200 

Pennsylvania Avenue, NW; Washington, DC 20460

• Hand Delivery or Express Mail: Send comments (no facsimiles or tapes) to Docket 

Coordinator, Headquarters; U.S. Environmental Protection Agency; CERCLA Docket 

Office; 1301 Constitution Avenue, NW; EPA West, Room 3334, Washington, DC 20004. 

Such deliveries are accepted only during the Docket’s normal hours of operation (8:30 

a.m. to 4:30 p.m., Monday through Friday, excluding federal holidays). 

Instructions: Direct your comments to Docket number EPA-HQ-SFUND-2010-1086. EPA's 

policy is that all comments received will be included in the public Docket without change and 

may be made available online at www.regulations.gov, including any personal information 

provided, unless the comment includes information claimed to be Confidential Business 

Information (CBI) or other information disclosure is restricted by statute. Do not submit 

information that you consider to be CBI or otherwise protected, through www.regulations.gov or 

email. The www.regulations.gov website is an “anonymous access” system; that means EPA will 

not know your identity or contact information unless you provide it in the body of your comment. 

If you send an email comment directly to EPA without going through www.regulations.gov, your 

email address will be automatically captured and included as part of the comment that is placed in 

the public Docket and made available on the Internet. If you submit an electronic comment, EPA 

recommends that you include your name and other contact information in the body of your 
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comment and with any disk or CD-ROM you submit. If EPA cannot read your comment due to 

technical difficulties and cannot contact you for clarification, EPA may not be able to consider 

your comment. Electronic files should avoid the use of special characters, any form of encryption, 

and be free of any defects or viruses. For additional information about EPA’s public docket, 

visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm. 

Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although 

listed in the index, some information is not publicly available, e.g., CBI or other information 

disclosure is restricted by statute. Certain other material, such as copyrighted material, will be 

publicly available only in hard copy. Publicly available docket materials are available either 

electronically in http://www.regulations.gov or in hard copy at the CERCLA Docket Office; 1301 

Constitution Ave., NW; EPA West, Room 3334, Washington, DC. The Public Reading Room is 

open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding federal holidays. The 

telephone number for the Public Reading Room is (202) 566-1744 and the telephone number for 

the CERCLA Docket Office is (202) 566-0276.

 

FOR FURTHER INFORMATION CONTACT: Terry Jeng, phone: (703) 603-8852, email: 

jeng.terry@epa.gov, Site Assessment and Remedy Decisions Branch, Assessment and 

Remediation Division, Office of Superfund Remediation and Technology Innovation (Mail Code 

5204P), U.S. Environmental Protection Agency, 1200 Pennsylvania Avenue, NW, Washington, 

DC 20460; or the Superfund Hotline, phone (800) 424-9346 or (703) 412-9810 in the 

Washington, DC metropolitan area.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. General Information 
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A. What is EPA Seeking Comment On?

B. How Does this Action Apply to Me?

C. Tips for Preparing Your Comments to EPA

II. Statutory Authority

III. Background

A. Why is EPA Proposing an Addition to the HRS?

B. What is the History of the HRS?

C. What is the Impact of this Proposed Rule?

1.Impact on Current Cleanup Programs, Resources and Cost

2.Children’s Environmental Health and Environmental Justice

IV. Hazard Ranking System 

A. Purpose

B. Structure 

V. Approach to HRS Addition

A. General Approach

1.What is the Need for Regulatory Action on the HRS?

2.What Alternative Regulatory Options to this Action Were Considered by EPA?

3.What Public Outreach Activities Did EPA Conduct?

4.What Peer Review Process Did EPA Use?

5.How Did EPA Select the Approach for Including the Addition in the HRS?

B. Technical Considerations to Maintaining the Current HRS Structure and Algorithm

1. Maintaining the Current Ground Water, Surface Water, and Air Migration 

Pathways 

2. Addition of the New Component to Restructure and Rename the Soil Exposure 

Pathway 

C. Supporting Materials
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VI. Discussion of the Proposed SsI Addition to the HRS

A. Addition Within a Restructured Soil Exposure Pathway 

B. SsI Component Addition

1. New Definitions

2. Delineation of Areas of Subsurface Intrusion

a. Area of Observed Exposure (AOE)

b. Area of Subsurface Contamination (ASC)

c. Other Area of Subsurface Intrusion Considered: Potential Migration Zone

3. Likelihood of Exposure

a. Observed Exposure 

b. Potential for Exposure

c. Calculation of the Likelihood of Exposure Factor Category Value

4. Waste Characteristics

a. Toxicity/Degradation

b. Hazardous Waste Quantity

c. Calculation of the Waste Characteristics Factor Category Value

5. Targets

a. Identification of Eligible Targets 

b. Exposed Individual and Levels of Exposure

c. Population

d. Resources

e. Calculation of the Targets Factor Category Value

6. Calculation and Incorporation of the SsI Component Score into the HRS Site 

Score

a. Calculation of the SsI Component Score
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b. Incorporation of the SsI Component Score into the Soil Exposure and 

Subsurface Intrusion Pathway Score

c. Incorporation of the Soil Exposure and Subsurface Intrusion Pathway Score 

into a Site Score

7. Example Site Scoring Scenarios

VII. Summary of Proposed Updates to the HRS

VIII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review

B. Paperwork Reduction Act (PRA)

C. Regulatory Flexibility Act (RFA)

D. Unfunded Mandates Reform Act (UMRA)

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments

G. Executive Order 13045: Protection of Children from Environmental Health Risks and 

Safety Risks

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect 

Energy Supply, Distribution or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

K. Executive Order 12580: Superfund Implementation

I. General Information

A. What is EPA Seeking Comment On?
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EPA is proposing an addition of one new component to one part of the current Hazard Ranking 

System (HRS). No major structural changes to other parts of the HRS are proposed. EPA is 

seeking comments on the addition of the subsurface intrusion component to the HRS. Comments 

on unmodified parts of the HRS are not being requested and will not be considered if submitted. 

B. How Does this Action Apply to Me?

This action proposes an addition to the HRS. The HRS is used for evaluating the relative potential 

risk posed by the uncontrolled release, or potential release, of hazardous substances to human 

health or the environment. This addition will enable EPA to identify risks posed by subsurface 

intrusion of hazardous substances into regularly occupied structures for all populations who live 

and work in areas where the subsurface environment may create exposures. The agency considers 

that including the evaluation of subsurface intrusion in the HRS serves the public interest by 

widening EPA’s ability to evaluate these threats.

This proposed regulatory change expands the available options for EPA and organizations 

performing work on behalf of EPA (state and tribal partners) to evaluate potential threats to 

public health and the environment from subsurface intrusion contamination. State and tribal 

partners may receive financial assistance from EPA to evaluate sites through a Cooperative 

Agreement. EPA and states or tribes collaborate closely throughout the Cooperative Agreement 

process, particularly when identifying sites to be evaluated and establishing priorities for 

performing evaluations. As necessary, sites where subsurface intrusion threats exist may be 

evaluated using the HRS and, if warranted, proposed for placement on the NPL. EPA does not 

expect that this proposed change will result in additional site assessments being conducted per 

year or placement of more sites on the NPL per year. Rather, given potentially limited budgets 

and the possibility of increased costs for an SsI site assessment, EPA may conduct fewer 
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assessments per year. The pipeline of sites will be reviewed to identify those sites that pose the 

highest risk and prioritized accordingly. This is not a change to how EPA currently evaluates and 

prioritizes sites for the NPL; EPA will simply have an additional mechanism to address sites that 

pose the greatest risk. Because assessing the worst sites first is a priority, the regions will 

continue to identify the sites posing the highest risk or potential risk and develop a strategy to 

assess those sites in a timely manner, while balancing their other site assessment needs. 

The addition of a subsurface intrusion component to the HRS affirms that EPA is fulfilling its 

regulatory requirements by ensuring "to the maximum extent feasible, that the hazard ranking 

system accurately assesses the relative degree of risk to human health and the environment posed 

by sites and facilities subject to review." 42 USC 9605(c)(1), as mandated by the Superfund 

Amendments and Reauthorization Act (SARA) amendments to the Comprehensive 

Environmental Response, Compensation, and Liability Act (CERCLA).

This proposed addition is necessary because no present authority consistently and 

comprehensively addresses subsurface intrusion contamination across all non-federal potential 

sites, particularly when subsurface intrusion is the key exposure pathway. While most states have 

identified sites with subsurface intrusion contamination issues, not all states have subsurface 

intrusion programs, and states with subsurface intrusion remediation programs vary in their 

authority, resources, and remediation criteria. A redirection of resources available through 

Cooperative Agreement funding is expected to provide for greater national consistency in the 

identification and evaluation of subsurface intrusion sites. 

Additionally, EPA finalized the OSWER Technical Guide for Assessing and Mitigating the 

Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, in June 2015. This guide 

and this proposed addition to the HRS would further the agency’s efforts to establish national 
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consistency in evaluating vapor intrusion threats by enabling EPA to use remedial authority under 

CERCLA. 

This proposed regulatory change does not affect the status of sites currently on or proposed to be 

added to the NPL. 

C. Tips for Preparing Your Comments to EPA 

When submitting comments, remember to:

• Identify the rulemaking by Docket number EPA-HQ-SFUND-2010-1086 and other 

identifying information (subject heading, Federal Register date and page number).

• Explain why you agree or disagree, suggest alternatives, and substitute language for your 

requested changes.

• Describe any assumptions and provide any technical information and/or data that you 

used.

• If you estimate potential costs or burdens, explain how you arrived at your estimate in 

sufficient detail to allow for it to be reproduced.

• Provide specific examples to illustrate your concerns and suggest alternatives.

• Explain your views as clearly as possible.

• Make sure to submit your comments by the comment period deadline identified.

II. Statutory Authority

The authority for these proposed technical modifications to the HRS (40 CFR 300, Appendix A) 

is in section 105(a)(8)(A) of the Comprehensive Environmental Response, Compensation, and 
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Liability Act (CERCLA) enacted in 1980. Under this law, the National Oil and Hazardous 

Substances Pollution Contingency Plan (NCP) (40 CFR 300) must include criteria for 

determining priorities among releases or threatened releases for the purpose of taking remedial or 

removal actions. In 1986, Congress passed the Superfund Amendments and Reauthorization Act 

(SARA) (Pub. L. 99 499), which added section 105(c)(1) to CERCLA, requiring EPA to amend 

the HRS to assure "to the maximum extent feasible, that the hazard ranking system accurately 

assesses the relative degree of risk to human health and the environment posed by sites and 

facilities subject to review." Furthermore, CERCLA section 115 authorizes EPA to promulgate 

any regulations necessary to carry out the provisions of CERCLA. 

III. Background

EPA is proposing this addition to protect human health from the threat posed by subsurface 

intrusion. By adding this component to the HRS, EPA will be able to consider subsurface 

intrusion threats when evaluating sites for placement on the NPL and implement the requirements 

of CERCLA and the National Oil and Hazardous Substances Pollution Contingency Plan (NCP). 

This proposed addition is a technical modification to the current HRS that will allow EPA and its 

partners to more comprehensively address the releases of hazardous substances into the 

environment.

A. Why is EPA Proposing an Addition to the Hazard Ranking System?

Contaminant subsurface intrusion1 is defined as the migration of hazardous substances, 

 1 Subsurface intrusion, for the purposes of this preamble, refers to the intrusion of hazardous substances 

from the subsurface into a structure.
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pollutants, or contaminants2 from the subsurface environment, or more specifically, the surficial 

ground water into overlying structures and/or the unsaturated zone. Subsurface intrusion can 

result in people being exposed to harmful levels of hazardous substances and cause negative 

health effects. While subsurface intrusion can take multiple forms, the most common form of 

subsurface intrusion is vapor intrusion. There are several reasons why EPA is proposing today’s 

addition to the HRS.

First, the current HRS (40 CFR 300, Appendix A), promulgated December 14, 1990 (hereafter 

referred to as the current HRS), discussed in more detail in section IV of this preamble, does not 

consider the threat posed by subsurface intrusion in its evaluation of relative risk posed by a site; 

therefore, it does not provide a complete assessment of the relative risk that a site may pose to the 

public. The existing pathways used to evaluate threats posed by hazardous substances do not 

include those entering a regularly occupied structure from the subsurface. For example, the 

ground water migration pathway evaluates the threat posed by contaminated ground water if there 

is an indication that ground water is being consumed. Similarly, the soil exposure pathway 

evaluates the threat posed by contaminated surfaces (e.g., surface soils) if there is an indication of 

human exposure. The air migration pathway considers the threat posed by hazardous substances 

released to atmospheric air (ambient air), but does not address indoor air, and has no subsurface 

component. The surface water migration pathway does not cover subsurface intrusion as it only 

considers the threat posed by contaminated surface water bodies. 

In fact, in a May 2010 report3

, the Government Accountability Office (GAO) concluded that if 

 2 For the purpose of this preamble, the term “hazardous substances, pollutants or contaminants” will be 

referred to simply as “hazardous substances.” See section 1.1, of the current HRS for the definition of a 

hazardous substance.

3 EPA’s Estimated Costs to Remediate Existing Sites Exceed Current Funding Levels, and More Sites are 

Expected to Be Added to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-

380, May 2010.
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vapor intrusion sites “are not assessed and, if needed, listed on the NPL, some seriously 

contaminated hazardous waste sites with unacceptable human exposure may not otherwise be 

cleaned up.” The GAO recommended that EPA consider vapor intrusion as part of the NPL 

process; EPA agreed with the GAO recommendation. With the addition of a subsurface intrusion 

component, a site with vapor intrusion may qualify for the NPL, whereas presently the site may 

not have qualified using the threats evaluated in the current HRS. Therefore, without this 

addition, EPA may not be identifying the highest priority sites, that is, the sites posing the 

greatest risk to human health and the environment.

Second, EPA is offering today’s proposal because of the substantial public support for this action. 

EPA conducted outreach activities to determine the level of interest and support from the public. 

This included a Notice of Opportunity for Public Input (76 FR 5370, January 31, 2011) and four 

public listening sessions held across the country. More than 40 written comments, from a diverse 

group of private citizens, businesses, states, Native American tribes, environmental action groups, 

and other governmental agencies, were received during the public comment period. Of the public 

who attended the listening sessions and provided comments, the majority were supportive of the 

addition of a subsurface intrusion component to the HRS. In addition, five states and two tribes 

submitted comments—all in support of the addition. The Association of State and Territorial 

Solid Waste Management Officials (ASTSWMO) compiled and presented input from 14 states—

all but one favoring the addition of subsurface intrusion to the HRS. The comments opposing the 

HRS addition were, in general, from industry representatives. 

Third, to support development of this proposal, EPA evaluated the need for this proposed addition 

to the current HRS by identifying the scope of the subsurface intrusion contamination problem. 

These efforts to identify and classify sites that may pose a subsurface intrusion threat have 

resulted in the identification of 1,073 sites that may or may not qualify for the NPL but are 
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suspected of having vapor intrusion issues. Many of the sites in this inventory are currently listed 

in EPA’s Superfund Enterprise Management System4 (SEMS). Of the 1,073 identified sites:

• 328 sites are identified as having a suspected subsurface intrusion threat based on SEMS 

and Agency for Toxic Substances and Disease Registry (ATSDR) key word searches, as 

well as EPA or state self-identification, but for which no sampling data were obtained

• 532 sites are identified as having characteristics or evidence that indicate subsurface 

intrusion (e.g., volatile hazardous substance in ground water) may have occurred or will 

occur. 

• 202 sites are identified as having a subsurface intrusion threat documented by subslab, 

crawl space, or indoor air samples but insufficient HRS-required evaluation factors to 

qualify for the NPL.

• 11 sites are identified as having a subsurface intrusion threat with documented actual 

exposure of a sufficient number of targets and sufficient other HRS-required evaluation 

factors to suggest the site may qualify for the NPL.

EPA is also considering sites with another form of subsurface intrusion, namely, intrusion of 

contaminated ground water into regularly occupied structures—which is an emerging issue. For 

example, a site was discovered where shallow (surficial) ground water contaminated with 

chromium had intruded into residential basements and after the water receded, or evaporated, a 

precipitate of chromium remained as a residue. The presence of this residue posed a significant 

threat to public health; however, the site could not be evaluated under the current HRS due to the 

lack of a mechanism to evaluate human exposure resulting from intrusion of contaminated ground 

water (subsurface intrusion contamination). The only viable option to place the site on the NPL 

 4 This information was previously stored in a predecessor database called the Comprehensive 

Environmental Response, Compensation and Liability Information System (CERCLIS).
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was to rely on ATSDR to make a determination that the exposure at the site posed a significant 

threat to public health. The decision to include sites on the NPL based on a determination by the 

ATSDR is made infrequently because the HRS is the primary mechanism for placing a site on the 

NPL.

EPA regional site assessment programs have identified 7 additional sites where intrusion of 

contaminated ground water is a potential issue and the related threat cannot be evaluated using 

the current HRS. Under the proposed SsI addition, ground water intrusion would be evaluated 

using current conditions, which may involve situations where metals have precipitated from water 

or where volatile substances have entered a structure via infiltrating ground water. 

As EPA further explores this emerging issue, the agency considers it likely that other ground 

water intrusion sites requiring evaluation will be identified. The inventory of sites, identified by 

EPA, with a possible threat from contaminated vapor or ground water intruding into overlying 

regularly occupied structures is not representative of the magnitude of the potential scope of sites 

with subsurface intrusion contamination. EPA identified these sites based on currently available 

information to initially assess the subsurface intrusion problem. In the case of vapor intrusion, 

certain states undertook comprehensive efforts to identify and evaluate subsurface intrusion 

threats, which resulted in the identification of a proportionately higher number of sites with 

potential vapor intrusion problems in those states. In the case of ground water intrusion, the issue 

is still emerging. For these reasons, EPA recognizes that a degree of inherent uncertainty is 

associated with compiling an inventory of sites with potential subsurface intrusion problems and 

that additional analysis is necessary, especially in cases where little information exists. See 

Appendix A of the Technical Support Document for this proposed addition (Proposal TSD) for 

the inventory of vapor intrusion sites. As additional information is gathered and new sites are 

added to SEMS and undergo the site assessment process, the number of sites with subsurface 
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intrusion threats is likely to change. Nevertheless, the aforementioned illustrates that there 

currently exists at least 1,073 sites that have significant actual or potential human exposure due to 

subsurface intrusion, but because of the shortcomings of the current HRS, cannot be identified as 

high priority releases and, as necessary, included on the NPL. 

It is also important to emphasize that the inventory of sites compiled (where subsurface intrusion 

has been identified as a possible issue) does not represent a list of sites that will be placed on the 

NPL. EPA recognizes that, in many instances, additional information is needed to verify the 

presence, and to determine the nature/extent, of a subsurface intrusion problem. As such, the 

inventory should not be considered a list of NPL candidate sites. EPA notes that less than 5% of 

all sites evaluated through the site assessment process are actually added to the NPL. This 

percentage is not expected to change significantly with this addition to the HRS.

Finally, EPA has concluded that for non-federal facilities no other national program is able to 

consistently and comprehensively evaluate and, if warranted, address subsurface intrusion 

contamination. This topic is further discussed in section V.A.2 of this preamble.

B. What is the History of the Hazard Ranking System?

In 1980, Congress enacted CERCLA (42 U.S.C. 9601 et seq.), commonly called Superfund, in 

response to the dangers posed by uncontrolled releases of hazardous substances into the 

environment. To implement section 105 (a)(8)(A) of CERCLA and Executive Order 12316 (46 

FR 42237, August 20, 1981), EPA revised the NCP on several occasions, with the most recent 

comprehensive revision occurring on March 8, 1990 (55 FR 8666). The NCP sets forth the 

guidelines and procedures needed for responding to releases, or potential releases, of hazardous 

substances. Section 105(a)(8)(A) of CERCLA required EPA to establish:
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[C]riteria for determining priorities among releases or threatened releases [of 

hazardous substances] throughout the United States for the purpose of taking 

remedial action and, to the extent practicable, taking into account the potential 

urgency of such action, for the purpose of taking removal action. Criteria and 

priorities...shall be based upon relative risk or danger to public health or welfare 

or the environment...taking into account to the extent possible the population at 

risk, the hazard potential of hazardous substances at such facilities, the potential 

for contamination of drinking water supplies, the potential for direct human 

contact [and] the potential for destruction of sensitive ecosystems....

To meet this requirement and provide criteria to set priorities, EPA adopted the HRS as Appendix 

A to the NCP (47 FR 31180, July 16, 1982). The HRS was last revised on December 14, 1990 (55 

FR 51532) to include the evaluation of additional threats to ensure a complete assessment of the 

relative risk that a site may pose to the public. The HRS is a scoring system used to assess the 

relative risk associated with actual or potential releases of hazardous substances from a site based 

on the information that can be collected in a limited one to two day site inspection (SI). The HRS 

is designed to be applied consistently to each site, enabling sites to be ranked relative to each 

other with respect to actual or potential hazards. As EPA explained when it originally adopted the 

HRS, "the HRS is a means for applying uniform technical judgment regarding the potential 

hazards presented by a facility relative to other facilities. It does not address the feasibility, 

desirability, or degree of cleanup required."5

 (47 FR 31220, July 16, 1982). 

 5 Although the HRS is designed to assess the relative risk of a site compared to other sites, it is not designed 

to be used as a site-specific quantitative risk assessment. Such an assessment is conducted later in the 

Superfund process, as necessary.
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Section 105(a)(8)(B) of CERCLA requires that the statutory criteria described in section 

105(a)(8)(A) be used to prepare a list of national priorities among the known releases, or 

threatened releases throughout the United States. The list, which is Appendix B of the NCP, is the 

NPL. 

The HRS is a crucial part of the agency’s program to address the identification and cleanup of 

actual and potential releases of hazardous substances because the HRS score is the primary 

criterion for determining whether a site is to be included on the NPL. The NPL (Appendix B to 

40 CFR 300) includes those sites that emerge as potentially posing the most serious threats to 

public health and the environment and may warrant remedial investigation and possible cleanup 

under CERCLA. Only sites on the NPL are eligible for Superfund-financed remedial actions. 

Emergency removal and enforcement actions can be conducted at any site, whether or not it is on 

the NPL.

In 1986, Congress passed the Superfund Amendments and Reauthorization Act (SARA) (Pub. L. 

99 499), which added section 105(c)(1) to CERCLA, requiring EPA to amend the HRS to assure 

"to the maximum extent feasible, that the hazard ranking system accurately assesses the relative 

degree of risk to human health and the environment posed by sites and facilities subject to 

review." The HRS was previously amended in 1990. This proposed action will amend the HRS to 

add a subsurface intrusion component to the evaluation. 

C. What is the Impact of this Proposed Rule?

1. Impact on Current Cleanup Programs, Resources and Cost
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This proposed addition to the HRS will have the most significant impact on EPA’s Superfund 

cleanup program. The current HRS considers releases to the ground water, surface water and air, 

as well as direct exposure to contamination such as soil in identifying the highest priority 

releases. If promulgated, this proposed rule will not impact the way the current HRS addresses 

these releases. However, in the course of present HRS assessments, sometimes subsurface 

intrusion issues are coincident with a ground water or soil contamination problem. The HRS 

presently does not consider the threat posed by sites with subsurface intrusion problems and 

direct human exposure, when ground water is not being used as a drinking water source or 

surficial soils are not contaminated. If promulgated, this proposed rule will for the first time allow 

the EPA site assessment program to address sites with only subsurface intrusion issues and no 

coincidental exposure. When hazardous substances are released and enter the subsurface 

environment, they can move from the subsurface into buildings as a gas, vapor, or liquid. The 

addition of a subsurface intrusion component to the HRS would enable EPA to directly evaluate 

at sites the relative degree of risk posed by human exposure to hazardous substances that enter 

regularly occupied structures through the subsurface environment.

To the extent practicable, EPA attempts to score all pathways that pose significant threats. If the 

contribution of a pathway is minimal to the overall score, in general, that pathway will not be 

scored. This proposed regulatory change would expand available options for EPA and 

organizations performing work on behalf of EPA (state and tribal partners) to evaluate potential 

threats to public health and the environment from hazardous waste sites. This modification to the 

HRS, by itself, only augments the criteria for applying the HRS. EPA also does not expect this 

proposed rulemaking to affect the status of sites currently on or proposed to the NPL. Sites that 

are currently on or proposed to the NPL have already been evaluated under another pathway (i.e., 

ground water migration, air migration, surface water migration, or soil exposure) and, in 
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accordance with section 105(c)(3) of CERCLA, as amended, would not be re-evaluated. Proposal 

of this addition also will not disrupt EPA’s listing of sites. 

Because federal agencies currently address subsurface intrusion issues as part of their 

environmental programs, it is unlikely that a significant number of sites will be added to the NPL. 

However, it could lead to an increase in site assessment activities and related costs. Executive 

Order 12580 delegates broad CERCLA authority to federal agencies for responding to actual and 

potential releases of hazardous substances where a release is either on, or the sole source of the 

release is from, any facility or vessel under the jurisdiction, custody, or control of the federal 

agency. Federal agencies are required to exercise this authority consistent with the requirements 

of CERCLA section 120, as amended, and implement regulations under the NCP, for both NPL 

and non-NPL sites. Therefore, federal agencies are in a position to proactively identify and 

respond to risks posed by subsurface intrusion of hazardous substances into regularly occupied 

structures for all populations who live and work in areas where the subsurface environment may 

create exposures. If it is determined that releases of hazardous substances pose immediate threats 

to public health and the environment, EPA fully expects that the appropriate federal agency will 

continue to undertake response actions to address such threats. In fact, some federal agencies, 

including EPA, have developed or are developing new or updated agency-specific policy and 

guidance documents to address subsurface intrusion threats. 

This proposed addition will impact both resources and costs to federal cleanup programs. EPA 

does not expect that this proposed change will result in additional site assessments being 

conducted per year or placement of more sites on the NPL per year. Rather, given potentially 

limited budgets and the possibility of increased costs for a subsurface intrusion (SsI) site 

assessment, EPA may conduct fewer assessments per year. The pipeline of sites will be reviewed 

to identify those sites that pose the highest risk and prioritized accordingly. This is not a change 
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to how EPA currently evaluates and prioritizes sites for the NPL; EPA will simply have an 

additional mechanism to address sites that pose the greatest risk. Because assessing the worst 

sites first is a priority, the regions will continue to identify the sites posing the highest risk or 

potential risk and develop a strategy to assess those sites in a timely manner, while balancing 

their other site assessment needs. 

The proposed addition, which could lead to the inclusion of a site on the NPL, does not itself 

impose any costs on outside parties; it does not establish that EPA will necessarily undertake 

response actions, nor does it require any action by a private party or determine liability for site 

response costs. Costs are limited to screening relevant sites for subsurface intrusion 

contamination during site inspections and the resulting HRS evaluation and documentation record 

preparation. Costs that arise from site remedial responses are the result of site-specific decisions 

made post-listing, not directly from the act of listing itself. 

Later decisions that consider information collected under the proposed addition could separately 

have specific economic costs and benefits (e.g., remediation costs and reduced risk), but these 

impacts are contingent upon a series of separate and sequential actions after listing a site on the 

NPL. The addition of subsurface intrusion to the HRS is several regulatory steps removed from 

imposing costs on private entities.

The HRS addition may increase the costs to government agencies conducting assessments at 

subsurface intrusion sites because the scope of a typical site inspection may need to be expanded 

or may require more expensive sampling to collect information for an SsI evaluation. SsI 

sampling may require additional sampling and different sample types than those collected at other 

sites. This may result in an increase in some site assessment costs at some sites with possible 

subsurface intrusion issues. However, SsI site assessment costs at some other sites may be 
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comparable to, or even less than, sites scored under the existing HRS. For example, a site 

assessment requiring sampling of deep ground water monitoring wells under the existing HRS 

may cost as much as, or more, than sampling conducted at sites with possible subsurface intrusion 

issues. The exact cost of any sampling at a site, including sites with possible SsI issues, varies 

greatly based on site-specific factors (e.g., number and type of samples required, difficulty in 

establishing sources of contamination or attribution of releases, number of HRS pathways being 

evaluated, and availability of data from previous sampling events). Additionally, any newly 

increased costs to government agencies conducting assessments at SsI sites are expected to be 

minimal because federal agencies should already be identifying and addressing subsurface 

intrusion as part of their environmental programs. Any increase in the cost of site assessments 

conducted by EPA for SsI sites will require EPA to realign and prioritize its site assessment 

budget to address sites with subsurface intrusion. The addition of an SsI component to the HRS is 

not expected to result in additional site assessment funding to account for any increase in site 

assessment costs. Instead, the pipeline of sites in each region will continue to be reviewed under 

the current site assessment process. If it is found that SsI-contaminated sites potentially pose a 

greater risk than other sites, then these sites will be prioritized over other sites with contamination 

not resulting from subsurface intrusion. Therefore, the proposed addition will likely result in less 

effort being conducted in some site assessment areas to free resources to be used for SsI 

evaluations.

2. Children’s Environmental Health and Environmental Justice

This rulemaking is not subject to Executive Order 13045, Protection of Children from 

Environmental Health Risks and Safety Risks because this rulemaking is expected to only have 
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moderate costs6 and this executive order only applies to significant rulemakings. EPA has also 

found that this rulemaking will have no direct impact on communities considered under 

Executive Order 12898, Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations. 

Although the rule will not have any direct impact on human health or risk within minority or lowincome populations located near potential SsI sites, populations of concern under Executive 

Order 12898, EPA did consider whether the proposed action might have contingent impacts on 

these communities if future actions affect remediation of these sites. This analysis concluded that 

potentially affected sites are located in areas that have slightly higher concentrations of minority 

populations and populations below the poverty line than surrounding areas. Therefore, any future 

actions addressing risks in these communities would not contribute to disproportionate adverse 

impacts on human health.

IV. Hazard Ranking System

A. Purpose

The current HRS serves as a screening tool to evaluate the potential for uncontrolled hazardous 

substances to cause human health problems or environmental damage at one site relative to other 

sites evaluated. The pre-remedial portion of the Superfund program—the portion prior to placing 

sites on the NPL—is intended to identify those sites that present the highest priority for further 

investigation and possible cleanup under CERCLA. (See Figure 1 for a general depiction of the 

Superfund Site Assessment process.) During Pre-CERCLA screening, which is the first step of 

 6 The regulatory impact analysis (RIA) found this rulemaking will only have moderate costs and will not be 

a significant rulemaking. The RIA for this rulemaking can be found in the official Docket for this action.
Page 23 of 214

the pre-remedial process, EPA determines if there is indication of a possible significant release. If 

so, EPA determines if a substance in the release is regulated by CERCLA, whether it is already 

being addressed, and whether any statutorily mandated limitations on CERCLA response may 

exist. If EPA determines the release meets these requirements, then the suspected release is listed 

in EPA’s Superfund Enterprise Management System (SEMS). 

Determining hazardous substance, pollutant, or contaminant can be addressed by CERCLA 

requires the application of site-specific facts to CERCLA statutory requirements and EPA policy. 

One such statutory requirement is CERCLA’s limit on response actions to some naturally 

occurring substances. CERCLA expressly limits any response actions taken in response to a 

release, or threat of release, of a naturally occurring substance in its unaltered form from a 

location where it is naturally found, from products which are part of the structure, or into drinking 

water supplies due to deterioration of the system. (see CERCLA section 104(a)(3) and 104(a)(4) 

for additional guidance on limitations on response and exception to limitations). Therefore, even 

though a naturally occurring substance in its unaltered form may potentially be regulated by 

CERCLA, the response actions taken in response to these releases, or threat of releases, may be 

expressly limited by CERCLA. For example, although radon and asbestos may qualify as a 

CERCLA hazardous substance, CERCLA section 104(a)(3) may limit responses to releases of 

radon or asbestos in some situations where the release is from building products or occurs from in 

situ natural sources, but section 104(a)(4) identifies specific circumstances that, if present, would 

allow CERCLA response in such situations. (see also EPA OSWER Directive 9360.3-12, 

Response Actions at Sites with Contamination Inside Buildings, August 12, 1993 (discussing 

when EPA Regional Decision Teams may implement early actions under the Superfund 

Accelerated Cleanup Model (SACM)). If EPA finds an eligible release of a CERCLA eligible 

substance and response actions are permissible under CERCLA, then EPA proceeds to address 

the release under CERCLA. This may include a preliminary assessment. 
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A preliminary assessment uses readily available data to determine if there is evidence of an 

unacceptable potential threat. If based on the results of a preliminary assessment, EPA determines 

that a site warrants further screening under the CERCLA remedial program, the agency initiates a 

site inspection as specified in the NCP (40 CFR 300.420). The site inspection usually includes the 

collection of samples for chemical analysis. Such samples aid in ascertaining what substances are 

present at the site and whether they are being released. The purpose of the site inspection is to 

determine if there is an actual or potential threat to human health or the environment, to 

determine if there is an immediate threat to people or the environment in the area, and to collect 

sufficient data to enable the site to be scored using the HRS.

Figure 1. Superfund Site Assessment Process
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EPA has designed the Superfund program to focus its resources on the highest priority sites. 

Consequently, the initial studies, the preliminary assessment and site inspection, which are 

performed on a large number of sites, are relatively modest in scope and cost compared to the 

remedial investigations and feasibility studies subsequently performed on NPL sites. Because of 

the need to carry out the initial studies expeditiously, EPA elected to place certain constraints on 

the data requirement for an HRS evaluation. The required HRS data should be information that, 

for most sites, can be collected during a screening level site inspection or that are already 

available. Thus, the HRS does not rely on data that require extensive sampling or repeated 

sampling over a long period of time. The HRS has also been designed so that it can be applied 

consistently to a wide variety of sites. The HRS is not a tool for conducting quantitative risk 

assessment and was designed to be a measure of relative risk among sites rather than absolute risk 

at an individual site. 

The narrow technical modifications being proposed reflect the agency’s actions to encompass 

additional risks posed by releases of hazardous substances and to address the SARA statutory 

requirement that EPA amend the HRS to assure “to the maximum extent feasible, that the HRS 

accurately assesses the relative degree of risk to human health and the environment posed by sites 

subject to review.” Thus, the fundamental purpose and structure of the HRS approach will not be 

changed when the HRS is amended to include consideration of subsurface intrusion.

B. Structure

The current HRS (40 CFR 300, Appendix A) evaluates four pathways in projecting the relative 

threat a site poses:
Page 26 of 214

• The ground water migration pathway evaluates the likelihood that hazardous substances 

will migrate to ground water and contaminate aquifers and drinking water wells that draw 

on those aquifers. 

• The surface water migration pathway evaluates the likelihood that hazardous substances 

can enter surface water and affect people or the environment. Threats to human health 

and the environment included in this pathway include drinking water (DW), the human 

food chain (HFC) (i.e., hazardous substances accumulate in the aquatic organisms that 

humans in turn consume), and sensitive environments (ENV). The surface water 

migration pathway is also divided into two "components" reflecting different mechanisms 

for contaminant transport within each component (i.e., overland/flood migration to 

surface water component and ground water to surface water migration component).

• The air migration pathway evaluates the likelihood of release of hazardous substances 

into the atmosphere and the number of people and sensitive environments actually or 

potentially exposed to hazardous substances carried in the ambient (outdoor) air, 

including gases and particulates. The air migration pathway does not evaluate releases to 

indoor air originating from the subsurface.

• The soil exposure pathway evaluates the potential threats to humans and terrestrial 

environments posed by direct, physical contact with, and subsequent ingestion of, 

hazardous substances. This pathway includes threats to people living on property where 

hazardous substances are present in the surface/subsurface, including contaminated soils 

(resident population threat), and to people living nearby with access to the contaminated 

area (nearby population threat).

Figure 2 illustrates the general structure of the current HRS.
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Figure 2. Structure of the Current HRS

Ground Water Migration Pathway Score (SGW)

Surface Water Migration Pathway Score (SSW)

Soil Exposure Pathway Score (SSE)

Air Migration Pathway Score (SA)

The scoring system for each pathway is based on a number of individual factors associated 

with risk-related conditions at the site. These factors are grouped into three factor categories as 

discussed below. These categories include factors that are used to characterize the relative risk at 

the site. 

1. Likelihood of release/exposure (i.e., likelihood that hazardous substances have been 

released or potentially could be released from a source into the environment, or that 

people or sensitive environments could come into contact with hazardous 

substances). 

2. Waste characteristics (i.e., toxicity, mobility, and/or persistence of the substances in 

the environment and the quantity of the hazardous substances that have or could be 

released). 
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3. Targets (i.e., people or sensitive environments actually or potentially exposed to the 

release).

An HRS score is determined for a site by summing the score for the four pathways. 

Specifically, the score for each pathway is obtained by evaluating a set of factors that characterize

the potential of the release to cause harm via that pathway. The factors, which represent toxicity 

of the hazardous substance, or substances, at a site, waste quantity, and population are multiplied 

by a weighting factor, yielding the factor value; the factor values are used to assign factor 

category values. The factor category values are then multiplied together to develop a score for the 

pathway being evaluated. Finally, the pathway scores are combined according to the root-meansquare equation presented below to determine the HRS score for the site. See also Table 2-1 of 

the proposed addition (section 2.1.2) for additional discussion regarding the method for 

calculating an HRS site score.

SS = �SSgggg

2 + SSss2 + SSssss2 + SSaa2

4

S = site score

Sgw = ground water migration pathway score

Ssw = surface water migration pathway score

Sse = soil exposure pathway score

Sa = air migration pathway score

By using this formula to assign a site score, the HRS score will be low if all pathway scores are 

low. However, the final score can be relatively high if one pathway score is high. This approach 

was chosen to ensure that the site scores do not deemphasize single-pathway problems, 
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underestimating their importance. EPA considers this an important requirement for the HRS 

scoring methodology because some extremely dangerous sites pose threats through only one 

pathway. For example, leaking drums of hazardous substances can contaminate drinking water 

wells, but if the drums are buried deeply enough and the hazardous substances are not very 

volatile, they may not release any hazardous substances to the air or to surface water.

It should be emphasized that the existing pathways can address subsurface contamination if it 

enters into ground water (in the ground water migration pathway), if it enters into surface water 

(in the surface water migration pathway), if it enters into ambient air (in the air migration 

pathway) from the soil surface or if it leads to surface soil contamination (in the soil exposure 

pathway). However, none of these scenarios address intrusion from the subsurface into regularly 

occupied structures. 

Finally, it should also be emphasized that the HRS score does not represent a specific level of risk 

at a site. Rather, the score serves as a screening-level indicator of the relative risk among sites 

reflecting the hazardous substance releases or potential releases at sites based on the criteria 

identified in CERCLA. 

V. Approach to HRS Addition

The following sections detail EPA’s comprehensive approach to the consideration of exposures to 

hazardous substances due to subsurface intrusion and the relevant scientific and technical 

considerations in developing this proposed rule. 
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A. General Approach

1. What Is the Need for Regulatory Action on the HRS? 

Without an evaluation of threats posed by subsurface intrusion contamination, the HRS is not a 

complete assessment and omits a known pathway of human exposure to contamination. EPA 

considers the addition of subsurface intrusion to the HRS to be consistent with CERCLA section 

105 because it will improve the agency’s ability to identify sites of the highest priority for further 

investigation and will enhance EPA’s ability, in dialogue with other federal agencies and the 

states and tribes, to determine the most appropriate state or federal authority to address sites. As is 

currently the case, EPA often defers to other state and federal cleanup authorities based on the 

site assessments and HRS evaluations. While some states/tribes have programs to address 

subsurface intrusion contamination, they often have limited authority and resources, and variable 

remediation criteria. The availability of the federal remedial authority and the more 

comprehensive site assessment program should complement and strengthen these programs. 

Other EPA programs such as the Resource Conservation and Recovery Act (RCRA) and the 

Brownfields program have limited authority and ability to address all subsurface intrusion threats. 

The RCRA Corrective Action/Enforcement is only applicable at sites subject to RCRA permitting 

or sites reachable by RCRA's enforcement activities. Furthermore, RCRA is a state delegated 

program and not all states recognize subsurface intrusion as a significant issue, and those that do 

may have variable remediation criteria. RCRA sites with subsurface intrusion issues may not be 

addressed in all states. Also, governmental entities with site-specific Brownfields assessment 

and/or revolving loan fund cleanup may only use grant funds on the selected eligible property. 

While subsurface intrusion sites may be eligible for Brownfields cleanup grants, site or propertyspecific limitations may not allow for permanent remediation where multiple properties may be 
Page 31 of 214

involved or where Brownfields grant funds, as limited by statute, may not be adequate to fund 

long-term cleanups7

. 

EPA’s removal program has the ability to quickly respond to immediate threats to public health 

and the environment from the release of hazardous substances, such as subsurface intrusion into a 

structure through a removal action. A removal action can be implemented regardless of NPL 

status to eliminate or reduce the threat of a release, or a potential release, of hazardous 

substances, pollutants or contaminants that pose an imminent and substantial danger to public 

health. However, removal actions are short-term responses to immediate threats and are not 

intended to necessarily serve as a method for dealing with long term issues such as ground water 

contamination. Generally, EPA considers vapor intrusion mitigation systems as “interim” or 

“early” response actions to promptly reduce threats to human health. Installation of vapor 

intrusion mitigation systems addresses temporary human health problems, but fails to address the 

source of the problem. 

The NCP expresses the preference for response actions that eliminate or substantially reduce the 

level of contamination in the source medium to acceptable levels, thereby achieving a permanent 

remedy. U.S. EPA, OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion 

Pathway from Subsurface Vapor Sources to Indoor Air, OSWER Publication 9200.2-154, June 

2015. OSWER’s VI guidance states:

The preferred long-term response to the intrusion of vapors into buildings is to 

eliminate or substantially reduce the level of contamination in the subsurface 

 7 EPA’s Estimated Costs to Remediate Existing Sites Exceed Current Funding Levels, and More Sites are 

Expected to Be Added to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-

380, May 2010.
Page 32 of 214

vapor source (e.g., groundwater, subsurface soil, sewer lines) by vapor-forming 

chemicals to acceptable-risk levels, thereby achieving a permanent remedy. 

Remediation of the groundwater plume or a source of vapor-forming chemicals 

in the vadose zone will eventually eliminate potential exposure pathways and can 

include the following actions, among others: removal of contaminated soil via 

excavation; removal of contaminated groundwater with pump-and-treat 

approaches; decontaminating and/or rehabilitating sewer lines that harbor vaporforming chemicals; and, treatment of contaminated soil and groundwater in situ, 

using technologies such as soil vapor extraction, multiphase extraction, and 

bioremediation, or natural attenuation. 

In the case of vapor intrusion resulting from a subsurface contaminant plume, failing to address 

the source of contamination and the resulting plume may result in an increased exposure to

individuals due to migration and expansion of the plume over time. In this instance, individuals in 

regularly occupied structures that were previously unaffected by the plume may become 

negatively impacted by subsurface intrusion. Additionally, a subsurface contaminant plume in a 

lesser-developed area has the potential to impact future development if left untreated.

There are several other concerns related to only addressing subsurface intrusion problems with a 

vapor mitigation system. The first concern is that vapor mitigation systems require ongoing 

monitoring and maintenance throughout the life of the system. Periodic inspections of the vapor 

mitigation system are necessary to make sure it is operating as designed. Over time the system 

can degrade, and maintenance will also be necessary, such as replacing the fan in an active subslab depressurization system. Non-mechanical failures of the system can occur as well, such as, 

electric power failure, turning off the fan or ignoring a damaged system. 
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A vapor intrusion mitigation system is a tool for protecting human health, but may not contribute 

to the Superfund program’s goal of cleaning up uncontrolled hazardous waste sites. Furthermore, 

EPA still lacks a mechanism to assess human health hazards from vapor intrusion in the current 

HRS model, and therefore cannot currently evaluate the threat of vapor intrusion as part of its 

ranking of sites for placement on the NPL.

Under the Superfund remedial program for NPL sites, subsurface intrusion is only addressed at 

sites placed on the NPL based on threats from other pathways. That is, subsurface intrusion issues 

are addressed later in the remedial process after placement on the NPL. For example, this may be 

done as part of EPA’s five-year review process. Sites with only subsurface intrusion issues are 

not being included on the NPL due to the lack of a subsurface intrusion component in the HRS. 

Therefore, many sites, especially those not evaluated under another HRS pathway or those not 

scoring high enough under another HRS pathway, may not be addressed for threats due to 

subsurface intrusion because they may not qualify for placement on the NPL. As the Government 

Accountability Office (GAO) states in its May 2010 report:

EPA may not be listing some sites that pose health risks that are serious enough 

that the sites should be considered for inclusion on the NPL. While EPA is 

assessing vapor intrusion contamination at listed NPL sites, EPA does not assess 

the relative risks posed by vapor intrusion when deciding which sites to include 

on the NPL. By not including these risks, states may be left to remediate those 

sites without federal assistance, and given states’ constrained budgets, some 

states may not have the ability to clean up these sites on their own...However, if 

these sites are not assessed and, if needed, listed on the NPL, some seriously 

contaminated hazardous waste sites with unacceptable human exposure may not 

otherwise be cleaned up.
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EPA proposes the addition of the subsurface component to ensure the HRS does not omit 

this known pathway of human exposure to contamination and provides a mechanism for 

complete assessment of SsI threats to human health and the environment.

2. What Alternative Regulatory Options to this Action Were Considered by EPA?

EPA considered alternatives to this proposed regulatory action for addressing the need to evaluate 

subsurface intrusion threats as discussed below.

Specifically, EPA considered whether existing programs adequately address the risks associated 

with subsurface intrusion at contaminated sites, as discussed in the previous section. If one or 

more programs were in place to adequately address concerns from subsurface intrusion, this could 

obviate the need for EPA action. However, no other authority consistently and comprehensively 

addresses subsurface intrusion across all potential non-federal sites, particularly when subsurface 

intrusion is the key exposure route. In particular, state programs vary significantly in addressing 

subsurface intrusion. In fact, not all states have subsurface intrusion programs, and states with 

programs vary in their authority, resources, and remediation criteria. The 2004 Interstate 

Technology and Regulatory Council’s (ITRC) Vapor Intrusion Team developed and conducted an 

on-line survey of state, federal, and tribal agencies regarding vapor intrusion regulations, policy, 

and guidance. Ninety-six percent (96%) of survey respondents consider vapor intrusion a 

concern; however, only 11% have a procedure for evaluating vapor intrusion codified into law, 

while a larger number of states have developed, or are developing, guidance for addressing vapor 

intrusion issues. A majority of the states that responded to the survey expressed that their 

processes for addressing vapor intrusion were only informally adopted by their agencies, and 

most defer to EPA. The 2009 Vapor Intrusion Pathway: A Guide for State and Territorial Federal 
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Facilities Managers study also surveyed state and territorial subsurface intrusion programs. 

According to this study, there were no states with a statute directly addressing vapor intrusion or 

identifying requirements for assessing the risk. Nine states had regulations that address vapor 

intrusion specifically; three states had regulations under development. Thirty-four states either 

have guidance for addressing vapor intrusion or are in the process of developing guidance. In 

addition, the Association of State and Territorial Solid Waste Management Officials 

(ASTSWMO) has expressed support for the proposed rule and has requested that EPA take 

leadership on this issue. Since vapor intrusion is projected to be the most significant component 

of subsurface intrusion, these responses would apply to subsurface intrusion as well. As 

previously discussed in section V.A.1 of this preamble, other federal programs were reviewed; 

while some programs could address subsurface intrusion at some sites, they cannot 

comprehensively address all sites (federal and non-federal).

Two other mechanisms currently exist to place sites on the NPL. First, each state can designate a 

single site to the NPL as a state top priority site regardless of its HRS score; this can be done only 

once. (see NCP, 40 CFR 300.425(c)(2)). This state-designated sites option has been implemented 

for 44 states/territories, and the remaining state options would not be sufficient to address the 

subsurface intrusion issue nationally and comprehensively, given the projected number of sites 

with subsurface intrusion problems. Second, sites may be added in response to a health advisory 

from the ATSDR. (See NCP, 40 CFR 300.425(c)(3)). However, the ATSDR mechanism was 

designed to be used only when the Agency for Toxic Substances and Disease Registry (ATSDR) 

designated the threat found to warrant immediate dissociation from the release and other criteria 

are met. This is not a mechanism that can be used uniformly and consistently. It is highly 

resource intensive and may not comprehensively address all chronic threats.
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Furthermore, CERCLA section 105 clearly mandates that EPA implement the HRS to take into 

account “to the extent possible the population at risk, the hazard potential of hazardous 

substances..., the potential for contamination of drinking water supplies and the potential for 

direct human contact.” When the HRS was last revised in 1990, the technology to detect and 

evaluate subsurface intrusion threats was not sufficiently developed. For example, there were no 

health-based benchmark concentration values for residences or standardized technologies for 

sampling indoor air, precision of analytical equipment prior to computerization was limited, and 

associations between contaminated ground water and soil vapors were not well understood. 

However, it is now possible for subsurface intrusion threats to be evaluated comprehensively. 

Therefore, it is now appropriate, given the potential that subsurface intrusion presents for direct 

human contact, to add to the HRS the consideration of threats due to subsurface intrusion.

3. What Public Outreach Activities Did EPA Conduct?

Before making the decision to issue this proposed rulemaking, EPA conducted outreach activities 

to determine interest and support from the public. Thus, on January 31, 2011, EPA published a 

“Notice of Opportunity for Public Input” (76 FR 5370, January 31, 2011) soliciting stakeholder 

comment on whether to include a subsurface intrusion component in the HRS. Additionally, EPA 

sent letters to all federally recognized tribes, asking for their comments on the FR document. 

During the 75-day public comment period on this action, four listening sessions were held 

throughout the country (Arlington, VA; San Francisco, CA; Albuquerque, NM; and Edison, NJ). 

The comments made by a majority of speakers, including members of the public, at the listening 

sessions were supportive of the potential addition of a subsurface intrusion component into the 

HRS. Of the 43 written comments received during the public comment period, 35 were in support 

of adding a vapor intrusion component to the HRS, 6 comments (generally from industry 

representatives) were opposed to this addition, and 2 comments were neutral. The comments 
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received during the public listening sessions and in response to the “Notice of Opportunity for 

Public Input” have been reviewed and considered in the development of this proposed 

rulemaking. EPA has also established a public website, http://www.epa.gov/superfund/vaporintrusion-and-superfund-program, providing background information on why this addition to the 

HRS is being considered.

4. What Peer Review Process Did EPA Use? 

This proposed rule consists of narrow technical modifications and is an expansion of the current 

HRS, which was peer reviewed by the agency’s Science Advisory Board (SAB). The 1988 SAB 

review was comprehensive and addressed the basic structure and concepts of the HRS. This 

proposed addition adheres to the basic structure and concepts of the current HRS, and thus, is 

consistent with the recommendations of the SAB. The 1988 SAB report focused on the following 

issues: 

• The overall algorithm for the HRS; 

• The inclusion of exposure in the HRS; 

• How the HRS could be evaluated in the future; 

• Work that could be done to provide better documentation for the next revision of the 

HRS; 

• The types of toxicity the HRS should address and how it should do so; 

• Distances from an uncontrolled hazardous waste site that are relevant when considering 

air pollutants from sites; and 

• The feasibility of including waste concentration in the HRS and whether large volume 

waste sites had been treated differently than others in the HRS. 
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The 1988 SAB report is available in the public docket for this proposed rulemaking.

During development of this proposed HRS update, EPA determined that several subsurface 

intrusion-specific issues warranted external independent scientific peer review. As a result, EPA 

has identified elements that have undergone peer review including:

• Consideration of potential for subsurface exposure (intrusion) into regularly occupied 

structures;

• Determination of hazardous waste quantity for the subsurface intrusion component;

• Population scoring;

• Evaluating populations in multi-story and multi-subunit structures; and

• Evaluation of target values for workers.

The results of the 2011 peer review of the proposed addition are discussed in the Summary of 

Peer Review Comments and Suggested Responses on the Addition of a Subsurface Intrusion 

Component to the HRS, which is available in the public docket for this proposed rulemaking. 

This proposed addition reflects modifications made as a result of EPA’s peer review process.

5. How did EPA Select the Approach for Including the Addition in the HRS?

The following six concepts were used as the basis for evaluating possible approaches to the HRS 

addition and the selection of a preferred approach:

1. Limit the proposed addition to the existing HRS structure to avoid confusion by 

minimizing the portions of the present HRS that would need to be revised.

2. Utilize the existing HRS basic structure and scoring algorithm, and maintain the relative 

weighting of the different pathways. 
Page 39 of 214

3. Base technical decisions on sound and proven science.

4. Ensure the HRS acts as an effective screening tool and minimizes unnecessary resource 

expenditures, while also minimizing the erroneous inclusion or exclusion of sites for 

possible NPL placement.

5. Assemble and utilize conceptual site models, case studies, and sensitivity analyses to test 

the model. 

6. Ensure that an HRS scoring evaluation of the soil exposure and subsurface intrusion 

pathway can be completed using the information and level of effort that are typical of a 

site inspection or expanded site inspection (ESI).

In the process of developing the proposed rule, EPA identified multiple options that are consistent 

with the above concepts. Based on literature reviews and agency experience, EPA projected the 

range of conditions at which the proposed addition might be applied. Using the basic structure of 

the current HRS, EPA tested each option by simulating the scores for typical scenarios. Using the 

results of these studies, EPA selected the option that best met the above criteria. To verify that the 

selected option would provide comparable results at actual sites, EPA tested the scoring algorithm 

using existing subsurface intrusion data from actual sites. The results of these studies demonstrate 

that the proposed addition functioned as expected. See section 8.0 of the Technical Support 

Document for this proposed addition (Proposal TSD) for supplemental information regarding 

EPA’s testing efforts.

B. Technical Considerations to Maintaining the Current HRS Structure and Algorithm

1. Maintaining the Current Ground Water, Surface Water, and Air Migration Pathways 
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The current approach for scoring the ground water, surface water, and air migration pathways is 

not being altered by the proposed addition of a subsurface intrusion component. Therefore, EPA 

is not soliciting comments on these pathways and will not respond to comments that are 

submitted on these pathways.

2. Addition of the New Component to the Soil Exposure Pathway 

EPA is proposing to add the subsurface intrusion threat to the present soil exposure pathway, 

which already considers direct exposure to receptors. This pathway is proposed to be restructured 

and renamed the soil exposure and subsurface intrusion pathway. The restructured pathway will 

retain unchanged the existing two soil exposure threats (resident population and nearby 

population) in the pathway as one component. The threat posed by subsurface intrusion is 

proposed to be added as a new component. 

The internal structure of the soil exposure component, including the two soil exposure threats 

within that component, remains unchanged. Therefore, EPA is not soliciting comments on the soil 

exposure component of the proposed soil exposure and subsurface intrusion pathway, nor will it 

respond to comments that are submitted on the soil exposure component.

The soil exposure pathway was selected for modification because its structure already focuses on 

populations actually coming into or potentially coming into direct contact with hazardous 

substances. The present soil exposure pathway addresses direct contact with contamination 

outside of structures. The new subsurface intrusion component also addresses direct contact with 

contamination that has already been demonstrated to have entered into regularly occupied 

structures or where the contamination is present beneath the regularly occupied structures and is 
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likely to enter into regularly occupied structures. See section VI.A of this preamble for further 

discussion.

C. Supporting Materials

The proposed addition to the HRS is discussed in the following primary documents: (1) the 

proposed rule, (2) this preamble, (3) the Proposal TSD (including all supporting appendices), (4) 

the regulatory impact analysis (RIA). The proposed rule identifies the proposed changes to the 

NCP and focuses on the specific mechanics of scoring sites with the new component. This 

preamble provides an overview of the proposed HRS addition, along with an explanation of any 

modifications and the supporting justification. The Proposal TSD contains a more detailed 

explanation of the technical basis for the proposed additions to the HRS, along with descriptions 

of the options considered, analyses that were used to evaluate the performance of the new 

subsurface intrusion component, and technical literature that was used in the development of the 

addition. The Proposal TSD is available to help guide the evaluation of subsurface intrusion sites. 

The Proposal TSD follows the same general outline as the preamble, with one section describing 

the necessary narrow technical modifications that affect multiple pathways, and the remaining 

sections describing the addition of the subsurface intrusion component to the current soil 

exposure pathway. The Proposal TSD contains a description of the current HRS, the options 

considered, and the technical justifications for the option chosen. In addition, the Proposal TSD 

references other supporting documents that provide an even greater level of detail on the 

proposed additions. 

These four documents are available to the public in the Docket for this rulemaking. To facilitate 

public review, EPA has prepared an index to the proposed rule, the preamble to the proposed rule, 
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and the Proposal TSD with detailed cross referencing of issues. This index is available in the 

public Docket. See the ADDRESSES section of this preamble for further information. 

VI. Discussion of the Proposed SsI Addition to the HRS

This section first discusses why the evaluation of the relative risk posed by subsurface intrusion 

has been added as a component to the same HRS pathway as for soil exposure. It then discusses 

how the evaluation will be performed using a structure consistent with the other threats, 

components, and pathways in the HRS, but taking into account the unique parameters impacting 

the probability of exposure to subsurface intrusion. 

A. Addition Within a Restructured Soil Exposure Pathway

EPA is proposing to add the evaluation of the relative risk posed by subsurface intrusion of 

hazardous substances into regularly occupied structures by restructuring the soil exposure 

pathway in the current HRS to include subsurface intrusion. As noted previously, no changes are 

being proposed for the other three pathways in the present HRS. The restructured soil exposure 

pathway is proposed to be renamed the soil exposure and subsurface intrusion pathway to reflect 

both components of the restructured pathway. See Figure 3 for a depiction of how the proposed 

addition fits into the HRS structure.
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Figure 3. HRS Structure with Proposed Addition

Ground Water Migration Pathway Score (SGW)

Surface Water Migration Pathway Score (SSW)

Soil Exposure and Subsurface Intrusion Pathway Score (SSESSI)

Air Migration Pathway Score (SA)

The threat posed by subsurface intrusion is proposed to be added to the soil exposure pathway 

because both consider the relative risk posed by direct contact with existing contamination areas. 

As identified in the preamble to the 1988 Federal Register document proposing the current HRS 

(53 FR 51997-52000, December 23, 1988), the soil exposure pathway, proposed in 1988 to be 

named the “onsite exposure” pathway, was added to the HRS to address the threat posed by direct 

contact with existing contamination and focused on ingestion of contaminated soil. This is in 

contrast with the other existing HRS pathways, which evaluate the relative risk posed by actual or 

potential migration of contamination from an original release location (called a “source” in HRS 

terminology) via ground water, surface water, or ambient air to other locations where exposure 

may occur. Given that the relative risk posed by subsurface intrusion is also due to direct contact 
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with contamination already present in, or likely to be intruding into, regularly occupied structures 

and no further migration away from the existing contamination areas need occur, EPA considers 

it appropriate to incorporate the subsurface intrusion threat in the same direct exposure pathway 

that includes the soil exposure relative risk. See section 6.0 of the 1988 Revised HRS Technical 

Support Document (1988 Revised HRS TSD) for supplemental information (originally referred to 

as the onsite exposure pathway).

The existing soil exposure pathway will be retained as one component of the restructured 

pathway, with the two threats within the present soil exposure pathway, resident and nearby 

populations, being retained as threats within the soil exposure component. The scoring of the soil 

exposure component will remain unaltered, but the score will be assigned as the soil exposure 

component score, not the pathway score. (See section 5.1 of the Proposed HRS Addition.) The 

proposed subsurface intrusion component will be added as a new component of the restructured 

soil exposure and subsurface intrusion pathway. As discussed in greater detail below, it will have 

the same basic structure, scoring, and weighting as other parts of the HRS. 

The score for the restructured pathway is based on a combination of the two component scores—

soil exposure and subsurface intrusion. The soil exposure component score is added to the 

subsurface intrusion component score to determine the pathway score. The two component scores 

are proposed to be additive because the populations may be subjected to exposures via both 

routes: the soil exposure component reflects exposures to people when outside a structure and 

focuses on ingestion and the subsurface intrusion component reflects exposures inside a structure 

and focuses on inhalation. Hence, the addition of the two component scores reflects the potential 

cumulative risk of multiple exposure routes and is not double counting the relative risk. 

A maximum pathway score is not contingent on scoring both the soil exposure and subsurface 

intrusion components. It is possible for a site to have only one component evaluated and still 
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reach the maximum pathway score. Because the scoring of the soil exposure component is not 

being altered, this component would contribute the same score to the overall site score absent the 

addition of subsurface intrusion.

B. Subsurface Intrusion Component Addition 

The structure of the current HRS is basically the same for all individual pathways, components, 

and/or threats. This structure was first used in the original HRS (47 FR 31220, 1982) and was 

only slightly altered when the HRS was revised in 1990 (55 FR 51532, December 14, 1990) to fit 

pathway-specific parameters and to address comments on the proposed rule. See also section 2.2 

of the 1988 Revised HRS TSD for supplemental information. The design of the HRS reflects a 

conceptual understanding of how hazardous substance releases from CERCLA sites can result in 

risks to public health and welfare and the environment. The risk scenario at these sites is a 

function of:

• The probability of exposure to (or releases to a medium in a migration pathway of) 

hazardous substances,

• The expected magnitude and duration of the releases or exposures,

• The toxicity or other potential adverse effects to a receptor (target) from the releases,

• The probability that the release will reach a receptor and the expected change in the 

concentration of hazardous substances during the movement from the location of the 

contamination to the receptors,

• The expected dose to the receptor, and

• The expected number and character of the receptors. 
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The above considerations are addressed in three factor categories: likelihood of exposure (or 

release), waste characteristics, and targets. 

The following subsections describe the structure of the proposed subsurface intrusion component 

and how this structure is consistent conceptually with the existing structure of the other HRS 

pathways and components: 1) new definitions, 2) delineation of areas of subsurface intrusion, 3) 

likelihood of exposure, 4) waste characteristics, 5) targets, and 6) calculating and incorporating 

the subsurface intrusion component score into the HRS site score. For background on why this 

structure was selected by EPA and peer reviewed by the SAB, see section 2.0 of the 1988 

Revised HRS TSD.

1. New Definitions – See section 1.1 of the Proposed HRS Addition8

EPA is proposing that 14 new definitions be added to the HRS, section 1.1, with additional 

modifications to existing definitions. EPA is adding these new definitions to aid the site evaluator 

in establishing the environmental boundaries that are being evaluated in this component (e.g., 

contamination in or above the surficial aquifer), in identifying factors unique to the subsurface 

intrusion component (e.g., channelized flow through which soil gas transports with no resistance), 

and to ensure consistent application of the HRS. 

2. Delineation of Areas of Subsurface Intrusion – See section 5.2.0 of the Proposed HRS 

Addition

EPA is proposing to include in the subsurface intrusion component evaluation two areas in which 

exposure due to subsurface intrusion contamination exists or is likely to exist: 1) areas of 

 8 For references to a specific section of the proposed HRS addition, please refer to the regulatory text of 

the proposed rulemaking.
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observed exposure—areas in which contaminant intrusion into regularly occupied structures has 

been documented, and 2) areas of subsurface contamination—areas in which subsurface 

contamination underlying regularly occupied structures (such as in surficial ground water or soil 

vapor) has been documented, but at which either sampling of indoor air has not documented that 

subsurface contamination has entered a regularly occupied structure or no sampling of indoor air 

has been undertaken. See Figure 4 for an illustration of the two areas. Additionally, special 

considerations are given to buildings with multiple subunits and multiple levels (e.g., apartment 

buildings) when establishing areas of subsurface intrusion. For a more detailed discussion on the 

selection of these areas, see section 6.0 of the Proposal TSD.

a. Area of Observed Exposure (AOE) – See section 5.2.0 of the Proposed HRS Addition

EPA is proposing to identify an area (or areas) of observed exposure at a site based on the 

location of regularly occupied structures with documented contamination resulting from 

subsurface intrusion attributable to the site being evaluated. The area encompassed by such 

structures constitutes the area of observed exposure (AOE). Other regularly occupied structures 

within this encompassed area (or areas) will also be inferred to be in the AOE unless available 

information indicates otherwise. Populations occupying structures within the AOE are considered 

exposed to subsurface contamination for HRS scoring purposes, and thus, are included in the 

HRS evaluation. See section 6.0 of the Proposal TSD for further discussion on the delineation of 

an AOE and the rationale for the inclusion of this area in an HRS evaluation.

b. Area of Subsurface Contamination (ASC) – See section 5.2.0 of the Proposed HRS Addition 

EPA is proposing to also identify an area (or areas) of subsurface contamination as an area 

outside that of the AOE, but for which subsurface contamination has been documented at levels 
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meeting observed release criteria (contamination at levels significantly above background and the 

significant increase can be attributed at least in part to the site). The contamination would be 

present either in surficial ground water samples, in subslab or semi-enclosed or enclosed crawl 

space samples, in subsurface soil samples, or in soil gas samples in the unsaturated zone. An ASC 

may also include regularly occupied structures where indoor air sampling has not documented 

that an observed exposure has occurred. (See current HRS section 2.3 for observed release 

criteria.) In addition, EPA is proposing to limit the delineation of an ASC based on the location of 

subsurface volatile hazardous substances. However, non-volatile hazardous substances may be 

used to establish an ASC if they have also been documented in an observed exposure. 

Populations in regularly occupied structures within an ASC are considered potentially 

contaminated, but are weighted less in the HRS evaluation than those populations in an AOE. The 

populations in an ASC are assigned a weighting value ranging from 0.1 to 0.9 depending on such 

factors as the distance of subsurface contamination to a regularly occupied structure’s foundation 

and the sample media (see section 5.2.1.3.2.3 of the Proposed HRS Addition). The ASC is 

included in the HRS evaluation because there is currently contamination below regularly 

occupied structures in the ASC, and although a sampling event has not documented intrusion into 

these structures, based on previous studies, it is likely that intrusion has occurred or could occur 

when suitable climatic and lifestyle conditions were or are present. The populations in the ASC 

are weighted less to reflect the relatively lower demonstrated risk in the ASC in comparison to the 

AOE. See section 6.0 of the Proposal TSD for further discussion on the delineation of an ASC 

and the rationale for inclusion of this area in an HRS evaluation.
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Figure 4. Areas of Observed Exposure or Subsurface Contamination 

c. Other Area of Subsurface Intrusion Considered: Potential Migration Zone

In the three current HRS migration pathways (ground water, surface water, and air migration 

pathways), a projected present and future migration distance called the target distance limit is 

assigned based on studies performed when the HRS was revised in 1988. Targets (receptors) 

within that distance are considered either actually or potentially exposed and the values assigned 

to these receptors are weighted based on the level of contamination, the distance from a source, 

and the possible amount of hazardous substance dilution. 

As a result, EPA considered including within the subsurface intrusion component an approach for 

incorporating populations subject to future migration (outside the ASC) similar to that used for 

the ground water migration pathway. The approach included a standard 4-mile distance (modified 

if site-specific geologic information indicates otherwise) radiating either in all directions or only 
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in the probable downgradient direction from each source at a site to establish this future migration

zone. This approach could account for the possibility of future horizontal migration of either 

volatile substances in contaminated ground water or as a soil gas beyond the demonstrated 

boundaries of the subsurface contamination and subsequently into regularly occupied structures 

(i.e., a potential future migration zone). This might happen, for example, if hazardous substance 

plumes expand or migrate due to the additional release of hazardous substances, shift side-to-side 

due to ground water gradient changes resulting from seasonal variations or tidal influences, or 

change direction due to the sequencing of dry and wet years or pumping at municipal water 

supply or other well fields. Additionally, natural and anthropogenic influences, such as utility 

corridors, fracture patterns, karst features, or buried stream channels or other geologic 

heterogeneity may alter or enhance hazardous substance migration. 

However, EPA’s confidence in the present science to accurately project hazardous substance 

migration through both the ground water and the unsaturated zone is limited. Several fate and 

transport models, many based on the Johnson and Ettinger Model, currently exist and are used to 

project vapor migration and predict contaminant vapor intrusion into a structure. The ability of a 

site assessor to accurately evaluate the potential future migration of subsurface hazardous 

substances would rely heavily on the ability to gather site-specific data in all areas of future 

migration in the relatively short time period and with minimal resources available when data 

collection for an HRS evaluation is performed (i.e., during the site inspection). EPA’s review of 

existing models indicate that in most instances, to obtain acceptable projections, extensive sitespecific data collection efforts and often multiple rounds of site investigations are required to 

develop an accurate model for projecting the future extent of vapor migration, especially in the 

unsaturated zone. As discussed in section 2.5 of the 1988 TSD, the “...misapplication of a model 

or the use of incomplete data would, of course, result in less accuracy... [and] a very conservative 
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model may also increase the frequency with which sites that do not pose significant risks are 

placed on the NPL.” 

Therefore, after thorough review of this option, the agency has chosen not to include the 

consideration of future subsurface contaminant migration in the proposed subsurface intrusion 

component. The possibility of placing sites on the NPL based on speculative projections with no 

demonstrated risk of actual exposure is too significant. The exclusion of this option in the 

proposed HRS addition does not directly prevent a site from being considered for listing on the 

NPL based on demonstrated intrusion, nor does it restrict future investigations from expanding 

the site boundaries or re-evaluating a site if further studies indicate that the extent of 

contamination at a site may have increased due to future migration. Please refer to section 6.0 of 

the Proposal TSD for supplemental information regarding consideration of a potential migration 

zone.

3. Likelihood of Exposure – See section 5.2.1.1 of the Proposed HRS Addition

A key factor considered in the HRS relative risk ranking is whether any exposure has occurred 

and if not, whether there is a probability that exposure could occur. This is termed the likelihood 

of exposure for the subsurface intrusion component. For purposes of an exposure assessment, not 

only must subsurface intrusion have occurred, but the structure must be regularly occupied. 

Consistent with other HRS pathways and components, likelihood of exposure is evaluated in two 

ways within the proposed subsurface intrusion component. The first step is to determine whether 

contamination has entered a regularly occupied structure; if this has occurred, “observed 

exposure” is established. If an observed exposure can be demonstrated in at least one structure, 

the likelihood of exposure category value is assigned the highest possible score. If observed 

exposure has not been documented, the second step is to evaluate the “potential for exposure.” 
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The potential for exposure factor is assigned a score lower than that given when an observed 

exposure has been documented. How to evaluate the likelihood of exposure is discussed below. 

See section 4.0 of the Proposal TSD for supplemental information regarding likelihood of 

exposure.

a. Observed Exposure – See section 5.2.1.1.1 of the Proposed HRS Addition

For HRS purposes, an observed exposure is established if it can be documented that a hazardous 

substance from the site being evaluated has moved through the subsurface and has entered at least 

one regularly occupied structure. When it can be documented that subsurface intrusion has 

occurred, the likelihood of exposure is assigned its maximum value. The HRS identifies for all 

the pathways a consistent approach for establishing observed exposure (or observed release in 

migration pathways) and is discussed in section 2.3 of the current HRS. Also, the requirements 

for establishing observed exposure (or observed releases) are equivalent to those used to establish 

releases throughout the HRS. See section 2.6 of the 1988 Revised HRS TSD for supplemental 

information. Consistent with the current HRS structure, EPA is proposing to establish observed 

exposure in the subsurface intrusion component by any of the following methods:

i. Observed Exposure by Direct Observation – See section 5.2.1.1.1 of the Proposed HRS 

Addition

The identification of an observed exposure by direct observation can be based on a solid, liquid, 

or gaseous hazardous substance attributable to the site being observed or known to have entered a 

regularly occupied structure from the subsurface. This finding will generally require the 

observation that a solid, liquid, or gas is entering the structure, and can be documented from a 

sample of the material that shows the hazardous substance is present due to the release from the 
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site being evaluated. For example, this type of direct exposure could be documented if: 1) 

contaminated vapors are found in a sample from a sump open to the regularly occupied structure, 

and 2) the same hazardous substances are found in subsurface samples collected beneath the 

regularly occupied structure or otherwise can be demonstrated as having emanated from known 

contamination underlying the structure. Another example would be if chromium precipitate is 

found in basements subject to ground water flooding and it is known that a chromium 

contaminant plume is present, and its presence is not from indoor sources. In neither example 

would a significant increase above a background contaminant level be required. For exposures to 

intruded ground water, EPA is proposing documented observed exposure by direct observation as 

the only method for establishing likelihood of exposure. Figure 5 below depicts an additional 

example of documenting observed exposure by direct observation through collection of a 

contaminated water sample taken from the sump of an occupied structure that is known to be 

subject to flooding. Other methods may also be used to establish direct observation depending on 

site-specific conditions. See section 4.0 of the Proposal TSD for further information.
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Figure 5. Observed Exposure by Direct Observation Example: Intrusion of Contaminated 

Ground Water 

ii. Observed Exposure by Chemical Analysis – See section 5.2.1.1.1 of the Proposed HRS 

Addition

Observed exposure by “chemical analysis” is established by comparing hazardous substance 

concentrations in background and release samples that have been chemically analyzed. The 

concentration of one or more hazardous substance in one or more indoor air sample taken from a 

regularly occupied structure (termed the “release sample”) is compared to the concentration at 

appropriate background locations and under appropriate background conditions. If the chemical 

analyses document a significant increase over background levels and if at least part of the 

significant increase can be shown to be attributable to a release from the site being evaluated, 

then observed exposure by chemical analysis has been documented. This option for establishing 

observed exposure differs from observed exposure by direct observation in that comparison of the 
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hazardous substance concentration in a release sample to a background level is required. This 

method for establishing observed exposure by chemical analysis is outlined in detail below.

Background levels for this situation, in some cases, may be determined by chemical analysis of 

samples from similar environments collected from outside the area impacted by the release, or 

releases, from the site being evaluated. While the appropriate sample locations to be used to 

establish this background level will vary based on site-specific conditions, an appropriate 

background level needs to account for both outdoor air concentrations and indoor air 

concentrations in structures of similar construction type (e.g., basement, slab-on-grade) within the 

vicinity. This is to ensure that the background level represents the concentration of a hazardous 

substance in the absence of the subsurface intrusion. In some cases it may be possible to use 

published studies on typical background concentrations in establishing an appropriate background 

level. See section 4.0 of the Proposal TSD for further discussion on background levels.

The first step in determining if observed exposure by chemical analysis has occurred is to 

document that the magnitude of the difference between the background level concentration and 

the release sample concentration is sufficient to rule out the possibility that neither the difference 

nor the similarity is due to variation in site conditions; and to ensure the sampling and analytical 

procedures are precise and can be replicated. The magnitude of this “significant increase” was 

established for all HRS pathways based on studies peer reviewed by the Science Advisory Board 

when the HRS was last revised in 1990. See section 2.6 of the 1988 Revised HRS TSD for 

supplemental information. 

A significant increase is generally identified to have occurred if the release sample hazardous 

substance concentration is above quantification limits and at least three times the background 

level, provided the background sample concentrations for the hazardous substance are found at or 
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above appropriate detection limits. If the hazardous substance background level is below the 

appropriate detection limit, any quantifiable level of the hazardous substance detected in the 

targeted structure is considered to have a concentration significantly above background.

The second step in determining if observed exposure by chemical analysis has occurred is to 

document that at least part of the significant increase can be attributed to a release from the site 

being evaluated. This step is required for establishing observed releases or observed exposures in 

all HRS pathways. See section 2.3 of the current HRS and section 2.6 of the 1988 Revised HRS 

TSD for supplemental information. This step is conducted to ensure that the increase is due to the 

release being evaluated and not from other potential contaminant sources located in the vicinity. 

(See section 4.0 of the Proposal TSD for further discussion.) For the proposed subsurface 

intrusion component, establishing significant increase over background is particularly critical 

because many of the projected intrusion contaminants are solvents and, in particular, chlorinated 

solvents. Chlorinated solvents are commonly found in multiple household and commercial 

cleaning products and in various consumer goods found in regularly occupied structures. These 

products present a substantial challenge for discerning the contribution from the environmental 

release that is being evaluated. Therefore, it is critical that a significant increase in these 

hazardous substances be documented as coming from the subsurface and not simply emanating 

from these products. 

It is suggested that the evidence to support this determination include multiple lines of evidence, 

including determining outdoor air hazardous substance concentrations; finding the hazardous 

substance at the source facility, site, or release being investigated; and finding the hazardous 

substance in subsurface samples. (See section 4.0 of the Proposal TSD regarding lines of 

evidence.) In addition, actions should be taken to ensure that sources of the hazardous substances 

inside a structure (e.g., household chemicals) have been removed from the structure prior to 
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sampling. Establishing attribution to the site in some situations, however, may be straightforward 

to document, such as when the hazardous substance is manmade, unique, and not used in 

consumer products and thus, there would be no need to follow all the steps identified above to 

establish attribution. EPA expects that future advancement in methods for establishing the source 

of indoor contamination will be helpful for drawing conclusions about attribution. 

In summary, if it is demonstrated that there is a significant increase in hazardous substance levels 

in a regularly occupied structure and it is demonstrated that the significant increase in the 

contamination is in part due to the release from subsurface intrusion being evaluated, then an 

observed release by chemical analysis has been established.

b. Potential for Exposure – See section 5.2.1.1.2 of the Proposed HRS Addition

When an observed exposure has not been established, EPA is proposing to evaluate the potential 

for exposure within structures located in an ASC using the subsurface intrusion component. 

Given that within an ASC, contamination has been demonstrated to be below or in the subsurface 

encompassing regularly occupied structures, it is probable that exposure to the intruding 

hazardous substance has occurred but that sampling has not been performed at the time the 

exposure took place. As explained in section 4.0 of the Proposal TSD, the factors affecting when 

intrusion will occur and the rate of subsurface intrusion are extremely time-, site-, and climatespecific. Sampling may not have been performed in these structures for a number of reasons, or, 

even if performed during the limited time period (due to resource limitations, site inspections are 

conducted over a limited period of time, usually 1 to 2 days) of a site inspection, the sampling 

may have been conducted during conditions in which the subsurface intrusion was not occurring, 

or occurring at levels not detectable or differentiable from that in background sources of the 

hazardous substance. Therefore, it is important that the potential for exposure be included as a 
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consideration when evaluating subsurface intrusion threats, especially when volatile substances 

are documented in the subsurface below regularly occupied structures. 

As also explained in section 4.0 of the Proposal TSD, EPA is proposing to evaluate the potential 

for exposure for the subsurface intrusion component using the same concept and framework used 

to estimate the potential to release in other pathways. (See section 2.3 of current HRS.) As 

depicted in Figure 6 below, this involves predicting the probability of exposure in an area of 

subsurface contamination based on structural containment features of the regularly occupied 

structure and the route characteristics in the subsurface, including hazardous substance physical 

and chemical properties and physical subsurface properties that influence the probability that 

intrusion is occurring.

Figure 6. Potential for Exposure
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i. Structure Containment – See section 5.2.1.1.2.1 of the Proposed HRS Addition

Containment within the current HRS is used to consider barriers that restrict the movement of 

hazardous substances. See the preamble to the 1988 Revised HRS (53 FR 51985, December 23, 

1988) for supplemental information. For the proposed subsurface intrusion component, the 

containment features considered represent structural features that block the movement of 

hazardous substances so as to minimize or prevent indoor exposures resulting from subsurface 

intrusion into a regularly occupied structure. As is consistent with the current HRS, EPA is 

proposing containment factor values that range from zero to ten where a low containment factor 

value indicates a low chance for exposure. For example, in Table 5-12 of the proposed HRS 

addition, a structure with no visible open preferential pathways from the subsurface has a lower 

containment value than a structure with documented open preferential pathways because open 

preferential pathways (e.g., sumps, foundation cracks) represent a situation in which a greater 

probability for subsurface intrusion to occur is present. Populations in structures that show no 

possible SsI intrusion route are not evaluated in this new component. Supplemental information 

regarding containment and the factor values specified in Table 5-12 is provided in section 4.0 of 

the Proposal TSD.

ii. Route Characteristics – See section 5.2.1.1.2 of the Proposed HRS Addition

The HRS uses “route characteristics” to index the relative degree to which hazardous substances 

move into or have already moved into specific areas, such as from a source into ground water, or 

for the subsurface intrusion component into a regularly occupied structure (see the 1988 TSD and 

section 4.0 of the Proposal TSD for supplemental information). These characteristics represent 

the physical and chemical properties of the specific hazardous substances and the media in which 
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they must have moved through or could move through. To determine which route characteristics 

are appropriate for evaluating potential exposure to subsurface hazardous substances, EPA 

examined the literature to identify the modeling methods that are currently used to estimate the 

levels of hazardous substance exposure. Numerous route characteristics and the relationship of 

these and site-specific input requirements were identified. EPA also gave careful consideration to 

ensure that route characteristic factors may be measured or calculated on a site-specific basis in a 

manner appropriate with current HRS evaluations. See section 4.0 of the Proposal TSD for 

supplemental information evaluated as part of this process. 

EPA reviewed existing sensitivity analyses and performed further analyses to evaluate the 

intrinsic relationships among the examined route characteristics to identify those that have the 

greatest impact on potential for exposure. Based on the agency’s analysis, three factors 

represented the greatest impact on potential for exposure and for which sufficient site-specific 

information could be collected during a site inspection: 1) depth to contamination, 2) vertical 

migration, and 3) vapor migration potential. These three factors are described in the following 

sections.

a. Depth to Contamination – See section 5.2.1.1.2.2 of the Proposed HRS Addition

The depth to contamination factor represents the vertical distance between contamination (either 

in soil, soil gas, or surficial ground water) and the lowest horizontal point of an overlying 

regularly occupied structure (e.g., a basement floor). This distance represents how far a hazardous 

substance would have to travel through the subsurface to intrude into that structure. Based on 

available data, the probability of exposure decreases as the depth to contamination increases. In 

addition, as part of EPA’s sensitivity analysis in developing route characteristics, at depths 

greater than 150 feet it became increasingly unlikely that exposure would occur. This is reflected 
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in Table 5-13 (section 5.2.1.1.2.2 of the Proposed HRS Addition). EPA is proposing depth to 

contamination factor values ranging from zero to ten, where increasing depth results in a lower 

factor value. 

EPA is also proposing to give special consideration in two situations in which it is likely that 

exposure has occurred. One situation is when subsurface profiles may be impacted by 

channelized flow features, such as fractured bedrock or karst. The other situation is at locations 

where the contamination is measured directly below the structure (e.g., in subslab or 

enclosed/semi-enclosed crawl space samples). These features reflect a situation with a high 

probability of exposure to intruded hazardous substances because of limited resistance to 

migration of the substances into the structure. See section 4.0 of the Proposal TSD for 

supplemental information on how the depths to contamination were weighted when assigning the 

factor values to different distances..

b. Vertical Migration – See section 5.2.1.1.2.3 of the Proposed HRS Addition

The vertical migration factor considers the geologic makeup of materials between a regularly 

occupied structure and the hazardous substance plume and the rate at which substances are likely 

to have moved through the materials. EPA is proposing to index vertical migration based on two 

factors: effective porosity (or equivalently, the permeability) of geologic materials and the 

thickness of the lowest porosity layer.

Factor values for effective porosity (as it relates to permeability) of geologic materials range from 

one to four and are based solely on the typical range of porosity of subsurface materials (e.g., 

gravel, sand, silt and clay). These factor values are used in conjunction with the thickness of the 
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lowest porosity layer (greater than 1 foot thickness) to establish a vertical migration factor value, 

ranging from one to fifteen. 

As part of the vertical migration factor, EPA identified soil moisture content to potentially be a 

significant route characteristic variable. Thus, to incorporate soil moisture in EPA’s assessment of 

potential for exposure, the agency used published “average soil moisture content” values for

specific soil types. These averages were used to develop effective porosity/permeability factor 

values. See section 4.0 of the Proposal TSD for supplemental information.

c. Vapor Migration Potential – See section 5.2.1.1.2.4 of the Proposed HRS Addition

The vapor migration potential factor is based on hazardous substance-specific chemical 

properties, including both the vapor pressure and Henry’s constant values for hazardous 

substances associated with the site. This factor evaluates the volatile nature of these hazardous 

substances and is projected to be the most influential route characteristic factor on calculating 

potential for exposure based on a sensitivity analysis using subsurface migration modeling. When 

calculating the vapor migration potential, a factor value is determined only for the most volatile 

hazardous substance based on vapor pressure and Henry’s constant values. Those values are used 

to establish the vapor migration potential factor value. See section 4.0 of the Proposal TSD for 

supplemental information on this topic.

iii. Calculation of the Potential for Exposure Factor Value – See section 5.2.1.1.2.5 of the 

Proposed HRS Addition
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Consistent with potential to release determinations in the HRS, the potential for exposure for this 

component is calculated by summing all route characteristic factor values and multiplying the 

sum by the containment factor value to determine a potential for exposure factor value.

c. Calculation of the Likelihood of Exposure Factor Category Value – See section 5.2.1.1.3 of 

the Proposed HRS Addition

As in all HRS pathways and components, the likelihood of exposure factor category value is 

assigned based on the higher of the observed exposure (or release) value or the potential for 

exposure (or release) value. The maximum value assigned for the likelihood of exposure factor 

category is 550 and is assigned if observed exposure is documented. If observed exposure is not 

documented, the value assigned when evaluating potential for exposure ranges between 0 and 

500. This approach is consistent with the current HRS structure. See sections 2.2 of the 1988 

Revised HRS TSD for supplemental information regarding this approach.

4. Waste Characteristics – See section 5.2.1.2 of the Proposed HRS Addition

The waste characteristics factor category is based on factors that are related to the relative risk 

considerations included in the basic HRS structure: 1) the toxicity or other potential adverse 

effects to a receptor from the releases, 2) the potential to degrade in the subsurface prior to 

intruding into a regularly occupied structure, and 3) the expected magnitude and duration of the 

exposure. The factors considered in determining the waste characteristics factor category value 

are the toxicity of the hazardous substances, the ability of the hazardous substance to degrade , 

and an estimate of the quantity of the hazardous substances to which occupants could be exposed. 

Consistent with the soil exposure component, the assigned factor values are multiplied together to 

determine this category value for the subsurface intrusion component. (See sections 2.2 and 2.4 of 
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the 1988 Revised HRS TSD for further discussion on the structure of this factor category and how 

it fits within the overall HRS structure.) How and why these factors are proposed to be included 

in this factor category is discussed below.

a. Toxicity/Degradation – See section 5.2.1.2.1 of the Proposed HRS Addition

The combined toxicity/degradation factor includes consideration of both the toxicity and the 

possibility for degradation of hazardous substances being evaluated for HRS purposes.

The toxicity factor in the overall HRS structure reflects the toxicity of a hazardous substance 

associated with a release or exposure, and is assigned the same factor value for all the pathways 

and components in the current HRS. As in all HRS pathways and components, it is proposed to 

be assigned the same corresponding factor value as for other parts of the HRS. The rationale for 

the assignment of the factor value is discussed in the section 2.3 of the 1988 Revised HRS TSD. 

This toxicity factor is based on the toxicity of the substances present at a site. In the HRS 

addition, a different factor value is proposed to be assigned to each hazardous substance that an 

occupant has been or is potentially exposed to. The factor value is driven by the magnitude of 

each hazardous substance’s acute and chronic toxicity to humans. The toxicity factor value is 

directly related to the concentration at which the hazardous substance is known to have a health 

effect: the more toxic the chemical, the higher the toxicity value. Any hazardous substance 

identified in an observed exposure within the AOE or meeting the observed release criteria in 

either the AOE or ASC will be assigned a toxicity factor value. The method for assigning this 

value is contained in section 2.4.1.1 of the current HRS (40 CFR 300, Appendix A) and is 

discussed in section 2.3 of the 1988 Revised HRS TSD. 
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The degradation factor represents the possibility for a substance to degrade in the subsurface prior 

to intruding into a regularly occupied structure. The potential of a substance to degrade has been 

identified as a significant factor in numerous studies evaluating the potential for intrusion by a 

vapor. The possibility that a substance may degrade is both a substance- and location-specific 

evaluation that is influenced by factors such as molecular structure, makeup of the immediate 

subsurface geology, and the presence or absence of oxygen within intervening unsaturated soils. 

Because many of the site-specific characteristics impacting the rate of degradation are considered 

beyond the scope of a typical site investigation, EPA is proposing to evaluate degradation based 

on the substance being evaluated, the depth to contamination, and if appropriate environmental 

conditions are present to ensure that sufficient degradation will occur to diminish the threat. 

Based on EPA’s review of the current literature and research on this topic, the assigned 

degradation factor is limited to three possible factor values, two for substances that are readily 

degradable and the appropriate environmental factors are present, and one for when either of 

these parameters are not present. 

EPA seeks public input on the following question regarding the degradation factor:

Is there a way to determine the presence and extent of biologically active soil at a site during a 

limited site investigation? If so, what soil characteristics should EPA consider to determine 

whether biologically active soil is documented to be present?

EPA proposes the degradation factor also be based on the half-life of a substance, with the halflife being determined by biodegradation and hydrolysis rates. If this information is not available 

then a hazardous substance’s estimated half-life will be based on the substance’s chemical 

structure, unless available information indicates otherwise. Substances with relatively low 
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structural complexity, such as petroleum and petroleum-like substances (having straight carbon 

chain or simple ring structures), have the greatest potential to degrade in the subsurface while 

halogenated and poly-aromatic ringed substances (e.g., tetrachloroethylene, PCBs) are less likely 

to significantly degrade as result of subsurface microbial activity. 

If it has been documented that a hazardous substance has been found to have entered a regularly 

occupied structure, regardless of the substance or the site conditions, the degradation value is 

assigned to reflect the likelihood that the substance is not significantly degrading in the

subsurface. Also, if the substance is a daughter, or degradation product, of a parent substance that 

is also present, then the degradation factor will reflect this relationship. Parent and daughter 

substances are assigned values to reflect that the daughter substance will be continuously created 

by degradation of the parent substance. See also section 5.0 of the Proposal TSD for additional 

discussion regarding the inclusion of a degradation factor.

The toxicity and degradation factors are multiplied together to assign a combined factor value. If 

multiple substances are present, the highest combined factor value is selected for use in 

determining the waste characteristics factor category value, as discussed below.

b. Hazardous Waste Quantity – See section 5.2.1.2.2 of the Proposed HRS Addition

In the basic HRS structure used in all pathways and components, the hazardous waste quantity 

factor reflects the risk consideration related to the magnitude and duration of either the release for 

a migration pathway or the exposure for an exposure pathway. In other words, for an exposure 

pathway, the risk posed by a release of hazardous substances is directly related to the amount of 

hazardous substances to which receptors (targets) are exposed and the length of the exposure. 
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As explained in the preamble to the 1990 HRS and in the 1988 Revised HRS TSD, an estimate of 

the waste quantity associated with a site was the best surrogate for the amount of hazardous 

substances that receptors were exposed to and that the duration of the exposure was probably 

correlated to the magnitude of the exposure. In the current three migration pathways (ground 

water, surface water, air), the hazardous waste quantity factor reflects the total amount of 

hazardous substances in sources at the site to take into account not only where contamination has 

already migrated to, but also future migration of contamination to other locations. For the soil 

exposure pathway, however, the estimate does not include the total amount in or released from 

the site sources, but only the amount of hazardous substance in the top two feet of contaminated 

soils sources and in the surface portions of other source types in an area of observed 

contamination. (See section 5.0.1 of the current HRS.)

EPA is proposing that since the subsurface intrusion component also focuses on exposure and not 

the amount of hazardous substances that might migrate to targets in the future, the waste quantity 

factor value for this component should also reflect only the amount of hazardous substances that 

people currently are exposed to, that is, the amount in regularly occupied structures. EPA is 

proposing a four-tiered hierarchical approach consistent with the current HRS (see section III.C 

of the preamble of the current HRS (55 FR 51542, December 14, 1990)) as well as minimum 

waste quantity factors (see section 2.4.2 of the current HRS). The minimum waste quantity 

factors are included because of insufficient information at many sites to adequately estimate 

waste quantity with confidence, as discussed in section I of the preamble to the current HRS (55 

FR 51533, December 14, 1990). The current HRS establishes a minimum waste quantity factor 

value of 10 for each pathway or component at sites with no actually contaminated targets and a 

waste quantity factor value of 100 for the migration pathways if observed exposure has been 

documented. (See section 2.4.2 of the current HRS.)
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It is proposed for the estimation of waste quantity for the subsurface intrusion component, that 

regularly occupied structures within the AOE and ASC be considered. For sites at which the 

component waste quantity (the sum waste quantities for all occupied structures in the AOE and 

ASC) is below 10, it is proposed that a minimum factor of 10 should apply the same as in other 

pathways and components. This minimum factor reflects that in a limited site inspection, it is 

likely that information on the actual waste quantity at a site may not be available and a lower 

value would likely underestimate the actual conditions. Furthermore, if any target is subject to 

Level I or II contaminant concentrations a minimum hazardous waste quantity factor value of 100 

could be assigned.

EPA seeks public input on the following question regarding the calculation of hazardous waste 

quantity: How could EPA further take into account the differences in dilution and air exchange 

rates in large industrial buildings as compared to smaller residential and commercial structures 

when calculating the hazardous waste quantity for the HRS SsI Addition?

The component waste quantity is the sum of all the waste quantities for all the regularly occupied 

structures found in both the AOE and ASC. The component waste quantity factor value assigned 

is then based on the magnitude of this sum, subject to minimum values. See section 5.0 of the 

Proposal TSD for supplemental information regarding this topic.

c. Calculation of the Waste Characteristics Factor Category Value – See section 5.2.1.2.3 of the 

Proposed HRS Addition

As in all HRS pathways and components, the waste characteristics category value is the product 

of the toxicity/degradation factor value (or the functional equivalent) and the hazardous waste 

quantity factor value, scaled so as to be weighted consistently in all pathways. Similar to the 
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likelihood of exposure factor category, the waste characteristics factor category is subject to a 

maximum value to maintain the balance between factor categories. This approach is consistent 

with the current HRS structure. See sections 2.2 and 2.4 of the 1988 Revised HRS TSD for 

supplemental information regarding this approach.

5. Targets – See section 5.2.1.3 of the Proposed HRS Addition

The targets factor is based upon estimates of the expected dose to each receptor and the number 

and type of receptors present. In a human health risk assessment, it is critical to understand the 

nature and extent of exposure to individuals, populations, and resources. The relative risk 

assessment embodied within the current HRS uses the targets factor as an index of the nature and 

extent of exposure to individuals, populations, resources, if appropriate for the migration or 

exposure route being evaluated, sensitive environments. This will remain the same in the 

proposed HRS addition, except sensitive environments will not be considered an eligible target. 

a. Identification of Eligible Targets – See section 5.2.1.3 of the Proposed HRS Addition.

The target factors evaluated by all pathways under the current HRS include the following:

• The most exposed individual (i.e., nearest well for ground water migration, nearest 

intake for drinking water threat, food chain individual for human food chain threat, 

resident individual for resident population threat, and nearest individual for nearby 

population threat and air migration),

• Populations (including residents, workers, students, and those in daycare),

• Resources (including economic and cultural uses of contaminated resources), 
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• Sensitive environments (except for the ground water migration pathway). (Examples of 

sensitive environments include government designated protected areas (e.g., national 

wildlife refuge), wetlands and critical habitat known to be used by a State or Federallydesignated threatened or endangered species.)

See sections 2.5 and 5.1.3 of the current HRS for supplemental information on how eligible 

targets are identified.

Given that the subsurface intrusion component is proposed to be included as an exposure 

component within the modified soil exposure and subsurface intrusion pathway, the agency is 

proposing to use the same target categories used in the current soil exposure pathway, including 

exposed individual, resident populations, workers, and resources. However, unlike the current soil 

exposure pathway, workers are proposed to be evaluated as exposed individuals and as part of the 

population within an area of subsurface contamination instead of being evaluated under a separate 

worker factor value. See section 5.2.0 and its subsections of the proposed HRS addition. 

Additionally, sensitive environments are not being considered as eligible targets because 

exposures related to subsurface intrusion are limited to indoor areas and it is unlikely that 

sensitive environments would be exposed. See section 5.2.1.3 of the proposed HRS addition.

EPA seeks public input on the following question regarding subsurface source strength:

The HRS SsI Addition considers source strength in delineating ASCs and AOEs, in scoring 

likelihood of exposure, in assigning waste quantity specifically when estimating hazardous 

constituent quantity and in weighting targets in an ASC. The HRS algorithm for all pathways 

incorporates the consideration of source strength in determining an HRS site score. Could EPA 

further take into account source strength in performing an HRS evaluation?
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b. Exposed Individual and Levels of Exposure – See section 5.2.1.3.1 of the Proposed HRS 

Addition

This section introduces the methods used to identify and establish the levels of contamination and 

benchmarks proposed to be used within the subsurface intrusion component. Additionally, the 

exposed individual factor is discussed, as well as how to apply a factor value based on the 

benchmarks and the resulting levels of exposure.

i. Identifying Levels of Exposure and Benchmarks for Subsurface Intrusion – See 

section 5.2.1.3.1 of the Proposed HRS Addition

For all current HRS pathways, the magnitude of the values assigned to the individual and 

population factors depend on the concentration of the contamination to which the receptors 

(targets) are exposed. If receptors are exposed to hazardous substance levels that meet observed 

release criteria, they are identified as actually contaminated; however, if the receptors are not 

exposed to hazardous substances that meet the observed release criteria but are within the target 

area being evaluated, they may be considered potentially contaminated. Potential targets are 

evaluated because a typical site inspection may not identify the extent of contamination. A site 

inspection typically includes 1 to 3 days of sampling and investigation activities. These limited 

investigations may not adequately characterize the annual or longer term indoor exposure levels 

(see page 4 of the 1988 SAB report and section 6.0 of the Proposal TSD), especially in the case of 

subsurface intrusion where seasonal and temporal fluctuations can significantly impact the rate of 

subsurface intrusion. 

Actually contaminated targets are further divided into two categories based on whether the 

hazardous substance concentrations are above standard health-based benchmarks (or for 
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environmental receptors, ambient water quality criteria). If so, they are identified as Level I; if 

they are not, they are identified as Level II. See section 2.5.2 of the current HRS for a discussion 

of applicable benchmarks. 

EPA is proposing to use a similar target weighting structure in the subsurface intrusion 

component. (See sections 5.2.1.3.1 and 5.2.1.3.2 of the proposed HRS addition.) Those targets in 

the AOE are considered actually contaminated, whereas, those in the ASC are considered 

potentially contaminated. The targets in an AOE are further divided into Level I and II, based on 

whether the hazardous substance concentrations are at or above identified health-based 

benchmarks. EPA is proposing to use the following benchmarks for the subsurface intrusion 

component:

• Screening concentrations for cancer 

• Screening concentrations for noncancer toxicological responses

Targets associated with an observed exposure by direct observation are only considered subject to 

Level II contamination in all parts of the HRS and EPA is proposing that this remains consistent 

in the subsurface intrusion component. Furthermore, because intrusion by contaminated ground 

water is documented by direct observation only, targets residing within a structure subject to 

intrusion by contaminated ground water are also proposed to be evaluated as Level II (see section 

2.5 of the proposed HRS addition). 

The targets within an ASC are also further divided based on the type of sample (e.g., gas, soil, 

water) and the distance of the sample from the targets (e.g., the depth of the sample below the 

structure). Weighting factors ranging from 0.1 to 0.9 are then assigned accordingly as discussed 

below in section 5.c.ii. See also section III.H. of the preamble to the current HRS (55 FR 51547, 

December 14, 1990) for supplemental information.
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ii. Exposed Individual – See section 5.2.1.3.1 of the Proposed HRS Addition

The standard HRS approach for scoring targets includes a measure reflecting the maximum level 

of exposure to individuals. The evaluation of exposed individuals is proposed to include 

individuals living, attending school or day care, or working in a regularly occupied structure. The 

reasonably maximally exposed individuals are those individuals in the eligible target population 

that are expected to be exposed to the highest concentration of the hazardous substance in 

question for a significant time. See section V.C.9 of the preamble to the proposed 1988 HRS (53 

FR 51978, December 28, 1988) for supplemental information.

EPA is proposing to retain the basic scoring approach used throughout the current HRS for 

evaluating the exposed individual factor. As is consistent with all pathways, a value of 50 points 

is assigned if there is any individual exposed to Level I concentrations or 45 points if there is any 

individual exposed to Level II concentrations. If there are no individuals exposed to Level I or 

Level II concentrations, but at least one individual is living, attending school or day care, or 

working in a regularly occupied structure within an ASC, EPA proposes to assign a value of 20. 

See section 2.5 of the current HRS for supplemental information as to how EPA addresses 

exposed individuals within the HRS structure. 

c. Population – See section 5.2.1.3.2 of the Proposed HRS Addition

The population factor is evaluated using media-specific, health-based benchmarks as discussed 

above. EPA proposes the population factor include all populations qualifying as exposed 

individuals, including residents, students, workers and those attending day care. However, 

workers are weighted slightly differently than other exposed individuals to reflect that a worker’s 
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exposure is limited to the time present in a workplace. Additionally, as workers may be employed 

on a full-time or part-time basis, the number of workers present in a structure or subunit is 

proposed to be adjusted by an appropriate factor reflecting this difference in exposure durations. 

EPA is proposing to retain the current scoring methodology for weighting populations used 

throughout the HRS, with actual exposure more heavily weighted than those potentially exposed. 

The proposed subsurface intrusion component will evaluate populations based on the number of 

individuals located within an identified AOE (i.e., those populations exposed to Level I and Level 

II concentrations) and the number of individuals located within an ASC (i.e., potential 

contamination as determined based on subsurface sampling), which is further subdivided as 

described in subsection ii below. 

i. Weighting of Targets in the Area of Observed Exposure (AOE) – See sections 

5.2.1.3.2.1 and 5.2.1.3.2.2 of the Proposed HRS Addition

EPA is proposing to establish an AOE based on documented contamination meeting observed 

exposure criteria (either by direct observation or chemical analysis). Consistent with the 

weighting of populations throughout the HRS (see section 2.5 of the current HRS), the proposed 

subsurface intrusion component will weight targets subject to Level I contaminant concentrations 

by a factor of 10 and weight targets subject to Level II contaminant concentrations by a factor of 

1. As noted previously, eligible populations also include individuals working in regularly 

occupied structures. However, the number of workers present in a regularly occupied structure 

will be adjusted to reflect that their exposure is limited to the time they are in a workplace. 

Therefore, the number of full- and part-time workers in a structure or subunit will be identified 

and divided by an appropriate factor prior to being summed with the number of other individuals 

present. If information is unavailable to classify a worker as full- or part-time, that worker will be 

evaluated as full-time.
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For example, if a single residence occupied by a family of four was observed to be exposed to 

hazardous substance concentrations above a media-specific, health-based benchmark, the number 

of residents would be multiplied by 10 for a factor value of 40. However, if that same family was 

exposed to a hazardous substance and the hazardous substance concentration was below the 

applicable benchmark but met the criteria for observed exposure, the number of residents would 

be multiplied by 1 for a factor value of 4. To ensure the entire population within an AOE is 

included in the HRS evaluation, both Level I and Level II factor values are counted and summed 

together. 

Within the AOE, EPA is proposing to consider as actually contaminated those populations in 

regularly occupied structures for which observed exposures have not been established but the 

structures are surrounded by regularly occupied structures in which observed exposures have 

been identified, unless evidence indicates otherwise. This action is proposed because it is 

considered likely that if these structures were sampled during the correct conditions, observed 

exposures would be identified at levels similar to those in surrounding structures. Targets inferred 

to be exposed to this contamination will be weighted as Level II as there are no actual sample 

results to compare against benchmarks. However, EPA has included an exception to allow for 

situations where site-specific conditions clearly document that there may be no observed 

exposures in these structures. The rule language states that targets can be inferred to have 

observed exposures in these situations “unless available information indicates otherwise”. This 

concept of inferred exposure is also included in the existing soil exposure pathway and in the air 

migration pathway. 

In the case of multi-story/multi-subunit structures, all regularly occupied subunits on a level with 

an observed exposure and all levels below are considered to be within an AOE, unless available 
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information indicates otherwise. For multi-story/multi-subunit structures located within an AOE, 

but where an observed exposure has not been documented, only those regularly occupied spaces 

on the lowest level are considered to be within an AOE, unless available information indicates 

otherwise. (See sections 5.0.1 and 6.3 of the current HRS.)

ii. Weighting of Targets in the Area of Subsurface Contamination (ASC) – See 

section 5.2.1.3.2.3 of the Proposed HRS Addition

EPA is proposing to establish an ASC as defined by documented ground water, subslab, soil, 

semi-enclosed or enclosed crawl space, or soil gas contamination meeting observed release 

criteria. These areas are included in the subsurface intrusion component due to the potential that 

limited sampling conducted during a site inspection may not identify that subsurface intrusion is 

occurring because of the high temporal and spatial variability associated with detecting 

subsurface intrusion. Temporal and spatial differences can significantly impact the rate at which 

volatile hazardous substances enter a structure. However, when an ASC has been defined, that 

area represents a location where subsurface hazardous substances have the potential to intrude 

into a structure. EPA is limited in its extent of preliminary screening activities, and a single 

indoor air sampling event is unlikely to identify the full threat posed by subsurface intrusion.

As is consistent with the 1990 HRS, EPA is proposing to weight these potentially exposed targets 

at a value less than those targets that have been identified to be actually exposed. Due to the 

variability in subsurface intrusion rates, the potential weighting factor values for targets within an 

ASC range from 0.1 to 0.9 and depend on where the subsurface contamination has been found. 

Using EPA’s vapor intrusion attenuation factors published in 2012 and basic subsurface 

contaminant transport concepts, EPA developed a relatively proportional weighting for potential 

targets based on the sampling media being considered. This range of weighting factors represents 
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the proportional probability of a target to be exposed as a result of contaminant intrusion from the 

subsurface in a variety of likely sampling scenarios. The potential target weighting factors 

presented in the proposed addition do not directly correspond to attenuation factors in themselves. 

Instead, the relative weighting between these values is based on the published attenuation factors. 

These weighting factors are presented in this manner to project that contaminants found in a 

crawl space sample, for example, are more likely to attenuate less before entering into an 

overlying structure, and thus more likely to pose a threat, as opposed to those found in a shallow 

ground water sample. 

EPA is proposing that the weighting of potential targets also reflect the distance to or the depth at 

which contamination is found. For any contamination found at a horizontal or vertical distance of 

five feet or less from a regularly occupied structure’s foundation, EPA is proposing to assign a 

minimum weighting factor of 0.4 regardless of the sample medium. Similarly, EPA is proposing 

to assign a weighing factor of 0.1 to any contamination found or inferred at depths greater than 30 

feet regardless of sampling medium. These minimum weighting values are in response to an 

analysis of the data used in deriving published attenuation values. The attenuation values were 

published based on real-world sampling data collected from numerous sites across the United 

States. The majority of sampling data collected as part of this effort came from sites where 

contamination was generally found at depths less than 30 feet. Therefore, EPA considers the 

attenuation factors and relative weightings between them to only be appropriate for shallower 

depths. The minimum value for the upper five feet allows consideration of sites where 

contamination is found at extremely shallow depths and therefore has a minimal vertical distance 

to travel before intruding into a regularly occupied structure. 

In the case of multi-story/multi-subunit structures, all regularly occupied subunits on a level 

above one where an observed exposure has been documented or inferred, or where a gaseous 
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indoor air sample meeting observed release criteria is present, are considered to be located within 

an ASC, unless available information indicates otherwise. For multi-story/multi-subunit 

structures located only within an ASC, only those regularly occupied subunits within the lowest 

level are considered in an HRS evaluation. 

EPA proposes eligible populations include individuals living in, or attending school or day care in 

the structure, and workers in regularly occupied structures. The number of workers is adjusted to 

reflect that their exposure is limited to the time they are in a workplace. Therefore, the number of 

full- and part-time workers in a structure or subunit will be divided by an appropriate factor prior 

to being summed with the number of other individuals present. If information is unavailable to 

classify a worker as full- or part-time, that worker will be evaluated as full-time.

The proposed weighting factors for exposed individuals in any structure within an ASC are based 

on the probability of contamination entering into occupied structures from the subsurface. The 

weighting factors reflect depth to contamination, sample type, and media. The magnitude of the 

factor is also based on attenuation factors from current scientific literature including EPA’s 2012 

vapor intrusion attenuation factors publication. Additional information regarding this analysis is 

presented in section 6.0 of the Proposal TSD.

d. Resources – See section 5.2.1.3.3 of the Proposed HRS Addition

The resources target factor is evaluated in all pathways under the current HRS. A factor value of 

five is assigned if at least one resource is present and a factor value of zero if no resource is 

present. Eligible resources are pathway-, component-, or threat-specific. These resources 

represent uses of a contaminated medium or area where exposures occur and are not covered by 

the other identified targets. For example, resources within the air migration pathway include 

commercial agriculture or silviculture and major/designated recreation areas. The resident 
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population threat also includes commercial livestock production or grazing. See section III.I of 

the preamble to the current HRS (55 FR 51549, December 14, 1990) for supplemental 

information.

Because subsurface intrusion is limited to indoor spaces, EPA is proposing to include regularly 

occupied structures that are located within a defined AOE or ASC (as previously discussed in 

section VI.B.2 of this preamble) and in which populations, not including those already counted as 

exposed individuals, may be exposed to contamination due to subsurface intrusion. For example, 

libraries, recreational facilities, and religious or tribal structures used by individuals, may qualify 

as eligible resources.

e. Calculation of the Targets Factor Category Value – See section 5.2.1.3.4 of the Proposed 

HRS Addition

As is done throughout the HRS, EPA is proposing to sum all of the target factor values together 

to establish a target factor category value in calculating the proposed subsurface intrusion 

component score. Unlike the likelihood of exposure and waste characteristics factor category 

values in all HRS pathways, which are subject to maximum values, the target factor category is 

not limited in the current HRS. This is to ensure that all individuals, populations, resources, and 

sensitive environments are included; thereby, representing the full relative risk associated with 

the identified threat. It is also consistent with the direction of CERCLA section 105 to amend the 

HRS “to the maximum extent feasible” to address “the relative degree of risk to human health and 

the environment” by putting the emphasis on the number of receptors exposed to contamination.

6. Calculation and Incorporation of the SsI Component Score into the HRS Site Score 
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The following subsections summarize the calculation of the subsurface intrusion component 

score, how the component score is then used in the calculation of the soil exposure and 

subsurface intrusion pathway score, and how, in turn, the pathway score is subsequently 

incorporated into the HRS site score. 

a. Calculation of the SsI Component Score – See section 5.2.2 of the Proposed HRS Addition 

EPA is proposing to calculate the subsurface intrusion component score using the same algorithm 

as in other components and pathways of the HRS. (See section 2.2 of the 1988 Revised HRS 

TSD.) This involves multiplying the likelihood of exposure factor category value times the waste 

characteristics factor category value times the targets factor category value and dividing that 

value by a weighting factor so that it has equal magnitude to other component scores (subject to a 

maximum value). The values are multiplied to reflect that it is the product of these values that 

represents a relative risk level. 

In a relative risk (or in a site-specific risk) assessment, the use of the product of the factor 

category values is considered appropriate because the magnitude of each of the factor category

values reflects the probability of exposure occurring: likelihood of releases reflects the probability 

of exposure actually occurring, waste characteristics reflects the probable quantity and duration of 

the exposure, and targets reflect the probable number of receptors at risk. Thus, since each factor 

category value reflects a probability in a series of events, the overall probability associated with 

the series is the product of the individual probabilities. For example, if any factor category value 

is zero, such as when there are no targets exposed or potentially exposed to subsurface intrusion, 

the component score is zero, consistent with there being no risk due to subsurface intrusion. 
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b. Incorporation of the SsI Component Score into the Soil Exposure and Subsurface Intrusion 

Pathway Score – See section 5.3 of the Proposed HRS Addition

The score for this restructured pathway is proposed to be a combination of two component scores. 

The subsurface intrusion component score is added to the soil exposure component score (subject 

to a maximum value) to determine the pathway score. The two component scores are proposed to 

be additive because the populations may be subjected to exposures separately via both routes: the 

soil exposure component reflects exposures to people when outside a structure and focuses on 

ingestion, while the subsurface intrusion component reflects exposures to people when inside a 

structure and focuses on inhalation. Hence, the addition of the two component scores reflects the 

cumulative potential risk and is not double counting the relative risk. 

In addition, a pathway score can be assigned without scoring both the soil exposure and 

subsurface intrusion components using this approach. It is possible for a site to have only one 

component evaluated and still reach the same pathway score as under the current HRS. It should 

be observed that because the scoring of the soil exposure component is not being altered, the soil 

exposure component would contribute the same score to the overall site score as it would if the 

subsurface intrusion component is not added.

c. Incorporation of the Soil Exposure and Subsurface Intrusion Pathway Score into a Site Score 

– See section 2.1.1 of the Proposed HRS Addition

EPA is not proposing any changes to the methodology used to assign an overall site score due to 

the addition of the subsurface intrusion component to the soil exposure pathway and renaming 

that pathway the soil exposure and subsurface intrusion pathway. The overall site score remains a 

function of four pathway scores and the same weighting is given to each pathway score as in the 
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current HRS. See section 2.2 of the 1988 Revised HRS TSD for supplemental information on 

why the existing methodology was chosen.

7. Example Site Scoring Scenarios

To evaluate the proposed subsurface intrusion component and factor category weighting, EPA 

developed three conceptual site scenarios: one that would not qualify for the NPL (score below 

28.50); one that would qualify marginally for the NPL (score of about 28.50); and one that should 

clearly qualify for the NPL (site score considerably above 28.50).

The first scenario consists of a ground water plume contaminated with a hazardous substance 

with moderate toxicity that underlies approximately 3 acres of a residential neighborhood 

comprised of single-family detached homes. Indoor air samples have been collected from inside 

two homes and have reported hazardous substance concentrations above background, but below 

the applicable benchmarks. Additionally, several other occupied structures were sampled for 

indoor air and subslab contaminant concentrations; however, no other detections of hazardous 

substances were observed. This site would not qualify for the NPL based on available information 

(i.e., score below 28.50).

The second scenario also consists of a ground water plume contaminated with a hazardous 

substance with moderate toxicity as in the first scenario, but it has a considerably larger plume 

and more targets. The ground water plume underlies approximately 20 acres of a residential 

neighborhood and commercial area comprised of single-family detached homes, a daycare 

facility, and a single-story office building. Indoor air samples collected inside 19 homes, the 

daycare facility, and office building have hazardous substance concentrations above the 

applicable benchmark. Indoor air samples in 5 homes, the daycare facility with approximately 25 
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children enrolled and 6 full-time and 2 part-time workers, and the office building with 18 fulltime workers have hazardous substance concentrations above background, but below the 

applicable benchmark. The homes and daycare facility were checked for indoor sources of 

hazardous substances prior to sampling and such sources were removed if found. This site would 

likely qualify for the NPL based on available information (i.e., score of about 28.50).

The third scenario consists of a ground water plume contaminated with a highly toxic hazardous 

substance and a larger number of targets than the second scenario. The plume underlies

approximately 25 acres of a residential neighborhood and hazardous substance concentrations 

above a benchmark were detected in indoor air samples from 25 homes and one daycare with 

approximately 25 children enrolled and 5 full-time workers. Hazardous substance concentrations 

above background but below benchmarks were detected within 15 homes. The homes and 

daycare facility were checked for indoor sources of hazardous substances prior to sampling and 

such sources were removed if found. Based on available information, this site would qualify for 

the NPL and would likely achieve the maximum HRS score for a single component and pathway 

(i.e., 50.00).

Further evaluation of the varying factor values and resulting HRS site scores, along with further 

discussion of these three scenarios is presented in section 8.1.c of the Proposal TSD.

VII. Summary of Proposed Updates to the HRS (Sections 2, 5, 6, and 7)

A. Addition of an SsI Component to the HRS (Sections 2, 5, and 7)

1. The proposed addition of a subsurface intrusion component is proposed to be added to the 

existing Soil Exposure pathway as section 5.2 in Chapter 5 to the current HRS. The new 
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pathway name is proposed as the Soil Exposure and Subsurface Intrusion Pathway. The 

existing method for evaluating the soil exposure threat will remain unchanged.

2. Chapter 2: Evaluations Common to All Pathways is proposed to be updated to reflect the 

addition of the subsurface intrusion component to the existing soil exposure pathway. The 

evaluations for the current four pathways remain unchanged and a comparable evaluation will 

be added for the subsurface intrusion component. 

3. Chapter 7: Sites Containing Radioactive Substances currently reflects how radioactive 

substances are evaluated in the context of the four current HRS pathways. Updates will be 

made to reflect how radioactive substances are evaluated using the proposed subsurface 

intrusion component.

B. Terminology Updates Affecting Specific Sections of the HRS (Sections 2 , 5 & 6)

During the development of this proposed addition to the HRS, the agency determined that the 

following terms should be updated to reflect current terminology and procedures used by EPA in 

performing risk assessments. 

1. Ambient Water Quality Criteria: Ambient Water Quality Criteria (AWQC) are now identified 

also as National Recommended Water Quality Criteria (NRWQC). In addition, the acute 

AWQC are now identified as the Criterion Maximum Concentration (CMC) and the chronic 

criteria are referred to as the Criterion Continuous Concentration (CCC). (See section 1.1 of 

the proposed HRS addition.) These criteria are used to determine the level of threat to 

environmental targets.

2. Reference Concentrations: For inhalation exposures, EPA is adopting the use of Reference 

Concentrations (RfCs) instead of Reference Doses (RfDs) when determining non-cancer 
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related risk levels. RfCs are used in determining the level of threat to human targets due to 

possible inhalation and when determining the toxicity of the substances.

3. Cancer Unit Risk: For inhalation exposures, EPA is adopting the use of Inhalation Unit Risk 

(IUR) instead of cancer slope factors in determining cancer-related risk levels. IURs are used 

in determining the level of threat to human targets due to possible inhalation and when 

determining the toxicity of the substances.

4. Weight-of-Evidence Groupings: The 2005 EPA weight-of-evidence groupings supporting the 

designation of a substance as a human carcinogen have been incorporated into the HRS 

algorithm for determining the toxicity factor value. (The former EPA weight-of-evidence 

categories included as part of the 1990 HRS have been retained as EPA has not yet completed 

assigning all substances to the revised categories and are doing so at the time the EPA 

substance literature reviews are updated.)

 

VIII. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at 

http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review

This action is a significant regulatory action that was submitted to the Office of Management and 

Budget (OMB) for review. This action may raise novel legal or policy issues arising out of legal 
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mandates, the President’s priorities, or the principles set forth in the EO. Any changes made in 

response to OMB recommendations have been documented in the docket.

EPA prepared an analysis of the potential costs and benefits associated with this action. This 

analysis, Addition of a Subsurface Intrusion (SsI) Component to the Hazard Ranking System 

(HRS): Regulatory Impact Analysis is available in the docket for this action.

B. Paperwork Reduction Act (PRA)

This action does not impose any new information collection burden under the PRA. OMB has 

previously approved the information collection activities contained in the existing regulations and 

has assigned OMB control number 2050-0095. 

This proposed regulatory change will only affect how EPA and organizations performing work on 

behalf of EPA (state or tribal partners) conduct site assessments and HRS scoring at sites where 

certain environmental conditions exist. This proposed regulatory change will result in data 

collection at these types of sites to allow evaluation under the HRS. EPA expects that the total 

number of site assessments performed and the number of sites added to the NPL per year will not 

increase, but rather expects that there will be a realignment and reprioritization of its internal 

resources and state cooperative agreement funding. 

C. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of 

small entities under the RFA. This action will not impose any requirements on small entities. This 

proposed regulatory change enables the HRS evaluation to directly consider human exposure to 
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hazardous substances that enter building structures through subsurface intrusion. This addition to 

the HRS would not impose direct impacts on any other entities. For additional discussion on this 

subject see section 4.9 of the Regulatory Impact Analysis (see the docket for this action). 

D. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, 

and does not significantly or uniquely affect small governments. The action imposes no 

enforceable duty on any state, local, or tribal governments or the private sector. 

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct effects on the 

states, on the relationship between the national government and the states, or on the distribution 

of power and responsibilities among the various levels of government. 

F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments

This action does not have tribal implications, as specified in Executive Order 13175. EPA’s 

evaluation of a site using the HRS does not impose any costs on a tribe (except those already in a 

cooperative agreement relationship with EPA). Thus, Executive Order 13175 does not apply to 

this action. 

Although Executive Order 13175 does not apply to this action, EPA consulted with tribal officials 

through meetings and correspondence, including a letter sent to all federally recognized tribes 

asking for comment on the “Notice of Opportunity for Public Input” that was published in the 
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Federal Register on January 31, 2011 (76 FR 5370), and public listening sessions regarding the 

decision to proceed with the development of this action. All tribal comments indicated support for 

this action. 

EPA specifically solicits additional comment on this proposed action from tribal officials.

G. Executive Order 13045: Protection of Children from Environmental Health Risks and Safety 

Risks

EPA interprets Executive Order 13045 as applying only to those regulatory actions that concern 

environmental health or safety risks that EPA has reason to believe may disproportionately affect 

children, per the definition of “covered regulatory action” in section 2-202 of the Executive 

Order. This action is not subject to Executive Order 13045 because it does not concern an 

environmental health risk or safety risk. 

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy 

Supply, Distribution or Use

This action is not a “significant energy action” because it is not likely to have a significant 

adverse effect on the supply, distribution, or use of energy. The site assessment activities affected 

by this rule are limited in scope and number and rely on existing energy distribution systems. 

Further, we have concluded that this proposed rule would not significantly expand the energy 

demand for site assessments, and would not require an entity to conduct any action that would 

require significant energy use, that would significantly affect energy supply, distribution, or 

usage. Thus, Executive Order 13211 does not apply to this action.
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I. National Technology Transfer and Advancement Act

This rulemaking does not involve technical standards. 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

EPA believes the human health or environmental risk addressed by this action will not have 

potential disproportionately high and adverse human health or environmental effects on minority, 

low-income or indigenous populations. The results of this evaluation are contained in section 

III.C.4 of this preamble and section 4.3 (and all subsections) and Appendix C of the Regulatory 

Impact Analysis for this proposed rulemaking. A copy of the Addition of a Subsurface Intrusion 

(SsI) Component to the Hazard Ranking System (HRS): Regulatory Impact Analysis is available 

in the docket for this action. 

K. Executive Order 12580 – Superfund Implementation

Executive Order 12580, section 1(d), states that revisions to the NCP shall be made in 

consultation with members of the National Response Team (NRT) prior to publication for notice 

and comment. Revisions shall also be made in consultation with the Director of the Federal 

Emergency Management Agency (FEMA) and the Nuclear Regulatory Commission (NRC) to 

avoid inconsistent or duplicative requirements in the emergency planning responsibilities of those 

agencies. Executive Order 12580 delegates responsibility for revision of the NCP to EPA.

The agency has complied with Executive Order 12580 to the extent that it is related to the 

addition of a new component to the HRS, through consultation with members of the NRT.
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List of Subjects in 40 CFR Part 300

Environmental protection, Air pollution control, Chemicals, Hazardous substances, Hazardous 

waste, Intergovernmental relations, Natural resources, Oil pollution, Penalties, Reporting and 

recordkeeping requirements, Superfund, Water pollution control, Water supply.

Dated: 

Gina McCarthy, 

Administrator

For the reasons set out in the preamble, Title 40, Chapter 1 of the Code of Federal 

Regulations is proposed to be amended as follows:

Appendix A of Part 300—Hazard Ranking System 

1. The authority citation for Appendix A of Part 300 is revised to read as follows:

Authority: 33 U.S.C. 1321(c)(2); 42 U.S.C. 9601–9657; E.O. 13626, 77 FR 56749, 3 CFR, 

2013 Comp., p. 306; E.O. 12777, 56 FR 54757, 3 CFR, 1991 Comp., p. 351; E.O. 12580, 52 

FR 2923, 3 CFR, 1987 Comp., p. 193.Amend Appendix A to Part 300:

2. Amend Appendix A to Part 300: 
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a. In section 1.1 by: 

i. Amending the definition “Ambient Water Quality Criteria (AWQC),” by 

removing the definition heading “Ambient Water Quality Criteria (AWQC),” and 

adding “Ambient Water Quality Criteria (AWQC)/National Recommended 

Water Quality Criteria”, in its place; and removing the text “maximum acute or 

chronic toxicity” and adding “maximum acute (Criteria Maximum Concentration 

or CMC) or chronic (Criterion Continuous Concentration or CCC) toxicity.” in

its place;

ii. Adding in alphabetical order the definitions “Channelized flow” and “Crawl 

space”; 

iii. Revising the definition “Distance weight”;

iv. Revising the definition “Half-life”;

v. Removing from the definition “HRS “pathway” the text “soil,” and adding “soil 

exposure and subsurface intrusion,” in its place; 

vi. Adding in alphabetical order the definitions “Indoor air”, “Inhalation Unit Risk 

(IUR)”, “Occupied structures”, “Preferential subsurface intrusion pathways”; 

and “Reference concentration (RfC)”;

vii. Revising the definition “Reference dose (RfD)”; “Screening concentration”, and 

“Slope factor (also referred to as cancer potency factor)”; 
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viii. Adding in alphabetical order the definitions “Soil gas” , “Soil porosity”; 

“Subslab”, “Subsurface Intrusion”, “Surficial ground water”, “Unit Risk”, and 

“Unsaturated Zone”; and

ix. Revising the introductory text of the definition “Weight-of-evidence”.

b. In section 2.0 by:

i. Adding section 2.0 Evaluations Common to all Pathways; and

ii. Removing section 2.0 Evaluations Common to all Pathways.

c. In section 5.0 by:

i. Adding section 5.0 Soil Exposure and Subsurface Intrusion Pathway; and

ii. Removing section 5.0 Soil Exposure and Subsurface Intrusion Pathway.

d. In section 6.0 by: 

i. Revising Table 6-14, entitled “Health-Based Benchmarks for Hazardous 

Substances in Air”.

e. In section 7.0 by:

i. Revising the table entitled “Table 7-1. HRS Factors Evaluated Differently 

For Radionuclides”

ii. Under Table 7-1, revising the 2nd undesignated paragraph.

iii. Revising sections 7.1, 7.1.1, and 7.1.2;
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iv. Revising section 7.2.3;

v. Revising section 7.2.4 by

vi. Removing text “Biodegradation” and adding “degradation” in its place;

vii. Revising section 7.2.5;

a. Revising sections 7.2.5.1, 7.2.5.1.1, 7.2.5.1.2, and 7.2.5.1.3;

b. Amending section 7.2.5.2, by revising the introductory text and the 

last paragraph, and adding a paragraph to the end of the section;

c. Revising section 7.2.5.3;

viii. Revising sections 7.3, 7.3.1, and 7.3.2; and

ix. Adding section 7.3.3.
Page 94 of 214

The revisions and additions read as follows: 

Appendix A of Part 300—Hazard Ranking System 

* * * * *

1.1 Definitions

* * * * *

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.

* * * * *

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.

* * * * *

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]. 

* * * * *

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 determining surface 

water persistence: biodegradation, hydrolysis, photolysis, radioactive decay, and volatilization. 
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The HRS considers two decay processes for determining subsurface intrusion degradation: 

biodegradation and hydrolysis.

* * * * *

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/m3 in 

air. 

* * * * *

Occupied structures: Structures with enclosed air space, either where people are present on a 

regular basis or that were previously occupied but vacated due to a site-related hazardous 

substance(s).

* * * * *

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. 

* * * * *

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.
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* * * * *

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-ofevidence 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. 

* * * * *

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.

* * * * *
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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 the surficial ground water into overlying structures.

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

* * * * *

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/m3 

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 surficial water table (excluding localized or 

perched water).

* * * * *

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 groupings,

depending on the date EPA updated the profile, include either:

* * * * * 

2.0 Evaluations Common to Multiple Pathways
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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: 
Page 99 of 214

SS = �SSgg

2 + SSss2 + SSssssssssss

2 + SSaa

2

4

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.
Page 100 of 214

TABLE 2-1 SAMPLE PATHWAY SCORESHEET
Page 101 of 214

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)
Page 102 of 214

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.

a 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.

b 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 exposure 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, degradation and/or bioaccumulation 

(or ecosystem bioaccumulation) potential, as appropriate to the pathway (component 

or threat). 

 Evaluating hazardous waste quantity. 

– Combining hazardous waste quantity with the other waste characteristics factors. 

 Determining waste characteristics factor category value.
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• 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 

and subsurface intrusion 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, component 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, areas of observed exposure or areas of 

subsurface contamination) at the site. 
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• Hazardous substances associated with these sources (or areas of observed contamination, 

areas of observed exposure or areas of subsurface 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 and subsurface intrusion pathway, identify areas of observed 

contamination, areas of observed exposure, and/or areas of subsurface contamination at the site 

(see sections 5.1.0 and 5.2.0).
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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: ___

Area of observed exposure: ___

Area of subsurface contamination: ___

B. Hazardous substances associated with the source. 

Hazardous 

Substance

Available to Pathway

Air Ground 

water

(GW)

Surface Water 

(SW)

Soil Exposure/Subsurface Intrusion (SESSI)

Soil Exposure Subsurface Intrusion

Gas Particulate Overland/

flood

GW 

to 

SW

Resident Nearby Area of Observed 

Exposure

Area of Subsurface 

Contamination

..... ..... ..... ..... ..... ..... ..... ..... ..... .....

..... ..... ..... ..... ..... ..... ..... ..... ..... .....

..... ..... ..... ..... ..... ..... ..... ..... ..... .....

..... ..... ..... ..... ..... ..... ..... ..... ..... .....

..... ..... ..... ..... ..... ..... ..... ..... ..... .....

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 
Page 106 of 214

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 component of the soil exposure and 

subsurface intrusion pathway, consider only those hazardous substances that meet the criteria for 

observed contamination for that area (see section 5.1.0) to be associated with that area when 

evaluating the pathway.

For an area of observed exposure or area of subsurface contamination (see section 5.2.0) in the 

subsurface intrusion component of the soil exposure and subsurface intrusion pathway, consider 

only those hazardous substances that:

• Meet the criteria for observed exposure, or

• Meet the criteria for observed release in an area of subsurface contamination and has a vapor 

pressure greater than or equal to one torr or a Henry’s constant greater than or equal to 10-5

atm-m3

/mol, or 

• Meet the criteria for an observed release in a structure within, or in a sample from below, an 

area of observed exposure and has a vapor pressure greater than or equal to one torr or a 

Henry’s constant greater than or equal to 10-5 atm-m3

/mol.
Page 107 of 214

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).
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• 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 and subsurface intrusion pathway, consider the following 

hazardous substances available to the pathway: 

• Soil exposure component—resident population threat.

– All hazardous substances that meet the criteria for observed contamination at the site (see 

section 5.1.0).

• Soil exposure component—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 5.1.2.1.1).

• Subsurface intrusion component.

– All hazardous substances that meet the criteria for observed exposure at the site (see 

section 5.2.0).

– All hazardous substances with a vapor pressure greater than or equal to one torr or a 

Henry’s constant greater than or equal to 10-5 atm-m3

/mol that meet the criteria for an 

observed release in an area of subsurface contamination (see section 5.2.0). 

– All hazardous substances that meet the criteria for an observed release in a structure 

within, or in a sample from below, an area of observed exposure (see section 5.2.0).

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 
Page 109 of 214

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 component and for establishing areas of observed exposure and areas of 

subsurface contamination in the subsurface intrusion component of the soil exposure and 

subsurface intrusion pathway, see section 5.1.0 and section 5.2.0). Separate criteria apply to 

radionuclides (see section 7.1.1).

TABLE 2-3 OBSERVED RELEASE CRITERIA FOR CHEMICAL ANALYSIS

Sample Measurement < Sample Quantitation Limit a

No observed release is established.

Sample Measurement ≥ Sample Quantitation Limit a

An observed release is established as follows:

• 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.a
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• 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.

a 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, degradation, and/or bioaccumulation (or ecosystem 

bioaccumulation) potential.

2.4.1 Selection of substance potentially posing greatest hazard. For all pathways 

(components and threats), select the hazardous substance potentially posing the greatest hazard 

for the pathway (component or threat) and use that substance in evaluating the waste 

characteristics category of the pathway (component 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 component of the soil exposure and subsurface intrusion pathway, base the 

selection on the toxicity factor alone. For the subsurface intrusion component of the soil exposure 

and subsurface intrusion pathway, base the selection on the toxicity factor value for the substance, 

combined with its degradation factor value. Evaluation of the toxicity factor is specified in 

section 2.4.1.1. Use and evaluation of the mobility, persistence, degradation, and/or 

bioaccumulation (or ecosystem bioaccumulation) potential factors vary by pathway (component 
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or threat) and are specified under the appropriate pathway (component or threat) section. Section 

2.4.1.2 identifies the specific factors that are combined with toxicity in evaluating each pathway 

(component 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 weightof-evidence ratings for carcinogenicity for all exposure routes except inhalation. Use 

inhalation unit risk (IUR) for inhalation exposure. If an inhalation unit risk or a slope factor is 

not available for a substance, use its ED10 value to estimate a slope factor as follows: 

Slope factor = 1

6 (ED10)

• Noncancer toxicological responses of chronic exposure—use reference dose (RfD) or 

reference concentration (RfC) values as applicable. 

• 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:
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• If RfD/RfC and slope factor/inhalation unit risk values are 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/RfC or slope factor/inhalation unit risk 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/RfC or slope factor/inhalation unit risk). 

• If neither an RfD/RfC nor slope factor/inhalation unit risk 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/RfC and slope 

factor/IUR values are not available. 

• If neither an RfD/RfC, nor slope factor/inhalation unit risk, 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)

Reference dose (RfD) (mg/kg-day) Assigned 

Value

RfD < 0.0005 10,000

0.0005 ≤ RfD < 0.005 1,000

0.005 ≤ RfD < 0.05 100

0.05 ≤ RfD < 0.5 10
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Chronic Toxicity (Human)

Reference dose (RfD) (mg/kg-day) Assigned 

Value

0.5 ≤ RfD 1

RfD not available 0

Reference concentration (RfC) 

(mg/m3)

Assigned 

value

RfC < 0.0001 10,000

0.0001 ≤ RfC < 0.006 1,000

0.006 ≤ RfC < 0.2 100

0.2 ≤ RfC < 2.0 10

2.0 ≤ RfC 1

RfC not available 0

Carcinogenicity (Human)

Weight-of-evidencea/Slope factor (mg/kg-day)-1 Assigned 

value

A or Carcinogenic to humans B or Likely to be 

carcinogenic to humans

C or Suggestive 

evidence of 

carcinogenic 

potential

0.5 ≤ SFb 5 ≤ SF 50 ≤ SF 10,000

0.05 ≤ SF < 0.5 0.5 ≤ SF < 5 5 ≤ SF < 50 1,000

SF < 0.05 0.05 ≤ SF < 0.5 0.5 ≤ SF < 5 100

_ _ _ SF < 0.05 SF < 0.5 10

Slope factor not available. Slope factor not available. Slope factor not 

available.

0
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Weight-of-evidencea/Inhalation unit risk (μg/m3)

A or Carcinogenic to humans B or Likely to be 

carcinogenic to humans

C or Suggestive 

evidence of 

carcinogenic 

potential

Assigned 

value

0.00004 ≤ IURc 0.0004 ≤ IUR 0.004 ≤ IUR 10,000

0.00001 ≤ IUR < 0.00004 0.0001 ≤ IUR < 0.0004

0.001 ≤ IUR < 

0.004

1,000

IUR < 0.00001 0.00001 ≤ IUR < 0.0001

0.0001 ≤ IUR < 

0.001

100

_ _ _ < 0.00001 IUR < 0.0001 10

Inhalation unit risk not available.

Inhalation unit risk not 

available.

Inhalation unit risk 

not available.

0

a A, B, and C, as well as Carcinogenic to humans, Likely to be carcinogenic to humans, and Suggestive evidence 

of carcinogenic potential 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), as well as 

inadequate information to assess carcinogenic potential and not likely to be carcinogenic to humans a value of 0 

for carcinogenicity.

b SF = Slope factor.

c IUR = Inhalation Unit Risk.
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Acute Toxicity (Human)

Oral LD50

(mg/kg)

Dermal LD50

(mg/kg)

Dust or mist LC50

(mg/l)

Gas or vapor LC50

(ppm)

Assigned 

value

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

LD 50 < 5 LD50 < 2 LC50 < 0.2 LC50 < 20 1,000

5 ≤ LD50 < 50 2 ≤ LD50 < 20 0.2 ≤ LC50 < 2 20 ≤ LC50 <200 100

50 ≤ LD50 < 500 20 ≤ LD50 < 200 2 ≤ LC50 <20 200 ≤ LC50 <2,000 10

500 ≤ LD50 200 ≤ LD50 20 ≤ LC50 2,000 ≤ LC50 1

LD50 not 

available

LD50 not available LD50 not available LD50 not available 0

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 
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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). 
Page 117 of 214

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 and subsurface intrusion pathway, determine toxicity and 

toxicity/degradation factor values as follows:

• Soil exposure and subsurface intrusion—soil exposure component.

• 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 (see section 

5.1.1.2.1).

• Soil exposure and subsurface intrusion—subsurface intrusion component.

• Determine a combined human toxicity/degradation factor value for each hazardous 

substance being evaluated that: 

 Meets the criteria for observed exposure, or

 Meets the criteria for observed release in an area of subsurface contamination and has 

a vapor pressure greater than or equal to one torr or a Henry’s constant greater than 

or equal to 10-5 atm-m3

/mol, or 

 Meets the criteria for an observed release in a structure within, or in a sample from 

below, an area of observed exposure and has a vapor pressure greater than or equal to 

one torr or a Henry’s constant greater than or equal to 10-5 atm-m3

/mol. 
Page 118 of 214

• Select the hazardous substance with the highest combined factor value and use that 

substance in evaluating the waste characteristics factor category (see sections 5.2.1.2.1 

and 5.2.1.2). 

2.4.2 Hazardous waste quantity. Evaluate the hazardous waste quantity factor by first 

assigning each source (or area of observed contamination, area of observed exposure or area of 

subsurface 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 component of the soil 

exposure and subsurface intrusion 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 component of the soil exposure and subsurface 

intrusion pathway. 
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In evaluating the hazardous waste quantity factor for the subsurface intrusion component of the 

soil exposure and subsurface intrusion pathway, allocate to each area of observed exposure or 

area of subsurface contamination only those hazardous substances and hazardous wastestreams 

that contain hazardous substances that: 

• Meet the criteria for observed exposure, or

• Meet the criteria for observed release in an area of subsurface contamination and has a vapor 

pressure greater than or equal to one torr or a Henry’s constant greater than or equal to 10-5

atm-m3

/mol, or 

• Meet the criteria for an observed release in a structure within, or in a sample from below, an 

area of observed exposure and has a vapor pressure greater than or equal to one torr or a 

Henry’s constant greater than or equal to 10-5 atm-m3

/mol.

Do not consider other hazardous substances or hazardous wastestreams at the site in evaluating 

this factor for the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway. When determining the hazardous waste quantity for multi-subunit structures, use the 

procedures identified in section 5.2.1.2.2.

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.
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For the soil exposure component of the soil exposure and subsurface intrusion pathway, assign a 

source hazardous waste quantity value to each area of observed contamination, as applicable to 

the threat being evaluated.

For the subsurface intrusion component of the soil exposure and subsurface intrusion pathway, 

assign a source hazardous waste quantity value to each regularly occupied structure within an 

area of observed exposure or an area of subsurface contamination that has a structure containment 

factor value greater than 0.

For determining all hazardous waste quantity calculations except for an unallocated source or an 

area of subsurface contamination, evaluate 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. For an area of subsurface 

contamination, evaluate non-radioactive hazardous substances using only the last two measures 

and evaluate radioactive hazardous substances using hazardous wastestream quantity only. See 

also section 7.0 regarding the evaluation of radioactive substances.

Separate criteria apply for assigning a source hazardous waste quantity value for radionuclides 

(see section 7.2.5).
Page 121 of 214

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:
Page 122 of 214

• 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 and subsurface intrusion pathway—soil exposure component, assign the 

area of observed contamination a value using the Tier A equation of Table 5-2 (section 

5.1.1.2.2).

• For the soil exposure and subsurface intrusion pathway—subsurface intrusion component, 

assign the area of observed exposure a value using the Tier A equation of Table 5-18 (section 

5.2.1.2.2).

If the hazardous constituent quantity for the source (or area of observed contamination or area of 

observed exposure) 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 or 

area of observed exposure] 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 or area of observed exposure) 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 or area of observed exposure) 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
Page 123 of 214

Tier Measure Units Equation for assigning value a

A Hazardous constituent quantity (C) lb C 

Bb Hazardous wastestream quantity (W) lb W/5,000 

Cb Volume (V)

Landfill yd3 V/2,500

Surface impoundment yd3 V/2.5

Surface impoundment 

(buried/backfilled)

yd3 V/2.5

Drums c gallon V/2.5

Tanks and containers other than drums yd3 V/2.5

Contaminated soil yd3 V/2,500

Pile yd3 V/2.5

Other yd3 V/2.5

Db Area (A)

Landfill ft2 A/3,400

Surface impoundment ft2 A/13

Surface impoundment 

(buried/backfilled)

ft2 A/13

Land treatment ft2 A/270

Pile d ft2 A/13

Contaminated soil ft2 A/34,000

a Do not round to nearest integer.

b Convert volume to mass when necessary: 1 ton=2,000 pounds=1 cubic yard=4 drums=200 gallons.
Page 124 of 214

c If actual volume of drums is unavailable, assume 1 drum=50 gallons.

d 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 or area of observed exposure) 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 or area of observed exposure). 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 and subsurface intrusion pathway – soil exposure component, assign the 

area of observed contamination a value using the Tier B equation of Table 5-2 (section 

5.1.1.2.2).

• For the soil exposure and subsurface intrusion pathway – subsurface intrusion component, 

assign the area of observed exposure a value using the Tier B equation of Table 5-18 (section 

5.2.1.2.2).

Do not evaluate the volume and area measures described below if the source is the unallocated 

source or if the following condition applies:
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• 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, area of observed exposure, or area of subsurface 

contamination). For the soil exposure and subsurface intrusion pathway, restrict the use of the 

volume measure to those areas of observed contamination, areas of observed exposure, or areas of 

subsurface contamination as specified in sections 5.1.1.2.2 and 5.2.1.2.2.

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 and subsurface intrusion pathway – soil exposure component, assign 

the area of observed contamination a value for volume using the appropriate Tier C equation 

of Table 5-2 (section 5.1.1.2.2).
Page 126 of 214

• For the soil exposure and subsurface intrusion pathway – subsurface intrusion 

component, assign the value based on the volume of the regularly occupied structures 

within the area of observed exposure or area of subsurface contamination using the Tier 

C equation of Table 5-18 (section 5.2.1.2.2).

If the volume of the source (or volume of the area of observed contamination, area of observed 

exposure, or area of subsurface 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 and subsurface 

intrusion pathway), assign the source (or area of observed contamination, area of observed 

exposure, or area of subsurface 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, area of observed exposure, or area of subsurface 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 and subsurface intrusion pathway—soil exposure component, assign the 

area of observed contamination a value for area using the appropriate Tier D equation of 

Table 5-2 (section 5.1.1.2.2).

• For the soil exposure and subsurface intrusion pathway—subsurface intrusion component, 

assign a value based on the area of regularly occupied structures within the area of observed 
Page 127 of 214

exposure or area of subsurface contamination using the Tier D equation of Table 5-18 

(section 5.2.1.2.2).

2.4.2.1.5 Calculation of source hazardous waste quantity value. Select the highest of the 

values assigned to the source (or areas of observed contamination, areas of observed exposure, or 

areas of subsurface 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, areas of observed exposure, or areas of subsurface 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

Hazardous waste quantity value Assigned value

0 0

1a to 100 1 b

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
Page 128 of 214

a If the hazardous waste quantity value is greater than 0, but less than 1, round it to 1 as specified in text.

b 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 
Page 129 of 214

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 component of the soil exposure and subsurface intrusion 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.

For the subsurface intrusion component of the soil exposure and subsurface intrusion pathway, if 

the hazardous constituent quantity is adequately determined for all areas of observed exposure, 

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 exposure, 

assign either the value from Table 2-6 or assign a factor value as follows:

• If any target for the subsurface intrusion component 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 this component.

• If none of the targets for the subsurface intrusion component is subject to Level I or Level II 

concentrations and if there has been a removal action, assign a factor value as follows:
Page 130 of 214

– Determine the 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 component.

– 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 component.

• Otherwise, if none of the targets for the subsurface intrusion component is subject to Level I 

or Level II concentrations and there has not been a removal action, assign a value from Table 

2-6 or a value of 10, whichever is greater.

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 (component 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 

1x108

. Based on this waste characteristics product, assign a waste characteristics factor category 

value to the pathway (component or threat) from Table 2-7.
Page 131 of 214

TABLE 2-7 WASTE CHARACTERISTICS FACTOR CATEGORY VALUES

Waste characteristics product Assigned value

0 0

Greater than 0 to less than 10 1

10 to less than 1x102 2

1x102 to less than 1x103 3

1x103 to less than 1x104 6

1x104 to less than 1x105 10

1x105 to less than 1x106 18

1x106 to less than 1x107 32

1x107 to less than 1x108 56

1x108 to less than 1x109 100

1x109 to less than 1x1010 180

1x1010 to less than 1x1011 320

1x1011 to less than 1x1012 560

1x1012 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:
Page 132 of 214

• A maximum product of 1x1012, and

• A maximum product exclusive of the bioaccumulation (or ecosystem bioaccumulation) 

potential factor of 1x 108

. 

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, component, and threat). 

• Human population. 

• Resources (these vary by pathway, component, and threat). 

• Sensitive environments (included for the surface water migration pathway, air migration 

pathway, and soil exposure component of the soil exposure and subsurface intrusion 

pathway).

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 or observed exposure) 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 
Page 133 of 214

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 or observed exposure) 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 mediaspecific 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 or observed exposure) 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 or observed exposures based on direct observation, assign 

Level II to targets as specified in sections 3, 4, 5, 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. 
Page 134 of 214

– 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 unless reduced by a 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 component of the soil exposure and subsurface intrusion and air migration 

pathways.) 

• Sum Level II targets. 

• Multiply potential targets in all but the soil exposure and subsurface intrusion pathway 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. For targets within an area of subsurface contamination in the 
Page 135 of 214

subsurface intrusion component of the soil exposure and subsurface intrusion pathway, 

multiply by a weighting factor as directed in section 5.2.1.3.2.3.

• 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 (component or threat) being evaluated. 

• Compare the concentrations of hazardous substances in the sample (or comparable samples) 

to their benchmark concentrations for the pathway (component 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 (component 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 or observed 

exposure) 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 
Page 136 of 214

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/National Recommended Water Quality 

Criteria) 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. 
Page 137 of 214

• Screening concentration for cancer corresponding to that concentration that corresponds to 

the 10−6 individual cancer risk for inhalation exposures (air migration pathway or subsurface 

intrusion component of the soil exposure and subsurface intrusion pathway) or for oral 

exposures (ground water migration pathway; drinking water and human food chain threats in 

surface water migration pathway; and soil exposure and subsurface intrusion pathway). 

• Screening concentration for noncancer toxicological responses corresponding to the RfC for 

inhalation exposures (air migration pathway and subsurface intrusion component of the soil 

exposure and subsurface intrusion pathway) or RfD for oral exposures (ground water 

migration pathway; drinking water and human food chain threats in surface water migration 

pathway; and soil exposure and subsurface intrusion pathway). 

Select the benchmark(s) applicable to the pathway (component 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 (component 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 or observed exposure) 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 or observed exposure) for the sample (or comparable samples) or is eligible to be 
Page 138 of 214

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 either a carcinogen 

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), calculate an index I for the sample location as follows:

I = � Ci

SCi

n

i=1

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 or RfC is available, calculate an index J for the 

sample location as follows: 
Page 139 of 214

I = � Cj

CRj

m

j=1

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 or RfC 

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 (component 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 (component 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.

* * * * *
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5.0 Soil Exposure and Subsurface Intrusion Pathway

5.0.1 Exposure components. Evaluate the soil exposure and subsurface intrusion pathway 

based on two exposure components:

• Soil exposure component (see section 5.1).

• Subsurface intrusion component (see section 5.2).

Score one or both components considering their relative importance. If only one component is 

scored, assign its score as the soil exposure and subsurface intrusion pathway score. If both 

components are scored, sum the two scores and assign it as the soil exposure and subsurface 

intrusion pathway score, subject to a maximum of 100.
Page 141 of 214

Figure 5-1 Overview of the Soil Exposure and Subsurface Intrusion Pathway

Observed Contamination

 Area with Resident

 Targets

Attractiveness/ 

Accessibility

Area of Contamination

Observed Exposure

Potential for Exposure

• Structure Containment

• Depth to Contamination

• Vertical Migration

• Vapor Migration Potential

Toxicity

• Chronic

• Carcinogenic

• Acute

Hazardous Waste Quantity

• Hazardous Constituent Quantity

• Hazardous Wastestream Quantity

• Volume

• Area

Toxicity

• Chronic

• Carcinogenic

• Acute

Degradation

Hazardous Waste Quantity

• Hazardous Constituent Quantity

• Hazardous Wastestream Quantity

• Volume

• Area

Resident Individual

Resident Population

• Level I Concentrations

• Level II Concentrations

Workers

Resources

Terrestrial Sensitive Environments

Nearby Individual

Population Within One Mile

Exposed Individual

Population

• Level I Concentrations

• Level II Concentrations

• Population on ASC

Resources

Resident 

Population

Nearby Population

Subsurface Intrusion 

Component

Toxicity

• Chronic

• Carcinogenic

• Acute

Hazardous Waste Quantity

• Hazardous Constituent Quantity

• Hazardous Wastestream Quantity

• Volume

• Area

Likelihood of Exposure (LE) Waste Characteristics (WC) Targets (T)

Likelihood of Exposure (LE) Waste Characteristics (WC) Targets (T)

+

+

Likelihood of Exposure (LE) Waste Characteristics (WC) Targets (T)

X X

X X

X X

Soil Exposure 

Component
Page 142 of 214

5.1 Soil exposure component. Evaluate the soil exposure component based on two threats: Resident 

population threat and nearby population threat. Evaluate both threats based on three factor categories: Likelihood 

of exposure, waste characteristics, and targets. Figure 5-1 indicates the factors included within each factor 

category for each type of threat. 

Determine the soil exposure component score (Sse) in terms of the factor category values as follows: 

Sse = ∑ (LEi)(WCi)(Ti) 2

i=1

SF

where: 

LEi=Likelihood of exposure factor category value for threat i (that is, resident population threat or nearby 

population threat). 

WCi=Waste characteristics factor category value for threat i. 

Ti=Targets factor category value for threat i. 

SF=Scaling factor. 

Table 5-1 outlines the specific calculation procedure. 
Page 143 of 214
Page 144 of 214

TABLE 5-1 SOIL EXPOSURE COMPONENT SCORESHEET

Factor categories and factors Maximum 

value

Value 

assigned

Resident Population Threat

Likelihood of Exposure

1. Likelihood of Exposure 550

Waste Characteristics

2. Toxicity (a)

3. Hazardous Waste Quantity (a)

4. Waste Characteristics 100

Targets

5. Resident Individual 50

6. Resident Population:

6a. Level I Concentrations (b)

6b. Level II Concentrations (b)

6c. Resident Population (lines 6a + 6b) (b)

7. Workers 15

8. Resources 5

9. Terrestrial Sensitive Environments (c)

10. Targets (lines 5 + 6c + 7 + 8 + 9) (b)

Resident Population Threat Score

11. Resident Population Threat (lines 1x4x10) (b)
Page 145 of 214

Factor categories and factors Maximum 

value

Value 

assigned

Nearby Population Threat

Likelihood of Exposure

12. Attractiveness/Accessibility 100

13. Area of Contamination 100

14. Likelihood of Exposure 500

Waste Characteristics

15. Toxicity (a)

16. Hazardous Waste Quantity (a)

17. Waste Characteristics 100

Targets

18. Nearby Individual 1

19. Population Within 1 Mile (b)

20. Targets (lines 18 + 19) (b)

Nearby Population Threat Score

21. Nearby Population Threat (lines 14x17x20) (b)

Soil Exposure Component Score

22. Soil Exposure Component Score d (Sse), (lines [11+21] /82,500, subject to a maximum 

of 100)

100

a Maximum value applies to waste characteristics category.

b Maximum value not applicable.

c No specific maximum value applies to factor. However, pathway score based solely on terrestrial sensitive 
Page 146 of 214

environments is limited to maximum of 60.

d Do not round to nearest integer.

5.1.0 General considerations. Evaluate the soil exposure component based on areas of 

observed contamination: 

• Consider observed contamination to be present at sampling locations where analytic evidence 

indicates that:

– A hazardous substance attributable to the site is present at a concentration significantly 

above background levels for the site (see Table 2-3 in section 2.3 for the criteria for 

determining analytical significance), and

– This hazardous substance, if not present at the surface, is covered by 2 feet or less of 

cover material (for example, soil). 

• Establish areas of observed contamination based on sampling locations at which there is 

observed contamination as follows: 

– For all sources except contaminated soil, if observed contamination from the site is 

present at any sampling location within the source, consider that entire source to be an 

area of observed contamination. 

– For contaminated soil, consider both the sampling location(s) with observed 

contamination from the site and the area lying between such locations to be an area of 

observed contamination, unless available information indicates otherwise. 

• If an area of observed contamination (or portion of such an area) is covered by a permanent, 

or otherwise maintained, essentially impenetrable material (for example, asphalt) that is not 

more than 2 feet thick, exclude that area (or portion of the area) in evaluating the soil 

exposure component. 
Page 147 of 214

• For an area of observed contamination, consider only those hazardous substances that meet 

the criteria for observed contamination for that area to be associated with that area in 

evaluating the soil exposure component (see section 2.2.2). 

If there is observed contamination, assign scores for the resident population threat and the nearby 

population threat, as specified in sections 5.1.1 and 5.1.2. If there is no observed contamination, 

assign the soil exposure component of the soil exposure and subsurface intrusion pathway a score 

of 0. 

5.1.1 Resident population threat. Evaluate the resident population threat only if there is 

an area of observed contamination in one or more of the following locations: 

• Within the property boundary of a residence, school, or day care center and within 200 feet 

of the respective residence, school, or day care center, or 

• Within a workplace property boundary and within 200 feet of a workplace area, or 

• Within the boundaries of a resource specified in section 5.1.1.3.4, or 

• Within the boundaries of a terrestrial sensitive environment specified in section 5.1.1.3.5. 

If not, assign the resident population threat a value of 0, enter this value in Table 5-1, and proceed 

to the nearby population threat (section 5.1.2). 

5.1.1.1 Likelihood of exposure. Assign a value of 550 to the likelihood of exposure factor 

category for the resident population threat if there is an area of observed contamination in one or 

more locations listed in section 5.1.1. Enter this value in Table 5-1. 

5.1.1.2 Waste characteristics. Evaluate waste characteristics based on two factors: toxicity 

and hazardous waste quantity. Evaluate only those hazardous substances that meet the criteria for 

observed contamination at the site (see section 5.1.0). 
Page 148 of 214

5.1.1.2.1 Toxicity. Assign a toxicity factor value to each hazardous substance as specified in 

section 2.4.1.1. Use the hazardous substance with the highest toxicity factor value to assign the 

value to the toxicity factor for the resident population threat. Enter this value in Table 5-1.

5.1.1.2.2 Hazardous waste quantity. Assign a hazardous waste quantity factor value as 

specified in section 2.4.2. In estimating the hazardous waste quantity, use Table 5-2 and: 

• Consider only the first 2 feet of depth of an area of observed contamination, except as 

specified for the volume measure. 

• Use the volume measure (see section 2.4.2.1.3) only for those types of areas of observed 

contamination listed in Tier C of Table 5-2. In evaluating the volume measure for these listed 

areas of observed contamination, use the full volume, not just the volume within the top 2 

feet. 

• Use the area measure (see section 2.4.2.1.4), not the volume measure, for all other types of 

areas of observed contamination, even if their volume is known. 

Enter the value assigned in Table 5-1. 

TABLE 5-2 HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SOIL EXPOSURE COMPONENT

Tier Measure Units Equation for assigning valuea

A Hazardous Constituent Quantity (C) lb C

Bb Hazardous Wastestream Quantity (W) lb W/5,000

Cb Volume (V)

Surface Impoundmentc yd3 V/2.5

Drumsd gallon V/500
Page 149 of 214

Tier Measure Units Equation for assigning valuea

Tanks and Containers Other Than Drums yd3 V/2.5

Db Area (A)

Landfill ft2 A/34,000

Surface Impoundment ft2 A/13

Surface Impoundment (Buried/backfilled) ft2 A/13

Land treatment ft2 A/270

Pilee ft2 A/34

Contaminated Soil ft2 A/34,000

a Do not round nearest integer.

b Convert volume to mass when necessary: 1 ton=2,000 pounds=1 cubic yard=4 drums=200 gallons.

c Use volume measure only for surface impoundments containing hazardous substances present as liquids. Use 

area measures in Tier D for dry surface impoundments and for buried/backfilled surface impoundments.

d If actual volume of drums is unavailable, assume 1 drum=50 gallons.

e Use land surface area under pile, not surface area of pile.

5.1.1.2.3 Calculation of waste characteristics factor category value. Multiply the toxicity and 

hazardous waste quantity factor values, subject to a maximum product of 1 x 108

. Based on this 

product, assign a value from Table 2-7 (section 2.4.3.1) to the waste characteristics factor 

category. Enter this value in Table 5-1.

5.1.1.3 Targets. Evaluate the targets factor category for the resident population threat based 

on five factors: resident individual, resident population, workers, resources, and terrestrial 

sensitive environments.
Page 150 of 214

In evaluating the targets factor category for the resident population threat, count only the 

following as targets: 

• Resident individual — a person living or attending school or day care on a property with an 

area of observed contamination and whose residence, school, or day care center, respectively, 

is on or within 200 feet of the area of observed contamination. 

• Worker — a person working on a property with an area of observed contamination and

whose workplace area is on or within 200 feet of the area of observed contamination. 

• Resources located on an area of observed contamination, as specified in section 5.1.1. 

• Terrestrial sensitive environments located on an area of observed contamination, as specified 

in section 5.1.1. 

5.1.1.3.1 Resident individual. Evaluate this factor based on whether there is a resident 

individual, as specified in section 5.1.1.3, who is subject to Level I or Level II concentrations. 

First, determine those areas of observed contamination subject to Level I concentrations and those 

subject to Level II concentrations as specified in sections 2.5.1 and 2.5.2. Use the health-based 

benchmarks from Table 5-3 in determining the level of contamination. Then assign a value to the 

resident individual factor as follows: 

• Assign a value of 50 if there is at least one resident individual for one or more areas subject 

to Level I concentrations. 

• Assign a value of 45 if there is no such resident individuals, but there is at least one resident 

individual for one or more areas subject to Level II concentrations. 

• Assign a value of 0 if there is no resident individual. 

Enter the value assigned in Table 5-1. 
Page 151 of 214

5.1.1.3.2 Resident population. Evaluate resident population based on two factors: Level I 

concentrations and Level II concentrations. Determine which factor applies as specified in 

sections 2.5.1 and 2.5.2, using the health-based benchmarks from Table 5-3. Evaluate populations 

subject to Level I concentrations as specified in section 5.1.1.3.2.1 and populations subject to 

Level II concentrations as specified in section 5.1.1.3.2.2. 

TABLE 5-3 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN SOILS

Screening concentration for cancer corresponding to that concentration that corresponds to the 10−6

individual cancer risk for oral exposures.

Screening concentration for noncancer toxicological responses corresponding to the Reference Dose 

(RfD) for oral exposures.

Count only those persons meeting the criteria for resident individual as specified in section 

5.1.1.3. In estimating the number of people living on property with an area of observed 

contamination, when the estimate is based on the number of residences, multiply each residence 

by the average number of persons per residence for the county in which the residence is located. 

5.1.1.3.2.1 Level I concentrations. Sum the number of resident individuals subject to Level I 

concentrations and multiply this sum by 10. Assign the resulting product as the value for this 

factor. Enter this value in Table 5-1.

5.1.1.3.2.2 Level II concentrations. Sum the number of resident individuals subject to Level II 

concentrations. Do not include those people already counted under the Level I concentrations 

factor. Assign this sum as the value for this factor. Enter this value in Table 5-1.
Page 152 of 214

5.1.1.3.2.3 Calculation of resident population factor value. Sum the factor values for Level I 

concentrations and Level II concentrations. Assign this sum as the resident population factor 

value. Enter this value in Table 5-1. 

5.1.1.3.3 Workers. Evaluate this factor based on the number of workers that meet the section 

5.1.1.3 criteria. Assign a value for these workers using Table 5-4. Enter this value in Table 5-1. 

TABLE 5-4 FACTOR VALUES FOR WORKERS 

Number of workers Assigned value

0 0

1 to 100 5

101 to 1,000 10

Greater than 1,000 15

5.1.1.3.4 Resources. Evaluate the resources factor as follows: 

• Assign a value of 5 to the resources factor if one or more of the following is present on an 

area of observed contamination at the site: 

– Commercial agriculture. 

– Commercial silviculture. 

– Commercial livestock production or commercial livestock grazing. 

• Assign a value of 0 if none of the above are present. 

Enter the value assigned in Table 5-1. 
Page 153 of 214

5.1.1.3.5 Terrestrial sensitive environments. Assign value(s) from Table 5-5 to each terrestrial 

sensitive environment that meets the eligibility criteria of section 5.1.1.3. 

Calculate a value (ES) for terrestrial sensitive environments as follows: 

ES = �Si

n

i=1

where: 

Si=Value(s) assigned from Table 5-5 to terrestrial sensitive environment i. 

n=Number of terrestrial sensitive environments meeting section 5.1.1.3 criteria. 

Because the pathway score based solely on terrestrial sensitive environments is limited to a 

maximum of 60, determine the value for the terrestrial sensitive environments factor as follows: 

TABLE 5-5 TERRESTRIAL SENSITIVE ENVIRONMENTS RATING VALUES

Terrestrial sensitive environments Assigned 

value

Terrestrial critical habitata for Federal designated endangered or threatened species

 National Park

 Designated Federal Wilderness Area

 National Monument 

100

Terrestrial habitat known to be used by Federal designated or proposed threatened or 

endangered species

 National Preserve (terrestrial)

 National or State Terrestrial Wildlife Refuge

75
Page 154 of 214

Terrestrial sensitive environments Assigned 

value

 Federal land designated for protection of natural ecosystems

 Administratively proposed Federal Wilderness Area

 Terrestrial areas utilized for breeding by large or dense aggregations of animalsb

Terrestrial habitat known to be used by State designated endangered or threatened 

species

 Terrestrial habitat known to be used by species under review as to its Federal 

designated endangered or

 threatened status

50

State lands designated for wildlife or game management

 State designated Natural Areas

 Particular areas, relatively small in size, important to maintenance of unique biotic 

communities

25

a Critical habitat as defined in 50 CFR 424.02.

b Limit to vertebrate species.

• Multiply the values assigned to the resident population threat for likelihood of exposure (LE), 

waste characteristics (WC), and ES. Divide the product by 82,500. 

– If the result is 60 or less, assign the value ES as the terrestrial sensitive environments 

factor value. 

– If the result exceeds 60, calculate a value EC as follows:

EC = (60)(82,500)

(LE)(WC)
Page 155 of 214

Assign the value EC as the terrestrial sensitive environments factor value. Do not round this value 

to the nearest integer.

Enter the value assigned for the terrestrial sensitive environments factor in Table 5-1.

5.1.1.3.6 Calculation of resident population targets factor category value. Sum the values for 

the resident individual, resident population, workers, resources, and terrestrial sensitive 

environments factors. Do not round to the nearest integer. Assign this sum as the targets factor 

category value for the resident population threat. Enter this value in Table 5-1.

5.1.1.4 Calculation of resident population threat score. Multiply the values for likelihood 

of exposure, waste characteristics, and targets for the resident population threat, and round the 

product to the nearest integer. Assign this product as the resident population threat score. Enter 

this score in Table 5-1. 

5.1.2 Nearby population threat. Include in the nearby population only those individuals 

who live or attend school within a 1-mile travel distance of an area of observed contamination at 

the site and who do not meet the criteria for resident individual as specified in section 5.1.1.3.

Do not consider areas of observed contamination that have an attractiveness/accessibility factor 

value of 0 (see section 5.1.2.1.1) in evaluating the nearby population threat.

5.1.2.1 Likelihood of exposure. Evaluate two factors for the likelihood of exposure factor 

category for the nearby population threat: attractiveness/accessibility and area of contamination.
Page 156 of 214

5.1.2.1.1 Attractiveness/accessibility. Assign a value for attractiveness/accessibility from 

Table 5-6 to each area of observed contamination, excluding any land used for residences. Select 

the highest value assigned to the areas evaluated and use it as the value for the 

attractiveness/accessibility factor. Enter this value in Table 5-1.

5.1.2.1.2 Area of contamination. Evaluate area of contamination based on the total area of the 

areas of observed contamination at the site. Count only the area(s) that meet the criteria in section 

5.1.0 and that receive an attractiveness/accessibility value greater than 0. Assign a value to this 

factor from Table 5-7. Enter this value in Table 5-1.

TABLE 5-6 ATTRACTIVENESS/ACCESSIBILITY VALUES

Area of observed contamination Assigned 

value

Designated recreational area 100

Regularly used for public recreation (for example, fishing, hiking, softball) 75

Accessible and unique recreational area (for example, vacant lots in urban area) 75

Moderately accessible (may have some access improvements, for example, gravel road), 

with some public recreation use

50

Slightly accessible (for example, extremely rural area with no road improvement), with 

some public recreation use

25

Accessible, with no public recreation use 10

Surrounded by maintained fence or combination of maintained fence and natural barriers 5

Physically inaccessible to public, with no evidence of public recreation use 0

TABLE 5-7 AREA OF CONTAMINATION FACTOR VALUES
Page 157 of 214

Total area of the areas of observed contamination (square feet) Assigned value

Less than or equal to 5,000 5

Greater than 5,000 to 125,000 20

Greater than 125,000 to 250,000 40

Greater than 250,000 to 375,000 60

Greater than 375,000 to 500,000 80

Greater than 500,000 100

5.1.2.1.3 Likelihood of exposure factor category value. Assign a value from Table 5-8 to the 

likelihood of exposure factor category, based on the values assigned to the 

attractiveness/accessibility and area of contamination factors. Enter this value in Table 5-1.

TABLE 5-8 NEARBY POPULATION LIKELIHOOD OF EXPOSURE FACTOR VALUES

Area of contamination factor value Attractiveness/accessibility factor value

100 75 50 25 10 5 0

100 500 500 375 250 125 50 0

80 500 375 250 125 50 25 0

60 375 250 125 50 25 5 0

40 250 125 50 25 5 5 0

20 125 50 25 5 5 5 0

5 50 25 5 5 5 5 0
Page 158 of 214

5.1.2.2 Waste characteristics. Evaluate waste characteristics based on two factors: toxicity 

and hazardous waste quantity. Evaluate only those hazardous substances that meet the criteria for 

observed contamination (see section 5.1.0) at areas that can be assigned an 

attractiveness/accessibility factor value greater than 0.

5.1.2.2.1 Toxicity. Assign a toxicity factor value as specified in section 2.4.1.1 to each 

hazardous substance meeting the criteria in section 5.1.2.2. Use the hazardous substance with the 

highest toxicity factor value to assign the value to the toxicity factor for the nearby population 

threat. Enter this value in Table 5-1. 

5.1.2.2.2 Hazardous waste quantity. Assign a value to the hazardous waste quantity factor as 

specified in section 5.1.1.2.2, except: consider only those areas of observed contamination that 

can be assigned an attractiveness/accessibility factor value greater than 0. Enter the value 

assigned in Table 5-1. 

5.1.2.2.3 Calculation of waste characteristics factor category value. Multiply the toxicity and 

hazardous waste quantity factor values, subject to a maximum product of 1 x 108

. Based on this 

product, assign a value from Table 2-7 (section 2.4.3.1) to the waste characteristics factor 

category. Enter this value in Table 5-1. 

5.1.2.3 Targets. Evaluate the targets factory category for the nearby population threat based 

on two factors: nearby individual and population within a 1-mile travel distance from the site.

5.1.2.3.1 Nearby individual. If one or more persons meet the section 5.1.1.3 criteria for a 

resident individual, assign this factor a value of 0. Enter this value in Table 5-1. 

If no person meets the criteria for a resident individual, determine the shortest travel distance 

from the site to any residence or school. In determining the travel distance, measure the shortest 
Page 159 of 214

overland distance an individual would travel from a residence or school to the nearest area of 

observed contamination for the site with an attractiveness/accessibility factor value greater than 0. 

If there are no natural barriers to travel, measure the travel distance as the shortest straight-line 

distance from the residence or school to the area of observed contamination. If natural barriers 

exist (for example, a river), measure the travel distance as the shortest straight-line distance from 

the residence or school to the nearest crossing point and from there as the shortest straight-line 

distance to the area of observed contamination. Based on the shortest travel distance, assign a 

value from Table 5-9 to the nearest individual factor. Enter this value in Table 5-1. 

TABLE 5-9 NEARBY INDIVIDUAL FACTOR VALUES

Travel distance for nearby individual (miles) Assigned value

Greater than 0 to 1/4 1a

Greater than 1/4 to 1 0

a Assign a value of 0 if one or more persons meet the section 5.1.1.3 criteria for resident individual.

5.1.2.3.2 Population within 1 mile. Determine the population within each travel distance 

category of Table 5-10. Count residents and students who attend school within this travel 

distance. Do not include those people already counted in the resident population threat. 

Determine travel distances as specified in section 5.1.2.3.1. 

In estimating residential population, when the estimate is based on the number of residences, 

multiply each residence by the average number of persons per residence for the county in which 

the residence is located. 

Based on the number of people included within a travel distance category, assign a distanceweighted population value for that travel distance from Table 5-10. 
Page 160 of 214

Calculate the value for the population within 1 mile factor (PN) as follows: 

PN = 1

10�Wi

3

i=1

where: 

Wi=Distance-weighted population value from Table 5-10 for travel distance category i. 

If PN is less than 1, do not round it to the nearest integer; if PN is 1 or more, round to the nearest 

integer. Enter this value in Table 5-1. 

5.1.2.3.3 Calculation of nearby population targets factor category value. Sum the values for 

the nearby individual factor and the population within 1 mile factor. Do not round this sum to the 

nearest integer. Assign this sum as the targets factor category value for the nearby population 

threat. Enter this value in Table 5-1. 

TABLE 5-10 DISTANCE WEIGHTED POPULATION VALUES FOR NEARBY POPULATION THREATa

Travel 

distance 

category 

(miles)

Number of people within the travel distance category

0 1 to 

10 

11 

to 

30

31 

to 

100 

101 

to 

300

301 to 

1,000 

1,001 

to 

3,000

3,001 to 

10,000

10,001 

to 

30,000

30,001 to 

100,000

100,001 

to 

300,000

300,001 to 

1,000,000

Greater than 

0 to 1/4

0 0.1 0.4 1.0 4 13 41 130 408 1,303 4,081 13,034

Greater than 

1/4 to 1/2

0 0.05 0.2 0.7 2 7 20 65 204 652 2,041 6,517

Greater than 

1/2 to 1

0 0.02 0.1 0.3 1 3 10 33 102 326 1,020 3,258
Page 161 of 214

a Round the number of people present within a travel distance category to nearest integer. Do not round the 

assigned distance-weighted population value to nearest integer.

5.1.2.4 Calculation of nearby population threat score. Multiply the values for likelihood 

of exposure, waste characteristics, and targets for the nearby population threat, and round the 

product to the nearest integer. Assign this product as the nearby population threat score. Enter this 

score in Table 5-1.

5.1.3 Calculation of soil exposure component score. Sum the resident population 

threat score and the nearby population threat score, and divide the sum by 82,500. Assign the 

resulting value, subject to a maximum of 100, as the soil exposure component score (Sse). Enter 

this score in Table 5-1.
Page 162 of 214

5.2 Subsurface intrusion component. Evaluate the subsurface intrusion component based 

on three factor categories: likelihood of exposure, waste characteristics, and targets. Figure 5-1 

indicates the factors included within each factor category for the subsurface intrusion component. 

Determine the component score (Sssi) in terms of the factor category values as follows: 

Sssi = (LE)(WC)(T)

SF

where: 

LE=Likelihood of exposure factor category value. 

WC=Waste characteristics factor category value. 

T=Targets factor category value. 

SF=Scaling factor. 

Table 5-11 outlines the specific calculation procedure. 
Page 163 of 214

TABLE 5-11 SUBSURFACE INTRUSION COMPONENT SCORESHEET

Factor Categories and Factors Maximum

Value

Value

Assigned

Subsurface Intrusion Component

Likelihood of Exposure:

1. Observed Exposure 550

2. Potential for Exposure

2a. Structure Containment 10

2b. Depth to contamination 10

2c. Vertical Migration 15

2d. Vapor Migration Potential 25

3. Potential for Exposure (lines 2a * (2b+2c+2d), subject to a maximum of 500) 500

4. Likelihood of Exposure (higher of lines 1 or 3) 550 

Waste Characteristics:

5. Toxicity/Degradation (a) 

6. Hazardous Waste Quantity (a) 

7. Waste Characteristics (subject to a maximum of 100) 100 

Targets:

8. Exposed Individual 50 

9. Population:

 9a. Level I Concentrations (b) 
Page 164 of 214

 9b. Level II Concentrations (b) 

9c. Population within an Area of Subsurface Contamination (b)

 9d. Total Population (lines 9a + 9b + 9c) (b) 

10. Resources 5 

11. Targets (lines 8 + 9d + 10) (b) 

Subsurface Intrusion Component Score

12. Subsurface Intrusion Component (lines 4 x 7 x 11)/82,500c (subject to a 

maximum of 100)

100 

Soil Exposure and Subsurface Intrusion Pathway Score

13. Soil Exposure Component + Subsurface Intrusion Component (subject to a 

maximum of 100)

100 

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

c

Do not round to the nearest integer.

5.2.0 General considerations. The subsurface intrusion component evaluates the threats 

from hazardous substances that have or could intrude into regularly occupied structures via 

surficial ground water or the unsaturated zone. Evaluate the subsurface intrusion component 

based on the actual or potential intrusion of hazardous substances into a regularly occupied 

structures that has structure containment value greater than zero ; or actual or potential intrusion 

of hazardous substances exists in the unsaturated zone or the surficial ground water below the 

regularly occupied structures. These structures may or may not have subunits. Subunits are 

partitioned areas within a structure with separate heating, ventilating, and air conditioning 

(HVAC) systems or distinctly different air exchange rates. Subunits include regularly occupied 

partitioned tenant spaces such as office suites, apartments, condos, common or shared areas, and 
Page 165 of 214

portions of residential, commercial or industrial structures with separate heating, ventilating, and 

air conditioning (HVAC) systems.

In evaluating the subsurface intrusion component, consider the following:

• Area(s) of observed exposure: An area of observed exposure is delineated by regularly 

occupied structures with documented contamination meeting observed exposure criteria; an 

area of observed exposure includes regularly occupied structures with samples meeting 

observed exposure criteria or inferred to be within an area of observed exposure based on 

samples meeting observed exposure criteria (see section 5.2.1.1.1 Observed Exposure). 

Establish areas of observed exposure as follows: 

– For regularly occupied structures that have no subunits, consider both the regularly 

occupied structures containing sampling location(s) meeting observed exposure criteria 

for the site and the regularly occupied structure(s) in the area lying between such 

locations to be an area of observed exposure (i.e., inferred to be in an area of observed 

exposure), unless available information indicates otherwise. 

– In multi-story, multi-subunit , regularly occupied structures, consider all subunits on a 

level with sampling locations meeting observed exposure criteria from the site and all 

levels below, if any, to be within an area of observed exposure, unless available 

information indicates otherwise.

– In multi-tenant structures, that do not have a documented observed exposure, but are 

located in an area lying between locations where observed exposures have been 

documented, consider only those regularly occupied subunits, if any, on the lowest level 

of the structure, to be within an area of observed exposure (i.e., inferred to be in an area 

of observed exposure, unless available information indicates otherwise.
Page 166 of 214

• Area(s) of subsurface contamination: An area of subsurface contamination is delineated by 

sampling locations meeting observed release criteria for subsurface intrusion, excluding areas 

of observed exposure (see Table 2-3 in section 2.3). The area within an area of subsurface 

contamination includes potentially exposed populations. If the significant increase in 

hazardous substance levels cannot be attributed at least in part to the site and cannot be 

attributed to other sites, attribution can be established based on the presence of hazardous 

substances in the area of subsurface contamination. Establish areas of subsurface 

contamination as follows:

– Exclude those areas that contain structures meeting the criteria defined as an area of 

observed exposure.

– Consider both the sampling location(s) with subsurface contamination meeting observed 

release criteria from the site and the area lying between such locations to be an area of 

subsurface contamination (i.e., inferred to be in an area of subsurface contamination), 

unless available information indicates otherwise.

– Evaluate an area of subsurface contamination based on hazardous substances that: 

 Meet the criteria for observed exposure, or

 Meet the criteria for observed release in an area of subsurface contamination and 

have a vapor pressure greater than or equal to one torr or a Henry’s constant greater 

than or equal to 10-5 atm-m3

/mol, or 

 Meet the criteria for an observed release in a structure within, or in a sample from 

below, an area of observed exposure and has a vapor pressure greater than or equal to 

one torr or a Henry’s constant greater than or equal to 10-5 atm-m3

/mol.

See Section 7.0 for establishing an area of subsurface contamination based on the presence of 

radioactive hazardous substances.
Page 167 of 214

– Evaluate all structures with no subunits to be in an area of subsurface contamination if 

they are lying between locations of subsurface intrusion samples meeting observed 

release criteria. 

– Evaluate multi-subunit structures as follows:

 If an observed exposure has been documented based on a gaseous indoor air sample, 

consider all regularly occupied subunit(s), if any, on the level immediately above the 

level where an observed exposure has been documented (or has been inferred to be 

within an area of observed exposure), to be within an area of subsurface 

contamination, unless available information indicates otherwise.

 If observed release criteria have been met based on a gaseous indoor air sample 

collected from a level not regularly occupied, consider all regularly occupied 

subunit(s), if any, on the level immediately above the level where the observed 

release criteria has been documented, to be within an area of subsurface 

contamination, unless available information indicates otherwise.

 If an observed exposure has been documented based on an intruded liquid or 

particulate sample, do not consider any regularly occupied subunit(s) above the level 

where an observed exposure has been documented to be within an area of subsurface 

contamination, unless available information indicates otherwise.

 If any regularly occupied multi-subunit structure is inferred to be in an area of 

subsurface contamination, consider only those regularly occupied subunit(s), if any, 

on the lowest level, to be within an area of subsurface contamination, unless 

available information indicates otherwise.

If there is no area of observed exposure and no area of subsurface contamination, assign a score 

of 0 for the subsurface intrusion component.
Page 168 of 214

5.2.1 Subsurface intrusion component. Evaluate this component only if there is an area 

of observed exposure or area of subsurface contamination: 

• Within or underlying a residence, school, day care center, workplace, or

• Within or underlying a resource specified in section 5.2.1.3.3.

5.2.1.1 Likelihood of exposure. Assign a value of 550 to the likelihood of exposure factor 

category for the subsurface intrusion component if there is an area of observed exposure in one or 

more locations listed in section 5.2.1. Enter this value in Table 5-11. 

5.2.1.1.1 Observed exposure. Establish observed exposure in a regularly occupied 

structure by demonstrating that a hazardous substance has been released into a regularly occupied 

structure via the subsurface. Base this demonstration on either of the following criterion: 

• Direct observation: 

– A solid, liquid or gaseous material that contains one or more hazardous substances 

attributable to the site has been observed entering a regularly occupied structure through 

migration via the subsurface or is known to have entered a regularly occupied structure 

via the subsurface, or 

– When evidence supports the inference of subsurface intrusion of a material that contains 

one or more hazardous substances associated with the site into a regularly occupied 

structure, demonstrated adverse effects associated with that release may be used to 

establish observed exposure.

• Chemical analysis: 
Page 169 of 214

– Analysis of indoor samples indicates that the concentration of hazardous substance(s) has 

increased significantly above the background concentration for the site for that type of 

sample (see section 2.3). 

– Some portion of the significant increase must be attributable to the site to establish the 

observed exposure. Documentation of this attribution should account for possible 

concentrations of the hazardous substance(s) in outdoor air or from materials found in the 

regularly occupied structure, and should provide a rationale for the increase being from 

subsurface intrusion.

If observed exposure can be established in a regularly occupied structure, assign an observed 

exposure factor value of 550, enter this value in Table 5-11, and proceed to section 5.2.1.1.3. If 

no observed exposure can be established, assign an observed exposure factor value of 0, enter this 

value in Table 5-11, and proceed to section 5.2.1.1.2. 

5.2.1.1.2 Potential for exposure. Evaluate potential for exposure only if an observed 

exposure cannot be established, but an area of subsurface contamination has been delineated. 

Evaluate potential for exposure based only on the presence of hazardous substances with a vapor 

pressure greater than or equal to one torr or a Henry’s constant greater than or equal to 10-5 atmm3

/mol. Evaluate potential for exposure for each area of subsurface contamination based on four 

factors: structure containment (see section 5.2.1.1.2.1), depth to contamination (see section 

5.2.1.1.2.2), vertical migration (see section 5.2.1.1.2.3) and vapor migration potential (see section 

5.2.1.1.2.4). For each area of subsurface contamination, assign the highest value for each factor. 

If information is insufficient to calculate any single factor value used to calculate the potential for 

exposure factor values at an identified area of subsurface contamination, information collected for 

another area of subsurface contamination at the site may be used when evaluating potential for 

exposure. Calculate the potential for exposure value for the site as specified in section 5.2.1.1.2.5.
Page 170 of 214

5.2.1.1.2.1 Structure containment. Calculate containment for eligible hazardous substances 

within this component as directed in Table 5-12 and enter this value into Table 5-11. Assign each 

regularly occupied structure within an area of subsurface contamination the highest appropriate 

structure containment value from Table 5-12 and use the regularly occupied structure at the site 

with the highest structure containment value in performing the potential for exposure calculation. 

Assign a structure containment factor value of 10 to any regularly occupied structure located 

within an area of observed exposure that is established based on documented surficial ground 

water intrusion, unless available information indicates otherwise.

TABLE 5-12 STRUCTURE CONTAINMENT

No. Evidence of Structure Containment

Assigned 

value

1.

Regularly occupied structure with evidence of subsurface intrusion, including 

documented observed exposure or sampling of bio or inert gases, such as methane 

and radon.

10

2.

Regularly occupied structure with open preferential pathways from the subsurface 

(e.g., sumps, foundation cracks, unsealed utility lines).

10

3.

Regularly occupied structure with an engineered vapor migration barrier system that 

does not address all preferential pathways.

7

4.

Regularly occupied structure with an engineered passive vapor mitigation system 

without documented institutional controls (e.g., deed restrictions) or evidence of 

regular maintenance and inspection.

6

5.

Regularly occupied structure with no visible open preferential pathways from the 

subsurface (e.g., sumps, foundation cracks, unsealed utility lines).

4
Page 171 of 214

6.

Regularly occupied structure with an engineered passive vapor mitigation system 

(e.g., passive venting) with documented institutional controls (e.g., deed restrictions) 

or evidence of regular maintenance and inspection.

3

7.

Regularly occupied structure with an engineered, active vapor mitigation system 

(e.g., active venting) without documented institutional controls (e.g., deed 

restrictions) and funding in place for on-going operation, inspection and 

maintenance.

2

8. Regularly occupied structure with unknown containment features. 2

9.

Regularly occupied structure with a permanent engineered, active vapor mitigation 

system (e.g., active venting) with documented institutional controls (e.g., deed 

restrictions) and funding in place for on-going operation, inspection and 

maintenance. This does not include mitigation systems installed as part of a 

removal or other temporary response by federal, state or tribal authorities. 

0

10.

Regularly occupied structure with a foundation raised greater than 6 feet (e.g., 

structure on stilts) or structure that has been built, and maintained, in a manner to 

prevent subsurface intrusion

0

5.2.1.1.2.2 Depth to contamination. Assign each area of subsurface contamination a depth to 

contamination based on the least depth to either contaminated crawl space or subsurface media 

underlying a regularly occupied structure. Measure this depth to contamination based on the 

distance between the lowest point of a regularly occupied structure to the highest known point of 

hazardous substances eligible to be evaluated. Use any regularly occupied structure within an area 

of subsurface contamination with a structure containment factor greater than zero. Subtract from 

the depth to contamination the thickness of any subsurface layer composed of features that would 

allow channelized flow (e.g., karst, lava tubes, open fractures).
Page 172 of 214

Based on this calculated depth, assign a factor value from Table 5-13. If the necessary 

information is available at multiple locations, calculate the depth to contamination at each 

location. Use the location having the least depth to contamination to assign the factor value. Enter 

this value in Table 5-11.

TABLE 5-13 DEPTH TO CONTAMINATION

Depth range1, 2

Depth to Contamination

Assigned Value

0 to 10 ft.(Including subslab and 

semi-enclosed or enclosed crawl 

space contamination)

10

>10 to 20ft 8

>20 to 50ft 6

>50 to 100ft 4

>100 to 150ft 2

>150ft 0

1 If any part of the subsurface profile has channelized flow features, assign that portion of the subsurface profile a 

depth of 0.

2 Measure elevation below any regularly occupied structure within an area of subsurface contamination at a site. Select 

the regularly occupied structure with the least depth to contamination below a structure. 

5.2.1.1.2.3 Vertical migration. Evaluate the vertical migration factor for each area of 

subsurface contamination based on the geologic materials in the interval between the lowest point 

of a regularly occupied structure and the highest known point of hazardous substances in the 

subsurface. Use any regularly occupied structure either within an area of subsurface 

contamination or overlying subsurface soil gas or ground water contamination. Assign a value to 

the vertical migration factor as follows: 

• If the depth to contamination (see section 5.2.1.1.2.2) is 10 feet or less, assign a value of 15.
Page 173 of 214

• Do not consider layers or portions of layers within the first 10 feet of the depth to 

contamination. 

• If, for the interval identified above, all layers that underlie a portion of a regularly occupied 

structure at the site are karst or otherwise allow channelized flow, assign a value of 15. 

• Otherwise: 

– Select the lowest effective porosity/permeability layer(s) from within the above interval. 

Consider only layers at least 1 foot thick. (If site-specific data is not available, use the 

layer with the highest value assigned in Table 5-14.)

– Assign a value for individual layers from Table 5-14. 

– If more than one layer has the same assigned porosity/permeability value, include all such 

layers and sum their thicknesses. Assign a thickness of 0 feet to a layer with channelized 

flow features found within any area of subsurface contamination at the site.

– Assign a value from Table 5-15 to the vertical migration factor, based on the thickness 

and assigned porosity/permeability value of the lowest effective porosity/permeability 

layer(s). 

Determine vertical migration only at locations within an area of subsurface contamination at the 

site. If the necessary subsurface geologic information is available at multiple locations, evaluate 

the vertical migration factor at each location. Use the location having the highest vertical 

migration factor value to assign the factor value. Enter this value in Table 5-11.

TABLE 5-14 EFFECTIVE POROSITY/PERMEABILITY OF GEOLOGIC MATERIALS

Type of Material

Assigned 

Porosity/Permeability 

Value
Page 174 of 214

Gravel; highly permeable fractured igneous and metamorphic rocks; permeable basalt; 

karst limestones and dolomites.

1

Sand; sandy clays; sandy loams; loamy sands; sandy silts; sediments that are 

predominantly sand; highly permeable till (coarse-grained, unconsolidated or compact 

and highly fractured); peat; moderately permeable limestones and dolomites (no karst); 

moderately permeable sandstone; moderately permeable fractured igneous and 

metamorphic rocks.

2

Silt; loams; silty loams; loesses; silty clays; sediments that are predominantly silts; 

moderately permeable till (fine-grained, unconsolidated till, or compact till with some 

fractures); low permeability limestones and dolomites (no karst); low permeability 

sandstone; low permeability fractured igneous and metamorphic rocks.

3

Clay; low permeability till (compact unfractured till); shale; unfractured metamorphic 

and igneous rocks.

4

TABLE 5-15 VERTICAL MIGRATION FACTOR VALUES a

Assigned 

Porosity/

Permeability 

Value

Thickness of Lowest Porosity Layer(s)b (feet)

0 to 5

Greater than 

5 to 10

Greater than 

10 to 20

Greater than 

20 to 50

Greater than 

50 to 100

Greater than 

100 to 150

1 15 15 14 11 8 6

2 15 14 12 9 6 4

3 15 13 10 7 5 2

4 15 12 9 6 3 1

a If depth to contamination is 10 feet or less or if, for the interval being evaluated, all layers that underlie a portion of 

the structure at the site are karst or have other channelized flow features, assign a value of 15.

b Consider only layers at least 1 foot thick.
Page 175 of 214

5.2.1.1.2.4 Vapor migration potential. Evaluate this factor for each area of subsurface 

contamination as follows: 

• If the depth to contamination (see section 5.2.1.1.2.2) is 10 feet or less, assign a value of 25.

• Assign a value for vapor migration potential to each of the gaseous hazardous substances 

associated with the area of subsurface contamination (see section 2.2.2) as follows: 

– Assign values from Table 5-16 for both vapor pressure and Henry's constant to each 

hazardous substance. If Henry's constant cannot be determined for a hazardous substance, 

assign that hazardous substance a value of 2 for the Henry's constant component. 

– Sum the two values assigned to each hazardous substance. 

– Based on this sum, assign each hazardous substance a value from Table 5-17 for vapor 

migration potential.

• Assign a value for vapor migration potential to each area of subsurface contamination as 

follows: 

– Select the hazardous substance associated with the area of subsurface contamination with 

the highest vapor migration potential value and assign this value as the vapor migration 

potential factor for the area of subsurface contamination. 

Enter this value in Table 5-11.

TABLE 5-16 VALUES FOR VAPOR PRESSURE AND HENRY’S CONSTANT

Vapor Pressure (Torr) Assigned Value

Greater than 10 3
Page 176 of 214

1 to 10 2

Less than 1 0

Henry’s Constant (atm-m3/mol) Assigned Value

Greater than 10-3 3

Greater than 10-4 to 10-3 2

10-5 to 10-4 1

Less than 10-5 0

TABLE 5-17 VAPOR MIGRATION POTENTIAL FACTOR VALUES FOR A HAZARDOUS SUBSTANCE

Sum of Values for Vapor Pressure and 

Henry’s Constant

Assigned Value

0 0

1 or 2 5

3 or 4 15

5 or 6 25

5.2.1.1.2.5 Calculation of potential for exposure factor value. For each identified area of 

subsurface contamination, sum the factor values for depth to contamination, vertical migration 

and vapor migration potential, and multiply this sum by the factor value for structure 

containment. Select the highest product for any area of subsurface contamination and assign this 

value as the potential for exposure factor value for the component. Enter this value in Table 5-11.

5.2.1.1.3 Calculation of likelihood of exposure factor category value. If observed exposure 

is established for the site, assign the observed exposure factor value of 550 as the likelihood of 

exposure factor category value for the site. Otherwise, assign the potential for exposure factor 
Page 177 of 214

value for the component as the likelihood of exposure value. Enter the value assigned in Table 5-

11.

5.2.1.2 Waste characteristics. Evaluate waste characteristics based on two factors: 

toxicity/degradation and hazardous waste quantity. 

5.2.1.2.1 Toxicity/degradation. For each hazardous substance, assign a toxicity factor 

value, a degradation factor value and a combined toxicity/degradation factor value as specified in 

sections 2.2.3, 2.4.1.2 and 5.2.1.2.1.1 through 5.2.1.2.1.3.

5.2.1.2.1.1 Toxicity. Assign a toxicity factor value to each hazardous substance as specified 

in sections 2.2.2 and 2.4.1.1. 

5.2.1.2.1.2 Degradation. Assign a degradation factor value to each hazardous substance as 

follows: 

• For any hazardous substance that meets the criteria for an observed exposure, assign that 

substance a degradation factor value of 1.

• For all hazardous substances at the site that meet subsurface intrusion observed release 

criteria but not observed exposure criteria, assign a degradation factor value of 1 if the depth 

to contamination below an area of subsurface contamination or area of observed exposure is 

less than 10 feet or if available evidence suggests that there is less than 10 feet of biologically 

active soil in the subsurface anywhere underneath a regularly occupied structure within an 

area of subsurface contamination or area of observed exposure. 

For all other situations first calculate the half-life for each hazardous substance that meets 

subsurface intrusion observed release criteria as follows:
Page 178 of 214

The half-life or a substance in the subsurface is defined for HRS purposes as the time required to 

reduce the initial concentration in the subsurface by one-half as a result of the combined decay 

processes of two components: biodegradation and hydrolysis.

Estimate the half-life (t1/2) of a hazardous substance as follows: 

t1/2 = 1

 1/h + 1/b

where: 

h=Hydrolysis half-life. 

b=Biodegradation half-life. 

If one of these component half-lives cannot be estimated for the hazardous substance from 

available data, delete that component half-life from the above equation.

If no half-life information is available for a hazardous substance and the substance is not already 

assigned a value of 1, unless information indicates otherwise, all straight-chain and simple-ring 

structure substances will be considered to have a half-life less than 30 days if not the hazardous 

substance will be assigned a half-life of greater than 100 days.

Based on the hazardous substance’s assigned half-life the degradation factor is assigned as 

follows:
Page 179 of 214

• For all hazardous substances at the site that meet subsurface intrusion observed release 

criteria but not observed exposure criteria, assign a degradation factor value of 0.1, if:

– The depth to contamination at the site is greater than or equal to 10 feet, but not if 

available evidence suggests that at least 10 feet of biologically active soil is not present in 

the subsurface anywhere underneath a structure within an area of subsurface 

contamination or area of observed exposure, and

– The hazardous substance has a half-life of 30 days or less.

• For all hazardous substances at the site that meet subsurface intrusion observed release 

criteria but not observed exposure criteria, assign a degradation factor value of 0.5, if:

– The depth to contamination at the site is greater than 30 feet, but not if available evidence 

suggests that at least 30 feet of biologically active soil is not present in the subsurface 

anywhere underneath a structure within an area of subsurface contamination or area of 

observed exposure, and

– The hazardous substance has a half-life equal to or less than 100 days.

• For all other situations assign a degradation factor of 1 for all hazardous substances at the site 

that meet subsurface intrusion observed release criteria. 

In addition, for hazardous substances that meet observed release criteria, have a parent-daughter 

degradation relationship, and the daughter substance is found only in samples with a depth greater 

than 10 feet, assign the daughter substance degradation factor value as follows:

1. Identify the shallowest subsurface sample that contains the daughter substance.

2. Determine if the selected sample or another sample from the same relative position in the 

media of concern, or in a shallower sample, contains the parent substance.
Page 180 of 214

3. If the parent substance is not present in the identified samples, assign the degradation factor 

value for the daughter substance based on the half-life for the daughter substance.

4. If the parent substance is present in a sample from the same relative position in the subsurface 

or in a shallower sample, compare the half-life-based degradation factor value for the 

daughter substance to the degradation factor value assigned to the parent substance. Assign 

the greater of the two values as the degradation factor value for the daughter substance.

5.2.1.2.1.3 Calculation of toxicity/degradation factor value. Assign each substance a 

toxicity/biodegradation value by multiplying the toxicity factor value by the degradation factor 

value. Use the hazardous substance with the highest combined toxicity/degradation value to 

assign the factor value to the toxicity/degradation factor for the subsurface intrusion threat. Enter 

this value in Table 5-11.

5.2.1.2.2 Hazardous waste quantity. Assign a hazardous waste quantity factor value as 

specified in section 2.4.2. Consider only those regularly occupied structures with a non-zero 

structure containment value. In estimating the hazardous waste quantity, use Tables 2-5 and 5-18 

and: 

• For Tier A, hazardous constituent quantity, use the mass of constituents found in the regularly 

occupied structure(s) where the observed exposure has been identified.

– For multi-subunit structures, when calculating Tier A, use the mass of constituents found 

in the regularly occupied subunit space(s) where the observed exposure has been 

identified.

• For Tier B, hazardous wastestream quantity, use the flow-through volume of the regularly 

occupied structures where the observed exposure has been identified.
Page 181 of 214

– For multi-subunit structures, when calculating Tier B, use the flow-through volume of the 

regularly occupied subunit spaces where the observed exposure has been identified.

• For Tier C, volume, use the volume divisor listed in Tier C of Table 5-18. Volume is 

calculated for those regularly occupied structures located within areas of observed exposure 

with observed or inferred intrusion and within areas of subsurface contamination. 

– In evaluating the volume measure for these listed areas of observed exposure and areas of 

subsurface contamination based on a gaseous/vapor intrusion or the potential for 

gaseous/vapor intrusion, consider the following:

 Calculate the volume of each regularly occupied structure based on actual data. If 

unknown, use a ceiling height of 8 feet. 

 For multi-subunit structures, when calculating Tier C, calculate volume for those 

subunit spaces with observed or inferred exposure and all other regularly occupied 

subunit spaces on that level, unless available information indicates otherwise. If the 

structure has multiple stories, also include the volume of all regularly occupied 

subunit spaces below the floor with an observed exposure and one story above, 

unless evidence indicates otherwise.

 For multi-subunit structures within an area of subsurface contamination and no 

observed or inferred exposure, consider only the volume of the regularly occupied 

subunit spaces on the lowest story, unless available information indicates otherwise.

– In evaluating the volume measure for these listed areas of observed exposure and areas of 

subsurface contamination where intrusion of contaminated ground water has occurred, do 

not calculate the volume of each regularly occupied structure. Instead, consider only the 

volume of contaminated ground water known to have intruded into a regularly occupied 

structure.
Page 182 of 214

• For Tier D, area, if volume is unknown, use the area divisor listed in Tier D of Table 5-18 for 

those regularly occupied structures within areas of observed exposure with observed or 

inferred intrusion and within areas of subsurface contamination. In evaluating the area 

measure for these listed areas of observed exposure and areas of subsurface contamination, 

calculate the area of each regularly occupied structure (including multi-subunit structures) 

based on actual footprint area data. If the actual footprint area of the structure(s) is unknown, 

use an area of 1,740 square feet for each structure (or subunit space).

TABLE 5-18 HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SUBSURFACE INTRUSION COMPONENT

Tier Measure Units Equation for assigning 

valuea

A Hazardous Constituent Quantity (C) lb C

Bb Hazardous Wastestream Quantity (W) lb W/5,000

Cb,c Volume (V)

Regularly occupied structure(s) in areas of observed exposure or 

subsurface contamination

yd3 V/2.5

Db,d Area (A)

Regularly occupied structure(s) in areas of observed exposure or 

subsurface contamination

ft2 A/13

a Do not round to the nearest integer.

b Convert volume to mass when necessary: 1 ton=2,000 pounds=1 cubic yard=4 drums=200 gallons.

c Calculate volume of each regularly occupied structure or subunit space in areas of observed exposure and areas of 

subsurface contamination – Assume 8-foot ceiling height unless actual value is known.

d Calculate area of the footprint of each regularly occupied structure in areas of observed exposure and areas of 

subsurface contamination. If the footprint area of a regularly occupied structure is unknown, use 1,740 square feet as 

the footprint area of the structure or subunit space.
Page 183 of 214

For the subsurface intrusion component, if the hazardous constituent quantity is adequately 

determined for all areas of observed exposure, 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 exposure or if one or more areas of subsurface contamination are 

present, assign either the value from Table 2-6 or assign a factor value as follows:

• If any target for the subsurface intrusion component 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 this component.

• If none of the targets for the subsurface intrusion component is subject to Level I or Level II 

concentrations and if there has been a removal action that does not permanently interrupt 

target exposure from subsurface intrusion, assign a factor value as follows:

– Determine the 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 component.

– 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 component.
Page 184 of 214

• Otherwise, if none of the targets for the subsurface intrusion component is subject to Level I 

or Level II concentrations and there has not been a removal action, assign a minimum value 

of 10.

Enter the value assigned in Table 5-11. 

5.2.1.2.3 Calculation of waste characteristics factor category value. Multiply the 

toxicity/degradation and hazardous waste quantity factor values, subject to a maximum product of 

1x108

. Based on this product, assign a value from Table 2-7 (section 2.4.3.1) to the waste 

characteristics factor category. Enter this value in Table 5-11.

5.2.1.3 Targets. Evaluate the targets factor category for the subsurface intrusion threat based on 

three factors: exposed individual, population, and resources in regularly occupied structures. 

Evaluate only those targets within areas of observed exposure and areas of subsurface 

contamination (see section 5.2.0).

In evaluating the targets factor category for the subsurface intrusion threat, count only the 

following as targets: 

• Exposed individual—a person living, attending school or day care, or working in a regularly 

occupied structure with observed exposure or in a structure within an area of observed 

exposure or within an area of subsurface contamination.

• Population—exposed individuals in a regularly occupied structure within an area of observed 

exposure or within an area of subsurface contamination.

• Resources—located within an area of observed exposure or within an area of subsurface 

contamination as specified in section 5.2.1.3.3. 
Page 185 of 214

If a former structure that has been vacated due to subsurface intrusion attributable to the site, 

count the initial targets as if they were still residing in the structure. In addition, if a removal 

action has occurred that has not completely mitigated the release, count the initial targets as if the 

removal action has not permanently interrupted target exposure from subsurface intrusion.

For populations residing in or working in a multi-subunit structure with multiple stories in an area 

of observed exposure or area of subsurface contamination, count these targets as follows:

• If there is no observed exposure within the structure, include in the evaluation only those 

targets, if any, in the lowest occupied level, unless available information indicates otherwise.

• If there is an observed exposure in any level, include in the evaluation those targets in that 

level, the level above and all levels below, unless available information indicates otherwise. 

(The weighting of these targets is specified in Section 5.2.1.3.2.) 

5.2.1.3.1 Exposed individual. Evaluate this factor based on whether there is an exposed 

individual, as specified in sections 2.5.1, 2.5.2 and 5.2.1.3, who is subject to Level I or Level II 

concentrations.

First, determine those regularly occupied structures or partitioned subunit(s) within structures in 

an area of observed exposure subject to Level I concentrations and those subject to Level II 

concentrations as specified as follows (see section 5.2.0):

• Level I Concentrations: For contamination resulting from subsurface intrusion, compare the 

hazardous substance concentrations in any sample meeting the observed exposure by 
Page 186 of 214

chemical analysis criteria to the appropriate benchmark. Use the health-based benchmarks 

from Table 5-19 to determine the level of contamination. 

– If the sample is from a structure with no subunits and the concentration equals or exceeds 

the appropriate benchmark, assign Level I concentrations to the entire structure. 

– If the sample is from a subunit within a structure and the concentration from that subunit 

equals or exceeds the appropriate benchmark, assign Level I concentrations to that 

subunit. 

• Level II Concentrations: Structures, or subunits within structures, with one or more samples 

that meet observed exposure by chemical analysis criteria but do not equal or exceed the 

appropriate benchmark; structures, or subunits, that have an observed exposure by direct 

observation; and structures inferred to be in an area of observed exposure based on samples 

meeting observed exposure, are assigned Level II concentrations. 

o For all regularly occupied structures, or subunits in such structures, in an area of observed 

exposure that are not assigned Level I concentrations, assign Level II concentrations.

Then assign a value to the exposed individual factor as follows: 

• Assign a value of 50 if there is at least one exposed individual in one or more regularly 

occupied structures subject to Level I concentrations. 

• Assign a value of 45 if there are no Level I exposed individuals, but there is at least one 

exposed individual in one or more regularly occupied structures subject to Level II 

concentrations. 

• Assign a value of 20 if there is no Level I or Level II exposed individual but there is at least 

one individual in a regularly occupied structure within an area of subsurface contamination.
Page 187 of 214

Enter the value assigned in Table 5-11. 

5.2.1.3.2 Population. Evaluate population based on three factors: Level I concentrations, 

Level II concentrations, and population within an area of subsurface contamination. Determine 

which factors apply as specified in section 5.2.1.3.1, using the health-based benchmarks from 

Table 5-19. Evaluate populations subject to Level I and Level II concentrations as specified in 

section 2.5. 

TABLE 5-19 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN THE SUBSURFACE INTRUSION 

COMPONENT

Screening concentration for cancer corresponding to that concentration that corresponds to the 10−6 individual cancer 

risk using the inhalation unit risk. For oral exposures use the oral cancer slope factor. 

Screening concentration for noncancer toxicological responses corresponding to the reference dose (RfD) for oral 

exposure and the reference concentration (RfC) for inhalation exposures. 

Count only those persons meeting the criteria for population as specified in section 5.2.1.3. In 

estimating the number of individuals in structures in an area of observed exposure or area of 

subsurface contamination if the actual number of residents is not known, multiply each residence 

by the average number of persons per residence for the county in which the residence is located. 

5.2.1.3.2.1 Level I concentrations. Assign the population subject to Level I concentrations as 

follows:

1. Identify all exposed individuals regularly present in a structure, or if the structure has 

subunits, identify those regularly present in each subunit, located in an area of observed 

exposure subject to Level I concentrations as described in sections 5.2.0 and 5.2.1.3.1. 

Identify only once per structure those exposed individuals that are using more than one 
Page 188 of 214

eligible subunit of the same structure (e.g., using a common or shared area and other parts of 

the same structure).

2. For each structure or subunit count the number of individuals residing in or attending school 

or day care in the structure or subunit. 

3. Count the number of full-time and part-time workers in the structure or subunit(s) subject to 

Level I concentrations. If information is unavailable to classify a worker as full- or part-time, 

evaluate that worker as being full-time. Divide the number of full-time workers by 3 and the 

number of part-time workers by 6, and then sum these products with the number of other 

individuals for each structure or subunit. 

4. Sum this combined value for all structures, or subunits, within areas of observed exposure 

and multiply this sum by 10. 

Assign the resulting product as the combined population factor value subject to Level I 

concentrations for the site. Enter this value in line 9a of Table 5-11.

5.2.1.3.2.2 Level II concentrations. Assign the population subject to Level II concentrations 

as follows:

1. Identify all exposed individuals regularly present in an eligible structure, or if the structure 

has subunits, identify those regularly present in each subunit, located in an area of observed 

exposure subject to Level II concentrations as described in sections 5.2.0 and 5.2.1.3.1. 

Identify only once per structure those exposed individuals that are using more than one 

eligible subunit of the same structure (e.g., using a common or shared area and other parts of 

the same structure).

2. Do not include exposed individuals already counted under the Level I concentrations factor. 
Page 189 of 214

3. For each structure or subunit(s), count the number of individuals residing in or attending 

school or day care in the structure, or subunit, subject to Level II concentrations. 

4. Count the number of full-time and part-time workers in the structure or subunit(s) subject to 

Level II concentrations. If information is unavailable to classify a worker as full- or part-time, 

evaluate that worker as being full-time. Divide the number of full-time workers by 3 and the 

number of part-time workers by 6, and then sum these products with the number of other 

individuals for each structure or subunit.

5. Sum the combined population value for all structures within the areas of observed exposure 

for the site.

Assign this sum as the combined population value subject to Level II contamination for this 

factor. Enter this value in line 9b of Table 5-11.

5.2.1.3.2.3 Population within area(s) of subsurface contamination. Assign the population in 

area(s) of subsurface contamination factor value as follows, unless available information indicates 

otherwise (see sections 5.2.0 and 5.2.1.3.1):

1. Identify the regularly occupied structures with a structure containment value greater than zero 

and the eligible population associated with the structures or portions of structures in each area 

of subsurface contamination:

• For each regularly occupied structure or portion of a structure in an area of subsurface 

contamination, sum the number of all individuals residing in or attending school or day 

care, in the structure or portion of the structure in the area of subsurface contamination. 

• Count the number of full-time and part-time workers regularly present in each structure 

or portion of a structure in an area of subsurface contamination. If information is 

unavailable to classify a worker as full- or part-time, evaluate that worker as being full-
Page 190 of 214

time. Divide the number of full-time workers by 3 and the number of part-time workers 

by 6. Sum these products with the number of individuals residing in or attending school 

or day care in the structure. 

• Use this sum as the population for the structure.

2. Estimate the depth or distance to contamination at each regularly occupied structure within an 

area of subsurface contamination based on available sampling data, and categorize each 

eligible structure based on the depth or distance to contamination and sampling media as 

presented in Table 5-20. Weight the population in each structure using the appropriate 

weighting factors in Table 5-20. If samples from multiple media are available, use the sample 

that results in the highest weighting factor.

3. Sum the weighted population in all structures within the area(s) of subsurface contamination 

and assign this sum as the population subject subsurface contamination factor value. Enter 

this value in line 9c of Table 5-11.

TABLE 5-20 WEIGHTING FACTOR VALUES FOR POPULATIONS WITHIN AN AREA OF SUBSURFACE CONTAMINATION

Eligible Populationsa in Structuresb within an Area of Subsurface Contamination

Population 

Weighting 

Factor

Population in a structure with levels of contamination in a semi-enclosed or enclosed 

crawl space sample meeting observed release criteria

or

Population in a subunit of a multi-story structure within an area of subsurface 

contamination located directly above a level in an area of observed exposure or a 

gaseous indoor air sample meeting observed release criteria

0.9

Population in a structure where levels of contaminants meeting observed release criteria 

are found in any sampling media at or within five feet horizontally or vertically of the

structure foundation 

0.4
Page 191 of 214

Population occupying a structure where levels of contaminants meeting observed release 

criteria are found or inferred based on any underlying non-ground water subsurface 

sample at a depth less than or equal to 30 feet

or 

Population in a structure within an area of subsurface contamination where levels of 

contaminants meeting observed release criteria are inferred based on semi-enclosed or 

enclosed crawl space samples in surrounding structures 

0.2

Population in a structure where levels of contaminants meeting observed release criteria 

are found or inferred based on underlying ground water samples greater than five feet 

from the structure foundation 

or

Population in a structure where levels of contaminants meeting observed release criteria 

are found or inferred based on any underlying sample at depths greater than 30 feet 

0.1

a Eligible populations include residents (including individuals living in, or attending school or day care in the structure), and workers 

in regularly occupied structures (see HRS Section 5.2.1.3). b Eligible structures may include single- or multi-tenant structures where 

eligible populations reside, attend school or day care, or work. These structures may also be mixed use structures.

5.2.1.3.2.4 Calculation of population factor value. Sum the factor values for Level I 

concentrations, Level II concentrations, and population in the area(s) of subsurface 

contamination. Assign this sum as the population factor value. Enter this value in line 9d of Table 

5-11. 

5.2.1.3.3 Resources. Evaluate the resources factor as follows:

• Assign a value of 5 if a resource structure (e.g., library, church, tribal facility) is present and 

regularly occupied within either an area of observed exposure or area of subsurface 

contamination.

• Assign a value of 0 if there is no resource structure within an area of observed exposure or 

area of subsurface contamination. 
Page 192 of 214

Enter the value assigned in Table 5-11. 

5.2.1.3.4 Calculation of targets factor category value. Sum the values for the exposed 

individual, population, and resources factors. Do not round to the nearest integer. Assign this sum 

as the targets factor category value for the subsurface intrusion component. Enter this value in 

Table 5-11.

5.2.2 Calculation of subsurface intrusion component score. Multiply the factor 

category values for likelihood of exposure, waste characteristics and targets and round the 

product to the nearest integer. Divide the product by 82,500. Assign the resulting value, subject to 

a maximum of 100, as the subsurface intrusion component score and enter this score in Table 5-

11.

5.3 Calculation of the soil exposure and subsurface intrusion pathway score:

Sum the soil exposure component score and subsurface intrusion component. Assign the resulting 

value, subject to a maximum of 100, as the soil exposure and subsurface intrusion pathway score 

(Ssessi). Enter this score in Table 5-11.
Page 193 of 214

6.0 Air Migration Pathway

TABLE 6-14 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN AIR

• Concentration corresponding to National Ambient Air Quality Standard (NAAQS). 

• Concentration corresponding to National Emission Standards for Hazardous Air Pollutants 

(NESHAPs).

• Screening concentration for cancer corresponding to that concentration that corresponds to the 

10-6 individual cancer risk for inhalation exposures.

• Screening concentration for noncancer toxicological responses corresponding to the Reference 

Concentration (RfC) for inhalation exposures.

Sites Containing Radioactive Substances

* * * * *
Page 194 of 214

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status 

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Likelihood of 

Release

Likelihood of 

Release

Likelihood of 

Exposure

Likelihood of 

Exposure

Likelihood of 

Release

Observed 

Release

Yes Observed 

Release

Yes Observed 

Contamination

Yes Observed 

Exposure

Yes Observed 

Release

Yes

Potential to 

Release

No Potential to 

Release

No Attractiveness/ 

Accessibility to 

Nearby 

Residents

No Potential for 

Exposure

Yes Gas Potential to 

Release

No

Containment No Overland Flow 

Containment

No Area of 

Contamination

No Structure 

Containment

No Gas

Containment

No

Net Precipitation No Runoff No Depth to 

Contamination

Yes Gas Source 

Type

No

Depth to Aquifer No Distance to 

Surface water

No Vertical 

migration

No Gas Migration 

Potential

No

Travel Time No Flood Frequency No Vapor Migration 

Potential

No Particulate 

Potential to 

Release

No
Page 195 of 214

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status 

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Flood 

Containment

No Particulate 

Containment

No

Particulate 

Source Type

No

Particulate 

Migration 

Potential

No

Waste 

Characteristics

Waste

Characteristics

Waste 

Characteristics

Waste 

Characteristics

Waste 

Characteristics

Toxicity Yes Toxicity/

Ecotoxicity

Yes/Y

es

Toxicity Yes Toxicity/ 

Degradation

Yes Toxicity Yes

Mobility No Persistence/

Mobility

Yes/N

o

Hazardous 

Waste Quantity

Yes Hazardous 

Waste Quantity

Yes Mobility No

Hazardous 

Waste Quantity

Yes Bioaccumulation PotentialNo Hazardous 

Waste Quantity

Yes
Page 196 of 214

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status 

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Hazardous 

Waste Quantity

Yes

Targets Targets Targets Targets Targets

Nearest Well Yes b Nearest Intake Yes b Resident 

Individual

Yes b Exposed

Individual

Yes b Nearest 

Individual

Yes b

Population Yes b Drinking Water 

Population

Yes b Resident 

Population

Yes b Population Yes b Population Yes b

Resources No Resources No Workers No Resources No Resources No

Wellhead 

Protection Area

No Sensitive 

Environments

Yes b Resources No Sensitive 

Environments

No

Human Food 

Chain Individual

Yes b Terrestrial 

Sensitive 

Environments

No

Human Food 

Chain Population

Yes b Nearby 

Individual

No
Page 197 of 214

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status 

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Population 

Within 1 Mile

No

a Factors evaluated differently are denoted by “yes”; factors not evaluated differently are denoted by “no”.

b Difference is in the determination of Level I and Level II concentrations.
Page 198 of 214

* * * * *

* * * These differences apply largely to the soil exposure and subsurface intrusion pathway 

and to sites containing mixed radioactive and other hazardous substances. * * *

7.1 Likelihood of release/likelihood of exposure. Evaluate likelihood of release for the 

three migration pathways and likelihood of exposure for the soil exposure and subsurface 

intrusion pathway as specified in sections 2 through 6, except: establish an observed release, 

observed contamination, and/or observed exposure as specified in section 7.1.1. When an 

observed release or exposure cannot be established for a migration pathway or the subsurface 

intrusion component of the soil exposure and subsurface intrusion pathway, evaluate potential to 

release as specified in section 7.1.2. When observed contamination cannot be established, do not 

evaluate the soil exposure component of the soil exposure and subsurface intrusion pathway.

7.1.1 Observed release/observed contamination/observed exposure. For radioactive 

substances, establish an observed release for each migration pathway by demonstrating that the 

site has released a radioactive substance to the pathway (or watershed or aquifer, as appropriate); 

establish observed contamination or observed exposure for the soil exposure and subsurface 

intrusion pathway as indicated below. * * * 

* * * * *

– For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, a material that contains one or more radionuclides has been observed 

entering a regularly occupied structure via the subsurface or is known to have entered 

a regularly occupied structure via the subsurface. Also, when evidence supports the 
Page 199 of 214

inference of subsurface intrusion of a material that contains one or more 

radionuclides by the site into a regularly occupied structure, demonstrated adverse 

effects associated with that release may also be used to establish observed exposure 

by direct observation. 

• Analysis of radionuclide concentrations in samples appropriate to the pathway (that is, 

ground water, soil, air, indoor air, surface water, benthic, or sediment samples): 

* * * * *

 Some portion of the increase must be attributable to the site to establish the 

observed release (or observed contamination or observed exposure), and 

 For the soil exposure component of the soil exposure and subsurface intrusion 

pathway only, the radionuclide must also be present at the surface or covered by 

2 feet or less of cover material (for example, soil) to establish observed 

contamination.

* * * * *

 If the sample quantitation limit cannot be established: 

° If the sample analysis was performed under the EPA Contract Laboratory 

Program, use the EPA contract-required quantitation limit (CRQL) in place 

of the sample quantitation limit in establishing an observed release (or 

observed contamination or observed exposure). 

* * * * *
Page 200 of 214

 For the soil exposure component of the soil exposure and subsurface intrusion 

pathway only, the radionuclide must also be present at the surface or covered by 

2 feet or less of cover material (for example, soil) to establish observed 

contamination.

• Gamma radiation measurements (applies only to observed contamination or observed 

exposure in the soil exposure and subsurface intrusion pathway): 

– The gamma radiation exposure rate, as measured in microroentgens per hour (μR/hr) 

using a survey instrument held 1 meter above the ground surface or floor or walls of 

a structure (or 1 meter away from an aboveground source for the soil exposure 

component), equals or exceeds 2 times the site-specific background gamma radiation 

exposure rate. 

* * * * *

For the three migration pathways and for the subsurface intrusion component of the soil 

exposure and subsurface intrusion pathway, if an observed release or observed exposure can be 

established for the pathway (or threat, aquifer, or watershed, as appropriate), assign the pathway 

(or threat, aquifer, or watershed) an observed release or observed exposure factor value of 550 

and proceed to section 7.2. If an observed release or observed exposure cannot be established, 

assign an observed release or observed exposure factor value of 0 and proceed to section 7.1.2.

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if 

observed contamination can be established, assign the likelihood of exposure factor for resident 

population a value of 550 if there is an area of observed contamination in one or more locations 
Page 201 of 214

listed in section 5.1.1; evaluate the likelihood of exposure factor for nearby population as 

specified in section 5.1.2.1; and proceed to section 7.2. If observed contamination cannot be 

established, do not evaluate the soil exposure component of the soil exposure and subsurface 

intrusion pathway.

At sites containing mixed radioactive and other hazardous substances, evaluate observed 

release (or observed contamination or observed exposure) separately for radionuclides as 

described in this section and for other hazardous substances as described in sections 2 through 6. 

For the three migration pathways and the subsurface intrusion component of the soil exposure 

and subsurface intrusion pathway, if an observed release or observed exposure can be established 

based on either radionuclides or other hazardous substances, or both, assign the pathway (or 

threat, aquifer, or watershed) an observed release or observed exposure factor value of 550 and 

proceed to section 7.2. If an observed release or observed exposure cannot be established based 

on either radionuclides or other hazardous substances, assign an observed release or observed 

exposure factor value of 0 and proceed to section 7.1.2. 

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if 

observed contamination can be established based on either radionuclides or other hazardous 

substances, or both, assign the likelihood of exposure factor for resident population a value of 550 

if there is an area of observed contamination in one or more locations listed in section 5.1.1; 

evaluate the likelihood of exposure factor for nearby population as specified in section 5.1.2.1; 

and proceed to section 7.2. If observed contamination cannot be established based on either 

radionuclides or other hazardous substances, do not evaluate the soil exposure component of the 

soil exposure and subsurface intrusion pathway. 
Page 202 of 214

7.1.2 Potential to release/potential for exposure. For the three migration pathways and 

the subsurface intrusion component of the soil exposure and subsurface intrusion pathway, 

evaluate potential to release or potential for exposure for sites containing radionuclides in the 

same manner as specified for sites containing other hazardous substances. * * * For the 

subsurface intrusion component of the soil exposure and subsurface intrusion pathway, if the 

potential for exposure is based on the presence of gamma emitting radioactive substances, assign 

a potential for exposure factor value of 500 only if the contamination is found within 2 feet 

beneath a regularly occupied structure, otherwise assign a potential for exposure factor value of 0.

For sites containing mixed radioactive and other hazardous substances, evaluate potential to 

release or potential for exposure considering radionuclides and other hazardous substances 

together. Evaluate potential to release for each migration pathway and the potential for exposure 

for the subsurface intrusion component of the soil exposure and subsurface intrusion pathway as 

specified in sections 3 through 6, as appropriate.

* * * * *

7.2.3 Persistence/Degradation. For In determining the surface water persistence factor 

for radionuclides, evaluate the surface water persistence this factor based solely on half-life; do 

not include sorption to sediments in the evaluation as is done for nonradioactive hazardous 

substances. Assign a persistence factor value from Table 4-10 (section 4.1.2.2.1.2) to each 

radionuclide based on half-life (t 1/2) calculated as follows:

t1/2 = 1

1

r+1

v

where:
Page 203 of 214

r=Radioactive half-life.

v=Volatilization half-life.

If the volatilization half-life cannot be estimated for a radionuclide from available data, delete

it from the equation. Select the portion of Table 4-10 to use in assigning the persistence factor 

value as specified in section 4.1.2.2.1.2.

At sites containing mixed radioactive and other hazardous substances, evaluate the 

persistence factor separately for each radionuclide and for each nonradioactive hazardous 

substance, even if the available data indicate that they are combined chemically. Assign a 

persistence factor value to each radionuclide as specified in this section and to each 

nonradioactive hazardous substance as specified in section 4.1.2.2.1.2. When combined 

chemically, assign a single persistence factor value based on the higher of the two values assigned 

(individually) to the radioactive and nonradioactive components.

In determining the subsurface intrusion degradation factor for radionuclides, when evaluating 

this factor based solely on half-life. Assign a degradation factor value from section 5.2.1.2.1.2 to 

each radionuclide based on half-life (t1/2) calculated as follows:

t1/2 = 1

 1/r

where: 

h=Hydrolysis half-life. 
Page 204 of 214

b=Biodegradation half-life. 

r= Radioactive half-life.

At sites containing mixed radioactive and other hazardous substances, evaluate the persistence or 

degradation factor separately for each radionuclide and for each nonradioactive hazardous 

substance, even if the available data indicate that they are combined chemically. Assign a 

persistence or degradation factor value to each radionuclide as specified in this section and to 

each nonradioactive hazardous substance as specified in sections 4.1.2.2.1.2 and 5.2.1.2.1.2. 

When combined chemically, assign a single persistence or degradation factor value based on the 

higher of the two values assigned (individually) to the radioactive and nonradioactive 

components.

7.2.4 Selection of substance potentially posing greatest hazard. For the subsurface 

intrusion component of the soil exposure and subsurface intrusion pathway and each migration

pathway (or threat, aquifer, or watershed, as appropriate), select the radioactive substance or 

nonradioactive hazardous substance that potentially poses the greatest hazard based on its toxicity 

factor value, combined with the applicable mobility, persistence, degradation and/or 

bioaccumulation (or ecosystem bioaccumulation) potential factor values. Combine these factor 

values as specified in sections 2 through 6. For the soil exposure component of the soil exposure 

and subsurface intrusion pathway, base the selection on the toxicity factor alone (see sections 2

and 5).

* * * * *
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7.2.5.1 Source hazardous waste quantity for radionuclides. * * * For the soil exposure 

component of the soil exposure and subsurface intrusion pathway, assign a source hazardous 

waste quantity value to each area of observed contamination, as applicable to the threat being 

evaluated. For the subsurface intrusion component, assign a source hazardous waste quantity 

value to each regularly occupied structure located within areas of observed exposure or areas of 

subsurface contamination. Allocate hazardous substances and hazardous wastestreams to specific 

sources (or areas of observed contamination, area of observed exposure or area of subsurface 

contamination) as specified in sections 2.4.2 and 5.2.0.

7.2.5.1.1 Radionuclide constituent quantity (Tier A). Evaluate radionuclide constituent 

quantity for each source (or area of observed contamination or area of observed exposure) based 

on the activity content of the radionuclides allocated to the source (or area of observed 

contamination or area of observed exposure) as follows:

• Estimate the net activity content (in curies) for the source (or area of observed 

contamination or area of observed exposure) based on: 

* * * * *

where: 

N=Estimated net activity content (in curies) for the source (or area of observed contamination

or area of observed exposure). 

V=Total volume of material (in cubic yards) in a source (or area of observed contamination 

or area of observed exposure) containing radionuclides. 
Page 206 of 214

ACi=Activity concentration above the respective background concentration (in pCi/g) for 

each radionuclide i allocated to the source (or area of observed contamination or area of observed 

exposure). 

n=Number of radionuclides allocated to the source (or area of observed contamination or area 

of observed exposure) above the respective background concentrations. or, 

* * * * *

where:

N=Estimated net activity content (in curies) for the source (or area of observed contamination 

or area of observed exposure). 

V=Total volume of material (in gallons) in a source (or area of observed contamination or 

area of observed exposure) containing radionuclides. 

ACi=Activity concentration above the respective background concentration (in pCi/1) for 

each radionuclide i allocated to the source (or area of observed contamination or area of observed 

exposure). 

n=Number of radionuclides allocated to the source (or area of observed contamination or area 

of observed exposure) above the respective background concentrations. 

• Estimate volume for the source (or volume for the area of observed contamination or area 

of observed exposure) based on records or measurements.
Page 207 of 214

• For the soil exposure component, in estimating the volume for areas of observed 

contamination, do not include more than the first 2 feet of depth, except: for those types of areas 

of observed contamination listed in Tier C of Table 5-2 (section 5.1.1.2.2) * * *. 

• For the subsurface intrusion component, in estimating the volume for areas of observed 

exposure, only use the volume of air in the regularly occupied structures where observed 

exposure has been documented.

• Convert from curies of radionuclides to equivalent pounds of nonradioactive hazardous 

substances by multiplying the activity estimate for the source (or area of observed contamination 

or area of observed exposure) by 1,000. 

• Assign this resulting product as the radionuclide constituent quantity value for the source 

(or area of observed contamination or area of observed exposure).

If the radionuclide constituent quantity for the source (or area of observed contamination or 

area of observed exposure) is adequately determined (that is, the total activity of all radionuclides 

in the source and releases from the source [or in the area of observed contamination or area of 

observed exposure] is known or is estimated with reasonable confidence), do not evaluate the 

radionuclide wastestream quantity measure in section 7.2.5.1.2. * * * If the radionuclide 

constituent quantity is not adequately determined, assign the source (or area of observed 

contamination or area of observed exposure) a value for radionuclide constituent quantity based 

on the available data and proceed to section 7.2.5.1.2.
Page 208 of 214

7.2.5.1.2 Radionuclide wastestream quantity (Tier B). Evaluate radionuclide wastestream 

quantity for the source (or area of observed contamination, area of observed exposure, or area of 

subsurface contamination) based on the activity content of radionuclide wastestreams allocated to 

the source (or area of observed contamination, area of observed exposure, or area of subsurface 

contamination) as follows:

• Estimate the total volume (in cubic yards or in gallons) of wastestreams containing 

radionuclides allocated to the source (or area of observed contamination, area of observed 

exposure, or area of subsurface contamination). 

* * * * *

• Assign the resulting value as the radionuclide wastestream quantity value for the source

(or area of observed contamination, area of observed exposure, or area of subsurface 

contamination). 

• For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, estimate the total wastestream volume for all regularly occupied structures located 

within areas of observed exposure with observed or inferred intrusion and within areas of 

subsurface contamination. Calculate the volume of each regularly occupied structure based on 

actual data. If unknown, use a ceiling height of 8 feet.

7.2.5.1.3 Calculation of source hazardous waste quantity value for radionuclides. Select 

the higher of the values assigned to the source ( or area of observed contamination, area of

observed exposure, and/or area of subsurface contamination) for radionuclide constituent quantity 
Page 209 of 214

and radionuclide wastestream quantity. Assign this value as the source hazardous waste quantity 

value for the source (or area of observed contamination, area of observed exposure, or area of 

subsurface contamination). * * * 

7.2.5.2 Calculation of hazardous waste quantity factor value for radionuclides. Sum the 

source hazardous waste quantity values assigned to all sources (or area of observed 

contamination, area of observed exposure, or area of subsurface 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. * * * 

* * * * *

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if the 

radionuclide 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. * * 

* 

For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, if the radionuclide constituent quantity is adequately determined for all areas of 

observed exposure, assign the value from Table 2-6 as the hazardous waste quantity factor value. 

If the radionuclide constituent quantity is not adequately determined for one or more areas of 

observed exposure, assign either the value from Table 2-6 or a value of 10, whichever is greater, 

as the hazardous waste quantity factor value. 

7.2.5.3 Calculation of hazardous waste quantity factor value for sites containing mixed 

radioactive and other hazardous substances. For each source (or area of observed contamination, 
Page 210 of 214

area of observed exposure, or area of subsurface contamination) containing mixed radioactive and 

other hazardous substances, calculate two source hazardous waste quantity values — one based 

on radionuclides as specified in sections 7.2.5.1 through 7.2.5.1.3 and the other based on the 

nonradioactive hazardous substances as specified in sections 2.4.2.1 through 2.4.2.1.5, and 

sections 5.1.1.2.2, 5.1.2.2.2 and 5.2.1.2.2 (that is, determine each value as if the other type of 

substance was not present). Sum the two values to determine a combined source hazardous waste 

quantity value for the source (or area of observed contamination, area of observed exposure, or 

area of subsurface contamination). * * * 

Use this combined source hazardous waste quantity value to calculate the hazardous waste 

quantity factor value for the pathway as specified in section 2.4.2.2, except: if either the 

hazardous constituent quantity or the radionuclide constituent quantity, or both, are not 

adequately determined for one or more sources (or one or more portions of sources or releases 

remaining after a removal action) or for one or more areas of observed contamination, areas of

observed exposure, or areas of subsurface contamination, as applicable, assign the value from 

Table 2-6 or the default value applicable for the pathway, whichever is greater, as the hazardous 

waste quantity factor value for the pathway. 
Page 211 of 214

7.3 Targets. For radioactive substances, evaluate the targets factor category as specified 

in section 2.5 and sections 3 through 6, except: establish Level I and Level II concentrations at 

sampling locations as specified in sections 7.3.1 and 7.3.2 and establish weighting factors for 

populations associated with an area of subsurface contamination in the subsurface intrusion 

component of the soil exposure and subsurface intrusion pathway as specified in section 7.3.3.

For all pathways (components and threats), use the same target distance limits for sites 

containing radioactive substances as is specified in sections 3 through 6 for sites containing 

nonradioactive hazardous substances. At sites containing mixed radioactive and other hazardous 

substances, include all sources ( or areas of observed contamination, areas of observed exposure, 

or areas of subsurface contamination) at the site in identifying the applicable targets for the 

pathway. 

7.3.1 Level of contamination at a sampling location. * * * 

• Select the benchmarks from section 7.3.2 applicable to the pathway (or component or 

threat) being evaluated. 

• Compare the concentrations of radionuclides in the sample (or comparable samples) to 

their benchmark concentrations for the pathway (or component or threat) as specified in section 

7.3.2. Treat comparable samples as specified in section 2.5.1. 

* * * * * 

 
Page 212 of 214

• If none of the radionuclides eligible to be evaluated for the sampling location have an 

applicable benchmark, assign Level II to the actual contamination at that sampling location for 

the pathway (or component or threat). 

• In making the comparison, consider only those samples, and only those radionuclides in 

the sample, that meet the criteria for an observed release (or observed contamination or observed 

exposure) for the pathway, except: tissue samples from aquatic human food chain organisms may 

also be used for the human food chain threat of the surface water pathway as specified in sections 

4.1.3.3 and 4.2.3.3. 

7.3.2 Comparison to benchmarks. * * * 

* * * * *

• Uranium Mill Tailings Radiation Control Act (UMTRCA) standards—soil exposure 

component of the soil exposure and subsurface intrusion pathway only.

• Screening concentration for cancer corresponding to that concentration that corresponds 

to the 10-6 individual cancer risk for inhalation exposures (air migration pathway and subsurface 

intrusion component of the soil exposure and subsurface intrusion pathway) or for oral exposures 

(ground water migration pathway; drinking water or human food chain threats in surface water 

migration pathway; and soil exposure and subsurface intrusion pathway). 

– For the soil exposure and subsurface intrusion pathway, include two screening 

concentrations for cancer — one for ingestion of surface materials and one for 

external radiation exposures from gamma-emitting radionuclides in surface materials. 
Page 213 of 214

Select the benchmark(s) applicable to the pathway (component or threat) being evaluated. 

Compare the concentration of each radionuclide from the sampling location to its benchmark 

concentration(s) for that pathway (component or threat). Use only those samples and only those 

radionuclides in the sample that meet the criteria for an observed release (or observed 

contamination or observed exposure) 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 radionuclide from any sample equals or exceeds its benchmark 

concentration, consider the sampling location to be subject to Level I concentrations for that 

pathway (component or threat). If more than one benchmark applies to the radionuclide, assign 

Level I if the radionuclide concentration equals or exceeds the lowest applicable benchmark 

concentration. In addition, for the soil exposure and subsurface intrusion pathway, assign Level I 

concentrations at the sampling location if measured gamma radiation exposure rates equal or 

exceed 2 times the background level (see section 7.1.1). 

If no radionuclide individually equals or exceeds its benchmark concentration, but more than 

one radionuclide either meets the criteria for an observed release (or observed contamination or 

observed exposure) for the sample or is eligible to be evaluated for a tissue sample (see sections 

4.1.3.3 and 4.2.3.3), calculate a value for index I for these radionuclides as specified in section 

2.5.2. * * * 

* * * Use only those samples and only those substances in the sample that meet the criteria 

for an observed release (or observed contamination or observed exposure) 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. * * * 
Page 214 of 214

If no radionuclide or other hazardous substance individually exceed a benchmark 

concentration, but more than one radionuclide or other hazardous substance either meets the 

criteria for an observed release (or observed contamination or observed exposure) for the sample 

or is eligible to be evaluated for a tissue sample, calculate an index I for both types of substances 

as specified in section 2.5.2. * * * 

7.3.3 Weighting of targets within an area of subsurface contamination. For the 

subsurface intrusion component of the soil exposure and subsurface intrusion pathway, assign a 

weighting factor as specified in section 5.2.1.3.2.3 except when an area of subsurface 

contamination is bound by gamma radiation exposure rates meeting observed release criteria with 

a depth to contamination of 2 feet or less. For those populations residing, working, or attending 

school or day care in a structure, assign a weighting factor of 0.9.