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6560-50-P

EO12866_CERCLA Part 300 HRS SsI Addition 2050 AG67 NPRM NPRM_20150701

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].

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 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 2

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 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 

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 SsI Addition

A. General Approach
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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

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

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

Deleted: <#>Workers¶
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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?

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 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 

Deleted: hazardous waste sites

Deleted: . However, because subsurface intrusion sites have the 

potential to pose a higher level of risk than other exposure routes 

based on site-specific factors such as exposure duration, EPA

expects that there will be a realignment and reprioritization of its 

internal resources and Cooperative Agreement funding toward 

subsurface intrusion evaluations. ¶

Deleted: N

Deleted: sites 

Deleted: as an issue
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HRS would further the agency’s efforts to establish national 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 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

EPAis 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.

Deleted: ¶

¶

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

Contaminant subsurface intrusion1 is defined as the migration of hazardous substances, 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 report,3 the Government Accountability Office (GAO) concluded that if 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 suspected of having vapor intrusion issues. Many of the sites in this 

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

into a structure.

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.

Deleted: As such, none of the existing pathways address the threat 

posed by hazardous substances entering a regularly occupied 

structure from the subsurface.

Deleted: As such, none of the existing pathways address the threat 

posed by hazardous substances entering a regularly occupied 

structure from the subsurface.
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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 selfidentification, 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 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 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 

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

Compensation and Liability Information System (CERCLIS). 

Deleted: 8

Deleted: ¶

¶
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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 HRS?

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: 

[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). 

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.

 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.

Deleted: ¶

¶

Deleted: Finally, in addition to the significant number of sites with 

possible subsurface intrusion issues, the public support for this 

addition,addition and the limitation in the HRS to address subsurface 

intrusion issues, 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.¶

Deleted: threat 

Deleted: 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.

Deleted: appear 

Deleted: to

Deleted: e

Deleted: that 
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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

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 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 

Deleted: subsurface intrusion

Deleted: subsurface intrusion

Deleted: 

Deleted: within 

Deleted: consider at sites

Deleted: The current HRS considers releases to the ground water, 

surface water, air, and 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. 

Moved (insertion) [3]

Moved up [3]: 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. 

Deleted: In these cases, the HRS documentation record may 

include a brief qualitative discussion to present a more complete 

picture of the conditions and hazards at the site. ¶

¶

Deleted: ¶

Deleted: ¶

Deleted: already 

Deleted: have the authority to 

Deleted: ,

Deleted: this proposed rule is anticipated to have minimal impacts 

on federal facility cleanup programs. 

Deleted: n
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 10

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 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 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 low-income 

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 

 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. 

Deleted: ¶

Deleted: ¶

Deleted: ¶

Deleted: ¶
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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 preremedial 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 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. 

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.

Deleted: site discovery

Deleted: SEMS

Deleted: what substances are a

Deleted: that 

Deleted: is a site-specific inquiry that requires 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 12

Figure 1. Superfund Site Assessment Process

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:
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 13

• 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.

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). 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 14

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). 

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-mean-square 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, 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 SsI 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. 

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 
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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 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 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 vaporforming 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 vapor-forming 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 

 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.

Deleted: However, because

Deleted: ; those

Deleted: may have 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 16

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 sub-slab 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. 

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.

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 

Deleted: While EPA’s removal program can provide a response to 

an emerging subsurface intrusion issue, it may not be appropriate in 

all cases, such as when actions are needed at a complex site to 

provide more comprehensive protection of human health and the 

environment. EPA’s removal program may only address subsurface 

intrusion when contaminant levels cause an imminent threat as 

opposed to a long-term threat. Also, the removal program may only 

be preventing the exposure to subsurface contamination via 

intrusion, and not addressing the underlying cause of the 

contamination. ¶

¶

Deleted: GAO 
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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 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, 400 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.

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 notice. During the 75-day public comment period on this notice, 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 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/sites/npl/hrsaddition.htm, 

providing background information on why this addition to the HRS is being considered. 

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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. 

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. 

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 

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Deleted: Additional information, such as the official charge 

questions and other documents provided to the 2011 peer reviewers 

and the original peer review comments received on this proposed 

addition to the HRS are also available in the public docket for this 

proposed rulemaking. Comments and EPA responses associated with 

the five charge questions are discussed in Appendix E, Peer Review 

ResponsesSummary of Peer Review Comments and Responses, of 

the Proposal TSD.
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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 

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 likely to enter into regularly occupied structures. See section VI.A of this preamble for further discussion.

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. 

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on the proposed addition and agency responses.
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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, 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.

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 notice 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 

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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 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 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. 

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) 
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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 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 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 semienclosed 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. 

 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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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.

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 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 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 24

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 site-specific 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 

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.” 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:

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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 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.

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 hazardous substance concentration in a release 
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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 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 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. 

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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 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.

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Figure 6. Potential for Exposure

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 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-
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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 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 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 
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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

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 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 half-life 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 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. 

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 

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low structural complexity, such as petroleum and petroleum-like 

substances (having straight carbon chain or simple ring structures), 

have the greatest potential to biodegrade 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. Therefore, if a substance is 

likely to biodegrade based on its structure and if there are 

appropriate environmental conditions present, the biodegradation 

factor value will be less than if the substances are not biodegradable 

or if environmental conditions are not conducive to biodegradation.¶

¶

In addition, 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 biodegradation value is 

adjusted accordingly. See also section 5.0 of the Proposal TSD for 

additional discussion regarding the inclusion of a biodegradation 

factor.¶

¶

The toxicity and biodegradation 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.¶
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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.)

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 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 

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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), 

• 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 Federally-designated 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?

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 

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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 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.

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 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 

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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 fulland 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.

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 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 

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clearly document that there could be no observed exposures in these 

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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 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 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

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¶

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Workers – See section 5.2.1.3.3 of the Proposed HRS Addition¶

¶

EPA is proposing to include a worker factor in the subsurface 

intrusion component. Such a consideration is included in all of the 

existing HRS pathways. This factor reflects the possibility that 

indoor workers can be exposed in a workplace to contamination 

from the subsurface. This worker factor evaluation is proposed to be 

structured the same as in the current soil exposure pathway. 

Currently, for the resident population threat of the soil exposure 

pathway, a factor value for workers is assigned based on the total 

number of workers present within the area established by observed 

contamination criteria. These factor values range from 0 to 15, 

depending on the number of workers exposed to outdoor 

contamination. (See section 5.1.3.3 of the current HRS.)¶

¶

EPA is proposing to mirror the procedures in the resident population 

threat in the soil exposure component for assigning a worker factor 

value in the subsurface intrusion component. (See Table 5-4 in 

section 5.1.3.3 of the current HRS.) However, since it is anticipated 

that workers spend a larger percentage of their time indoors and 

therefore, may be more exposed to contamination resulting from 

subsurface intrusion than to outdoor soil (or other surface) 

contamination, EPA is proposing to assign workers exposed to 

subsurface intrusion a higher weighting (i.e., ranging from 0 to 45). 

See section 6.0 of the Proposal TSD for supplemental information 

regarding EPA’s decision to include workers within the subsurface 

intrusion component, as well as the basis for the weightings. ¶

¶

It should also be noted that, in developing the current HRS, EPA

investigated statutory, regulatory, and policy conditions that might 

restrict the inclusion of workers in target populations. This analysis 

found no broad statutory or regulatory authority for excluding 

workers covered by the Occupational Safety and Health 

Administration (OSHA) regulations from consideration as targets in 

the HRS when they are exposed to a CERCLA release. Although the 

definition of a release under CERCLA section 101(22) excludes 

“any release which results in exposure to persons solely within a 

workplace,” it only does so for purposes of claims by workers who 

are covered by state worker compensation laws. The legislative 

history of section 101(22) specifically anticipated that authority 

under CERCLA might, in appropriate cases, be used to respond to 

releases that migrate into a workplace. Thus, EPA concluded that 

there are no broad statutory or regulatory restrictions against 

evaluating CERCLA releases at OSHA-regulated facilities. 

Therefore, EPA has determined that the evaluation of the potential 

threat to workers posed by subsurface intrusion is not restricted at 

OSHA-regulated facilities. EPA will use the HRS to evaluate 

releases and threats of a release to the environment, including 

situations where those releases may have entered a workplace; 

however EPA will generally not use the HRS to evaluate 

occupational sources of contamination that have not been released or 

threatened to release to the environment. ¶

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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 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 

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. 

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

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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 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 children 

enrolled and 6 full-time and 2 part-time workers, and the office building with 18 full-time 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 

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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 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, and 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 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.)

 

Deleted: ¶

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These terminology updates are reflected in the HRS addition and do not represent any substantive change in the 

application of the HRS.

VIII. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at http://www2.epa.gov/lawsregulations/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 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 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. 
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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 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.

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
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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.

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
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Addition of Subsurface Intrusion to the Hazard Ranking System (HRS)

Rule Language Redline in Response to EO 12866 Review

Table of Contents

1.1 Definitions 46

2.0 Evaluations Common to Multiple Pathways 52

2.1 Overview.............................................................................................................................................. 52

2.1.1 Calculation of HRS site score 52

2.1.2 Calculation of pathway score 52

2.1.3 Common evaluations 52

2.2 Characterize sources........................................................................................................................... 54

2.2.1 Identify sources 54

2.2.2 Identify hazardous substances associated with a source 55

2.2.3 Identify hazardous substances available to a pathway 56

2.3 Likelihood of release............................................................................................................................ 56

2.4 Waste characteristics ............................................................................................................................... 57

2.4.1 Selection of substance potentially posing greatest hazard. 57

2.4.2 Hazardous waste quantity. 61

2.4.3 Waste characteristics factor category value. 66

2.4.3.1 Factor category value. 66

2.5 Targets...................................................................................................................................................... 67

2.5.1 Determination of level of actual contamination at a sampling location. 68

2.5.2 Comparison to benchmarks. 69

5.0 Soil Exposure and Subsurface Intrusion Pathway 71

5.0.1 Exposure components. 71

5.1 Soil exposure component. ................................................................................................................... 73

5.1.0 General considerations 75

5.1.1 Resident population threat 75

5.1.2 Nearby population threat 80

5.1.3 Calculation of soil exposure component score 83

5.2 Subsurface intrusion component................................................................................................................. 84

5.2.0 General considerations 86

5.2.1 Subsurface intrusion component 87
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5.2.2 Calculation of subsurface intrusion component score 98

5.3 Calculation of the soil exposure and subsurface intrusion pathway score.......................................... 99

6.0 Air Migration Pathway* 100

7.0 Sites Containing Radioactive Substances 101

7.1 Likelihood of release/likelihood of exposure........................................................................................... 102

7.1.1 Observed release/observed contamination/observed exposure 102

7.1.2 Potential to release/potential for exposure 104

7.2 Waste characteristics ............................................................................................................................. 104

7.2.1 Human toxicity 104

7.2.2 Ecosystem toxicity 105

7.2.3 Persistence/Degradation 105

7.2.4 Selection of substance potentially posing greatest hazard 106

7.2.5 Hazardous waste quantity 106

7.3 Targets.................................................................................................................................................... 109

7.3.1 Level of contamination at a sampling location 109

7.3.2 Comparison to benchmarks 110

7.3.3 Weighting of targets within an area of subsurface contamination 111

Tables

TABLE 2-1 SAMPLE PATHWAY SCORESHEET ...........................................................................................................53

TABLE 2-2 SAMPLE SOURCE CHARACTERIZATION WORKSHEET.............................................................................55

TABLE 2-3 OBSERVED RELEASE CRITERIA FOR CHEMICAL ANALYSIS ...................................................................57

TABLE 2-4 TOXICITY FACTOR EVALUATION............................................................................................................58

TABLE 2-5 HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS ..................................................................63

TABLE 2-6 HAZARDOUS WASTE QUANTITY FACTOR VALUES ................................................................................65

TABLE 2-7 WASTE CHARACTERISTICS FACTOR CATEGORY VALUES......................................................................66

TABLE 5-1 SOIL EXPOSURE COMPONENT SCORESHEET...........................................................................................74

TABLE 5-2 HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SOIL EXPOSURE COMPONENT...........76

TABLE 5-3 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN SOILS.............................................77

TABLE 5-4 FACTOR VALUES FOR WORKERS............................................................................................................78

TABLE 5-5 TERRESTRIAL SENSITIVE ENVIRONMENTS RATING VALUES .................................................................79

TABLE 5-6 ATTRACTIVENESS/ACCESSIBILITY VALUES ...........................................................................................80

TABLE 5-7 AREA OF CONTAMINATION FACTOR VALUES ........................................................................................80

TABLE 5-8 NEARBY POPULATION LIKELIHOOD OF EXPOSURE FACTOR VALUES....................................................81

TABLE 5-9 NEARBY INDIVIDUAL FACTOR VALUES .................................................................................................81

TABLE 5-10 DISTANCE WEIGHTED POPULATION VALUES FOR NEARBY POPULATION THREATa ............................82

TABLE 5-11 SUBSURFACE INTRUSION COMPONENT SCORESHEET...........................................................................85

TABLE 5-12 STRUCTURE CONTAINMENT .................................................................................................................88

TABLE 5-13 DEPTH TO CONTAMINATION.................................................................................................................89
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TABLE 5-14 EFFECTIVE POROSITY/PERMEABILITY OF GEOLOGIC MATERIALS.......................................................90

TABLE 5-15 VERTICAL MIGRATION FACTOR VALUES a ...........................................................................................90

TABLE 5-16 VALUES FOR VAPOR PRESSURE AND HENRY’S CONSTANT..................................................................91

TABLE 5-17 VAPOR MIGRATION POTENTIAL FACTOR VALUES FOR A HAZARDOUS SUBSTANCE...........................91

TABLE 5-18 HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SUBSURFACE INTRUSION 

COMPONENT .............................................................................................................................................................94

TABLE 5-19 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN THE SUBSURFACE INTRUSION 

COMPONENT .............................................................................................................................................................96

TABLE 5-20 WEIGHTING FACTOR VALUES FOR POPULATIONS WITHIN AN AREA OF SUBSURFACE 

CONTAMINATION......................................................................................................................................................98

TABLE 6-14 HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN AIR.............................................100

TABLE 7-1 HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES .......................................................101

TABLE 7-2 TOXICITY FACTOR VALUES FOR RADIONUCLIDES ...............................................................................105

Figures

Figure 5-1 Overview of the Soil Exposure and Subsurface Intrusion Pathway .......................................................72
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1.1 Definitions

Acute toxicity: Measure of toxicological responses that result from a single exposure to a substance or from 

multiple exposures within a short period of time (typically several days or less). Specific measures of acute 

toxicity used within the HRS include lethal dose50 (LD50) and lethal concentration50 (LC50), typically measured 

within a 24-hour to 96-hour period. 

Ambient Aquatic Life Advisory Concentrations (AALACs): EPA's advisory concentration limit for acute or chronic 

toxicity to aquatic organisms as established under section 304(a)(1) of the Clean Water Act, as amended. 

Ambient Water Quality Criteria (AWQC)/National Recommended Water Quality Criteria: EPA's maximum acute 

(Criteria Maximum Concentration or CMC) or chronic (Criterion Continuous Concentration or CCC) toxicity 

concentrations for protection of aquatic life and its uses as established under section 304(a)(1) of the Clean Water 

Act, as amended. 

Bioconcentration factor (BCF): Measure of the tendency for a substance to accumulate in the tissue of an aquatic 

organism. BCF is determined by the extent of partitioning of a substance, at equilibrium, between the tissue of an 

aquatic organism and water. As the ratio of concentration of a substance in the organism divided by the 

concentration in water, higher BCF values reflect a tendency for substances to accumulate in the tissue of aquatic 

organisms. [unitless]. 

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

CERCLA: Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended (Pub. 

L. 96-510, as amended). 

Channelized flow: Natural geological or manmade features such as karst, fractures, lava tubes and utility conduits 

(e.g., sewer lines), which allow ground water and/or soil gas to move through the subsurface environment more 

easily.

Chronic toxicity: Measure of toxicological responses that result from repeated exposure to a substance over an 

extended period of time (typically 3 months or longer). Such responses may persist beyond the exposure or may 

not appear until much later in time than the exposure. HRS measures of chronic toxicity include Reference Dose 

(RfD) and Reference Concentration (RfC) values. 

Contract Laboratory Program (CLP): Analytical program developed for CERCLA waste site samples to fill the 

need for legally defensible analytical results supported by a high level of quality assurance and documentation. 

Contract-Required Detection Limit (CRDL): Term equivalent to contract-required quantitation limit, but used 

primarily for inorganic substances. 

Contract-Required Quantitation Limit (CRQL): Substance-specific level that a CLP laboratory must be able to 

routinely and reliably detect in specific sample matrices. It is not the lowest detectable level achievable, but rather 

the level that a CLP laboratory should reasonably quantify. The CRQL may or may not be equal to the 

quantitation limit of a given substance in a given sample. For HRS purposes, the term CRQL refers to both the 

contract-required quantitation limit and the contract-required detection limit. 

Crawl space: The enclosed or semi-enclosed area between a regularly occupied structure’s foundation (e.g., pier 

and beam construction) and the ground surface. Crawl space samples are collected to determine the concentration 

of hazardous substances in the air beneath a regularly occupied structure.

Curie (Ci): Measure used to quantify the amount of radioactivity. One curie equals 37 billion nuclear 

transformations per second, and one picocurie (pCi) equals 10−12 Ci. 
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Decay product: Isotope formed by the radioactive decay of some other isotope. This newly formed isotope 

possesses physical and chemical properties that are different from those of its parent isotope, and may also be 

radioactive. 

Detection Limit (DL): Lowest amount that can be distinguished from the normal random "noise" of an analytical 

instrument or method. For HRS purposes, the detection limit used is the method detection limit (MDL) or, for 

real-time field instruments, the detection limit of the instrument as used in the field. 

Dilution weight: Parameter in the HRS surface water migration pathway that reduces the point value assigned to 

targets as the flow or depth of the relevant surface water body increases. [unitless]. 

Distance weight: Parameter in the HRS air migration pathway, ground water migration pathway, and the soil 

exposure component of the soil exposure and subsurface intrusion pathway that reduces the point value assigned 

to targets as their distance increases from the site. [unitless]. 

Distribution coefficient (Kd): Measure of the extent of partitioning of a substance between geologic materials (for 

example, soil, sediment, rock) and water (also called partition coefficient). The distribution coefficient is used in 

the HRS in evaluating the mobility of a substance for the ground water migration pathway. [ml/g]. 

ED10 (10 percent effective dose): Estimated dose associated with a 10 percent increase in response over control 

groups. For HRS purposes, the response considered is cancer. [milligrams toxicant per kilogram body weight per 

day (mg/kg-day)]. 

Food and Drug Administration Action Level (FDAAL): Under section 408 of the Federal Food, Drug and 

Cosmetic Act, as amended, concentration of a poisonous or deleterious substance in human food or animal feed at 

or above which FDA will take legal action to remove adulterated products from the market. Only FDAALs 

established for fish and shellfish apply in the HRS. 

Half-life: Length of time required for an initial concentration of a substance to be halved as a result of loss 

through decay. The HRS considers five decay processes for determining surface water persistence: 

biodegradation, hydrolysis, photolysis, radioactive decay, and volatilization. The HRS considers two decay 

processes for determining subsurface intrusion degradation: biodegradation and hydrolysis. 

Hazardous substance: CERCLA hazardous substances, pollutants, and contaminants as defined in CERCLA 

sections 101(14) and 101(33), except where otherwise specifically noted in the HRS. 

Hazardous wastestream: Material containing CERCLA hazardous substances (as defined in CERCLA section 

101[14]) that was deposited, stored, disposed, or placed in, or that otherwise migrated to, a source. 

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

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

targets). 

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

HRS "pathway": Set of HRS factor categories combined to produce a score to measure relative risks posed by a 

site in one of four environmental pathways (that is, ground water, surface water, soil exposure and subsurface 

intrusion, and air). 

HRS "site score": Composite of the four HRS pathway scores. 
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Henry's law constant: Measure of the volatility of a substance in a dilute solution of water at equilibrium. It is the 

ratio of the vapor pressure exerted by a substance in the gas phase over a dilute aqueous solution of that substance 

to its concentration in the solution at a given temperature. For HRS purposes, use the value reported at or near 25° 

C. [atmosphere-cubic meters per mole (atm-m3

/mol)]. 

Hydrolysis: Chemical reaction of a substance with water. 

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. 

Karst: Terrain with characteristics of relief and drainage arising from a high degree of rock solubility in natural 

waters. The majority of karst occurs in limestones, but karst may also form in dolomite, gypsum, and salt 

deposits. Features associated with karst terrains typically include irregular topography, sinkholes, vertical shafts, 

abrupt ridges, caverns, abundant springs, and/or disappearing streams. Karst aquifers are associated with karst 

terrain. 

LC50 (lethal concentration, 50 percent): Concentration of a substance in air [typically micrograms per cubic meter 

(μg/m3)] or water [typically micrograms per liter (μg/l)] that kills 50 percent of a group of exposed organisms. 

The LC50 is used in the HRS in assessing acute toxicity. 

LD50 (lethal dose, 50 percent): Dose of a substance that kills 50 percent of a group of exposed organisms. The 

LD50 is used in the HRS in assessing acute toxicity [milligrams toxicant per kilogram body weight (mg/kg)]. 

Maximum Contaminant Level (MCL): Under section 1412 of the Safe Drinking Water Act, as amended, the 

maximum permissible concentration of a substance in water that is delivered to any user of a public water supply. 

Maximum Contaminant Level Goal (MCLG): Under section 1412 of the Safe Drinking Water Act, as amended, a 

nonenforceable concentration for a substance in drinking water that is protective of adverse human health effects 

and allows an adequate margin of safety. 

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

sample or blank. 

Mixed radioactive and other hazardous substances: Material containing both radioactive hazardous substances 

and nonradioactive hazardous substances, regardless of whether these types of substances are physically 

separated, combined chemically, or simply mixed together. 

National Ambient Air Quality Standards (NAAQS): Primary standards for air quality established under sections 

108 and 109 of the Clean Air Act, as amended. 

National Emission Standards for Hazardous Air Pollutants (NESHAPs): Standards established for substances 

listed under section 112 of the Clean Air Act, as amended. Only those NESHAPs promulgated in ambient 

concentration units apply in the HRS. 

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).

Deleted: Multi-tenant structures: Structures that contain more than 

one commercial, residential, or industrial tenant space (e.g., 

apartment buildings, shopping centers, day care centers). They may 

or may not have multiple stories, and the tenant space may be on 

more than one floor.
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Octanol-water partition coefficient (Kow [or P]): Measure of the extent of partitioning of a substance between 

water and octanol at equilibrium. The Kow is determined by the ratio between the concentration in octanol divided 

by the concentration in water at equilibrium. [unitless]. 

Organic carbon partition coefficient (Koc): Measure of the extent of partitioning of a substance, at equilibrium, 

between organic carbon in geologic materials and water. The higher the Koc, the more likely a substance is to bind 

to geologic materials than to remain in water. [ml/g]. 

Photolysis: Chemical reaction of a substance caused by direct absorption of solar energy (direct photolysis) or 

caused by other substances that absorb solar energy (indirect photolysis). 

Preferential subsurface intrusion pathways: Subsurface features such as animal burrows, cracks in walls, spaces 

around utility lines or drains through which a hazardous substance moves more easily into a regularly occupied 

structure. 

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

Radioactive decay: Process of spontaneous nuclear transformation, whereby an isotope of one element is 

transformed into an isotope of another element, releasing excess energy in the form of radiation. 

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

undergo radioactive decay. 

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

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

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

"radioisotope" are used synonymously. 

Reference concentration (RfC): An estimate of a continuous inhalation exposure to the human population that is 

likely to be without an appreciable risk of deleterious effects during a lifetime. 

Reference dose (RfD): An estimate of a daily oral exposure to the human population that is likely to be without an 

appreciable risk of deleterious effects during a lifetime. 

Removal action: Action that removes hazardous substances from the site for proper disposal or destruction in a 

facility permitted under the Resource Conservation and Recovery Act or the Toxic Substances Control Act or by 

the Nuclear Regulatory Commission. 

Roentgen (R): Measure of external exposures to ionizing radiation. One roentgen equals that amount of x-ray or 

gamma radiation required to produce ions carrying a charge of 1 electrostatic unit (esu) in 1 cubic centimeter of 

dry air under standard conditions. One microroentgen (μR) equals 10−6 R. 

Sample quantitation limit (SQL): Quantity of a substance that can be reasonably quantified given the limits of 

detection for the methods of analysis and sample characteristics that may affect quantitation (for example, 

dilution, concentration). 

Screening concentration: Media-specific benchmark concentration for a hazardous substance that is used in the 

HRS for comparison with the concentration of that hazardous substance in a sample from that media. The 

screening concentration for a specific hazardous substance corresponds to its reference concentration for 

inhalation exposures or reference dose for oral exposures, as appropriate, and, if the substance is a human 

carcinogen with either a weight-of-evidence classification of A, B, or C, or a weight-of-evidence classification of 
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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. 

Site: Area(s) where a hazardous substance has been deposited, stored, disposed, or placed, or has otherwise come 

to be located. Such areas may include multiple sources and may include the area between sources. 

Slope factor (also referred to as cancer potency factor): Estimate of the probability of response (for example, 

cancer) per unit intake of a substance over a lifetime. The slope factor is typically used to estimate upper-bound 

probability of an individual developing cancer as a result of exposure to a particular level of a human carcinogen 

with either a weight-of-evidence classification of A, B, or C, or a weight-of-evidence classification of 

carcinogenic to humans, likely to be carcinogenic to humans or having suggestive evidence of carcinogenic 

potential. [(mg/kg-day)−1 for non-radioactive substances and (pCi)−1 for radioactive substances].

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

Soil porosity: The degree to which the total volume of soil is permeated with pores or cavities through which 

fluids (including air or gas) can move. It is typically calculated as the ratio of the pore spaces within the soil to the 

overall volume of the soil.

Source: Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils 

that have become contaminated from migration of a hazardous substance. Sources do not include those volumes 

of air, ground water, surface water, or surface water sediments that have become contaminated by migration, 

except: in the case of either a ground water plume with no identified source or contaminated surface water 

sediments with no identified source, the plume or contaminated sediments may be considered a source. 

Subslab: The area immediately beneath a regularly occupied structure with a basement foundation or a slab-ongrade 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.

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

varies by HRS pathway. 

Unit Risk: The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent 

(i.e., hazardous substance) at a concentration of 1 μg/L in water, or 1 μg/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).

Uranium Mill Tailings Radiation Control Act (UMTRCA) Standards: Standards for radionuclides established 

under sections 102, 104, and 108 of the Uranium Mill Tailings Radiation Control Act, as amended. 

Vapor pressure: Pressure exerted by the vapor of a substance when it is in equilibrium with its solid or liquid 

form at a given temperature. For HRS purposes, use the value reported at or near 25° C. [atmosphere or torr]. 

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

liquid to a gas. 
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Water solubility: Maximum concentration of a substance in pure water at a given temperature. For HRS purposes, 

use the value reported at or near 25 °C. [milligrams per liter (mg/L)]. 

Weight-of-evidence: EPA classification system for characterizing the evidence supporting the designation of a 

substance as a human carcinogen. The EPA weight-of-evidence groupings, depending on the date EPA updated 

the profile, include either: 

• Group A: Human carcinogen—sufficient evidence of carcinogenicity in humans. 

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

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

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

• Group D: Not classifiable as to human carcinogenicity—applicable when there is no animal evidence, or 

when human or animal evidence is inadequate. 

• Group E: Evidence of noncarcinogenicity for humans. 

or:

• Carcinogenic to humans.

• Likely to be carcinogenic to humans.

• Suggestive evidence of carcinogenic potential.

• Inadequate information to assess carcinogenic potential.

• Not likely to be carcinogenic to humans.
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2.0 Evaluations Common to Multiple Pathways

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

• Ground Water Migration (Sgw). 

• Surface Water Migration (Ssw). 

• Soil Exposure and Subsurface Intrusion (Ssessi). 

• Air Migration (Sa). 

The ground water and air migration pathways use single threat evaluations, while the surface water migration and 

soil exposure and subsurface intrusion pathways use multiple threat evaluations. Three threats are evaluated for 

the surface water migration pathway: drinking water, human food chain, and environmental. These threats are 

evaluated for two separate migration components--overland/flood migration and ground water to surface water 

migration. Two components are evaluated for the soil exposure and subsurface intrusion pathway: soil exposure 

and subsurface intrusion. The soil exposure component evaluates two threats: resident population and nearby 

population, and the subsurface intrusion component is a single threat evaluation.

The HRS is structured to provide a parallel evaluation for each of these pathways, components and threats. This 

section focuses on these parallel evaluations, starting with the calculation of the HRS site score and the individual 

pathway scores.

2.1.1 Calculation of HRS site score. Scores are first calculated for the individual pathways as specified 

in sections 2 through 7 and then are combined for the site using the following root-mean-square equation to 

determine the overall HRS site score, which ranges from 0 to 100: 

SS = �SSgggg

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.
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TABLE 2-1 SAMPLE PATHWAY SCORESHEET

Factor Category Maximum 

value

Value 

assigned

Likelihood of Release:

1. Observed Release 550

2. Potential to Release 500

3. Likelihood of Release (higher of lines 1 and 2) 550

Waste Characteristics:

4. Toxicity/Mobility (a)

5. Hazardous Waste Quantity (a)

6. Waste Characteristics 100

Targets:

7. Nearest Individual

7a. Level I 50

7b. Level II 45

7c. Potential Contamination 20

7d. Nearest Individual (higher of lines 7a, 7b, or 7c) 50

8. Population (b)

8a. Level I (b)

8b. Level II (b)

8c. Potential Contamination (b)

8d. Total Population (lines 8a+8b+8c)

9. Resources 5

10. Sensitive Environments (b)

10a. Actual Contamination (b)

10b. Potential Environments (b)

10c. Sensitive Environments (lines 10a+10b) (b)

11. Targets (lines 7d+8d+9+10c) (b)

12. Pathway Score is the product of Likelihood of Release, Waste Characteristics, and 

Targets, divided by 82,500. Pathway scores are limited to a maximum of 100 points.

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.
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• 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.

• 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. 

• 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 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 have a vapor pressure 

greater than or equal to 1 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 have a vapor pressure greater than or equal to 1 torr or a Henry’s constant greater than or equal 

to 10-5 atm-m3/mol.
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2.2.3 Identify hazardous substances available to a pathway. In evaluating each migration pathway, 

consider the following hazardous substances available to migrate from the sources at the site to the pathway: 

• Ground water migration. 

– Hazardous substances that meet the criteria for an observed release (see section 2.3) to ground water. 

– All hazardous substances associated with a source with a ground water containment factor value greater 

than 0 (see section 3.1.2.1).

• Surface water migration—overland/flood component. 

– Hazardous substances that meet the criteria for an observed release to surface water in the watershed 

being evaluated. 

– All hazardous substances associated with a source with a surface water containment factor value greater 

than 0 for the watershed (see sections 4.1.2.1.2.1.1 and 4.1.2.1.2.2.1).

• Surface water migration—ground water to surface water component. 

– Hazardous substances that meet the criteria for an observed release to ground water. 

– All hazardous substances associated with a source with a ground water containment factor value greater 

than 0 (see sections 4.2.2.1.2 and 3.1.2.1).

• Air migration. 

– Hazardous substances that meet the criteria for an observed release to the atmosphere. 

– All gaseous hazardous substances associated with a source with a gas containment factor value greater 

than 0 (see section 6.1.2.1.1). 

– All particulate hazardous substances associated with a source with a particulate containment factor value 

greater than 0 (see section 6.1.2.2.1).

• For each migration pathway, in those instances when the specific source(s) containing the hazardous 

substance cannot be documented, consider that hazardous substance to be available to migrate to the pathway 

when it can be associated (see section 2.2.2) with at least one source having a containment factor value 

greater than 0 for that pathway. 

In evaluating the soil exposure 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 1 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 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 

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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

• 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 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 weight-of-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: 

Deleted: bio

Deleted: bio

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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:

• 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

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)
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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

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.

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 LC50 not available LC50 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 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 60

human toxicity factor value for all hazardous substances available to the pathway. For hazardous substances 

having usable toxicity data for multiple exposure routes (for example, inhalation and ingestion), consider all 

exposure routes and use the highest assigned value, regardless of exposure route, as the toxicity factor value.

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

Separate criteria apply for assigning factor values for human toxicity and ecosystem toxicity for radionuclides 

(see sections 7.2.1 and 7.2.2).

2.4.1.2 Hazardous substance selection. For each hazardous substance evaluated for a migration pathway 

(or threat), combine the human toxicity factor value (or ecosystem toxicity factor value) for the hazardous 

substance with a mobility, persistence, and/or bioaccumulation (or ecosystem bioaccumulation) potential factor 

value as follows:

• Ground water migration. 

– Determine a combined human toxicity/mobility factor value for the hazardous substance (see section 

3.2.1).

• Surface water migration—overland/flood migration component. 

– Determine a combined human toxicity/persistence factor value for the hazardous substance for the 

drinking water threat (see section 4.1.2.2.1). 

– Determine a combined human toxicity/persistence/bioaccumulation factor value for the hazardous 

substance for the human food chain threat (see section 4.1.3.2.1). 

– Determine a combined ecosystem toxicity/persistence/bioaccumulation factor value for the hazardous 

substance for the environmental threat (see section 4.1.4.2.1). 

• Surface water migration—ground water to surface water migration component. 

– Determine a combined human toxicity/mobility/persistence factor value for the hazardous substance for 

the drinking water threat (see section 4.2.2.2.1). 

– Determine a combined human toxicity/mobility/persistence/bioaccumulation factor value for the 

hazardous substance for the human food chain threat (see section 4.2.3.2.1). 

– Determine a combined ecosystem toxicity/mobility/persistence/bioaccumulation factor value for the 

hazardous substance for the environmental threat (see section 4.2.4.2.1). 

• Air migration. 

• Determine a combined human toxicity/mobility factor value for the hazardous substance (see section 

6.2.1). 

Determine each combined factor value for a hazardous substance by multiplying the individual factor values 

appropriate to the pathway (or threat). For each migration pathway (or threat) being evaluated, select the 

hazardous substance with the highest combined factor value and use that substance in evaluating the waste 

characteristics factor category of the pathway (or threat).

For the soil exposure 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

Deleted: bio

Deleted: bio
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 Meets the criteria for observed release in an area of subsurface contamination and has a vapor 

pressure greater than or equal to 1 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 1 torr or a Henry’s constant 

greater than or equal to 10-5 atm-m3/mol.

• 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 and 5.2.1.2.1). 

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. 

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 have a vapor pressure 

greater than or equal to 1 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 have a vapor pressure greater than or equal to 1 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.

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.

Deleted: tenant 
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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).

2.4.2.1.1 Hazardous constituent quantity. Evaluate hazardous constituent quantity for the source (or area of 

observed contamination) based solely on the mass of CERCLA hazardous substances (as defined in CERCLA 

section 101(14), as amended) allocated to the source (or area of observed contamination), except:

• For a hazardous waste listed pursuant to section 3001 of the Solid Waste Disposal Act, as amended by the 

Resource Conservation and Recovery Act of 1976 (RCRA), 42 U.S.C. 6901 et seq., determine its mass for the 

evaluation of this measure as follows: 

– If the hazardous waste is listed solely for Hazard Code T (toxic waste), include only the mass of 

constituents in the hazardous waste that are CERCLA hazardous substances and not the mass of the entire 

hazardous waste. 

– If the hazardous waste is listed for any other Hazard Code (including T plus any other Hazard Code), 

include the mass of the entire hazardous waste.

• For a RCRA hazardous waste that exhibits the characteristics identified under section 3001 of RCRA, as 

amended, determine its mass for the evaluation of this measure as follows: 

– If the hazardous waste exhibits only the characteristic of toxicity (or only the characteristic of EP 

toxicity), include only the mass of constituents in the hazardous waste that are CERCLA hazardous 

substances and not the mass of the entire hazardous waste. 

– If the hazardous waste exhibits any other characteristic identified under section 3001 (including any other 

characteristic plus the characteristic of toxicity [or the characteristic of EP toxicity]), include the mass of 

the entire hazardous waste.

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

• For the migration pathways, assign the source a value for hazardous constituent quantity using the Tier A 

equation of Table 2-5. 

• For the soil exposure 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 
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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

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.

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.
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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:

• 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).

• 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. 
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• 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 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

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 consideration of the removal 

action or a value of 100, whichever is greater, as the hazardous waste quantity factor value for the 

pathway. 
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 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:

– 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.

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
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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:

• 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 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 
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samples from aquatic human food chain organisms are at or above benchmark values. Such tissue 

samples may be used in addition to media-specific concentrations only as specified in sections 4.1.3.3 

and 4.2.3.3.

– Level II: 

 Media-specific concentrations for the target meet the criteria for an observed release (or observed 

contamination 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. 

– 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 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).
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 69

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 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. 

• 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 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.
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 70

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: 

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. 
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 71

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.
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 72

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
Non-Federal Register Language Version – Draft – Do Not Cite, Quote or Release – December 11, 2015 73

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 74 of 111

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)

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)
Page 75 of 111

Factor categories and factors Maximum 

value

Value 

assigned

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 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. 

• 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 
Page 76 of 111

• 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). 

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

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
Page 77 of 111

Tier Measure Units Equation for assigning valuea

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. 

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. 

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 healthbased 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.
Page 78 of 111

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.

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. 

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. 
Page 79 of 111

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

 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

75

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)

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. 
Page 80 of 111

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.

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

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
Page 81 of 111

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

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 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
Page 82 of 111

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 distance-weighted population 

value for that travel distance from Table 5-10. 

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

a Round the number of people present within a travel distance category to nearest integer. Do not round the assigned distanceweighted 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.
Page 83 of 111

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 84 of 111

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 85 of 111

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) 

 9b. Level II Concentrations (b) 

9c. Population within Area(s) 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.

Deleted: Biod

Deleted: an 

Deleted: <#>Workers ...

Deleted: + 11

Deleted: 2
Page 86 of 111

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 structure that has a 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 structure. 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 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.

• 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 1 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 have a vapor pressure greater than or equal to 1 torr or a Henry’s constant 

greater than or equal to 10-5 atm-m3/mol.

Deleted: (s)

Deleted: (s)

Deleted: which are either contaminated

Deleted: contaminated

Deleted: C

Deleted: 

Deleted: with

Deleted: rom

Deleted: 

Deleted: multi-tenantregularly

Deleted: both the subunit(s) stenant space(s)

Deleted: observed exposure 

Deleted: and all other regularly occupied subunit(s) stenant 

spaces on that level 

Deleted: that have multiple stories,

Deleted: and 

Deleted: not

Deleted: an

Deleted: (

Deleted: )tenant spaces

Deleted: at which there is contamination

Moved (insertion) [2]

Deleted: with

Deleted: areas 

Moved up [2]: Exclude those areas meeting the criteria 

defined as an area of observed exposure.

Deleted: only 
Page 87 of 111

See Section 7.0 for establishing an area of subsurface contamination based on the presence of radioactive 

hazardous substances.

– 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.

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: 

– 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 

Deleted: the

Deleted: SsI

Deleted: tenant 

Deleted: tenant spaces

Deleted: tenant spaces

Deleted: no observed exposure has been documented in 

Deleted: subunit(s) of a 

Deleted: that 

Deleted: 4
Page 88 of 111

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 1 torr or a Henry’s 

constant greater than or equal to 10-5 atm-m3/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.

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

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

Deleted: 10-5
Page 89 of 111

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).

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.

• 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.

Deleted: n
Page 90 of 111

– 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

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.

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: 

Deleted: highest 
Page 91 of 111

– 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

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 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. 

Deleted: ¶

¶

¶

Deleted: Greater than 10-3

Deleted: 10-5 to 10-3

...

Deleted: 10-5

Deleted: 5

Deleted: 7

Deleted: 5

Deleted: 7

Deleted: bio
Page 92 of 111

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:

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:

• 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 

Deleted: bio

Deleted: bio

Deleted: bio

Deleted: <#>¶
Page 93 of 111

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.

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.

– 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.

Deleted: 3

Deleted: bio

Deleted: bio

Deleted: bio

Deleted: bio

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 
Page 94 of 111

 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.

• 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 multisubunit 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.

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.

Deleted: tenant 

Deleted: tenant 

Deleted: tenant 

Deleted: residential 

Deleted: residential tenant

Deleted: tenant 

Deleted: tenant 
Page 95 of 111

– 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 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. 

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 chemical analysis criteria to the 

Deleted: bio

Deleted: four 

Deleted: workers, 

Deleted: 4

Deleted: removed 

Deleted: had not occurred unless it has been demonstrated that no 

significant risk remains

Deleted: tenant 

Deleted: areas of observed exposure
Page 96 of 111

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.

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 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. 

Deleted: in determining

Deleted: single residential 

Deleted: assume 

Deleted: multi-subunit,

Deleted: single or multi-story, residential

Deleted: Use the health-based benchmarks from Table 5-19 in 

determining the level of contamination. 

Deleted: such 

Deleted: living 

Deleted: living or attending school or day care 

Deleted: people living 

Deleted: when the estimate is based on the number of residences,

Deleted: factor value 

Deleted: n

Deleted: eligible 

Deleted: ,
Page 97 of 111

3. Count the number of full-time and part-time workers in the structure or subunit 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 quotients 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. 

3. For each structure or subunit, 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 quotients 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 fullor 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. Sum these quotients 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 

Deleted: ,

Deleted: exposed individuals 

Deleted: is

Deleted: product

Deleted: factor value 

Moved (insertion) [1]

Deleted: those people

Deleted: (s)

Deleted: this

Deleted: product

Deleted: .Sum the number of exposed individuals subject to 

Level II concentrations. Do not include those people already 

counted under the Level I concentrations factor. Also, include 

those people who regularly occupy a structure within areas of 

inferred exposure in an area of observed exposure . 

Moved up [1]: Do not include those people already counted 

under the Level I concentrations factor. 

Deleted: an 

Deleted: 0

Deleted: s

Deleted: ose

Deleted: s

Deleted: ose

Deleted: ,
Page 98 of 111

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

Population in 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. 

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 

Deleted: for this factor

Deleted: residing 

Deleted: residing 

Deleted: tenant 

Deleted: space 

Deleted: story 

Deleted: with an observed exposure

Deleted: residing 

Deleted: residing 

Deleted: residing 

Deleted: residing 

Deleted: residing 

Deleted: and

Deleted: 

Deleted: students

Deleted: .2.3

Deleted: Workers are excluded from this evaluation (see HRS 

Sections 5.2.1.3 and 5.2.1.3.3).¶

Deleted: or

Deleted: y 

Deleted: 5.2.1.3.3 Workers. Evaluate this factor based on the 

number of workers that meet the section 5.2.1.3 criteria. Assign a 

value for these workers using Table 5-21. Enter this value in Table 

5-11.¶

TABLE 5-21 WORKER FACTOR VALUES¶

Number of Workers ...

Deleted: 4

Deleted: 5

Deleted: workers, 
Page 99 of 111

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 100 of 111

6.0 Air Migration Pathway*

*The only section of 6.0 that has been updated from the 1990 HRS is Table 6-14; specifically the final bullet;

Reference Concentration (RfC) has replaced Reference Dose (RfD).

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.
Page 101 of 111

7.0 Sites Containing Radioactive Substances

In general, radioactive substances are hazardous substances under CERCLA and should be considered in HRS 

scoring. Releases of certain radioactive substances are, however, excluded from the definition of "release" in 

section 101(22) of CERCLA, as amended, and should not be considered in HRS scoring. 

Evaluate sites containing radioactive substances using the instructions specified in sections 2 through 6, 

supplemented by the instructions in this section. Those factors denoted with a "yes" in Table 7-1 are evaluated 

differently for sites containing radioactive substances than for sites containing only nonradioactive hazardous 

substances, while those denoted with a "no" are not evaluated differently and are not addressed in this section. 

TABLE 7-1 HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status aSurface 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

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/Yes Toxicity Yes Toxicity/ Degradation Yes Toxicity Yes

Mobility No Persistence/

Mobility Yes/No Hazardous Waste Quantity Yes Hazardous Waste Quantity Yes Mobility No

Hazardous 

Waste Quantity Yes BioaccumulationPotential No Hazardous Waste Quantity Yes

Hazardous 

Waste Quantity Yes

Targets Targets Targets Targets Targets

Nearest Well Yes b Nearest Intake Yes b Resident 

Individual Yes b ExposedIndividual 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

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.

In general, sites containing mixed radioactive and other hazardous substances involve more evaluation than sites 

containing only radionuclides. For sites containing mixed radioactive and other hazardous substances, HRS 

Deleted: Bio

Deleted: d

Deleted: Workers

Deleted: Resources

Deleted: No
Page 102 of 111

factors are evaluated based on considerations of both the radioactive substances and the other hazardous 

substances in order to derive a single set of factor values for each factor category in each of the four pathways. 

Thus, the HRS score for these sites reflects the combined potential hazards posed by both the radioactive and 

other hazardous substances. 

Section 7 is organized by factor category, similar to sections 3 through 6. Pathway-specific differences in 

evaluation criteria are specified under each factor category, as appropriate. These differences apply largely to the 

soil exposure and subsurface intrusion pathway and to sites containing mixed radioactive and other hazardous 

substances. All evaluation criteria specified in sections 2 through 6 must be met, except where modified in section 

7. 

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. Base these demonstrations on 

one or more of the following, as appropriate to the pathway being evaluated: 

• Direct observation: 

– For each migration pathway, a material that contains one or more radionuclides has been seen entering the 

atmosphere, surface water, or ground water, as appropriate, or is known to have entered ground water or 

surface water through direct deposition, or 

– For the surface water migration pathway, a source area containing radioactive substances has been 

flooded at a time that radioactive substances were present and one or more radioactive substances were in 

contact with the flood waters. 

– 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 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): 

– For radionuclides that occur naturally and for radionuclides that are ubiquitous in the environment: 

 Measured concentration (in units of activity, for example, pCi per kilogram [pCi/kg], pCi per liter 

[pCi/L], pCi per cubic meter [pCi/m3

]) of a given radionuclide in the sample are at a level that: 

o Equals or exceeds a value 2 standard deviations above the mean site-specific background 

concentration for that radionuclide in that type of sample, or 

o Exceeds the upper-limit value of the range of regional background concentration values for that 

specific radionuclide in that type of sample.

 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. 
Page 103 of 111

– For man-made radionuclides without ubiquitous background concentrations in the environment: 

 Measured concentration (in units of activity) of a given radionuclide in a sample equals or exceeds 

the sample quantitation limit for that specific radionuclide in that type of media and is attributable to 

the site. 

 However, if the radionuclide concentration equals or exceeds its sample quantitation limit, but its 

release can also be attributed to one or more neighboring sites, then the measured concentration of 

that radionuclide must also equal or exceed a value either 2 standard deviations above the mean 

concentration of that radionuclide contributed by those neighboring sites or 3 times its background 

concentration, whichever is lower. 

 If the sample quantitation limit cannot be established: 

o 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). 

o If the sample analysis is not performed under the EPA Contract Laboratory Program, use the 

detection limit in place of the sample quantitation limit. 

 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. 

– Some portion of the increase must be attributable to the site to establish observed contamination. The 

gamma-emitting radionuclides do not have to be within 2 feet of the surface of the source. 

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 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. 
Page 104 of 111

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. 

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. Base the evaluation on the physical and chemical properties of the radionuclides, not on 

their level of radioactivity. 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 Waste characteristics. For radioactive substances, evaluate the human toxicity factor, the ecosystem 

toxicity factor, the surface water persistence factor, and the hazardous waste quantity factor as specified in the 

following sections. Evaluate all other waste characteristic factors as specified in sections 2 through 6. 

7.2.1 Human toxicity. For radioactive substances, evaluate the human toxicity factor as specified below, not 

as specified in section 2.4.1.1. 

Assign human toxicity factor values to those radionuclides available to the pathway based on quantitative doseresponse parameters for cancer risks as follows: 

• Evaluate radionuclides only on the basis of carcinogenicity and assign all radionuclides to weight-of-evidence 

category A. 

• Assign a human toxicity factor value from Table 7-2 to each radionuclide based on its slope factor (also 

referred to as cancer potency factor). 

– For each radionuclide, use the higher of the slope factors for inhalation and ingestion to assign the factor 

value. 

– If only one slope factor is available for the radionuclide, use it to assign the toxicity factor value. 

– If no slope factor is available for the radionuclide, assign that radionuclide a toxicity factor value of 0 and 

use other radionuclides for which a slope factor is available to evaluate the pathway. 

• If all radionuclides available to a particular pathway are assigned a human toxicity factor value of 0 (that is, 

no slope factor is available for all the radionuclides), use a default human toxicity factor value of 1,000 as the 

human toxicity factor value for all radionuclides available to the pathway.

At sites containing mixed radioactive and other hazardous substances, evaluate the toxicity factor separately for 

the radioactive and other hazardous substances and assign each a separate toxicity factor value. This applies 

regardless of whether the radioactive and other hazardous substances are physically separated, combined 

chemically, or simply mixed together. Assign toxicity factor values to the radionuclides as specified above and to 

the other hazardous substances as specified in section 2.4.1.1. 

At sites containing mixed radioactive and other hazardous substances, if all radionuclides available to a particular 

pathway are assigned a human toxicity factor value of 0, use a default human toxicity factor value of 1,000 for all 
Page 105 of 111

those radionuclides even if nonradioactive hazardous substances available to the pathway are assigned human 

toxicity factor values greater than 0. Similarly, if all nonradioactive hazardous substances available to the pathway 

are assigned a human toxicity factor value of 0, use a default human toxicity factor value of 100 for all these 

nonradioactive hazardous substances even if radionuclides available to the pathway are assigned human toxicity 

factor values greater than 0. 

7.2.2 Ecosystem toxicity. For the surface water environmental threat (see sections 4.1.4 and 4.2.4). assign 

an ecosystem toxicity factor value to radionuclides (alone or combined chemically or mixed with other hazardous 

substances) using the same slope factors and procedures specified for the human toxicity factor in section 7.2.1, 

except: use a default of 100, not 1,000, if all radionuclides eligible to be evaluated for ecosystem toxicity receive 

an ecosystem toxicity factor value of 0. 

TABLE 7-2 TOXICITY FACTOR VALUES FOR RADIONUCLIDES

Cancer slope factor a (SF) (pCi)-1 Assigned value

3x10-11 ≤ SF 10,000

3x10-12 ≤ SF<3x10-11 1,000

SF<3x10-12 100

SF not available for the radionuclide 0

a Radionuclide slope factors are estimates of age-averaged, individual lifetime total excess cancer risk per picocurie of radionuclide inhaled or 

ingested.

At sites containing mixed radioactive and other hazardous substances, evaluate the ecosystem toxicity factor 

separately for the radioactive and other hazardous substances and assign each a separate ecosystem toxicity factor 

value. This applies regardless of whether the radioactive and other hazardous substances are physically separated, 

combined chemically, or simply mixed together. Assign ecosystem toxicity factor values to the radionuclides as 

specified above and to the other hazardous substances as specified in sections 4.1.4.2.1.1 and 4.2.4.2.1.1. If all 

radionuclides available to a particular pathway are assigned an ecosystem toxicity factor value of 0, use a default 

ecosystem toxicity factor value of 100 for all these radionuclides even if nonradioactive hazardous substances 

available to the pathway are assigned ecosystem toxicity factor values greater than 0. Similarly, if all 

nonradioactive hazardous substances available to the pathway are assigned an ecosystem toxicity factor value of 

0, use a default ecosystem toxicity factor value of 100 for all these nonradioactive hazardous substances even if 

radionuclides available to the pathway are assigned ecosystem toxicity factor values greater than 0.

7.2.3 Persistence/Degradation.

In determining the surface water persistence factor for radionuclides, evaluate 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 (t1/2) 

calculated as follows:

t1/2 = 1

1

r+1

v

where:

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.

Deleted: For 

Deleted: the surface water persistence

Deleted: 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.¶
Page 106 of 111

In determining the subsurface intrusion degradation factor for radionuclides, evaluate this factor based on halflife. 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: 

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).

7.2.5 Hazardous waste quantity. To calculate the hazardous waste quantity factor value for sites 

containing radioactive substances, evaluate source hazardous waste quantity (see section 2.4.2.1) using only the 

following two measures in the following hierarchy (these measures are consistent with Tiers A and B for 

nonradioactive hazardous substances in sections 2.4.2.1.1 and 2.4.2.1.2):

• Radionuclide constituent quantity (Tier A). 

• Radionuclide wastestream quantity (Tier B).

7.2.5.1 Source hazardous waste quantity for radionuclides. For each migration pathway, assign a source 

hazardous waste quantity value to each source having a containment factor value greater than 0 for the pathway 

being evaluated. 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: 

– Manifests, or 

Deleted: when 

Deleted: inge

Deleted: solely 

Deleted: 1/h + 1/b + 

Deleted: h=Hydrolysis half-life. ¶

b=Biodegradation half-life. ¶

Deleted: bio
Page 107 of 111

– Either of the following equations, as applicable:

N = 9.1x10−7(V)�ACi

n

i=1

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. 

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,

N = 3.8x10−12(V)�ACi

n

i=1

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. 

• 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), include the entire depth, not just that within 2 feet of the 

surface. 

• 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. 

Instead, assign radionuclide wastestream quantity a value of 0 and proceed to section 7.2.5.1.3. If the radionuclide 

constituent quantity is not adequately determined, assign the source (or area of observed contamination or area of 
Page 108 of 111

observed exposure) a value for radionuclide constituent quantity based on the available data and proceed to 

section 7.2.5.1.2.

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). 

• Divide the volume in cubic yards by 0.55 (or the volume in gallons by 110) to convert to the activity content 

expressed in terms of equivalent pounds of nonradioactive hazardous substances. 

• 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 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). Do not round to the nearest integer. 

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 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 this pathway from Table 2-6 (section 2.4.2.2). 

For a migration pathway, if the radionuclide constituent quantity is adequately determined (see section 7.2.5.1.1) 

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 radionuclide 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 7.3), 

assign either the value from Table 2-6 or a value of 100, whichever is greater, as the hazardous waste quantity 

factor value for that pathway. 

• If none of the targets for that pathway is subject to Level I or Level II concentrations, assign a factor value as 

follows: 

– If there has been no removal action, assign either the value from Table 2-6 or a value of 10, whichever is 

greater, as the hazardous waste quantity factor value for that pathway. 

– If there has been a removal action: 

 Determine values from Table 2-6 with and without consideration of the removal action. 

 If the value that would be assigned from Table 2-6 without consideration of the removal action 

would be 100 or greater, assign either the value from Table 2-6 with consideration of the removal 

action or a value of 100, whichever is greater, as the hazardous waste quantity factor value for the 

pathway. 
Page 109 of 111

 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 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. If the radionuclide 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 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, 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). Do not round this value to the nearest integer. 

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. 

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. Determine whether Level I or Level II 

concentrations apply at a sampling location (and thus to the associated targets) as follows: 

• 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 110 of 111

• Determine which level applies based on this comparison. 

• 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. Use the following media specific benchmarks (expressed in activity 

units, for example, pCi/l for water, pCi/kg for soil and for aquatic human food chain organisms, and pCi/m3 for 

air) for making the comparisons for the indicated pathway (or threat): 

• Maximum Contaminant Levels (MCLs)—ground water migration pathway and drinking water threat in 

surface water migration pathway. 

• 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. 

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. If I equals or exceeds 1, assign Level I to the sampling location. 

If I is less than 1, assign Level II. 

At sites containing mixed radioactive and other hazardous substances, establish the level of contamination for 

each sampling location considering radioactive substances and nonradioactive hazardous substances separately. 

Compare the concentration of each radionuclide and each nonradioactive hazardous substance from the sampling 

location to its respective benchmark concentration(s). 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. If the concentration of one or more applicable radionuclides or other hazardous substances 

from any sample equals or exceeds its benchmark concentration, consider the sampling location to be subject to 

Level I concentrations. If more than one benchmark applies to a radionuclide or other hazardous substance, assign 
Page 111 of 111

Level I if the concentration of the radionuclide or other hazardous substance equals or exceeds its lowest 

applicable benchmark concentration.

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. Sum the index I values for the two types of 

substances. If the value, individually or combined, equals or exceeds 1, assign Level I to the sample location. If it 

is less than 1, calculate an index J for the nonradioactive hazardous substances as specified in section 2.5.2. If J 

equals or exceeds 1, assign Level I to the sampling location. If J is less than 1, assign Level II. 

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. Deleted: in