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6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 300

EPA-HQ-SFUND-2010-1086; FRL_XXXX-X 

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 60 DAYS FROM 

PUBLICATION IN THE FEDERAL REGISTER].

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-SFUND-2010-1086, to 

the Federal eRulemaking Portal: http://www.regulations.gov. Follow the online instructions for 

submitting comments. Comments may not be edited once submitted, and may publish in the docket 
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without changes by the EPA. Do not submit electronically any information you consider to be 

Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. If 

you need to include CBI as part of your comment, please 

visit http://www.epa.gov/dockets/comments.html for instructions. Multimedia submissions (audio, video, 

etc.) must be accompanied by a written comment. The written comment is considered the official 

comment and should include discussion of all points you wish to make.

For additional submission methods, the full EPA public comment policy, and general guidance on 

making effective comments, please visit http://www.epa.gov/dockets/comments.html.

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?

II. Statutory Authority

III. Background

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

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

C. What is the Impact of this Proposed Rule?

1.Impact on Current Cleanup Programs

2.Impact on Resources and Costs

3.Children’s Environmental Health
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4.Environmental Justice

IV. Hazard Ranking System 

A. Purpose

B. Structure 

V. Approach to HRS Addition

A. General Approach

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

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

3.What Public Outreach Activities Did EPA Conduct?

4.What Peer Review Process Did EPA Use?

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

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

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

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

C. Supporting Materials

VI. Discussion of the Proposed Subsurface Intrusion Addition to the HRS

A. Addition Within a Restructured Soil Exposure and Subsurface Intrusion Pathway 

B. Subsurface Intrusion 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 Areas of Subsurface Intrusion Considered: Potential Migration Zone

3. Likelihood of Exposure

a. Observed Exposure 

b. Potential for Exposure
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c. Calculation of the Likelihood of Exposure Factor Category Value

4. Waste Characteristics

a. Toxicity/Biodegradation

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. Workers

e. Resources

f. Calculation of the Targets Factor Category Value

6. Calculation and Incorporation of the Subsurface Intrusion Component Score into the 

HRS Site Score

a. Calculation of the Subsurface Intrusion Component Score

b. Incorporation of the Subsurface Intrusion 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. Terminology Updates Affecting Specific Sections of HRS (Sections 2 & 5)

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

C. Regulatory Flexibility Act
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D. Unfunded Mandates Reform Act

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 that Significantly Affect Energy Supply, Distribution, or Usage

I. National Technology Transfer and Advancement Act

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

Populations and Low Income Populations

K. Executive Order 12580: Superfund Implementation

I. General Information

A. What is EPA Seeking Comment On?

EPA is proposing an addition of one new component to one part of the current HRS. No major 

structural changes to other parts of the HRS are proposed. Therefore, comments are only being solicited 

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.
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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 hazardous waste sites. 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. 

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. 

No authority consistently and comprehensively addresses subsurface intrusion contamination across 

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

have identified subsurface intrusion sites as an issue, 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. 

Furthermore, EPA is planning on issuing a final guidance document for evaluating the vapor intrusion 

threat after all public comments have been addressed. (The most common form of the intrusion of 

contaminants from the subsurface into an overlying structure is vapor intrusion.) This document, OSWER 

Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Sources to 

Indoor Air, is an update of the agency’s 2002 draft guidance, OSWER Draft Guidance for Evaluating the

Vapor Intrusion into Indoor Air Pathway from Ground Water and Soils.
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This proposed regulatory change does not affect the status of sites currently on or proposed to be 

added to the NPL. 

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

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

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

contaminants2 from the subsurface environment, or more specifically, the unsaturated zone and/or the 

surficial ground water into overlying structures. 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.

 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.
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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 (see section III.B 

of this preamble). 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. As such, none of the existing 

pathways address the threat posed by hazardous substances entering a regularly occupied structure from 

the subsurface.

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 

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

to Be Added to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-380, May 2010.
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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 inventory are currently listed in the Superfund Enterprise 

Management System (SEMS). Of the 1,073 identified sites:

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

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

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

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

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

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

space, or indoor air samples but insufficient HRS-required evaluation factors to qualify for the 

NPL.
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• 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 would qualify for the NPL.

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

intrusion of contaminated ground water into regularly occupied structures, which appears to be an 

emerging issue. Recently, a site was discovered where shallow (surficial) ground water had intruded into 

residential basements. After the water receded or evaporated, a precipitate of chromium remained as a 

residue in the basements. Although EPA considers the presence of this residue as posing a significant 

threat to public health, the current HRS does not include a mechanism to evaluate human exposure 

resulting from subsurface intrusion. A determination by the ATSDR that such exposure posed a 

significant threat was relied upon to place this site on the NPL. The decision to include sites on the NPL 

based on a determination by the ATSDR is made infrequently, as the HRS is the primary mechanism for 

placing a site on the NPL and therefore needs to comprehensively assess site threat. EPA regional site 

assessment programs have identified 8 additional sites where intrusion of contaminated ground water is a 

potential issue and the related threat cannot be evaluated using the current HRS. By including ground 

water intrusion as part of a subsurface intrusion component to the HRS, it will be possible for EPA to 

accurately evaluate sites where contaminated ground water has impacted regularly occupied structures. 

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.

EPA’s identification of sites with a possible threat from contaminated vapor or ground water 

intruding into overlying regularly occupied structures does not represent a comprehensive assessment of 

the potential scope of the subsurface intrusion threat. EPA identified these sites based on currently 

available information to serve as the basis for initially assessing the subsurface intrusion problem in 
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general. 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 there exists a degree of 

inherent uncertainty 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 subsurface 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 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 as 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, in addition to the significant number of sites with possible subsurface intrusion issues, the 

public support for this 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.
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B. What is the History of the Hazard Ranking System?

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

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

implement section 105 (a)(8)(A) of CERCLA and Executive Order 12316 (46 FR 42237, August 20, 

1981), EPA revised the NCP on several occasions, with the most recent comprehensive revision occurring 

on March 8, 1990 (55 FR 8666). The NCP sets forth the guidelines and procedures needed for responding 

to releases or potential releases of hazardous substances. Section 105(a)(8)(A) of CERCLA required EPA

to establish: 

...criteria 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). The HRS is a scoring system used to assess the relative threat 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 uniformly 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" (47 FR 31220, July 16, 1982). Although the HRS 
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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.

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 appear to pose the most serious threats to public health and the environment and that may 

warrant remedial investigation and possible cleanup under CERCLA. Only sites on the NPL are eligible 

for Superfund-financed remedial actions. Emergency removal and enforcement actions can be conducted 

at any site, whether or not it is on the NPL.

In 1986, Congress passed the Superfund Amendments and Reauthorization Act (SARA) (Pub. L. 99-499), 

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

extent feasible, that the hazard ranking system accurately assesses the relative degree of risk to human 

health and the environment posed by sites and facilities subject to review." The HRS was previously 

amended in 1990. This proposed action will amend the HRS to add a subsurface intrusion component to 

the evaluation. 

C. What is the Impact of this Proposed Rule?

1. Impact on Current Cleanup Programs

When hazardous substances are released and enter the subsurface environment, they can move within 

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 consider at sites the relative degree of risk posed by human 
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exposure to hazardous substances that enter regularly occupied structures through the subsurface 

environment. 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. 

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. 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. 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. 

As a matter of policy, EPA does not delay listing a site to incorporate new data or score new 

pathways if the listing decision would not be affected by such action. EPA must balance the need to fully 

characterize a site and to perform the analysis with the limited resources available to collect and analyze 

site data. For this reason, EPA generally will not score additional pathways upon receiving new data as 

long as the site still meets the HRS cutoff score. However, any additional data characterizing site 

conditions collected during the HRS evaluation process could provide useful information during the 
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Remedial Investigation( RI). It is important to note that the HRS is intended to be a “rough list” of 

prioritized hazardous sites; a “first step in a process--nothing more, nothing less” Eagle Picher Indus. v. 

EPA, 759 F.2d 922, 932 (D.C. Cir. 1985) (Eagle Picher II). EPA would like to investigate each possible 

site completely and thoroughly prior to evaluating the site for proposal to the NPL, but must reconcile the 

need for certainty before action with the need for procedures that are expeditious and limited in scope to 

identify potentially hazardous sites. The D.C. Circuit Court of Appeals has found EPA's approach to 

solving this conundrum to be “reasonable and fully in accord with Congressional intent.” Eagle Picher 

Industries, Inc. v. EPA, (759 F.2d 905 (D.C. Cir. 1985) (Eagle Picher I)).

EPA’s site assessment program is operated strictly by the public sector. EPA uses funds that are 

appropriated annually and reside in the Superfund Trust Fund, while other federal agencies that conduct 

CERCLA site assessments use funds appropriated annually to them for the purposes of implementing 

CERCLA at their properties. Because subsurface intrusion sites have the potential to pose a high level of 

risk to residents and occupants of structures, EPA expects that there will be a realignment and 

reprioritization of its internal resources and in Cooperative Agreement funding to address sites with 

subsurface intrusion as a priority. Therefore the proposed rule is likely to result in less effort in some site 

assessment areas to free resources that can be used for subsurface intrusion evaluations. For this reason, 

EPA considers that this proposed rule is not expected to result in either an increase in the number of site 

assessments per year or in the placement of more sites on the NPL per year. 

Because federal agencies already have the authority to address subsurface intrusion issues as part of 

their environmental programs, this proposed rule is anticipated to have minimal impacts on federal 

facility cleanup programs. 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 nonNPL sites. Therefore, federal agencies are in a position to proactively identify and respond to risks posed 
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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. 

2. Impact on Resources and Costs

No present program consistently and comprehensively addresses subsurface intrusion contamination 

across non-federal possible sites, particularly when subsurface intrusion is the primary threat. While most 

states have identified subsurface intrusion sites as an issue, not all states have subsurface intrusion 

programs, and states with programs vary in their authority, resources, and remediation criteria. A 

redirection of resources available through EPA’s Cooperative Agreement funding is expected to provide 

for greater national consistency in the identification and evaluation of sites with subsurface intrusion 

contamination. Additionally, EPA is planning to issue new vapor intrusion guidance in the near future. 

Together with this proposed rule, EPA will provide a comprehensive framework to guide evaluation of 

subsurface intrusion sites across the country.

Conducting an HRS evaluation imposes no direct costs on any private entity. After EPA or a 

state/tribe conducts a site assessment and EPA determines that a site is eligible for the NPL, additional 

regulatory actions must be taken before any costs or other impacts would occur. Placement of a site on the 

NPL does not ascribe liability or in any way impose requirements on private parties. 

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.
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3. Children’s Environmental Health

EPA interprets Executive Order 13045, Protection of Children from Environmental Health Risks and 

Safety Risks, as applying only to those regulatory actions that concern environmental health or safety 

risks that EPA believes may disproportionately affect children. This rulemaking is expected to have only 

moderate costs and no direct impact on human health risk. In addition, any impacts resulting from future 

actions to remediate sites with SsI contamination would reduce the human health risk at these sites. 

Therefore, EPA concludes that this rule will have no significant impacts on children’s health.

4. Environmental Justice

This rulemaking is expected to have only moderate costs and no impact on human health risk. 

Therefore, the rule has no impacts on communities considered under Executive Order 12898, Federal 

Actions to Address Environmental Justice in Minority Populations and Low-Income Populations.

Therefore, EPA did not conduct a complete environmental justice analysis. However, EPA did consider 

whether the proposed action might have contingent impacts on minority or low-income populations 

located near potential SsI sites if future actions affect remediation of these sites. This analysis concluded 

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

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

addressing risks in these communities would not contribute to disproportionate adverse impacts on human 

health.

A regulatory impact analysis (RIA) of the three regulatory options considered in the development of 

this rulemaking was also conducted. The option selected as the preferred option was chosen as it is 

consistent with the 1990 HRS, serves as an effective screening tool that would both minimize unnecessary 

resource expenditures and the erroneous inclusion or exclusion of sites from possible placement on the 

NPL, and is based on proven science. The RIA for this rulemaking can be found in the official Docket for 

this action. 
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Before making the decision to proceed with this action, EPA also conducted outreach activities to 

determine interest and support from the public. On January 31, 2011, EPA published a “Notice of 

Opportunity for Public Input (76 FR 5370, January 31, 2011) soliciting public input on whether to include 

a subsurface intrusion component in the HRS. EPA also sent letters to all federally recognized tribes 

asking for their comments on the FR notice. Four public listening sessions were also held throughout the 

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

IV. Hazard Ranking System

A. Purpose

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

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

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

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

possible cleanup under CERCLA. (See Figure 1 for a general depiction of the Superfund Site Assessment 

process.) During site discovery, which is the first step of the pre-remedial process, possible releases of 

hazardous substances are listed in EPA’s SEMS. If based on the results of a preliminary assessment (an 

assessment made using existing data to determine if there is evidence of a potential threat), EPA

determines that a site warrants further investigation, 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.
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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 
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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:

• 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 
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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)
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The scoring system for each pathway is based on a number of individual factors associated with riskrelated conditions at the site. These factors are grouped into three factor categories as discussed below. 

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

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

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

environments could come into contact with hazardous substances). 

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

and the quantity of the hazardous substances that have or could be released). 

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(s) 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.
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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. 
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V. Approach to HRS Addition

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

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

in developing this proposed rule. 

A. General Approach

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

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

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

the addition of subsurface intrusion to the HRS to be consistent with CERCLA section 105 because it will 

improve the agency’s ability to identify sites of the highest priority for further investigation and will 

enhance EPA’s ability, in dialogue with other federal agencies and the states and tribes, to determine the 

most appropriate state or federal authority to address sites. As is currently the case, EPA often defers to 

other state and federal cleanup authorities based on the site assessments and HRS evaluations. While 

some states/tribes have programs to address subsurface intrusion contamination, they often have limited 

authority and resources, and variable remediation criteria. The availability of the federal remedial 

authority and the more comprehensive site assessment program should complement and strengthen these 

programs. 

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

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

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

reachable by RCRA's enforcement activities. However, because RCRA is a state delegated program and 

not all states recognize subsurface intrusion as a significant issue; 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 
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Brownfields cleanup grants, site or property-specific 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 cleanups4

. 

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. 

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 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 

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

to Be Added to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-380, May 2010.
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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.

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

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

subsurface intrusion threats as discussed below.

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

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

were in place to adequately address concerns from subsurface intrusion, this could obviate the need for 

EPA action. However, no other authority consistently and comprehensively addresses subsurface 

intrusion across all potential non-federal sites, particularly when subsurface intrusion is the key exposure 

route. In particular, state programs vary significantly in addressing subsurface intrusion. In fact, not all 

states have subsurface intrusion programs, and states with programs vary in their authority, resources, and 

remediation criteria. The 2004 Interstate Technology and Regulatory Council’s (ITRC) Vapor Intrusion 

Team developed and conducted an on-line survey of state, federal, and tribal agencies regarding vapor 

intrusion regulations, policy, and guidance. Ninety-six percent (96%) of survey respondents consider 

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

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

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

addressing vapor intrusion were only informally adopted by their agencies, and most defer to EPA. The 

2009 Vapor Intrusion Pathway: A Guide for State and Territorial Federal 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 
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of developing guidance. In addition, the 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 43 

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 

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 
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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 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. 

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: 
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• 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 intrusionspecific 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-tenant 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 Proposed Addition of a Subsurface Intrusion 

Component to the HRS, which is available in the public docket for this proposed rulemaking. 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 
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with the five charge questions are discussed in Appendix E, Peer Review Responses, of the Proposal 

TSD. 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 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 
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data from actual sites. The results of these studies demonstrate that the proposed addition functioned as 

expected. See section 8.0 of the 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 Restructure and Rename 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 
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beneath the regularly occupied structures and is likely to enter into regularly occupied structures. See 

section VI.A of this preamble for further discussion.

C. Supporting Materials

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

rule, (2) this preamble, (3) the Proposal TSD (including supporting materials), (4) the RIA, and (5) a 

summary document of peer review comments on the proposed addition and agency responses. The 

proposed rule identifies the proposed changes to the NCP and focuses on the specific mechanics of 

scoring sites with the new component. This preamble provides an overview of the proposed HRS 

addition, along with an explanation of any modifications and the supporting justification. The Proposal 

TSD contains a more detailed explanation of the technical basis for the proposed additions to the HRS, 

along with descriptions of the options considered, analyses that were used to evaluate the performance of 

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

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

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

narrow technical modifications that affect multiple pathways, and the remaining sections describing the 

addition of the subsurface intrusion component to the current soil exposure pathway. The Proposal TSD 

contains a description of the current HRS, the options considered, and the technical justifications for the 

option chosen. In addition, the Proposal TSD references other supporting documents that provide an even 

greater level of detail on the proposed additions. 

These five 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 Subsurface Intrusion Addition to the HRS
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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 and Subsurface Intrusion Pathway

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

substances into regularly occupied structures by restructuring the soil exposure pathway in the current 

HRS to include subsurface intrusion. As noted previously, no changes are being proposed for the other 

three pathways in the present HRS. The restructured soil exposure pathway is proposed to be renamed the 

soil exposure and subsurface intrusion pathway to reflect both components of the restructured pathway.

See Figure 3 for a depiction of how the proposed addition fits into the HRS structure.
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Figure 3. HRS Structure with Proposed Addition

Ground Water Migration Pathway Score (SGW)

Surface Water Migration Pathway Score (SSW)

Soil Exposure and Subsurface Intrusion Pathway Score (SSESSI)

Air Migration Pathway Score (SA)

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

both consider the relative risk posed by direct contact with existing contamination areas. As identified in 

the preamble to the 1988 Federal Register 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 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, 
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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 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 scoressoil 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 
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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 pathwayspecific 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(s) 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 discussion describes 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 in subsections: 1) new definitions, 2) delineation of areas of subsurface intrusion, 3) 

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

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

selected by EPA and peer reviewed by the SAB, see section 2.0 of the 1988 Revised HRS TSD.
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1. New Definitions – See section 1.1 of the Proposed HRS Addition5

EPA is proposing that 15 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 tenant spaces with 

multiple stories (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 

 5 For references to a specific section of the Proposed HRS Addition, please refer to the regulatory text of the

proposed rulemaking.
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observed exposure (AOE). Other regularly occupied structures within this encompassed area (or areas) 

will also be considered 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 semi-enclosed or enclosed crawl space samples, in subsurface soil samples, 

or in soil gas samples in the unsaturated zone. An ASC may also include regularly occupied structures 

where indoor air sampling has not documented that an observed exposure has occurred. (See current HRS 

section 2.3 for observed release criteria.) In addition, EPA is proposing to limit the delineation of an ASC 

based on the location of subsurface volatile hazardous substances. However, non-volatile hazardous 

substances may be used to establish an ASC if they have also been documented in an observed exposure. 

Populations in regularly occupied structures within an ASC are considered potentially contaminated, 

but are weighted less in the HRS evaluation than those populations in an AOE. The populations in an 

ASC are assigned a weighting value ranging from 0.1 to 0.9 depending on such factors as the distance of 

subsurface contamination to a regularly occupied structure’s foundation and the sample media (see 

section 5.2.1.3.2.3 of the Proposed HRS Addition). The ASC is included in the HRS evaluation because 

there is currently contamination below regularly occupied structures in the ASC, and although a sampling 

event has not documented intrusion into these structures, based on previous studies, it is likely that 

intrusion has occurred or could occur when suitable climatic and lifestyle conditions were or are present. 
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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 
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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 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 indicated 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 
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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 reevaluating 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 
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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 one of the 

following methods:

i. Observed Exposure by Direct Observation – See section 5.2.1.1.1 of the Proposed HRS Addition

The identification of an observed exposure by direct observation can be based on a solid, liquid, or 

gaseous hazardous substance attributable to the site being observed or known to have entered a regularly 

occupied structure from the subsurface. This finding will in general 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(s) 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 been 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 
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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 for establishing 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 
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of one or more hazardous substance(s) in one or more indoor air sample(s) taken from a regularly 

occupied structure (termed the “release sample”) is compared to the concentration(s) 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(s) 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 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(s) 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 documenting 

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 simply 

due to variation in site conditions and in the replicability and precision of the sampling and analytical 

procedures. 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. 
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A significant increase is generally identified to have occurred if the release sample hazardous 

substance concentration is 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 detection limit, any 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 that at least 

part of the significant increase must 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 

to ensure that the increase is due to the release being evaluated and not from other contaminant sources 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 
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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 attribution conclusions. 

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 the 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(s) 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 climate-specific. Sampling may not have been performed in these 

structures for a number of reasons, or, even if performed during the limited time period of a site 

inspection, the sampling may have been conducted during conditions that the subsurface intrusion was not 

occurring or occurring at levels not detectable or differentiable from that in background sources of the 

hazardous substance. (Due to resource limitations, site inspections are conducted over a limited period of 

time, usually 1 to 2 days.) 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 area(s) of subsurface contamination based 
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on structural containment features of the regularly occupied structure and the route characteristics in the 

subsurface, including hazardous substance physical and chemical properties and physical subsurface 

properties that influence the probability that intrusion is occurring.

Figure 6. Potential for Exposure

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 the barriers that are present 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 
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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. 

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-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 
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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 had 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 and this fact 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 
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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 migration modeling. When calculating the vapor migration potential, a factor 

value is determined only for the most volatile hazardous substance based on vapor pressure and Henry’s 

constant values. Those values are used to establish the vapor migration potential factor value. See section 

4.0 of the Proposal TSD for supplemental information on this topic.

iii. Calculation of the Potential for Exposure Factor Value – See section 5.2.1.1.2.5 of the Proposed 

HRS Addition
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Consistent with potential to release determinations in the HRS, the potential for exposure for this 

component is calculated by summing all route characteristic factor values and multiplying the sum by the 

containment factor value to determine a potential for exposure factor value.

c. Calculation of the Likelihood of Exposure Factor Category Value – See section 5.2.1.1.3 of the 

Proposed HRS Addition

As in all HRS pathways and components, the likelihood of exposure factor category value is assigned 

based on the higher of the observed exposure (or release) value or the potential for exposure (or release) 

value. The maximum value assigned for the likelihood of exposure factor category is 550 and is assigned 

if observed exposure is documented. If observed exposure is not documented, the value assigned when 

evaluating potential for exposure ranges between 0 and 500. This approach is consistent with the current 

HRS structure. See sections 2.2 of the 1988 Revised HRS TSD for supplemental information regarding 

this approach.

4. Waste Characteristics – See section 5.2.1.2 of the Proposed HRS Addition

The waste characteristics factor category is based on factors that are related to the relative risk 

considerations included in the basic HRS structure: 1) the toxicity or other potential adverse effects to a 

receptor from the releases, 2) the potential to biodegrade 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 biodegradability of the hazardous substance, 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.
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a. Toxicity/Biodegradation – See section 5.2.1.2.1 of the Proposed HRS Addition

The combined toxicity/biodegradation factor includes consideration of both the toxicity and the 

possibility for biodegradation 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. 

The biodegradation factor represents the possibility for a substance to biodegrade in the subsurface 

prior to intruding into a regularly occupied structure. The potential of a substance to biodegrade has been 

identified as a significant factor in numerous studies evaluating the potential for intrusion by a vapor. The 

possibility that a substance may biodegrade 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 biodegradation are considered 

beyond the scope of a typical site investigation, EPA is proposing to evaluate biodegradation based on the 
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substance being evaluated and if appropriate environmental conditions are present to ensure that sufficient 

biodegradation will occur to diminish the threat. Based on EPA’s review of the current literature and 

research on this topic, the assigned biodegradation factor is limited to only two possible factor values, one 

for substances that are readily biodegradable and the appropriate environmental factors are present, and 

one for when either of these parameters are not present. 

It is EPA’s determination that 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 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.

b. Hazardous Waste Quantity – See section 5.2.1.2.2 of the Proposed HRS Addition

In the basic HRS structure used in all pathways and components, the hazardous waste quantity factor 

reflects the risk consideration related to the magnitude and duration of either the release for a migration 

pathway or the exposure for an exposure pathway. In other words, for an exposure pathway, the risk 

posed by a release of hazardous substances is directly related to the amount of hazardous substances to 

which receptors (targets) are exposed and the length of the exposure. 
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As explained in the preamble to the 1990 HRS and in the 1988 Revised HRS TSD, an estimate of the 

waste quantity associated with a site was the best surrogate for the amount of hazardous substances that 

receptors were exposed to and that the duration of the exposure was probably correlated to the magnitude 

of the exposure. In the current three migration pathways (ground water, surface water, air), the hazardous 

waste quantity factor reflects the total amount of hazardous substances in sources at the site to take into 

account not only where contamination has already migrated to, but also future migration of contamination 

to other locations. For the soil exposure pathway, however, the estimate does not include the total amount 

in or released from the site sources, but only the amount of hazardous substance in the top 2 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 fourtiered 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 
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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 Level I or II contaminant concentrations a minimum 

hazardous waste quantity factor value of 100 could be assigned.

The component waste quantity is the sum of all the waste quantities for all the regularly occupied 

structures found in both the AOE(s) and ASC(s). 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/biodegradation 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 to individuals, populations, and resources. The relative risk assessment embodied 

within the HRS uses the targets factor as an index of the nature and extent of exposure to individuals, 

populations, resources, and sensitive environments. 
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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, 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.

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 
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individual factor is discussed, as well as how to apply a factor value based on the benchmarks and the 

resulting levels of exposure.

i. Identifying Levels of Exposure and Benchmarks for Subsurface Intrusion – See section 5.2.1.3.1 

of the Proposed HRS Addition

For all current HRS pathways, the magnitude of the values assigned to the individual and population 

factors depend on the concentration of the contamination to which the receptors (targets) are exposed. If 

receptors are exposed to hazardous substance levels that meet observed release criteria, they are identified 

as actually contaminated; however, if the receptors are not exposed to hazardous substances that meet the 

observed release criteria but are within the target area being evaluated, they may be considered potentially 

contaminated. Potential targets are evaluated because a typical site inspection may not identify the extent 

of contamination. A site inspection typically includes 1 to 3 days of sampling and investigation activities. 

These limited investigations may not adequately characterize the annual or longer term indoor exposure 

levels (see page 4 of the 1988 SAB report and section 6.0 of the Proposal TSD), especially in the case of 

subsurface intrusion where seasonal and temporal fluctuations can significantly impact the rate of 

subsurface intrusion. 

Actually contaminated targets are further divided into two categories based on whether the hazardous 

substance concentrations are above standard health-based benchmarks (or for 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:
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• Screening concentrations for cancer 

• Screening concentrations for noncancer toxicological responses

Because targets associated with an observed exposure by direct observation are only considered 

subject to Level II contamination in all parts of the HRS and intrusion by contaminated ground water is 

documented by direct observation only, targets residing within a structure subject to intrusion by 

contaminated ground water are 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 is to include a measure reflecting the maximum level 

of exposure to individuals. 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 or attending school or day care 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.
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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, 

and does not include workers. EPA is proposing to retain the current scoring methodology for weighting 

populations used throughout the HRS, with actual exposure more heavily weighted than those potentially 

exposed. The proposed subsurface intrusion component will evaluate populations based on the number of 

individuals located within an identified AOE (i.e., those populations exposed to Level I and Level II 

concentrations) and the number of individuals located within an ASC (i.e., potential contamination as 

determined based on subsurface sampling), which is further subdivided as described in subsection ii 

below.

i. Weighting of Targets in the Area of Observed Exposure (AOE) – See sections 5.2.1.3.2.1 and 

5.2.1.3.2.2 of the Proposed HRS Addition

EPA is proposing to establish an AOE based on documented contamination meeting observed 

exposure criteria (either by direct observation or chemical analysis). Consistent with the weighting of 

populations throughout the HRS (see section 2.5 of the current HRS), the proposed subsurface intrusion 

component will weight targets subject to Level I contaminant concentrations by a factor of 10 and weight 

targets subject to Level II contaminant concentrations by a factor of 1. 

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 an observed exposures has not been established but the structures 
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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 sampled 

during the correct conditions, observed exposure would be identified in these structures 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 the requirement of “unless available information indicates otherwise” to allow for situations 

where site-specific conditions clearly document that there could be no observed exposures in these 

structures. 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-tenant structures, all regularly occupied tenant spaces 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-tenant structures located within an AOE, but where 

an observed exposure has not been documented, only those regularly occupied tenant 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, semienclosed or enclosed crawl space, or soil gas contamination meeting observed release criteria. These areas 

are included in the subsurface intrusion component due to the potential that limited sampling conducted 

during a site inspection may not identify that subsurface intrusion is occurring because of the high 

temporal and spatial variability associated with detecting subsurface intrusion. Temporal and spatial 

differences can significantly impact the rate at which volatile hazardous substances enter a structure. 

However, when an ASC has been defined, that area represents a location where subsurface hazardous 

substances have the potential to intrude into a structure. EPA is limited in its extent of preliminary 
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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 
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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-tenant structures, all regularly occupied tenant spaces 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-tenant structures located only within an ASC, only 

those regularly occupied tenant spaces within the lowest level are considered in an HRS evaluation. 

The proposed weighting factors for targets within an ASC include:

• 0.9 - Population residing in a structure with levels of contamination in a semi-enclosed or 

enclosed crawl space sample meeting observed release criteria.

• 0.9 - Population residing in a tenant space of a multi-story structure within an area of subsurface 

contamination located directly above a story with an observed exposure or a gaseous indoor air 

sample meeting observed release criteria.

• 0.4 - Population residing in a structure where levels of contaminants meeting observed release 

criteria are found in any sampling media at or within 5 feet horizontally or vertically of the 

structure foundation.

• 0.2 - Population residing 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

• 0.2 - Population residing 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
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• 0.1 - Population residing in a structure where levels of contaminants meeting observed release 

criteria are found or inferred based on underlying ground water samples greater than 5 feet from 

the structure foundation.

• 0.1 - Population residing 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.

Additional information regarding this analysis is presented in section 6.0 of the Proposal TSD.

d. 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 
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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 OSHAregulated 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. 

e. Resources – See section 5.2.1.3.4 of the Proposed HRS Addition

The resources target factor is evaluated in all pathways under the current HRS. A factor value of five 

is assigned if at least one resource is present and a factor value of zero if no resource is present. Eligible 

resources are pathway-, component-, or threat-specific. These resources represent uses of a contaminated 

medium or area where exposures occur and are not covered by the other identified targets. For example, 

resources within the air migration pathway include commercial agriculture or silviculture and 

major/designated recreation areas. The resident 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 
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VI.B.2 of this preamble) and in which populations may be exposed to contamination due to subsurface 

intrusion. For example, libraries, churches, and tribal facilities may qualify as eligible resources.

f. Calculation of the Targets Factor Category Value – See section 5.2.1.3.5 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 Subsurface Intrusion 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 Subsurface Intrusion 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 
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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 Subsurface Intrusion Component Score into the Soil Exposure and Subsurface 

Intrusion Pathway Score – See section 5.3 of the Proposed HRS Addition

The score for this restructured pathway is proposed to be a combination of two component scores. 

The subsurface intrusion component score is added to the soil exposure component score (subject to a 

maximum value) to determine the pathway score. The two component scores are proposed to be additive 

because the populations may be subjected to exposures separately via both routes: the soil exposure 

component reflects exposures to people when outside a structure and focuses on ingestion, while the 

subsurface intrusion component reflects exposures to people when inside a structure and focuses on 

inhalation. Hence, the addition of the two component scores reflects the cumulative potential risk and is 

not double counting the relative risk. 

In addition, a pathway score can be assigned without scoring both the soil exposure and subsurface 

intrusion components using this approach. It is possible for a site to have only one component evaluated 

and still reach the same pathway score as under the current HRS. It should be observed that because the 

scoring of the soil exposure component is not being altered, the soil exposure component would 
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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 singlefamily 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. 
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The ground water plume underlies approximately 20 acres of a residential neighborhood comprised of 

single-family detached homes. Indoor air samples collected inside 14 homes have hazardous substance 

concentrations above the applicable benchmark. Indoor air samples in 5 homes and one daycare facility 

with approximately 25 children enrolled have hazardous substance concentrations above background, but 

below the applicable benchmark. The homes and daycare facility were checked for indoor sources of 

hazardous substances prior to sampling and such sources were removed if found. This site would likely 

qualify for the NPL based on available information (i.e., score of about 28.50).

The third scenario consists of a ground water plume contaminated with a highly toxic hazardous 

substance and a larger number of targets than the second scenario. The plume underlies approximately 25 

acres of a residential neighborhood and hazardous substance concentrations above a benchmark were 

detected in indoor air samples from 25 homes and one daycare with approximately 25 children enrolled. 

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. Terminology Updates Affecting Specific Sections of the HRS (Sections 2 & 5)

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. 

A. 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 
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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.

B. 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.

C. 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.

D. 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.)

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/laws-regulations/laws-and-executive-orders.

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

Regulation and Regulatory Review

This action is a significant regulatory action that was submitted to the Office of Management and 

Budget (OMB) for review. This action may raise novel legal or policy issues arising out of legal 

mandates, the President’s priorities, or the principles set forth in the EO. Any changes made in response 
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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 2050-0179 (formerly 2050-0095 prior to consolidation). 

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 change, 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)
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This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, 

and does not significantly or uniquely affect small governments. The action imposes no enforceable duty 

on any state, local, or tribal governments or the private sector. 

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct effects on the 

states, on the relationship between the national government and the states, or on the distribution of power 

and responsibilities among the various levels of government. 

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

This action does not have tribal implications, as specified in Executive Order 13175. EPA’s 

evaluation of a site using the HRS does not impose any costs on a tribe (except those already in a 

cooperative agreement relationship with EPA). Thus, Executive Order 13175 does not apply to this 

action. 

Although Executive Order 13175 does not apply to this action, EPA consulted with tribal officials 

through meetings and correspondence, including a letter sent to all federally recognized tribes asking for 

comment on the “Notice of Opportunity for Public Input” that was published in the 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 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 
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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.

For the reasons set out in the preamble, Title 40, Chapter 1 of the Code of Federal Regulations is 

proposed to be amended as follows:

Appendix A of Part 300 – Hazard Ranking System [AMENDED]
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1. The authority citation for Appendix A of Part 300 continues to read as follows:

Authority: 42 U.S.C. 9601-9657; 33 U.S.C. 1321(d); E.O. 11735, 38 FR 21243; E.O. 12580, 52 FR 

2923; E.O. 12777, 56 FR 54757

2. Section 1.0 of Appendix A is amended by revising the text of subsection 1.1 as follows: 

a. In the definition of “Ambient Water Quality Criteria (AWQC),” replace “Ambient Water Quality 

Criteria (AWQC)” with “Ambient Water Quality Criteria (AWQC)/National Recommended Water 

Quality Criteria”; in the text of this definition, replace “maximum acute or chronic toxicity” with 

“maximum acute (Criteria Maximum Concentration or CMC) or chronic (Criterion Continuous 

Concentration or CCC) toxicity.”

b. Following the definition of “CERCLA,” add a definition for “Channelized flow.”

c. In the text of the definition for Chronic toxicity, replace the words “Reference Dose (RfD) 

values” with the words “Reference Dose (RfD) and Reference Concentration (RfC) values.”

d. Following the definition of “Contract-Required Detection Limit (CRDL),” add a definition for 

“Crawl space.”

e. Revise the text of the definition of “Distance weight.” f. In the text of the definition for “HRS 

‘pathway’,” replace the words “soil exposure” with the words “soil exposure and subsurface intrusion.”

g. Following the definition of “Hydrolysis,” add definitions for “Indoor air” and “Inhalation Unit 

Risk (IUR).”
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h. Following the definition of “Mixed radioactive and other hazardous substances,” add a definition 

for “Multi-tenant structures.”

i. Following the definition of “National Emission Standards for Hazardous Air Pollutants 

(NESHAPs),” add a definition for “Occupied structures.”

j. Following the definition of “Photolysis,” add a definition for “Preferential subsurface intrusion 

pathways.”

k. Following the definition of “Radionuclide/radioisotope,” add a definition for “Reference 

concentration (RfC).”

l. Revise the text of the definition of “Reference dose (RfD).”

m. Revise the definition of “Screening concentration.” 

n. Revise the definition of “Slope factor (also referred to as cancer potency factor).” 

o. Following the definition of “Slope factor (also referred to as cancer potency factor),” add 

definitions for “Soil gas” and “Soil porosity.”

p. Following the definition of “Source,” add definitions for “Subslab,” “Subsurface Intrusion” and 

“Surficial ground water.”

q. Following the definition of “Target Distance Limit,” add definitions for “Unit Risk” and 

“Unsaturated Zone.”

r. Revise the definition of “Weight-of-evidence.”

1.1 Definitions

* * * * *
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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. 

* * * * *

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: * * * HRS measures of chronic toxicity include Reference Dose (RfD) and 

Reference Concentration (RfC) values.

* * * * *

Crawl space: The enclosed or semi-enclosed area between a regularly occupied structure’s 

foundation (e.g., pier and beam construction) and the ground surface. Crawl space samples are collected 

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

* * * * *

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

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

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

* * * * *
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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). 

* * * * *

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. 

* * * * *

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.

* * * * *

Occupied structures: Structures with enclosed air space, either where people are present on a regular 

basis or that were previously occupied but vacated due to a site-related hazardous substance(s).

* * * * *

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

walls, spaces around utility lines or drains through which a hazardous substance moves more easily into a 

regularly occupied structure. 

* * * * *
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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.

* * * * *

Screening concentration: * * * The screening concentration for a specific hazardous substance 

corresponds to its reference concentration for inhalation exposures or reference dose for oral exposures, 

as appropriate, and, if the substance is a human carcinogen with either a weight-of-evidence classification 

of A, B, or C, or a weight-of-evidence classification of carcinogenic to humans, likely to be carcinogenic 

to humans or suggestive evidence of carcinogenic potential, to that concentration that corresponds to its 

10−6 individual lifetime excess cancer risk for inhalation exposures or for oral exposures, as appropriate. 

* * * * *

Slope factor (also referred to as cancer potency factor): * * * 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-ofevidence classification of carcinogenic to humans, likely to be carcinogenic to humans or having 

suggestive evidence of carcinogenic potential. [(mg/kg-day)−1 for non-radioactive substances and (pCi)−1

for radioactive substances].

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

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

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

within the soil to the overall volume of the soil.
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* * * * *

Subslab: The area immediately beneath a regularly occupied structure with a basement foundation or 

a slab-on-grade foundation. Subslab samples are collected to determine the concentration of hazardous 

substances in the soil gas beneath a home or building. 

Subsurface Intrusion: The migration of hazardous substances from the unsaturated zone and/or the 

surficial ground water into overlying structures.

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

* * * * *

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

to an agent (i.e., hazardous substance) at a concentration of 1 μg/L in water, or 1 μg/m3 in air.

Unsaturated Zone: The portion of subsurface between the land surface and the zone of saturation. It 

extends from the ground surface to the surficial water table (excluding localized or perched water).

* * * * *

Weight-of-evidence: * * * The EPA weight-of-evidence groupings, depending on the date EPA

updated the profile, include either:

* * * * * 

or:

• Carcinogenic to humans.

• Likely to be carcinogenic to humans.

• Suggestive evidence of carcinogenic potential.
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• Inadequate information to assess carcinogenic potential.

• Not likely to be carcinogenic to humans.

3. Section 2.0 of Appendix A is amended as follows:

a. By revising subsection 2.1 and subordinate subsections 2.1.1, 2.1.2 and 2.1.3.

b. By revising subsection 2.2 and subordinate subsections 2.2.1, 2.2.2 and 2.2.3. 

c. By revising subsection 2.3.

d. By revising subsection 2.4 and subordinate subsections 2.4.1, 2.4.1.1, 2.4.1.2 , 2.4.2, 2.4.2.1, 

2.4.2.1.1, 2.4.2.1.2, 2.4.2.1.3, 2.4.2.1.4, 2.4.2.1.5, 2.4.2.2, and 2.4.3.1.

e. By revising Table 2-4 to add RfC and IUR assigned values.

f. By revising subsection 2.5 and subordinate subsections 2.5.1 and 2.5.2.

2.0 Evaluations Common to Multiple Pathways

2.1 Overview

* * * * *

• Soil Exposure and Subsurface Intrusion (Ssessi). 

* * * * *

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. * * * 

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

SS = �SSgggg

2 + SSss2 + SSssssssssss

2 + SSaa

2

4

2.1.2 Calculation of 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.) * * * 

2.1.3 Common evaluations. * * *

* * * * * 

– 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). 

* * * * *

– Scoring observed release (or observed exposure or observed contamination). * * *
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 Combining toxicity with mobility, persistence, biodegradation, and/or bioaccumulation (or 

ecosystem bioaccumulation) potential, as appropriate to the pathway (component or threat). 

* * * * * 

* * * However, the evaluations differ in certain respects for the soil exposure and subsurface 

intrusion pathway. * * * 

Section 2 focuses on evaluations common at the pathway, component and threat levels. * * * 

2.2 Characterize sources. * * *

• 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). 

* * * * *

2.2.1 Identify sources. * * * 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. * * *
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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 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 observed release associated with that area when 

evaluating the pathway (see section 5.2.0). Furthermore, evaluate an area of subsurface contamination 

based only on the presence of hazardous substances with a vapor pressure greater than or equal to 10-9

torr. Consider those hazardous substances within an area of subsurface contamination with a vapor 

pressure less than 10-5

, only if they have also been documented to be in an observed exposure (see section 

5.2.0).

2.2.3 Identify hazardous substances available to a 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).
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• 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 less than 10-5 torr that meet the criteria for an 

observed release in the area of subsurface contamination at the site (see section 5.2.0).

− All hazardous substances with a vapor pressure less than 10-5 torr but greater than or equal to 

10-9 torr that meet the criteria for observed release in an area of subsurface contamination, 

and that have also been documented to be in an observed exposure at the site (see section 

5.2.0).

2.3 Likelihood of release. * * *

* * * (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.) * * *

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, biodegradation, 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 
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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 biodegradation factor value. Evaluation of the toxicity factor is specified 

in section 2.4.1.1. Use and evaluation of the mobility, persistence, biodegradation, 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. * * *

* * * * *

• Cancer—Use slope factors (also referred to as cancer potency factors) combined with weight-ofevidence ratings for carcinogenicity for all exposure routes except inhalation. * * * 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: 

* * * * *

• Noncancer toxicological responses of chronic exposure—use reference dose (RfD) or reference 

concentration (RfC) values as applicable. 

* * * * *

• 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. 
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• 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. 
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TABLE 2-4. TOXICITY FACTOR EVALUATION
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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)

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
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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.

* * * * *

c IUR = Inhalation Unit Risk.

* * * * * 

2.4.1.2 Hazardous substance selection. * * *

* * * * *

For the soil exposure and subsurface intrusion pathway, determine toxicity and 

toxicity/biodegradation factor values as follows:

• Soil exposure and subsurface intrusion—soil exposure component.
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– 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/biodegradation factor value for each hazardous substance being evaluated that 

meets the criteria for observed exposure or meets the criteria for observed release and has a vapor 

pressure greater than or equal to 10-9 torr within an area of subsurface contamination. Those hazardous 

substances with a vapor pressure less than 10-5 torr but greater than or equal to 10-9 torr that meet the 

criteria for observed release in an area of subsurface contamination must also have been documented to be 

in an observed exposure at the site. Select the hazardous substance with the highest combined factor value 

and use that substance in evaluating the waste characteristics factor category (see section 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 soil exposure component of the soil exposure 

and subsurface intrusion pathway, allocate to each area of observed contamination only those hazardous 

substances that meet the criteria for observed contamination for that area of observed contamination and 

only those hazardous wastestreams that contain hazardous substances that meet the criteria for observed 

contamination for that area of observed contamination. Do not consider other hazardous substances or 

hazardous wastestreams at the site in evaluating this factor for the soil exposure component of the soil 

exposure and subsurface intrusion pathway. 
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In evaluating the hazardous waste quantity factor for the subsurface intrusion component of the soil 

exposure and subsurface intrusion pathway, allocate to each area of observed exposure or area of 

subsurface contamination only those hazardous substances that meet the criteria for observed exposure or 

observed release for that area of observed exposure or area of subsurface contamination and only those 

hazardous wastestreams that contain hazardous substances that meet the criteria for observed exposure or 

observed release for that area of observed exposure or area of subsurface contamination. Furthermore, 

include only those hazardous substances in an area of subsurface contamination with a vapor pressure 

greater than or equal to 10-9 torr. 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-tenant structures, 

use the procedures identified in section 5.2.1.2.2.

2.4.2.1 Source hazardous waste quantity. * * *

For the soil exposure component of the soil exposure and subsurface intrusion pathway, assign a 

source hazardous waste quantity value to each area of observed contamination, as applicable to the threat 

being evaluated.

For the subsurface intrusion component of the soil exposure and subsurface intrusion pathway, assign 

a source hazardous waste quantity value to each regularly occupied structure within an area of observed 

exposure or an area of subsurface contamination that has a structure containment factor value greater than 

0.

For determining all hazardous waste quantity calculations except for an unallocated source or an area 

of subsurface contamination, evaluate using the following four measures in the following hierarchy:

* * * * *
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* * * 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.

* * * * *

2.4.2.1.1 Hazardous constituent quantity. * * *

* * * * *

• For the soil exposure and subsurface intrusion pathway—soil exposure component, assign the 

area of observed contamination a value using the Tier A equation of Table 5-2 (section 5.1.1.2.2).

• For the soil exposure and subsurface intrusion pathway—subsurface intrusion component, assign 

the area of observed exposure a value using the Tier A equation of Table 5-18 (section 5.2.1.2.2).

If the hazardous constituent quantity for the source (or area of observed contamination or area of 

observed exposure) is adequately determined (that is, the total mass of all CERCLA hazardous substances 

in the source and releases from the source [or in the area of observed contamination or area of observed 

exposure] is known or is estimated with reasonable confidence), do not evaluate the other three measures 

discussed below. Instead assign these other three measures a value of 0 for the source (or area of observed 

contamination or area of observed exposure) and proceed to section 2.4.2.1.5. 

If the hazardous constituent quantity is not adequately determined, assign the source (or area of 

observed contamination or area of observed exposure) a value for hazardous constituent quantity based on 

the available data and proceed to section 2.4.2.1.2.

* * * * *
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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.

* * * * * 

• 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).

* * * * *

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.

* * * * * 
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• 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). * * *

* * * * *

• 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 5-19 (section 5.2.1.2.2).
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2.4.2.1.5 Calculation of source hazardous waste quantity value. Select the highest of the values 

assigned to the source (or area of observed contamination, area of observed exposure, or area of 

subsurface contamination) for the hazardous constituent quantity, hazardous wastestream quantity, 

volume, and area measures. * * *

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, area of observed exposure, or area of subsurface contamination) for the pathway being 

evaluated and round this sum to the nearest integer, except: if the sum is greater than 0, but less than 1, 

round it to 1. * * *

* * * * *

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if the 

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. * * *

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:
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− 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

* * * * *

2.4.3.1 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.

* * * * *

2.5 Targets. * * *

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

* * * * *
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• 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).

* * * * *

• 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). * * * 

– 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. * * *

* * * * *

− 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. * * * 

 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 

* * * * *
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• * * * The value assigned is 20 unless reduced by a distance or dilution weight appropriate to the 

pathway.

* * * * *

• * * * (Level I is not used for sensitive environments in the soil exposure component of the soil 

exposure and subsurface intrusion and air migration pathways.) 

• * * * 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. * * * 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.

* * * * *

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

• 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. 

* * * * *
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• If none of the hazardous substances eligible to be evaluated for the sampling location has an 

applicable benchmark, assign Level II to the actual contamination at that sampling location for the 

pathway (component or threat).

In making the comparison, consider only those samples, and only those hazardous substances in the 

sample, that meet the criteria for an observed release (or observed contamination or observed exposure) 

for the pathway, except: tissue samples from aquatic human food chain organisms may also be used as 

specified in sections 4.1.3.3 and 4.2.3.3 of the surface water-human food chain threat. * * *

 

* * * * *

2.5.2 Comparison to benchmarks. * * * 

* * * * *

• EPA Ambient Water Quality Criteria (AWQC/National Recommended Water Quality Criteria) 

for protection of aquatic life—environmental threat in surface water migration pathway. 

* * * * *

• 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). 
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• 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 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.

For those hazardous substances that are carcinogens (that is, those having either a carcinogen weightof-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:

* * * * *
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For those hazardous substances for which an RfD or RfC is available, calculate an index J for the 

sample location as follows:

* * * * *

CRj=Screening concentration for noncancer toxicological responses corresponding to RfD or RfC for 

applicable exposure (inhalation or oral) for hazardous substance j.

* * * * *

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). * * *

* * * * * 
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4. Section 5.0 of Appendix A is amended as follows: 

a. By revising the designation of subsections as follows:

Section or 

Subsection

Remove Add

5.0 n/a 5.0.1 and 

5.1

5.0.1 5.0.1 5.1.0

5.1 5.1 5.1.1

5.1.1 5.1.1 5.1.1.1

5.1.2 5.1.2 5.1.1.2

5.1.2.1 5.1.2.1 5.1.1.2.1

5.1.2.2 5.1.2.2 5.1.1.2.2

5.1.2.3 5.1.2.3 5.1.1.2.3

5.1.3 5.1.3 5.1.1.3

5.1.3.1 5.1.3.1 5.1.1.3.1

5.1.3.2 5.1.3.2 5.1.1.3.2

5.1.3.2.1 5.1.3.2.1 5.1.1.3.2.1

5.1.3.2.2 5.1.3.2.2 5.1.1.3.2.2

5.1.3.2.3 5.1.3.2.3 5.1.1.3.2.3

5.1.3.3 5.1.3.3 5.1.1.3.3

5.1.3.4 5.1.3.4 5.1.1.3.4

5.1.3.5 5.1.3.5 5.1.1.3.5

5.1.3.6 5.1.3.6 5.1.1.3.6
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Section or 

Subsection

Remove Add

5.1.4 5.1.4 5.1.1.4

5.2 5.2 5.1.2

5.2.1 5.2.1 5.1.2.1

5.2.1.1 5.2.1.1 5.1.2.1.1

5.2.1.2 5.2.1.2 5.1.2.1.2

5.2.1.3 5.2.1.3 5.1.2.1.3

5.2.2 5.2.2 5.1.2.2

5.2.2.1 5.2.2.1 5.1.2.2.1

5.2.2.2 5.2.2.2 5.1.2.2.2

5.2.2.3 5.2.2.3 5.1.2.2.3

5.2.3 5.2.3 5.1.2.3

5.2.3.1 5.2.3.1 5.1.2.3.1

5.2.3.2 5.2.3.2 5.1.2.3.2

5.2.3.3 5.2.3.3 5.1.2.3.3

5.2.4 5.2.4 5.1.2.4

5.3 5.3 5.1.3

b. By revising Section 5.0 to divide the introductory text into two numbered subsections (5.0.1 and 

5.1) as indicated above, and revising text.

c. By redesignating subsection 5.0.1 as subsection 5.1.0 as indicated above, and revising text.
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d. By revising the title of Figure 5-1 from “Overview of Soil Exposure Pathway” to “Overview of the 

Soil Exposure and Subsurface Intrusion Pathway” and adding a subsurface intrusion component to the 

Figure.

e. By revising the title of Table 5-1 from “Soil Exposure Pathway Scoresheet” to “Soil Exposure 

Component Scoresheet,” reformatting the table, and revising the last two rows of the table to change “Soil 

Exposure Pathway” to “Soil Exposure Component” and “Ss” to “Sse.”

f. By redesignating subsection 5.1 as subsection 5.1.1, redesignating subsection 5.1.1 as subsection 

5.1.1.1, redesignating subsection 5.1.2 as subsection 5.1.1.2, as indicated above, and revising references 

contained in these subsections to reflect changed subsection designations. 

g. By redesignating subsection 5.1.2.1 as subsection 5.1.1.2.1, redesignating subsection 5.1.2.2 as 

subsection 5.1.1.2.2, and redesignating subsection 5.1.2.3 as subsection 5.1.1.2.3, as indicated above.

h. By revising the title of Table 5-2 to remove “Pathway” and add in its place “Component.”

i. By redesignating subsection 5.1.3 as 5.1.1.3, redesignating subsection 5.1.3.1 as subsection 

5.1.1.3.1, redesignating subsection 5.1.3.2 as subsection 5.1.1.3.2, redesignating subsection 5.1.3.2.1 as 

subsection 5.1.1.3.2.1, redesignating subsection 5.1.3.2.2 as 5.1.1.3.2.2, redesignating subsection 5.1.3.2.3 

as subsection 5.1.1.3.2.3, redesignating subsection 5.1.3.3 as subsection 5.1.1.3.3, redesignating 

subsection 5.1.3.4 as subsection 5.1.1.3.4, redesignating subsection 5.1.3.5 as subsection 5.1.1.3.5, 

redesignating subsection 5.1.3.6 as subsection 5.1.1.3.6, and redesignating subsection 5.1.4 as subsection 

5.1.1.4, as indicated above, and revising references contained in these subsections to reflect changed 

subsection designations. 

j. By redesignating subsection 5.2 as subsection 5.1.2, redesignating subsection 5.2.1 as subsection 

5.1.2.1, redesignating subsection 5.2.1.1 as subsection 5.1.2.1.1, redesignating subsection 5.2.1.2 as 

subsection 5.1.2.1.2, redesignating subsection 5.2.1.3 as subsection 5.1.2.1.3, redesignating subsection 
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5.2.2 as subsection 5.1.2.2, redesignating subsection 5.2.2.1 as subsection 5.1.2.2.1, redesignating 

subsection 5.2.2.2 as subsection 5.1.2.2.2, redesignating subsection 5.2.2.3 as subsection 5.1.2.2.3, 

redesignating subsection 5.2.3 as subsection 5.1.2.3, redesignating subsection 5.2.3.1 as subsection 

5.1.2.3.1, redesignating subsection 5.2.3.2 as subsection 5.1.2.3.2, redesignating subsection 5.2.3.3 as 

subsection 5.1.2.3.3, and redesignating subsection 5.2.4 as subsection 5.1.2.4, as indicated above, and 

revising references contained in these subsections to reflect changed subsection designations. 

k. By redesignating subsection 5.3 as subsection 5.1.3 as indicated above, and revising text.

l. By adding subsection 5.2 and subordinate subsections 5.2.0, 5.2.1, 5.2.1.1, 5.2.1.1.1, 5.2.1.1.2, 

5.2.1.1.2.1, 5.2.1.1.2.2, 5.2.1.1.2.3, 5.2.1.1.2.4, 5.2.1.1.2.5, 5.2.1.1.3, 5.2.1.2, 5.2.1.2.1, 5.2.1.2.1.1, 

5.2.1.2.1.2, 5.2.1.2.1.3, 5.2.1.2.2, 5.2.1.2.3, 5.2.1.3, 5.2.1.3.1, 5.2.1.3.2, 5.2.1.3.2.1, 5.2.1.3.2.2, 

5.2.1.3.2.3, 5.2.1.3.2.4, 5.2.1.3.3, 5.2.1.3.4, 5.2.1.3.5 and 5.2.2.

m. By adding Tables 5-11, 5-12, 5-13, 5-14, 5-15, 5-16, 5-17, 5-18, 5-19, 5-20 and 5-21. 

n. By adding subsection 5.3.

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 
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scored, sum the two scores and assign it as the soil exposure and subsurface intrusion pathway score, 

subject to a maximum of 100.

5.1 Soil exposure component. Evaluate the soil exposure component based on two threats: 

Resident population threat and nearby population 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

* * * * *
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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

Biodegradation

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

Workers

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

Figure 5-1 Overview of the Soil Exposure and Subsurface Intrusion Pathway

Soil Exposure 

Component
Page 109 of 162

TABLE 5-1. SOIL EXPOSURE COMPONENT SCORESHEET

Factor categories and factors

Maximum 

value

Value 

assigned

Resident Population Threat

Likelihood of Exposure

1. Likelihood of Exposure 550

Waste Characteristics

2. Toxicity (a)

3. Hazardous Waste Quantity (a)

4. Waste Characteristics 100

Targets

5. Resident Individual 50

6. Resident Population:

6a. Level I Concentrations (b)

6b. Level II Concentrations (b)

6c. Resident Population (lines 6a + 6b) (b)

7. Workers 15

8. Resources 5

9. Terrestrial Sensitive Environments (c)

10. Targets (lines 5 + 6c + 7 + 8 + 9) (b)

Resident Population Threat Score

11. Resident Population Threat (lines 1x4x10) (b)
Page 110 of 162

Factor categories and factors

Maximum 

value

Value 

assigned

Nearby Population Threat

Likelihood of Exposure

12. Attractiveness/Accessibility 100

13. Area of Contamination 100

14. Likelihood of Exposure 500

Waste Characteristics

15. Toxicity (a)

16. Hazardous Waste Quantity (a)

17. Waste Characteristics 100

Targets

18. Nearby Individual 1

19. Population Within 1 Mile (b)

20. Targets (lines 18 + 19) (b)

Nearby Population Threat Score

21. Nearby Population Threat (lines 14x17x20) (b)

Soil Exposure Component Score

22. Soil Exposure Component Score d (Sse), (lines [11+21] /82,500, subject 

to a maximum of 100)

100

a Maximum value applies to waste characteristics category.

b Maximum value not applicable.

c No specific maximum value applies to factor. However, pathway score based solely on terrestrial sensitive 
Page 111 of 162

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: 

* * * * *

• 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. * * * 

* * * * *

• Within the boundaries of a resource specified in section 5.1.1.3.4, or 

• Within the boundaries of a terrestrial sensitive environment specified in section 5.1.1.3.5. 
Page 112 of 162

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. * * * 

5.1.1.2 Waste characteristics. * * * 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. * * * 

5.1.1.2.2 Hazardous waste quantity. * * * 

* * * * *

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

* * * * *

5.1.1.2.3 Calculation of waste characteristics factor category value. * * * 

* * * * *

5.1.1.3 Targets * * * 

* * * * *

• 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. 
Page 113 of 162

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. 

* * * * *

5.1.1.3.2 Resident population.* * * 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. 

* * * * *

Count only those persons meeting the criteria for resident individual as specified in section 

5.1.1.3. * * *

5.1.1.3.2.1 Level I concentrations. * * *

5.1.1.3.2.2 Level II concentrations. * * *

5.1.1.3.2.3 Calculation of resident population factor value. * * *

5.1.1.3.3 Workers. Evaluate this factor based on the number of workers that meet the 

section 5.1.1.3 criteria. * * * 

* * * * *

5.1.1.3.4 Resources. * * * 

* * * * *

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. 
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* * * * *

n=Number of terrestrial sensitive environments meeting section 5.1.1.3 criteria. 

* * * * *

5.1.1.3.6 Calculation of resident population targets factor category value. * * *

5.1.1.4 Calculation of resident population threat score. * * *

5.1.2 Nearby population threat. 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. * * *

5.1.2.1.1 Attractiveness/accessibility. * * *

5.1.2.1.2 Area of contamination. * * * Count only the area(s) that meet the criteria in 

section 5.1.0 and that receive an attractiveness/accessibility value greater than 0. * * *

5.1.2.1.3 Likelihood of exposure factor category value. * * * 

* * * * *

5.1.2.2 Waste characteristics. * * * 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.
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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. * * *

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. * * *

5.1.2.2.3 Calculation of waste characteristics factor category value. * * *

5.1.2.3 Targets. * * *

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. * * *

* * * * *

TABLE 5-9. NEARBY INDIVIDUAL FACTOR VALUES

* * * * * 

 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 travel distances as specified in 

section 5.1.2.3.1. 

* * * * *

5.1.2.3.3 Calculation of nearby population targets factor category value. * * *

5.1.2.4 Calculation of nearby population threat score. * * *
Page 116 of 162

5.1.3 Calculation of soil exposure component score. * * * Assign the resulting value, 

subject to a maximum of 100, as the soil exposure component score (Sse). * * *

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 117 of 162

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/Biodegradation (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 an Area of Subsurface Contamination (b)
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 9d. Total Population (lines 9a + 9b + 9c) (b) 

10. Workers 45 

11. Resources 5 

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

Subsurface Intrusion Component Score

13. Subsurface Intrusion Component (lines 4 x 7 x 12)/82,500c (subject to a 

maximum of 100)

100

 

Soil Exposure and Subsurface Intrusion Pathway Score

14. Soil Exposure Component + Subsurface Intrusion Component (subject to 

a maximum of 100)

100

 

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

c

Do not round to the nearest integer.

5.2.0 General considerations. The subsurface intrusion component evaluates the threats 

from hazardous substances that have or could intrude into regularly occupied structures via 

surficial ground water or the unsaturated zone. Evaluate the subsurface intrusion component 

based on the actual or potential intrusion of hazardous substances into a regularly occupied 

structure(s), or contaminants from the unsaturated zone or the surficial ground water below the 

regularly occupied structure(s). 

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 which are either contaminated or 
Page 119 of 162

inferred to be contaminated (see section 5.2.1.1.1 Observed Exposure). Establish areas of 

observed exposure as follows: 

– Consider both the sampling location(s) with observed exposure from the site and the area 

lying between such locations to be an area of observed exposure, unless available 

information indicates otherwise. 

– In multi-tenant structures, consider both the tenant space(s) with observed exposure from 

the site and all other regularly occupied tenant spaces on that level and all levels below, if 

any, to be within an area of observed exposure, unless available information indicates 

otherwise.

– In multi-tenant structures that have multiple stories and have not documented an observed 

exposure, but are located in an area lying between locations where observed exposures 

have been documented, consider only those regularly occupied tenant spaces, if any, on 

the lowest level, to be within 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 based on sampling 

locations at which there is contamination as follows:

– Consider both the sampling location(s) with subsurface contamination from the site 

and the area lying between such locations to be an area of subsurface contamination, 

unless available information indicates otherwise.
Page 120 of 162

– Exclude those areas meeting the criteria defined as an area of observed exposure.

– Evaluate an area of subsurface contamination based only on the presence of 

hazardous substances with a vapor pressure greater than or equal to 10-9 torr that also 

meet the criteria for an observed release for any location associated with an area of 

subsurface contamination (see section 2.2.2). Consider those hazardous substances 

with a vapor pressure less than 10-5

, only if they have also been documented to be in 

an observed exposure. See Section 7.0 for establishing an area of subsurface 

contamination based on the presence of radioactive hazardous substances.

– Evaluate multi-tenant structures as follows:

 If an observed exposure has been documented based on a gaseous indoor air 

sample, consider all regularly occupied tenant spaces, 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 

tenant spaces, 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 tenant spaces above 

the level where an observed exposure has been documented to be within an area 

of subsurface contamination, unless available information indicates otherwise.

 If no observed exposure has been documented in any regularly occupied tenant 

space(s) of a multi-tenant structure that is located within an area lying between 
Page 121 of 162

locations where subsurface contamination sampling meets the criteria for an 

observed release, consider only those regularly occupied tenant spaces, 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 the component only if there is an 

area of observed exposure or area of subsurface contamination: 

• Within or underlying a residence, school, or day care center, or 

• Within or underlying a workplace, or 

• Within or underlying a resource specified in section 5.2.1.3.4.

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: 
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– 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 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. 
Page 123 of 162

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 10-5 torr. 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
Page 124 of 162

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

9.

Regularly occupied structure with an 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

0
Page 125 of 162

No. Evidence of Structure Containment

Assigned 

value

10.

Regularly occupied structure with a foundation raised greater than 6 feet (e.g., 

structure on stilts)

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
Page 126 of 162

>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 
Page 127 of 162

such layers and sum their thicknesses. Assign a thickness of 0 feet to a layer with 

channelized flow features found within any area of subsurface contamination at the 

site.

– Assign a value from Table 5-15 to the vertical migration factor, based on the 

thickness and assigned porosity/permeability value of the highest 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 

3
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fractures); low permeability limestones and dolomites (no karst); low permeability 

sandstone; low permeability fractured igneous and metamorphic rocks.

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: 
Page 129 of 162

– 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

Greater than 10-3 to 10 2

10-5 to 10-3 1

Less than 10-5 0

Henry’s Constant (atm-m3/mol) Assigned Value

Greater than 10-3 3
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Greater than 10-5 to 10-3 2

10-7 to 10-5 1

Less than 10-7 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/biodegradation and hazardous waste quantity. 
Page 131 of 162

5.2.1.2.1 Toxicity/biodegradation. For each hazardous substance, assign a toxicity factor 

value, a biodegradation factor value and a combined toxicity/biodegradation factor value as 

specified in sections 2.2.3 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 Biodegradation. Assign a biodegradation factor value to each hazardous 

substance as follows:

• For any hazardous substance that meets the criteria for an observed exposure assign a 

biodegradation factor value of 1.

For all hazardous substances not meeting observed exposure criteria:

• For any petroleum or petroleum-like hazardous substance that is known to readily 

biodegrade in the subsurface (e.g., non-halogenated straight chain and single ring hydrocarbons), 

assign a biodegradation factor value of 0.5 if depth to contamination is greater than 30 feet, unless 

available evidence suggests that 30 feet of biologically active soil is not present in the subsurface. 

Otherwise, assign a biodegradation factor value of 1.

• For all other hazardous substances being evaluated, assign a biodegradation factor value 

of 1.

5.2.1.2.1.3 Calculation of toxicity/biodegradation factor value. Assign each substance a 

toxicity/biodegradation value by multiplying the toxicity factor value by the biodegradation factor 
Page 132 of 162

value. Use the hazardous substance with the highest combined toxicity/biodegradation value to 

assign the factor value to the toxicity/biodegradation 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-tenant structures, when calculating Tier A, use the mass of constituents 

found in the regularly occupied tenant 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-tenant structures, when calculating Tier B, use the flow-through volume of 

the regularly occupied tenant 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. 
Page 133 of 162

– 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-tenant structures, when calculating Tier C, calculate volume for those 

tenant spaces with observed or inferred exposure and all other regularly occupied 

tenant spaces on that level, unless available information indicates otherwise. If 

the structure has multiple stories, also include the volume of all regularly 

occupied tenant spaces below the floor with an observed exposure and one story 

above, unless evidence indicates otherwise.

 For multi-tenant structures within an area of subsurface contamination and no 

observed or inferred exposure, consider only the volume of the regularly 

occupied tenant 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 multi-tenant structures) based on actual 
Page 134 of 162

footprint area data. If the actual footprint area of the structure(s) is unknown, use an area of 1,740 

square feet for each residential structure (or residential tenant 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 tenant 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 tenant 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:
Page 135 of 162

• 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 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/biodegradation and hazardous waste quantity factor values, subject to a maximum 
Page 136 of 162

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 four factors: exposed individual, population, workers, 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 or attending school or day care in a structure with 

observed exposure, or living in a structure within an area of observed exposure or within an area 

of subsurface contamination.

• Population—an exposed individual or a person living or attending school or day care in a 

structure within an area of observed exposure or within an area of subsurface contamination.

• Worker—a person working in a 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 5.2.1.3.4. 

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 removed the release, count the initial targets as if the 

removal action had not occurred unless it has been demonstrated that no significant risk remains.
Page 137 of 162

For populations residing in or working in a multi-tenant 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 areas of observed exposure subject to Level I concentrations and those 

subject to Level II concentrations as specified as follows:

• For contamination resulting from subsurface intrusion, compare the hazardous substance 

concentrations in any sample meeting the observed exposure by chemical analysis criteria to the 

appropriate benchmark. If the concentration equals or exceeds the appropriate benchmark, 

assume Level I concentrations. For those regularly occupied structures and regularly occupied 

tenant spaces in a multi-tenant structure that are located within an area of observed exposure for 

which subsurface intrusion is inferred, or if there is an observed exposure by direct observation 

only, assume Level II concentrations. 

Use the health-based benchmarks from Table 5-19 in determining the level of contamination. 

Then assign a value to the exposed individual factor as follows: 
Page 138 of 162

• 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 such exposed individuals, but there is at least one 

exposed individual living 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 living or attending school or day care in a 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 people living in structures in an area of observed exposure or area of 
Page 139 of 162

subsurface contamination, when the estimate is based on the number of residences, if actual 

number 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. Sum the number of exposed 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 line 9a of Table 5-11.

5.2.1.3.2.2 Level II concentrations. 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 . Assign this sum as the value for this factor. Enter this 

value in line 9b of Table 5-11.

5.2.1.3.2.3 Population within an area of subsurface contamination. The population in an 

area of subsurface contamination is determined as follows, unless available information indicates 

otherwise:

1. Identify the regularly occupied structures and the population associated with each 

category:

• For single tenant structures, count all residents and students residing in or attending 

school or daycare in those structures.

• In multi-tenant structures where an area of subsurface contamination includes only those 

regularly occupied tenant spaces on the lowest level, count the number of residents and students 

residing in or attending school or daycare in those tenant spaces.
Page 140 of 162

• In multi-tenant structures with multiple stories where an area of subsurface contamination 

includes regularly occupied tenant spaces in the level directly above a level with an observed

exposure, sum the number of residents and students residing or attending school or daycare in 

those tenant spaces.

• In multi-tenant structures with multiple stories where an area of subsurface contamination 

includes regularly occupied tenant spaces in the level directly above a level with a gaseous indoor 

air sample meeting observed release criteria, sum the number of residents and students residing in 

or attending school or daycare in those tenant spaces.

2. Estimate the depth or distance to contamination at each regularly occupied structure 

within an area of subsurface contamination based on available sampling data, and categorize each 

eligible structure based on the depth or distance to contamination and sampling media as 

presented in Table 5-20. Weight the population in each structure using the appropriate weighting 

factors in Table 5-20. If samples from multiple media are available, use the sample that results in 

the highest weighting factor.

3. Sum the weighted population in the area(s) of subsurface contamination and assign this 

sum as the value for this factor. 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 residing in a structure with levels of contamination in a semi-enclosed or 

enclosed crawl space sample meeting observed release criteria

or

0.9
Page 141 of 162

Eligible Populationsa in Structuresb within an Area of Subsurface Contamination

Population 

Weighting 

Factor

Population residing in a tenant space of a multi-story structure within an area of 

subsurface contamination located directly above a story with an observed exposure or a 

gaseous indoor air sample meeting observed release criteria

Population residing 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 residing 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 residing 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 residing 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 residing 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 and students attending school or daycare (see HRS Section 5.2.1.3.2.3). Workers are excluded 

from this evaluation (see HRS Sections 5.2.1.3 and 5.2.1.3.3).

bEligible structures may include single- or multi-tenant structures where eligible populations reside or attend school or daycare. They 

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 
Page 142 of 162

contamination. Assign this sum as the population factor value. Enter this value in line 9d of Table 

5-11. 

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 Assigned Value

0 0

1-100 15

Greater than 100 -1000 30

Greater than 1000 45

5.2.1.3.4 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. 
Page 143 of 162

5.2.1.3.5 Calculation of targets factor category value. Sum the values for the exposed 

individual, population, workers, and resources factors. Do not round to the nearest integer. Assign 

this sum as the targets factor category value for the subsurface intrusion component. Enter this 

value in Table 5-11.

5.2.2 Calculation of subsurface intrusion component score. Multiply the factor 

category values for likelihood of exposure, waste characteristics and targets and round the 

product to the nearest integer. Divide the product by 82,500. Assign the resulting value, subject to 

a maximum of 100, as the subsurface intrusion component score and enter this score in Table 5-

11.

5.3 Calculation of the soil exposure and subsurface intrusion pathway score: Sum 

the soil exposure component score and subsurface intrusion component. Assign the resulting 

value, subject to a maximum of 100, as the soil exposure and subsurface intrusion pathway score 

(Ssessi). Enter this score in Table 5-11.

5. Section 6.0 of Appendix A is amended as follows:

6.0 Air Migration Pathway

* * * * * 

Table 6-14. * * *

* * * * * 

• Screening concentration for noncancer toxicological responses corresponding to the 

Reference Concentration (RfC) for inhalation exposures.

* * * * * 
Page 144 of 162

6. Section 7.0 of Appendix A is amended as follows: 

a. By revising Table 7-1.

b. By revising section 7.0.

c. By revising subsections 7.1, 7.1.1, and 7.1.2.

d. By revising subsection 7.2.4.

e. By revising subsections 7.2.5.1, 7.2.5.1.1, 7.2.5.1.2, 7.2.5.1.3, 7.2.5.2, and 7.2.5.3. 

f. By revising subsections 7.3, 7.3.1, and 7.3.2.

g. By adding subsection 7.3.3.

7.0 Sites Containing Radioactive Substances

* * * * *
Page 145 of 162

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Likelihood of

Release

Likelihood of 

Release

Likelihood of 

Exposure

Likelihood of 

Exposure

Likelihood of 

Release

Observed 

Release

Yes Observed 

Release

Yes Observed 

Contamination

Yes Observed 

Exposure

Yes Observed 

Release

Yes

Potential to 

Release

No Potential to 

Release

No Attractiveness/ 

Accessibility to 

Nearby 

Residents

No Potential for 

Exposure

Yes Gas Potential to 

Release

No

Containment No Overland Flow 

Containment

No Area of 

Contamination

No Structure 

Containment

No Gas 

Containment

No

Net Precipitation No Runoff No Depth to 

Contamination

Yes Gas Source 

Type

No

Depth to Aquifer No Distance to 

Surface water

No Vertical 

migration

No Gas Migration 

Potential

No

Travel Time No Flood Frequency No Vapor Migration 

Potential

No Particulate 

Potential to 

Release

No
Page 146 of 162

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Flood 

Containment

No Particulate 

Containment

No

Particulate 

Source Type

No

Particulate 

Migration 

Potential

No

Waste 

Characteristics

Waste

Characteristics

Waste 

Characteristics

Waste 

Characteristics

Waste 

Characteristics

Toxicity Yes Toxicity/

Ecotoxicity

Yes/Y

es

Toxicity Yes Toxicity/ 

Biodegradation

Yes Toxicity Yes

Mobility No Persistence/

Mobility

Yes/N

o

Hazardous 

Waste Quantity

Yes Hazardous 

Waste Quantity

Yes Mobility No

Hazardous 

Waste Quantity

Yes Bioaccumulation PotentialNo Hazardous 

Waste Quantity

Yes
Page 147 of 162

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Hazardous 

Waste Quantity

Yes

Targets Targets Targets Targets Targets

Nearest Well Yes b Nearest Intake Yes b Resident 

Individual

Yes b Exposed

Individual

Yes b Nearest 

Individual

Yes b

Population Yes b Drinking Water 

Population

Yes b Resident 

Population

Yes b Population Yes b Population Yes b

Resources No Resources No Workers No Workers No Resources No

Wellhead 

Protection Area

No Sensitive 

Environments

Yes b Resources No Resources No Sensitive 

Environments

No

Human Food 

Chain Individual

Yes b Terrestrial 

Sensitive 

Environments

No

Human Food 

Chain Population

Yes b Nearby 

Individual

No
Page 148 of 162

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCLIDES

Ground Water 

Pathway

Status a

Surface Water 

Pathway

Status

a

Soil Exposure 

Component of 

SESSI Pathway

Status a

Subsurface 

Intrusion 

Component of 

SESSI Pathway

Status a Air Pathway Status a

Population 

Within 1 Mile

No

a Factors evaluated differently are denoted by “yes”; factors not evaluated differently are denoted by “no”.

b Difference is in the determination of Level I and Level II concentrations.
Page 149 of 162

* * * * *

* * * These differences apply largely to the soil exposure and subsurface intrusion pathway 

and to sites containing mixed radioactive and other hazardous substances. * * *

7.1 Likelihood of release/likelihood of exposure. Evaluate likelihood of release for the 

three migration pathways and likelihood of exposure for the soil exposure and subsurface 

intrusion pathway as specified in sections 2 through 6, except: establish an observed release, 

observed contamination, and/or observed exposure as specified in section 7.1.1. When an 

observed release or exposure cannot be established for a migration pathway or the subsurface 

intrusion component of the soil exposure and subsurface intrusion pathway, evaluate potential to 

release as specified in section 7.1.2. When observed contamination cannot be established, do not 

evaluate the soil exposure component of the soil exposure and subsurface intrusion pathway.

7.1.1 Observed release/observed contamination/observed exposure. For radioactive 

substances, establish an observed release for each migration pathway by demonstrating that the 

site has released a radioactive substance to the pathway (or watershed or aquifer, as appropriate); 

establish observed contamination or observed exposure for the soil exposure and subsurface 

intrusion pathway as indicated below. * * * 

* * * * *

– For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, a material that contains one or more radionuclides has been observed 

entering a regularly occupied structure via the subsurface or is known to have entered 

a regularly occupied structure via the subsurface. Also, when evidence supports the 

inference of subsurface intrusion of a material that contains one or more 
Page 150 of 162

radionuclides by the site into a regularly occupied structure, demonstrated adverse 

effects associated with that release may also be used to establish observed exposure 

by direct observation. 

• Analysis of radionuclide concentrations in samples appropriate to the pathway (that is, 

ground water, soil, air, indoor air, surface water, benthic, or sediment samples): 

* * * * *

 Some portion of the increase must be attributable to the site to establish the 

observed release (or observed contamination or observed exposure), and 

 For the soil exposure component of the soil exposure and subsurface intrusion 

pathway only, the radionuclide must also be present at the surface or covered by 

2 feet or less of cover material (for example, soil) to establish observed 

contamination.

* * * * *

 If the sample quantitation limit cannot be established: 

° If the sample analysis was performed under the EPA Contract Laboratory 

Program, use the EPA contract-required quantitation limit (CRQL) in place 

of the sample quantitation limit in establishing an observed release (or 

observed contamination or observed exposure). 

* * * * *

 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 
Page 151 of 162

2 feet or less of cover material (for example, soil) to establish observed 

contamination.

• Gamma radiation measurements (applies only to observed contamination or observed 

exposure in the soil exposure and subsurface intrusion pathway): 

– The gamma radiation exposure rate, as measured in microroentgens per hour (μR/hr) 

using a survey instrument held 1 meter above the ground surface or floor or walls of 

a structure (or 1 meter away from an aboveground source for the soil exposure 

component), equals or exceeds 2 times the site-specific background gamma radiation 

exposure rate. 

* * * * *

For the three migration pathways and for the subsurface intrusion component of the soil 

exposure and subsurface intrusion pathway, if an observed release or observed exposure can be 

established for the pathway (or threat, aquifer, or watershed, as appropriate), assign the pathway 

(or threat, aquifer, or watershed) an observed release or observed exposure factor value of 550 

and proceed to section 7.2. If an observed release or observed exposure cannot be established, 

assign an observed release or observed exposure factor value of 0 and proceed to section 7.1.2.

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if 

observed contamination can be established, assign the likelihood of exposure factor for resident 

population a value of 550 if there is an area of observed contamination in one or more locations 

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 
Page 152 of 162

established, do not evaluate the soil exposure component of the soil exposure and subsurface 

intrusion pathway.

At sites containing mixed radioactive and other hazardous substances, evaluate observed 

release (or observed contamination or observed exposure) separately for radionuclides as 

described in this section and for other hazardous substances as described in sections 2 through 6. 

For the three migration pathways and the subsurface intrusion component of the soil exposure 

and subsurface intrusion pathway, if an observed release or observed exposure can be established 

based on either radionuclides or other hazardous substances, or both, assign the pathway (or 

threat, aquifer, or watershed) an observed release or observed exposure factor value of 550 and 

proceed to section 7.2. If an observed release or observed exposure cannot be established based 

on either radionuclides or other hazardous substances, assign an observed release or observed 

exposure factor value of 0 and proceed to section 7.1.2. 

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if 

observed contamination can be established based on either radionuclides or other hazardous 

substances, or both, assign the likelihood of exposure factor for resident population a value of 550 

if there is an area of observed contamination in one or more locations listed in section 5.1.1; 

evaluate the likelihood of exposure factor for nearby population as specified in section 5.1.2.1; 

and proceed to section 7.2. If observed contamination cannot be established based on either 

radionuclides or other hazardous substances, do not evaluate the soil exposure component of the 

soil exposure and subsurface intrusion pathway. 

7.1.2 Potential to release/potential for exposure. For the three migration pathways and 

the subsurface intrusion component of the soil exposure and subsurface intrusion pathway, 

evaluate potential to release or potential for exposure for sites containing radionuclides in the 

same manner as specified for sites containing other hazardous substances. * * * For the 
Page 153 of 162

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.

* * * * *

7.2.4 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, biodegradation 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).

* * * * *
Page 154 of 162

7.2.5.1 Source hazardous waste quantity for radionuclides. * * * For the soil exposure 

component of the soil exposure and subsurface intrusion pathway, assign a source hazardous 

waste quantity value to each area of observed contamination, as applicable to the threat being 

evaluated. For the subsurface intrusion component, assign a source hazardous waste quantity 

value to each regularly occupied structure located within areas of observed exposure or areas of 

subsurface contamination. Allocate hazardous substances and hazardous wastestreams to specific 

sources (or areas of observed contamination, area of observed exposure or area of subsurface 

contamination) as specified in sections 2.4.2 and 5.2.0.

7.2.5.1.1 Radionuclide constituent quantity (Tier A). Evaluate radionuclide constituent 

quantity for each source (or area of observed contamination or area of observed exposure) based 

on the activity content of the radionuclides allocated to the source (or area of observed 

contamination or area of observed exposure) as follows:

• Estimate the net activity content (in curies) for the source (or area of observed 

contamination or area of observed exposure) based on: 

* * * * *

where: 

N=Estimated net activity content (in curies) for the source (or area of observed contamination 

or area of observed exposure). 

V=Total volume of material (in cubic yards) in a source (or area of observed contamination 

or area of observed exposure) containing radionuclides. 
Page 155 of 162

ACi=Activity concentration above the respective background concentration (in pCi/g) for 

each radionuclide i allocated to the source (or area of observed contamination or area of observed 

exposure). 

n=Number of radionuclides allocated to the source (or area of observed contamination or area 

of observed exposure) above the respective background concentrations. or, 

* * * * *

where:

N=Estimated net activity content (in curies) for the source (or area of observed contamination 

or area of observed exposure). 

V=Total volume of material (in gallons) in a source (or area of observed contamination or 

area of observed exposure) containing radionuclides. 

ACi=Activity concentration above the respective background concentration (in pCi/1) for

each radionuclide i allocated to the source (or area of observed contamination or area of observed 

exposure). 

n=Number of radionuclides allocated to the source (or area of observed contamination or area 

of observed exposure) above the respective background concentrations. 

• Estimate volume for the source (or volume for the area of observed contamination or area 

of observed exposure) based on records or measurements.

• 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) * * *. 
Page 156 of 162

• For the subsurface intrusion component, in estimating the volume for areas of observed 

exposure, only use the volume of air in the regularly occupied structures where observed 

exposure has been documented.

• Convert from curies of radionuclides to equivalent pounds of nonradioactive hazardous 

substances by multiplying the activity estimate for the source (or area of observed contamination 

or area of observed exposure) by 1,000. 

• Assign this resulting product as the radionuclide constituent quantity value for the source 

(or area of observed contamination or area of observed exposure).

If the radionuclide constituent quantity for the source (or area of observed contamination or 

area of observed exposure) is adequately determined (that is, the total activity of all radionuclides 

in the source and releases from the source [or in the area of observed contamination or area of 

observed exposure] is known or is estimated with reasonable confidence), do not evaluate the 

radionuclide wastestream quantity measure in section 7.2.5.1.2. * * * If the radionuclide 

constituent quantity is not adequately determined, assign the source (or area of observed 

contamination or area of observed exposure) a value for radionuclide constituent quantity based 

on the available data and proceed to section 7.2.5.1.2.

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:
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• Estimate the total volume (in cubic yards or in gallons) of wastestreams containing 

radionuclides allocated to the source (or area of observed contamination, area of observed 

exposure, or area of subsurface contamination). 

* * * * *

• Assign the resulting value as the radionuclide wastestream quantity value for the source

(or area of observed contamination, area of observed exposure, or area of subsurface 

contamination). 

• For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, estimate the total wastestream volume for all regularly occupied structures located 

within areas of observed exposure with observed or inferred intrusion and within areas of 

subsurface contamination. Calculate the volume of each regularly occupied structure based on 

actual data. If unknown, use a ceiling height of 8 feet.

7.2.5.1.3 Calculation of source hazardous waste quantity value for radionuclides. Select 

the higher of the values assigned to the source ( or area of observed contamination, area of

observed exposure, and/or area of subsurface contamination) for radionuclide constituent quantity 

and radionuclide wastestream quantity. Assign this value as the source hazardous waste quantity 

value for the source (or area of observed contamination, area of observed exposure, or area of 

subsurface contamination). * * * 

7.2.5.2 Calculation of hazardous waste quantity factor value for radionuclides. Sum the 

source hazardous waste quantity values assigned to all sources (or area of observed 
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contamination, area of observed exposure, or area of subsurface contamination) for the pathway 

being evaluated and round this sum to the nearest integer, except: if the sum is greater than 0, but 

less than 1, round it to 1. * * * 

* * * * *

For the soil exposure component of the soil exposure and subsurface intrusion pathway, if the 

radionuclide constituent quantity is adequately determined for all areas of observed 

contamination, assign the value from Table 2-6 as the hazardous waste quantity factor value. * * 

* 

For the subsurface intrusion component of the soil exposure and subsurface intrusion 

pathway, if the radionuclide constituent quantity is adequately determined for all areas of 

observed exposure, assign the value from Table 2-6 as the hazardous waste quantity factor value. 

If the radionuclide constituent quantity is not adequately determined for one or more areas of 

observed exposure, assign either the value from Table 2-6 or a value of 10, whichever is greater, 

as the hazardous waste quantity factor value. 

7.2.5.3 Calculation of hazardous waste quantity factor value for sites containing mixed 

radioactive and other hazardous substances. For each source (or area of observed contamination, 

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 
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quantity value for the source (or area of observed contamination, area of observed exposure, or 

area of subsurface contamination). * * * 

Use this combined source hazardous waste quantity value to calculate the hazardous waste 

quantity factor value for the pathway as specified in section 2.4.2.2, except: if either the 

hazardous constituent quantity or the radionuclide constituent quantity, or both, are not 

adequately determined for one or more sources (or one or more portions of sources or releases 

remaining after a removal action) or for one or more areas of observed contamination, areas of

observed exposure, or areas of subsurface contamination, as applicable, assign the value from 

Table 2-6 or the default value applicable for the pathway, whichever is greater, as the hazardous 

waste quantity factor value for the pathway. 

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. * * * 
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• 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. 

* * * * * 

 

• If none of the radionuclides eligible to be evaluated for the sampling location have an 

applicable benchmark, assign Level II to the actual contamination at that sampling location for 

the pathway (or component or threat). 

• In making the comparison, consider only those samples, and only those radionuclides in 

the sample, that meet the criteria for an observed release (or observed contamination or observed 

exposure) for the pathway, except: tissue samples from aquatic human food chain organisms may 

also be used for the human food chain threat of the surface water pathway as specified in sections 

4.1.3.3 and 4.2.3.3. 

7.3.2 Comparison to benchmarks. * * * 

* * * * *

• Uranium Mill Tailings Radiation Control Act (UMTRCA) standards—soil exposure 

component of the soil exposure and subsurface intrusion pathway only.
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• 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 
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observed exposure) for the sample or is eligible to be evaluated for a tissue sample (see sections 

4.1.3.3 and 4.2.3.3), calculate a value for index I for these radionuclides as specified in section 

2.5.2. * * * 

* * * Use only those samples and only those substances in the sample that meet the criteria 

for an observed release (or observed contamination or observed exposure) for the pathway 

except: tissue samples from aquatic human food chain organisms may be used as specified in 

sections 4.1.3.3 and 4.2.3.3. * * * 

If no radionuclide or other hazardous substance individually exceed a benchmark 

concentration, but more than one radionuclide or other hazardous substance either meets the 

criteria for an observed release (or observed contamination or observed exposure) for the sample 

or is eligible to be evaluated for a tissue sample, calculate an index I for both types of substances 

as specified in section 2.5.2. * * * 

7.3.3 Weighting of targets within an area of subsurface contamination. For the 

subsurface intrusion component of the soil exposure and subsurface intrusion pathway, assign a 

weighting factor as specified in section 5.2.1.3.2.3 except when an area of subsurface 

contamination is bound by gamma radiation exposure rates meeting observed release criteria with 

a depth to contamination of 2 feet or less. For those populations residing in or attending school or 

daycare in a structure, assign a weighting factor of 0.9.