10372 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 300 

[EPA–HQ–SFUND–2010–1086; FRL–9925– 

69–OLEM] 

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 subsurface intrusion 

contamination to be evaluated for 

placement of sites 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 must be received on 

or before April 29, 2016. 

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. 

Once submitted, comments cannot be 

edited or withdrawn. The EPA may 

publish any comment received to its 

public docket. Do not submit 

electronically any information you 

consider to be Confidential Business 

Information (CBI) or other information 

whose disclosure is restricted by statute. 

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. The EPA will 

generally not consider comments or 

comment contents located outside of the 

primary submission (i.e. on the Web, 

cloud, or other file sharing system). For 

additional submission methods, the full 

EPA public comment policy, 

information about CBI or multimedia 

submissions, and general guidance on 

making effective comments, please visit 

http://www.epa.gov/dockets/

commenting-epa-dockets. 

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: The 

information presented in this preamble 

is organized as follows: 

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 HRS? 

B. What is the history of the HRS? 

C. What is the impact of this proposed 

rule? 

1. Impact on Current Cleanup Programs, 

Resources and Cost 

2. Children’s Environmental Health and 

Environmental Justice 

IV. Hazard Ranking System 

A. Purpose 

B. Structure 

V. Approach to HRS Addition 

A. General Approach 

1. What is the need for regulatory action on 

the HRS? 

2. What alternative regulatory options to 

this action were considered by EPA? 

3. What public outreach activities did EPA 

conduct? 

4. What peer review process did EPA use? 

5. How did EPA select the approach for 

including the addition in the HRS? 

B. Technical Considerations To 

Maintaining The Current HRS Structure 

and Algorithm 

1. Maintaining the Current Ground Water, 

Surface Water, and Air Migration 

Pathways 

2. Addition of the New Component to 

Restructure and Rename the Soil 

Exposure Pathway 

C. Supporting Materials 

VI. Discussion of the Proposed SsI Addition 

to the HRS 

A. Addition Within a Restructured Soil 

Exposure Pathway 

B. SsI Component Addition 

1. New Definitions 

2. Delineation of Areas of Subsurface 

Intrusion 

a. Area of Observed Exposure (AOE) 

b. Area of Subsurface Contamination (ASC) 

c. Other Area of Subsurface Intrusion 

Considered: Potential Migration Zone 

3. Likelihood of Exposure 

a. Observed Exposure 

b. Potential for Exposure 

c. Calculation of the Likelihood of 

Exposure Factor Category Value 

4. Waste Characteristics 

a. Toxicity/Degradation 

b. Hazardous Waste Quantity 

c. Calculation of the Waste Characteristics 

Factor Category Value 

5. Targets 

a. Identification of Eligible Targets 

b. Exposed Individual and Levels of 

Exposure 

c. Population 

d. Resources 

e. Calculation of the Targets Factor 

Category Value 

6. Calculation and Incorporation of the SsI 

Component Score Into the HRS Site 

Score 

a. Calculation of the SsI Component Score 

b. Incorporation of the SsI Component 

Score Into the Soil Exposure and 

Subsurface Intrusion Pathway Score 

c. Incorporation of the Soil Exposure and 

Subsurface Intrusion Pathway Score Into 

a Site Score 

7. Example Site Scoring Scenarios 

VII. Summary of Proposed Updates to the 

HRS 

VIII. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

B. Paperwork Reduction Act (PRA) 

C. Regulatory Flexibility Act (RFA) 

D. Unfunded Mandates Reform Act 

(UMRA) 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Executive Order 12580: Superfund 

Implementation 

I. General Information 

A. What is EPA seeking comment on? 

EPA is proposing an addition of one 

new component to one part of the 

current Hazard Ranking System (HRS). 

No major structural changes to other 

parts of the HRS are proposed. EPA is 

seeking comments on the addition of the 

subsurface intrusion component to the 

HRS. Comments on unmodified parts of 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10373 

1Subsurface 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. 

the HRS are not being requested and 

will not be considered if submitted. 

B. How does this action apply to me? 

This action proposes an addition to 

the HRS. The HRS is used for evaluating 

the relative potential risk posed by the 

uncontrolled release, or potential 

release, of hazardous substances to 

human health or the environment. This 

addition will enable EPA to identify 

risks posed by subsurface intrusion of 

hazardous substances into regularly 

occupied structures for all populations 

who live and work in areas where the 

subsurface environment may create 

exposures. The agency considers that 

including the evaluation of subsurface 

intrusion in the HRS serves the public 

interest by widening EPA’s ability to 

evaluate these threats. 

This proposed regulatory change 

expands the available options for EPA 

and organizations performing work on 

behalf of EPA (state and tribal partners) 

to evaluate potential threats to public 

health and the environment from 

subsurface intrusion contamination. 

State and tribal partners may receive 

financial assistance from EPA to 

evaluate sites through a Cooperative 

Agreement. EPA and states or tribes 

collaborate closely throughout the 

Cooperative Agreement process, 

particularly when identifying sites to be 

evaluated and establishing priorities for 

performing evaluations. As necessary, 

sites where subsurface intrusion threats 

exist may be evaluated using the HRS 

and, if warranted, proposed for 

placement on the NPL. EPA does not 

expect that this proposed change will 

result in additional site assessments 

being conducted per year or placement 

of more sites on the NPL per year. 

Rather, given potentially limited 

budgets and the possibility of increased 

costs for an SsI site assessment, EPA 

may conduct fewer assessments per 

year. The pipeline of sites will be 

reviewed to identify those sites that 

pose the highest risk and prioritized 

accordingly. This is not a change to how 

EPA currently evaluates and prioritizes 

sites for the NPL; EPA will simply have 

an additional mechanism to address 

sites that pose the greatest risk. Because 

assessing the worst sites first is a 

priority, EPA will continue to identify 

the sites posing the highest risk or 

potential risk and develop a strategy to 

assess those sites in a timely manner, 

while balancing their other site 

assessment needs. 

The addition of a subsurface intrusion 

component to the HRS affirms that EPA 

is fulfilling its regulatory requirements 

by ensuring ‘‘to the maximum extent 

feasible, that the hazard ranking system 

accurately assesses the relative degree of 

risk to human health and the 

environment posed by sites and 

facilities subject to review.’’ 42 U.S.C. 

9605(c)(1), as mandated by the 

Superfund Amendments and 

Reauthorization Act (SARA) 

amendments to the Comprehensive 

Environmental Response, 

Compensation, and Liability Act 

(CERCLA). 

This proposed addition is necessary 

because no present authority 

consistently and comprehensively 

addresses subsurface intrusion 

contamination across all non-federal 

potential sites, particularly when 

subsurface intrusion is the key exposure 

pathway. While most states have 

identified sites with subsurface 

intrusion contamination issues, not all 

states have subsurface intrusion 

programs, and states with subsurface 

intrusion remediation programs vary in 

their authority, resources, and 

remediation criteria. A redirection of 

resources available through Cooperative 

Agreement funding is expected to 

provide for greater national consistency 

in the identification and evaluation of 

subsurface intrusion sites. 

Additionally, EPA finalized the 

OSWER Technical Guide for Assessing 

and Mitigating the Vapor Intrusion 

Pathway from Subsurface Vapor Sources 

to Indoor Air, in June 2015. This guide 

and this proposed addition to the HRS 

would further the agency’s efforts to 

establish national consistency in 

evaluating vapor intrusion threats by 

enabling EPA to use remedial authority 

under CERCLA. 

This proposed regulatory change does 

not affect the status of sites currently on 

or proposed to be added to the NPL. 

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 

EPA is proposing this addition to 

protect human health from the threat 

posed by subsurface intrusion. By 

adding this component to the HRS, EPA 

will be able to consider subsurface 

intrusion threats when evaluating sites 

for placement on the NPL and 

implement the requirements of CERCLA 

and the National Oil and Hazardous 

Substances Pollution Contingency Plan 

(NCP). This proposed addition is a 

technical modification to the current 

HRS that will allow EPA and its 

partners to more comprehensively 

address the releases of hazardous 

substances into the environment. 

A. Why is EPA proposing an addition to 

the Hazard Ranking System? 

Contaminant subsurface intrusion 1 is 

defined as the migration of hazardous 

substances, pollutants, or 

contaminants 2 from the subsurface 

environment, or more specifically, the 

surficial ground water into overlying 

structures and/or the unsaturated zone. 

Subsurface intrusion can result in 

people being exposed to harmful levels 

of hazardous substances and cause 

negative health effects. While 

subsurface intrusion can take multiple 

forms, the most common form of 

subsurface intrusion is vapor intrusion. 

There are several reasons why EPA is 

proposing this addition to the HRS. 

First, the current HRS (40 CFR 300, 

Appendix A), promulgated December 

14, 1990 (hereafter referred to as the 

current HRS), discussed in more detail 

in section IV of this preamble, does not 

consider the threat posed by subsurface 

intrusion in its evaluation of relative 

risk posed by a site; therefore, it does 

not provide a complete assessment of 

the relative risk that a site may pose to 

the public. The existing pathways used 

to evaluate threats posed by hazardous 

substances do not include those 

entering a regularly occupied structure 

from the subsurface. For example, the 

ground water migration pathway 

evaluates the threat posed by 


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10374 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

3EPA’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. 

4This information was previously stored in a 

predecessor database called the Comprehensive 

Environmental Response, Compensation and 

Liability Information System (CERCLIS). 

contaminated ground water if there is an 

indication that ground water is being 

consumed. Similarly, the soil exposure 

pathway evaluates the threat posed by 

contaminated surfaces (e.g., surface 

soils) if there is an indication of human 

exposure. The air migration pathway 

considers the threat posed by hazardous 

substances released to atmospheric air 

(ambient air), but does not address 

indoor air, and has no subsurface 

component. The surface water migration 

pathway does not cover subsurface 

intrusion as it only considers the threat 

posed by contaminated surface water 

bodies. 

In fact, in a May 2010 report,3 the 

Government Accountability Office 

(GAO) concluded that if vapor intrusion 

sites ‘‘are not assessed and, if needed, 

listed on the NPL, some seriously 

contaminated hazardous waste sites 

with unacceptable human exposure may 

not otherwise be cleaned up.’’ The GAO 

recommended that EPA consider vapor 

intrusion as part of the NPL process; 

EPA agreed with the GAO 

recommendation. With the addition of a 

subsurface intrusion component, a site 

with vapor intrusion may qualify for the 

NPL, whereas presently the site may not 

have qualified using the threats 

evaluated in the current HRS. Therefore, 

without this addition, EPA may not be 

identifying the sites that most warrant 

further investigation. 

Second, EPA is offering this proposal 

because of the substantial public 

support for this action. EPA conducted 

outreach activities to determine the 

level of interest and support from the 

public. This included a Notice of 

Opportunity for Public Input (76 FR 

5370, January 31, 2011) and four public 

listening sessions held across the 

country. More than 40 written 

comments, from a diverse group of 

private citizens, businesses, states, 

American Indian 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 EPA’s 

Superfund Enterprise Management 

System 4 (SEMS). Of the 1,073 identified 

sites: 

• 328 sites are identified as having a 

suspected subsurface intrusion threat 

based on SEMS and Agency for Toxic 

Substances and Disease Registry 

(ATSDR) key word searches, as well as 

EPA or state self-identification, but for 

which no sampling data were obtained 

• 532 sites are identified as having 

characteristics or evidence that indicate 

subsurface intrusion (e.g., volatile 

hazardous substance in ground water) 

may have occurred or will occur. 

• 202 sites are identified as having a 

subsurface intrusion threat documented 

by subslab, crawl space, or indoor air 

samples but insufficient HRS-required 

evaluation factors to qualify for the NPL. 

• 11 sites are identified as having a 

subsurface intrusion threat with 

documented actual exposure of a 

sufficient number of targets and 

sufficient other HRS-required evaluation 

factors to suggest the site may qualify 

for the NPL. 

EPA is also considering sites with 

another form of subsurface intrusion, 

namely, intrusion of contaminated 

ground water into regularly occupied 

structures—which is an emerging issue. 

For example, a site was discovered 

where shallow (surficial) ground water 

contaminated with chromium had 

intruded into residential basements and 

after the water receded, or evaporated, 

a precipitate of chromium remained as 

a residue. The presence of this residue 

posed a significant threat to public 

health; however, the site could not be 

evaluated under the current HRS due to 

the lack of a mechanism to evaluate 

human exposure resulting from 

intrusion of contaminated ground water 

(subsurface intrusion contamination). 

The only viable option to place the site 

on the NPL was to rely on ATSDR to 

make a determination that the exposure 

at the site posed a significant threat to 

public health. The decision to include 

sites on the NPL based on a 

determination by the ATSDR is made 

infrequently because the HRS is the 

primary mechanism for placing a site on 

the NPL. 

EPA regional site assessment 

programs have identified 7 additional 

sites where intrusion of contaminated 

ground water is a potential issue and the 

related threat cannot be evaluated using 

the current HRS. Under the proposed 

SsI addition, ground water intrusion 

would be evaluated using current 

conditions, which may involve 

situations where metals have 

precipitated from water or where 

volatile substances have entered a 

structure via infiltrating ground water. 

As EPA further explores this emerging 

issue, the agency considers it likely that 

other ground water intrusion sites 

requiring evaluation will be identified. 

The inventory of sites, identified by 

EPA, with a possible threat from 

contaminated vapor or ground water 

intruding into overlying regularly 

occupied structures is not representative 

of the magnitude of the potential scope 

of sites with subsurface intrusion 

contamination. EPA identified these 

sites based on currently available 

information to initially assess the 

subsurface intrusion problem. In the 

case of vapor intrusion, certain states 

undertook comprehensive efforts to 

identify and evaluate subsurface 

intrusion threats, which resulted in the 

identification of a proportionately 

higher number of sites with potential 

vapor intrusion problems in those 

states. In the case of ground water 

intrusion, the issue is still emerging. For 

these reasons, EPA recognizes that a 

degree of inherent uncertainty is 

associated with compiling an inventory 

of sites with potential subsurface 

intrusion problems and that additional 

analysis is necessary, especially in cases 

where little information exists. See 

Appendix A of the Technical Support 

Document for this proposed addition 

(Proposal TSD) for the inventory of 

vapor intrusion sites. As additional 

information is gathered and new sites 

are added to SEMS and undergo the site 

assessment process, the number of sites 

with subsurface intrusion threats is 

likely to change. Nevertheless, the 

aforementioned illustrates that there 

currently exists at least 1,073 sites that 

have significant actual or potential 

human exposure due to subsurface 

intrusion, but because of the 

shortcomings of the current HRS, cannot 

be evaluated to determine if they 

warrant addition to the NPL. 


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Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10375 

5Although the HRS is designed to assess the 

relative risk of a site compared to other sites, it is 

not designed to be used as a site-specific 

quantitative risk assessment. Such an assessment is 

conducted later in the Superfund process, as 

necessary. 

It is also important to emphasize that 

the inventory of sites compiled (where 

subsurface intrusion has been identified 

as a possible issue) does not represent 

a list of sites that will be placed on the 

NPL. EPA recognizes that, in many 

instances, additional information is 

needed to verify the presence, and to 

determine the nature/extent, of a 

subsurface intrusion problem. As such, 

the inventory should not be considered 

a list of NPL candidate sites. EPA notes 

that less than 5% of all sites evaluated 

through the site assessment process are 

actually added to the NPL. This 

percentage is not expected to change 

significantly with this addition to the 

HRS. 

Finally, EPA has concluded that for 

non-federal facilities no other national 

program is able to consistently and 

comprehensively evaluate and, if 

warranted, address subsurface intrusion 

contamination. This topic is further 

discussed in section V.A.2 of this 

preamble. 

B. What is the history of the hazard 

ranking system? 

In 1980, Congress enacted CERCLA 

(42 U.S.C. 9601 et seq.), commonly 

called Superfund, in response to the 

dangers posed by uncontrolled releases 

of hazardous substances into the 

environment. To implement section 105 

(a)(8)(A) of CERCLA and Executive 

Order 12316 (46 FR 42237, August 20, 

1981), EPA revised the NCP on several 

occasions, with the most recent 

comprehensive revision occurring on 

March 8, 1990 (55 FR 8666). The NCP 

sets forth the guidelines and procedures 

needed for responding to releases, or 

potential releases, of hazardous 

substances. Section 105(a)(8)(A) of 

CERCLA required EPA to establish: 

[C]riteria for determining priorities among 

releases or threatened releases [of hazardous 

substances] throughout the United States for 

the purpose of taking remedial action and, to 

the extent practicable, taking into account the 

potential urgency of such action, for the 

purpose of taking removal action. Criteria 

and priorities . . . shall be based upon 

relative risk or danger to public health or 

welfare or the environment . . . taking into 

account to the extent possible the population 

at risk, the hazard potential of hazardous 

substances at such facilities, the potential for 

contamination of drinking water supplies, 

the potential for direct human contact [and] 

the potential for destruction of sensitive 

ecosystems. . . . 

To meet this requirement and provide 

criteria to set priorities, EPA adopted 

the HRS as Appendix A to the NCP (47 

FR 31180, July 16, 1982). The HRS was 

last revised on December 14, 1990 (55 

FR 51532) to include the evaluation of 

additional threats to ensure a complete 

assessment of the relative risk that a site 

may pose to the public. The HRS is a 

scoring system used to assess the 

relative risk associated with actual or 

potential releases of hazardous 

substances from a site based on the 

information that can be collected in a 

limited, typically one to two day site 

inspection (SI). The HRS is designed to 

be applied consistently to each site, 

enabling sites to be ranked relative to 

each other with respect to actual or 

potential hazards. As EPA explained 

when it originally adopted the HRS, 

‘‘the HRS is a means for applying 

uniform technical judgment regarding 

the potential hazards presented by a 

facility relative to other facilities. It does 

not address the feasibility, desirability, 

or degree of cleanup required.’’ 5 (47 FR 

31220, July 16, 1982). 

Section 105(a)(8)(B) of CERCLA 

requires that the statutory criteria 

described in section 105(a)(8)(A) be 

used to prepare a list of national 

priorities among the known releases, or 

threatened releases throughout the 

United States. The list, which is 

Appendix B of the NCP, is the NPL. 

The HRS is a crucial part of the 

agency’s program to address the 

identification and cleanup of actual and 

potential releases of hazardous 

substances because the HRS score is the 

primary criterion for determining 

whether a site is to be included on the 

NPL. The NPL (Appendix B to 40 CFR 

300) includes those sites that emerge as 

potentially posing the most serious 

threats to public health and the 

environment and may warrant remedial 

investigation and possible cleanup 

under CERCLA. Only sites on the NPL 

are eligible for Superfund-financed 

remedial actions. Removal and 

enforcement actions can be conducted 

at any site, whether or not it is on the 

NPL. 

In 1986, Congress passed the 

Superfund Amendments and 

Reauthorization Act (SARA) (Pub. L. 99 

499), which added section 105(c)(1) to 

CERCLA, requiring EPA to amend the 

HRS to assure ‘‘to the maximum extent 

feasible, that the hazard ranking system 

accurately assesses the relative degree of 

risk to human health and the 

environment posed by sites and 

facilities subject to review.’’ The HRS 

was previously amended in 1990. This 

proposed action will amend the HRS to 

add a subsurface intrusion component 

to the evaluation. 

C. What is the impact of this proposed 

rule? 

1. Impact on Current Cleanup Programs, 

Resources and Cost 

This proposed addition to the HRS 

will have the most significant impact on 

EPA’s Superfund cleanup program. The 

current HRS considers releases to the 

ground water, surface water and air, as 

well as direct exposure to 

contamination such as soil in 

identifying releases which warrant 

further investigation. 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 at sites by subsurface intrusion 

problems and direct human exposure, 

when ground water is not being used as 

a drinking water source or surficial soils 

are not contaminated. If promulgated, 

this proposed rule will for the first time 

allow the EPA site assessment program 

to address sites with only subsurface 

intrusion issues and no coincidental 

exposure. When hazardous substances 

are released and enter the subsurface 

environment, they can move from the 

subsurface into buildings as a gas, 

vapor, or liquid. The addition of a 

subsurface intrusion component to the 

HRS would enable EPA to directly 

evaluate at sites the relative degree of 

risk posed by human exposure to 

hazardous substances that enter 

regularly occupied structures through 

the subsurface environment. 

To the extent practicable, EPA 

attempts to score all pathways that pose 

significant threats. If the contribution of 

a pathway is minimal to the overall 

score, in general, that pathway will not 

be scored. This proposed regulatory 

change would expand available options 

for EPA and organizations performing 

work on behalf of EPA (state and tribal 

partners) to evaluate potential threats to 

public health and the environment from 

hazardous waste sites. This 

modification to the HRS, by itself, only 

augments the criteria for applying the 

HRS. EPA also does not expect this 

proposed rulemaking to affect the status 

of sites currently on or proposed to the 

NPL. Sites that are currently on or 

proposed to the NPL have already been 

evaluated under another pathway (i.e., 

ground water migration, air migration, 

surface water migration, or soil 

exposure) and, consistent with section 

105(c)(3) of CERCLA, as amended, 

would not be re-evaluated. Proposal of 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
10376 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

6The regulatory impact analysis (RIA) found this 

rulemaking will only have moderate costs and will 

not be a significant rulemaking. The RIA for this 

rulemaking can be found in the official Docket for 

this action. 

this addition also will not disrupt EPA’s 

listing of sites. 

Because federal agencies currently 

address subsurface intrusion issues as 

part of their environmental programs, it 

is unlikely that a significant number of 

sites will be added to the NPL. 

However, it could lead to an increase in 

site assessment activities and related 

costs. Executive Order 12580 delegates 

broad CERCLA authority to federal 

agencies for responding to actual and 

potential releases of hazardous 

substances where a release is either on, 

or the sole source of the release is from, 

any facility or vessel under the 

jurisdiction, custody, or control of the 

federal agency. Federal agencies are 

required to exercise this authority 

consistent with the requirements of 

CERCLA section 120, as amended, and 

implement regulations under the NCP, 

for both NPL and non-NPL sites. 

Therefore, federal agencies are in a 

position to proactively identify and 

respond to risks posed by subsurface 

intrusion of hazardous substances into 

regularly occupied structures for all 

populations who live and work in areas 

where the subsurface environment may 

create exposures. If it is determined that 

releases of hazardous substances pose 

immediate threats to public health and 

the environment, EPA fully expects that 

the appropriate federal agency will 

continue to undertake response actions 

to address such threats. In fact, some 

federal agencies, including EPA, have 

developed or are developing new or 

updated agency-specific policy and 

guidance documents to address 

subsurface intrusion threats. 

This proposed addition will impact 

both resources and costs to federal 

cleanup programs. EPA does not expect 

that this proposed change will result in 

additional site assessments being 

conducted per year or placement of 

more sites on the NPL per year. Rather, 

given potentially limited budgets and 

the possibility of increased costs for a 

subsurface intrusion (SsI) site 

assessment, EPA may conduct fewer 

assessments per year. The pipeline of 

sites will be reviewed to identify those 

sites that pose the highest risk and 

prioritized accordingly. This is not a 

change to how EPA currently evaluates 

and prioritizes sites for the NPL; EPA 

will simply have an additional 

mechanism to address sites that pose 

the greatest risk. Because assessing the 

worst sites first is a priority, EPA will 

continue to identify the sites posing the 

highest risk or potential risk and 

develop a strategy to assess those sites 

in a timely manner, while balancing 

their other site assessment needs. 

The proposed addition, which could 

lead to the inclusion of a site on the 

NPL, does not itself impose any costs on 

outside parties; it does not establish that 

EPA will necessarily undertake 

response actions, nor does it require any 

action by a private party or determine 

liability for site response costs. Costs are 

limited to screening relevant sites for 

subsurface intrusion contamination 

during site inspections and the resulting 

HRS evaluation and documentation 

record preparation. Costs that arise from 

site remedial responses are the result of 

site-specific decisions made post-listing, 

not directly from the act of listing itself. 

Later decisions that consider 

information collected under the 

proposed addition could separately 

have specific economic costs and 

benefits (e.g., remediation costs and 

reduced risk), but these impacts are 

contingent upon a series of separate and 

sequential actions after listing a site on 

the NPL. The addition of subsurface 

intrusion to the HRS is several 

regulatory steps removed from imposing 

costs on private entities. 

The HRS addition may increase the 

costs to government agencies 

conducting assessments at subsurface 

intrusion sites because the scope of a 

typical site inspection may need to be 

expanded or may require more 

expensive sampling to collect 

information for an SsI evaluation. SsI 

sampling may require additional 

sampling and different sample types 

than those collected at other sites. This 

may result in an increase in some site 

assessment costs at some sites with 

possible subsurface intrusion issues. 

However, SsI site assessment costs at 

some other sites may be comparable to, 

or even less than, sites scored under the 

existing HRS. For example, a site 

assessment requiring sampling of deep 

ground water monitoring wells under 

the existing HRS may cost as much as, 

or more, than sampling conducted at 

sites with possible subsurface intrusion 

issues. The exact cost of any sampling 

at a site, including sites with possible 

SsI issues, varies greatly based on sitespecific factors (e.g., number and type of 

samples required, difficulty in 

establishing sources of contamination or 

attribution of releases, number of HRS 

pathways being evaluated, and 

availability of data from previous 

sampling events). Additionally, any 

newly increased costs to government 

agencies conducting assessments at SsI 

sites are expected to be minimal because 

federal agencies should already be 

identifying and addressing subsurface 

intrusion as part of their environmental 

programs. Any increase in the cost of 

site assessments conducted by EPA for 

SsI sites will require EPA to realign and 

prioritize its site assessment budget to 

address sites with subsurface intrusion. 

The addition of an SsI component to the 

HRS is not expected to result in 

additional site assessment funding to 

account for any increase in site 

assessment costs. Instead, the pipeline 

of sites will continue to be reviewed 

under the current site assessment 

process. If it is found that SsIcontaminated sites potentially pose a 

greater risk than other sites, then these 

sites will be prioritized over other sites. 

EPA will develop a strategy to assess 

these sites in a timely manner, while 

balancing other site assessment needs. 

2. Children’s Environmental Health and 

Environmental Justice 

This rulemaking is not subject to 

Executive Order 13045, Protection of 

Children from Environmental Health 

Risks and Safety Risks because this 

rulemaking is expected to only have 

moderate costs 6 and this executive 

order only applies to significant 

rulemakings. EPA has also found that 

this rulemaking will have no direct 

impact on communities considered 

under Executive Order 12898, Federal 

Actions to Address Environmental 

Justice in Minority Populations and 

Low-Income Populations. 

Although the rule will not have any 

direct impact on human health or risk 

within minority or low-income 

populations located near potential SsI 

sites, populations of concern under 

Executive Order 12898, EPA did 

consider whether the proposed action 

might have contingent impacts on these 

communities if future actions affect 

remediation of these sites. This analysis 

concluded that potentially affected sites 

are located in areas that have slightly 

higher concentrations of minority 

populations and populations below the 

poverty line than surrounding areas. 

Therefore, any future actions addressing 

risks in these communities would not 

contribute to disproportionate adverse 

impacts on human health. 

IV. Hazard Ranking System 

A. Purpose 

The current HRS serves as a screening 

tool to evaluate the potential for 

uncontrolled hazardous substances to 

cause human health problems or 

environmental damage at one site 

relative to other sites evaluated. The 

pre-remedial portion of the Superfund 


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program—the portion prior to placing 

sites on the NPL—is intended to 

identify those sites which warrant 

further investigation and possible 

cleanup under CERCLA. (See Figure 1 

for a general depiction of the Superfund 

Site Assessment process.) During PreCERCLA screening, which is the first 

step of the pre-remedial process, EPA 

determines if there is indication of a 

possible significant release. If so, EPA 

determines if a substance in the release 

is regulated by CERCLA, whether it is 

already being addressed, and whether 

any statutorily mandated limitations on 

CERCLA response may exist. If EPA 

determines the release meets these 

requirements, then the suspected release 

is listed in EPA’s Superfund Enterprise 

Management System (SEMS). 

Determining whether hazardous 

substances, pollutants, or contaminants 

can be addressed by CERCLA requires 

the application of site-specific facts to 

CERCLA statutory requirements and 

EPA policy. One such statutory 

requirement is CERCLA’s limit on 

response actions to some naturally 

occurring substances. CERCLA 

expressly limits any response actions 

taken in response to a release, or threat 

of release, of a naturally occurring 

substance in its unaltered form from a 

location where it is naturally found, 

from products which are part of a 

structure, or into drinking water 

supplies due to deterioration of the 

system. (See CERCLA section 104(a)(3) 

and 104(a)(4) for additional guidance on 

limitations on response and exception 

to limitations). Therefore, even though a 

naturally occurring substance in its 

unaltered form may potentially be 

regulated by CERCLA, the response 

actions taken in response to these 

releases, or threat of releases, may be 

expressly limited by CERCLA. For 

example, although radon and asbestos 

may qualify as a CERCLA hazardous 

substance, CERCLA section 104(a)(3) 

may limit responses to releases of radon 

or asbestos in some situations where the 

release is from building products or 

occurs from in situ natural sources, but 

section 104(a)(4) identifies specific 

circumstances that, if present, would 

allow CERCLA response in such 

situations. (See also EPA OSWER 

Directive 9360.3–12, Response Actions 

at Sites with Contamination Inside 

Buildings, August 12, 1993). If EPA 

finds an eligible release of a CERCLA 

eligible substance and response actions 

are permissible under CERCLA, then 

EPA proceeds to address the release 

under CERCLA. This may include a 

preliminary assessment. 

A preliminary assessment uses readily 

available data to determine if there is 

evidence of an unacceptable potential 

threat. If based on the results of a 

preliminary assessment, EPA 

determines that a site warrants further 

screening under the CERCLA remedial 

program, the agency initiates a site 

inspection as specified in the NCP (40 

CFR 300.420). The site inspection 

usually includes the collection of 

samples for chemical analysis. Such 

samples aid in ascertaining what 

substances are present at the site and 

whether they are being released. The 

purpose of the site inspection is to 

determine if there is an actual or 

potential threat to human health or the 

environment, to determine if there is an 

immediate threat to people or the 

environment in the area, and to collect 

sufficient data to enable the site to be 

scored using the HRS. 


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EPA has designed the Superfund 

program to focus its resources on sites 

that warrant further investigation. 

Consequently, the initial studies, the 

preliminary assessment and site 

inspection, which are performed on a 

large number of sites, are relatively 

modest in scope and cost compared to 

the remedial investigations and 

feasibility studies subsequently 

performed on NPL sites. Because of the 

need to carry out the initial studies 

expeditiously, EPA elected to place 

certain constraints on the data 

requirement for an HRS evaluation. The 

required HRS data should be 

information that, for most sites, can be 

collected during a screening level site 

inspection or that are already available. 

Thus, the HRS does not rely on data that 

require extensive sampling or repeated 

sampling over a long period of time. The 

HRS has also been designed so that it 

can be applied consistently to a wide 

variety of sites. The HRS is not a tool 

for conducting quantitative risk 

assessment and was designed to be a 

measure of relative risk among sites 

rather than absolute risk at an 

individual site. 

The narrow technical modifications 

being proposed reflect the agency’s 

actions to encompass additional risks 

posed by releases of hazardous 

substances and to address the SARA 

statutory requirement that EPA amend 

the HRS to assure ‘‘to the maximum 

extent feasible, that the HRS accurately 

assesses the relative degree of risk to 

human health and the environment 

posed by sites subject to review.’’ Thus, 

the fundamental purpose and structure 

of the HRS approach will not be 

changed when the HRS is amended to 

include consideration of subsurface 

intrusion. 

B. Structure 

The current HRS (40 CFR 300, 

Appendix A) evaluates four pathways in 

projecting the relative threat a site 

poses: 

• The ground water migration 

pathway evaluates the likelihood that 

hazardous substances will migrate to 

ground water and contaminate aquifers 

and drinking water wells that draw on 

those aquifers. 

• The surface water migration 

pathway evaluates the likelihood that 

hazardous substances can enter surface 

water and affect people or the 

environment. Threats to human health 

and the environment included in this 

pathway include drinking water (DW), 

the human food chain (HFC) (i.e., 

hazardous substances accumulate in the 

aquatic organisms that humans in turn 

consume), and sensitive environments 

(ENV). The surface water migration 

pathway is also divided into two 

‘‘components’’ reflecting different 

mechanisms for contaminant transport 

within each component (i.e., overland/ 

flood migration to surface water 

component and ground water to surface 

water migration component). 

• The air migration pathway 

evaluates the likelihood of release of 

hazardous substances into the 

atmosphere and the number of people 

and sensitive environments actually or 

potentially exposed to hazardous 

substances carried in the ambient 

(outdoor) air, including gases and 

particulates. The air migration pathway 

does not evaluate releases to indoor air 

originating from the subsurface. 

• The soil exposure pathway 

evaluates the potential threats to 

humans and terrestrial environments 

posed by direct, physical contact with, 

and subsequent ingestion of, hazardous 

substances. This pathway includes 

threats to people living on property 

where hazardous substances are present 

in the surface/subsurface, including 

contaminated soils (resident population 

threat), and to people living nearby with 

access to the contaminated area (nearby 

population threat). 

Figure 2 illustrates the general 

structure of the current HRS. 


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The scoring system for each pathway 

is based on a number of individual 

factors associated with risk-related 

conditions at the site. These factors are 

grouped into three factor categories as 

discussed below. These categories 

include factors that are used to 

characterize the relative risk at the site. 

1. Likelihood of release/exposure 

(i.e., likelihood that hazardous 

substances have been released or 

potentially could be released from a 

source into the environment, or that 

people or sensitive environments could 

come into contact with hazardous 

substances). 

2. Waste characteristics (i.e., toxicity, 

mobility, and/or persistence of the 

substances in the environment and the 

quantity of the hazardous substances 

that have or could be released). 

3. Targets (i.e., people or sensitive 

environments actually or potentially 

exposed to the release). 

An HRS score is determined for a site 

by summing the score for the four 

pathways. Specifically, the score for 

each pathway is obtained by evaluating 

a set of factors that characterize the 

potential of the release to cause harm 

via that pathway. The factors, which 

represent toxicity of the hazardous 

substance, or substances, at a site, waste 

quantity, and population are multiplied 

by a weighting factor, yielding the factor 

value; the factor values are used to 

assign factor category values. The factor 

category values are then multiplied 

together to develop a score for the 

pathway being evaluated. Finally, the 

pathway scores are combined according 

to the root-mean-square equation 

presented below to determine the HRS 

score for the site. See also Table 2–1 of 

the proposed addition (section 2.1.2) for 

additional discussion regarding the 

method for calculating an HRS site 

score. 

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. 


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7EPA’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. 

Finally, it should also be emphasized 

that the HRS score does not represent a 

specific level of risk at a site. Rather, the 

score serves as a screening-level 

indicator of the relative risk among sites 

reflecting the hazardous substance 

releases or potential releases at sites 

based on the criteria identified in 

CERCLA. 

V. Approach to HRS Addition 

The following sections detail EPA’s 

comprehensive approach to the 

consideration of exposures to hazardous 

substances due to subsurface intrusion 

and the relevant scientific and technical 

considerations in developing this 

proposed rule. 

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 for 

further investigation and will enhance 

EPA’s ability, in dialogue with other 

federal agencies and the states and 

tribes, to determine the most 

appropriate state or federal authority to 

address sites. As is currently the case, 

EPA often defers to other state and 

federal cleanup authorities based on the 

site assessments and HRS evaluations. 

While some states/tribes have programs 

to address subsurface intrusion 

contamination, they often have limited 

authority and resources, and variable 

remediation criteria. The availability of 

the federal remedial authority and the 

more comprehensive site assessment 

program should complement and 

strengthen these programs. 

Other EPA programs such as the 

Resource Conservation and Recovery 

Act (RCRA) and the Brownfields 

program have limited authority and 

ability to address all subsurface 

intrusion threats. The RCRA Corrective 

Action/Enforcement is only applicable 

at sites subject to RCRA permitting or 

sites reachable by RCRA’s enforcement 

activities. Furthermore, RCRA is a state 

delegated program and not all states 

recognize subsurface intrusion as a 

significant issue, and those that do may 

have variable remediation criteria. 

RCRA sites with subsurface intrusion 

issues may not be addressed in all 

states. Also, governmental entities with 

site-specific Brownfields assessment 

and/or revolving loan fund cleanup may 

only use grant funds on the selected 

eligible property. While subsurface 

intrusion sites may be eligible for 

Brownfields cleanup grants, site or 

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 cleanups.7

EPA’s removal program has the ability 

to quickly respond to immediate threats 

to public health and the environment 

from the release of hazardous 

substances, such as subsurface intrusion 

into a structure through a removal 

action. A removal action can be 

implemented regardless of NPL status to 

eliminate or reduce the threat of a 

release, or a potential release, of 

hazardous substances, pollutants or 

contaminants that pose an imminent 

and substantial danger to public health. 

However, removal actions are not 

intended to necessarily serve as a 

method for dealing with long term 

issues such as ground water 

contamination. Generally, EPA 

considers vapor intrusion mitigation 

systems as ‘‘interim’’ or ‘‘early’’ 

response actions to promptly reduce 

threats to human health. Installation of 

vapor intrusion mitigation systems 

addresses temporary human health 

problems, but fails to address the source 

of the problem. 

The NCP expresses the preference for 

response actions that eliminate or 

substantially reduce the level of 

contamination in the source medium to 

acceptable levels, thereby achieving a 

permanent remedy. U.S. EPA, OSWER 

Technical Guide for Assessing and 

Mitigating the Vapor Intrusion Pathway 

from Subsurface Vapor Sources to 

Indoor Air, OSWER Publication 9200.2– 

154, June 2015. OSWER’s VI guidance 

states: 

The preferred long-term response to the 

intrusion of vapors into buildings is to 

eliminate or substantially reduce the level of 

contamination in the subsurface vapor source 

(e.g., groundwater, subsurface soil, sewer 

lines) by vapor-forming chemicals to 

acceptable-risk levels, thereby achieving a 

permanent remedy. Remediation of the 

groundwater plume or a source of vaporforming chemicals in the vadose zone will 

eventually eliminate potential exposure 

pathways and can include the following 

actions, among others: removal of 

contaminated soil via excavation; removal of 

contaminated groundwater with pump-andtreat approaches; decontaminating and/or 

rehabilitating sewer lines that harbor vaporforming chemicals; and, treatment of 

contaminated soil and groundwater in situ, 

using technologies such as soil vapor 

extraction, multiphase extraction, and 

bioremediation, or natural attenuation. 

In the case of vapor intrusion 

resulting from a subsurface contaminant 

plume, failing to address the source of 

contamination and the resulting plume 

may result in an increased exposure to 

individuals due to migration and 

expansion of the plume over time. In 

this instance, individuals in regularly 

occupied structures that were 

previously unaffected by the plume may 

become negatively impacted by 

subsurface intrusion. Additionally, a 

subsurface contaminant plume in a 

lesser-developed area has the potential 

to impact future development if left 

untreated. 

There are several other concerns 

related to only addressing subsurface 

intrusion problems with a vapor 

mitigation system. The first concern is 

that vapor mitigation systems require 

ongoing monitoring and maintenance 

throughout the life of the system. 

Periodic inspections of the vapor 

mitigation system are necessary to make 

sure it is operating as designed. Over 

time the system can degrade, and 

maintenance will also be necessary, 

such as replacing the fan in an active 

sub-slab depressurization system. Nonmechanical failures of the system can 

occur as well, such as, electric power 

failure, turning off the fan or ignoring a 

damaged system. 

A vapor intrusion mitigation system is 

a tool for protecting human health, but 

may not contribute to the Superfund 

program’s goal of cleaning up 

uncontrolled hazardous waste sites. 

Furthermore, EPA still lacks a 

mechanism to assess human health 

hazards from vapor intrusion in the 

current HRS model, and therefore 

cannot currently evaluate the threat of 

vapor intrusion as part of its ranking of 

sites for placement on the NPL. 

Under the Superfund remedial 

program for NPL sites, subsurface 

intrusion is only addressed at sites 

placed on the NPL based on threats from 

other pathways. That is, subsurface 

intrusion issues are addressed later in 

the remedial process after placement on 

the NPL. For example, this may be done 

as part of EPA’s five-year review 

process. Sites with only subsurface 

intrusion issues are not being included 

on the NPL due to the lack of a 

subsurface intrusion component in the 

HRS. Therefore, many sites, especially 

those not evaluated under another HRS 

pathway or those not scoring high 

enough under another HRS pathway, 


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may not be addressed for threats due to 

subsurface intrusion because they may 

not qualify for placement on the NPL. 

As the Government Accountability 

Office (GAO) states in its May 2010 

report: 

EPA may not be listing some sites that pose 

health risks that are serious enough that the 

sites should be considered for inclusion on 

the NPL. While EPA is assessing vapor 

intrusion contamination at listed NPL sites, 

EPA does not assess the relative risks posed 

by vapor intrusion when deciding which 

sites to include on the NPL. By not including 

these risks, states may be left to remediate 

those sites without federal assistance, and 

given states’ constrained budgets, some states 

may not have the ability to clean up these 

sites on their own . . . However, if these 

sites are not assessed and, if needed, listed 

on the NPL, some seriously contaminated 

hazardous waste sites with unacceptable 

human exposure may not otherwise be 

cleaned up. 

EPA proposes the addition of the 

subsurface component to ensure the 

HRS does not omit this known pathway 

of human exposure to contamination 

and provides a mechanism for complete 

assessment of SsI threats to human 

health and the environment. 

2. What alternative regulatory options to 

this action were considered by EPA? 

EPA considered alternatives to this 

proposed regulatory action for 

addressing the need to evaluate 

subsurface intrusion threats as 

discussed below. 

Specifically, EPA considered whether 

existing programs adequately address 

the risks associated with subsurface 

intrusion at contaminated sites, as 

discussed in the previous section. If one 

or more programs were in place to 

adequately address concerns from 

subsurface intrusion, this could obviate 

the need for EPA action. However, no 

other authority consistently and 

comprehensively addresses subsurface 

intrusion across all potential nonfederal 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 of 

developing guidance. In addition, the 

Association of State and Territorial 

Solid Waste Management Officials 

(ASTSWMO) has expressed support for 

the proposed rule and has requested 

that EPA take leadership on this issue. 

Since vapor intrusion is projected to be 

the most significant component of 

subsurface intrusion, these responses 

would apply to subsurface intrusion as 

well. As previously discussed in section 

V.A.1 of this preamble, other federal 

programs were reviewed; while some 

programs could address subsurface 

intrusion at some sites, they cannot 

comprehensively address all sites 

(federal and non-federal). 

Two other mechanisms currently exist 

to place sites on the NPL. First, each 

state can designate a single site to the 

NPL as a state top priority site 

regardless of its HRS score; this can be 

done only once. (see NCP, 40 CFR 

300.425(c)(2)). This state-designated 

sites option has been implemented for 

44 states/territories, and the remaining 

state options would not be sufficient to 

address the subsurface intrusion issue 

nationally and comprehensively, given 

the projected number of sites with 

subsurface intrusion problems. Second, 

sites may be added in response to a 

health advisory from the ATSDR. (See 

NCP, 40 CFR 300.425(c)(3)). However, 

the ATSDR mechanism was designed to 

be used only when the Agency for Toxic 

Substances and Disease Registry 

(ATSDR) designated the threat found to 

warrant immediate dissociation from 

the release and other criteria are met. 

This is not a mechanism that can be 

used uniformly and consistently. It is 

highly resource intensive and may not 

comprehensively address all chronic 

threats. 

Furthermore, CERCLA section 105 

clearly mandates that EPA implement 

the HRS to take into account ‘‘to the 

extent possible the population at risk, 

the hazard potential of hazardous 

substances . . . , the potential for 

contamination of drinking water 

supplies and the potential for direct 

human contact.’’ When the HRS was last 

revised in 1990, the technology to detect 

and evaluate subsurface intrusion 

threats was not sufficiently developed. 

For example, there were no health-based 

benchmark concentration values for 

residences or standardized technologies 

for sampling indoor air, precision of 

analytical equipment prior to 

computerization was limited, and 

associations between contaminated 

ground water and soil vapors were not 

well understood. However, it is now 

possible for subsurface intrusion threats 

to be evaluated comprehensively. 

Therefore, it is now appropriate, given 

the potential that subsurface intrusion 

presents for direct human contact, to 

add to the HRS the consideration of 

threats due to subsurface intrusion. 

3. What public outreach activities did 

EPA conduct? 

Before making the decision to issue 

this proposed rulemaking, EPA 

conducted outreach activities to 

determine interest and support from the 

public. Thus, on January 31, 2011, EPA 

published a ‘‘Notice of Opportunity for 

Public Input’’ (76 FR 5370, January 31, 

2011) soliciting stakeholder comment 

on whether to include a subsurface 

intrusion component in the HRS. 

Additionally, EPA sent letters to all 

federally recognized tribes, asking for 

their comments on the FR document. 

During the 75-day public comment 

period on this action, four listening 

sessions were held throughout the 

country (Arlington, VA; San Francisco, 

CA; Albuquerque, NM; and Edison, NJ). 

The comments made by a majority of 

speakers, including members of the 

public, at the listening sessions were 

supportive of the potential addition of a 

subsurface intrusion component into the 

HRS. Of the 43 written comments 

received during the public comment 

period, 35 were in support of adding a 

vapor intrusion component to the HRS, 

6 comments (generally from industry 

representatives) were opposed to this 

addition, and 2 comments were neutral. 

The comments 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 Web site, http://

www.epa.gov/superfund/vaporintrusion-and-superfund-program, 

providing background information on 


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why this addition to the HRS is being 

considered. 

4. What peer review process did EPA 

use? 

This proposed rule consists of narrow 

technical modifications and is an 

expansion of the current HRS, which 

was peer reviewed by the agency’s 

Science Advisory Board (SAB). The 

1988 SAB review was comprehensive 

and addressed the basic structure and 

concepts of the HRS. This proposed 

addition adheres to the basic structure 

and concepts of the current HRS, and 

thus, is consistent with the 

recommendations of the SAB. The 1988 

SAB report focused on the following 

issues: 

• The overall algorithm for the HRS; 

• The inclusion of exposure in the 

HRS; 

• How the HRS could be evaluated in 

the future; 

• Work that could be done to provide 

better documentation for the next 

revision of the HRS; 

• The types of toxicity the HRS 

should address and how it should do so; 

• Distances from an uncontrolled 

hazardous waste site that are relevant 

when considering air pollutants from 

sites; and 

• The feasibility of including waste 

concentration in the HRS and whether 

large volume waste sites had been 

treated differently than others in the 

HRS. 

The 1988 SAB report is available in the 

public docket for this proposed 

rulemaking. 

During development of this proposed 

HRS update, EPA determined that 

several subsurface intrusion-specific 

issues warranted external independent 

scientific peer review. As a result, EPA 

has identified elements that have 

undergone peer review including: 

• Consideration of potential for 

subsurface exposure (intrusion) into 

regularly occupied structures; 

• Determination of hazardous waste 

quantity for the subsurface intrusion 

component; 

• Population scoring; 

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

• Evaluation of target values for 

workers. 

The results of the 2011 peer review of 

the proposed addition are discussed in 

the Summary of Peer Review Comments 

and Suggested Responses on the 

Addition of a Subsurface Intrusion 

Component to the HRS, which is 

available in the public docket for this 

proposed rulemaking. This proposed 

addition reflects modifications made as 

a result of EPA’s peer review process. 

5. How did EPA select the approach for 

including the addition in the HRS? 

The following six concepts were used 

as the basis for evaluating possible 

approaches to the HRS addition and the 

selection of a preferred approach: 

1. Limit the proposed addition to the 

existing HRS structure to avoid 

confusion by minimizing the portions of 

the present HRS that would need to be 

revised. 

2. Utilize the existing HRS basic 

structure and scoring algorithm, and 

maintain the relative weighting of the 

different pathways. 

3. Base technical decisions on sound 

and proven science. 

4. Ensure the HRS acts as an effective 

screening tool and minimizes 

unnecessary resource expenditures, 

while also minimizing the erroneous 

inclusion or exclusion of sites for 

possible NPL placement. 

5. Assemble and utilize conceptual 

site models, case studies, and sensitivity 

analyses to test the model. 

6. Ensure that an HRS scoring 

evaluation of the soil exposure and 

subsurface intrusion pathway can be 

completed using the information and 

level of effort that are typical of a site 

inspection or expanded site inspection 

(ESI). 

In the process of developing the 

proposed rule, EPA identified multiple 

options that are consistent with the 

above concepts. Based on literature 

reviews and agency experience, EPA 

projected the range of conditions at 

which the proposed addition might be 

applied. Using the basic structure of the 

current HRS, EPA tested each option by 

simulating the scores for typical 

scenarios. Using the results of these 

studies, EPA selected the option that 

best met the above criteria. To verify 

that the selected option would provide 

comparable results at actual sites, EPA 

tested the scoring algorithm using 

existing subsurface intrusion data from 

actual sites. The results of these studies 

demonstrate that the proposed addition 

functioned as expected. See section 8.0 

of the Technical Support Document for 

this proposed addition (Proposal TSD) 

for supplemental information regarding 

EPA’s testing efforts. 

B. Technical Considerations to 

Maintaining the Current HRS Structure 

and Algorithm 

1. Maintaining the Current Ground 

Water, Surface Water, and Air Migration 

Pathways 

The current approach for scoring the 

ground water, surface water, and air 

migration pathways is not being altered 

by the proposed addition of a subsurface 

intrusion component. Therefore, EPA is 

not soliciting comments on these 

pathways and will not respond to 

comments that are submitted on these 

pathways. 

2. Addition of the New Component to 

the Soil Exposure Pathway 

EPA is proposing to add the 

subsurface intrusion threat to the 

present soil exposure pathway, which 

already considers direct exposure to 

receptors. This pathway is proposed to 

be restructured and renamed the soil 

exposure and subsurface intrusion 

pathway. The restructured pathway will 

retain unchanged the existing two soil 

exposure threats (resident population 

and nearby population) in the pathway 

as one component. The threat posed by 

subsurface intrusion is proposed to be 

added as a new component. 

The internal structure of the soil 

exposure component, including the two 

soil exposure threats within that 

component, remains unchanged. 

Therefore, EPA is not soliciting 

comments on the soil exposure 

component of the proposed soil 

exposure and subsurface intrusion 

pathway, nor will it respond to 

comments that are submitted on the soil 

exposure component. 

The soil exposure pathway was 

selected for modification because its 

structure already focuses on populations 

actually coming into or potentially 

coming into direct contact with 

hazardous substances. The present soil 

exposure pathway addresses direct 

contact with contamination outside of 

structures. The new subsurface 

intrusion component also addresses 

direct contact with contamination that 

has already been demonstrated to have 

entered into regularly occupied 

structures or where the contamination is 

present beneath the regularly occupied 

structures and is likely to enter into 

regularly occupied structures. See 

section VI.A of this preamble for further 

discussion. 

C. Supporting Materials 

The proposed addition to the HRS is 

discussed in the following primary 

documents: (1) The proposed rule, (2) 

this preamble, (3) the Proposal TSD 

(including all supporting appendices), 

(4) the regulatory impact analysis (RIA). 

The proposed rule identifies the 

proposed changes to the NCP and 

focuses on the specific mechanics of 

scoring sites with the new component. 

This preamble provides an overview of 

the proposed HRS addition, along with 

an explanation of any modifications and 

the supporting justification. The 

Proposal TSD contains a more detailed 


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explanation of the technical basis for the 

proposed additions to the HRS, along 

with descriptions of the options 

considered, analyses that were used to 

evaluate the performance of the new 

subsurface intrusion component, and 

technical literature that was used in the 

development of the addition. The 

Proposal TSD is available to help guide 

the evaluation of subsurface intrusion 

sites. The Proposal TSD follows the 

same general outline as the preamble, 

with one section describing the 

necessary narrow technical 

modifications that affect multiple 

pathways, and the remaining sections 

describing the addition of the 

subsurface intrusion component to the 

current soil exposure pathway. The 

Proposal TSD contains a description of 

the current HRS, the options 

considered, and the technical 

justifications for the option chosen. In 

addition, the Proposal TSD references 

other supporting documents that 

provide an even greater level of detail 

on the proposed additions. 

These four documents are available to 

the public in the Docket for this 

rulemaking. To facilitate public review, 

EPA has prepared an index to the 

proposed rule, the preamble to the 

proposed rule, and the Proposal TSD 

with detailed cross referencing of issues. 

This index is available in the public 

Docket. See the ADDRESSES section of 

this preamble for further information. 

VI. Discussion of the Proposed SsI 

Addition to the HRS 

This section first discusses why the 

evaluation of the relative risk posed by 

subsurface intrusion has been added as 

a component to the same HRS pathway 

as for soil exposure. It then discusses 

how the evaluation will be performed 

using a structure consistent with the 

other threats, components, and 

pathways in the HRS, but taking into 

account the unique parameters 

impacting the probability of exposure to 

subsurface intrusion. 

A. Addition Within a Restructured Soil 

Exposure Pathway 

EPA is proposing to add the 

evaluation of the relative risk posed by 

subsurface intrusion of hazardous 

substances into regularly occupied 

structures by restructuring the soil 

exposure pathway in the current HRS to 

include subsurface intrusion. As noted 

previously, no changes are being 

proposed for the other three pathways 

in the present HRS. The restructured 

soil exposure pathway is proposed to be 

renamed the soil exposure and 

subsurface intrusion pathway to reflect 

both components of the restructured 

pathway. See Figure 3 for a depiction of 

how the proposed addition fits into the 

HRS structure. 


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8 For references to a specific section of the 

proposed HRS addition, please refer to the 

regulatory text of the proposed rulemaking. 

The threat posed by subsurface 

intrusion is proposed to be added to the 

soil exposure pathway because both 

consider the relative risk posed by 

direct contact with existing 

contamination areas. As identified in 

the preamble to the 1988 Federal 

Register document proposing the 

current HRS (53 FR 51997–52000, 

December 23, 1988), the soil exposure 

pathway, proposed in 1988 to be named 

the ‘‘onsite exposure’’ pathway, was 

added to the HRS to address the threat 

posed by direct contact with existing 

contamination and focused on ingestion 

of contaminated soil. This is in contrast 

with the other existing HRS pathways, 

which evaluate the relative risk posed 

by actual or potential migration of 

contamination from an original release 

location (called a ‘‘source’’ in HRS 

terminology) via ground water, surface 

water, or ambient air to other locations 

where exposure may occur. Given that 

the relative risk posed by subsurface 

intrusion is also due to direct contact 

with contamination already present in, 

or likely to be intruding into, regularly 

occupied structures and no further 

migration away from the existing 

contamination areas need occur, EPA 

considers it appropriate to incorporate 

the subsurface intrusion threat in the 

same direct exposure pathway that 

includes the soil exposure relative risk. 

See section 6.0 of the 1988 Revised HRS 

Technical Support Document (1988 

Revised HRS TSD) for supplemental 

information (originally referred to as the 

onsite exposure pathway). 

The existing soil exposure pathway 

will be retained as one component of 

the restructured pathway, with the two 

threats within the present soil exposure 

pathway, resident and nearby 

populations, being retained as threats 

within the soil exposure component. 

The scoring of the soil exposure 

component will remain unaltered, but 

the score will be assigned as the soil 

exposure component score, not the 

pathway score. (See section 5.1 of the 

Proposed HRS Addition.) The proposed 

subsurface intrusion component will be 

added as a new component of the 

restructured soil exposure and 

subsurface intrusion pathway. As 

discussed in greater detail below, it will 

have the same basic structure, scoring, 

and weighting as other parts of the HRS. 

The score for the restructured 

pathway is based on a combination of 

the two component scores—soil 

exposure and subsurface intrusion. The 

soil exposure component score is added 

to the subsurface intrusion component 

score to determine the pathway score. 

The two component scores are proposed 

to be additive because the populations 

may be subjected to exposures via both 

routes: The soil exposure component 

reflects exposures to people when 

outside a structure and focuses on 

ingestion and the subsurface intrusion 

component reflects exposures inside a 

structure and focuses on inhalation. 

Hence, the addition of the two 

component scores reflects the potential 

cumulative risk of multiple exposure 

routes and is not double counting the 

relative risk. 

A maximum pathway score is not 

contingent on scoring both the soil 

exposure and subsurface intrusion 

components. It is possible for a site to 

have only one component evaluated and 

still reach the maximum pathway score. 

Because the scoring of the soil exposure 

component is not being altered, this 

component would contribute the same 

score to the overall site score absent the 

addition of subsurface intrusion. 

B. Subsurface Intrusion Component 

Addition 

The structure of the current HRS is 

basically the same for all individual 

pathways, components, and/or threats. 

This structure was first used in the 

original HRS (47 FR 31220, 1982) and 

was only slightly altered when the HRS 

was revised in 1990 (55 FR 51532, 

December 14, 1990) to fit 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 and the expected 

change in the concentration of 

hazardous substances during the 

movement from the location of the 

contamination to the receptors, 

• The expected dose to the receptor, 

and 

• The expected number and character 

of the receptors. 

The above considerations are addressed 

in three factor categories: likelihood of 

exposure (or release), waste 

characteristics, and targets. 

The following subsections describe 

the structure of the proposed subsurface 

intrusion component and how this 

structure is consistent conceptually 

with the existing structure of the other 

HRS pathways and components: (1) 

New definitions, (2) delineation of areas 

of subsurface intrusion, (3) likelihood of 

exposure, (4) waste characteristics, (5) 

targets, and (6) calculating and 

incorporating the subsurface intrusion 

component score into the HRS site 

score. For background on why this 

structure was selected by EPA and peer 

reviewed by the SAB, see section 2.0 of 

the 1988 Revised HRS TSD. 

1. New Definitions—See Section 1.1 of 

the Proposed HRS Addition 8

EPA is proposing that 14 new 

definitions be added to the HRS, section 

1.1, with additional modifications to 

existing definitions. EPA is adding these 

new definitions to aid the site evaluator 

in establishing the environmental 

boundaries that are being evaluated in 

this component (e.g., contamination in 

or above the surficial aquifer), in 

identifying factors unique to the 

subsurface intrusion component (e.g., 

channelized flow through which soil gas 

transports with no resistance), and to 

ensure consistent application of the 

HRS. 

2. Delineation of Areas of Subsurface 

Intrusion—See Section 5.2.0 of the 

Proposed HRS Addition 

EPA is proposing to include in the 

subsurface intrusion component 

evaluation two areas in which exposure 

due to subsurface intrusion 

contamination exists or is likely to exist: 

(1) Areas of observed exposure—areas in 

which contaminant intrusion into 

regularly occupied structures has been 

documented, and (2) areas of subsurface 

contamination—areas in which 

subsurface contamination underlying 

regularly occupied structures (such as in 

surficial ground water or soil vapor) has 

been documented, but at which either 

sampling of indoor air has not 

documented that subsurface 

contamination has entered a regularly 

occupied structure or no sampling of 

indoor air has been undertaken. See 

Figure 4 for an illustration of the two 

areas. Additionally, special 

considerations are given to buildings 

with multiple subunits and multiple 

levels (e.g., apartment buildings) when 

establishing areas of subsurface 

intrusion. For a more detailed 

discussion on the selection of these 

areas, see section 6.0 of the Proposal 

TSD. 


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a. Area of Observed Exposure (AOE)— 

See Section 5.2.0 of the Proposed HRS 

Addition 

EPA is proposing to identify an area 

(or areas) of observed exposure at a site 

based on the location of regularly 

occupied structures with documented 

contamination resulting from subsurface 

intrusion attributable to the site being 

evaluated. The area encompassed by 

such structures constitutes the area of 

observed exposure (AOE). Other 

regularly occupied structures within 

this encompassed area (or areas) will 

also be inferred to be in the AOE unless 

available information indicates 

otherwise. Populations occupying 

structures within the AOE are 

considered exposed to subsurface 

contamination for HRS scoring 

purposes, and thus, are included in the 

HRS evaluation. See section 6.0 of the 

Proposal TSD for further discussion on 

the delineation of an AOE and the 

rationale for the inclusion of this area in 

an HRS evaluation. 

b. Area of Subsurface Contamination 

(ASC)—See Section 5.2.0 of the 

Proposed HRS Addition 

EPA is proposing to also identify an 

area (or areas) of subsurface 

contamination as an area outside that of 

the AOE, but for which subsurface 

contamination has been documented at 

levels meeting observed release criteria 

(contamination at levels significantly 

above background and the significant 

increase can be attributed at least in part 

to the site). The contamination would be 

present either in surficial ground water 

samples, in subslab or semi-enclosed or 

enclosed crawl space samples, in 

subsurface soil samples, or in soil gas 

samples in the unsaturated zone. An 

ASC may also include regularly 

occupied structures where indoor air 

sampling has not documented that an 

observed exposure has occurred. (See 

current HRS section 2.3 for observed 

release criteria.) In addition, EPA is 

proposing to limit the delineation of an 

ASC based on the location of subsurface 

volatile hazardous substances. However, 

non-volatile hazardous substances may 

be used to establish an ASC if they have 

also been documented in an observed 

exposure. 

Populations in regularly occupied 

structures within an ASC are considered 

potentially contaminated, but are 

weighted less in the HRS evaluation 

than those populations in an AOE. The 

populations in an ASC are assigned a 

weighting value ranging from 0.1 to 0.9 

depending on such factors as the 

distance of subsurface contamination to 

a regularly occupied structure’s 

foundation and the sample media (see 

section 5.2.1.3.2.3 of the Proposed HRS 

Addition). The ASC is included in the 

HRS evaluation because there is 

currently contamination below regularly 

occupied structures in the ASC, and 

although a sampling event has not 

documented intrusion into these 

structures, based on previous studies, it 

is likely that intrusion has occurred or 

could occur when suitable climatic and 

lifestyle conditions were or are present. 

The populations in the ASC are 

weighted less to reflect the relatively 

lower demonstrated risk in the ASC in 

comparison to the AOE. See section 6.0 

of the Proposal TSD for further 

discussion on the delineation of an ASC 

and the rationale for inclusion of this 

area in an HRS evaluation. 

c. Other Area of Subsurface Intrusion 

Considered: Potential Migration Zone 

In the three current HRS migration 

pathways (ground water, surface water, 

and air migration pathways), a projected 

present and future migration distance 

called the target distance limit is 

assigned based on studies performed 

when the HRS was revised in 1988. 

Targets (receptors) within that distance 

are considered either actually or 

potentially exposed and the values 

assigned to these receptors are weighted 

based on the level of contamination, the 


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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 sitespecific data in all areas of future 

migration in the relatively short time 

period and with minimal resources 

available when data collection for an 

HRS evaluation is performed (i.e., 

during the site inspection). EPA’s 

review of existing models indicate that 

in most instances, to obtain acceptable 

projections, extensive site-specific data 

collection efforts and often multiple 

rounds of site investigations are 

required to develop an accurate model 

for projecting the future extent of vapor 

migration, especially in the unsaturated 

zone. As discussed in section 2.5 of the 

1988 TSD, the ‘‘. . . misapplication of 

a model or the use of incomplete data 

would, of course, result in less 

accuracy . . . [and] a very conservative 

model may also increase the frequency 

with which sites that do not pose 

significant risks are placed on the NPL.’’ 

Therefore, after thorough review of 

this option, the agency has chosen not 

to include the consideration of future 

subsurface contaminant migration in the 

proposed subsurface intrusion 

component. The possibility of placing 

sites on the NPL based on speculative 

projections with no demonstrated risk of 

actual exposure is too significant. The 

exclusion of this option in the proposed 

HRS addition does not directly prevent 

a site from being considered for listing 

on the NPL based on demonstrated 

intrusion, nor does it restrict future 

investigations from expanding the site 

boundaries or re-evaluating a site if 

further studies indicate that the extent 

of contamination at a site may have 

increased due to future migration. 

Please refer to section 6.0 of the 

Proposal TSD for supplemental 

information regarding consideration of a 

potential migration zone. 

3. Likelihood of Exposure—See Section 

5.2.1.1 of the Proposed HRS Addition 

A key factor considered in the HRS 

relative risk ranking is whether any 

exposure has occurred and if not, 

whether there is a probability that 

exposure could occur. This is termed 

the likelihood of exposure for the 

subsurface intrusion component. For 

purposes of an exposure assessment, not 

only must subsurface intrusion have 

occurred, but the structure must be 

regularly occupied. Consistent with 

other HRS pathways and components, 

likelihood of exposure is evaluated in 

two ways within the proposed 

subsurface intrusion component. The 

first step is to determine whether 

contamination has entered a regularly 

occupied structure; if this has occurred, 

‘‘observed exposure’’ is established. If 

an observed exposure can be 

demonstrated in at least one structure, 

the likelihood of exposure category 

value is assigned the highest possible 

score. If observed exposure has not been 

documented, the second step is to 

evaluate the ‘‘potential for exposure.’’ 

The potential for exposure factor is 

assigned a score lower than that given 

when an observed exposure has been 

documented. How to evaluate the 

likelihood of exposure is discussed 

below. See section 4.0 of the Proposal 

TSD for supplemental information 

regarding likelihood of exposure. 

a. Observed Exposure—See Section 

5.2.1.1.1 of the Proposed HRS Addition 

For HRS purposes, an observed 

exposure is established if it can be 

documented that a hazardous substance 

from the site being evaluated has moved 

through the subsurface and has entered 

at least one regularly occupied 

structure. When it can be documented 

that subsurface intrusion has occurred, 

the likelihood of exposure is assigned 

its maximum value. The HRS identifies 

for all the pathways a consistent 

approach for establishing observed 

exposure (or observed release in 

migration pathways) and is discussed in 

section 2.3 of the current HRS. Also, the 

requirements for establishing observed 

exposure (or observed releases) are 

equivalent to those used to establish 

releases throughout the HRS. See 

section 2.6 of the 1988 Revised HRS 

TSD for supplemental information. 

Consistent with the current HRS 

structure, EPA is proposing to establish 

observed exposure in the subsurface 

intrusion component by any of the 

following methods: 

i. Observed Exposure by Direct 

Observation—See Section 5.2.1.1.1 of 

the Proposed HRS Addition 

The identification of an observed 

exposure by direct observation can be 

based on a solid, liquid, or gaseous 

hazardous substance attributable to the 

site being observed or known to have 

entered a regularly occupied structure 

from the subsurface. This finding will 

generally require the observation that a 

solid, liquid, or gas is entering the 

structure, and can be documented from 

a sample of the material that shows the 

hazardous substance is present due to 

the release from the site being 

evaluated. For example, this type of 

direct exposure could be documented if: 

(1) Contaminated vapors are found in a 

sample from a sump open to the 

regularly occupied structure, and (2) the 

same hazardous substances are found in 

subsurface samples collected beneath 

the regularly occupied structure or 

otherwise can be demonstrated as 

having emanated from known 

contamination underlying the structure. 

Another example would be if chromium 

precipitate is found in basements 

subject to ground water flooding and it 

is known that a chromium contaminant 

plume is present, and its presence is not 

from indoor sources. In neither example 

would a significant increase above a 

background contaminant level be 

required. For exposures to intruded 

ground water, EPA is proposing 


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documented observed exposure by 

direct observation as the only method 

for establishing likelihood of exposure. 

Figure 5 below depicts an additional 

example of documenting observed 

exposure by direct observation through 

collection of a contaminated water 

sample taken from the sump of an 

occupied structure that is known to be 

subject to flooding. Other methods may 

also be used to establish direct 

observation depending on site-specific 

conditions. See section 4.0 of the 

Proposal TSD for further information. 

ii. Observed Exposure by Chemical 

Analysis—See Section 5.2.1.1.1 of the 

Proposed HRS Addition 

Observed exposure by ‘‘chemical 

analysis’’ is established by comparing 

hazardous substance concentrations in 

background and release samples that 

have been chemically analyzed. The 

concentration of one or more hazardous 

substance in one or more indoor air 

sample taken from a regularly occupied 

structure (termed the ‘‘release sample’’) 

is compared to the concentration at 

appropriate background locations and 

under appropriate background 

conditions. If the chemical analyses 

document a significant increase over 

background levels and if at least part of 

the significant increase can be shown to 

be attributable to a release from the site 

being evaluated, then observed exposure 

by chemical analysis has been 

documented. This option for 

establishing observed exposure differs 

from observed exposure by direct 

observation in that comparison of the 

hazardous substance concentration in a 

release sample to a background level is 

required. This method for establishing 

observed exposure by chemical analysis 

is outlined in detail below. 

Background levels for this situation, 

in some cases, may be determined by 

chemical analysis of samples from 

similar environments collected from 

outside the area impacted by the release, 

or releases, from the site being 

evaluated. While the appropriate sample 

locations to be used to establish this 

background level will vary based on 

site-specific conditions, an appropriate 

background level needs to account for 

both outdoor air concentrations and 

indoor air concentrations in structures 

of similar construction type (e.g., 

basement, slab-on-grade) within the 

vicinity. This is to ensure that the 

background level represents the 

concentration of a hazardous substance 

in the absence of the subsurface 

intrusion. In some cases it may be 

possible to use published studies on 

typical background concentrations in 

establishing an appropriate background 

level. See section 4.0 of the Proposal 

TSD for further discussion on 

background levels. 

The first step in determining if 

observed exposure by chemical analysis 

has occurred is to document that the 

magnitude of the difference between the 

background level concentration and the 

release sample concentration is 

sufficient to rule out the possibility that 

neither the difference nor the similarity 

is due to variation in site conditions; 

and to ensure the sampling and 

analytical procedures are precise and 

can be replicated. The magnitude of this 

‘‘significant increase’’ was established 

for all HRS pathways based on studies 

peer reviewed by the Science Advisory 

Board when the HRS was last revised in 

1990. See section 2.6 of the 1988 

Revised HRS TSD for supplemental 

information. 

A significant increase is generally 

identified to have occurred if the release 

sample hazardous substance 

concentration is above quantification 

limits and at least three times the 

background level, provided the 

background sample concentrations for 

the hazardous substance are found at or 

above appropriate detection limits. If 

the hazardous substance background 


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level is below the appropriate detection 

limit, any quantifiable level of the 

hazardous substance detected in the 

targeted structure is considered to have 

a concentration significantly above 

background. 

The second step in determining if 

observed exposure by chemical analysis 

has occurred is to document that at least 

part of the significant increase can be 

attributed to a release from the site 

being evaluated. This step is required 

for establishing observed releases or 

observed exposures in all HRS 

pathways. See section 2.3 of the current 

HRS and section 2.6 of the 1988 Revised 

HRS TSD for supplemental information. 

This step is conducted to ensure that the 

increase is due to the release being 

evaluated and not from other potential 

contaminant sources located in the 

vicinity. (See section 4.0 of the Proposal 

TSD for further discussion.) For the 

proposed subsurface intrusion 

component, establishing significant 

increase over background is particularly 

critical because many of the projected 

intrusion contaminants are solvents 

and, in particular, chlorinated solvents. 

Chlorinated solvents are commonly 

found in multiple household and 

commercial cleaning products and in 

various consumer goods found in 

regularly occupied structures. These 

products present a substantial challenge 

for discerning the contribution from the 

environmental release that is being 

evaluated. Therefore, it is critical that a 

significant increase in these hazardous 

substances be documented as coming 

from the subsurface and not simply 

emanating from these products. 

It is suggested that the evidence to 

support this determination include 

multiple lines of evidence, including 

determining outdoor air hazardous 

substance concentrations; finding the 

hazardous substance at the source 

facility, site, or release being 

investigated; and finding the hazardous 

substance in subsurface samples. (See 

section 4.0 of the Proposal TSD 

regarding lines of evidence.) In addition, 

actions should be taken to ensure that 

sources of the hazardous substances 

inside a structure (e.g., household 

chemicals) have been removed from the 

structure prior to sampling. Establishing 

attribution to the site in some situations, 

however, may be straightforward to 

document, such as when the hazardous 

substance is manmade, unique, and not 

used in consumer products and thus, 

there would be no need to follow all the 

steps identified above to establish 

attribution. EPA expects that future 

advancement in methods for 

establishing the source of indoor 

contamination will be helpful for 

drawing conclusions about attribution. 

In summary, if it is demonstrated that 

there is a significant increase in 

hazardous substance levels in a 

regularly occupied structure and it is 

demonstrated that the significant 

increase in the contamination is in part 

due to the release from subsurface 

intrusion being evaluated, then an 

observed release by chemical analysis 

has been established. 

b. Potential for Exposure—See Section 

5.2.1.1.2 of the Proposed HRS Addition 

When an observed exposure has not 

been established, EPA is proposing to 

evaluate the potential for exposure 

within structures located in an ASC 

using the subsurface intrusion 

component. Given that within an ASC, 

contamination has been demonstrated to 

be below or in the subsurface 

encompassing regularly occupied 

structures, it is probable that exposure 

to the intruding hazardous substance 

has occurred but that sampling has not 

been performed at the time the exposure 

took place. As explained in section 4.0 

of the Proposal TSD, the factors 

affecting when intrusion will occur and 

the rate of subsurface intrusion are 

extremely time-, site-, and climatespecific. Sampling may not have been 

performed in these structures for a 

number of reasons, or, even if performed 

during the limited time period (due to 

resource limitations, site inspections are 

conducted over a limited period of time, 

usually 1 to 2 days) of a site inspection, 

the sampling may have been conducted 

during conditions in which the 

subsurface intrusion was not occurring, 

or occurring at levels not detectable or 

differentiable from that in background 

sources of the hazardous substance. 

Therefore, it is important that the 

potential for exposure be included as a 

consideration when evaluating 

subsurface intrusion threats, especially 

when volatile substances are 

documented in the subsurface below 

regularly occupied structures. 

As also explained in section 4.0 of the 

Proposal TSD, EPA is proposing to 

evaluate the potential for exposure for 

the subsurface intrusion component 

using the same concept and framework 

used to estimate the potential to release 

in other pathways. (See section 2.3 of 

current HRS.) As depicted in Figure 6 

below, this involves predicting the 

probability of exposure in an area of 

subsurface contamination based on 

structural containment features of the 

regularly occupied structure and the 

route characteristics in the subsurface, 

including hazardous substance physical 

and chemical properties and physical 

subsurface properties that influence the 

probability that intrusion is occurring. 


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i. Structure Containment—See Section 

5.2.1.1.2.1 of the Proposed HRS 

Addition 

Containment within the current HRS 

is used to consider barriers that restrict 

the movement of hazardous substances. 

See the preamble to the 1988 Revised 

HRS (53 FR 51985, December 23, 1988) 

for supplemental information. For the 

proposed subsurface intrusion 

component, the containment features 

considered represent structural features 

that block the movement of hazardous 

substances so as to minimize or prevent 

indoor exposures resulting from 

subsurface intrusion into a regularly 

occupied structure. As is consistent 

with the current HRS, EPA is proposing 

containment factor values that range 

from zero to ten where a low 

containment factor value indicates a low 

chance for exposure. For example, in 

Table 5–12 of the proposed HRS 

addition, a structure with no visible 

open preferential pathways from the 

subsurface has a lower containment 

value than a structure with documented 

open preferential pathways because 

open preferential pathways (e.g., sumps, 

foundation cracks) represent a situation 

in which a greater probability for 

subsurface intrusion to occur is present. 

Populations in structures that show no 

possible SsI intrusion route are not 

evaluated in this new component. 

Supplemental information regarding 

containment and the factor values 

specified in Table 5–12 is provided in 

section 4.0 of the Proposal TSD. 

ii. Route Characteristics—See Section 

5.2.1.1.2 of the Proposed HRS Addition 

The HRS uses ‘‘route characteristics’’ 

to index the relative degree to which 

hazardous substances move into or have 

already moved into specific areas, such 

as from a source into ground water, or 

for the subsurface intrusion component 

into a regularly occupied structure (see 

the 1988 TSD and section 4.0 of the 

Proposal TSD for supplemental 

information). These characteristics 

represent the physical and chemical 

properties of the specific hazardous 

substances and the media in which they 

must have moved through or could 

move through. To determine which 

route characteristics are appropriate for 

evaluating potential exposure to 

subsurface hazardous substances, EPA 

examined the literature to identify the 

modeling methods that are currently 

used to estimate the levels of hazardous 

substance exposure. Numerous route 

characteristics and the relationship of 

these and site-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 sitespecific information could be collected 

during a site inspection: (1) Depth to 

contamination, (2) vertical migration, 

and (3) vapor migration potential. These 

three factors are described in the 

following sections. 

a. Depth to Contamination—See Section 

5.2.1.1.2.2 of the Proposed HRS 

Addition 

The depth to contamination factor 

represents the vertical distance between 

contamination (either in soil, soil gas, or 

surficial ground water) and the lowest 

horizontal point of an overlying 

regularly occupied structure (e.g., a 


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basement floor). This distance 

represents how far a hazardous 

substance would have to travel through 

the subsurface to intrude into that 

structure. Based on available data, the 

probability of exposure decreases as the 

depth to contamination increases. In 

addition, as part of EPA’s sensitivity 

analysis in developing route 

characteristics, at depths greater than 

150 feet it became increasingly unlikely 

that exposure would occur. This is 

reflected in Table 5–13 (section 

5.2.1.1.2.2 of the Proposed HRS 

Addition). EPA is proposing depth to 

contamination factor values ranging 

from zero to ten, where increasing depth 

results in a lower factor value. 

EPA is also proposing to give special 

consideration in two situations in which 

it is likely that exposure has occurred. 

One situation is when subsurface 

profiles may be impacted by 

channelized flow features, such as 

fractured bedrock or karst. The other 

situation is at locations where the 

contamination is measured directly 

below the structure (e.g., in subslab or 

enclosed/semi-enclosed crawl space 

samples). These features reflect a 

situation with a high probability of 

exposure to intruded hazardous 

substances because of limited resistance 

to migration of the substances into the 

structure. See section 4.0 of the Proposal 

TSD for supplemental information on 

how the depths to contamination were 

weighted when assigning the factor 

values to different distances. 

b. Vertical Migration—See Section 

5.2.1.1.2.3 of the Proposed HRS 

Addition 

The vertical migration factor 

considers the geologic makeup of 

materials between a regularly occupied 

structure and the hazardous substance 

plume and the rate at which substances 

are likely to have moved through the 

materials. EPA is proposing to index 

vertical migration based on two factors: 

Effective porosity (or equivalently, the 

permeability) of geologic materials and 

the thickness of the lowest porosity 

layer. 

Factor values for effective porosity (as 

it relates to permeability) of geologic 

materials range from one to four and are 

based solely on the typical range of 

porosity of subsurface materials (e.g., 

gravel, sand, silt and clay). These factor 

values are used in conjunction with the 

thickness of the lowest porosity layer 

(greater than 1 foot thickness) to 

establish a vertical migration factor 

value, ranging from one to fifteen. 

As part of the vertical migration 

factor, EPA identified soil moisture 

content to potentially be a significant 

route characteristic variable. Thus, to 

incorporate soil moisture in EPA’s 

assessment of potential for exposure, the 

agency used published ‘‘average soil 

moisture content’’ values for specific 

soil types. These averages were used to 

develop effective porosity/permeability 

factor values. See section 4.0 of the 

Proposal TSD for supplemental 

information. 

c. Vapor Migration Potential—See 

Section 5.2.1.1.2.4 of the Proposed HRS 

Addition 

The vapor migration potential factor 

is based on hazardous substancespecific 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 

Consistent with potential to release 

determinations in the HRS, the potential 

for exposure for this component is 

calculated by summing all route 

characteristic factor values and 

multiplying the sum by the containment 

factor value to determine a potential for 

exposure factor value. 

c. Calculation of the Likelihood of 

Exposure Factor Category Value—See 

Section 5.2.1.1.3 of the Proposed HRS 

Addition 

As in all HRS pathways and 

components, the likelihood of exposure 

factor category value is assigned based 

on the higher of the observed exposure 

(or release) value or the potential for 

exposure (or release) value. The 

maximum value assigned for the 

likelihood of exposure factor category is 

550 and is assigned if observed 

exposure is documented. If observed 

exposure is not documented, the value 

assigned when evaluating potential for 

exposure ranges between 0 and 500. 

This approach is consistent with the 

current HRS structure. See sections 2.2 

of the 1988 Revised HRS TSD for 

supplemental information regarding this 

approach. 

4. Waste Characteristics—See Section 

5.2.1.2 of the Proposed HRS Addition 

The waste characteristics factor 

category is based on factors that are 

related to the relative risk 

considerations included in the basic 

HRS structure: (1) The toxicity or other 

potential adverse effects to a receptor 

from the releases, (2) the potential to 

degrade in the subsurface prior to 

intruding into a regularly occupied 

structure, and (3) the expected 

magnitude and duration of the 

exposure. The factors considered in 

determining the waste characteristics 

factor category value are the toxicity of 

the hazardous substances, the ability of 

the hazardous substance to degrade, and 

an estimate of the quantity of the 

hazardous substances to which 

occupants could be exposed. Consistent 

with the soil exposure component, the 

assigned factor values are multiplied 

together to determine this category 

value for the subsurface intrusion 

component. (See sections 2.2 and 2.4 of 

the 1988 Revised HRS TSD for further 

discussion on the structure of this factor 

category and how it fits within the 

overall HRS structure.) How and why 

these factors are proposed to be 

included in this factor category is 

discussed below. 

a. Toxicity/Degradation—See Section 

5.2.1.2.1 of the Proposed HRS Addition 

The combined toxicity/degradation 

factor includes consideration of both the 

toxicity and the possibility for 

degradation of hazardous substances 

being evaluated for HRS purposes. 

The toxicity factor in the overall HRS 

structure reflects the toxicity of a 

hazardous substance associated with a 

release or exposure, and is assigned the 

same factor value for all the pathways 

and components in the current HRS. As 

in all HRS pathways and components, it 

is proposed to be assigned the same 

corresponding factor value as for other 

parts of the HRS. The rationale for the 

assignment of the factor value is 

discussed in the section 2.3 of the 1988 

Revised HRS TSD. This toxicity factor is 

based on the toxicity of the substances 

present at a site. In the HRS addition, a 

different factor value is proposed to be 

assigned to each hazardous substance 

that an occupant has been or is 

potentially exposed to. The factor value 

is driven by the magnitude of each 

hazardous substance’s acute and 

chronic toxicity to humans. The toxicity 

factor value is directly related to the 

concentration at which the hazardous 


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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 degradation factor represents the 

possibility for a substance to degrade in 

the subsurface prior to intruding into a 

regularly occupied structure. The 

potential of a substance to degrade has 

been identified as a significant factor in 

numerous studies evaluating the 

potential for intrusion by a vapor. The 

possibility that a substance may degrade 

is both a substance- and locationspecific evaluation that is influenced by 

factors such as molecular structure, 

makeup of the immediate subsurface 

geology, and the presence or absence of 

oxygen within intervening unsaturated 

soils. 

Because many of the site-specific 

characteristics impacting the rate of 

degradation are considered beyond the 

scope of a typical site investigation, EPA 

is proposing to evaluate degradation 

based on the substance being evaluated, 

the depth to contamination, and if 

appropriate environmental conditions 

are present to ensure that sufficient 

degradation will occur to diminish the 

threat. Based on EPA’s review of the 

current literature and research on this 

topic, the assigned degradation factor is 

limited to three possible factor values, 

two for substances that are readily 

degradable and the appropriate 

environmental factors are present, and 

one for when either of these parameters 

are not present. 

EPA seeks public input on the 

following question regarding the 

degradation factor: Is there a way to 

determine the presence and extent of 

biologically active soil at a site during 

a limited site investigation? If so, what 

soil characteristics should EPA consider 

to determine whether biologically active 

soil is documented to be present? 

EPA proposes the degradation factor 

also be based on the half-life of a 

substance, with the half-life being 

determined by biodegradation and 

hydrolysis rates. If this information is 

not available then a hazardous 

substance’s estimated half-life will be 

based on the substance’s chemical 

structure, unless available information 

indicates otherwise. Substances with 

relatively low structural complexity, 

such as petroleum and petroleum-like 

substances (having straight carbon chain 

or simple ring structures), have the 

greatest potential to degrade in the 

subsurface while halogenated and polyaromatic ringed substances (e.g., 

tetrachloroethylene, PCBs) are less 

likely to significantly degrade as result 

of subsurface microbial activity. 

If it has been documented that a 

hazardous substance has been found to 

have entered a regularly occupied 

structure, regardless of the substance or 

the site conditions, the degradation 

value is assigned to reflect the 

likelihood that the substance is not 

significantly degrading in the 

subsurface. Also, if the substance is a 

daughter, or degradation product, of a 

parent substance that is also present, 

then the degradation factor will reflect 

this relationship. Parent and daughter 

substances are assigned values to reflect 

that the daughter substance will be 

continuously created by degradation of 

the parent substance. See also section 

5.0 of the Proposal TSD for additional 

discussion regarding the inclusion of a 

degradation factor. 

The toxicity and degradation factors 

are multiplied together to assign a 

combined factor value. If multiple 

substances are present, the highest 

combined factor value is selected for use 

in determining the waste characteristics 

factor category value, as discussed 

below. 

b. Hazardous Waste Quantity—See 

Section 5.2.1.2.2 of the Proposed HRS 

Addition 

In the basic HRS structure used in all 

pathways and components, the 

hazardous waste quantity factor reflects 

the risk consideration related to the 

magnitude and duration of either the 

release for a migration pathway or the 

exposure for an exposure pathway. In 

other words, for an exposure pathway, 

the risk posed by a release of hazardous 

substances is directly related to the 

amount of hazardous substances to 

which receptors (targets) are exposed 

and the length of the exposure. 

As explained in the preamble to the 

1990 HRS and in the 1988 Revised HRS 

TSD, an estimate of the waste quantity 

associated with a site was the best 

surrogate for the amount of hazardous 

substances that receptors were exposed 

to and that the duration of the exposure 

was probably correlated to the 

magnitude of the exposure. In the 

current three migration pathways 

(ground water, surface water, air), the 

hazardous waste quantity factor reflects 

the total amount of hazardous 

substances in sources at the site to take 

into account not only where 

contamination has already migrated to, 

but also future migration of 

contamination to other locations. For 

the soil exposure pathway, however, the 

estimate does not include the total 

amount in or released from the site 

sources, but only the amount of 

hazardous substance in the top two feet 

of contaminated soils sources and in the 

surface portions of other source types in 

an area of observed contamination. (See 

section 5.0.1 of the current HRS.) 

EPA is proposing that since the 

subsurface intrusion component also 

focuses on exposure and not the amount 

of hazardous substances that might 

migrate to targets in the future, the 

waste quantity factor value for this 

component should also reflect only the 

amount of hazardous substances that 

people currently are exposed to, that is, 

the amount in regularly occupied 

structures. EPA is proposing a 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 is proposed that a minimum 

factor of 10 should apply the same as in 

other pathways and components. This 

minimum factor reflects that in a 

limited site inspection, it is likely that 

information on the actual waste quantity 

at a site may not be available and a 

lower value would likely underestimate 

the actual conditions. Furthermore, if 

any target is subject to Level I or II 

contaminant concentrations a minimum 

hazardous waste quantity factor value of 

100 could be assigned. 

EPA seeks public input on the 

following question regarding the 

calculation of hazardous waste quantity: 

How could EPA further take into 

account the differences in dilution and 


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air exchange rates in large industrial 

buildings as compared to smaller 

residential and commercial structures 

when calculating the hazardous waste 

quantity for the HRS SsI Addition? 

The component waste quantity is the 

sum of all the waste quantities for all 

the regularly occupied structures found 

in both the AOE and ASC. The 

component waste quantity factor value 

assigned is then based on the magnitude 

of this sum, subject to minimum values. 

See section 5.0 of the Proposal TSD for 

supplemental information regarding this 

topic. 

c. Calculation of the Waste 

Characteristics Factor Category Value— 

See Section 5.2.1.2.3 of the Proposed 

HRS Addition 

As in all HRS pathways and 

components, the waste characteristics 

category value is the product of the 

toxicity/degradation factor value (or the 

functional equivalent) and the 

hazardous waste quantity factor value, 

scaled so as to be weighted consistently 

in all pathways. Similar to the 

likelihood of exposure factor category, 

the waste characteristics factor category 

is subject to a maximum value to 

maintain the balance between factor 

categories. This approach is consistent 

with the current HRS structure. See 

sections 2.2 and 2.4 of the 1988 Revised 

HRS TSD for supplemental information 

regarding this approach. 

5. Targets—See section 5.2.1.3 of the 

Proposed HRS Addition 

The targets factor is based upon 

estimates of the expected dose to each 

receptor and the number and type of 

receptors present. In a human health 

risk assessment, it is critical to 

understand the nature and extent of 

exposure to individuals, populations, 

and resources. The relative risk 

assessment embodied within the current 

HRS uses the targets factor as an index 

of the nature and extent of exposure to 

individuals, populations, resources, if 

appropriate for the migration or 

exposure route being evaluated, 

sensitive environments. This will 

remain the same in the proposed HRS 

addition, except sensitive environments 

will not be considered an eligible target. 

a. Identification of Eligible Targets—See 

Section 5.2.1.3 of the Proposed HRS 

Addition. 

The target factors evaluated by all 

pathways under the current HRS 

include the following: 

• The most exposed individual (i.e., 

nearest well for ground water migration, 

nearest intake for drinking water threat, 

food chain individual for human food 

chain threat, resident individual for 

resident population threat, and nearest 

individual for nearby population threat 

and air migration), 

• Populations (including residents, 

workers, students, and those in 

daycare), 

• Resources (including economic and 

cultural uses of contaminated 

resources), 

• Sensitive environments (except for 

the ground water migration pathway). 

(Examples of sensitive environments 

include government designated 

protected areas (e.g., national wildlife 

refuge), wetlands and critical habitat 

known to be used by a State or 

Federally-designated threatened or 

endangered species.) 

See sections 2.5 and 5.1.3 of the 

current HRS for supplemental 

information on how eligible targets are 

identified. 

Given that the subsurface intrusion 

component is proposed to be included 

as an exposure component within the 

modified soil exposure and subsurface 

intrusion pathway, the agency is 

proposing to use the same target 

categories used in the current soil 

exposure pathway, including exposed 

individual, resident populations, 

workers, and resources. However, 

unlike the current soil exposure 

pathway, workers are proposed to be 

evaluated as exposed individuals and as 

part of the population within an area of 

subsurface contamination instead of 

being evaluated under a separate worker 

factor value. See section 5.2.0 and its 

subsections of the proposed HRS 

addition. Additionally, sensitive 

environments are not being considered 

as eligible targets because exposures 

related to subsurface intrusion are 

limited to indoor areas and it is unlikely 

that sensitive environments would be 

exposed. See section 5.2.1.3 of the 

proposed HRS addition. 

EPA seeks public input on the 

following question regarding subsurface 

source strength: The HRS SsI Addition 

considers source strength in delineating 

ASCs and AOEs, in scoring likelihood of 

exposure, in assigning waste quantity 

specifically when estimating hazardous 

constituent quantity and in weighting 

targets in an ASC. The HRS algorithm 

for all pathways incorporates the 

consideration of source strength in 

determining an HRS site score. Could 

EPA further take into account source 

strength in performing an HRS 

evaluation? 

b. Exposed Individual and Levels of 

Exposure—See Section 5.2.1.3.1 of the 

Proposed HRS Addition 

This section introduces the methods 

used to identify and establish the levels 

of contamination and benchmarks 

proposed to be used within the 

subsurface intrusion component. 

Additionally, the exposed individual 

factor is discussed, as well as how to 

apply a factor value based on the 

benchmarks and the resulting levels of 

exposure. 

i. Identifying Levels of Exposure and 

Benchmarks for Subsurface Intrusion— 

See Section 5.2.1.3.1 of the Proposed 

HRS Addition 

For all current HRS pathways, the 

magnitude of the values assigned to the 

individual and population factors 

depend on the concentration of the 

contamination to which the receptors 

(targets) are exposed. If receptors are 

exposed to hazardous substance levels 

that meet observed release criteria, they 

are identified as actually contaminated; 

however, if the receptors are not 

exposed to hazardous substances that 

meet the observed release criteria but 

are within the target area being 

evaluated, they may be considered 

potentially contaminated. Potential 

targets are evaluated because a typical 

site inspection may not identify the 

extent of contamination. A site 

inspection typically includes 1 to 3 days 

of sampling and investigation activities. 

These limited investigations may not 

adequately characterize the annual or 

longer term indoor exposure levels (see 

page 4 of the 1988 SAB report 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 


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on whether the hazardous substance 

concentrations are at or above identified 

health-based benchmarks. EPA is 

proposing to use the following 

benchmarks for the subsurface intrusion 

component: 

• Screening concentrations for cancer 

• Screening concentrations for 

noncancer toxicological responses 

Targets associated with an observed 

exposure by direct observation are only 

considered subject to Level II 

contamination in all parts of the HRS 

and EPA is proposing that this remains 

consistent in the subsurface intrusion 

component. Furthermore, because 

intrusion by contaminated ground water 

is documented by direct observation 

only, targets residing within a structure 

subject to intrusion by contaminated 

ground water are also proposed to be 

evaluated as Level II (see section 2.5 of 

the proposed HRS addition). 

The targets within an ASC are also 

further divided based on the type of 

sample (e.g., gas, soil, water) and the 

distance of the sample from the targets 

(e.g., the depth of the sample below the 

structure). Weighting factors ranging 

from 0.1 to 0.9 are then assigned 

accordingly as discussed below in 

section 5.c.ii. See also section III.H. of 

the preamble to the current HRS (55 FR 

51547, December 14, 1990) for 

supplemental information. 

ii. Exposed Individual—See Section 

5.2.1.3.1 of the Proposed HRS Addition 

The standard HRS approach for 

scoring targets includes a measure 

reflecting the maximum level of 

exposure to individuals. The evaluation 

of exposed individuals is proposed to 

include individuals living, attending 

school or day care, or working in a 

regularly occupied structure. The 

reasonably maximally exposed 

individuals are those individuals in the 

eligible target population that are 

expected to be exposed to the highest 

concentration of the hazardous 

substance in question for a significant 

time. See section V.C.9 of the preamble 

to the proposed 1988 HRS (53 FR 51978, 

December 28, 1988) for supplemental 

information. 

EPA is proposing to retain the basic 

scoring approach used throughout the 

current HRS for evaluating the exposed 

individual factor. As is consistent with 

all pathways, a value of 50 points is 

assigned if there is any individual 

exposed to Level I concentrations or 45 

points if there is any individual exposed 

to Level II concentrations. If there are no 

individuals exposed to Level I or Level 

II concentrations, but at least one 

individual is living, attending school or 

day care, or working in a regularly 

occupied structure within an ASC, EPA 

proposes to assign a value of 20. See 

section 2.5 of the current HRS for 

supplemental information as to how 

EPA addresses exposed individuals 

within the HRS structure. 

c. Population—See Section 5.2.1.3.2 of 

the Proposed HRS Addition 

The population factor is evaluated 

using media-specific, health-based 

benchmarks as discussed above. EPA 

proposes the population factor include 

all populations qualifying as exposed 

individuals, including residents, 

students, workers and those attending 

day care. However, workers are 

weighted slightly differently than other 

exposed individuals to reflect that a 

worker’s exposure is limited to the time 

present in a workplace. Additionally, as 

workers may be employed on a full-time 

or part-time basis, the number of 

workers present in a structure or 

subunit is proposed to be adjusted by an 

appropriate factor reflecting this 

difference in exposure durations. EPA is 

proposing to retain the current scoring 

methodology for weighting populations 

used throughout the HRS, with actual 

exposure more heavily weighted than 

those potentially exposed. The proposed 

subsurface intrusion component will 

evaluate populations based on the 

number of individuals located within an 

identified AOE (i.e., those populations 

exposed to Level I and Level II 

concentrations) and the number of 

individuals located within an ASC (i.e., 

potential contamination as determined 

based on subsurface sampling), which is 

further subdivided as described in 

subsection ii below. 

i. Weighting of Targets in the Area of 

Observed Exposure (AOE)—See 

Sections 5.2.1.3.2.1 and 5.2.1.3.2.2 of 

the Proposed HRS Addition 

EPA is proposing to establish an AOE 

based on documented contamination 

meeting observed exposure criteria 

(either by direct observation or chemical 

analysis). Consistent with the weighting 

of populations throughout the HRS (see 

section 2.5 of the current HRS), the 

proposed subsurface intrusion 

component will weight targets subject to 

Level I contaminant concentrations by a 

factor of 10 and weight targets subject to 

Level II contaminant concentrations by 

a factor of 1. As noted previously, 

eligible populations also include 

individuals working in regularly 

occupied structures. However, the 

number of workers present in a 

regularly occupied structure will be 

adjusted to reflect that their exposure is 

limited to the time they are in a 

workplace. Therefore, the number of 

full- and part-time workers in a 

structure or subunit will be identified 

and divided by an appropriate factor 

prior to being summed with the number 

of other individuals present. If 

information is unavailable to classify a 

worker as full- or part-time, that worker 

will be evaluated as full-time. 

For example, if a single residence 

occupied by a family of four was 

observed to be exposed to hazardous 

substance concentrations above a 

media-specific, health-based 

benchmark, the number of residents 

would be multiplied by 10 for a factor 

value of 40. However, if that same 

family was exposed to a hazardous 

substance and the hazardous substance 

concentration was below the applicable 

benchmark but met the criteria for 

observed exposure, the number of 

residents would be multiplied by 1 for 

a factor value of 4. To ensure the entire 

population within an AOE is included 

in the HRS evaluation, both Level I and 

Level II factor values are counted and 

summed together. 

Within the AOE, EPA is proposing to 

consider as actually contaminated those 

populations in regularly occupied 

structures for which observed exposures 

have not been established but the 

structures are surrounded by regularly 

occupied structures in which observed 

exposures have been identified, unless 

evidence indicates otherwise. This 

action is proposed because it is 

considered likely that if these structures 

were sampled during the correct 

conditions, observed exposures would 

be identified at levels similar to those in 

surrounding structures. Targets inferred 

to be exposed to this contamination will 

be weighted as Level II as there are no 

actual sample results to compare against 

benchmarks. However, EPA has 

included an exception to allow for 

situations where site-specific conditions 

clearly document that there may be no 

observed exposures in these structures. 

The rule language states that targets can 

be inferred to have observed exposures 

in these situations ‘‘unless available 

information indicates otherwise’’. This 

concept of inferred exposure is also 

included in the existing soil exposure 

pathway and in the air migration 

pathway. 

In the case of multi-story/multisubunit structures, all regularly 

occupied subunits on a level with an 

observed exposure and all levels below 

are considered to be within an AOE, 

unless available information indicates 

otherwise. For multi-story/multisubunit structures located within an 

AOE, but where an observed exposure 

has not been documented, only those 


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regularly occupied spaces on the lowest 

level are considered to be within an 

AOE, unless available information 

indicates otherwise. (See sections 5.0.1 

and 6.3 of the current HRS.) 

ii. Weighting of Targets in the Area of 

Subsurface Contamination (ASC)—See 

Section 5.2.1.3.2.3 of the Proposed HRS 

Addition 

EPA is proposing to establish an ASC 

as defined by documented ground 

water, subslab, soil, semi-enclosed or 

enclosed crawl space, or soil gas 

contamination meeting observed release 

criteria. These areas are included in the 

subsurface intrusion component due to 

the potential that limited sampling 

conducted during a site inspection may 

not identify that subsurface intrusion is 

occurring because of the high temporal 

and spatial variability associated with 

detecting subsurface intrusion. 

Temporal and spatial differences can 

significantly impact the rate at which 

volatile hazardous substances enter a 

structure. However, when an ASC has 

been defined, that area represents a 

location where subsurface hazardous 

substances have the potential to intrude 

into a structure. EPA is limited in its 

extent of preliminary screening 

activities, and a single indoor air 

sampling event is unlikely to identify 

the full threat posed by subsurface 

intrusion. 

As is consistent with the 1990 HRS, 

EPA is proposing to weight these 

potentially exposed targets at a value 

less than those targets that have been 

identified to be actually exposed. Due to 

the variability in subsurface intrusion 

rates, the potential weighting factor 

values for targets within an ASC range 

from 0.1 to 0.9 and depend on where the 

subsurface contamination has been 

found. Using EPA’s vapor intrusion 

attenuation factors published in 2012 

and basic subsurface contaminant 

transport concepts, EPA developed a 

relatively proportional weighting for 

potential targets based on the sampling 

media being considered. This range of 

weighting factors represents the 

proportional probability of a target to be 

exposed as a result of contaminant 

intrusion from the subsurface in a 

variety of likely sampling scenarios. The 

potential target weighting factors 

presented in the proposed addition do 

not directly correspond to attenuation 

factors in themselves. Instead, the 

relative weighting between these values 

is based on the published attenuation 

factors. These weighting factors are 

presented in this manner to project that 

contaminants found in a crawl space 

sample, for example, are more likely to 

attenuate less before entering into an 

overlying structure, and thus more 

likely to pose a threat, as opposed to 

those found in a shallow ground water 

sample. 

EPA is proposing that the weighting 

of potential targets also reflect the 

distance to or the depth at which 

contamination is found. For any 

contamination found at a horizontal or 

vertical distance of five feet or less from 

a regularly occupied structure’s 

foundation, EPA is proposing to assign 

a minimum weighting factor of 0.4 

regardless of the sample medium. 

Similarly, EPA is proposing to assign a 

weighing factor of 0.1 to any 

contamination found or inferred at 

depths greater than 30 feet regardless of 

sampling medium. These minimum 

weighting values are in response to an 

analysis of the data used in deriving 

published attenuation values. The 

attenuation values were published 

based on real-world sampling data 

collected from numerous sites across the 

United States. The majority of sampling 

data collected as part of this effort came 

from sites where contamination was 

generally found at depths less than 30 

feet. Therefore, EPA considers the 

attenuation factors and relative 

weightings between them to only be 

appropriate for shallower depths. The 

minimum value for the upper five feet 

allows consideration of sites where 

contamination is found at extremely 

shallow depths and therefore has a 

minimal vertical distance to travel 

before intruding into a regularly 

occupied structure. 

In the case of multi-story/multisubunit structures, all regularly 

occupied subunits on a level above one 

where an observed exposure has been 

documented or inferred, or where a 

gaseous indoor air sample meeting 

observed release criteria is present, are 

considered to be located within an ASC, 

unless available information indicates 

otherwise. For multi-story/multisubunit structures located only within 

an ASC, only those regularly occupied 

subunits within the lowest level are 

considered in an HRS evaluation. 

EPA proposes eligible populations 

include individuals living in, or 

attending school or day care in the 

structure, and workers in regularly 

occupied structures. The number of 

workers is adjusted to reflect that their 

exposure is limited to the time they are 

in a workplace. Therefore, the number 

of full- and part-time workers in a 

structure or subunit will be divided by 

an appropriate factor prior to being 

summed with the number of other 

individuals present. If information is 

unavailable to classify a worker as fullor part-time, that worker will be 

evaluated as full-time. 

The proposed weighting factors for 

exposed individuals in any structure 

within an ASC are based on the 

probability of contamination entering 

into occupied structures from the 

subsurface. The weighting factors reflect 

depth to contamination, sample type, 

and media. The magnitude of the factor 

is also based on attenuation factors from 

current scientific literature including 

EPA’s 2012 vapor intrusion attenuation 

factors publication. Additional 

information regarding this analysis is 

presented in section 6.0 of the Proposal 

TSD. 

d. Resources—See Section 5.2.1.3.3 of 

the Proposed HRS Addition 

The resources target factor is 

evaluated in all pathways under the 

current HRS. A factor value of five is 

assigned if at least one resource is 

present and a factor value of zero if no 

resource is present. Eligible resources 

are pathway-, component-, or threatspecific. These resources represent uses 

of a contaminated medium or area 

where exposures occur and are not 

covered by the other identified targets. 

For example, resources within the air 

migration pathway include commercial 

agriculture or silviculture and major/

designated recreation areas. The 

resident population threat also includes 

commercial livestock production or 

grazing. See section III.I of the preamble 

to the current HRS (55 FR 51549, 

December 14, 1990) for supplemental 

information. 

Because subsurface intrusion is 

limited to indoor spaces, EPA is 

proposing to include regularly occupied 

structures that are located within a 

defined AOE or ASC (as previously 

discussed in section VI.B.2 of this 

preamble) and in which populations, 

not including those already counted as 

exposed individuals, may be exposed to 

contamination due to subsurface 

intrusion. For example, libraries, 

recreational facilities, and religious or 

tribal structures used by individuals, 

may qualify as eligible resources. 

e. Calculation of the Targets Factor 

Category Value—See Section 5.2.1.3.4 of 

the Proposed HRS Addition 

As is done throughout the HRS, EPA 

is proposing to sum all of the target 

factor values together to establish a 

target factor category value in 

calculating the proposed subsurface 

intrusion component score. Unlike the 

likelihood of exposure and waste 

characteristics factor category values in 

all HRS pathways, which are subject to 

maximum values, the target factor 


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category is not limited in the current 

HRS. This is to ensure that all 

individuals, populations, resources, and 

sensitive environments are included; 

thereby, representing the full relative 

risk associated with the identified 

threat. It is also consistent with the 

direction of CERCLA section 105 to 

amend the HRS ‘‘to the maximum extent 

feasible’’ to address ‘‘the relative degree 

of risk to human health and the 

environment’’ by putting the emphasis 

on the number of receptors exposed to 

contamination. 

6. Calculation and Incorporation of the 

SsI Component Score Into the HRS Site 

Score 

The following subsections summarize 

the calculation of the subsurface 

intrusion component score, how the 

component score is then used in the 

calculation of the soil exposure and 

subsurface intrusion pathway score, and 

how, in turn, the pathway score is 

subsequently incorporated into the HRS 

site score. 

a. Calculation of the SsI Component 

Score—See Section 5.2.2 of the 

Proposed HRS Addition 

EPA is proposing to calculate the 

subsurface intrusion component score 

using the same algorithm as in other 

components and pathways of the HRS. 

(See section 2.2 of the 1988 Revised 

HRS TSD.) This involves multiplying 

the likelihood of exposure factor 

category value times the waste 

characteristics factor category value 

times the targets factor category value 

and dividing that value by a weighting 

factor so that it has equal magnitude to 

other component scores (subject to a 

maximum value). The values are 

multiplied to reflect that it is the 

product of these values that represents 

a relative risk level. 

In a relative risk (or in a site-specific 

risk) assessment, the use of the product 

of the factor category values is 

considered appropriate because the 

magnitude of each of the factor category 

values reflects the probability of 

exposure occurring: Likelihood of 

releases reflects the probability of 

exposure actually occurring, waste 

characteristics reflects the probable 

quantity and duration of the exposure, 

and targets reflect the probable number 

of receptors at risk. Thus, since each 

factor category value reflects a 

probability in a series of events, the 

overall probability associated with the 

series is the product of the individual 

probabilities. For example, if any factor 

category value is zero, such as when 

there are no targets exposed or 

potentially exposed to subsurface 

intrusion, the component score is zero, 

consistent with there being no risk due 

to subsurface intrusion. 

b. Incorporation of the SsI Component 

Score Into the Soil Exposure and 

Subsurface Intrusion Pathway Score— 

See Section 5.3 of the Proposed HRS 

Addition 

The score for this restructured 

pathway is proposed to be a 

combination of two component scores. 

The subsurface intrusion component 

score is added to the soil exposure 

component score (subject to a maximum 

value) to determine the pathway score. 

The two component scores are proposed 

to be additive because the populations 

may be subjected to exposures 

separately via both routes: The soil 

exposure component reflects exposures 

to people when outside a structure and 

focuses on ingestion, while the 

subsurface intrusion component reflects 

exposures to people when inside a 

structure and focuses on inhalation. 

Hence, the addition of the two 

component scores reflects the 

cumulative potential risk and is not 

double counting the relative risk. 

In addition, a pathway score can be 

assigned without scoring both the soil 

exposure and subsurface intrusion 

components using this approach. It is 

possible for a site to have only one 

component evaluated and still reach the 

same pathway score as under the 

current HRS. It should be observed that 

because the scoring of the soil exposure 

component is not being altered, the soil 

exposure component would contribute 

the same score to the overall site score 

as it would if the subsurface intrusion 

component is not added. 

c. Incorporation of the Soil Exposure 

and Subsurface Intrusion Pathway Score 

Into a Site Score—See Section 2.1.1 of 

the Proposed HRS Addition 

EPA is not proposing any changes to 

the methodology used to assign an 

overall site score due to the addition of 

the subsurface intrusion component to 

the soil exposure pathway and renaming 

that pathway the soil exposure and 

subsurface intrusion pathway. The 

overall site score remains a function of 

four pathway scores and the same 

weighting is given to each pathway 

score as in the current HRS. See section 

2.2 of the 1988 Revised HRS TSD for 

supplemental information on why the 

existing methodology was chosen. 

7. Example Site Scoring Scenarios 

To evaluate the proposed subsurface 

intrusion component and factor category 

weighting, EPA developed three 

conceptual site scenarios: One that 

would not qualify for the NPL (score 

below 28.50); one that would qualify 

marginally for the NPL (score of about 

28.50); and one that should clearly 

qualify for the NPL (site score 

considerably above 28.50). 

The first scenario consists of a ground 

water plume contaminated with a 

hazardous substance with moderate 

toxicity that underlies approximately 3 

acres of a residential neighborhood 

comprised of single-family detached 

homes. Indoor air samples have been 

collected from inside two homes and 

have reported hazardous substance 

concentrations above background, but 

below the applicable benchmarks. 

Additionally, several other occupied 

structures were sampled for indoor air 

and subslab contaminant 

concentrations; however, no other 

detections of hazardous substances were 

observed. This site would not qualify for 

the NPL based on available information 

(i.e., score below 28.50). 

The second scenario also consists of 

a ground water plume contaminated 

with a hazardous substance with 

moderate toxicity as in the first 

scenario, but it has a considerably larger 

plume and more targets. The ground 

water plume underlies approximately 

20 acres of a residential neighborhood 

and commercial area comprised of 

single-family detached homes, a daycare 

facility, and a single-story office 

building. Indoor air samples collected 

inside 19 homes, the daycare facility, 

and office building have hazardous 

substance concentrations above the 

applicable benchmark. Indoor air 

samples in 5 homes, the daycare facility 

with approximately 25 children 

enrolled and 6 full-time and 2 part-time 

workers, and the office building with 18 

full-time workers have hazardous 

substance concentrations above 

background, but below the applicable 

benchmark. The homes and daycare 

facility were checked for indoor sources 

of hazardous substances prior to 

sampling and such sources were 

removed if found. This site would likely 

qualify for the NPL based on available 

information (i.e., score of about 28.50). 

The third scenario consists of a 

ground water plume contaminated with 

a highly toxic hazardous substance and 

a larger number of targets than the 

second scenario. The plume underlies 

approximately 25 acres of a residential 

neighborhood and hazardous substance 

concentrations above a benchmark were 

detected in indoor air samples from 25 

homes and one daycare with 

approximately 25 children enrolled and 

5 full-time workers. Hazardous 

substance concentrations above 

background but below benchmarks were 


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detected within 15 homes. The homes 

and daycare facility were checked for 

indoor sources of hazardous substances 

prior to sampling and such sources were 

removed if found. Based on available 

information, this site would qualify for 

the NPL and would likely achieve the 

maximum HRS score for a single 

component and pathway (i.e., 50.00). 

Further evaluation of the varying 

factor values and resulting HRS site 

scores, along with further discussion of 

these three scenarios is presented in 

section 8.1.c of the Proposal TSD. 

VII. Summary of Proposed Updates to 

the HRS (Sections 2, 5, 6, and 7) 

A. Addition of an SsI Component to the 

HRS (Sections 2, 5, and 7) 

1. Chapter 5 

The proposed addition of a subsurface 

intrusion component is proposed to be 

added to the existing Soil Exposure 

pathway as section 5.2 in Chapter 5 to 

the current HRS. The new pathway 

name is proposed as the Soil Exposure 

and Subsurface Intrusion Pathway. The 

existing method for evaluating the soil 

exposure threat will remain unchanged. 

2. Chapter 2 

Evaluations Common to All Pathways 

is proposed to be updated to reflect the 

addition of the subsurface intrusion 

component to the existing soil exposure 

pathway. The evaluations for the 

current four pathways remain 

unchanged and a comparable evaluation 

will be added for the subsurface 

intrusion component. 

3. Chapter 7 

Sites Containing Radioactive 

Substances currently reflects how 

radioactive substances are evaluated in 

the context of the four current HRS 

pathways. Updates will be made to 

reflect how radioactive substances are 

evaluated using the proposed subsurface 

intrusion component. 

B. Terminology Updates Affecting 

Specific Sections of the HRS (Sections 2, 

5 and 6) 

During the development of this 

proposed addition to the HRS, the 

agency determined that the following 

terms should be updated to reflect 

current terminology and procedures 

used by EPA in performing risk 

assessments. 

1. Ambient Water Quality Criteria 

Ambient Water Quality Criteria 

(AWQC) are now identified also as 

National Recommended Water Quality 

Criteria (NRWQC). In addition, the acute 

AWQC are now identified as the 

Criterion Maximum Concentration 

(CMC) and the chronic criteria are 

referred to as the Criterion Continuous 

Concentration (CCC). (See section 1.1 of 

the proposed HRS addition.) These 

criteria are used to determine the level 

of threat to environmental targets. 

2. Reference Concentrations 

For inhalation exposures, EPA is 

adopting the use of Reference 

Concentrations (RfCs) instead of 

Reference Doses (RfDs) when 

determining non-cancer related risk 

levels. RfCs are used in determining the 

level of threat to human targets due to 

possible inhalation and when 

determining the toxicity of the 

substances. 

3. Cancer Unit Risk 

For inhalation exposures, EPA is 

adopting the use of Inhalation Unit Risk 

(IUR) instead of cancer slope factors in 

determining cancer-related risk levels. 

IURs are used in determining the level 

of threat to human targets due to 

possible inhalation and when 

determining the toxicity of the 

substances. 

4. Weight-of-Evidence Groupings 

The 2005 EPA weight-of-evidence 

groupings supporting the designation of 

a substance as a human carcinogen have 

been incorporated into the HRS 

algorithm for determining the toxicity 

factor value. (The former EPA weight-ofevidence categories included as part of 

the 1990 HRS have been retained as 

EPA has not yet completed assigning all 

substances to the revised categories and 

are doing so at the time the EPA 

substance literature reviews are 

updated.) 

VIII. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive Orders can be 

found at http://www2.epa.gov/lawsregulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

This action is a significant regulatory 

action that was submitted to the Office 

of Management and Budget (OMB) for 

review. This action may raise novel 

legal or policy issues arising out of legal 

mandates, the President’s priorities, or 

the principles set forth in the EO. Any 

changes made in response to OMB 

recommendations have been 

documented in the docket. 

EPA prepared an analysis of the 

potential costs and benefits associated 

with this action. This analysis, Addition 

of a Subsurface Intrusion (SsI) 

Component to the Hazard Ranking 

System (HRS): Regulatory Impact 

Analysis is available in the docket for 

this action. 

B. Paperwork Reduction Act (PRA) 

This action does not impose any new 

information collection burden under the 

PRA. OMB has previously approved the 

information collection activities 

contained in the existing regulations 

and has assigned OMB control number 

2050–0095. 

This proposed regulatory change will 

only affect how EPA and organizations 

performing work on behalf of EPA (state 

or tribal partners) conduct site 

assessments and HRS scoring at sites 

where certain environmental conditions 

exist. This proposed regulatory change 

will result in data collection at these 

types of sites to allow evaluation under 

the HRS. EPA expects that the total 

number of site assessments performed 

and the number of sites added to the 

NPL per year will not increase, but 

rather expects that there will be a 

realignment and reprioritization of its 

internal resources and state cooperative 

agreement funding. 

C. Regulatory Flexibility Act (RFA) 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the RFA. This action will not 

impose any requirements on small 

entities. This proposed regulatory 

change enables the HRS evaluation to 

directly consider human exposure to 

hazardous substances that enter 

building structures through subsurface 

intrusion. This addition to the HRS 

would not impose direct impacts on any 

other entities. For additional discussion 

on this subject see section 4.9 of the 

Regulatory Impact Analysis (see the 

docket for this action). 

D. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain any 

unfunded mandate as described in 

UMRA, 2 U.S.C. 1531–1538, and does 

not significantly or uniquely affect small 

governments. The action imposes no 

enforceable duty on any state, local, or 

tribal governments or the private sector. 

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 


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responsibilities among the various 

levels of government. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have tribal 

implications, as specified in Executive 

Order 13175. EPA’s evaluation of a site 

using the HRS does not impose any 

costs on a tribe (except those already in 

a cooperative agreement relationship 

with EPA). Thus, Executive Order 13175 

does not apply to this action. 

Although Executive Order 13175 does 

not apply to this action, EPA consulted 

with tribal officials through meetings 

and correspondence, including a letter 

sent to all federally recognized tribes 

asking for comment on the ‘‘Notice of 

Opportunity for Public Input’’ that was 

published in the Federal Register on 

January 31, 2011 (76 FR 5370), and 

public listening sessions regarding the 

decision to proceed with the 

development of this action. All tribal 

comments indicated support for this 

action. 

EPA specifically solicits additional 

comment on this proposed action from 

tribal officials. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

EPA interprets Executive Order 13045 

as applying only to those regulatory 

actions that concern environmental 

health or safety risks that EPA has 

reason to believe may 

disproportionately affect children, per 

the definition of ‘‘covered regulatory 

action’’ in section 2–202 of the 

Executive Order. This action is not 

subject to Executive Order 13045 

because it does not concern an 

environmental health risk or safety risk. 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The site assessment activities affected 

by this rule are limited in scope and 

number and rely on existing energy 

distribution systems. Further, we have 

concluded that this proposed rule 

would not significantly expand the 

energy demand for site assessments, and 

would not require an entity to conduct 

any action that would require 

significant energy use, that would 

significantly affect energy supply, 

distribution, or usage. Thus, Executive 

Order 13211 does not apply to this 

action. 

I. National Technology Transfer and 

Advancement Act 

This rulemaking does not involve 

technical standards. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

EPA believes the human health or 

environmental risk addressed by this 

action will not have potential 

disproportionately high and adverse 

human health or environmental effects 

on minority, low-income or indigenous 

populations. The results of this 

evaluation are contained in section 

III.C.4 of this preamble and section 4.3 

(and all subsections) and Appendix C of 

the Regulatory Impact Analysis for this 

proposed rulemaking. A copy of the 

Addition of a Subsurface Intrusion (SsI) 

Component to the Hazard Ranking 

System (HRS): Regulatory Impact 

Analysis is available in the docket for 

this action. 

K. Executive Order 12580: Superfund 

Implementation 

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: February 3, 2016. 

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: 

PART 300—NATIONAL OIL AND 

HAZARDOUS SUBSTANCES 

POLLUTION CONTINGENCY PLAN 

■ 1. The authority citation for part 300 

continues to read as follows: 

Authority: 33 U.S.C. 1251 et seq. 

■ 2. Amend Appendix A to Part 300: 

■ a. In section 1.1 by: 

■ i. Amending by removing the 

definition heading ‘‘Ambient Water 

Quality Criteria (AWQC) and adding 

‘‘Ambient Water Quality Criteria 

(AWQC)/National Recommended Water 

Quality Criteria’’, in its place; and 

removing the text ‘‘maximum acute or 

chronic toxicity’’ and adding 

‘‘maximum acute (Criteria Maximum 

Concentration or CMC) or chronic 

(Criterion Continuous Concentration or 

CCC) toxicity.’’ in its place; 

■ ii. Adding in alphabetical order the 

definitions ‘‘Channelized flow’’ and 

‘‘Crawl space’’; 

■ iii. Revising the definitions ‘‘Distance 

weight’’ and ‘‘Half-life’’; 

■ iv. Amending the definition ‘‘HRS 

pathway’’ by removing the word ‘‘soil,’’ 

and adding ‘‘soil exposure and 

subsurface intrusion,’’ in its place; 

■ v. Adding in alphabetical order the 

definitions ‘‘Indoor air’’, ‘‘Inhalation 

Unit Risk (IUR)’’, ‘‘Occupied 

structures’’, ‘‘Preferential subsurface 

intrusion pathways’’; and ‘‘Reference 

concentration (RfC)’’; 

■ vi. Revising the definition ‘‘Reference 

dose (RfD)’’; ‘‘Screening concentration’’, 

and ‘‘Slope factor (also referred to as 

cancer potency factor)’’; 

■ vii. Adding in alphabetical order the 

definitions ‘‘Soil gas’’, ‘‘Soil porosity’’; 

‘‘Subslab’’, ‘‘Subsurface Intrusion’’, 

‘‘Surficial ground water’’, ‘‘Unit Risk’’, 

and ‘‘Unsaturated Zone’’; and 

■ viii. Revising the introductory text of 

the definition ‘‘Weight-of-evidence’’. 

■ b. Revising section 2.0 to include 

sections 2.0 through 2.5.2; 

■ c. Revising section 5.0 to include 

sections 5.0 through 5.3; 

■ d. In section 6.0 by revising Table 6– 

14, entitled ‘‘Health-Based Benchmarks 

for Hazardous Substances in Air’’; and 

■ e. In section 7.0 by: 

■ i. Revising the table entitled ‘‘Table 7– 

1. HRS Factors Evaluated Differently For 

Radionuclides’’; 

■ ii. Under Table 7–1, the second 

undesignated paragraph, revising the 

third sentence ; 

■ iii. Revising sections 7.1, 7.1.1, and 

7.1.2; 7.2.3; 7.2.4; 7.2.5.1, 7.2.5.1.1 

through 7.2.5.1.3; 7.2.5.2; 7.2.5.3; 7.3, 

7.3.1, and 7.3.2; and 

■ iv. Adding section 7.3.3. 

The revisions and additions read as 

follows: 


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Appendix A of Part 300—Hazard 

Ranking System 

* * * * * 

1.1 Definitions 

* * * * * 

Channelized flow: Natural geological 

or manmade features such as karst, 

fractures, lava tubes, and utility 

conduits (e.g., sewer lines), which allow 

ground water and/or soil gas to move 

through the subsurface environment 

more easily. 

* * * * * 

Crawl space: The enclosed or semienclosed area between a regularly 

occupied structure’s foundation (e.g., 

pier and beam construction) and the 

ground surface. Crawl space samples are 

collected to determine the concentration 

of hazardous substances in the air 

beneath a regularly occupied structure. 

* * * * * 

Distance weight: Parameter in the 

HRS air migration pathway, ground 

water migration pathway, and the soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway that reduces the point value 

assigned to targets as their distance 

increases from the site. [unitless]. 

* * * * * 

Half-life: Length of time required for 

an initial concentration of a substance to 

be halved as a result of loss through 

decay. The HRS considers five decay 

processes for determining surface water 

persistence: Biodegradation, hydrolysis, 

photolysis, radioactive decay, and 

volatilization. The HRS considers two 

decay processes for determining 

subsurface intrusion degradation: 

Biodegradation and hydrolysis. 

* * * * * 

Indoor air: The air present within a 

structure. 

Inhalation Unit Risk (IUR): The 

upper-bound excess lifetime cancer risk 

estimated to result from continuous 

exposure to an agent (i.e., hazardous 

substance) at a concentration of 1mg/m3 

in air. 

* * * * * 

Occupied structures: Structures with 

enclosed air space, either where people 

are present on a regular basis or that 

were previously occupied but vacated 

due to a site-related hazardous 

substance(s). 

* * * * * 

Preferential subsurface intrusion 

pathways: Subsurface features such as 

animal burrows, cracks in walls, spaces 

around utility lines or drains through 

which a hazardous substance moves 

more easily into a regularly occupied 

structure. 

* * * * * 

Reference concentration (RfC): An 

estimate of a continuous inhalation 

exposure to the human population that 

is likely to be without an appreciable 

risk of deleterious effects during a 

lifetime. 

Reference dose (RfD): An estimate of 

a daily oral exposure to the human 

population that is likely to be without 

an appreciable risk of deleterious effects 

during a lifetime. 

* * * * * 

Screening concentration: Mediaspecific benchmark concentration for a 

hazardous substance that is used in the 

HRS for comparison with the 

concentration of that hazardous 

substance in a sample from that media. 

The screening concentration for a 

specific hazardous substance 

corresponds to its reference 

concentration for inhalation exposures 

or reference dose for oral exposures, as 

appropriate, and, if the substance is a 

human carcinogen with either a 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, to 

that concentration that corresponds to 

its 10 ¥6 individual lifetime excess 

cancer risk for inhalation exposures or 

for oral exposures, as appropriate. 

* * * * * 

Slope factor (also referred to as cancer 

potency factor): Estimate of the 

probability of response (for example, 

cancer) per unit intake of a substance 

over a lifetime. The slope factor is 

typically used to estimate upper-bound 

probability of an individual developing 

cancer as a result of exposure to a 

particular level of a human carcinogen 

with either a weight-of-evidence 

classification of A, B, or C, or a weightof-evidence classification of 

carcinogenic to humans, likely to be 

carcinogenic to humans or having 

suggestive evidence of carcinogenic 

potential. [(mg/kg-day) ¥1 for nonradioactive 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. 

* * * * * 

Subslab: The area immediately 

beneath a regularly occupied structure 

with a basement foundation or a slabon-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 mg/L in water, or 1 

mg/m3 in air. 

Unsaturated Zone: The portion of 

subsurface between the land surface and 

the zone of saturation. It extends from 

the ground surface to the surficial water 

table (excluding localized or perched 

water). 

* * * * * 

Weight-of-evidence: EPA 

classification system for characterizing 

the evidence supporting the designation 

of a substance as a human carcinogen. 

The EPA weight-of-evidence groupings, 

depending on the date EPA updated the 

profile, include either: 

* * * * * 

2.0 Evaluations Common to Multiple 

Pathways 

2.1 Overview. The HRS site score (S) 

is the result of an evaluation of four 

pathways: 

• Ground Water Migration (Sgw). 

• Surface Water Migration (Ssw). 

• Soil Exposure and Subsurface 

Intrusion (Ssessi). 

• Air Migration (Sa). 

The ground water and air migration 

pathways use single threat evaluations, 

while the surface water migration and 

soil exposure and subsurface intrusion 

pathways use multiple threat 

evaluations. Three threats are evaluated 

for the surface water migration pathway: 

Drinking water, human food chain, and 

environmental. These threats are 

evaluated for two separate migration 

components—overland/flood migration 

and ground water to surface water 

migration. Two components are 

evaluated for the soil exposure and 

subsurface intrusion pathway: Soil 

exposure and subsurface intrusion. The 

soil exposure component evaluates two 

threats: Resident population and nearby 

population, and the subsurface 


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intrusion component is a single threat 

evaluation. 

The HRS is structured to provide a 

parallel evaluation for each of these 

pathways, components and threats. This 

section focuses on these parallel 

evaluations, starting with the 

calculation of the HRS site score and the 

individual pathway scores. 

2.1.1 Calculation of HRS site score. 

Scores are first calculated for the 

individual pathways as specified in 

sections 2 through 7 and then are 

combined for the site using the 

following root-mean-square equation to 

determine the overall HRS site score, 

which ranges from 0 to 100: 

2.1.2 Calculation of pathway score. 

Table 2–1, which is based on the air 

migration pathway, illustrates the basic 

parameters used to calculate a pathway 

score. As Table 2–1 shows, each 

pathway (component or threat) score is 

the product of three ‘‘factor categories’’: 

Likelihood of release, waste 

characteristics, and targets. (The soil 

exposure and subsurface intrusion 

pathway uses likelihood of exposure 

rather than likelihood of release.) Each 

of the three factor categories contains a 

set of factors that are assigned numerical 

values and combined as specified in 

sections 2 through 7. The factor values 

are rounded to the nearest integer, 

except where otherwise noted. 

2.1.3 Common evaluations. 

Evaluations common to all four HRS 

pathways include: 

• Characterizing sources. 

— Identifying sources (and, for the soil 

exposure and subsurface intrusion 

pathway, areas of observed 

contamination, areas of observed 

exposure and/or areas of subsurface 

contamination (see sections 5.1.0 and 

5.2.0)). 

— Identifying hazardous substances 

associated with each source (or area of 

observed contamination, or observed 

exposure, or subsurface 

contamination). 

— Identifying hazardous substances 

available to a pathway. 

TABLE 2–1—SAMPLE PATHWAY SCORESHEET

Factor category Maximum value Value assigned 

Likelihood of Release 

1. Observed Release ................................................................................................................................... 550 

2. Potential to Release ................................................................................................................................ 500 

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

Waste Characteristics 

4. Toxicity/Mobility ....................................................................................................................................... (a) 

5. Hazardous Waste Quantity ..................................................................................................................... (a) 

6. Waste Characteristics .............................................................................................................................. 100 

Targets 

7. Nearest Individual: ..............................

7a. Level I ............................................................................................................................................. 50 

7b. Level II ............................................................................................................................................ 45 

7c. Potential Contamination ................................................................................................................. 20 

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

8. Population ................................................................................................................................................ (b) 

8a. Level I ............................................................................................................................................. (b) 

8b. Level II ............................................................................................................................................ (b) 

8c. Potential Contamination ................................................................................................................. (b) 

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

9. Resources ................................................................................................................................................ 5 

10. Sensitive Environments ......................................................................................................................... (b) 

10a. Actual Contamination ................................................................................................................... (b) 

10b. Potential Environments ................................................................................................................ (b) 

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

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

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

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

a Maximum value applies to waste characteristics category. The product of lines 4 and 5 is used in Table 2–7 to derive the value for the waste 

characteristics factor category. bThere is no limit to the human population or sensitive environments factor values. However, the pathway score based solely on sensitive environments is limited to a maximum of 60 points. 

• Scoring likelihood of release (or 

likelihood of exposure) factor 

category. 

—Scoring observed release (or 

observed exposure or observed 

contamination). 

—Scoring potential to release when 

there is no observed release. 

• Scoring waste characteristics factor 

category. 

—Evaluating toxicity. 

D Combining toxicity with mobility, 

persistence, degradation and/or 

bioaccumulation (or ecosystem 

bioaccumulation) potential, as 

appropriate to the pathway 

(component or threat). 

D Evaluating hazardous waste 

quantity. 

—Combining hazardous waste 

quantity with the other waste 

characteristics factors. 

D Determining waste characteristics 

factor category value. 

• Scoring targets factor category. 


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—Determining level of contamination 

for targets. 

These evaluations are essentially 

identical for the three migration 

pathways (ground water, surface water, 

and air). However, the evaluations differ 

in certain respects for the soil exposure 

and subsurface intrusion pathway. 

Section 7 specifies modifications that 

apply to each pathway when evaluating 

sites containing radioactive substances. 

Section 2 focuses on evaluations 

common at the pathway, component 

and threat levels. Note that for the 

ground water and surface water 

migration pathways, separate scores are 

calculated for each aquifer (see section 

3.0) and each watershed (see sections 

4.1.1.3 and 4.2.1.5) when determining 

the pathway scores for a site. Although 

the evaluations in section 2 do not vary 

when different aquifers or watersheds 

are scored at a site, the specific factor 

values (for example, observed release, 

hazardous waste quantity, toxicity/

mobility) that result from these 

evaluations can vary by aquifer and by 

watershed at the site. This can occur 

through differences both in the specific 

sources and targets eligible to be 

evaluated for each aquifer and 

watershed and in whether observed 

releases can be established for each 

aquifer and watershed. Such differences 

in scoring at the aquifer and watershed 

level are addressed in sections 3 and 4, 

not section 2. 

2.2 Characterize sources. Source 

characterization includes identification 

of the following: 

• Sources (and areas of observed 

contamination, areas of observed 

exposure or areas of subsurface 

contamination) at the site. 

• Hazardous substances associated 

with these sources (or areas of observed 

contamination, areas of observed 

exposure or areas of subsurface 

contamination). 

• Pathways potentially threatened by 

these hazardous substances. 

Table 2–2 presents a sample 

worksheet for source characterization. 

2.2.1 Identify sources. For the three 

migration pathways, identify the 

sources at the site that contain 

hazardous substances. Identify the 

migration pathway(s) to which each 

source applies. For the soil exposure 

and subsurface intrusion pathway, 

identify areas of observed 

contamination, areas of observed 

exposure, and/or areas of subsurface 

contamination at the site (see sections 

5.1.0 and 5.2.0). 

Table 2–2—Sample Source 

Characterization Worksheet 

Source:ll 

A. Source dimensions and hazardous 

waste quantity. 

Hazardous constituent quantity:ll 

Hazardous wastestream quantity:ll 

Volume:ll 

Area:ll 

Area of observed contamination:ll 

Area of observed exposure:ll 

Area of subsurface contamination:ll 

B. Hazardous substances associated 

with the source. 


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Hazardous 

substance 

Available to pathway 

Air Ground 

water 

(GW) 

Surface water 

(SW) 

Soil exposure/subsurface intrusion (SESSI) 

Gas Particulate 

Overland/ 

flood GW to SW 

Soil exposure 

Subsurface 

intrusion 

Resident Nearby 

Area of observed 

exposure 

Area of subsurface 

contamination 

............................................................................ .................... ........................ .................... .................... .................... .................... .................... .................... .......................................................................................................... .................... ........................ .................... .................... .................... .................... .................... .................... .......................................................................................................... .................... ........................ .................... .................... .................... .................... .................... .................... .......................................................................................................... .................... ........................ .................... .................... .................... .................... .................... .................... .......................................................................................................... .................... ........................ .................... .................... .................... .................... .................... .................... ..............................


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2.2.2 Identify hazardous substances 

associated with a source. For each of the 

three migration pathways, consider 

those hazardous substances documented 

in a source (for example, by sampling, 

labels, manifests, oral or written 

statements) to be associated with that 

source when evaluating each pathway. 

In some instances, a hazardous 

substance can be documented as being 

present at a site (for example, by labels, 

manifests, oral or written statements), 

but the specific source(s) containing that 

hazardous substance cannot be 

documented. For the three migration 

pathways, in those instances when the 

specific source(s) cannot be documented 

for a hazardous substance, consider the 

hazardous substance to be present in 

each source at the site, except sources 

for which definitive information 

indicates that the hazardous substance 

was not or could not be present. 

For an area of observed contamination 

in the soil exposure component of the 

soil exposure and subsurface intrusion 

pathway, consider only those hazardous 

substances that meet the criteria for 

observed contamination for that area 

(see section 5.1.0) to be associated with 

that area when evaluating the pathway. 

For an area of observed exposure or 

area of subsurface contamination (see 

section 5.2.0) in the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway, consider only those hazardous 

substances that: 

• Meet the criteria for observed 

exposure, or 

• Meet the criteria for observed 

release in an area of subsurface 

contamination and has a vapor pressure 

greater than or equal to one torr or a 

Henry’s constant greater than or equal to 

10¥5 atm-m3/mol, or 

• Meet the criteria for an observed 

release in a structure within, or in a 

sample from below, an area of observed 

exposure and has a vapor pressure 

greater than or equal to one torr or a 

Henry’s constant greater than or equal to 

10¥5 atm-m3/mol. 

2.2.3 Identify hazardous substances 

available to a pathway. In evaluating 

each migration pathway, consider the 

following hazardous substances 

available to migrate from the sources at 

the site to the pathway: 

• Ground water migration. 

— Hazardous substances that meet the 

criteria for an observed release (see 

section 2.3) to ground water. 

— All hazardous substances associated 

with a source with a ground water 

containment factor value greater than 

0 (see section 3.1.2.1). 

• Surface water migration—overland/ 

flood component. 

— Hazardous substances that meet the 

criteria for an observed release to 

surface water in the watershed being 

evaluated. 

— All hazardous substances associated 

with a source with a surface water 

containment factor value greater than 

0 for the watershed (see sections 

4.1.2.1.2.1.1 and 4.1.2.1.2.2.1). 

• Surface water migration—ground 

water to surface water component. 

— Hazardous substances that meet the 

criteria for an observed release to 

ground water. 

— All hazardous substances associated 

with a source with a ground water 

containment factor value greater than 

0 (see sections 4.2.2.1.2 and 3.1.2.1). 

• Air migration. 

— Hazardous substances that meet the 

criteria for an observed release to the 

atmosphere. 

— All gaseous hazardous substances 

associated with a source with a gas 

containment factor value greater than 

0 (see section 6.1.2.1.1). 

— All particulate hazardous substances 

associated with a source with a 

particulate containment factor value 

greater than 0 (see section 6.1.2.2.1). 

• For each migration pathway, in 

those instances when the specific 

source(s) containing the hazardous 

substance cannot be documented, 

consider that hazardous substance to be 

available to migrate to the pathway 

when it can be associated (see section 

2.2.2) with at least one source having a 

containment factor value greater than 0 

for that pathway. 

In evaluating the soil exposure and 

subsurface intrusion pathway, consider 

the following hazardous substances 

available to the pathway: 

• Soil exposure component—resident 

population threat. 

— All hazardous substances that meet 

the criteria for observed 

contamination at the site (see section 

5.1.0). 

• Soil exposure component—nearby 

population threat. 

— All hazardous substances that meet 

the criteria for observed 

contamination at areas with an 

attractiveness/accessibility factor 

value greater than 0 (see section 

5.1.2.1.1). 

• Subsurface intrusion component. 

— All hazardous substances that meet 

the criteria for observed exposure at 

the site (see section 5.2.0). 

—All hazardous substances with a 

vapor pressure greater than or equal to 

one torr or a Henry’s constant greater 

than or equal to 10¥5 atm-m3/mol that 

meet the criteria for an observed 

release in an area of subsurface 

contamination (see section 5.2.0). 

—All hazardous substances that meet 

the criteria for an observed release in 

a structure within, or in a sample 

from below, an area of observed 

exposure (see section 5.2.0). 

2.3 Likelihood of release. Likelihood 

of release is a measure of the likelihood 

that a waste has been or will be released 

to the environment. The likelihood of 

release factor category is assigned the 

maximum value of 550 for a migration 

pathway whenever the criteria for an 

observed release are met for that 

pathway. If the criteria for an observed 

release are met, do not evaluate 

potential to release for that pathway. 

When the criteria for an observed 

release are not met, evaluate potential to 

release for that pathway, with a 

maximum value of 500. The evaluation 

of potential to release varies by 

migration pathway (see sections 3, 4 and 

6). 

Establish an observed release either 

by direct observation of the release of a 

hazardous substance into the media 

being evaluated (for example, surface 

water) or by chemical analysis of 

samples appropriate to the pathway 

being evaluated (see sections 3, 4 and 6). 

The minimum standard to establish an 

observed release by chemical analysis is 

analytical evidence of a hazardous 

substance in the media significantly 

above the background level. Further, 

some portion of the release must be 

attributable to the site. Use the criteria 

in Table 2–3 as the standard for 

determining analytical significance. 

(The criteria in Table 2–3 are also used 

in establishing observed contamination 

for the soil exposure component and for 

establishing areas of observed exposure 

and areas of subsurface contamination 

in the subsurface intrusion component 

of the soil exposure and subsurface 

intrusion pathway, see section 5.1.0 and 

section 5.2.0). Separate criteria apply to 

radionuclides (see section 7.1.1). 

TABLE 2–3—OBSERVED RELEASE CRITERIA FOR CHEMICAL ANALYSIS

Sample Measurement < Sample Quantitation Limit.a

No observed release is established. 


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TABLE 2–3—OBSERVED RELEASE CRITERIA FOR CHEMICAL ANALYSIS—Continued 

Sample Measurement ≥ Sample Quantitation Limit.a

An observed release is established as follows: 

• If the background concentration is not detected (or is less than the detection limit), an observed release is established when the sample 

measurement equals or exceeds the sample quantitation limit.a

• If the background concentration equals or exceeds the detection limit, an observed release is established when the sample measurement 

is 3 times or more above the background concentration. a If the sample quantitation limit (SQL) cannot be established, determine if there is an observed release as follows: 

If the sample analysis was performed under the EPA Contract Laboratory Program, use the EPA contract-required quantitation limit (CRQL) in 

place of the SQL. 

If the sample analysis is not performed under the EPA Contract Laboratory Program, use the detection limit (DL) in place of the SQL. 

2.4 Waste characteristics. The waste 

characteristics factor category includes 

the following factors: Hazardous waste 

quantity, toxicity, and as appropriate to 

the pathway or threat being evaluated, 

mobility, persistence, degradation, and/ 

or bioaccumulation (or ecosystem 

bioaccumulation) potential. 

2.4.1 Selection of substance 

potentially posing greatest hazard. For 

all pathways (components and threats), 

select the hazardous substance 

potentially posing the greatest hazard 

for the pathway (component or threat) 

and use that substance in evaluating the 

waste characteristics category of the 

pathway (component or threat). For the 

three migration pathways (and threats), 

base the selection of this hazardous 

substance on the toxicity factor value for 

the substance, combined with its 

mobility, persistence, and/or 

bioaccumulation (or ecosystem 

bioaccumulation) potential factor 

values, as applicable to the migration 

pathway (or threat). For the soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway, base the selection on the 

toxicity factor alone. For the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway, base the selection on the 

toxicity factor value for the substance, 

combined with its degradation factor 

value. Evaluation of the toxicity factor is 

specified in section 2.4.1.1. Use and 

evaluation of the mobility, persistence, 

degradation, and/or bioaccumulation (or 

ecosystem bioaccumulation) potential 

factors vary by pathway (component or 

threat) and are specified under the 

appropriate pathway (component or 

threat) section. Section 2.4.1.2 identifies 

the specific factors that are combined 

with toxicity in evaluating each 

pathway (component or threat). 

2.4.1.1 Toxicity factor. Evaluate 

toxicity for those hazardous substances 

at the site that are available to the 

pathway being scored. For all pathways 

and threats, except the surface water 

environmental threat, evaluate human 

toxicity as specified below. For the 

surface water environmental threat, 

evaluate ecosystem toxicity as specified 

in section 4.1.4.2.1.1. 

Establish human toxicity factor values 

based on quantitative dose-response 

parameters for the following three types 

of toxicity: 

• Cancer—Use slope factors (also 

referred to as cancer potency factors) 

combined with weight-of-evidence 

ratings for carcinogenicity for all 

exposure routes except inhalation. Use 

inhalation unit risk (IUR) for inhalation 

exposure. If an inhalation unit risk or a 

slope factor is not available for a 

substance, use its ED10 value to estimate 

a slope factor as follows: 

• Noncancer toxicological responses 

of chronic exposure—use reference dose 

(RfD) or reference concentration (RfC) 

values as applicable. 

• Noncancer toxicological responses 

of acute exposure—use acute toxicity 

parameters, such as the LD50. 

Assign human toxicity factor values to 

a hazardous substance using Table 2–4, 

as follows: 

• If RfD/RfC and slope factor/

inhalation unit risk values are available 

for the hazardous substance, assign the 

substance a value from Table 2–4 for 

each. Select the higher of the two values 

assigned and use it as the overall 

toxicity factor value for the hazardous 

substance. 

• If either an RfD/RfC or slope factor/ 

inhalation unit risk value is available, 

but not both, assign the hazardous 

substance an overall toxicity factor 

value from Table 2–4 based solely on 

the available value (RfD/RfC or slope 

factor/inhalation unit risk). 

• If neither an RfD/RfC nor slope 

factor/inhalation unit risk value is 

available, assign the hazardous 

substance an overall toxicity factor 

value from Table 2–4 based solely on 

acute toxicity. That is, consider acute 

toxicity in Table 2–4 only when both 

RfD/RfC and slope factor/IUR values are 

not available. 

• If neither an RfD/RfC, nor slope 

factor/inhalation unit risk, nor acute 

toxicity value is available, assign the 

hazardous substance an overall toxicity 

factor value of 0 and use other 

hazardous substances for which 

information is available in evaluating 

the pathway. 

TABLE 2–4—TOXICITY FACTOR

EVALUATION

Assigned 

value 

Chronic toxicity (Human) 

Reference dose (RfD) (mg/kgday): 

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): 

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) 

A or Carcinogenic to humans B or Likely to be carcinogenic to 

humans 

C or Suggestive evidence of 

carcinogenic potential Assigned value 

Weight-of-evidence a/Slope factor (mg/kg-day)¥1

0.5 ≤ SF b ............................................... 5 ≤ SF ................................................... 50 ≤ SF ................................................. 10,000 


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Carcinogenicity (Human) 

A or Carcinogenic to humans B or Likely to be carcinogenic to 

humans 

C or Suggestive evidence of 

carcinogenic potential Assigned value 

0.05 ≤ SF < 0.5 ...................................... 0.5 ≤ SF < 5 .......................................... 5 ≤ SF < 50 ........................................... 1,000 

SF < 0.05 ............................................... 0.05 ≤ SF < 0.5 ..................................... 0.5 ≤ SF < 5 .......................................... 100 

— SF < 0.05 .............................................. SF < 0.5 ................................................ 10 

Slope factor not available ...................... Slope factor not available ..................... Slope factor not available ..................... 0 

Weight-of-evidence a/Inhalation unit risk (μg/m3) 

0.00004 ≤ IUR c ...................................... 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 

aA, 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. bSF = Slope factor. 

c IUR = Inhalation Unit Risk. 

Acute Toxicity (Human) 

Oral LD50

(mg/kg) 

Dermal LD50

(mg/kg) 

Dust or mist LC50

(mg/l) 

Gas or vapor LC50

(ppm) Assigned value 

LD50 < 5 ................................. LD50 < 2 ............................... LC50 < 0.2 ............................ LC50 < 20 ............................. 1,000 

5 ≤ LD50 < 50 ........................ 2 ≤ LD50 < 20 ....................... 0.2 ≤ LC50 < 2 ...................... 20 ≤ LC50 <200 .................... 100 

50 ≤ LD50 < 500 .................... 20 ≤ LD50 < 200 ................... 2 ≤ LC50 <20 ........................ 200 ≤ LC50 <2,000 ............... 10 

500 ≤ LD50 ............................. 200 ≤ LD50 ............................ 20 ≤ LC50 .............................. 2,000 ≤ LC50 ......................... 1 

LD50 not available .................. LD50 not available ................ LD50 not available ................ LD50 not available ................ 0 

If a toxicity factor value of 0 is 

assigned to all hazardous substances 

available to a particular pathway (that 

is, insufficient toxicity data are available 

for evaluating all the substances), use a 

default value of 100 as the overall 

human toxicity factor value for all 

hazardous substances available to the 

pathway. For hazardous substances 

having usable toxicity data for multiple 

exposure routes (for example, inhalation 

and ingestion), consider all exposure 

routes and use the highest assigned 

value, regardless of exposure route, as 

the toxicity factor value. 

For HRS purposes, assign both 

asbestos and lead (and its compounds) 

a human toxicity factor value of 10,000. 

Separate criteria apply for assigning 

factor values for human toxicity and 

ecosystem toxicity for radionuclides 

(see sections 7.2.1 and 7.2.2). 

2.4.1.2 Hazardous substance 

selection. For each hazardous substance 

evaluated for a migration pathway (or 

threat), combine the human toxicity 

factor value (or ecosystem toxicity factor 

value) for the hazardous substance with 

a mobility, persistence, and/or 

bioaccumulation (or ecosystem 

bioaccumulation) potential factor value 

as follows: 

• Ground water migration. 

—Determine a combined human 

toxicity/mobility factor value for the 

hazardous substance (see section 

3.2.1). 

• Surface water migration—overland/ 

flood migration component. 

—Determine a combined human 

toxicity/persistence factor value for 

the hazardous substance for the 

drinking water threat (see section 

4.1.2.2.1). 

—Determine a combined human 

toxicity/persistence/bioaccumulation 

factor value for the hazardous 

substance for the human food chain 

threat (see section 4.1.3.2.1). 

—Determine a combined ecosystem 

toxicity/persistence/bioaccumulation 

factor value for the hazardous 

substance for the environmental 

threat (see section 4.1.4.2.1). 

• Surface water migration—ground 

water to surface water migration 

component. 

—Determine a combined human 

toxicity/mobility/persistence factor 

value for the hazardous substance for 

the drinking water threat (see section 

4.2.2.2.1). 

—Determine a combined human 

toxicity/mobility/persistence/

bioaccumulation factor value for the 

hazardous substance for the human 

food chain threat (see section 

4.2.3.2.1). 

—Determine a combined ecosystem 

toxicity/mobility/persistence/

bioaccumulation factor value for the 

hazardous substance for the 

environmental threat (see section 

4.2.4.2.1). 

• Air migration. 

• Determine a combined human 

toxicity/mobility factor value for 

the hazardous substance (see 

section 6.2.1). 

Determine each combined factor value 

for a hazardous substance by 

multiplying the individual factor values 

appropriate to the pathway (or threat). 

For each migration pathway (or threat) 

being evaluated, select the hazardous 

substance with the highest combined 

factor value and use that substance in 

evaluating the waste characteristics 

factor category of the pathway (or 

threat). 

For the soil exposure and subsurface 

intrusion pathway, determine toxicity 

and toxicity/degradation factor values as 

follows: 

• Soil exposure and subsurface 

intrusion—soil exposure 

component. 

• Select the hazardous substance with 

the highest human toxicity factor 

value from among the substances 

that meet the criteria for observed 

contamination for the threat 

evaluated and use that substance in 

evaluating the waste characteristics 

factor category (see section 


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5.1.1.2.1). 

• Soil exposure and subsurface 

intrusion—subsurface intrusion 

component. 

• Determine a combined human 

toxicity/degradation factor value for 

each hazardous substance being 

evaluated that: 

D Meets the criteria for observed 

exposure, or 

D Meets the criteria for observed 

release in an area of subsurface 

contamination and has a vapor 

pressure greater than or equal to 

one torr or a Henry’s constant 

greater than or equal to 10¥5 atmm3/mol, or 

D Meets the criteria for an observed 

release in a structure within, or in 

a sample from below, an area of 

observed exposure and has a vapor 

pressure greater than or equal to 

one torr or a Henry’s constant 

greater than or equal to 10¥5 atmm3/mol. 

• Select the hazardous substance with 

the highest combined factor value 

and use that substance in evaluating 

the waste characteristics factor 

category (see sections 5.2.1.2.1 and 

5.2.1.2). 

2.4.2 Hazardous waste quantity. 

Evaluate the hazardous waste quantity 

factor by first assigning each source (or 

area of observed contamination, area of 

observed exposure or area of subsurface 

contamination) a source hazardous 

waste quantity value as specified below. 

Sum these values to obtain the 

hazardous waste quantity factor value 

for the pathway being evaluated. 

In evaluating the hazardous waste 

quantity factor for the three migration 

pathways, allocate hazardous 

substances and hazardous wastestreams 

to specific sources in the manner 

specified in section 2.2.2, except: 

Consider hazardous substances and 

hazardous wastestreams that cannot be 

allocated to any specific source to 

constitute a separate ‘‘unallocated 

source’’ for purposes of evaluating only 

this factor for the three migration 

pathways. Do not, however, include a 

hazardous substance or hazardous 

wastestream in the unallocated source 

for a migration pathway if there is 

definitive information indicating that 

the substance or wastestream could only 

have been placed in sources with a 

containment factor value of 0 for that 

migration pathway. 

In evaluating the hazardous waste 

quantity factor for the soil exposure 

component of the soil exposure and 

subsurface intrusion pathway, allocate 

to each area of observed contamination 

only those hazardous substances that 

meet the criteria for observed 

contamination, for that area of observed 

contamination and only those 

hazardous wastestreams that contain 

hazardous substances that meet the 

criteria for observed contamination for 

that area of observed contamination. Do 

not consider other hazardous substances 

or hazardous wastestreams at the site in 

evaluating this factor for the soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway. 

In evaluating the hazardous waste 

quantity factor for the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway, allocate to each area of 

observed exposure or area of subsurface 

contamination only those hazardous 

substances and hazardous wastestreams 

that contain hazardous substances that: 

• Meet the criteria for observed 

exposure, or 

• Meet the criteria for observed 

release in an area of subsurface 

contamination and has a vapor pressure 

greater than or equal to one torr or a 

Henry’s constant greater than or equal to 

10¥5 atm-m3/mol, or 

• Meet the criteria for an observed 

release in a structure within, or in a 

sample from below, an area of observed 

exposure and has a vapor pressure 

greater than or equal to one torr or a 

Henry’s constant greater than or equal to 

10¥5 atm-m3/mol. 

Do not consider other hazardous 

substances or hazardous wastestreams at 

the site in evaluating this factor for the 

subsurface intrusion component of the 

soil exposure and subsurface intrusion 

pathway. When determining the 

hazardous waste quantity for multisubunit structures, use the procedures 

identified in section 5.2.1.2.2. 

2.4.2.1 Source hazardous waste 

quantity. For each of the three migration 

pathways, assign a source hazardous 

waste quantity value to each source 

(including the unallocated source) 

having a containment factor value 

greater than 0 for the pathway being 

evaluated. Consider the unallocated 

source to have a containment factor 

value greater than 0 for each migration 

pathway. 

For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, assign a source 

hazardous waste quantity value to each 

area of observed contamination, as 

applicable to the threat being evaluated. 

For the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, assign a 

source hazardous waste quantity value 

to each regularly occupied structure 

within an area of observed exposure or 

an area of subsurface contamination that 

has a structure containment factor value 

greater than 0. 

For determining all hazardous waste 

quantity calculations except for an 

unallocated source or an area of 

subsurface contamination, evaluate 

using the following four measures in the 

following hierarchy: 

• Hazardous constituent quantity. 

• Hazardous wastestream quantity. 

• Volume. 

• Area. 

For the unallocated source, use only 

the first two measures. For an area of 

subsurface contamination, evaluate nonradioactive hazardous substances using 

only the last two measures and evaluate 

radioactive hazardous substances using 

hazardous wastestream quantity only. 

See also section 7.0 regarding the 

evaluation of radioactive substances. 

Separate criteria apply for assigning a 

source hazardous waste quantity value 

for radionuclides (see section 7.2.5). 

2.4.2.1.1 Hazardous constituent 

quantity. Evaluate hazardous 

constituent quantity for the source (or 

area of observed contamination) based 

solely on the mass of CERCLA 

hazardous substances (as defined in 

CERCLA section 101(14), as amended) 

allocated to the source (or area of 

observed contamination), except: 

• For a hazardous waste listed 

pursuant to section 3001 of the Solid 

Waste Disposal Act, as amended by the 

Resource Conservation and Recovery 

Act of 1976 (RCRA), 42 U.S.C. 6901 et 

seq., determine its mass for the 

evaluation of this measure as follows: 

—If the hazardous waste is listed solely 

for Hazard Code T (toxic waste), 

include only the mass of constituents 

in the hazardous waste that are 

CERCLA hazardous substances and 

not the mass of the entire hazardous 

waste. 

— If the hazardous waste is listed for 

any other Hazard Code (including T 

plus any other Hazard Code), include 

the mass of the entire hazardous 

waste. 

• For a RCRA hazardous waste that 

exhibits the characteristics identified 

under section 3001 of RCRA, as 

amended, determine its mass for the 

evaluation of this measure as follows: 

—If the hazardous waste exhibits only 

the characteristic of toxicity (or only 

the characteristic of EP toxicity), 

include only the mass of constituents 

in the hazardous waste that are 

CERCLA hazardous substances and 

not the mass of the entire hazardous 

waste. 

—If the hazardous waste exhibits any 

other characteristic identified under 


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section 3001 (including any other 

characteristic plus the characteristic 

of toxicity [or the characteristic of EP 

toxicity]), include the mass of the 

entire hazardous waste. 

Based on this mass, designated as C, 

assign a value for hazardous constituent 

quantity as follows: 

• For the migration pathways, assign 

the source a value for hazardous 

constituent quantity using the Tier A 

equation of Table 

2–5. 

• For the soil exposure and 

subsurface intrusion pathway—soil 

exposure component, assign the area of 

observed contamination a value using 

the Tier A equation of Table 5–2 

(section 5.1.1.2.2). 

• For the soil exposure and 

subsurface intrusion pathway— 

subsurface intrusion component, assign 

the area of observed exposure a value 

using the Tier A equation of Table 5–18 

(section 5.2.1.2.2). 

If the hazardous constituent quantity 

for the source (or area of observed 

contamination or area of observed 

exposure) is adequately determined 

(that is, the total mass of all CERCLA 

hazardous substances in the source and 

releases from the source [or in the area 

of observed contamination or area of 

observed exposure] is known or is 

estimated with reasonable confidence), 

do not evaluate the other three measures 

discussed below. Instead assign these 

other three measures a value of 0 for the 

source (or area of observed 

contamination or area of observed 

exposure) and proceed to section 

2.4.2.1.5. 

If the hazardous constituent quantity 

is not adequately determined, assign the 

source (or area of observed 

contamination or area of observed 

exposure) a value for hazardous 

constituent quantity based on the 

available data and proceed to section 

2.4.2.1.2. 

TABLE 2–5—HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS

Tier Measure Units 

Equation for 

assigning 

value a

A .............. Hazardous constituent quantity (C) ......................................................................................................... lb .............. C 

Bb ............ Hazardous wastestream quantity (W) ...................................................................................................... lb .............. W/5,000 

C b ............ Volume (V).

Landfill ...................................................................................................................................................... yd3 ........... V/2,500 

Surface impoundment .............................................................................................................................. yd3 ........... V/2.5 

Surface impoundment (buried/backfilled) ................................................................................................ yd3 ........... V/2.5 

Drums c ..................................................................................................................................................... gallon ....... V/2.5 

Tanks and containers other than drums .................................................................................................. yd3 ........... V/2.5 

Contaminated soil .................................................................................................................................... yd3 ........... V/2,500 

Pile ........................................................................................................................................................... yd3 ........... V/2.5 

Other ........................................................................................................................................................ yd3 ........... V/2.5 

D b ............ Area (A).

Landfill ...................................................................................................................................................... ft2 ............. A/3,400 

Surface impoundment .............................................................................................................................. ft2 ............. A/13 

Surface impoundment (buried/backfilled) ................................................................................................ ft2 ............. A/13 

Land treatment ......................................................................................................................................... ft2 ............. A/270 

Pile d ......................................................................................................................................................... ft2 ............. A/13 

Contaminated soil .................................................................................................................................... ft2 ............. A/34,000 

a Do not round to nearest integer. 

b Convert volume to mass when necessary: 1 ton = 2,000 pounds = 1 cubic yard = 4 drums = 200 gallons. 

c If actual volume of drums is unavailable, assume 1 drum = 50 gallons. 

d Use land surface area under pile, not surface area of pile. 

2.4.2.1.2 Hazardous wastestream 

quantity. Evaluate hazardous 

wastestream quantity for the source (or 

area of observed contamination or area 

of observed exposure) based on the mass 

of hazardous wastestreams plus the 

mass of any additional CERCLA 

pollutants and contaminants (as defined 

in CERCLA section 101[33], as 

amended) that are allocated to the 

source (or area of observed 

contamination or area of observed 

exposure). For a wastestream that 

consists solely of a hazardous waste 

listed pursuant to section 3001 of RCRA, 

as amended or that consists solely of a 

RCRA hazardous waste that exhibits the 

characteristics identified under section 

3001 of RCRA, as amended, include the 

mass of that entire hazardous waste in 

the evaluation of this measure. 

Based on this mass, designated as W, 

assign a value for hazardous 

wastestream quantity as follows: 

• For the migration pathways, assign 

the source a value for hazardous 

wastestream quantity using the Tier B 

equation of Table 

2–5. 

• For the soil exposure and 

subsurface intrusion pathway—soil 

exposure component, assign the area of 

observed contamination a value using 

the Tier B equation of Table 5–2 (section 

5.1.1.2.2). 

• For the soil exposure and 

subsurface intrusion pathway— 

subsurface intrusion component, assign 

the area of observed exposure a value 

using the Tier B equation of Table 5–18 

(section 5.2.1.2.2). 

Do not evaluate the volume and area 

measures described below if the source 

is the unallocated source or if the 

following condition applies: 

• The hazardous wastestream 

quantity for the source (or area of 

observed contamination) is adequately 

determined—that is, total mass of all 

hazardous wastestreams and CERCLA 

pollutants and contaminants for the 

source and releases from the source (or 

for the area of observed contamination) 

is known or is estimated with 

reasonable confidence. 

If the source is the unallocated source 

or if this condition applies, assign the 

volume and area measures a value of 0 

for the source (or area of observed 

contamination) and proceed to section 

2.4.2.1.5. Otherwise, assign the source 

(or area of observed contamination) a 

value for Hazardous wastestream 

quantity based on the available data and 

proceed to section 2.4.2.1.3. 


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2.4.2.1.3 Volume. Evaluate the 

volume measure using the volume of the 

source (or the volume of the area of 

observed contamination, area of 

observed exposure, or area of subsurface 

contamination). For the soil exposure 

and subsurface intrusion pathway, 

restrict the use of the volume measure 

to those areas of observed 

contamination, areas of observed 

exposure, or areas of subsurface 

contamination as specified in sections 

5.1.1.2.2 and 5.2.1.2.2. 

Based on the volume, designated as V, 

assign a value to the volume measure as 

follows: 

• For the migration pathways, assign 

the source a value for volume using the 

appropriate Tier C equation of Table 2– 

5. 

• For the soil exposure and 

subsurface intrusion pathway—soil 

exposure component, assign the area of 

observed contamination a value for 

volume using the appropriate Tier C 

equation of Table 5–2 (section 5.1.1.2.2). 

• For the soil exposure and 

subsurface intrusion pathway— 

subsurface intrusion component, assign 

the value based on the volume of the 

regularly occupied structures within the 

area of observed exposure or area of 

subsurface contamination using the Tier 

C equation of Table 5–18 (section 

5.2.1.2.2). 

If the volume of the source (or volume 

of the area of observed contamination, 

area of observed exposure, or area of 

subsurface contamination, if applicable) 

can be determined, do not evaluate the 

area measure. Instead, assign the area 

measure a value of 0 and proceed to 

section 2.4.2.1.5. If the volume cannot 

be determined (or is not applicable for 

the soil exposure and subsurface 

intrusion pathway), assign the source 

(or area of observed contamination, area 

of observed exposure, or area of 

subsurface contamination) a value of 0 

for the volume measure and proceed to 

section 2.4.2.1.4. 

2.4.2.1.4 Area. Evaluate the area 

measure using the area of the source (or 

the area of the area of observed 

contamination, area of observed 

exposure, or area of subsurface 

contamination). Based on this area, 

designated as A, assign a value to the 

area measure as follows: 

• For the migration pathways, assign 

the source a value for area using the 

appropriate Tier D equation of Table 2

–5. 

• For the soil exposure and 

subsurface intrusion pathway—soil 

exposure component, assign the area of 

observed contamination a value for area 

using the appropriate Tier D equation of 

Table 5–2 (section 5.1.1.2.2). 

• For the soil exposure and 

subsurface intrusion pathway— 

subsurface intrusion component, assign 

a value based on the area of regularly 

occupied structures within the area of 

observed exposure or area of subsurface 

contamination using the Tier D equation 

of Table 5–18 (section 5.2.1.2.2). 

2.4.2.1.5 Calculation of source 

hazardous waste quantity value. Select 

the highest of the values assigned to the 

source (or areas of observed 

contamination, areas of observed 

exposure, or areas of subsurface 

contamination) for the hazardous 

constituent quantity, hazardous 

wastestream quantity, volume, and area 

measures. Assign this value as the 

source hazardous waste quantity value. 

Do not round to the nearest integer. 

2.4.2.2 Calculation of hazardous 

waste quantity factor value. Sum the 

source hazardous waste quantity values 

assigned to all sources (including the 

unallocated source) or areas of observed 

contamination, areas of observed 

exposure, or areas of subsurface 

contamination for the pathway being 

evaluated and round this sum to the 

nearest integer, except: If the sum is 

greater than 0, but less than 1, round it 

to 1. Based on this value, select a 

hazardous waste quantity factor value 

for the pathway from Table 2–6. 

TABLE 2–6—HAZARDOUS WASTE

QUANTITY FACTOR VALUES

Hazardous waste quantity value Assigned 

value 

0 ................................................ 0 

1 a to 100 .................................. b 1 

Greater than 100 to 10,000 ...... 100 

Greater than 10,000 to 

1,000,000 .............................. 10,000 

Greater than 1,000,000 ............ 1,000,000 

a If the hazardous waste quantity value is 

greater than 0, but less than 1, round it to 1 

as specified in text. bFor the pathway, if hazardous constituent 

quantity is not adequately determined, assign 

a value as specified in the text; do not assign 

the value of 1. 

For a migration pathway, if the 

hazardous constituent quantity is 

adequately determined (see section 

2.4.2.1.1) for all sources (or all portions 

of sources and releases remaining after 

a removal action), assign the value from 

Table 2–6 as the hazardous waste 

quantity factor value for the pathway. If 

the hazardous constituent quantity is 

not adequately determined for one or 

more sources (or one or more portions 

of sources or releases remaining after a 

removal action) assign a factor value as 

follows: 

• If any target for that migration 

pathway is subject to Level I or Level II 

concentrations (see section 2.5), assign 

either the value from Table 2–6 or a 

value of 100, whichever is greater, as the 

hazardous waste quantity factor value 

for that pathway. 

• If none of the targets for that 

pathway is subject to Level I or Level II 

concentrations, assign a factor value as 

follows: 

—If there has been no removal action, 

assign either the value from Table 2– 

6 or a value of 10, whichever is 

greater, as the hazardous waste 

quantity factor value for that pathway. 

—If there has been a removal action: 

D Determine values from Table 2–6 

with and without consideration of 

the removal action. 

D If the value that would be assigned 

from Table 2–6 without 

consideration of the removal action 

would be 100 or greater, assign 

either the value from Table 2–6 

with consideration of the removal 

action or a value of 100, whichever 

is greater, as the hazardous waste 

quantity factor value for the 

pathway. 

D If the value that would be assigned 

from Table 2–6 without 

consideration of the removal action 

would be less than 100, assign a 

value of 10 as the hazardous waste 

quantity factor value for the 

pathway. 

For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, if the hazardous 

constituent quantity is adequately 

determined for all areas of observed 

contamination, assign the value from 

Table 2–6 as the hazardous waste 

quantity factor value. If the hazardous 

constituent quantity is not adequately 

determined for one or more areas of 

observed contamination, assign either 

the value from Table 2–6 or a value of 

10, whichever is greater, as the 

hazardous waste quantity factor value. 

For the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, if the 

hazardous constituent quantity is 

adequately determined for all areas of 

observed exposure, assign the value 

from Table 2–6 as the hazardous waste 

quantity factor value. If the hazardous 

constituent quantity is not adequately 

determined for one or more areas of 

observed exposure, assign either the 

value from Table 2–6 or assign a factor 

value as follows: 

• If any target for the subsurface 

intrusion component is subject to Level 

I or Level II concentrations (see section 

2.5), assign either the value from Table 

2–6 or a value of 100, whichever is 


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greater, as the hazardous waste quantity 

factor value for this component. 

• If none of the targets for the 

subsurface intrusion component is 

subject to Level I or Level II 

concentrations and if there has been a 

removal action, assign a factor value as 

follows: 

—Determine the values from Table 2–6 

with and without consideration of the 

removal action. 

—If the value that would be assigned 

from Table 2–6 without consideration 

of the removal action would be 100 or 

greater, assign either the value from 

Table 2–6 with consideration of the 

removal action or a value of 100, 

whichever is greater, as the hazardous 

waste quantity factor value for the 

component. 

—If the value that would be assigned 

from Table 2–6 without consideration 

of the removal action would be less 

than 100, assign a value of 10 as the 

hazardous waste quantity factor value 

for the component. 

• Otherwise, if none of the targets for 

the subsurface intrusion component is 

subject to Level I or Level II 

concentrations and there has not been a 

removal action, assign a value from 

Table 2–6 or a value of 10, whichever 

is greater. 

2.4.3 Waste characteristics factor 

category value. Determine the waste 

characteristics factor category value as 

specified in section 2.4.3.1 for all 

pathways and threats, except the surface 

water-human food chain threat and the 

surface water-environmental threat. 

Determine the waste characteristics 

factor category value for these latter two 

threats as specified in section 2.4.3.2. 

2.4.3.1 Factor category value. For 

the pathway (component or threat) 

being evaluated, multiply the toxicity or 

combined factor value, as appropriate, 

from section 2.4.1.2 and the hazardous 

waste quantity factor value from section 

2.4.2.2, subject to a maximum product 

of 1x108. Based on this waste 

characteristics product, assign a waste 

characteristics factor category value to 

the pathway (component or threat) from 

Table 2–7. 

TABLE 2–7—WASTE CHARACTERISTICS

FACTOR CATEGORY VALUES

Waste characteristics product Assigned 

value 

0 ................................................ 0 

Greater than 0 to less than 10 1 

10 to less than 1x102 ............... 2 

1x102 to less than 1x103 .......... 3 

1x103 to less than 1x104 .......... 6 

1x104 to less than 1x105 .......... 10 

1x105 to less than 1x106 .......... 18 

1x106 to less than 1x107 .......... 32 

TABLE 2–7—WASTE CHARACTERISTICS

FACTOR CATEGORY VALUES—Continued 

Waste characteristics product Assigned 

value 

1x107 to less than 1x108 .......... 56 

1x108 to less than 1x109 .......... 100 

1x109 to less than 1x1010 ........ 180 

1x1010 to less than 1x1011 ....... 320 

1x1011 to less than 1x1012 ....... 560 

1x1012 ....................................... 1,000 

2.4.3.2 Factor category value, 

considering bioaccumulation potential. 

For the surface water-human food chain 

threat and the surface waterenvironmental threat, multiply the 

toxicity or combined factor value, as 

appropriate, from section 2.4.1.2 and the 

hazardous waste quantity factor value 

from section 2.4.2.2, subject to: 

• A maximum product of 1x1012, and 

• A maximum product exclusive of 

the bioaccumulation (or ecosystem 

bioaccumulation) potential factor of 1x 

108. 

Based on the total waste 

characteristics product, assign a waste 

characteristics factor category value to 

these threats from Table 2–7. 

2.5 Targets. The types of targets 

evaluated include the following: 

• Individual (factor name varies by 

pathway, component, and threat). 

• Human population. 

• Resources (these vary by pathway, 

component, and threat). 

• Sensitive environments (included 

for the surface water migration pathway, 

air migration pathway, and soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway). 

The factor values that may be 

assigned to each type of target have the 

same range for each pathway for which 

that type of target is evaluated. The 

factor value for most types of targets 

depends on whether the target is subject 

to actual or potential contamination for 

the pathway and whether the actual 

contamination is Level I or Level II: 

• Actual contamination: Target is 

associated either with a sampling 

location that meets the criteria for an 

observed release (or observed 

contamination or observed exposure) for 

the pathway or with an observed release 

based on direct observation for the 

pathway (additional criteria apply for 

establishing actual contamination for 

the human food chain threat in the 

surface water migration pathway, see 

sections 4.1.3.3 and 4.2.3.3). Sections 3 

through 6 specify how to determine the 

targets associated with a sampling 

location or with an observed release 

based on direct observation. Determine 

whether the actual contamination is 

Level I or Level II as follows: 

—Level I: 

D Media-specific concentrations for 

the target meet the criteria for an 

observed release (or observed 

contamination or observed 

exposure) for the pathway and are 

at or above media-specific 

benchmark values. These 

benchmark values (see section 

2.5.2) include both screening 

concentrations and concentrations 

specified in regulatory limits (such 

as Maximum Contaminant Level 

(MCL) values), or 

D For the human food chain threat in 

the surface water migration 

pathway, concentrations in tissue 

samples from aquatic human food 

chain organisms are at or above 

benchmark values. Such tissue 

samples may be used in addition to 

media-specific concentrations only 

as specified in sections 4.1.3.3 and 

4.2.3.3. 

—Level II: 

D Media-specific concentrations for 

the target meet the criteria for an 

observed release (or observed 

contamination or observed 

exposure) for the pathway, but are 

less than media-specific 

benchmarks. If none of the 

hazardous substances eligible to be 

evaluated for the sampling location 

has an applicable benchmark, 

assign Level II to the actual 

contamination at the sampling 

location, or 

D 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 

D For the human food chain threat in 

the surface water migration 

pathway, concentrations in tissue 

samples from aquatic human food 

chain organisms, when applicable, 

are below benchmark values. 

—If a target is subject to both Level I 

and Level II concentrations for a 

pathway (or threat), evaluate the 

target using Level I concentrations for 

that pathway (or threat). 

• Potential contamination: Target is 

subject to a potential release (that is, 

target is not associated with actual 

contamination for that pathway or 

threat). 

Assign a factor value for individual 

risk as follows (select the highest value 

that applies to the pathway or threat): 

• 50 points if any individual is 

exposed to Level I concentrations. 

• 45 points if any individual is 

exposed to Level II concentrations. 


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• Maximum of 20 points if any 

individual is subject to potential 

contamination. The value assigned is 20 

unless reduced by a distance or dilution 

weight appropriate to the pathway. 

Assign factor values for population 

and sensitive environments as follows: 

• Sum Level I targets and multiply by 

10. (Level I is not used for sensitive 

environments in the soil exposure 

component of the soil exposure and 

subsurface intrusion and air migration 

pathways.) 

• Sum Level II targets. 

• Multiply potential targets in all but 

the soil exposure and subsurface 

intrusion pathway by distance or 

dilution weights appropriate to the 

pathway, sum, and divide by 10. 

Distance or dilution weighting accounts 

for diminishing exposure with 

increasing distance or dilution within 

the different pathways. For targets 

within an area of subsurface 

contamination in the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway, multiply by a weighting factor 

as directed in section 5.2.1.3.2.3. 

• Sum the values for the three levels. 

In addition, resource value points are 

assigned within all pathways for 

welfare-related impacts (for example, 

impacts to agricultural land), but do not 

depend on whether there is actual or 

potential contamination. 

2.5.1 Determination of level of 

actual contamination at a sampling 

location. Determine whether Level I 

concentrations or Level II 

concentrations apply at a sampling 

location (and thus to the associated 

targets) as follows: 

• Select the benchmarks applicable to 

the pathway (component or threat) 

being evaluated. 

• Compare the concentrations of 

hazardous substances in the sample (or 

comparable samples) to their benchmark 

concentrations for the pathway 

(component or threat), as specified in 

section 2.5.2. 

• Determine which level applies 

based on this comparison. 

• If none of the hazardous substances 

eligible to be evaluated for the sampling 

location has an applicable benchmark, 

assign Level II to the actual 

contamination at that sampling location 

for the pathway (component or threat). 

In making the comparison, consider 

only those samples, and only those 

hazardous substances in the sample, 

that meet the criteria for an observed 

release (or observed contamination or 

observed exposure) for the pathway, 

except: Tissue samples from aquatic 

human food chain organisms may also 

be used as specified in sections 4.1.3.3 

and 4.2.3.3 of the surface water-human 

food chain threat. If any hazardous 

substance is present in more than one 

comparable sample for the sampling 

location, use the highest concentration 

of that hazardous substance from any of 

the comparable samples in making the 

comparisons. 

Treat sets of samples that are not 

comparable separately and make a 

separate comparison for each such set. 

2.5.2 Comparison to benchmarks. 

Use the following media-specific 

benchmarks for making the comparisons 

for the indicated pathway (or threat): 

• Maximum Contaminant Level Goals 

(MCLGs)—ground water migration 

pathway and drinking water threat in 

surface water migration pathway. Use 

only MCLG values greater than 0. 

• Maximum Contaminant Levels 

(MCLs)—ground water migration 

pathway and drinking water threat in 

surface water migration pathway. 

• Food and Drug Administration 

Action Level (FDAAL) for fish or 

shellfish—human food chain threat in 

surface water migration pathway. 

• EPA Ambient Water Quality 

Criteria (AWQC/National 

Recommended Water Quality Criteria) 

for protection of aquatic life— 

environmental threat in surface water 

migration pathway. 

• EPA Ambient Aquatic Life 

Advisory Concentrations (AALAC)— 

environmental threat in surface water 

migration pathway. 

• National Ambient Air Quality 

Standards (NAAQS)—air migration 

pathway. 

• National Emission Standards for 

Hazardous Air Pollutants (NESHAPs)— 

air migration pathway. Use only those 

NESHAPs promulgated in ambient 

concentration units. 

• Screening concentration for cancer 

corresponding to that concentration that 

corresponds to the 10¥6 individual 

cancer risk for inhalation exposures (air 

migration pathway or subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway) or for oral exposures (ground 

water migration pathway; drinking 

water and human food chain threats in 

surface water migration pathway; and 

soil exposure and subsurface intrusion 

pathway). 

• Screening concentration for 

noncancer toxicological responses 

corresponding to the RfC for inhalation 

exposures (air migration pathway and 

subsurface intrusion component of the 

soil exposure and subsurface intrusion 

pathway) or RfD for oral exposures 

(ground water migration pathway; 

drinking water and human food chain 

threats in surface water migration 

pathway; and soil exposure and 

subsurface intrusion pathway). 

Select the benchmark(s) applicable to 

the pathway (component or threat) 

being evaluated as specified in sections 

3 through 6. Compare the concentration 

of each hazardous substance from the 

sampling location to its benchmark 

concentration(s) for that pathway 

(component or threat). Use only those 

samples and only those hazardous 

substances in the sample that meet the 

criteria for an observed release (or 

observed contamination or observed 

exposure) for the pathway, except: 

Tissue samples from aquatic human 

food chain organisms may be used as 

specified in sections 4.1.3.3 and 4.2.3.3. 

If the concentration of any applicable 

hazardous substance from any sample 

equals or exceeds its benchmark 

concentration, consider the sampling 

location to be subject to Level I 

concentrations for that pathway (or 

threat). If more than one benchmark 

applies to the hazardous substance, 

assign Level I if the concentration of the 

hazardous substance equals or exceeds 

the lowest applicable benchmark 

concentration. 

If no hazardous substance 

individually equals or exceeds its 

benchmark concentration, but more 

than one hazardous substance either 

meets the criteria for an observed 

release (or observed contamination or 

observed exposure) for the sample (or 

comparable samples) or is eligible to be 

evaluated for a tissue sample (see 

sections 4.1.3.3 and 4.2.3.3), calculate 

the indices I and J specified below based 

on these hazardous substances. 

For those hazardous substances that 

are carcinogens (that is, those having 

either a carcinogen weight-of-evidence 

classification of A, B, or C or a weightof-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: 

Where: 

Ci= Concentration of hazardous substance i 

in sample (or highest concentration of 

hazardous substance i from among 

comparable samples). 

SCi= Screening concentration for cancer 

corresponding to that concentration that 

corresponds to its 10¥6 individual 

cancer risk for applicable exposure 

(inhalation or oral) for hazardous 

substance i. 

n = Number of applicable hazardous 

substances in sample (or comparable 


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samples) that are carcinogens and for 

which an SCi is available. 

For those hazardous substances for 

which an RfD or RfC is available, 

calculate an index J for the sample 

location as follows: 

Where: 

Cj= Concentration of hazardous substance j 

in sample (or highest concentration of 

hazardous substance j from among 

comparable samples). 

CRj= Screening concentration for noncancer 

toxicological responses corresponding to 

RfD or RfC for applicable exposure 

(inhalation or oral) for hazardous 

substance j. 

m = Number of applicable hazardous 

substances in sample (or comparable 

samples) for which a CRj is available. 

If either I or J equals or exceeds 1, 

consider the sampling location to be 

subject to Level I concentrations for that 

pathway (component or threat). If both 

I and J are less than 1, consider the 

sampling location to be subject to Level 

II concentrations for that pathway 

(component or threat). If, for the 

sampling location, there are sets of 

samples that are not comparable, 

calculate I and J separately for each such 

set, and use the highest calculated 

values of I and J to assign Level I and 

Level II. 

See sections 7.3.1 and 7.3.2 for 

criteria for determining the level of 

contamination for radioactive 

substances. 

* * * * * 

5.0 Soil Exposure and Subsurface 

Intrusion Pathway 

5.0. Exposure components. Evaluate 

the soil exposure and subsurface 

intrusion pathway based on two 

exposure components: 

• Soil exposure component (see 

section 5.1). 

• Subsurface intrusion component 

(see section 5.2). 

Score one or both components 

considering their relative importance. If 

only one component is scored, assign its 

score as the soil exposure and 

subsurface intrusion pathway score. If 

both components are scored, sum the 

two scores and assign it as the soil 

exposure and subsurface intrusion 

pathway score, subject to a maximum of 

100. 


srobinson on DSK5SPTVN1PROD with PROPOSALS2EP29FE16.032</GPH>
5.1 Soil exposure component. 

Evaluate the soil exposure component population threat and nearby 

based on two threats: Resident 

based on three factor categories: 

population threat. Evaluate both threats 


EP29FE16.033</GPH>srobinson on DSK5SPTVN1PROD with PROPOSALS2

Soil Exposure 

Component 

Resident Population 

+ 

Nearby Population 

Subsurface Intrusion 

Component 

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

Likelihood of Exposure (LE) 

Observed Contamination 

Area with Resident 

Targets 

Likelihood of Exposure (LE) 

Attractiveness/ Accessibility Area of Contamination 

Likelihood of Exposure (LE) 

Observed Exposure 

Potential for Exposure 

Structure Containment 

Depth to Contamination 

Vertical Migration 

Vapor Migration Potential 

Waste Characteristics (WC) 

Toxicity X I• Chronic 

Carcinogenic 

Acute 

Hazardous Waste Quantity 

Hazardous Constituent Quantity Hazardous Wastestream Quantity 

Volume 

Area 

Waste Characteristics (WC) 

Toxicity X I• Chronic 

X 

Carcinogenic 

Acute 

Hazardous Waste Quantity 

Hazardous Constituent Quantity Hazardous Wastestream Quantity 

Volume 

Area 

+ 

Waste Characteristics (WC) 

Toxicity 

Chronic Carcinogenic 

Acute 

Degradation 

Hazardous Waste Quantity 

Hazardous Constituent Quantity Hazardous Wastestream Quantity 

Volume 

Area 

Targets (T

Resident Individual 

X I Resident Population 

Level I ConcenLevel II Conce

Workers 

Resources 

Terrestrial Sensitive 

Targets (T

Nearby Individual X I Population Within O

X 

Targets (T

Exposed Individu

Population 

Level I Conc

Level II Con

Population o

Resources 
10412 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

Likelihood of exposure, waste 

characteristics, and targets. Figure 5–1 

indicates the factors included within 

each factor category for each type of 

threat. 

Determine the soil exposure 

component score (Sse) in terms of the 

factor category values as follows: 

Where: 

LEi= Likelihood of exposure factor category 

value for threat i (that is, resident 

population threat or nearby population 

threat). 

WCi= Waste characteristics factor category 

value for threat i. 

Ti= Targets factor category value for threat i. 

SF = Scaling factor. 

Table 5–1 outlines the specific 

calculation procedure. 

TABLE 5–1—SOIL EXPOSURE COMPONENT SCORESHEET

Factor categories and factors Maximum 

value 

Value 

assigned 

Resident Population Threat 

Likelihood of Exposure 

1. Likelihood of Exposure ................................................................................................................................. 550 ........................

Waste Characteristics 

2. Toxicity ......................................................................................................................................................... (a) ........................

3. Hazardous Waste Quantity .......................................................................................................................... (a) ........................

4. Waste Characteristics .................................................................................................................................. 100 ........................

Targets 

5. Resident Individual ....................................................................................................................................... 50 ........................

6. Resident Population: 

6a. Level I Concentrations (b) ........................

6b. Level II Concentrations (b) ........................

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

7. Workers ........................................................................................................................................................ 15 ........................

8. Resources .................................................................................................................................................... 5 ........................

9. Terrestrial Sensitive Environments .............................................................................................................. (c) ........................

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

Resident Population Threat Score 

11. Resident Population Threat (lines 1x4x10) ................................................................................................ (b) ........................

Nearby Population Threat 

Likelihood of Exposure 

12. Attractiveness/Accessibility ........................................................................................................................ 100 ........................

13. Area of Contamination ............................................................................................................................... 100 ........................

14. Likelihood of Exposure ............................................................................................................................... 500 ........................

Waste Characteristics 

15. Toxicity ....................................................................................................................................................... (a) ........................

16. Hazardous Waste Quantity ........................................................................................................................ (a) ........................

17. Waste Characteristics ................................................................................................................................ 100 ........................

Targets 

18. Nearby Individual ........................................................................................................................................ 1 ........................

19. Population Within 1 Mile ............................................................................................................................ (b) ........................

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

Nearby Population Threat Score 

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

Soil Exposure Component Score 

22. Soil Exposure Component Score d (Sse), (lines [11+21]/82,500, subject to a maximum of 100) ............. 100 ........................

a Maximum value applies to waste characteristics category. 

b Maximum value not applicable. 

c No specific maximum value applies to factor. However, pathway score based solely on terrestrial sensitive environments is limited to maximum of 60. d Do not round to nearest integer. 

5.1.0 General considerations. 

Evaluate the soil exposure component 

based on areas of observed 

contamination: 

• Consider observed contamination to 

be present at sampling locations where 

analytic evidence indicates that: 

—A hazardous substance attributable to 

the site is present at a concentration 

significantly above background levels 

for the site (see Table 2–3 in section 

2.3 for the criteria for determining 

analytical significance), and 

—This hazardous substance, if not 

present at the surface, is covered by 

2 feet or less of cover material (for 

example, soil). 

• Establish areas of observed 

contamination based on sampling 

locations at which there is observed 

contamination as follows: 

—For all sources except contaminated 

soil, if observed contamination from 

the site is present at any sampling 

location within the source, consider 

that entire source to be an area of 

observed contamination. 

— For contaminated soil, consider both 

the sampling location(s) with 

observed contamination from the site 

and the area lying between such 

locations to be an area of observed 

contamination, unless available 

information indicates otherwise. 

• If an area of observed 

contamination (or portion of such an 

area) is covered by a permanent, or 

otherwise maintained, essentially 


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impenetrable material (for example, 

asphalt) that is not more than 2 feet 

thick, exclude that area (or portion of 

the area) in evaluating the soil exposure 

component. 

• For an area of observed 

contamination, consider only those 

hazardous substances that meet the 

criteria for observed contamination for 

that area to be associated with that area 

in evaluating the soil exposure 

component (see section 2.2.2). 

If there is observed contamination, 

assign scores for the resident population 

threat and the nearby population threat, 

as specified in sections 5.1.1 and 5.1.2. 

If there is no observed contamination, 

assign the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway a score of 0. 

5.1.1 Resident population threat. 

Evaluate the resident population threat 

only if there is an area of observed 

contamination in one or more of the 

following locations: 

• Within the property boundary of a 

residence, school, or day care center 

and within 200 feet of the respective 

residence, school, or day care center, or 

• Within a workplace property 

boundary and within 200 feet of a 

workplace area, or 

• Within the boundaries of a resource 

specified in section 5.1.1.3.4, or 

• Within the boundaries of a 

terrestrial sensitive environment 

specified in section 5.1.1.3.5. 

If not, assign the resident population 

threat a value of 0, enter this value in 

Table 5–1, and proceed to the nearby 

population threat (section 5.1.2). 

5.1.1.1 Likelihood of exposure. 

Assign a value of 550 to the likelihood 

of exposure factor category for the 

resident population threat if there is an 

area of observed contamination in one 

or more locations listed in section 5.1.1. 

Enter this value in Table 5–1. 

5.1.1.2 Waste characteristics. 

Evaluate waste characteristics based on 

two factors: Toxicity and hazardous 

waste quantity. Evaluate only those 

hazardous substances that meet the 

criteria for observed contamination at 

the site (see section 5.1.0). 

5.1.1.2.1 Toxicity. Assign a toxicity 

factor value to each hazardous 

substance as specified in section 2.4.1.1. 

Use the hazardous substance with the 

highest toxicity factor value to assign 

the value to the toxicity factor for the 

resident population threat. Enter this 

value in Table 5–1. 

5.1.1.2.2 Hazardous waste quantity. 

Assign a hazardous waste quantity 

factor value as specified in section 2.4.2. 

In estimating the hazardous waste 

quantity, use Table 5–2 and: 

• Consider only the first 2 feet of 

depth of an area of observed 

contamination, except as specified for 

the volume measure. 

• Use the volume measure (see 

section 2.4.2.1.3) only for those types of 

areas of observed contamination listed 

in Tier C of Table 5–2. In evaluating the 

volume measure for these listed areas of 

observed contamination, use the full 

volume, not just the volume within the 

top 2 feet. 

• Use the area measure (see section 

2.4.2.1.4), not the volume measure, for 

all other types of areas of observed 

contamination, even if their volume is 

known. 

Enter the value assigned in Table 5– 

1. 

TABLE 5–2—HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SOIL EXPOSURE COMPONENT

Tier Measure Units 

Equation for 

assigning 

value a

A .............. Hazardous Constituent Quantity (C) ....................................................................................................... lb .............. C 

Bb ............ Hazardous Wastestream Quantity (W) ................................................................................................... lb .............. W/5,000 

C b ............ Volume (V).

Surface Impoundment c ........................................................................................................................... yd 3 ........... V/2.5 

Drums d .................................................................................................................................................... gallon ....... V/500 

Tanks and Containers Other Than Drums ............................................................................................. yd 3 ........... V/2.5 

D b ............ Area (A).

Landfill ..................................................................................................................................................... ft 2 ............ A/34,000 

Surface Impoundment ............................................................................................................................. ft 2 ............ A/13 

Surface Impoundment (Buried/backfilled) ............................................................................................... ft 2 ............ A/13 

Land treatment ........................................................................................................................................ ft 2 ............ A/270 

Pile e ........................................................................................................................................................ ft 2 ............ A/34 

Contaminated Soil ................................................................................................................................... ft 2 ............ A/34,000 

a Do not round nearest integer. 

b Convert volume to mass when necessary: 1 ton = 2,000 pounds = 1 cubic yard = 4 drums = 200 gallons. 

c Use volume measure only for surface impoundments containing hazardous substances present as liquids. Use area measures in Tier D for 

dry surface impoundments and for buried/backfilled surface impoundments. d If actual volume of drums is unavailable, assume 1 drum = 50 gallons. 

e Use land surface area under pile, not surface area of pile. 

5.1.1.2.3 Calculation of waste 

characteristics factor category value. 

Multiply the toxicity and hazardous 

waste quantity factor values, subject to 

a maximum product of 1 × 108. Based 

on this product, assign a value from 

Table 2–7 (section 2.4.3.1) to the waste 

characteristics factor category. Enter this 

value in Table 5–1. 

5.1.1.3 Targets. Evaluate the targets 

factor category for the resident 

population threat based on five factors: 

Resident individual, resident 

population, workers, resources, and 

terrestrial sensitive environments. 

In evaluating the targets factor 

category for the resident population 

threat, count only the following as 

targets: 

• Resident individual—a person 

living or attending school or day care on 

a property with an area of observed 

contamination and whose residence, 

school, or day care center, respectively, 

is on or within 200 feet of the area of 

observed contamination. 

• Worker—a person working on a 

property with an area of observed 

contamination and whose workplace 

area is on or within 200 feet of the area 

of observed contamination. 

• Resources located on an area of 

observed contamination, as specified in 

section 5.1.1. 

• Terrestrial sensitive environments 

located on an area of observed 

contamination, as specified in section 

5.1.1. 

5.1.1.3.1 Resident individual. 

Evaluate this factor based on whether 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
10414 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

there is a resident individual, as 

specified in section 5.1.1.3, who is 

subject to Level I or Level II 

concentrations. 

First, determine those areas of 

observed contamination subject to Level 

I concentrations and those subject to 

Level II concentrations as specified in 

sections 2.5.1 and 2.5.2. Use the healthbased benchmarks from Table 5–3 in 

determining the level of contamination. 

Then assign a value to the resident 

individual factor as follows: 

• Assign a value of 50 if there is at 

least one resident individual for one or 

more areas subject to Level I 

concentrations. 

• Assign a value of 45 if there is no 

such resident individuals, but there is at 

least one resident individual for one or 

more areas subject to Level II 

concentrations. 

• Assign a value of 0 if there is no 

resident individual. 

Enter the value assigned in Table 5– 

1. 

5.1.1.3.2 Resident population. 

Evaluate resident population based on 

two factors: Level I concentrations and 

Level II concentrations. Determine 

which factor applies as specified in 

sections 2.5.1 and 2.5.2, using the 

health-based benchmarks from Table 5– 

3. Evaluate populations subject to Level 

I concentrations as specified in section 

5.1.1.3.2.1 and populations subject to 

Level II concentrations as specified in 

section 5.1.1.3.2.2. 

TABLE 5–3—HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN SOILS

Screening concentration for cancer corresponding to that concentration that corresponds to the 10¥6 individual cancer risk for oral exposures. 

Screening concentration for noncancer toxicological responses corresponding to the Reference Dose (RfD) for oral exposures. 

Count only those persons meeting the 

criteria for resident individual as 

specified in section 5.1.1.3. In 

estimating the number of people living 

on property with an area of observed 

contamination, when the estimate is 

based on the number of residences, 

multiply each residence by the average 

number of persons per residence for the 

county in which the residence is 

located. 

5.1.1.3.2.1 Level I concentrations. 

Sum the number of resident individuals 

subject to Level I concentrations and 

multiply this sum by 10. Assign the 

resulting product as the value for this 

factor. Enter this value in Table 5–1. 

5.1.1.3.2.2 Level II concentrations. 

Sum the number of resident individuals 

subject to Level II concentrations. Do 

not include those people already 

counted under the Level I 

concentrations factor. Assign this sum 

as the value for this factor. Enter this 

value in Table 5–1. 

5.1.1.3.2.3 Calculation of resident 

population factor value. Sum the factor 

values for Level I concentrations and 

Level II concentrations. Assign this sum 

as the resident population factor value. 

Enter this value in Table 5–1. 

5.1.1.3.3 Workers. Evaluate this 

factor based on the number of workers 

that meet the section 5.1.1.3 criteria. 

Assign a value for these workers using 

Table 5–4. Enter this value in Table 5– 

1. 

TABLE 5–4—FACTOR VALUES FOR

WORKERS

Number of workers Assigned 

value 

0 .................................................. 0 

1 to 100 ...................................... 5 

101 to 1,000 ............................... 10 

Greater than 1,000 ..................... 15 

5.1.1.3.4 Resources. Evaluate the 

resources factor as follows: 

• Assign a value of 5 to the resources 

factor if one or more of the following is 

present on an area of observed 

contamination at the site: 

—Commercial agriculture. 

—Commercial silviculture. 

—Commercial livestock production or 

commercial livestock grazing. 

• Assign a value of 0 if none of the 

above are present. 

Enter the value assigned in Table 5– 

1. 

5.1.1.3.5 Terrestrial sensitive 

environments. Assign value(s) from 

Table 5–5 to each terrestrial sensitive 

environment that meets the eligibility 

criteria of section 5.1.1.3. 

Calculate a value (ES) for terrestrial 

sensitive environments as follows: 

where: 

Si=Value(s) assigned from Table 5–5 to 

terrestrial sensitive environment i. 

n=Number of terrestrial sensitive 

environments meeting section 5.1.1.3 

criteria. 

Because the pathway score based 

solely on terrestrial sensitive 

environments is limited to a maximum 

of 60, determine the value for the 

terrestrial sensitive environments factor 

as follows: 

TABLE 5–5—TERRESTRIAL SENSITIVE ENVIRONMENTS RATING VALUES

Terrestrial sensitive environments Assigned value 

Terrestrial critical habitat a for Federal designated endangered or threatened species ............................................................... 100 

National Park 

Designated Federal Wilderness Area 

National Monument 

Terrestrial habitat known to be used by Federal designated or proposed threatened or endangered species .......................... 75 

National Preserve (terrestrial) 

National or State Terrestrial Wildlife Refuge 

Federal land designated for protection of natural ecosystems 

Administratively proposed Federal Wilderness Area 

Terrestrial areas utilized for breeding by large or dense aggregations of animals b

Terrestrial habitat known to be used by State designated endangered or threatened species ................................................... 50 

Terrestrial habitat known to be used by species under review as to its Federal designated endangered or threatened status 

State lands designated for wildlife or game management ............................................................................................................ 25 

State designated Natural Areas 

Particular areas, relatively small in size, important to maintenance of unique biotic communities 

a Critical habitat as defined in 50 CFR 424.02. 


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Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10415 

b Limit to vertebrate species. 

• Multiply the values assigned to the 

resident population threat for likelihood 

of exposure (LE), waste characteristics 

(WC), and ES. Divide the product by 

82,500. 

—If the result is 60 or less, assign the 

value ES as the terrestrial sensitive 

environments factor value. 

—If the result exceeds 60, calculate a 

value EC as follows: 

Assign the value EC as the terrestrial 

sensitive environments factor value. Do 

not round this value to the nearest 

integer. 

Enter the value assigned for the 

terrestrial sensitive environments factor 

in Table 5–1. 

5.1.1.3.6 Calculation of resident 

population targets factor category value. 

Sum the values for the resident 

individual, resident population, 

workers, resources, and terrestrial 

sensitive environments factors. Do not 

round to the nearest integer. Assign this 

sum as the targets factor category value 

for the resident population threat. Enter 

this value in Table 5–1. 

5.1.1.4 Calculation of resident 

population threat score. Multiply the 

values for likelihood of exposure, waste 

characteristics, and targets for the 

resident population threat, and round 

the product to the nearest integer. 

Assign this product as the resident 

population threat score. Enter this score 

in Table 5–1. 

5.1.2 Nearby population threat. 

Include in the nearby population only 

those individuals who live or attend 

school within a 1-mile travel distance of 

an area of observed contamination at the 

site and who do not meet the criteria for 

resident individual as specified in 

section 5.1.1.3. 

Do not consider areas of observed 

contamination that have an 

attractiveness/accessibility factor value 

of 0 (see section 5.1.2.1.1) in evaluating 

the nearby population threat. 

5.1.2.1 Likelihood of exposure. 

Evaluate two factors for the likelihood 

of exposure factor category for the 

nearby population threat: 

Attractiveness/accessibility and area of 

contamination. 

5.1.2.1.1 Attractiveness/

accessibility. Assign a value for 

attractiveness/accessibility from Table 

5–6 to each area of observed 

contamination, excluding any land used 

for residences. Select the highest value 

assigned to the areas evaluated and use 

it as the value for the attractiveness/

accessibility factor. Enter this value in 

Table 5–1. 

5.1.2.1.2 Area of contamination. 

Evaluate area of contamination based on 

the total area of the areas of observed 

contamination at the site. Count only 

the area(s) that meet the criteria in 

section 5.1.0 and that receive an 

attractiveness/accessibility value greater 

than 0. Assign a value to this factor from 

Table 5–7. Enter this value in Table 5– 

1. 

TABLE 5–6—ATTRACTIVENESS/ACCESSIBILITY VALUES

Area of observed contamination Assigned 

value 

Designated recreational area .............................................................................................................................................................. 100 

Regularly used for public recreation (for example, fishing, hiking, softball) ....................................................................................... 75 

Accessible and unique recreational area (for example, vacant lots in urban area) ........................................................................... 75 

Moderately accessible (may have some access improvements, for example, gravel road), with some public recreation use ......... 50

Slightly accessible (for example, extremely rural area with no road improvement), with some public recreation use ...................... 25 

Accessible, with no public recreation use ........................................................................................................................................... 10 

Surrounded by maintained fence or combination of maintained fence and natural barriers .............................................................. 5 

Physically inaccessible to public, with no evidence of public recreation use ..................................................................................... 0 

TABLE 5–7—AREA OF CONTAMINATION FACTOR VALUES

Total area of the areas of observed contamination (square feet) Assigned 

value 

Less than or equal to 5,000 ................................................................................................................................................................ 5 

Greater than 5,000 to 125,000 ............................................................................................................................................................ 20 

Greater than 125,000 to 250,000 ........................................................................................................................................................ 40 

Greater than 250,000 to 375,000 ........................................................................................................................................................ 60 

Greater than 375,000 to 500,000 ........................................................................................................................................................ 80 

Greater than 500,000 .......................................................................................................................................................................... 100 

5.1.2.1.3 Likelihood of exposure 

factor category value. Assign a value 

from Table 5–8 to the likelihood of 

exposure factor category, based on the 

values assigned to the attractiveness/

accessibility and area of contamination 

factors. Enter this value in Table 5–1. 

TABLE 5–8—NEARBY POPULATION LIKELIHOOD OF EXPOSURE FACTOR VALUES

Area of contamination 

factor value 

Attractiveness/accessibility factor value 

100 75 50 25 10 5 0 

100 ............................... 500 500 375 250 125 50 0 

80 ................................. 500 375 250 125 50 25 0 

60 ................................. 375 250 125 50 25 5 0 

40 ................................. 250 125 50 25 5 5 0 


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10416 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

TABLE 5–8—NEARBY POPULATION LIKELIHOOD OF EXPOSURE FACTOR VALUES—Continued 

Area of contamination 

factor value 

Attractiveness/accessibility factor value 

100 75 50 25 10 5 0 

20 ................................. 125 50 25 5 5 5 0 

5 ................................... 50 25 5 5 5 5 0 

5.1.2.2 Waste characteristics. 

Evaluate waste characteristics based on 

two factors: Toxicity and hazardous 

waste quantity. Evaluate only those 

hazardous substances that meet the 

criteria for observed contamination (see 

section 5.1.0) at areas that can be 

assigned an attractiveness/accessibility 

factor value greater than 0. 

5.1.2.2.1 Toxicity. Assign a toxicity 

factor value as specified in section 

2.4.1.1 to each hazardous substance 

meeting the criteria in section 5.1.2.2. 

Use the hazardous substance with the 

highest toxicity factor value to assign 

the value to the toxicity factor for the 

nearby population threat. Enter this 

value in Table 5–1. 

5.1.2.2.2 Hazardous waste quantity. 

Assign a value to the hazardous waste 

quantity factor as specified in section 

5.1.1.2.2, except: Consider only those 

areas of observed contamination that 

can be assigned an attractiveness/

accessibility factor value greater than 0. 

Enter the value assigned in Table 5–1. 

5.1.2.2.3 Calculation of waste 

characteristics factor category value. 

Multiply the toxicity and hazardous 

waste quantity factor values, subject to 

a maximum product of 1 × 108. Based 

on this product, assign a value from 

Table 2–7 (section 2.4.3.1) to the waste 

characteristics factor category. Enter this 

value in Table 5–1. 

5.1.2.3 Targets. Evaluate the targets 

factory category for the nearby 

population threat based on two factors: 

Nearby individual and population 

within a 1-mile travel distance from the 

site. 

5.1.2.3.1 Nearby individual. If one or 

more persons meet the section 5.1.1.3 

criteria for a resident individual, assign 

this factor a value of 0. Enter this value 

in Table 5–1. 

If no person meets the criteria for a 

resident individual, determine the 

shortest travel distance from the site to 

any residence or school. In determining 

the travel distance, measure the shortest 

overland distance an individual would 

travel from a residence or school to the 

nearest area of observed contamination 

for the site with an attractiveness/

accessibility factor value greater than 0. 

If there are no natural barriers to travel, 

measure the travel distance as the 

shortest straight-line distance from the 

residence or school to the area of 

observed contamination. If natural 

barriers exist (for example, a river), 

measure the travel distance as the 

shortest straight-line distance from the 

residence or school to the nearest 

crossing point and from there as the 

shortest straight-line distance to the area 

of observed contamination. Based on the 

shortest travel distance, assign a value 

from Table 5–9 to the nearest individual 

factor. Enter this value in Table 5–1. 

TABLE 5–9—NEARBY INDIVIDUAL

FACTOR VALUES

Travel distance for nearby 

individual 

(miles) 

Assigned 

value 

Greater than 0 to 1⁄4 ..................... a1 

Greater than 1⁄4 to 1 ..................... 0 

aAssign a value of 0 if one or more persons 

meet the section 5.1.1.3 criteria for resident 

individual. 

5.1.2.3.2 Population within 1 mile. 

Determine the population within each 

travel distance category of Table 5–10. 

Count residents and students who 

attend school within this travel 

distance. Do not include those people 

already counted in the resident 

population threat. Determine travel 

distances as specified in section 

5.1.2.3.1. 

In estimating residential population, 

when the estimate is based on the 

number of residences, multiply each 

residence by the average number of 

persons per residence for the county in 

which the residence is located. 

Based on the number of people 

included within a travel distance 

category, assign a distance-weighted 

population value for that travel distance 

from Table 5–10. 

Calculate the value for the population 

within 1 mile factor (PN) as follows: 

Where: 

Wi = Distance-weighted population value 

from Table 5–10 for travel distance 

category i. 

If PN is less than 1, do not round it 

to the nearest integer; if PN is 1 or more, 

round to the nearest integer. Enter this 

value in Table 5–1. 

5.1.2.3.3 Calculation of nearby 

population targets factor category value. 

Sum the values for the nearby 

individual factor and the population 

within 1 mile factor. Do not round this 

sum to the nearest integer. Assign this 

sum as the targets factor category value 

for the nearby population threat. Enter 

this value in Table 5–1. 


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Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10417 

TABLE 5–10—DISTANCE

WEIGHTED

POPULATION

VALUES FOR

NEARBY

POPULATION

THREAT a

Travel distance category 

(miles) 

Number of people within the travel distance category 

0 1 to 10 11 to 30 31 to 100 101 to 300 301 to 1,000 1,001 to 3,000 3,001 to 10,000 10,001 to 30,000 30,001 to 100,000 100,001 to 300,000 300,001 to 1,000,000 

Greater than 0 to 1⁄4 ........................... 0 0.1 0.4 1.0 4 13 41 130 408 1,303 4,081 13,034 Greater than 1⁄4 to 1⁄2 .......................... 0 0.05 0.2 0.7 2 7 20 65 204 652 2,041 6,517 Greater than 1⁄2 to 1 ........................... 0 0.02 0.1 0.3 1 3 10 33 102 326 1,020 3,258 

a Round the number of people present within a travel distance category to nearest integer. Do not round the assigned distance-weighted population value to nearest integer. 


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10418 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

5.1.2.4 Calculation of nearby 

population threat score. Multiply the 

values for likelihood of exposure, waste 

characteristics, and targets for the 

nearby population threat, and round the 

product to the nearest integer. Assign 

this product as the nearby population 

threat score. Enter this score in Table 

5–1. 

5.1.3 Calculation of soil exposure 

component score. Sum the resident 

population threat score and the nearby 

population threat score, and divide the 

sum by 82,500. Assign the resulting 

value, subject to a maximum of 100, as 

the soil exposure component score (Sse). 

Enter this score in Table 5–1. 

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: 

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. 

TABLE 5–11—SUBSURFACE INTRUSION COMPONENT SCORESHEET

Factor categories and factors Maximum 

value 

Value 

assigned 

Subsurface Intrusion Component 

Likelihood of Exposure: 

1. Observed Exposure 550 

2. Potential for Exposure: 

2a. Structure Containment ........................................................................................................................ 10 

2b. Depth to contamination ....................................................................................................................... 10 

2c. Vertical Migration ................................................................................................................................. 15 

2d. Vapor Migration Potential .................................................................................................................... 25 

3. Potential for Exposure (lines 2a * (2b + 2c + 2d), subject to a maximum of 500) ...................................... 500 

4. Likelihood of Exposure (higher of lines 1 or 3) ............................................................................................ 550 

Waste Characteristics: 

5. Toxicity/Degradation ..................................................................................................................................... (a)

6. Hazardous Waste Quantity .......................................................................................................................... (a)

7. Waste Characteristics (subject to a maximum of 100) ................................................................................ 100 

Targets: 

8. Exposed Individual ....................................................................................................................................... 50 

9. Population:.

9a. Level I Concentrations ........................................................................................................................ (b)

9b. Level II Concentrations ....................................................................................................................... (b)

9c. Population within an Area of Subsurface Contamination ................................................................... (b)

9d. Total Population (lines 9a + 9b + 9c) ................................................................................................. (b)

10. Resources .................................................................................................................................................. 5 

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

Subsurface Intrusion Component Score: 

12. Subsurface Intrusion Component (lines 4 × 7 × 11)/82,500 c (subject to a maximum of 100) ................. 100 

Soil Exposure and Subsurface Intrusion Pathway Score: 

13. Soil Exposure Component + Subsurface Intrusion Component (subject to a maximum of 100) ............. 100 

a Maximum value applies to waste characteristics category. 

b Maximum value not applicable. 

c Do not round to the nearest integer. 

5.2.0—General considerations. The 

subsurface intrusion component 

evaluates the threats from hazardous 

substances that have or could intrude 

into regularly occupied structures via 

surficial ground water or the 

unsaturated zone. Evaluate the 

subsurface intrusion component based 

on the actual or potential intrusion of 

hazardous substances into a regularly 

occupied structures that has structure 

containment value greater than zero; or 

actual or potential intrusion of 

hazardous substances exists in the 

unsaturated zone or the surficial ground 

water below the regularly occupied 

structures. These structures may or may 

not have subunits. Subunits are 

partitioned areas within a structure with 

separate heating, ventilating, and air 

conditioning (HVAC) systems or 

distinctly different air exchange rates. 

Subunits include regularly occupied 

partitioned tenant spaces such as office 

suites, apartments, condos, common or 

shared areas, and portions of residential, 

commercial or industrial structures with 

separate heating, ventilating, and air 

conditioning (HVAC) systems. 

In evaluating the subsurface intrusion 

component, consider the following: 

• Area(s) of observed exposure: An 

area of observed exposure is delineated 

by regularly occupied structures with 

documented contamination meeting 

observed exposure criteria; an area of 

observed exposure includes regularly 

occupied structures with samples 

meeting observed exposure criteria or 

inferred to be within an area of observed 

exposure based on samples meeting 

observed exposure criteria (see section 

5.2.1.1.1 Observed Exposure). Establish 

areas of observed exposure as follows: 

—For regularly occupied structures that 

have no subunits, consider both the 

regularly occupied structures 

containing sampling location(s) 

meeting observed exposure criteria for 

the site and the regularly occupied 

structure(s) in the area lying between 

such locations to be an area of 

observed exposure (i.e., inferred to be 

in an area of observed exposure), 

unless available information indicates 

otherwise. 


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Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10419 

—In multi-story, multi-subunit, 

regularly occupied structures, 

consider all subunits on a level with 

sampling locations meeting observed 

exposure criteria from the site and all 

levels below, if any, to be within an 

area of observed exposure, unless 

available information indicates 

otherwise. 

—In multi-tenant structures, that do not 

have a documented observed 

exposure, but are located in an area 

lying between locations where 

observed exposures have been 

documented, consider only those 

regularly occupied subunits, if any, 

on the lowest level of the structure, to 

be within an area of observed 

exposure (i.e., inferred to be in an area 

of observed exposure, unless available 

information indicates otherwise. 

• Area(s) of subsurface 

contamination: An area of subsurface 

contamination is delineated by 

sampling locations meeting observed 

release criteria for subsurface intrusion, 

excluding areas of observed exposure 

(see Table 2–3 in section 2.3). The area 

within an area of subsurface 

contamination includes potentially 

exposed populations. If the significant 

increase in hazardous substance levels 

cannot be attributed at least in part to 

the site and cannot be attributed to other 

sites, attribution can be established 

based on the presence of hazardous 

substances in the area of subsurface 

contamination. Establish areas of 

subsurface contamination as follows: 

— Exclude those areas that contain 

structures meeting the criteria defined 

as an area of observed exposure. 

— Consider both the sampling 

location(s) with subsurface 

contamination meeting observed 

release criteria from the site and the 

area lying between such locations to 

be an area of subsurface 

contamination (i.e., inferred to be in 

an area of subsurface contamination), 

unless available information indicates 

otherwise. 

— Evaluate an area of subsurface 

contamination based on hazardous 

substances that: 

D Meet the criteria for observed 

exposure, or 

D Meet the criteria for observed release 

in an area of subsurface 

contamination and have a vapor 

pressure greater than or equal to one 

torr or a Henry’s constant greater than 

or equal to 10¥5 atm-m3/mol, or 

D Meet the criteria for an observed 

release in a structure within, or in a 

sample from below, an area of 

observed exposure and has a vapor 

pressure greater than or equal to one 

torr or a Henry’s constant greater than 

or equal to 10¥5 atm-m3/mol. 

See Section 7.0 for establishing an 

area of subsurface contamination based 

on the presence of radioactive 

hazardous substances. 

— Evaluate all structures with no 

subunits to be in an area of subsurface 

contamination if they are lying 

between locations of subsurface 

intrusion samples meeting observed 

release criteria. 

— Evaluate multi-subunit structures as 

follows: 

D If an observed exposure has been 

documented based on a gaseous 

indoor air sample, consider all 

regularly occupied subunit(s), if any, 

on the level immediately above the 

level where an observed exposure has 

been documented (or has been 

inferred to be within an area of 

observed exposure), to be within an 

area of subsurface contamination, 

unless available information indicates 

otherwise. 

D If observed release criteria have been 

met based on a gaseous indoor air 

sample collected from a level not 

regularly occupied, consider all 

regularly occupied subunit(s), if any, 

on the level immediately above the 

level where the observed release 

criteria has been documented, to be 

within an area of subsurface 

contamination, unless available 

information indicates otherwise. 

D If an observed exposure has been 

documented based on an intruded 

liquid or particulate sample, do not 

consider any regularly occupied 

subunit(s) above the level where an 

observed exposure has been 

documented to be within an area of 

subsurface contamination, unless 

available information indicates 

otherwise. 

D If any regularly occupied multisubunit structure is inferred to be in 

an area of subsurface contamination, 

consider only those regularly 

occupied subunit(s), if any, on the 

lowest level, to be within an area of 

subsurface contamination, unless 

available information indicates 

otherwise. 

If there is no area of observed 

exposure and no area of subsurface 

contamination, assign a score of 0 for 

the subsurface intrusion component. 

5.2.1 Subsurface intrusion 

component. Evaluate this component 

only if there is an area of observed 

exposure or area of subsurface 

contamination: 

• Within or underlying a residence, 

school, day care center, workplace, or 

• Within or underlying a resource 

specified in section 5.2.1.3.3. 

5.2.1.1 Likelihood of exposure. 

Assign a value of 550 to the likelihood 

of exposure factor category for the 

subsurface intrusion component if there 

is an area of observed exposure in one 

or more locations listed in section 5.2.1. 

Enter this value in Table 5–11. 

5.2.1.1.1 Observed exposure. 

Establish observed exposure in a 

regularly occupied structure by 

demonstrating that a hazardous 

substance has been released into a 

regularly occupied structure via the 

subsurface. Base this demonstration on 

either of the following criterion: 

• Direct observation: 

— A solid, liquid or gaseous material 

that contains one or more hazardous 

substances attributable to the site has 

been observed entering a regularly 

occupied structure through migration 

via the subsurface or is known to have 

entered a regularly occupied structure 

via the subsurface, or 

— When evidence supports the 

inference of subsurface intrusion of a 

material that contains one or more 

hazardous substances associated with 

the site into a regularly occupied 

structure, demonstrated adverse 

effects associated with that release 

may be used to establish observed 

exposure. 

• Chemical analysis: 

— Analysis of indoor samples indicates 

that the concentration of hazardous 

substance(s) has increased 

significantly above the background 

concentration for the site for that type 

of sample (see section 2.3). 

— Some portion of the significant 

increase must be attributable to the 

site to establish the observed 

exposure. Documentation of this 

attribution should account for 

possible concentrations of the 

hazardous substance(s) in outdoor air 

or from materials found in the 

regularly occupied structure, and 

should provide a rationale for the 

increase being from subsurface 

intrusion. 

If observed exposure can be 

established in a regularly occupied 

structure, assign an observed exposure 

factor value of 550, enter this value in 

Table 5–11, and proceed to section 

5.2.1.1.3. If no observed exposure can be 

established, assign an observed 

exposure factor value of 0, enter this 

value in Table 5–11, and proceed to 

section 5.2.1.1.2. 

5.2.1.1.2 Potential for exposure. 

Evaluate potential for exposure only if 

an observed exposure cannot be 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
10420 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

established, but an area of subsurface 

contamination has been delineated. 

Evaluate potential for exposure based 

only on the presence of hazardous 

substances with a vapor pressure greater 

than or equal to one torr or a Henry’s 

constant greater than or equal to 10 5

atm-m3/mol. Evaluate potential for 

exposure for each area of subsurface 

contamination based on four factors: 

structure containment (see section 

5.2.1.1.2.1), depth to contamination (see 

section 5.2.1.1.2.2), vertical migration 

(see section 5.2.1.1.2.3) and vapor 

migration potential (see section 

5.2.1.1.2.4). For each area of subsurface 

contamination, assign the highest value 

for each factor. If information is 

insufficient to calculate any single factor 

value used to calculate the potential for 

exposure factor values at an identified 

area of subsurface contamination, 

information collected for another area of 

subsurface contamination at the site 

may be used when evaluating potential 

for exposure. Calculate the potential for 

exposure value for the site as specified 

in section 5.2.1.1.2.5. 

5.2.1.1.2.1 Structure containment. 

Calculate containment for eligible 

hazardous substances within this 

component as directed in Table 5–12 

and enter this value into Table 5–11. 

Assign each regularly occupied 

structure within an area of subsurface 

contamination the highest appropriate 

structure containment value from Table 

5–12 and use the regularly occupied 

structure at the site with the highest 

structure containment value in 

performing the potential for exposure 

calculation. Assign a structure 

containment factor value of 10 to any 

regularly occupied structure located 

within an area of observed exposure that 

is established based on documented 

surficial ground water intrusion, unless 

available information indicates 

otherwise. 

TABLE 5–12—STRUCTURE CONTAINMENT

No. Evidence of structure containment Assigned 

value 

1 ..................... Regularly occupied structure with evidence of subsurface intrusion, including documented observed exposure 

or sampling of bio or inert gases, such as methane and radon.

10 

2 ..................... Regularly occupied structure with open preferential pathways from the subsurface (e.g., sumps, foundation 

cracks, unsealed utility lines).

10 

3 ..................... Regularly occupied structure with an engineered vapor migration barrier system that does not address all preferential pathways.7 

4 ..................... Regularly occupied structure with an engineered passive vapor mitigation system without documented institutional controls (e.g., deed restrictions) or evidence of regular maintenance and inspection.6 

5 ..................... Regularly occupied structure with no visible open preferential pathways from the subsurface (e.g., sumps, 

foundation cracks, unsealed utility lines).

4 

6 ..................... Regularly occupied structure with an engineered passive vapor mitigation system (e.g., passive venting) with 

documented institutional controls (e.g., deed restrictions) or evidence of regular maintenance and inspection.

3 

7 ..................... Regularly occupied structure with an engineered, active vapor mitigation system (e.g., active venting) without 

documented institutional controls (e.g., deed restrictions) and funding in place for on-going operation, inspection and maintenance.

2 

8 ..................... Regularly occupied structure with unknown containment features ........................................................................ 2 

9 ..................... Regularly occupied structure with a permanent engineered, active vapor mitigation system (e.g., active venting) with documented institutional controls (e.g., deed restrictions) and funding in place for on-going operation, inspection and maintenance. This does not include mitigation systems installed as part of a removal 

or other temporary response by federal, state or tribal authorities.

0 

10 ................... Regularly occupied structure with a foundation raised greater than 6 feet (e.g., structure on stilts) or structure 

that has been built, and maintained, in a manner to prevent subsurface intrusion.

0 

5.2.1.1.2.2 Depth to contamination. 

Assign each area of subsurface 

contamination a depth to contamination 

based on the least depth to either 

contaminated crawl space or subsurface 

media underlying a regularly occupied 

structure. Measure this depth to 

contamination based on the distance 

between the lowest point of a regularly 

occupied structure to the highest known 

point of hazardous substances eligible to 

be evaluated. Use any regularly 

occupied structure within an area of 

subsurface contamination with a 

structure containment factor greater 

than zero. Subtract from the depth to 

contamination the thickness of any 

subsurface layer composed of features 

that would allow channelized flow (e.g., 

karst, lava tubes, open fractures). 

Based on this calculated depth, assign 

a factor value from Table 5–13. If the 

necessary information is available at 

multiple locations, calculate the depth 

to contamination at each location. Use 

the location having the least depth to 

contamination to assign the factor value. 

Enter this value in Table 5–11. 

TABLE 5–13—DEPTH TO

CONTAMINATION

Depth range 1 2

Depth to 

contamination 

assigned value 

0 to 10 ft.(Including subslab 

and semi-enclosed or enclosed crawl space contamination) ........................ 10 

>10 to 20 ft ........................... 8 

>20 to 50 ft ........................... 6 

>50 to 100 ft ......................... 4 

>100 to 150 ft ....................... 2 

>150 ft .................................. 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. 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 10421 

• Do not consider layers or portions 

of layers within the first 10 feet of the 

depth to contamination. 

• If, for the interval identified above, 

all layers that underlie a portion of a 

regularly occupied structure at the site 

are karst or otherwise allow channelized 

flow, assign a value of 15. 

• Otherwise: 

—Select the lowest effective porosity/

permeability layer(s) from within the 

above interval. Consider only layers at 

least 1 foot thick. (If site-specific data 

is not available, use the layer with the 

highest value assigned in Table 5–14.) 

—Assign a value for individual layers 

from Table 5–14. 

—If more than one layer has the same 

assigned porosity/permeability value, 

include all such layers and sum their 

thicknesses. Assign a thickness of 0 

feet to a layer with channelized flow 

features found within any area of 

subsurface contamination at the site. 

—Assign a value from Table 5–15 to the 

vertical migration factor, based on the 

thickness and assigned porosity/

permeability value of the lowest 

effective porosity/permeability 

layer(s). 

Determine vertical migration only at 

locations within an area of subsurface 

contamination at the site. If the 

necessary subsurface geologic 

information is available at multiple 

locations, evaluate the vertical 

migration factor at each location. Use 

the location having the highest vertical 

migration factor value to assign the 

factor value. Enter this value in Table 5– 

11. 

TABLE 5–14—EFFECTIVE POROSITY/PERMEABILITY OF GEOLOGIC MATERIALS

Type of material 

Assigned 

porosity/ 

permeability 

value 

Gravel; highly permeable fractured igneous and metamorphic rocks; permeable basalt; karst limestones and dolomites .............. 1 

Sand; sandy clays; sandy loams; loamy sands; sandy silts; sediments that are predominantly sand; highly permeable till 

(coarse-grained, unconsolidated or compact and highly fractured); peat; moderately permeable limestones and dolomites (no 

karst); moderately permeable sandstone; moderately permeable fractured igneous and metamorphic rocks .............................. 2 

Silt; loams; silty loams; loesses; silty clays; sediments that are predominantly silts; moderately permeable till (fine-grained, unconsolidated till, or compact till with some fractures); low permeability limestones and dolomites (no karst); low permeability

sandstone; low permeability fractured igneous and metamorphic rocks ........................................................................................ 3 

Clay; low permeability till (compact unfractured till); shale; unfractured metamorphic and igneous rocks ........................................ 4 

TABLE 5–15 VERTICAL MIGRATION FACTOR VALUES a

Assigned porosity/ 

permeability value 

Thickness of lowest porosity layer(s) b (feet) 

0 to 5 Greater than 

5 to 10 

Greater than 

10 to 20 

Greater than 

20 to 50 

Greater than 

50 to 100 

Greater than 

100 to 150 

1 ............................................................... 15 15 14 11 8 6 

2 ............................................................... 15 14 12 9 6 4 

3 ............................................................... 15 13 10 7 5 2 

4 ............................................................... 15 12 9 6 3 1 

a If depth to contamination is 10 feet or less or if, for the interval being evaluated, all layers that underlie a portion of the structure at the site 

are karst or have other channelized flow features, assign a value of 15. b Consider only layers at least 1 foot thick. 

5.2.1.1.2.4 Vapor migration 

potential. Evaluate this factor for each 

area of subsurface contamination as 

follows: 

• If the depth to contamination (see 

section 5.2.1.1.2.2) is 10 feet or less, 

assign a value of 25. 

• Assign a value for vapor migration 

potential to each of the gaseous 

hazardous substances associated with 

the area of subsurface contamination 

(see section 2.2.2) as follows: 

—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 

TABLE 5–16—VALUES FOR VAPOR

PRESSURE AND HENRY’S CONSTANT—Continued 

Vapor pressure (Torr) Assigned 

value 

1 to 10 .................................. 2 

Less than 1 ........................... 0 

Henry’s constant (atm-m3/ 

mol) 

Assigned 

value 

Greater than 10¥3 ................ 3 

Greater than 10¥4 to 10¥3 .. 2 

10¥5 to 10¥4 ........................ 1 

Less than 10¥5 .................... 0 


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TABLE 5–17—VAPOR MIGRATION POTENTIAL FACTOR VALUES FOR A

HAZARDOUS SUBSTANCE

Sum of values for vapor 

pressure and Henry’s constant 

Assigned 

value 

0 ............................................ 0 

1 or 2 .................................... 5 

3 or 4 .................................... 15 

5 or 6 .................................... 25 

5.2.1.1.2.5 Calculation of potential 

for exposure factor value. For each 

identified area of subsurface 

contamination, sum the factor values for 

depth to contamination, vertical 

migration and vapor migration 

potential, and multiply this sum by the 

factor value for structure containment. 

Select the highest product for any area 

of subsurface contamination and assign 

this value as the potential for exposure 

factor value for the component. Enter 

this value in Table 5–11. 

5.2.1.1.3 Calculation of likelihood of 

exposure factor category value. If 

observed exposure is established for the 

site, assign the observed exposure factor 

value of 550 as the likelihood of 

exposure factor category value for the 

site. Otherwise, assign the potential for 

exposure factor value for the component 

as the likelihood of exposure value. 

Enter the value assigned in Table 5–11. 

5.2.1.2 Waste characteristics. 

Evaluate waste characteristics based on 

two factors: Toxicity/degradation and 

hazardous waste quantity. 

5.2.1.2.1 Toxicity/degradation. For 

each hazardous substance, assign a 

toxicity factor value, a degradation 

factor value and a combined toxicity/

degradation factor value as specified in 

sections 2.2.3, 2.4.1.2 and 5.2.1.2.1.1 

through 5.2.1.2.1.3. 

5.2.1.2.1.1 Toxicity. Assign a 

toxicity factor value to each hazardous 

substance as specified in sections 2.2.2 

and 2.4.1.1. 

5.2.1.2.1.2 Degradation. Assign a 

degradation factor value to each 

hazardous substance as follows: 

• For any hazardous substance that 

meets the criteria for an observed 

exposure, assign that substance a 

degradation factor value of 1. 

• For all hazardous substances at the 

site that meet subsurface intrusion 

observed release criteria but not 

observed exposure criteria, assign a 

degradation factor value of 1 if the 

depth to contamination below an area of 

subsurface contamination or area of 

observed exposure is less than 10 feet or 

if available evidence suggests that there 

is less than 10 feet of biologically active 

soil in the subsurface anywhere 

underneath a regularly occupied 

structure within an area of subsurface 

contamination or area of observed 

exposure. 

For all other situations first calculate 

the half-life for each hazardous 

substance that meets subsurface 

intrusion observed release criteria as 

follows: 

The half-life or a substance in the 

subsurface is defined for HRS purposes 

as the time required to reduce the initial 

concentration in the subsurface by onehalf as a result of the combined decay 

processes of two components: 

biodegradation and hydrolysis. 

Estimate the half-life (t1/2) of a 

hazardous substance as follows: 

Where: 

h=Hydrolysis half-life. 

b=Biodegradation half-life. 

If one of these component half-lives 

cannot be estimated for the hazardous 

substance from available data, delete 

that component half-life from the above 

equation. 

If no half-life information is available 

for a hazardous substance and the 

substance is not already assigned a 

value of 1, unless information indicates 

otherwise, all straight-chain and simplering structure substances will be 

considered to have a half-life less than 

30 days if not the hazardous substance 

will be assigned a half-life of greater 

than 100 days. 

Based on the hazardous substance’s 

assigned half-life the degradation factor 

is assigned as follows: 

• For all hazardous substances at the 

site that meet subsurface intrusion 

observed release criteria but not 

observed exposure criteria, assign a 

degradation factor value of 0.1, if: 

—The depth to contamination at the site 

is greater than or equal to 10 feet, but 

not if available evidence suggests that 

at least 10 feet of biologically active 

soil is not present in the subsurface 

anywhere underneath a structure 

within an area of subsurface 

contamination or area of observed 

exposure, and 

—The hazardous substance has a halflife of 30 days or less. 

• For all hazardous substances at the 

site that meet subsurface intrusion 

observed release criteria but not 

observed exposure criteria, assign a 

degradation factor value of 0.5, if: 

—The depth to contamination at the site 

is greater than 30 feet, but not if 

available evidence suggests that at 

least 30 feet of biologically active soil 

is not present in the subsurface 

anywhere underneath a structure 

within an area of subsurface 

contamination or area of observed 

exposure, and 

—The hazardous substance has a halflife equal to or less than 100 days. 

• For all other situations assign a 

degradation factor of 1 for all hazardous 

substances at the site that meet 

subsurface intrusion observed release 

criteria. 

In addition, for hazardous substances 

that meet observed release criteria, have 

a parent-daughter degradation 

relationship, and the daughter substance 

is found only in samples with a depth 

greater than 10 feet, assign the daughter 

substance degradation factor value as 

follows: 

1. Identify the shallowest subsurface 

sample that contains the daughter 

substance. 

2. Determine if the selected sample or 

another sample from the same relative 

position in the media of concern, or in 

a shallower sample, contains the parent 

substance. 

3. If the parent substance is not 

present in the identified samples, assign 

the degradation factor value for the 

daughter substance based on the halflife for the daughter substance. 

4. If the parent substance is present in 

a sample from the same relative position 

in the subsurface or in a shallower 

sample, compare the half-life-based 

degradation factor value for the 

daughter substance to the degradation 

factor value assigned to the parent 

substance. Assign the greater of the two 

values as the degradation factor value 

for the daughter substance. 

5.2.1.2.1.3 Calculation of toxicity/

degradation factor value. Assign each 

substance a toxicity/biodegradation 

value by multiplying the toxicity factor 

value by the degradation factor value. 

Use the hazardous substance with the 

highest combined toxicity/degradation 

value to assign the factor value to the 

toxicity/degradation factor for the 

subsurface intrusion threat. Enter this 

value in Table 5–11. 

5.2.1.2.2 Hazardous waste quantity. 

Assign a hazardous waste quantity 

factor value as specified in section 2.4.2. 

Consider only those regularly occupied 

structures with a non-zero structure 

containment value. In estimating the 

hazardous waste quantity, use Tables 2– 

5 and 5–18 and: 

• For Tier A, hazardous constituent 

quantity, use the mass of constituents 

found in the regularly occupied 

structure(s) where the observed 

exposure has been identified. 

—For multi-subunit structures, when 

calculating Tier A, use the mass of 


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constituents found in the regularly 

occupied subunit space(s) where the 

observed exposure has been 

identified. 

• For Tier B, hazardous wastestream 

quantity, use the flow-through volume 

of the regularly occupied structures 

where the observed exposure has been 

identified. 

—For multi-subunit structures, when 

calculating Tier B, use the flowthrough volume of the regularly 

occupied subunit spaces where the 

observed exposure has been 

identified. 

• For Tier C, volume, use the volume 

divisor listed in Tier C of Table 5–18. 

Volume is calculated for those regularly 

occupied structures located within areas 

of observed exposure with observed or 

inferred intrusion and within areas of 

subsurface contamination. 

—In evaluating the volume measure for 

these listed areas of observed 

exposure and areas of subsurface 

contamination based on a gaseous/ 

vapor intrusion or the potential for 

gaseous/vapor intrusion, consider 

the following: 

• Calculate the volume of each 

regularly occupied structure based 

on actual data. If unknown, use a 

ceiling height of 8 feet. 

• For multi-subunit structures, when 

calculating Tier C, calculate volume 

for those subunit spaces with 

observed or inferred exposure and 

all other regularly occupied subunit 

spaces on that level, unless 

available information indicates 

otherwise. If the structure has 

multiple stories, also include the 

volume of all regularly occupied 

subunit spaces below the floor with 

an observed exposure and one story 

above, unless evidence indicates 

otherwise. 

• For multi-subunit structures within 

an area of subsurface contamination 

and no observed or inferred 

exposure, consider only the volume 

of the regularly occupied subunit 

spaces on the lowest story, unless 

available information indicates 

otherwise. 

—In evaluating the volume measure for 

these listed areas of observed 

exposure and areas of subsurface 

contamination where intrusion of 

contaminated ground water has 

occurred, do not calculate the 

volume of each regularly occupied 

structure. Instead, consider only the 

volume of contaminated ground 

water known to have intruded into 

a regularly occupied structure. 

• For Tier D, area, if volume is 

unknown, use the area divisor listed in 

Tier D of Table 5–18 for those regularly 

occupied structures within areas of 

observed exposure with observed or 

inferred intrusion and within areas of 

subsurface contamination. In evaluating 

the area measure for these listed areas 

of observed exposure and areas of 

subsurface contamination, calculate the 

area of each regularly occupied 

structure (including multi-subunit 

structures) based on actual footprint 

area data. If the actual footprint area of 

the structure(s) is unknown, use an area 

of 1,740 square feet for each structure 

(or subunit space). 

TABLE 5–18—HAZARDOUS WASTE QUANTITY EVALUATION EQUATIONS FOR SUBSURFACE INTRUSION COMPONENT

Tier Measure Units Equation for assigning 

value a

A Hazardous Constituent Quantity (C) ................................................................................................. lb ............ C. 

Bb Hazardous Wastestream Quantity (W) ............................................................................................. lb ............ W/5,000. 

C b c Volume (V). 

Regularly occupied structure(s) in areas of observed exposure or subsurface contamination ....... yd 3 ......... V/2.5. 

D b d Area (A). 

Regularly occupied structure(s) in areas of observed exposure or subsurface contamination ....... ft 2 ........... A/13. 

a Do not round to the nearest integer. 

b Convert volume to mass when necessary: 1 ton = 2,000 pounds = 1 cubic yard = 4 drums = 200 gallons. 

c Calculate volume of each regularly occupied structure or subunit space in areas of observed exposure and areas of subsurface contamination—Assume 8-foot ceiling height unless actual value is known. d Calculate area of the footprint of each regularly occupied structure in areas of observed exposure and areas of subsurface contamination. If 

the footprint area of a regularly occupied structure is unknown, use 1,740 square feet as the footprint area of the structure or subunit space. 

For the subsurface intrusion 

component, if the hazardous constituent 

quantity is adequately determined for 

all areas of observed exposure, assign 

the value from Table 2–6 as the 

hazardous waste quantity factor value. If 

the hazardous constituent quantity is 

not adequately determined for one or 

more areas of observed exposure or if 

one or more areas of subsurface 

contamination are present, assign either 

the value from Table 2–6 or assign a 

factor value as follows: 

• If any target for the subsurface 

intrusion component is subject to Level 

I or Level II concentrations (see section 

2.5), assign either the value from Table 

2–6 or a value of 100, whichever is 

greater, as the hazardous waste quantity 

factor value for this component. 

• If none of the targets for the 

subsurface intrusion component is 

subject to Level I or Level II 

concentrations and if there has been a 

removal action that does not 

permanently interrupt target exposure 

from subsurface intrusion, assign a 

factor value as follows: 

—Determine the values from Table 2–6 

with and without consideration of the 

removal action. 

—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. 


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10424 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

5.2.1.2.3 Calculation of waste 

characteristics factor category value. 

Multiply the toxicity/degradation and 

hazardous waste quantity factor values, 

subject to a maximum product of 1 × 

108. Based on this product, assign a 

value from Table 2–7 (section 2.4.3.1) to 

the waste characteristics factor category. 

Enter this value in Table 5–11. 

5.2.1.3 Targets. Evaluate the targets 

factor category for the subsurface 

intrusion threat based on three factors: 

Exposed individual, population, and 

resources in regularly occupied 

structures. Evaluate only those targets 

within areas of observed exposure and 

areas of subsurface contamination (see 

section 5.2.0). 

In evaluating the targets factor 

category for the subsurface intrusion 

threat, count only the following as 

targets: 

• Exposed individual—a person 

living, attending school or day care, or 

working in a regularly occupied 

structure with observed exposure or in 

a structure within an area of observed 

exposure or within an area of subsurface 

contamination. 

• Population—exposed individuals in 

a regularly occupied structure within an 

area of observed exposure or within an 

area of subsurface contamination. 

• Resources—located within an area 

of observed exposure or within an area 

of subsurface contamination as specified 

in section 5.2.1.3.3. 

If a former structure that has been 

vacated due to subsurface intrusion 

attributable to the site, count the initial 

targets as if they were still residing in 

the structure. In addition, if a removal 

action has occurred that has not 

completely mitigated the release, count 

the initial targets as if the removal 

action has not permanently interrupted 

target exposure from subsurface 

intrusion. 

For populations residing in or 

working in a multi-subunit structure 

with multiple stories in an area of 

observed exposure or area of subsurface 

contamination, count these targets as 

follows: 

• If there is no observed exposure 

within the structure, include in the 

evaluation only those targets, if any, in 

the lowest occupied level, unless 

available information indicates 

otherwise. 

• If there is an observed exposure in 

any level, include in the evaluation 

those targets in that level, the level 

above and all levels below, unless 

available information indicates 

otherwise. (The weighting of these 

targets is specified in Section 5.2.1.3.2.) 

5.2.1.3.1 Exposed individual. 

Evaluate this factor based on whether 

there is an exposed individual, as 

specified in sections 2.5.1, 2.5.2 and 

5.2.1.3, who is subject to Level I or 

Level II concentrations. 

First, determine those regularly 

occupied structures or partitioned 

subunit(s) within structures in an area 

of observed exposure subject to Level I 

concentrations and those subject to 

Level II concentrations as specified as 

follows (see section 5.2.0): 

• Level I Concentrations: For 

contamination resulting from subsurface 

intrusion, compare the hazardous 

substance concentrations in any sample 

meeting the observed exposure by 

chemical analysis criteria to the 

appropriate benchmark. Use the healthbased benchmarks from Table 5–19 to 

determine the level of contamination. 

—If the sample is from a structure with 

no subunits and the concentration 

equals or exceeds the appropriate 

benchmark, assign Level I 

concentrations to the entire structure. 

—If the sample is from a subunit within 

a structure and the concentration from 

that subunit equals or exceeds the 

appropriate benchmark, assign Level I 

concentrations to that subunit. 

• Level II Concentrations: Structures, 

or subunits within structures, with one 

or more samples that meet observed 

exposure by chemical analysis criteria 

but do not equal or exceed the 

appropriate benchmark; structures, or 

subunits, that have an observed 

exposure by direct observation; and 

structures inferred to be in an area of 

observed exposure based on samples 

meeting observed exposure, are assigned 

Level II concentrations. 

Æ For all regularly occupied 

structures, or subunits in such 

structures, in an area of observed 

exposure that are not assigned Level I 

concentrations, assign Level II 

concentrations. 

Then assign a value to the exposed 

individual factor as follows: 

• Assign a value of 50 if there is at 

least one exposed individual in one or 

more regularly occupied structures 

subject to Level I concentrations. 

• Assign a value of 45 if there are no 

Level I exposed individuals, but there is 

at least one exposed individual in one 

or more regularly occupied structures 

subject to Level II concentrations. 

• Assign a value of 20 if there is no 

Level I or Level II exposed individual 

but there is at least one individual in a 

regularly occupied structure within an 

area of subsurface contamination. 

Enter the value assigned in Table 5– 

11. 

5.2.1.3.2 Population. Evaluate 

population based on three factors: Level 

I concentrations, Level II 

concentrations, and population within 

an area of subsurface contamination. 

Determine which factors apply as 

specified in section 5.2.1.3.1, using the 

health-based benchmarks from Table 5– 

19. Evaluate populations subject to 

Level I and Level II concentrations as 

specified in section 2.5. 

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

Screening concentration for cancer corresponding to that concentration that corresponds to the 10¥6 individual cancer risk using the inhalation unit risk. For oral exposures use the oral cancer slope factor.

Screening concentration for noncancer toxicological responses corresponding to the reference dose (RfD) for oral exposure and the reference concentration (RfC) for inhalation exposures.

Count only those persons meeting the 

criteria for population as specified in 

section 5.2.1.3. In estimating the 

number of individuals in structures in 

an area of observed exposure or area of 

subsurface contamination if the actual 

number of residents is not known, 

multiply each residence by the average 

number of persons per residence for the 

county in which the residence is 

located. 

5.2.1.3.2.1 Level I concentrations. 

Assign the population subject to Level 

I concentrations as follows: 

1. Identify all exposed individuals 

regularly present in a structure, or if the 

structure has subunits, identify those 

regularly present in each subunit, 

located in an area of observed exposure 

subject to Level I concentrations as 

described in sections 5.2.0 and 5.2.1.3.1. 

Identify only once per structure those 

exposed individuals that are using more 

than one eligible subunit of the same 

structure (e.g., using a common or 

shared area and other parts of the same 

structure). 

2. For each structure or subunit count 

the number of individuals residing in or 


srobinson on DSK5SPTVN1PROD with PROPOSALS2
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attending school or day care in the 

structure or subunit. 

3. Count the number of full-time and 

part-time workers in the structure or 

subunit(s) subject to Level I 

concentrations. If information is 

unavailable to classify a worker as fullor part-time, evaluate that worker as 

being full-time. Divide the number of 

full-time workers by 3 and the number 

of part-time workers by 6, and then sum 

these products with the number of other 

individuals for each structure or 

subunit. 

4. Sum this combined value for all 

structures, or subunits, within areas of 

observed exposure and multiply this 

sum by 10. 

Assign the resulting product as the 

combined population factor value 

subject to Level I concentrations for the 

site. Enter this value in line 9a of Table 

5–11. 

5.2.1.3.2.2 Level II concentrations. 

Assign the population subject to Level 

II concentrations as follows: 

1. Identify all exposed individuals 

regularly present in an eligible 

structure, or if the structure has 

subunits, identify those regularly 

present in each subunit, located in an 

area of observed exposure subject to 

Level II concentrations as described in 

sections 5.2.0 and 5.2.1.3.1. Identify 

only once per structure those exposed 

individuals that are using more than one 

eligible subunit of the same structure 

(e.g., using a common or shared area 

and other parts of the same structure). 

2. Do not include exposed individuals 

already counted under the Level I 

concentrations factor. 

3. For each structure or subunit(s), 

count the number of individuals 

residing in or attending school or day 

care in the structure, or subunit, subject 

to Level II concentrations. 

4. Count the number of full-time and 

part-time workers in the structure or 

subunit(s) subject to Level II 

concentrations. If information is 

unavailable to classify a worker as fullor part-time, evaluate that worker as 

being full-time. Divide the number of 

full-time workers by 3 and the number 

of part-time workers by 6, and then sum 

these products with the number of other 

individuals for each structure or 

subunit. 

5. Sum the combined population 

value for all structures within the areas 

of observed exposure for the site. 

Assign this sum as the combined 

population value subject to Level II 

contamination for this factor. Enter this 

value in line 9b of Table 5–11. 

5.2.1.3.2.3 Population within area(s) 

of subsurface contamination. Assign the 

population in area(s) of subsurface 

contamination factor value as follows, 

unless available information indicates 

otherwise (see sections 5.2.0 and 

5.2.1.3.1): 

1. Identify the regularly occupied 

structures with a structure containment 

value greater than zero and the eligible 

population associated with the 

structures or portions of structures in 

each area of subsurface contamination: 

• For each regularly occupied 

structure or portion of a structure in an 

area of subsurface contamination, sum 

the number of all individuals residing in 

or attending school or day care, in the 

structure or portion of the structure in 

the area of subsurface contamination. 

• Count the number of full-time and 

part-time workers regularly present in 

each structure or portion of a structure 

in an area of subsurface contamination. 

If information is unavailable to classify 

a worker as full- or part-time, evaluate 

that worker as being full-time. Divide 

the number of full-time workers by 3 

and the number of part-time workers by 

6. Sum these products with the number 

of individuals residing in or attending 

school or day care in the structure. 

• Use this sum as the population for 

the structure. 

2. Estimate the depth or distance to 

contamination at each regularly 

occupied structure within an area of 

subsurface contamination based on 

available sampling data, and categorize 

each eligible structure based on the 

depth or distance to contamination and 

sampling media as presented in Table 

5–20. Weight the population in each 

structure using the appropriate 

weighting factors in Table 5–20. If 

samples from multiple media are 

available, use the sample that results in 

the highest weighting factor. 

3. Sum the weighted population in all 

structures within the area(s) of 

subsurface contamination and assign 

this sum as the population subject 

subsurface contamination factor value. 

Enter this value in line 9c of Table 5– 

11. 

TABLE 5–20—WEIGHTING FACTOR VALUES FOR POPULATIONS WITHIN AN AREA OF SUBSURFACE CONTAMINATION

Eligible populations a in structures b within an area of subsurface contamination 

Population 

weighting 

factor 

Population in a structure with levels of contamination in a semi-enclosed or enclosed crawl space sample meeting observed release criteria 

or 

Population in a subunit of a multi-story structure within an area of subsurface contamination located directly above a level in an 

area of observed exposure or a gaseous indoor air sample meeting observed release criteria ................................................... 0.9 

Population in a structure where levels of contaminants meeting observed release criteria are found in any sampling media at or 

within five feet horizontally or vertically of the structure foundation ................................................................................................ 0.4 

Population occupying a structure where levels of contaminants meeting observed release criteria are found or inferred based on 

any underlying non-ground water subsurface sample at a depth less than or equal to 30 feet 

or 

Population in a structure within an area of subsurface contamination where levels of contaminants meeting observed release 

criteria are inferred based on semi-enclosed or enclosed crawl space samples in surrounding structures .................................. 0.2 

Population in a structure where levels of contaminants meeting observed release criteria are found or inferred based on underlying ground water samples greater than five feet from the structure foundation 

or 

Population in a structure where levels of contaminants meeting observed release criteria are found or inferred based on any underlying sample at depths greater than 30 feet ............................................................................................................................... 0.1 

aEligible populations include residents (including individuals living in, or attending school or day care in the structure), and workers in regularly 

occupied structures (see HRS Section 5.2.1.3). bEligible structures may include single- or multi-tenant structures where eligible populations reside, attend school or day care, or work. These 

structures may also be mixed use structures. 


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5.2.1.3.2.4 Calculation of population 

factor value. Sum the factor values for 

Level I concentrations, Level II 

concentrations, and population in the 

area(s) of subsurface contamination. 

Assign this sum as the population factor 

value. Enter this value in line 9d of 

Table 5–11. 

5.2.1.3.3 Resources. Evaluate the 

resources factor as follows: 

• Assign a value of 5 if a resource 

structure (e.g., library, church, tribal 

facility) is present and regularly 

occupied within either an area of 

observed exposure or area of subsurface 

contamination. 

• Assign a value of 0 if there is no 

resource structure within an area of 

observed exposure or area of subsurface 

contamination. 

Enter the value assigned in Table 5– 

11. 

5.2.1.3.4 Calculation of targets factor 

category value. Sum the values for the 

exposed individual, population, and 

resources factors. Do not round to the 

nearest integer. Assign this sum as the 

targets factor category value for the 

subsurface intrusion component. Enter 

this value in Table 5–11. 

5.2.2 Calculation of subsurface 

intrusion component score. Multiply the 

factor category values for likelihood of 

exposure, waste characteristics and 

targets and round the product to the 

nearest integer. Divide the 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. 

6.0 Air Migration Pathway 

* * * * * 

TABLE 6–14—HEALTH-BASED BENCHMARKS FOR HAZARDOUS SUBSTANCES IN AIR

• Concentration corresponding to National Ambient Air Quality Standard (NAAQS). 

• Concentration corresponding to National Emission Standards for Hazardous Air Pollutants (NESHAPs). 

• Screening concentration for cancer corresponding to that concentration that corresponds to the 10¥6 individual cancer risk for inhalation exposures. 

• Screening concentration for noncancer toxicological responses corresponding to the Reference Concentration (RfC) for inhalation exposures. 

* * * * * 7.0 Sites Containing Radioactive 

Substances 

* * * * * 


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EP29FE16.040</GPH>srobinson on DSK5SPTVN1PROD with PROPOSALS2

TABLE 7-1. HRS FACTORS EVALUATED DIFFERENTLY FOR RADIONUCL

Soil Exposure Subsurface Ground Water Surface Water Status Intrusion 

Pathway Status • Pathway • Component of Status • Component of Status • 

SESSI Pathway SESSI Pathway 

Likelihood of Likelihood of Likelihood of Likelihood of 

Release Release Exposure Exposure 

Observed Yes Observed Yes Observed Yes Observed Yes 

Release Release Contamination Exposure 

Potential to No Potential to No Attractiveness/ No Potential for Yes 

Release Release Accessibility to Exposure 

Nearby Residents 

Containment No Overland Flow No Area of No Structure No 

Containment Contamination Containment 

Net Precipitation No Runoff No Depth to Yes 

Contamination 

Depth to Aquifer No Distance to No Vertical No 

Surface water migration 

Travel Time No Flood Frequency No Vapor Migration No 

Potential 

Flood No 

Containment 

Waste Waste Waste Waste 

Characteristics Characteristics Characteristics Characteristics 

Toxicity Yes Toxicity/ Yes/Y Toxicity Yes Toxicity/ Yes 

Ecotoxicity es Degradation 

Mobility No Persistence/ Yes/N Hazardous Yes Hazardous Yes 

Mobility 0 Waste Quantity Waste Quantity 

Hazardous Yes Bioaccumu- No 

Waste Quantity lation Potential 

Hazardous Yes 

Waste Quantity 

Targets Targets Targets Targets 

Nearest Well Yes b Nearest Intake Yes b Resident Yes b Exposed Yes b 
10428 Federal Register / Vol. 81, No. 39 / Monday, February 29, 2016 / Proposed Rules 

* * * * * 

* * * 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. 

Base these demonstrations on one or 

more of the following, as appropriate to 

the pathway being evaluated: 

• Direct observation: 

—For each migration pathway, a 

material that contains one or more 

radionuclides has been seen entering 

the atmosphere, surface water, or 

ground water, as appropriate, or is 

known to have entered ground water 

or surface water through direct 

deposition, or 

—For the surface water migration 

pathway, a source area containing 

radioactive substances has been 

flooded at a time that radioactive 

substances were present and one or 

more radioactive substances were in 

contact with the flood waters. 

—For the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, a 

material that contains one or more 

radionuclides has been observed 

entering a regularly occupied 

structure via the subsurface or is 

known to have entered a regularly 

occupied structure via the subsurface. 

Also, when evidence supports the 

inference of subsurface intrusion of a 

material that contains one or more 

radionuclides by the site into a 

regularly occupied structure, 

demonstrated adverse effects 

associated with that release may also 

be used to establish observed 

exposure by direct observation. 

• Analysis of radionuclide 

concentrations in samples appropriate 

to the pathway (that is, ground water, 

soil, air, indoor air, surface water, 

benthic, or sediment samples): 

—For radionuclides that occur naturally 

and for radionuclides that are 

ubiquitous in the environment: 

D Measured concentration (in units of 

activity, for example, pCi per kilogram 

[pCi/kg], pCi per liter [pCi/L], pCi per 

cubic meter [pCi/m3]) of a given 

radionuclide in the sample are at a level 

that: 

Æ Equals or exceeds a value 2 

standard deviations above the mean 

site-specific background concentration 

for that radionuclide in that type of 

sample, or 

Æ Exceeds the upper-limit value of 

the range of regional background 

concentration values for that specific 

radionuclide in that type of sample. 

D Some portion of the increase must 

be attributable to the site to establish the 

observed release (or observed 

contamination or observed exposure), 

and 

D 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. 

—For man-made radionuclides without 

ubiquitous background concentrations 

in the environment: 

D Measured concentration (in units of 

activity) of a given radionuclide in a 

sample equals or exceeds the sample 

quantitation limit for that specific 

radionuclide in that type of media and 

is attributable to the site. 

D However, if the radionuclide 

concentration equals or exceeds its 

sample quantitation limit, but its release 

can also be attributed to one or more 

neighboring sites, then the measured 

concentration of that radionuclide must 

also equal or exceed a value either 2 

standard deviations above the mean 

concentration of that radionuclide 

contributed by those neighboring sites 

or 3 times its background concentration, 

whichever is lower. 

D If the sample quantitation limit 

cannot be established: 

Æ 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). 

Æ If the sample analysis is not 

performed under the EPA Contract 

Laboratory Program, use the detection 

limit in place of the sample quantitation 

limit. 


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D For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway only, the 

radionuclide must also be present at the 

surface or covered by 2 feet or less of 

cover material (for example, soil) to 

establish observed contamination. 

• Gamma radiation measurements 

(applies only to observed contamination 

or observed exposure in the soil 

exposure and subsurface intrusion 

pathway): 

—The gamma radiation exposure rate, 

as measured in microroentgens per 

hour (mR/hr) using a survey 

instrument held 1 meter above the 

ground surface or floor or walls of a 

structure (or 1 meter away from an 

aboveground source for the soil 

exposure component), equals or 

exceeds 2 times the site-specific 

background gamma radiation 

exposure rate. 

—Some portion of the increase must be 

attributable to the site to establish 

observed contamination. The gammaemitting radionuclides do not have to 

be within 2 feet of the surface of the 

source. 

For the three migration pathways and 

for the subsurface intrusion component 

of the soil exposure and subsurface 

intrusion pathway, if an observed 

release or observed exposure can be 

established for the pathway (or threat, 

aquifer, or watershed, as appropriate), 

assign the pathway (or threat, aquifer, or 

watershed) an observed release or 

observed exposure factor value of 550 

and proceed to section 7.2. If an 

observed release or observed exposure 

cannot be established, assign an 

observed release or observed exposure 

factor value of 0 and proceed to section 

7.1.2. 

For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, if observed 

contamination can be established, 

assign the likelihood of exposure factor 

for resident population a value of 550 if 

there is an area of observed 

contamination in one or more locations 

listed in section 5.1.1; evaluate the 

likelihood of exposure factor for nearby 

population as specified in section 

5.1.2.1; and proceed to section 7.2. If 

observed contamination cannot be 

established, do not evaluate the soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway. 

At sites containing mixed radioactive 

and other hazardous substances, 

evaluate observed release (or observed 

contamination or observed exposure) 

separately for radionuclides as 

described in this section and for other 

hazardous substances as described in 

sections 2 through 6. 

For the three migration pathways and 

the subsurface intrusion component of 

the soil exposure and subsurface 

intrusion pathway, if an observed 

release or observed exposure can be 

established based on either 

radionuclides or other hazardous 

substances, or both, assign the pathway 

(or threat, aquifer, or watershed) an 

observed release or observed exposure 

factor value of 550 and proceed to 

section 7.2. If an observed release or 

observed exposure cannot be 

established based on either 

radionuclides or other hazardous 

substances, assign an observed release 

or observed exposure factor value of 0 

and proceed to section 7.1.2. 

For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, if observed 

contamination can be established based 

on either radionuclides or other 

hazardous substances, or both, assign 

the likelihood of exposure factor for 

resident population a value of 550 if 

there is an area of observed 

contamination in one or more locations 

listed in section 5.1.1; evaluate the 

likelihood of exposure factor for nearby 

population as specified in section 

5.1.2.1; and proceed to section 7.2. If 

observed contamination cannot be 

established based on either 

radionuclides or other hazardous 

substances, do not evaluate the soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway. 

7.1.2 Potential to release/potential 

for exposure. For the three migration 

pathways and the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, evaluate 

potential to release or potential for 

exposure for sites containing 

radionuclides in the same manner as 

specified for sites containing other 

hazardous substances. Base the 

evaluation on the physical and chemical 

properties of the radionuclides, not on 

their level of radioactivity. For the 

subsurface intrusion component of

the soil exposure and subsurface 

intrusion pathway, if the potential for 

exposure is based on the presence of 

gamma emitting radioactive substances, 

assign a potential for exposure factor 

value of 500 only if the contamination 

is found within 2 feet beneath a 

regularly occupied structure, otherwise 

assign a potential for exposure factor 

value of 0. 

For sites containing mixed radioactive 

and other hazardous substances, 

evaluate potential to release or potential 

for exposure considering radionuclides 

and other hazardous substances 

together. Evaluate potential to release 

for each migration pathway and the 

potential for exposure for the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway as specified in sections 3 

through 6, as appropriate. 

* * * * * 

7.2.3 Persistence/Degradation. In 

determining the surface water 

persistence factor for radionuclides, 

evaluate the surface water persistence 

this factor based solely on half-life; do 

not include sorption to sediments in the 

evaluation as is done for nonradioactive 

hazardous substances. Assign a 

persistence factor value from Table 4–10 

(section 4.1.2.2.1.2) to each radionuclide 

based on half-life (t 1/2) calculated as 

follows: 

Where: 

r = Radioactive half-life. 

v = Volatilization half-life. 

If the volatilization half-life cannot be 

estimated for a radionuclide from 

available data, delete it from the 

equation. Select the portion of Table 4– 

10 to use in assigning the persistence 

factor value as specified in section 

4.1.2.2.1.2. 

At sites containing mixed radioactive 

and other hazardous substances, 

evaluate the persistence factor 

separately for each radionuclide and for 

each nonradioactive hazardous 

substance, even if the available data 

indicate that they are combined 

chemically. Assign a persistence factor 

value to each radionuclide as specified 

in this section and to each 

nonradioactive hazardous substance as 

specified in section 4.1.2.2.1.2. When 

combined chemically, assign a single 

persistence factor value based on the 

higher of the two values assigned 

(individually) to the radioactive and 

nonradioactive components. 

In determining the subsurface 

intrusion degradation factor for 

radionuclides, when evaluating this 

factor based solely on half-life. Assign a 

degradation factor value from section 

5.2.1.2.1.2 to each radionuclide based 

on half-life (t1/2) calculated as follows: 

Where: 

r = Radioactive half-life. 

At sites containing mixed radioactive 

and other hazardous substances, 


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evaluate the persistence or degradation 

factor separately for each radionuclide 

and for each nonradioactive hazardous 

substance, even if the available data 

indicate that they are combined 

chemically. Assign a persistence or 

degradation factor value to each 

radionuclide as specified in this section 

and to each nonradioactive hazardous 

substance as specified in sections 

4.1.2.2.1.2 and 5.2.1.2.1.2. When 

combined chemically, assign a single 

persistence or degradation factor value 

based on the higher of the two values 

assigned (individually) to the 

radioactive and nonradioactive 

components. 

7.2.4 Selection of substance 

potentially posing greatest hazard. For 

the subsurface intrusion component of 

the soil exposure and subsurface 

intrusion pathway and each migration 

pathway (or threat, aquifer, or 

watershed, as appropriate), select the 

radioactive substance or nonradioactive 

hazardous substance that potentially 

poses the greatest hazard based on its 

toxicity factor value, combined with the 

applicable mobility, persistence, 

degradation and/or bioaccumulation (or 

ecosystem bioaccumulation) potential 

factor values. Combine these factor 

values as specified in sections 2 through 

6. For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, base the selection on 

the toxicity factor alone (see sections 2 

and 5). 

* * * * * 

7.2.5.1 Source hazardous waste 

quantity for radionuclides For each 

migration pathway, assign a source 

hazardous waste quantity value to each 

source having a containment factor 

value greater than 0 for the pathway 

being evaluated. For the soil exposure 

component of the soil exposure and 

subsurface intrusion pathway, assign a 

source hazardous waste quantity value 

to each area of observed contamination, 

as applicable to the threat being 

evaluated. For the subsurface intrusion 

component, assign a source hazardous 

waste quantity value to each regularly 

occupied structure located within areas 

of observed exposure or areas of 

subsurface contamination. Allocate 

hazardous substances and hazardous 

wastestreams to specific sources (or 

areas of observed contamination, area of 

observed exposure or area of subsurface 

contamination) as specified in sections 

2.4.2 and 5.2.0. 

7.2.5.1.1 Radionuclide constituent 

quantity (Tier A). Evaluate radionuclide 

constituent quantity for each source (or 

area of observed contamination or area 

of observed exposure) based on the 

activity content of the radionuclides 

allocated to the source (or area of 

observed contamination or area of 

observed exposure) as follows: 

• Estimate the net activity content (in 

curies) for the source (or area of 

observed contamination or area of 

observed exposure) based on: 

—Manifests, or 

—Either of the following equations, as 

applicable: 

Where: 

N = Estimated net activity content (in curies) 

for the source (or area of observed 

contamination or area of observed 

exposure). 

V = Total volume of material (in cubic yards) 

in a source (or area of observed 

contamination or area of observed 

exposure) containing radionuclides. 

ACi = Activity concentration above the 

respective background concentration (in 

pCi/g) for each radionuclide i allocated 

to the source (or area of observed 

contamination or area of observed 

exposure). 

n = Number of radionuclides allocated to the 

source (or area of observed 

contamination or area of observed 

exposure) above the respective 

background concentrations. 

or, 

Where: 

N = Estimated net activity content (in curies) 

for the source (or area of observed 

contamination or area of observed 

exposure). 

V = Total volume of material (in gallons) in 

a source (or area of observed 

contamination or area of observed 

exposure) containing radionuclides. 

ACi = Activity concentration above the 

respective background concentration (in 

pCi/1) for each radionuclide i allocated 

to the source (or area of observed 

contamination or area of observed 

exposure). 

n = Number of radionuclides allocated to the 

source (or area of observed 

contamination or area of observed 

exposure) above the respective 

background concentrations. 

• Estimate volume for the source (or 

volume for the area of observed 

contamination or area of observed 

exposure) based on records or 

measurements. 

• For the soil exposure component, in 

estimating the volume for areas of 

observed contamination, do not include 

more than the first 2 feet of depth, 

except: For those types of areas of 

observed contamination listed in Tier C 

of Table 5–2 (section 5.1.1.2.2), include 

the entire depth, not just that within 2 

feet of the surface. 

• For the subsurface intrusion 

component, in estimating the volume 

for areas of observed exposure, only use 

the volume of air in the regularly 

occupied structures where observed 

exposure has been documented. 

• Convert from curies of 

radionuclides to equivalent pounds of 

nonradioactive hazardous substances by 

multiplying the activity estimate for the 

source (or area of observed 

contamination or area of observed 

exposure) by 1,000. 

• Assign this resulting product as the 

radionuclide constituent quantity value 

for the source (or area of observed 

contamination or area of observed 

exposure). 

If the radionuclide constituent 

quantity for the source (or area of 

observed contamination or area of 

observed exposure) is adequately 

determined (that is, the total activity of 

all radionuclides in the source and 

releases from the source [or in the area 

of observed contamination or area of 

observed exposure] is known or is 

estimated with reasonable confidence), 

do not evaluate the radionuclide 

wastestream quantity measure in section 

7.2.5.1.2. Instead, assign radionuclide 

wastestream quantity a value of 0 and 

proceed to section 7.2.5.1.3. If the 

radionuclide constituent quantity is not 

adequately determined, assign the 

source (or area of observed 

contamination or area of observed 

exposure) a value for radionuclide 

constituent quantity based on the 

available data and proceed to section 

7.2.5.1.2. 

7.2.5.1.2 Radionuclide wastestream 

quantity (Tier B). Evaluate radionuclide 

wastestream quantity for the source (or 

area of observed contamination, area of 

observed exposure, or area of subsurface 

contamination) based on the activity 

content of radionuclide wastestreams 

allocated to the source (or area of 

observed contamination, area of 

observed exposure, or area of subsurface 

contamination) as follows: 

• Estimate the total volume (in cubic 

yards or in gallons) of wastestreams 

containing radionuclides allocated to 

the source (or area of observed 

contamination, area of observed 

exposure, or area of subsurface 

contamination). 

• Divide the volume in cubic yards by 

0.55 (or the volume in gallons by 110) 

to convert to the activity content 

expressed in terms of equivalent pounds 

of nonradioactive hazardous substances. 


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• Assign the resulting value as the 

radionuclide wastestream quantity 

value for the source (or area of observed 

contamination, area of observed 

exposure, or area of subsurface 

contamination). 

• For the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, estimate 

the total wastestream volume for all 

regularly occupied structures located 

within areas of observed exposure with 

observed or inferred intrusion and 

within areas of subsurface 

contamination. Calculate the volume of 

each regularly occupied structure based 

on actual data. If unknown, use a ceiling 

height of 8 feet. 

7.2.5.1.3 Calculation of source 

hazardous waste quantity value for 

radionuclides. Select the higher of the 

values assigned to the source (or area of 

observed contamination, area of 

observed exposure, and/or area of 

subsurface contamination) for 

radionuclide constituent quantity and 

radionuclide wastestream quantity. 

Assign this value as the source 

hazardous waste quantity value for the 

source (or area of observed 

contamination, area of observed 

exposure, or area of subsurface 

contamination). Do not round to the 

nearest integer. 

7.2.5.2 Calculation of hazardous 

waste quantity factor value for 

radionuclides. Sum the source 

hazardous waste quantity values 

assigned to all sources (or areas of 

observed contamination, areas of 

observed exposure, or areas of 

subsurface contamination) for the 

pathway being evaluated and round this 

sum to the nearest integer, except: If the 

sum is greater than 0, but less than 1, 

round it to 1. Based on this value, select 

a hazardous waste quantity factor value 

for this pathway from Table 2–6 (section 

2.4.2.2). 

For a migration pathway, if the 

radionuclide constituent quantity is 

adequately determined (see section 

7.2.5.1.1) for all sources (or all portions 

of sources and releases remaining after 

a removal action), assign the value from 

Table 2–6 as the hazardous waste 

quantity factor value for the pathway. If 

the radionuclide constituent quantity is 

not adequately determined for one or 

more sources (or one or more portions 

of sources or releases remaining after a 

removal action), assign a factor value as 

follows: 

• If any target for that migration 

pathway is subject to Level I or Level II 

concentrations (see section 7.3), assign 

either the value from Table 2–6 or a 

value of 100, whichever is greater, as the 

hazardous waste quantity factor value 

for that pathway. 

• If none of the targets for that 

pathway is subject to Level I or Level II 

concentrations, assign a factor value as 

follows: 

—If there has been no removal action, 

assign either the value from Table 2– 

6 or a value of 10, whichever is 

greater, as the hazardous waste 

quantity factor value for that pathway. 

—If there has been a removal action: 

D Determine values from Table 2–6 

with and without consideration of 

the removal action. 

D If the value that would be assigned 

from Table 2–6 without 

consideration of the removal action 

would be 100 or greater, assign 

either the value from Table 2–6 

with consideration of the removal 

action or a value of 100, whichever 

is greater, as the hazardous waste 

quantity factor value for the 

pathway. 

D If the value that would be assigned 

from Table 2–6 without 

consideration of the removal action 

would be less than 100, assign a 

value of 10 as the hazardous waste 

quantity factor value for the 

pathway. 

For the soil exposure component of 

the soil exposure and subsurface 

intrusion pathway, if the radionuclide 

constituent quantity is adequately 

determined for all areas of observed 

contamination, assign the value from 

Table 2–6 as the hazardous waste 

quantity factor value. If the radionuclide 

constituent quantity is not adequately 

determined for one or more areas of 

observed contamination, assign either 

the value from Table 2–6 or a value of 

10, whichever is greater, as the 

hazardous waste quantity factor value. 

For the subsurface intrusion 

component of the soil exposure and 

subsurface intrusion pathway, if the 

radionuclide constituent quantity is 

adequately determined for all areas of 

observed exposure, assign the value 

from Table 2–6 as the hazardous waste 

quantity factor value. If the radionuclide 

constituent quantity is not adequately 

determined for one or more areas of 

observed exposure, assign either the 

value from Table 2–6 or a value of 10, 

whichever is greater, as the hazardous 

waste quantity factor value. 

7.2.5.3 Calculation of hazardous 

waste quantity factor value for sites 

containing mixed radioactive and other 

hazardous substances. For each source 

(or area of observed contamination, area 

of observed exposure, or area of 

subsurface contamination) containing 

mixed radioactive and other hazardous 

substances, calculate two source 

hazardous waste quantity values—one 

based on radionuclides as specified in 

sections 7.2.5.1 through 7.2.5.1.3 and 

the other based on the nonradioactive 

hazardous substances as specified in 

sections 2.4.2.1 through 2.4.2.1.5, and 

sections 5.1.1.2.2, 5.1.2.2.2 and 5.2.1.2.2 

(that is, determine each value as if the 

other type of substance was not 

present). Sum the two values to 

determine a combined source hazardous 

waste quantity value for the source (or 

area of observed contamination, area of 

observed exposure, or area of subsurface 

contamination). Do not round this value 

to the nearest integer. 

Use this combined source hazardous 

waste quantity value to calculate the 

hazardous waste quantity factor value 

for the pathway as specified in section 

2.4.2.2, except: If either the hazardous 

constituent quantity or the radionuclide 

constituent quantity, or both, are not 

adequately determined for one or more 

sources (or one or more portions of 

sources or releases remaining after a 

removal action) or for one or more areas 

of observed contamination, areas of 

observed exposure, or areas of 

subsurface contamination, as applicable, 

assign the value from Table 2–6 or the 

default value applicable for the 

pathway, whichever is greater, as the 

hazardous waste quantity factor value 

for the pathway. 

7.3 Targets. For radioactive 

substances, evaluate the targets factor 

category as specified in section 2.5 and 

sections 3 through 6, except: Establish 

Level I and Level II concentrations at 

sampling locations as specified in 

sections 7.3.1 and 7.3.2 and establish 

weighting factors for populations 

associated with an area of subsurface 

contamination in the subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway as specified in section 7.3.3. 

For all pathways (components and 

threats), use the same target distance 

limits for sites containing radioactive 

substances as is specified in sections 3 

through 6 for sites containing 

nonradioactive hazardous substances. 

At sites containing mixed radioactive 

and other hazardous substances, include 

all sources (or areas of observed 

contamination, areas of observed 

exposure, or areas of subsurface 

contamination) at the site in identifying 

the applicable targets for the pathway. 

7.3.1 Level of contamination at a 

sampling location. Determine whether 

Level I or Level II concentrations apply 

at a sampling location (and thus to the 

associated targets) as follows: 


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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. 

• Determine which level applies 

based on this comparison. 

• If none of the radionuclides eligible 

to be evaluated for the sampling 

location have an applicable benchmark, 

assign Level II to the actual 

contamination at that sampling location 

for the pathway (or component or 

threat). 

• In making the comparison, consider 

only those samples, and only those 

radionuclides in the sample, that meet 

the criteria for an observed release (or 

observed contamination or observed 

exposure) for the pathway, except: 

Tissue samples from aquatic human 

food chain organisms may also be used 

for the human food chain threat of the 

surface water pathway as specified in 

sections 4.1.3.3 and 4.2.3.3. 

7.3.2 Comparison to benchmarks. 

Use the following media specific 

benchmarks (expressed in activity units, 

for example, pCi/l for water, pCi/kg for 

soil and for aquatic human food chain 

organisms, and pCi/m3 for air) for 

making the comparisons for the 

indicated pathway (or threat): 

• Maximum Contaminant Levels 

(MCLs)—ground water migration 

pathway and drinking water threat in 

surface water migration pathway. 

• Uranium Mill Tailings Radiation 

Control Act (UMTRCA) standards—soil 

exposure component of the soil 

exposure and subsurface intrusion 

pathway only. 

• Screening concentration for cancer 

corresponding to that concentration that 

corresponds to the 10¥6 individual 

cancer risk for inhalation exposures (air 

migration pathway and subsurface 

intrusion component of the soil 

exposure and subsurface intrusion 

pathway) or for oral exposures (ground 

water migration pathway; drinking 

water or human food chain threats in 

surface water migration pathway; and 

soil exposure and subsurface intrusion 

pathway). 

—For the soil exposure and subsurface 

intrusion pathway, include two 

screening concentrations for cancer— 

one for ingestion of surface materials 

and one for external radiation 

exposures from gamma-emitting 

radionuclides in surface materials. 

Select the benchmark(s) applicable to 

the pathway (component or threat) 

being evaluated. Compare the 

concentration of each radionuclide from 

the sampling location to its benchmark 

concentration(s) for that pathway 

(component or threat). Use only those 

samples and only those radionuclides in 

the sample that meet the criteria for an 

observed release (or observed 

contamination or observed exposure) for 

the pathway, except: Tissue samples 

from aquatic human food chain 

organisms may be used as specified in 

sections 4.1.3.3 and 4.2.3.3. If the 

concentration of any applicable 

radionuclide from any sample equals or 

exceeds its benchmark concentration, 

consider the sampling location to be 

subject to Level I concentrations for that 

pathway (component or threat). If more 

than one benchmark applies to the 

radionuclide, assign Level I if the 

radionuclide concentration equals or 

exceeds the lowest applicable 

benchmark concentration. In addition, 

for the soil exposure and subsurface 

intrusion pathway, assign Level I 

concentrations at the sampling location 

if measured gamma radiation exposure 

rates equal or exceed 2 times the 

background level (see section 7.1.1). 

If no radionuclide individually equals 

or exceeds its benchmark concentration, 

but more than one radionuclide either 

meets the criteria for an observed 

release (or observed contamination or 

observed exposure) for the sample or is 

eligible to be evaluated for a tissue 

sample (see sections 4.1.3.3 and 4.2.3.3), 

calculate a value for index I for these 

radionuclides as specified in section 

2.5.2. If I equals or exceeds 1, assign 

Level I to the sampling location. If I is 

less than 1, assign Level II. 

At sites containing mixed radioactive 

and other hazardous substances, 

establish the level of contamination for 

each sampling location considering 

radioactive substances and 

nonradioactive hazardous substances 

separately. Compare the concentration 

of each radionuclide and each 

nonradioactive hazardous substance 

from the sampling location to its 

respective benchmark concentration(s). 

Use only those samples and only those 

substances in the sample that meet the 

criteria for an observed release (or 

observed contamination or observed 

exposure) for the pathway except: 

Tissue samples from aquatic human 

food chain organisms may be used as 

specified in sections 4.1.3.3 and 4.2.3.3. 

If the concentration of one or more 

applicable radionuclides or other 

hazardous substances from any sample 

equals or exceeds its benchmark 

concentration, consider the sampling 

location to be subject to Level I 

concentrations. If more than one 

benchmark applies to a radionuclide or 

other hazardous substance, assign Level 

I if the concentration of the radionuclide 

or other hazardous substance equals or 

exceeds its lowest applicable 

benchmark concentration. 

If no radionuclide or other hazardous 

substance individually exceed a 

benchmark concentration, but more 

than one radionuclide or other 

hazardous substance either meets the 

criteria for an observed release (or 

observed contamination or observed 

exposure) for the sample or is eligible to 

be evaluated for a tissue sample, 

calculate an index I for both types of 

substances as specified in section 2.5.2. 

Sum the index I values for the two types 

of substances. If the value, individually 

or combined, equals or exceeds 1, assign 

Level I to the sample location. If it is 

less than 1, calculate an index J for the 

nonradioactive hazardous substances as 

specified in section 2.5.2. If J equals or 

exceeds 1, assign Level I to the sampling 

location. If J is less than 1, assign Level 

II. 

7.3.3 Weighting of targets within an 

area of subsurface contamination. For 

the subsurface intrusion component of 

the soil exposure and subsurface 

intrusion pathway, assign a weighting 

factor as specified in section 5.2.1.3.2.3 

except when an area of subsurface 

contamination is bound by gamma 

radiation exposure rates meeting 

observed release criteria with a depth to 

contamination of 2 feet or less. For those 

populations residing, working, or 

attending school or day care in a 

structure, assign a weighting factor of 

0.9. 

[FR Doc. 2016–02749 Filed 2–26–16; 8:45 am] 

BILLING CODE 6560–50–P 


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