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[Federal Register Volume 81, Number 39 (Monday, February 29, 2016)]
[Proposed Rules]
[Pages 10371-10432]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2016-02749]

[[Page 10371]]

Vol. 81

Monday,

No. 39

February 29, 2016

Part II

Environmental Protection Agency

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40 CFR Part 300

Addition of a Subsurface Intrusion Component to the Hazard Ranking 
System; Proposed Rule

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

[[Page 10372]]

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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.

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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 Once submitted, comments cannot be edited or withdrawn.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

[[Page 10373]]

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.
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    \1\ Subsurface intrusion, for the purposes of this preamble, 
refers to the intrusion of hazardous substances from the subsurface 
into a structure.
    \2\ For the purpose of this preamble, the term ``hazardous 
substances, pollutants or contaminants'' will be referred to simply 
as ``hazardous substances.'' See section 1.1, of the current HRS for 
the definition of a hazardous substance.
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    First, the current HRS (40 CFR 300, Appendix A), promulgated 
December 14, 1990 (hereafter referred to as the current HRS), discussed 
in more detail in section IV of this preamble, does not consider the 
threat posed by subsurface intrusion in its evaluation of relative risk 
posed by a site; therefore, it does not provide a complete assessment 
of the relative risk that a site may pose to the public. 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

[[Page 10374]]

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.
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    \3\ EPA's Estimated Costs to Remediate Existing Sites Exceed 
Current Funding Levels, and More Sites are Expected to Be Added to 
the National Priorities List, GAO Report to Congressional 
Requesters, GAO-10-380, May 2010.
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    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:
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    \4\ This information was previously stored in a predecessor 
database called the Comprehensive Environmental Response, 
Compensation and Liability Information System (CERCLIS).
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     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.

[[Page 10375]]

    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).
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    \5\ Although the HRS is designed to assess the relative risk of 
a site compared to other sites, it is not designed to be used as a 
site-specific quantitative risk assessment. Such an assessment is 
conducted later in the Superfund process, as necessary.
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    Section 105(a)(8)(B) of CERCLA requires that the statutory criteria 
described in section 105(a)(8)(A) be used to prepare a list of national 
priorities among the known releases, or threatened releases throughout 
the United States. The list, which is Appendix B of the NCP, is the 
NPL.
    The HRS is a crucial part of the agency's program to address the 
identification and cleanup of actual and potential releases of 
hazardous substances because the HRS score is the primary criterion for 
determining whether a site is to be included on the NPL. The NPL 
(Appendix B to 40 CFR 300) includes those sites that emerge as 
potentially posing the most serious threats to public health and the 
environment and may warrant remedial investigation and possible cleanup 
under CERCLA. Only sites on the NPL are eligible for Superfund-financed 
remedial actions. 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

[[Page 10376]]

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 site-specific 
factors (e.g., number and type of samples required, difficulty in 
establishing sources of contamination or attribution of releases, 
number of HRS pathways being evaluated, and availability of data from 
previous sampling events). Additionally, any newly increased costs to 
government agencies conducting assessments at SsI sites are expected to 
be minimal because federal agencies should already be identifying and 
addressing subsurface intrusion as part of their environmental 
programs. Any increase in the cost of site assessments conducted by EPA 
for SsI sites will require EPA to realign and prioritize its site 
assessment budget to address sites with subsurface intrusion. The 
addition of an SsI component to the HRS is not expected to result in 
additional site assessment funding to account for any increase in site 
assessment costs. Instead, the pipeline of sites will continue to be 
reviewed under the current site assessment process. If it is found that 
SsI-contaminated sites potentially pose a greater risk than other 
sites, then these sites will be prioritized over other sites. 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.
---------------------------------------------------------------------------

    \6\ The regulatory impact analysis (RIA) found this rulemaking 
will only have moderate costs and will not be a significant 
rulemaking. The RIA for this rulemaking can be found in the official 
Docket for this action.
---------------------------------------------------------------------------

    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

[[Page 10377]]

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 Pre-CERCLA screening, which 
is the first step of the pre-remedial process, EPA determines if there 
is indication of a possible significant release. If so, EPA determines 
if a substance in the release is regulated by CERCLA, whether it is 
already being addressed, and whether any statutorily mandated 
limitations on CERCLA response may exist. If EPA determines the release 
meets these requirements, then the suspected release is listed in EPA's 
Superfund Enterprise Management System (SEMS).
    Determining 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.
[GRAPHIC] [TIFF OMITTED] TP29FE16.022

[[Page 10378]]

    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.

[[Page 10379]]

[GRAPHIC] [TIFF OMITTED] TP29FE16.023

    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.
[GRAPHIC] [TIFF OMITTED] TP29FE16.024

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.

[[Page 10380]]

    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\
---------------------------------------------------------------------------

    \7\ EPA's Estimated Costs to Remediate Existing Sites Exceed 
Current Funding Levels, and More Sites are Expected to Be Added to 
the National Priorities List, GAO Report to Congressional 
Requesters, GAO-10-380, May 2010.
---------------------------------------------------------------------------

    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 vapor-forming 
chemicals in the vadose zone will eventually eliminate potential 
exposure pathways and can include the following actions, among 
others: removal of contaminated soil via excavation; removal of 
contaminated groundwater with pump-and-treat approaches; 
decontaminating and/or rehabilitating sewer lines that harbor vapor-
forming chemicals; and, treatment of contaminated soil and 
groundwater in situ, using technologies such as soil vapor 
extraction, multiphase extraction, and bioremediation, or natural 
attenuation.

    In the case of vapor intrusion resulting from a subsurface 
contaminant plume, failing to address the source of contamination and 
the resulting plume may result in an increased exposure to individuals 
due to migration and expansion of the plume over time. In this 
instance, individuals in regularly occupied structures that were 
previously unaffected by the plume may become negatively impacted by 
subsurface intrusion. Additionally, a subsurface contaminant plume in a 
lesser-developed area has the potential to impact future development if 
left untreated.
    There are several other concerns related to only addressing 
subsurface intrusion problems with a vapor mitigation system. The first 
concern is that vapor mitigation systems require ongoing monitoring and 
maintenance throughout the life of the system. Periodic inspections of 
the vapor mitigation system are necessary to make sure it is operating 
as designed. Over time the system can degrade, and maintenance will 
also be necessary, such as replacing the fan in an active sub-slab 
depressurization system. Non-mechanical failures of the system can 
occur as well, such as, electric power failure, turning off the fan or 
ignoring a damaged system.
    A vapor intrusion mitigation system is a tool for protecting human 
health, but may not contribute to the Superfund program's goal of 
cleaning up uncontrolled hazardous waste sites. Furthermore, EPA still 
lacks a mechanism to assess human health hazards from vapor intrusion 
in the current HRS model, and therefore cannot currently evaluate the 
threat of vapor intrusion as part of its ranking of sites for placement 
on the NPL.
    Under the Superfund remedial program for NPL sites, subsurface 
intrusion is only addressed at sites placed on the NPL based on threats 
from other pathways. That is, subsurface intrusion issues are addressed 
later in the remedial process after placement on the NPL. For example, 
this may be done as part of EPA's five-year review process. Sites with 
only subsurface intrusion issues are not being included on the NPL due 
to the lack of a subsurface intrusion component in the HRS. Therefore, 
many sites, especially those not evaluated under another HRS pathway or 
those not scoring high enough under another HRS pathway,

[[Page 10381]]

may not be addressed for threats due to subsurface intrusion because 
they may not qualify for placement on the NPL. As the Government 
Accountability Office (GAO) states in its May 2010 report:

EPA may not be listing some sites that pose health risks that are 
serious enough that the sites should be considered for inclusion on 
the NPL. While EPA is assessing vapor intrusion contamination at 
listed NPL sites, EPA does not assess the relative risks posed by 
vapor intrusion when deciding which sites to include on the NPL. By 
not including these risks, states may be left to remediate those 
sites without federal assistance, and given states' constrained 
budgets, some states may not have the ability to clean up these 
sites on their own . . . However, if these sites are not assessed 
and, if needed, listed on the NPL, some seriously contaminated 
hazardous waste sites with unacceptable human exposure may not 
otherwise be cleaned up.

    EPA proposes the addition of the subsurface component to ensure the 
HRS does not omit this known pathway of human exposure to contamination 
and provides a mechanism for complete assessment of SsI threats to 
human health and the environment.
2. What alternative regulatory options to this action were considered 
by EPA?
    EPA considered alternatives to this proposed regulatory action for 
addressing the need to evaluate subsurface intrusion threats as 
discussed below.
    Specifically, EPA considered whether existing programs adequately 
address the risks associated with subsurface intrusion at contaminated 
sites, as discussed in the previous section. If one or more programs 
were in place to adequately address concerns from subsurface intrusion, 
this could obviate the need for EPA action. However, no other authority 
consistently and comprehensively addresses subsurface intrusion across 
all potential non-federal sites, particularly when subsurface intrusion 
is the key exposure route. In particular, state programs vary 
significantly in addressing subsurface intrusion. In fact, not all 
states have subsurface intrusion programs, and states with programs 
vary in their authority, resources, and remediation criteria. The 2004 
Interstate Technology and Regulatory Council's (ITRC) Vapor Intrusion 
Team developed and conducted an on-line survey of state, federal, and 
tribal agencies regarding vapor intrusion regulations, policy, and 
guidance. Ninety-six percent (96%) of survey respondents consider vapor 
intrusion a concern; however, only 11% have a procedure for evaluating 
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/vapor-intrusion-and-superfund-program, providing background 
information on

[[Page 10382]]

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 multi-story and multi-subunit 
structures; and
     Evaluation of target values for workers.
    The results of the 2011 peer review of the proposed addition are 
discussed in the Summary of Peer Review Comments and Suggested 
Responses on the Addition of a Subsurface Intrusion Component to the 
HRS, which is available in the public docket for this proposed 
rulemaking. This proposed addition reflects modifications made as a 
result of EPA's peer review process.
5. How did EPA select the approach for including the addition in the 
HRS?
    The following six concepts were used as the basis for evaluating 
possible approaches to the HRS addition and the selection of a 
preferred approach:
    1. Limit the proposed addition to the existing HRS structure to 
avoid confusion by minimizing the portions of the present HRS that 
would need to be revised.
    2. Utilize the existing HRS basic structure and scoring algorithm, 
and maintain the relative weighting of the different pathways.
    3. Base technical decisions on sound and proven science.
    4. Ensure the HRS acts as an effective screening tool and minimizes 
unnecessary resource expenditures, while also minimizing the erroneous 
inclusion or exclusion of sites for possible NPL placement.
    5. Assemble and utilize conceptual site models, case studies, and 
sensitivity analyses to test the model.
    6. Ensure that an HRS scoring evaluation of the soil exposure and 
subsurface intrusion pathway can be completed using the information and 
level of effort that are typical of a site inspection or expanded site 
inspection (ESI).
    In the process of developing the proposed rule, EPA identified 
multiple options that are consistent with the above concepts. Based on 
literature reviews and agency experience, EPA projected the range of 
conditions at which the 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

[[Page 10383]]

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.
[GRAPHIC] [TIFF OMITTED] TP29FE16.025

[[Page 10384]]

    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 pathway-specific parameters and to address 
comments on the proposed rule. See also section 2.2 of the 1988 Revised 
HRS TSD for supplemental information. The design of the HRS reflects a 
conceptual understanding of how hazardous substance releases from 
CERCLA sites can result in risks to public health and welfare and the 
environment. The risk scenario at these sites is a function of:
     The probability of exposure to (or releases to a medium in 
a migration pathway of) hazardous substances,
     The expected magnitude and duration of the releases or 
exposures,
     The toxicity or other potential adverse effects to a 
receptor (target) from the releases,
     The probability that the release will reach a receptor and 
the expected change in the concentration of hazardous substances during 
the movement from the location of the contamination to the receptors,
     The expected dose to the receptor, and
     The expected number and character of the receptors.

The above considerations are addressed in three factor categories: 
likelihood of exposure (or release), waste characteristics, and 
targets.

    The following subsections describe the structure of the proposed 
subsurface intrusion component and how this structure is consistent 
conceptually with the existing structure of the other HRS pathways and 
components: (1) 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\
---------------------------------------------------------------------------

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

    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.

[[Page 10385]]

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.
[GRAPHIC] [TIFF OMITTED] TP29FE16.026

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

[[Page 10386]]

distance from a source, and the possible amount of hazardous substance 
dilution.
    As a result, EPA considered including within the subsurface 
intrusion component an approach for incorporating populations subject 
to future migration (outside the ASC) similar to that used for the 
ground water migration pathway. The approach included a standard 4-mile 
distance (modified if site-specific geologic information indicates 
otherwise) radiating either in all directions or only in the probable 
downgradient direction from each source at a site to establish this 
future migration zone. This approach could account for the possibility 
of future horizontal migration of either volatile substances in 
contaminated ground water or as a soil gas beyond the demonstrated 
boundaries of the subsurface contamination and subsequently into 
regularly occupied structures (i.e., a potential future migration 
zone). This might happen, for example, if hazardous substance plumes 
expand or migrate due to the additional release of hazardous 
substances, shift side-to-side due to ground water gradient changes 
resulting from seasonal variations or tidal influences, or change 
direction due to the sequencing of dry and wet years or pumping at 
municipal water supply or other well fields. Additionally, natural and 
anthropogenic influences, such as utility corridors, fracture patterns, 
karst features, or buried stream channels or other geologic 
heterogeneity may alter or enhance hazardous substance migration.
    However, EPA's confidence in the present science to accurately 
project hazardous substance migration through both the ground water and 
the unsaturated zone is limited. Several fate and transport models, 
many based on the Johnson and Ettinger Model, currently exist and are 
used to project vapor migration and predict contaminant vapor intrusion 
into a structure. The ability of a site assessor to accurately evaluate 
the potential future migration of subsurface hazardous substances would 
rely heavily on the ability to gather site-specific data in all areas 
of future migration in the relatively short time period and with 
minimal resources available when data collection for an HRS evaluation 
is performed (i.e., during the site inspection). EPA's review of 
existing models 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

[[Page 10387]]

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.
[GRAPHIC] [TIFF OMITTED] TP29FE16.027

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

[[Page 10388]]

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 climate-specific. 
Sampling may not have been performed in these structures for a number 
of reasons, or, even if performed during the limited time period (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.

[[Page 10389]]

[GRAPHIC] [TIFF OMITTED] TP29FE16.028

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 site-specific information could be collected 
during a site inspection: (1) Depth to contamination, (2) vertical 
migration, and (3) vapor migration potential. These three factors are 
described in the following sections.
a. Depth to Contamination--See Section 5.2.1.1.2.2 of the Proposed HRS 
Addition
    The depth to contamination factor represents the vertical distance 
between contamination (either in soil, soil gas, or surficial ground 
water) and the lowest horizontal point of an overlying regularly 
occupied structure (e.g., a

[[Page 10390]]

basement floor). This distance represents how far a hazardous substance 
would have to travel through the subsurface to intrude into that 
structure. Based on available data, the probability of exposure 
decreases as the depth to contamination increases. In addition, as part 
of EPA's sensitivity analysis in developing route characteristics, at 
depths greater than 150 feet it became increasingly unlikely that 
exposure would occur. This is reflected in Table 5-13 (section 
5.2.1.1.2.2 of the Proposed HRS Addition). EPA is proposing depth to 
contamination factor values ranging from zero to ten, where increasing 
depth results in a lower factor value.
    EPA is also proposing to give special consideration in two 
situations in which it is likely that exposure has occurred. One 
situation is when subsurface profiles may be impacted by channelized 
flow features, such as fractured bedrock or karst. The other situation 
is at locations where the contamination is measured directly below the 
structure (e.g., in subslab or enclosed/semi-enclosed crawl space 
samples). These features reflect a situation with a high probability of 
exposure to intruded hazardous substances because of limited resistance 
to migration of the substances into the structure. See section 4.0 of 
the Proposal TSD for supplemental information on how the depths to 
contamination were weighted when assigning the factor values to 
different distances.
b. Vertical Migration--See Section 5.2.1.1.2.3 of the Proposed HRS 
Addition
    The vertical migration factor considers the geologic makeup of 
materials between a regularly occupied structure and the hazardous 
substance plume and the rate at which substances are likely to have 
moved through the materials. EPA is proposing to index vertical 
migration based on two factors: Effective porosity (or equivalently, 
the permeability) of geologic materials and the thickness of the lowest 
porosity layer.
    Factor values for effective porosity (as it relates to 
permeability) of geologic materials range from one to four and are 
based solely on the typical range of porosity of subsurface materials 
(e.g., gravel, sand, silt and clay). These factor values are used in 
conjunction with the thickness of the lowest porosity layer (greater 
than 1 foot thickness) to establish a vertical migration factor value, 
ranging from one to fifteen.
    As part of the vertical migration factor, EPA identified soil 
moisture content to potentially be a significant route characteristic 
variable. Thus, to incorporate soil moisture in EPA's assessment of 
potential for exposure, the agency used published ``average soil 
moisture content'' values for specific soil types. These averages were 
used to develop effective porosity/permeability factor values. See 
section 4.0 of the Proposal TSD for supplemental information.
c. Vapor Migration Potential--See Section 5.2.1.1.2.4 of the Proposed 
HRS Addition
    The vapor migration potential factor is based on hazardous 
substance-specific chemical properties, including both the vapor 
pressure and Henry's constant values for hazardous substances 
associated with the site. This factor evaluates the volatile nature of 
these hazardous substances and is projected to be the most influential 
route characteristic factor on calculating potential for exposure based 
on a sensitivity analysis using subsurface 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

[[Page 10391]]

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 location-specific evaluation that 
is influenced by factors such as molecular structure, makeup of the 
immediate subsurface geology, and the presence or absence of oxygen 
within intervening unsaturated soils.
    Because many of the site-specific characteristics impacting the 
rate of degradation are considered beyond the scope of a typical site 
investigation, EPA is proposing to evaluate degradation based on the 
substance being evaluated, the depth to contamination, and if 
appropriate environmental conditions are present to ensure that 
sufficient degradation will occur to diminish the threat. Based on 
EPA's review of the current literature and research on this topic, the 
assigned degradation factor is limited to three possible factor values, 
two for substances that are readily degradable and the appropriate 
environmental factors are present, and one for when either of these 
parameters are not present.
    EPA seeks public input on the following question regarding the 
degradation factor: Is there a way to determine the presence and extent 
of biologically active soil at a site during a limited site 
investigation? If so, what soil characteristics should EPA consider to 
determine whether biologically active soil is documented to be present?
    EPA proposes the degradation factor also be based on the half-life 
of a substance, with the half-life being determined by biodegradation 
and hydrolysis rates. If this information is not available then a 
hazardous substance's estimated half-life will be based on the 
substance's chemical structure, unless available information indicates 
otherwise. Substances with relatively low structural complexity, such 
as petroleum and petroleum-like substances (having straight carbon 
chain or simple ring structures), have the greatest potential to 
degrade in the subsurface while halogenated and poly-aromatic ringed 
substances (e.g., tetrachloroethylene, PCBs) are less likely to 
significantly degrade as result of subsurface microbial activity.
    If it has been documented that a hazardous substance has been found 
to have entered a regularly occupied structure, regardless of the 
substance or the site conditions, the degradation value is assigned to 
reflect the likelihood that the substance is not significantly 
degrading in the subsurface. Also, if the substance is a daughter, or 
degradation product, of a parent substance that is also present, then 
the degradation factor will reflect this relationship. Parent and 
daughter substances are assigned values to reflect that the daughter 
substance will be continuously created by degradation of the parent 
substance. See also section 5.0 of the Proposal TSD for additional 
discussion regarding the inclusion of a degradation factor.
    The toxicity and degradation factors are multiplied together to 
assign a combined factor value. If multiple substances are present, the 
highest combined factor value is selected for use in determining the 
waste characteristics factor category value, as discussed below.
b. Hazardous Waste Quantity--See Section 5.2.1.2.2 of the Proposed HRS 
Addition
    In the basic HRS structure used in all pathways and components, the 
hazardous waste quantity factor reflects the risk consideration related 
to the magnitude and duration of either the release for a migration 
pathway or the exposure for an exposure pathway. In other words, for an 
exposure pathway, the risk posed by a release of hazardous substances 
is directly related to the amount of hazardous substances to which 
receptors (targets) are exposed and the length of the exposure.
    As explained in the preamble to the 1990 HRS and in the 1988 
Revised HRS TSD, an estimate of the waste quantity associated with a 
site was the best surrogate for the amount of hazardous substances that 
receptors were exposed to and that the duration of the exposure was 
probably correlated to the magnitude of the exposure. In the current 
three migration pathways (ground water, surface water, air), the 
hazardous waste quantity factor reflects the total amount of hazardous 
substances in sources at the site to take into account not only where 
contamination has already migrated to, but also future migration of 
contamination to other locations. For the soil exposure pathway, 
however, the estimate does not include the total amount in or released 
from the site sources, but only the amount of hazardous substance in 
the top two feet of contaminated soils sources and in the surface 
portions of other source types in an area of observed contamination. 
(See section 5.0.1 of the current HRS.)
    EPA is proposing that since the subsurface intrusion component also 
focuses on exposure and not the amount of hazardous substances that 
might migrate to targets in the future, the waste quantity factor value 
for this component should also reflect only the amount of hazardous 
substances that people currently are exposed to, that is, the amount in 
regularly occupied structures. EPA is proposing a four-tiered 
hierarchical approach consistent with the current HRS (see section 
III.C of the preamble of the current HRS (55 FR 51542, December 14, 
1990)) as well as minimum waste quantity factors (see section 2.4.2 of 
the current HRS). The minimum waste quantity factors are included 
because of insufficient information at many sites to adequately 
estimate waste quantity with confidence, as discussed in section I of 
the preamble to the current HRS (55 FR 51533, December 14, 1990). The 
current HRS establishes a minimum waste quantity factor value of 10 for 
each pathway or component at sites with no actually contaminated 
targets and a waste quantity factor value of 100 for the migration 
pathways if observed exposure has been documented. (See section 2.4.2 
of the current HRS.)
    It is proposed for the estimation of waste quantity for the 
subsurface intrusion component, that regularly occupied structures 
within the AOE and ASC be considered. For sites at which the component 
waste quantity (the sum waste quantities for all occupied structures in 
the AOE and ASC) is below 10, it is proposed that a minimum factor of 
10 should apply the same as in other pathways and components. This 
minimum factor reflects that in a limited site inspection, it is likely 
that information on the actual waste quantity at a site may not be 
available and a lower value would likely underestimate the actual 
conditions. Furthermore, if any target is subject to Level I or II 
contaminant concentrations a minimum hazardous waste quantity factor 
value of 100 could be assigned.
    EPA seeks public input on the following question regarding the 
calculation of hazardous waste quantity: How could EPA further take 
into account the differences in dilution and

[[Page 10392]]

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

[[Page 10393]]

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/multi-subunit structures, all regularly 
occupied subunits on a level with an observed exposure and all levels 
below are considered to be within an AOE, unless available information 
indicates otherwise. For multi-story/multi-subunit structures located 
within an AOE, but where an observed exposure has not been documented, 
only those

[[Page 10394]]

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/multi-subunit structures, all regularly 
occupied subunits on a level above one where an observed exposure has 
been documented or inferred, or where a gaseous indoor air sample 
meeting observed release criteria is present, are considered to be 
located within an ASC, unless available information indicates 
otherwise. For multi-story/multi-subunit structures located only within 
an ASC, only those regularly occupied subunits within the lowest level 
are considered in an HRS evaluation.
    EPA proposes eligible populations include individuals living in, or 
attending school or day care in the structure, and workers in regularly 
occupied structures. The number of workers is adjusted to reflect that 
their exposure is limited to the time they are in a workplace. 
Therefore, the number of full- and part-time workers in a structure or 
subunit will be divided by an appropriate factor prior to being summed 
with the number of other individuals present. If information is 
unavailable to classify a worker as full- or part-time, that worker 
will be evaluated as full-time.
    The proposed weighting factors for exposed individuals in any 
structure within an ASC are based on the probability of contamination 
entering into occupied structures from the subsurface. The weighting 
factors reflect depth to contamination, sample type, and media. The 
magnitude of the factor is also based on attenuation factors from 
current scientific literature including EPA's 2012 vapor intrusion 
attenuation factors publication. Additional information regarding this 
analysis is presented in section 6.0 of the Proposal TSD.
d. Resources--See Section 5.2.1.3.3 of the Proposed HRS Addition
    The resources target factor is evaluated in all pathways under the 
current HRS. A factor value of five is assigned if at least one 
resource is present and a factor value of zero if no resource is 
present. Eligible resources are pathway-, component-, or threat-
specific. These resources represent uses of a contaminated medium or 
area where exposures occur and are not covered by the other identified 
targets. For example, resources within the air migration pathway 
include commercial agriculture or silviculture and major/designated 
recreation areas. The resident population threat also includes 
commercial livestock production or grazing. See section III.I of the 
preamble to the current HRS (55 FR 51549, December 14, 1990) for 
supplemental information.
    Because subsurface intrusion is limited to indoor spaces, EPA is 
proposing to include regularly occupied structures that are located 
within a defined AOE or ASC (as previously discussed in section VI.B.2 
of this preamble) and in which populations, not including those already 
counted as exposed individuals, may be exposed to contamination due to 
subsurface intrusion. For example, libraries, recreational facilities, 
and religious or tribal structures used by individuals, may qualify as 
eligible resources.
e. Calculation of the Targets Factor Category Value--See Section 
5.2.1.3.4 of the Proposed HRS Addition
    As is done throughout the HRS, EPA is proposing to sum all of the 
target factor values together to establish a target factor category 
value in calculating the proposed subsurface intrusion component score. 
Unlike the likelihood of exposure and waste characteristics factor 
category values in all HRS pathways, which are subject to maximum 
values, the target factor

[[Page 10395]]

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

[[Page 10396]]

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-of-evidence categories included as part of the 1990 
HRS have been retained as EPA has not yet completed assigning all 
substances to the revised categories and are doing so at the time the 
EPA substance literature reviews are updated.)

VIII. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders 
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

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

    This action is a significant regulatory action that was submitted 
to the Office of Management and Budget (OMB) for review. This action 
may raise novel legal or policy issues arising out of legal 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

[[Page 10397]]

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

0
1. The authority citation for part 300 continues to read as follows:

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

0
2. Amend Appendix A to Part 300:
0
a. In section 1.1 by:
0
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;
0
ii. Adding in alphabetical order the definitions ``Channelized flow'' 
and ``Crawl space'';
0
iii. Revising the definitions ``Distance weight'' and ``Half-life'';
0
iv. Amending the definition ``HRS pathway'' by removing the word 
``soil,'' and adding ``soil exposure and subsurface intrusion,'' in its 
place;
0
v. Adding in alphabetical order the definitions ``Indoor air'', 
``Inhalation Unit Risk (IUR)'', ``Occupied structures'', ``Preferential 
subsurface intrusion pathways''; and ``Reference concentration (RfC)'';
0
vi. Revising the definition ``Reference dose (RfD)''; ``Screening 
concentration'', and ``Slope factor (also referred to as cancer potency 
factor)'';
0
vii. Adding in alphabetical order the definitions ``Soil gas'', ``Soil 
porosity''; ``Subslab'', ``Subsurface Intrusion'', ``Surficial ground 
water'', ``Unit Risk'', and ``Unsaturated Zone''; and
0
viii. Revising the introductory text of the definition ``Weight-of-
evidence''.
0
b. Revising section 2.0 to include sections 2.0 through 2.5.2;
0
c. Revising section 5.0 to include sections 5.0 through 5.3;
0
d. In section 6.0 by revising Table 6-14, entitled ``Health-Based 
Benchmarks for Hazardous Substances in Air''; and
0
e. In section 7.0 by:
0
i. Revising the table entitled ``Table 7-1. HRS Factors Evaluated 
Differently For Radionuclides'';
0
ii. Under Table 7-1, the second undesignated paragraph, revising the 
third sentence ;
0
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
0
iv. Adding section 7.3.3.
    The revisions and additions read as follows:

[[Page 10398]]

Appendix A of Part 300--Hazard Ranking System

* * * * *

1.1 Definitions

* * * * *
    Channelized flow: Natural geological or manmade features such as 
karst, fractures, lava tubes, and utility conduits (e.g., sewer lines), 
which allow ground water and/or soil gas to move through the subsurface 
environment more easily.
* * * * *
    Crawl space: The enclosed or semi-enclosed area between a regularly 
occupied structure's foundation (e.g., pier and beam construction) and 
the ground surface. Crawl space samples are collected to determine the 
concentration of hazardous substances in the air beneath a regularly 
occupied structure.
* * * * *
    Distance weight: Parameter in the HRS air migration pathway, ground 
water migration pathway, and the soil exposure component of the soil 
exposure and subsurface intrusion pathway that reduces the point value 
assigned to targets as their distance increases from the site. 
[unitless].
* * * * *
    Half-life: Length of time required for an initial concentration of 
a substance to be halved as a result of loss through decay. The HRS 
considers five decay processes for determining surface water 
persistence: Biodegradation, hydrolysis, photolysis, radioactive decay, 
and volatilization. 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 1[micro]g/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: Media-specific benchmark concentration for 
a hazardous substance that is used in the HRS for comparison with the 
concentration of that hazardous substance in a sample from that media. 
The screening concentration for a specific hazardous substance 
corresponds to its reference concentration for inhalation exposures or 
reference dose for oral exposures, as appropriate, and, if the 
substance is a human carcinogen with either a weight-of-evidence 
classification of A, B, or C, or a weight-of-evidence classification of 
carcinogenic to humans, likely to be carcinogenic to humans or 
suggestive evidence of carcinogenic potential, to that concentration 
that corresponds to its 10 -\6\ individual lifetime excess 
cancer risk for inhalation exposures or for oral exposures, as 
appropriate.
* * * * *
    Slope factor (also referred to as cancer potency factor): Estimate 
of the probability of response (for example, cancer) per unit intake of 
a substance over a lifetime. The slope factor is typically used to 
estimate upper-bound probability of an individual developing cancer as 
a result of exposure to a particular level of a human carcinogen with 
either a weight-of-evidence classification of A, B, or C, or a weight-
of-evidence classification of carcinogenic to humans, likely to be 
carcinogenic to humans or having suggestive evidence of carcinogenic 
potential. [(mg/kg-day) -\1\ for non-radioactive substances 
and (pCi) -\1\ for radioactive substances].
    Soil gas: The gaseous elements and compounds in the small spaces 
between particles of soil.
    Soil porosity: The degree to which the total volume of soil is 
permeated with pores or cavities through which fluids (including air or 
gas) can move. It is typically calculated as the ratio of the pore 
spaces within the soil to the overall volume of the soil.
* * * * *
    Subslab: The area immediately beneath a regularly occupied 
structure with a basement foundation or a slab-on-grade foundation. 
Subslab samples are collected to determine the concentration of 
hazardous substances in the soil gas beneath a home or building.
    Subsurface Intrusion: The migration of hazardous substances from 
the unsaturated zone and/or the surficial ground water into overlying 
structures.
    Surficial ground water: The uppermost saturated zone, typically 
unconfined.
* * * * *
    Unit Risk: The upper-bound excess lifetime cancer risk estimated to 
result from continuous exposure to an agent (i.e., hazardous substance) 
at a concentration of 1 [micro]g/L in water, or 1 [micro]g/m\3\ in air.
    Unsaturated Zone: The portion of subsurface between the land 
surface and the zone of saturation. It extends from the ground surface 
to the 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

[[Page 10399]]

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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.029

    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                  550
 of lines 1 and 2)................
------------------------------------------------------------------------
                          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                 50
     of lines 7a, 7b, or 7c)......
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                 (\b\)
     (lines 10a+10b)..............
11. Targets (lines 7d+8d+9+10c)...              (\b\)
12. Pathway Score is the product
 of Likelihood of Release, Waste
 Characteristics, and Targets,
 divided by 82,500. Pathway scores
 are limited to a maximum of 100
 points...........................
------------------------------------------------------------------------
\a\ Maximum value applies to waste characteristics category. The product
  of lines 4 and 5 is used in Table 2-7 to derive the value for the
  waste characteristics factor category.
\b\ There is no limit to the human population or sensitive environments
  factor values. However, the pathway score based solely on sensitive
  environments is limited to a maximum of 60 points.

 Scoring likelihood of release (or likelihood of exposure) 
factor category.
    --Scoring observed release (or observed exposure or observed 
contamination).
    --Scoring potential to release when there is no observed release.
 Scoring waste characteristics factor category.
    --Evaluating toxicity.
    [ssquf] Combining toxicity with mobility, persistence, degradation 
and/or bioaccumulation (or ecosystem bioaccumulation) potential, as 
appropriate to the pathway (component or threat).
    [ssquf] Evaluating hazardous waste quantity.
    --Combining hazardous waste quantity with the other waste 
characteristics factors.
    [ssquf] Determining waste characteristics factor category value.
 Scoring targets factor category.

[[Page 10400]]

--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:__
A. Source dimensions and hazardous waste quantity.
Hazardous constituent quantity:__
Hazardous wastestream quantity:__
Volume:__
Area:__
Area of observed contamination:__
Area of observed exposure:__
Area of subsurface contamination:__
B. Hazardous substances associated with the source.

[[Page 10401]]

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                    Available to pathway
                                                          --------------------------------------------------------------------------------------------------------------------------------------
                                                                        Air                                 Surface water (SW)               Soil exposure/subsurface intrusion (SESSI)
                                                          -------------------------------              -----------------------------------------------------------------------------------------
                   Hazardous substance                                                                                                     Soil exposure              Subsurface intrusion
                                                                                          Ground water                             -------------------------------------------------------------
                                                                Gas        Particulate        (GW)        Overland/     GW to SW                                   Area of
                                                                                                            flood                     Resident       Nearby       observed    Area of subsurface
                                                                                                                                                                  exposure       contamination
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                           ............  ...............  ............  ............  ............  ............  ............  ............  ..................
                                                           ............  ...............  ............  ............  ............  ............  ............  ............  ..................
                                                           ............  ...............  ............  ............  ............  ............  ............  ............  ..................
                                                           ............  ...............  ............  ............  ............  ............  ............  ............  ..................
                                                           ............  ...............  ............  ............  ............  ............  ............  ............  ..................
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

[[Page 10402]]

    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-m\3\/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-m\3\/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-m\3\/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\
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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.030

     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/kg-day):
    RfD m\3\):
    RfC Slope factor (mg/kg-day)-1
----------------------------------------------------------------------------------------------------------------
0.5 Inhalation unit risk ([mu]g/m3)
----------------------------------------------------------------------------------------------------------------
0.00004 kg)         Dermal LD50 (mg/kg)        (mg/l)               (ppm)          Assigned value
----------------------------------------------------------------------------------------------------------------
LD50 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

[[Page 10405]]

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:
    [ssquf] Meets the criteria for observed exposure, or
    [ssquf] Meets the criteria for observed release in an area of 
subsurface contamination and has a vapor pressure greater than or equal 
to one torr or a Henry's constant greater than or equal to 
10-5 atm-m\3\/mol, or
    [ssquf] Meets the criteria for an observed release in a structure 
within, or in a sample from below, an area of observed exposure and has 
a vapor pressure greater than or equal to one torr or a Henry's 
constant greater than or equal to 10-5 atm-m\3\/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-m\3\/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-m\3\/mol.
    Do not consider other hazardous substances or hazardous 
wastestreams at the site in evaluating this factor for the subsurface 
intrusion component of the soil exposure and subsurface intrusion 
pathway. When determining the hazardous waste quantity for multi-
subunit structures, use the procedures identified in section 5.2.1.2.2.
    2.4.2.1 Source hazardous waste quantity. For each of the three 
migration pathways, assign a source hazardous waste quantity value to 
each source (including the unallocated source) having a containment 
factor value greater than 0 for the pathway being evaluated. Consider 
the unallocated source to have a containment factor value greater than 
0 for each migration pathway.
    For the soil exposure component of the soil exposure and subsurface 
intrusion pathway, assign a source hazardous waste quantity value to 
each area of observed contamination, as applicable to the threat being 
evaluated.
    For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, assign a source hazardous waste quantity 
value to each regularly occupied structure within an area of observed 
exposure or an area of subsurface contamination that has a structure 
containment factor value greater than 0.
    For determining all hazardous waste quantity calculations except 
for an unallocated source or an area of subsurface contamination, 
evaluate using the following four measures in the following hierarchy:
     Hazardous constituent quantity.
     Hazardous wastestream quantity.
     Volume.
     Area.
    For the unallocated source, use only the first two measures. For an 
area of subsurface contamination, evaluate non-radioactive hazardous 
substances using only the last two measures and evaluate radioactive 
hazardous substances using hazardous wastestream quantity only. See 
also section 7.0 regarding the evaluation of radioactive substances.
    Separate criteria apply for assigning a source hazardous waste 
quantity value for radionuclides (see section 7.2.5).
    2.4.2.1.1 Hazardous constituent quantity. Evaluate hazardous 
constituent quantity for the source (or area of observed contamination) 
based solely on the mass of CERCLA hazardous substances (as defined in 
CERCLA section 101(14), as amended) allocated to the source (or area of 
observed contamination), except:
     For a hazardous waste listed pursuant to section 3001 of 
the Solid Waste Disposal Act, as amended by the Resource Conservation 
and Recovery Act of 1976 (RCRA), 42 U.S.C. 6901 et seq., determine its 
mass for the evaluation of this measure as follows:

--If the hazardous waste is listed solely for Hazard Code T (toxic 
waste), include only the mass of constituents in the hazardous waste 
that are CERCLA hazardous substances and not the mass of the entire 
hazardous waste.
-- If the hazardous waste is listed for any other Hazard Code 
(including T plus any other Hazard Code), include the mass of the 
entire hazardous waste.

     For a RCRA hazardous waste that exhibits the 
characteristics identified under section 3001 of RCRA, as amended, 
determine its mass for the evaluation of this measure as follows:

--If the hazardous waste exhibits only the characteristic of toxicity 
(or only the characteristic of EP toxicity), include only the mass of 
constituents in the hazardous waste that are CERCLA hazardous 
substances and not the mass of the entire hazardous waste.
--If the hazardous waste exhibits any other characteristic identified 
under

[[Page 10406]]

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
------------------------------------------------------------------------
                                                          Equation for
     Tier              Measure              Units       assigning value
                                                              \a\
------------------------------------------------------------------------
A.............  Hazardous constituent  lb............  C
                 quantity (C).
B \b\.........  Hazardous wastestream  lb............  W/5,000
                 quantity (W).
C \b\.........  Volume (V)...........
                Landfill.............  yd\3\.........  V/2,500
                Surface impoundment..  yd\3\.........  V/2.5
                Surface impoundment    yd\3\.........  V/2.5
                 (buried/backfilled).
                Drums \c\............  gallon........  V/2.5
                Tanks and containers   yd\3\.........  V/2.5
                 other than drums.
                Contaminated soil....  yd\3\.........  V/2,500
                Pile.................  yd\3\.........  V/2.5
                Other................  yd\3\.........  V/2.5
D \b\.........  Area (A).............
                Landfill.............  ft\2\.........  A/3,400
                Surface impoundment..  ft\2\.........  A/13
                Surface impoundment    ft\2\.........  A/13
                 (buried/backfilled).
                Land treatment.......  ft\2\.........  A/270
                Pile \d\.............  ft\2\.........  A/13
                Contaminated soil....  ft\2\.........  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.

[[Page 10407]]

    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
------------------------------------------------------------------------
                                                               Assigned
               Hazardous waste quantity value                   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.
\b\ For the pathway, if hazardous constituent quantity is not adequately
  determined, assign a value as specified in the text; do not assign the
  value of 1.

    For a migration pathway, if the hazardous constituent quantity is 
adequately determined (see section 2.4.2.1.1) for all sources (or all 
portions of sources and releases remaining after a removal action), 
assign the value from Table 2-6 as the hazardous waste quantity factor 
value for the pathway. If the hazardous constituent quantity is not 
adequately determined for one or more sources (or one or more portions 
of sources or releases remaining after a removal action) assign a 
factor value as follows:
     If any target for that migration pathway is subject to 
Level I or Level II concentrations (see section 2.5), assign either the 
value from Table 2-6 or a value of 100, whichever is greater, as the 
hazardous waste quantity factor value for that pathway.
     If none of the targets for that pathway is subject to 
Level I or Level II concentrations, assign a factor value as follows:

--If there has been no removal action, assign either the value from 
Table 2-6 or a value of 10, whichever is greater, as the hazardous 
waste quantity factor value for that pathway.
--If there has been a removal action:
    [ssquf] Determine values from Table 2-6 with and without 
consideration of the removal action.
    [ssquf] 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.
    [ssquf] 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

[[Page 10408]]

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 1x10\8\. 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
------------------------------------------------------------------------
                                                               Assigned
               Waste characteristics product                    value
------------------------------------------------------------------------
0..........................................................            0
Greater than 0 to less than 10.............................            1
10 to less than 1x10\2\....................................            2
1x10\2\ to less than 1x10\3\...............................            3
1x10\3\ to less than 1x10\4\...............................            6
1x10\4\ to less than 1x10\5\...............................           10
1x10\5\ to less than 1x10\6\...............................           18
1x10\6\ to less than 1x10\7\...............................           32
1x10\7\ to less than 1x10\8\...............................           56
1x10\8\ to less than 1x10\9\...............................          100
1x10\9\ to less than 1x10\10\..............................          180
1x10\10\ to less than 1x10\11\.............................          320
1x10\11\ to less than 1x10\12\.............................          560
1x10\12\...................................................        1,000
------------------------------------------------------------------------

    2.4.3.2 Factor category value, considering bioaccumulation 
potential. For the surface water-human food chain threat and the 
surface water-environmental threat, multiply the toxicity or combined 
factor value, as appropriate, from section 2.4.1.2 and the hazardous 
waste quantity factor value from section 2.4.2.2, subject to:
     A maximum product of 1x10\12\, and
     A maximum product exclusive of the bioaccumulation (or 
ecosystem bioaccumulation) potential factor of 1x 10\8\.
    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:
    [ssquf] 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
    [ssquf] 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:
    [ssquf] 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
    [ssquf] 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
    [ssquf] 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.

[[Page 10409]]

     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 weight-of-evidence classification of carcinogenic to humans, 
likely to be carcinogenic to humans, or suggestive evidence of 
carcinogenic potential), calculate an index I for the sample location 
as follows:
[GRAPHIC] [TIFF OMITTED] TP29FE16.031

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

[[Page 10410]]

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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.032

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.

[[Page 10411]]

[GRAPHIC] [TIFF OMITTED] TP29FE16.033

    5.1 Soil exposure component. Evaluate the soil exposure component 
based on two threats: Resident population threat and nearby population 
threat. Evaluate both threats based on three factor categories:

[[Page 10412]]

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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.034

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
------------------------------------------------------------------------
                                                               Value
      Factor categories and factors       Maximum  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 +            (\b\)  ..............
     6b)
    7. Workers..........................              15  ..............
    8. Resources........................               5  ..............
    9. Terrestrial Sensitive                         (c)  ..............
     Environments.......................
    10. Targets (lines 5 + 6c + 7 + 8 +            (\b\)  ..............
     9).................................
Resident Population Threat Score
    11. Resident Population Threat                 (\b\)  ..............
     (lines 1x4x10).....................
------------------------------------------------------------------------
                        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            (\b\)  ..............
     14x17x20)..........................
Soil Exposure Component Score
    22. Soil Exposure Component Score                100  ..............
     \d\ (Sse), (lines [11+21]/82,500,
     subject to a maximum of 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

[[Page 10413]]

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
------------------------------------------------------------------------
                                                         Equation for
     Tier              Measure             Units        assigning value
                                                              \a\
------------------------------------------------------------------------
A.............  Hazardous Constituent  lb...........  C
                 Quantity (C).
B \b\.........  Hazardous Wastestream  lb...........  W/5,000
                 Quantity (W).
C \b\.........  Volume (V)...........
                Surface Impoundment    yd \3\.......  V/2.5
                 \c\.
                Drums \d\............  gallon.......  V/500
                Tanks and Containers   yd \3\.......  V/2.5
                 Other Than Drums.
D \b\.........  Area (A).............
                Landfill.............  ft \2\.......  A/34,000
                Surface Impoundment..  ft \2\.......  A/13
                Surface Impoundment    ft \2\.......  A/13
                 (Buried/backfilled).
                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 x 10\8\. 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

[[Page 10414]]

there is a resident individual, as specified in section 5.1.1.3, who is 
subject to Level I or Level II concentrations.
    First, determine those areas of observed contamination subject to 
Level I concentrations and those subject to Level II concentrations as 
specified in sections 2.5.1 and 2.5.2. Use the health-based benchmarks 
from Table 5-3 in determining the level of contamination. Then assign a 
value to the resident individual factor as follows:
     Assign a value of 50 if there is at least one resident 
individual for one or more areas subject to Level I concentrations.
     Assign a value of 45 if there is no such resident 
individuals, but there is at least one resident individual for one or 
more areas subject to Level II concentrations.
     Assign a value of 0 if there is no resident individual.
    Enter the value assigned in Table 5-1.
    5.1.1.3.2 Resident population. Evaluate resident population based 
on two factors: Level I concentrations and Level II concentrations. 
Determine which factor applies as specified in sections 2.5.1 and 
2.5.2, using the 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
------------------------------------------------------------------------
                                                               Assigned
                      Number of workers                          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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.035

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                         100
 designated endangered or threatened species.........
 National Park
 Designated Federal Wilderness Area
 National Monument
Terrestrial habitat known to be used by Federal                       75
 designated or proposed threatened or endangered
 species.............................................
 National Preserve (terrestrial)
 National or State Terrestrial Wildlife Refuge
 Federal land designated for protection of natural
 ecosystems
 Administratively proposed Federal Wilderness Area
 Terrestrial areas utilized for breeding by large or
 dense aggregations of animals \b\
Terrestrial habitat known to be used by State                         50
 designated endangered or threatened species.........
 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                           25
 management..........................................
 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.

[[Page 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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.036

    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,                    75
 fishing, hiking, softball).............................
Accessible and unique recreational area (for example,                 75
 vacant lots in urban area).............................
Moderately accessible (may have some access                           50
 improvements, for example, gravel road), with some
 public recreation use..................................
Slightly accessible (for example, extremely rural area                25
 with no road improvement), with some public recreation
 use....................................................
Accessible, with no public recreation use...............              10
Surrounded by maintained fence or combination of                       5
 maintained fence and natural barriers..................
Physically inaccessible to public, with no evidence of                 0
 public recreation use..................................
------------------------------------------------------------------------

             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
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                             Attractiveness/accessibility factor value
   Area of contamination 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

[[Page 10416]]

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

    5.1.2.2 Waste characteristics. Evaluate waste characteristics based 
on two factors: Toxicity and hazardous waste quantity. Evaluate only 
those hazardous substances that meet the criteria for observed 
contamination (see section 5.1.0) at areas that can be assigned an 
attractiveness/accessibility factor value greater than 0.
    5.1.2.2.1 Toxicity. Assign a toxicity factor value as specified in 
section 2.4.1.1 to each hazardous substance meeting the criteria in 
section 5.1.2.2. Use the hazardous substance with the highest toxicity 
factor value to assign the value to the toxicity factor for the nearby 
population threat. Enter this value in Table 5-1.
    5.1.2.2.2 Hazardous waste quantity. Assign a value to the hazardous 
waste quantity factor as specified in section 5.1.1.2.2, except: 
Consider only those areas of observed contamination that can be 
assigned an attractiveness/accessibility factor value greater than 0. 
Enter the value assigned in Table 5-1.
    5.1.2.2.3 Calculation of waste characteristics factor category 
value. Multiply the toxicity and hazardous waste quantity factor 
values, subject to a maximum product of 1 x 10\8\. 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\....................  \a\1
Greater than \1/4\ to 1....................  0
------------------------------------------------------------------------
\a\ Assign a value of 0 if one or more persons meet the section 5.1.1.3
  criteria for resident individual.

    5.1.2.3.2 Population within 1 mile. Determine the population within 
each travel distance category of Table 5-10. Count residents and 
students who attend school within this travel distance. Do not include 
those people already counted in the resident population threat. 
Determine travel distances as specified in section 5.1.2.3.1.
    In estimating residential population, when the estimate is based on 
the number of residences, multiply each residence by the average number 
of persons per residence for the county in which the residence is 
located.
    Based on the number of people included within a travel distance 
category, assign a distance-weighted population value for that travel 
distance from Table 5-10.
    Calculate the value for the population within 1 mile factor (PN) as 
follows:
[GRAPHIC] [TIFF OMITTED] TP29FE16.037

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.

[[Page 10417]]

                                                         Table 5-10--Distance Weighted Population Values for Nearby Population Threat a
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                         Number of people within the travel distance category
                                     -----------------------------------------------------------------------------------------------------------------------------------------------------------
  Travel distance category  (miles)                                                                       301 to      1,001 to     3,001 to    10,001 to    30,001 to    100,001 to   300,001 to
                                           0         1 to 10      11 to 30    31 to 100    101 to 300     1,000        3,000        10,000       30,000      100,000      300,000     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.

[[Page 10418]]

    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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.038

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              500
     * (2b + 2c + 2d), subject to a
     maximum of 500)....................
    4. Likelihood of Exposure (higher of             550
     lines 1 or 3)......................
Waste Characteristics:
    5. Toxicity/Degradation.............           \(a)\
    6. Hazardous Waste Quantity.........           \(a)\
    7. Waste Characteristics (subject to             100
     a maximum of 100)..................
Targets:
    8. Exposed Individual...............              50
    9. Population:......................
        9a. Level I Concentrations......           \(b)\
        9b. Level II Concentrations.....           \(b)\
        9c. Population within an Area of           \(b)\
         Subsurface Contamination.......
        9d. Total Population (lines 9a +           \(b)\
         9b + 9c).......................
    10. Resources.......................               5
    11. Targets (lines 8 + 9d + 10).....           \(b)\
Subsurface Intrusion Component Score:
    12. Subsurface Intrusion Component               100
     (lines 4 x 7 x 11)/82,500 \c\
     (subject to a maximum of 100)......
Soil Exposure and Subsurface Intrusion
 Pathway Score:
    13. Soil Exposure Component +                    100
     Subsurface Intrusion Component
     (subject to a maximum of 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.

[[Page 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:

[ssquf] Meet the criteria for observed exposure, or
[ssquf] 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-m\3\/mol, or
[ssquf] 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-m\3\/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:

[ssquf] 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.
[ssquf] 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.
[ssquf] 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.
[ssquf] If any regularly occupied multi-subunit structure is inferred 
to be in an area of subsurface contamination, consider only those 
regularly occupied subunit(s), if any, on the lowest level, to be 
within an area of subsurface contamination, unless available 
information indicates otherwise.

    If there is no area of observed exposure and no area of subsurface 
contamination, assign a score of 0 for the subsurface intrusion 
component.
    5.2.1 Subsurface intrusion component. Evaluate this component only 
if there is an area of observed exposure or area of subsurface 
contamination:
     Within or underlying a residence, school, day care center, 
workplace, or
     Within or underlying a resource specified in section 
5.2.1.3.3.
    5.2.1.1 Likelihood of exposure. Assign a value of 550 to the 
likelihood of exposure factor category for the subsurface intrusion 
component if there is an area of observed exposure in one or more 
locations listed in section 5.2.1. Enter this value in Table 5-11.
    5.2.1.1.1 Observed exposure. Establish observed exposure in a 
regularly occupied structure by demonstrating that a hazardous 
substance has been released into a regularly occupied structure via the 
subsurface. Base this demonstration on either of the following 
criterion:
     Direct observation:

-- A solid, liquid or gaseous material that contains one or more 
hazardous substances attributable to the site has been observed 
entering a regularly occupied structure through migration via the 
subsurface or is known to have entered a regularly occupied structure 
via the subsurface, or
-- When evidence supports the inference of subsurface intrusion of a 
material that contains one or more hazardous substances associated with 
the site into a regularly occupied structure, demonstrated adverse 
effects associated with that release may be used to establish observed 
exposure.

     Chemical analysis:

-- Analysis of indoor samples indicates that the concentration of 
hazardous substance(s) has increased significantly above the background 
concentration for the site for that type of sample (see section 2.3).
-- Some portion of the significant increase must be attributable to the 
site to establish the observed exposure. Documentation of this 
attribution should account for possible concentrations of the hazardous 
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

[[Page 10420]]

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-
m\3\/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
------------------------------------------------------------------------
                                Evidence of structure
            No.                      containment          Assigned value
------------------------------------------------------------------------
1.........................  Regularly occupied structure              10
                             with evidence of subsurface
                             intrusion, including
                             documented observed
                             exposure or sampling of bio
                             or inert gases, such as
                             methane and radon.
2.........................  Regularly occupied structure              10
                             with open preferential
                             pathways from the
                             subsurface (e.g., sumps,
                             foundation cracks, unsealed
                             utility lines).
3.........................  Regularly occupied structure               7
                             with an engineered vapor
                             migration barrier system
                             that does not address all
                             preferential pathways.
4.........................  Regularly occupied structure               6
                             with an engineered passive
                             vapor mitigation system
                             without documented
                             institutional controls
                             (e.g., deed restrictions)
                             or evidence of regular
                             maintenance and inspection.
5.........................  Regularly occupied structure               4
                             with no visible open
                             preferential pathways from
                             the subsurface (e.g.,
                             sumps, foundation cracks,
                             unsealed utility lines).
6.........................  Regularly occupied structure               3
                             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.
7.........................  Regularly occupied structure               2
                             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.
8.........................  Regularly occupied structure               2
                             with unknown containment
                             features.
9.........................  Regularly occupied structure               0
                             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.
10........................  Regularly occupied structure               0
                             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.
------------------------------------------------------------------------

    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 to
                     Depth range 1 2                       contamination
                                                          assigned value
------------------------------------------------------------------------
0 to 10 ft.(Including subslab and semi-enclosed or                    10
 enclosed crawl space contamination)....................
>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.

[[Page 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
------------------------------------------------------------------------
                                                             Assigned
                                                             porosity/
                    Type of material                       permeability
                                                               value
------------------------------------------------------------------------
Gravel; highly permeable fractured igneous and                         1
 metamorphic rocks; permeable basalt; karst limestones
 and dolomites..........................................
Sand; sandy clays; sandy loams; loamy sands; sandy                     2
 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................
Silt; loams; silty loams; loesses; silty clays;                        3
 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......................................
Clay; low permeability till (compact unfractured till);                4
 shale; unfractured metamorphic and igneous rocks.......
------------------------------------------------------------------------

                                                      Table 5-15 Vertical Migration Factor Values a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                 Thickness of lowest porosity layer(s) \b\ (feet)
                                                         -----------------------------------------------------------------------------------------------
          Assigned porosity/ permeability value                           Greater than 5   Greater than    Greater than    Greater than    Greater than
                                                              0 to 5           to 10         10 to 20        20 to 50        50 to 100      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
1 to 10.................................................               2
Less than 1.............................................               0
------------------------------------------------------------------------
             Henry's constant (atm-m\3\/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
------------------------------------------------------------------------

[[Page 10422]]

   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 one-half as a result of the combined decay processes 
of two components: biodegradation and hydrolysis.
    Estimate the half-life (t1/2) of a hazardous substance 
as follows:
[GRAPHIC] [TIFF OMITTED] TP29FE16.039

Where:

h=Hydrolysis half-life.
b=Biodegradation half-life.

    If one of these component half-lives cannot be estimated for the 
hazardous substance from available data, delete that component half-
life from the above equation.
    If no half-life information is available for a hazardous substance 
and the substance is not already assigned a value of 1, unless 
information indicates otherwise, all straight-chain and simple-ring 
structure substances will be considered to have a half-life less than 
30 days if not the hazardous substance will be assigned a half-life of 
greater than 100 days.
    Based on the hazardous substance's assigned half-life the 
degradation factor is assigned as follows:
     For all hazardous substances at the site that meet 
subsurface intrusion observed release criteria but not observed 
exposure criteria, assign a degradation factor value of 0.1, if:

--The depth to contamination at the site is greater than or equal to 10 
feet, but not if available evidence suggests that at least 10 feet of 
biologically active soil is not present in the subsurface anywhere 
underneath a structure within an area of subsurface contamination or 
area of observed exposure, and
--The hazardous substance has a half-life of 30 days or less.

     For all hazardous substances at the site that meet 
subsurface intrusion observed release criteria but not observed 
exposure criteria, assign a degradation factor value of 0.5, if:

--The depth to contamination at the site is greater than 30 feet, but 
not if available evidence suggests that at least 30 feet of 
biologically active soil is not present in the subsurface anywhere 
underneath a structure within an area of subsurface contamination or 
area of observed exposure, and
--The hazardous substance has a half-life equal to or less than 100 
days.

     For all other situations assign a degradation factor of 1 
for all hazardous substances at the site that meet subsurface intrusion 
observed release criteria.
    In addition, for hazardous substances that meet observed release 
criteria, have a parent-daughter degradation relationship, and the 
daughter substance is found only in samples with a depth greater than 
10 feet, assign the daughter substance degradation factor value as 
follows:
    1. Identify the shallowest subsurface sample that contains the 
daughter substance.
    2. Determine if the selected sample or another sample from the same 
relative position in the media of concern, or in a shallower sample, 
contains the parent substance.
    3. If the parent substance is not present in the identified 
samples, assign the degradation factor value for the daughter substance 
based on the half-life for the daughter substance.
    4. If the parent substance is present in a sample from the same 
relative position in the subsurface or in a shallower sample, compare 
the half-life-based degradation factor value for the daughter substance 
to the degradation factor value assigned to the parent substance. 
Assign the greater of the two values as the degradation factor value 
for the daughter substance.
    5.2.1.2.1.3 Calculation of toxicity/degradation factor value. 
Assign each substance a toxicity/biodegradation value by multiplying 
the toxicity factor value by the degradation factor value. Use the 
hazardous substance with the highest combined toxicity/degradation 
value to assign the factor value to the toxicity/degradation factor for 
the subsurface intrusion threat. Enter this value in Table 5-11.
    5.2.1.2.2 Hazardous waste quantity. Assign a hazardous waste 
quantity factor value as specified in section 2.4.2. Consider only 
those regularly occupied structures with a non-zero structure 
containment value. In estimating the hazardous waste quantity, use 
Tables 2-5 and 5-18 and:
     For Tier A, hazardous constituent quantity, use the mass 
of constituents found in the regularly occupied structure(s) where the 
observed exposure has been identified.

--For multi-subunit structures, when calculating Tier A, use the mass 
of

[[Page 10423]]

constituents found in the regularly occupied subunit space(s) where the 
observed exposure has been identified.

     For Tier B, hazardous wastestream quantity, use the flow-
through volume of the regularly occupied structures where the observed 
exposure has been identified.

--For multi-subunit structures, when calculating Tier B, use the flow-
through volume of the regularly occupied subunit spaces where the 
observed exposure has been identified.

     For Tier C, volume, use the volume divisor listed in Tier 
C of Table 5-18. Volume is calculated for those regularly occupied 
structures located within areas of observed exposure with observed or 
inferred intrusion and within areas of subsurface contamination.

--In evaluating the volume measure for these listed areas of observed 
exposure and areas of subsurface contamination based on a gaseous/vapor 
intrusion or the potential for gaseous/vapor intrusion, consider the 
following:

     Calculate the volume of each regularly occupied structure 
based on actual data. If unknown, use a ceiling height of 8 feet.
     For multi-subunit structures, when calculating Tier C, 
calculate volume for those subunit spaces with observed or inferred 
exposure and all other regularly occupied subunit spaces on that level, 
unless available information indicates otherwise. If the structure has 
multiple stories, also include the volume of all regularly occupied 
subunit spaces below the floor with an observed exposure and one story 
above, unless evidence indicates otherwise.
     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
------------------------------------------------------------------------
                                                          Equation for
    Tier             Measure               Units        assigning value
                                                              \a\
------------------------------------------------------------------------
A            Hazardous Constituent    lb.............  C.
              Quantity (C).
B \b\        Hazardous Wastestream    lb.............  W/5,000.
              Quantity (W).
------------------------------------------------------------------------
C b c        Volume (V).
------------------------------------------------------------------------
             Regularly occupied       yd \3\.........  V/2.5.
              structure(s) in areas
              of observed exposure
              or subsurface
              contamination.
------------------------------------------------------------------------
D b d        Area (A).
------------------------------------------------------------------------
             Regularly occupied       ft \2\.........  A/13.
              structure(s) in areas
              of observed exposure
              or subsurface
              contamination.
------------------------------------------------------------------------
\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.

[[Page 10424]]

    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 x 10\8\. 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 health-based benchmarks from 
Table 5-19 to determine the level of contamination.

--If the sample is from a structure with no subunits and the 
concentration equals or exceeds the appropriate benchmark, assign Level 
I concentrations to the entire structure.
--If the sample is from a subunit within a structure and the 
concentration from that subunit equals or exceeds the appropriate 
benchmark, assign Level I concentrations to that subunit.

     Level II Concentrations: Structures, or subunits within 
structures, with one or more samples that meet observed exposure by 
chemical analysis criteria but do not equal or exceed the appropriate 
benchmark; structures, or subunits, that have an observed exposure by 
direct observation; and structures inferred to be in an area of 
observed exposure based on samples meeting observed exposure, are 
assigned Level II concentrations.
    [cir] 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

[[Page 10425]]

attending school or day care in the structure or subunit.
    3. Count the number of full-time and part-time workers in the 
structure or subunit(s) subject to Level I concentrations. If 
information is unavailable to classify a worker as full- or part-time, 
evaluate that worker as being full-time. Divide the number of full-time 
workers by 3 and the number of part-time workers by 6, and then sum 
these products with the number of other individuals for each structure 
or subunit.
    4. Sum this combined value for all structures, or subunits, within 
areas of observed exposure and multiply this sum by 10.
    Assign the resulting product as the combined population factor 
value subject to Level I concentrations for the site. Enter this value 
in line 9a of Table 5-11.
    5.2.1.3.2.2 Level II concentrations. Assign the population subject 
to Level II concentrations as follows:
    1. Identify all exposed individuals regularly present in an 
eligible structure, or if the structure has subunits, identify those 
regularly present in each subunit, located in an area of observed 
exposure subject to Level II concentrations as described in sections 
5.2.0 and 5.2.1.3.1. Identify only once per structure those exposed 
individuals that are using more than one eligible subunit of the same 
structure (e.g., using a common or shared area and other parts of the 
same structure).
    2. Do not include exposed individuals already counted under the 
Level I concentrations factor.
    3. For each structure or subunit(s), count the number of 
individuals residing in or attending school or day care in the 
structure, or subunit, subject to Level II concentrations.
    4. Count the number of full-time and part-time workers in the 
structure or subunit(s) subject to Level II concentrations. If 
information is unavailable to classify a worker as full- or part-time, 
evaluate that worker as being full-time. Divide the number of full-time 
workers by 3 and the number of part-time workers by 6, and then sum 
these products with the number of other individuals for each structure 
or subunit.
    5. Sum the combined population value for all structures within the 
areas of observed exposure for the site.
    Assign this sum as the combined population value subject to Level 
II contamination for this factor. Enter this value in line 9b of Table 
5-11.
    5.2.1.3.2.3 Population within area(s) of subsurface contamination. 
Assign the population in area(s) of subsurface contamination factor 
value as follows, unless available information indicates otherwise (see 
sections 5.2.0 and 5.2.1.3.1):
    1. Identify the regularly occupied structures with a structure 
containment value greater than zero and the eligible population 
associated with the structures or portions of structures in each area 
of subsurface contamination:
     For each regularly occupied structure or portion of a 
structure in an area of subsurface contamination, sum the number of all 
individuals residing in or attending school or day care, in the 
structure or portion of the structure in the area of subsurface 
contamination.
     Count the number of full-time and part-time workers 
regularly present in each structure or portion of a structure in an 
area of subsurface contamination. If information is unavailable to 
classify a worker as full- or part-time, evaluate that worker as being 
full-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
------------------------------------------------------------------------
                                                            Population
  Eligible populations \a\ in structures \b\ within an       weighting
            area of subsurface contamination                  factor
------------------------------------------------------------------------
Population in a structure with levels of contamination               0.9
 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...............................................
Population in a structure where levels of contaminants               0.4
 meeting observed release criteria are found in any
 sampling media at or within five feet horizontally or
 vertically of the structure foundation.................
Population occupying a structure where levels of                     0.2
 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.............................................
Population in a structure where levels of contaminants               0.1
 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................................................
------------------------------------------------------------------------
\a\ Eligible populations include residents (including individuals living
  in, or attending school or day care in the structure), and workers in
  regularly occupied structures (see HRS Section 5.2.1.3).
\b\ Eligible structures may include single- or multi-tenant structures
  where eligible populations reside, attend school or day care, or work.
  These structures may also be mixed use structures.

[[Page 10426]]

    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

* * * * *

[[Page 10427]]

[GRAPHIC] [TIFF OMITTED] TP29FE16.040

[[Page 10428]]

[GRAPHIC] [TIFF OMITTED] TP29FE16.041

* * * * *
    * * * 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:

    [ssquf] 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:
    [cir] Equals or exceeds a value 2 standard deviations above the 
mean site-specific background concentration for that radionuclide in 
that type of sample, or
    [cir] Exceeds the upper-limit value of the range of regional 
background concentration values for that specific radionuclide in that 
type of sample.
    [ssquf] Some portion of the increase must be attributable to the 
site to establish the observed release (or observed contamination or 
observed exposure), and
    [ssquf] 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:

    [ssquf] 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.
    [ssquf] 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.
    [ssquf] If the sample quantitation limit cannot be established:
    [cir] 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).
    [cir] If the sample analysis is not performed under the EPA 
Contract Laboratory Program, use the detection limit in place of the 
sample quantitation limit.

[[Page 10429]]

    [ssquf] 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 ([mu]R/hr) using a survey instrument held 1 meter above the ground 
surface or floor or walls of a structure (or 1 meter away from an 
aboveground source for the soil exposure component), equals or exceeds 
2 times the site-specific background gamma radiation exposure rate.
--Some portion of the increase must be attributable to the site to 
establish observed contamination. The gamma-emitting radionuclides do 
not have to be within 2 feet of the surface of the source.

    For the three migration pathways and for the subsurface intrusion 
component of the soil exposure and subsurface intrusion pathway, if an 
observed release or observed exposure can be established for the 
pathway (or threat, aquifer, or watershed, as appropriate), assign the 
pathway (or threat, aquifer, or watershed) an observed release or 
observed exposure factor value of 550 and proceed to section 7.2. If an 
observed release or observed exposure cannot be established, assign an 
observed release or observed exposure factor value of 0 and proceed to 
section 7.1.2.
    For the soil exposure component of the soil exposure and subsurface 
intrusion pathway, if observed contamination can be established, assign 
the likelihood of exposure factor for resident population a value of 
550 if there is an area of observed contamination in one or more 
locations listed in section 5.1.1; evaluate the likelihood of exposure 
factor for nearby population as specified in section 5.1.2.1; and 
proceed to section 7.2. If observed contamination cannot be 
established, do not evaluate the soil exposure component of the soil 
exposure and subsurface intrusion pathway.
    At sites containing mixed radioactive and other hazardous 
substances, evaluate observed release (or observed contamination or 
observed exposure) separately for radionuclides as described in this 
section and for other hazardous substances as described in sections 2 
through 6.
    For the three migration pathways and the subsurface intrusion 
component of the soil exposure and subsurface intrusion pathway, if an 
observed release or observed exposure can be established based on 
either radionuclides or other hazardous substances, or both, assign the 
pathway (or threat, aquifer, or watershed) an observed release or 
observed exposure factor value of 550 and proceed to section 7.2. If an 
observed release or observed exposure cannot be established based on 
either radionuclides or other hazardous substances, assign an observed 
release or observed exposure factor value of 0 and proceed to section 
7.1.2.
    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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.042

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:
[GRAPHIC] [TIFF OMITTED] TP29FE16.043

Where:

r = Radioactive half-life.

    At sites containing mixed radioactive and other hazardous 
substances,

[[Page 10430]]

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:
    [GRAPHIC] [TIFF OMITTED] TP29FE16.044
    

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,
    [GRAPHIC] [TIFF OMITTED] TP29FE16.045
    

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.

[[Page 10431]]

     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:

    [ssquf] Determine values from Table 2-6 with and without 
consideration of the removal action.
    [ssquf] 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.
    [ssquf] 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:

[[Page 10432]]

     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
```