[Federal Register Volume 81, Number 39 (Monday, February 29, 2016)]
[Proposed Rules]
[Pages 10371-10432]
[FR Doc No: 2016-02749]

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

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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 Once submitted, comments cannot be edited or withdrawn.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:<PHONE_NUMBER>,
email<EMAIL_ADDRESS>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<PHONE_NUMBER> or<PHONE_NUMBER> 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

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

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.

    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

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

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

    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.

[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.

    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,

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

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

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

    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\
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    \8\ For references to a specific section of the proposed HRS
addition, please refer to the regulatory text of the proposed
rulemaking.
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    EPA is proposing that 14 new definitions be added to the HRS,
section 1.1, with additional modifications to existing definitions. EPA
is adding these new definitions to aid the site evaluator in
establishing the environmental boundaries that are being evaluated in
this component (e.g., contamination in or above the surficial aquifer),
in identifying factors unique to the subsurface intrusion component
(e.g., channelized flow through which soil gas transports with no
resistance), and to ensure consistent application of the HRS.
2. Delineation of Areas of Subsurface Intrusion--See Section 5.2.0 of
the Proposed HRS Addition
    EPA is proposing to include in the subsurface intrusion component
evaluation two areas in which exposure due to subsurface intrusion
contamination exists or is likely to exist: (1) Areas of observed
exposure--areas in which contaminant intrusion into regularly occupied
structures has been documented, and (2) areas of subsurface
contamination--areas in which subsurface contamination underlying
regularly occupied structures (such as in surficial ground water or
soil vapor) has been documented, but at which either sampling of indoor
air has not documented that subsurface contamination has entered a
regularly occupied structure or no sampling of indoor air has been
undertaken. See Figure 4 for an illustration of the two areas.
Additionally, special considerations are given to buildings with
multiple subunits and multiple levels (e.g., apartment buildings) when
establishing areas of subsurface intrusion. For a more detailed
discussion on the selection of these areas, see section 6.0 of the
Proposal TSD.

a. Area of Observed Exposure (AOE)--See Section 5.2.0 of the Proposed
HRS Addition
    EPA is proposing to identify an area (or areas) of observed
exposure at a site based on the location of regularly occupied
structures with documented contamination resulting from subsurface
intrusion attributable to the site being evaluated. The area
encompassed by such structures constitutes the area of observed
exposure (AOE). Other regularly occupied structures within this
encompassed area (or areas) will also be inferred to be in the AOE
unless available information indicates otherwise. Populations occupying
structures within the AOE are considered exposed to subsurface
contamination for HRS scoring purposes, and thus, are included in the
HRS evaluation. See section 6.0 of the Proposal TSD for further
discussion on the delineation of an AOE and the rationale for the
inclusion of this area in an HRS evaluation.
b. Area of Subsurface Contamination (ASC)--See Section 5.2.0 of the
Proposed HRS Addition
    EPA is proposing to also identify an area (or areas) of subsurface
contamination as an area outside that of the AOE, but for which
subsurface contamination has been documented at levels meeting observed
release criteria (contamination at levels significantly above
background and the significant increase can be attributed at least in
part to the site). The contamination would be present either in
surficial ground water samples, in subslab or 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
<IP_ADDRESS>.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

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 <IP_ADDRESS> 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 <IP_ADDRESS>.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 <IP_ADDRESS>.1 of
the Proposed HRS Addition
    The identification of an observed exposure by direct observation
can be based on a solid, liquid, or gaseous hazardous substance
attributable to the site being observed or known to have entered a
regularly occupied structure from the subsurface. This finding will
generally require the observation that a solid, liquid, or gas is
entering the structure, and can be documented from a sample of the
material that shows the hazardous substance is present due to the
release from the site being evaluated. For example, this type of direct
exposure could be documented if: (1) Contaminated vapors are found in a
sample from a sump open to the regularly occupied structure, and (2)
the same hazardous substances are found in subsurface samples collected
beneath the regularly occupied structure or otherwise can be
demonstrated as having emanated from known contamination underlying the
structure. Another example would be if chromium precipitate is found in
basements subject to ground water flooding and it is known that a
chromium contaminant plume is present, and its presence is not from
indoor sources. In neither example would a significant increase above a
background contaminant level be required. For exposures to intruded
ground water, EPA is proposing

documented observed exposure by direct observation as the only method
for establishing likelihood of exposure. Figure 5 below depicts an
additional example of documenting observed exposure by direct
observation through collection of a contaminated water sample taken
from the sump of an occupied structure that is known to be subject to
flooding. Other methods may also be used to establish direct
observation depending on site-specific conditions. See section 4.0 of
the Proposal TSD for further information.
[GRAPHIC] [TIFF OMITTED] TP29FE16.027

ii. Observed Exposure by Chemical Analysis--See Section <IP_ADDRESS>.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

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 <IP_ADDRESS>.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.

[GRAPHIC] [TIFF OMITTED] TP29FE16.028

i. Structure Containment--See Section <IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS>.2.2 of the Proposed HRS
Addition
    The depth to contamination factor represents the vertical distance
between contamination (either in soil, soil gas, or surficial ground
water) and the lowest horizontal point of an overlying regularly
occupied structure (e.g., a

basement floor). This distance represents how far a hazardous substance
would have to travel through the subsurface to intrude into that
structure. Based on available data, the probability of exposure
decreases as the depth to contamination increases. In addition, as part
of EPA's sensitivity analysis in developing route characteristics, at
depths greater than 150 feet it became increasingly unlikely that
exposure would occur. This is reflected in Table 5-13 (section
<IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS> 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 <IP_ADDRESS>.1 of the Proposed HRS
Addition
    The combined toxicity/degradation factor includes consideration of
both the toxicity and the possibility for degradation of hazardous
substances being evaluated for HRS purposes.
    The toxicity factor in the overall HRS structure reflects the
toxicity of a hazardous substance associated with a release or
exposure, and is assigned the same factor value for all the pathways
and components in the current HRS. As in all HRS pathways and
components, it is proposed to be assigned the same corresponding factor
value as for other parts of the HRS. The rationale for the assignment
of the factor value is discussed in the section 2.3 of the 1988 Revised
HRS TSD. This toxicity factor is based on the toxicity of the
substances present at a site. In the HRS addition, a different factor
value is proposed to be assigned to each hazardous substance that an
occupant has been or is potentially exposed to. The factor value is
driven by the magnitude of each hazardous substance's acute and chronic
toxicity to humans. The toxicity factor value is directly related to
the concentration at which the hazardous

substance is known to have a health effect: The more toxic the
chemical, the higher the toxicity value. Any hazardous substance
identified in an observed exposure within the AOE or meeting the
observed release criteria in either the AOE or ASC will be assigned a
toxicity factor value. The method for assigning this value is contained
in section <IP_ADDRESS> 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 <IP_ADDRESS>.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

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 <IP_ADDRESS>.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 <IP_ADDRESS> 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 <IP_ADDRESS> 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 <IP_ADDRESS> 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 <IP_ADDRESS>.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 <IP_ADDRESS>.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 <IP_ADDRESS>.1 and <IP_ADDRESS>.2
of the proposed HRS addition.) Those targets in the AOE are considered
actually contaminated, whereas, those in the ASC are considered
potentially contaminated. The targets in an AOE are further divided
into Level I and II, based

on whether the hazardous substance concentrations are at or above
identified health-based benchmarks. EPA is proposing to use the
following benchmarks for the subsurface intrusion component:

 Screening concentrations for cancer
 Screening concentrations for noncancer toxicological responses

    Targets associated with an observed exposure by direct observation
are only considered subject to Level II contamination in all parts of