[Federal Register Volume 82, Number 5 (Monday, January 9, 2017)]
[Rules and Regulations]
[Pages 2760-2807]
[FR Doc No: 2016-30640]

Vol. 82

Monday,

No. 5

January 9, 2017

Part III

 Environmental Protection Agency

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40 CFR Part 300

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

Federal Register / Vol. 82, No. 5 / Monday, January 9, 2017 / Rules
and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 300

[EPA-HQ-SFUND-2010-1086; FRL-9956-58-OLEM]
RIN 2050-AG67

Addition of a Subsurface Intrusion Component to the Hazard
Ranking System

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is adding 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 NPL is a list
of national priorities among the known or threatened releases of
hazardous substances, pollutants or contaminants throughout the United
States. Sites on the NPL are priorities for further investigation to
determine if further response actions are warranted. The subsurface
intrusion component (this addition) expands the number of available
options for EPA and state and tribal organizations performing work on
behalf of EPA to evaluate actual and potential threats to public health
from releases of hazardous substances, pollutants, or contaminants.
This addition enables EPA to directly consider human exposure to
hazardous substances, pollutants, or contaminants that enter regularly
occupied structures through subsurface intrusion in assessing a site's
relative risk, and thus, enable sites with subsurface intrusion
contamination to be evaluated for placement on the NPL.

DATES: This final rule is effective February 8, 2017.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-SFUND-2010-1086. All documents in the docket are
listed on the http://www.regulations.gov Web site. Although listed in
the index, some information is not publicly available, e.g., CBI or
other information whose disclosure is restricted by statute. Certain
other material, such as copyrighted material, is not placed on the
Internet and will be publicly available only in hard copy form.
Publicly available docket materials are available electronically
through http://www.regulations.gov or in hard copy at the EPA Docket
Center Reading Room (see https://www.epa.gov/dockets/epa-docket-center-reading-room for more information).

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. Statutory Authority for Regulatory Change
II. Background
    A. The Hazard Ranking System
    B. Site Assessment and the Superfund Remedial Process
    C. Impact of the SsI Addition on Current Cleanup Programs,
Resources and Cost
    D. Impact of the Subsurface Intrusion Addition on the Hazard
Ranking System
III. Overview of the Final Rule
    A. HRS Structure With the Subsurface Intrusion Component
    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)
    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
    C. Testing the SsI Component
    1. Conceptual Site Model/Sensitivity Analysis
    2. Test Site (Tier 1) Summaries
    3. Pilot Study
IV. Summary of Changes to the HRS
    A. Changes Since Proposal
    B. Summary of Updates to the HRS (Sections 2, 5, 6, and 7)
V. Discussion of Major Comments
    A. Responses to Comments on EPA Questions Posed in the Proposed
Rule
    B. Major Comment Theme Summaries and Responses
VI. 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
    L. Congressional Review Act (CRA)

I. Statutory Authority for Regulatory Change

    EPA has revised the HRS, the principal mechanism for placing sites
on the NPL, to add a component for evaluating the threat or potential
threat posed by subsurface intrusion to protect human health and the
environment. Without an evaluation of threats posed by subsurface
intrusion contamination, the HRS is not a complete assessment because
it omits a known pathway of human exposure to contamination. The
addition of subsurface intrusion to the HRS is compliant with
Comprehensive Environmental Response, Compensation, and Liability Act
(CERCLA) Section 105(a)(8)(A), which requires EPA to prioritize sites
based on ``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. This addition to
the HRS also improves the agency's ability to identify priority sites
for further investigation and enhances 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. For
information on alternatives to this rulemaking that were considered for
addressing subsurface intrusion contamination, please see the

preamble to the proposed HRS SsI Addition [81 FR 10372, February 29,
2016].
    Additionally, the Government Accountability Office (GAO) stated in
its May 2010 report \1\:
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    \1\ 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 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
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otherwise be cleaned up.

    The authority for these technical modifications to the HRS is in
section 105(a)(8)(A) of CERCLA enacted in 1980. Under CERCLA, 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. 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.
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 NPL is Appendix B of the NCP (40 CFR 300,
Appendix B).
    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.'' In
addition, CERCLA section 115 authorizes EPA to promulgate any
regulations necessary to carry out the provisions of CERCLA.
    Furthermore, the Congressional Conference Report on SARA included
the absolute standard against which HRS revisions could be assessed:

    This standard is to be applied within the context of the purpose
for the National Priorities List; i.e., identifying for the States
and the public those facilities and sites which appear to warrant
remedial actions. * * * This standard does not, however, require the
Hazard Ranking System to be equivalent to detailed risk assessments,
quantitative or qualitative, such as might be performed as part of
remedial actions. The standard requires the Hazard Ranking System to
rank sites as accurately as the Agency believes is feasible using
information from preliminary assessments and site inspections * * *
Meeting this standard does not require long-term monitoring or an
accurate determination of the full nature and extent of
contamination at sites or the projected levels of exposure such as
might be done during remedial investigations and feasibility
studies. This provision is intended to ensure that the Hazard
Ranking System performs with a degree of accuracy appropriate to its
role in expeditiously identifying candidates for response actions.
[H.R. Rep. No. 962, 99th Cong., 2nd Sess. at 199-200 [1986]]

    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 in a more comprehensive manner.
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.
    This final rule ensures the HRS does not omit a known pathway of
human exposure to contamination due to subsurface intrusion of released
hazardous substances and provides a mechanism for assessing subsurface
intrusion threats and identifying sites for placement on the NPL.
Furthermore, these sites are now eligible for Superfund-financed
remedial actions.

II. Background

    The HRS is a crucial part of the agency's program for determining
which sites are a priority for further remedial investigation and
possible cleanup under CERCLA. To understand the importance of this
rulemaking it is necessary to understand the role of the HRS in
identifying sites for the NPL, the role of the HRS in the overall site
assessment and Superfund remedial process, and this final rule's
impacts on current and future Superfund activities. In addition, it is
also necessary to understand the impact of adding the SsI component to
the HRS.

A. The Hazard Ranking System

    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
preliminary assessment (PA) and site inspection (SI). The HRS is not a
tool for conducting a quantitative risk assessment and was designed to
be a measure of relative risk among sites rather than absolute site-
specific risk. As required by CERCLA, EPA has designed the Superfund
program to focus its resources on the priority sites. Consequently, the
initial studies--the PA and SI--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 expeditiously perform PAs and SIs, Congress
placed certain constraints on the data requirements 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 extended periods
of time. However, EPA allows for the expansion of the typical SI to
allow for additional data collection for more complex sites that cannot
be adequately characterized using standard SI methodologies. The HRS
has also been designed so that it can be applied consistently to a wide
variety of sites, enabling sites to be

ranked relative to each other with respect to actual or potential
hazards.
    Based on the state of the science, site specific data may be
collected beyond that which is normally available after a typical site
inspection. In these situations, the HRS in general, and the SsI
component, can incorporate that data into the HRS evaluation. For
example, the SsI component can use site-specific data as follows:
     Determination of the Hazardous Waste Quantity Factor
Value--If the mass of all hazardous substances can be adequately
determined (i.e., is known or can be estimated with reasonable
confidence), the HRS requires this estimate (identified as a Tier A
estimate) be used to assign the hazardous waste quantity for all
regularly occupied structures in an area of exposure (AOE) for which
this information is available. See section 2.4.2 and <IP_ADDRESS>.2 of the
HRS.
     Determining the extent of an ASC--If sufficient data are
available and state of the science shows there is no unacceptable risk
due to subsurface intrusion into a regularly occupied structure located
within an ASC, that structure or subunit can be excluded from the ASC.
Therefore, such structures would not be included in the evaluation of
the Hazardous Waste Quantity Factor or in the determination of other
factors evaluated based on structures or subunits within an ASC. See
section 5.2.0 of the HRS.
     Populations within the ASC--If sufficient structure-
specific concentration data is available and state of the science shows
there is no unacceptable risk of exposure to populations in a regularly
occupied structure in an ASC, those populations are not included in the
evaluation of the Targets Factor Category. See section <IP_ADDRESS> of the
HRS.
    EPA notes that if other site-specific information is available that
clearly demonstrates that the site does not pose an unacceptable risk
to human health via subsurface intrusion, there are points during the
PA or SI process, where further evaluation of the site for the
subsurface intrusion threat by the Superfund program can be terminated.
Please see section B. of this preamble for further information on the
Site Assessment process.
    As EPA explained when it originally adopted the HRS, ``the HRS is a
means for applying uniform technical judgment regarding the potential
hazards presented by a facility relative to other facilities. It does
not address the feasibility, desirability, or degree of cleanup
required.'' (47 FR 31220, July 16, 1982).
    The HRS uses a structured value analysis approach to scoring sites.
This approach assigns values to factors related to or indicative of
risk. The basic elements of the HRS are factors that are based on
information that can be collected in a limited screening assessment. A
scale of numerical rating values is provided for each factor and a
value is assigned to each factor based on conditions at the site.
Individual values are then weighted. The factors are grouped into three
factor categories--observed release/route characteristics, waste
characteristics, and targets--and are combined to obtain factor
category scores. Each factor category has a maximum value, as does each
of the component factors within the category. The relevant factor
category scores are multiplied together within each pathway and
normalized to obtain a pathway score. The pathway scores are combined
using a root-mean-square approach to calculate the overall site score;
that is, the final HRS score is the square root of the sum of the
squares of the pathway scores divided by the square root of the number
of HRS pathways. If all pathway scores are low, the HRS score will be
low. However, the final score will be relatively high even if only one
pathway score is high. EPA considers this an important requirement for
the HRS scoring because some extremely dangerous sites pose threats
through only one migration mode. For example, at a site, leaking drums
of hazardous substances may be contaminating drinking water wells,
thereby posing a significant threat via the groundwater migration
pathway. But if the drums are buried deeply enough and the hazardous
substances are not very volatile, the drums may not release any
hazardous substances and not pose a threat to the air or to surface
water.
    EPA emphasizes that the HRS score is a number between 0 and 100,
which reflects relative risk amongst candidate NPL sites. An HRS site
score is not a measure of actual site-specific risk.

B. Site Assessment and the Superfund Remedial Process

    EPA's Superfund remedial site assessment process evaluates sites to
ascertain if further investigation is needed for determining whether an
unacceptable risk is present.
    The majority of sites evaluated through the EPA's site assessment
program do not meet the criteria for possible placement on the NPL and
are ``screened out'' of the Superfund Remedial process. (See Figure 1.
Status of EPA's Site Assessments). Since EPA adopted the HRS, 52, 859
sites have been assessed under EPA's Superfund program. Of those sites,
1,782 were placed on the NPL, as of September 2016.
Site Assessment Strategy
    The site assessment process is structured as a series of limited
investigations which may include: (1) A Pre-CERCLA screening
assessment; (2) a preliminary assessment; and (3) a site inspection or
expanded site inspection (Figure 2. Site Assessment Process, below,
illustrates this process). If a site progresses through the site
assessment process for further investigation, the requirements for
documenting risk become increasingly rigorous. The following includes a
summary of the major phases of the site assessment process.
     A Pre-CERCLA Screening is an initial review of existing
information on a possible Superfund site. If a release of a hazardous
substance has occurred or if the potential of a hazardous substance to
release exists the site may be eligible for further remedial evaluation
under CERCLA authority. If further evaluation is warranted the site
should be entered into the remedial assessment active site inventory
for further assessment.
     The PA decision process parallels an HRS analysis, but
makes environmental ``worst-case'' assumptions of possible significant
risk regarding transport of contamination to receptors based on minimal
available information and professional judgment.
     The SI collects information to confirm the accuracy of the
PA assumptions. The information should be sufficient to support an HRS
evaluation with minimal further investigation.
     If placement on the NPL is pursued, the information
collected during the SI provides the basis for supporting the HRS
scoring scenario.

[GRAPHIC] [TIFF OMITTED] TR09JA17.058

    The following discussion provides further information on each of
these phases.
Pre-CERCLA Screening Assessment
    A Pre-CERCLA Screening is used to establish whether:
     A release or potential release of a hazardous substance
has occurred at a site;
     The site is eligible for further remedial assessment under
CERCLA authority;
     The site needs further attention under Superfund or
another cleanup program; and
     The site warrants entry into the federal Superfund
program's active site inventory for further assessment or response.
    Determining whether releases of hazardous substances, pollutants,
or contaminants can be addressed by CERCLA requires the application of
site-specific facts to CERCLA statutory requirements and EPA policy.
The initial determination as to whether a site warrants further
investigation is based on three site-specific facts including: (1)
Evidence of an actual release or potential to release; (2) targets
impacted by a release of contamination at the site; and (3)
documentation that a target has been exposed to a hazardous substance
released from the site. Examples of targets include populations,
drinking water wells, drinking water surface intakes, municipal wells,
fisheries and sensitive environments.
Preliminary Assessment
    A PA uses readily available data to determine if there is evidence
of a release that poses an unacceptable possible threat as specified in
the NCP (40 CFR 300.420).
     The PA is a limited-scope investigation performed by
States and/or EPA on every CERCLA site
     The PA may include the collection of readily available
information and an on- or off-site reconnaissance may be conducted
     The PA distinguishes, based on already existing
information, between sites that appear to pose little or no threat to
human health and the environment and sites that require further
investigation to determine if the threat to human health and the
environment is unacceptable.

[GRAPHIC] [TIFF OMITTED] TR09JA17.059

    If based on the results of a PA, EPA determines that a site
warrants further screening under the CERCLA remedial program, the
agency initiates a site inspection
Site Inspection
    The purpose of the SI is to collect the data necessary to perform
an HRS evaluation. An SI determines if a release of a hazardous
substance poses 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. EPA may expand the site inspection
scope as needed. This expanded site inspection (ESI) collects
additional data beyond what is collected in the standard site
inspection to evaluate sites for HRS scoring. ESIs are reserved for
more complex sites that cannot be adequately characterized using
standard site inspection methods.
     SI investigators typically collect waste and environmental
samples to determine the substances present at a site and whether they
are being released to the environment, as well as other information to
perform an HRS evaluation.
     EPA distinguishes, based on the information collected
during the SI, between sites that appear to pose little or no threat to
human health and the environment and sites that require further
investigation to determine if the threat to human health and the
environment exists.
     If the information indicates a threat, EPA determines the
best approach for addressing the threat, which can be placement on the
NPL or use of an alternative authority.
    If at any time in this site assessment process, EPA determines that
sufficient information indicates the site poses no unacceptable risk,
or if it can be addressed under alternative authorities it can be
removed from the process. Also, if an imminent or substantial
endangerment to public health is identified, EPA can initiate CERCLA
removal actions.

The NPL Rulemaking Process
    The NPL is a list of national priorities for further investigation
amongst the known or threatened releases of hazardous substances,
pollutants or contaminants throughout the United States. The list,
which is appendix B of the NCP (40 CFR part 300), is required under
section 105(a)(8)(B) of CERCLA, as amended. Section 105(a)(8)(B)
defines the NPL as a list of ``releases'' and the highest priority
``facilities'' and requires that the NPL be revised at least annually.
The NPL is intended primarily to guide the EPA in determining which
sites warrant further investigation to assess the nature and extent of
public health and environmental risks associated with a release of
hazardous substances, pollutants or contaminants. The NPL is of only
limited significance, however, as it does not assign liability to any
party or to the owner of any specific property. Also, placing a site on
the NPL does not mean that any remedial or removal action necessarily
need be taken.
    For purposes of listing, the NPL includes two sections, one of
sites that are generally evaluated and cleaned up by the EPA (the
``General Superfund section'') and one of sites that are owned or
operated by other federal agencies (the ``Federal Facilities
section''). With respect to the Federal Facilities sites, these sites
are generally being addressed by other federal agencies. Under
Executive Order 12580 (52 FR 2923, January 29, 1987) and CERCLA section
120, each federal agency is responsible for carrying out most response
actions at facilities under its own jurisdiction, custody or control,
although the EPA is responsible for preparing a Hazard Ranking System
(``HRS'') score and determining whether the facility is placed on the
NPL and having oversight authority at the sites for further actions.
NPL Site Selection Process
    The NPL is required to be revised annually and it is intended
primarily to guide EPA in determining which sites warrant further
investigation to assess the nature and extent of public health and
environmental risks associated with a release of hazardous substances,
pollutants or contaminants. This selection process is illustrated in
figure 3, below. Sites with HRS scores of 28.50 or greater are eligible
for placement on the NPL. Only non-Federal Facility sites on the NPL
are eligible for Superfund-financed remedial actions. Once a site is
determined to be NPL-caliber and a decision has been made that the
federal Superfund program should manage the site cleanup, EPA regions
apply a strong initial presumption in favor of placement on the NPL.
    Once the site is proposed for the NPL (i.e., announced in the
Federal Register), a 60-day comment period is initiated to allow the
public to comment on the proposal. EPA responds to all public comments,
and depending on the results of the public comment period, the site
could be removed from consideration for placement of the NPL; re-
proposed in the future due to public comments; or placed on the NPL.
Once the site is placed on the NPL, the rulemaking can be challenged in
court under the Administrative Procedure Act (APA). If no challenge is
made or if the court finds the rulemaking consistent with APA
requirements, it is then eligible for further investigation under the
Superfund remedial program. (Figure 3. Process for Placing a Site on
the NPL).
[GRAPHIC] [TIFF OMITTED] TR09JA17.060

C. Impact of the SsI Addition on Current Cleanup Programs, Resources
and Cost

    This SsI addition to the HRS will have the most significant impact
on EPA's Superfund cleanup program. This regulatory change expands
available options for EPA and organizations performing work on behalf
of EPA (state and tribal partners) to evaluate actual and potential
threats to public health and the environment from subsurface intrusion
contamination. This modification to the HRS, by itself, only augments
the criteria for applying the HRS. It has no effect on small
businesses.
    This final rule will not 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 have been shown to or are projected to qualify for placement on the
NPL. The method selected for including the SsI evaluation in the HRS
site score can only result in an increase in a site score, Therefore,
all sites qualifying for the NPL based on its HRS site score prior to
this final rule will continue to do so. It is consistent with section
105(c)(3) of CERCLA, as amended, that these sites will not be re-
evaluated. This final rule will not disrupt EPA's placement of sites on
the NPL.

    The possible impact on federal agencies other than EPA performing
Superfund actions will be less than that on private sites being
addressed by EPA. Federal agencies currently address subsurface
intrusion issues as part of their environmental programs and
authorities. 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. Many federal agencies, including EPA, have developed or
are developing new or updated agency-specific policy and guidance
documents to address subsurface intrusion threats.
    As a result of federal agency existing environmental programs and
authorities, this rulemaking is not anticipated to have a significant
impact to the resources and costs to federal cleanup programs.
    Since EPA's overall appropriated Superfund budget as well EPA's
cooperative agreement budget for performing site assessments will
continue to remain relatively steady, EPA anticipates that this final
rule will not result in additional site assessments nor the placement
of more sites on the NPL during any particular interval, but rather a
shift in the make-up of the type of sites included on the NPL. EPA will
continue to review sites as part of Superfund remedial site assessment
to determine whether sites are eligible for further remedial evaluation
under CERCLA authorities and prioritize sites that pose the highest
risk. This is not a change to how EPA currently evaluates and
prioritizes sites for the NPL. Because the level of effort required to
evaluate a site, regardless of pathway, varies on a site-by-site basis,
depending on the size and extent of contamination at the site, it
cannot be predicted with any certainly that there will be an increase
in cost or level of effort for any particular site due to this
rulemaking.
    This rulemaking, which could lead to the inclusion of a site on the
NPL that did not qualify for the NPL previously, 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. These costs are a
result of a release of hazardous substances and would not be incurred
if hazardous substances had not been released.
    Later Superfund-related decisions that consider information
collected under the HRS SsI 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. Therefore, addition
of subsurface intrusion to the HRS is several regulatory steps removed
from imposing costs on private entities.
    This rulemaking does not impose any requirements on small entities,
and therefore can be certified as no Significant Economic Impact on a
Substantial Number of Small Entities (SISNOSE). With the exception of
other federal agencies, site assessments are performed by EPA and on
behalf of EPA by states and tribes in cooperative agreement
partnerships with EPA. Under section 601 of the Regulatory Flexibility
Act, federal agencies do not fit under the definition of small
business, small entity, small organization or small governmental
jurisdiction.

D. Impact of the Subsurface Intrusion Addition on the Hazard Ranking
System

    This final rule, with the addition of a subsurface intrusion
component, does not change the purpose of the HRS, its fundamental
structure or its application. It does not change the balance between
the pathways or calculation of the overall HRS site score and the same
cutoff score to qualify a site for the NPL is maintained. The current
approach for scoring the ground water, surface water, and air migration
pathways is not being altered by the addition of a subsurface intrusion
component. EPA added the subsurface intrusion threat as a component to
the present soil exposure pathway because its structure already focuses
on populations actually or potentially coming into direct contact with
hazardous substances. The re-structured pathway is called the ``Soil
Exposure and Subsurface Intrusion'' pathway and now allows for the
consideration of the threat posed by subsurface contaminant intrusion.
The Soil Exposure and Subsurface Intrusion pathway retains the existing
two soil exposure threats (resident population and nearby population)
in the pathway as one component, with subsurface intrusion as the
second component.
    The narrow technical modifications resulting from this Final Rule
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 has not
changed with this amendment to the HRS to include the consideration of
subsurface intrusion.

III. Overview of the Final Rule

    This final rule revises the 1990 HRS to include a component for
evaluating the threats posed from subsurface intrusion. The following
sections discuss the structure of the HRS, the subsurface intrusion
component within the HRS, the major factors of the subsurface intrusion
addition, and 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. All sites that qualified for the
NPL under the 1990 HRS, would still qualify for the NPL under this
revised HRS. For a more comprehensive description and rationale of
changes, see the February 29, 2016 Proposed Rule [81 FR 10372, February
29, 2016].

A. HRS Structure With the Subsurface Intrusion Component

    EPA added the evaluation of the relative risk posed by subsurface
intrusion of hazardous substances into regularly occupied structures by
restructuring the soil exposure pathway from the 1990 HRS to include
subsurface intrusion. The soil exposure pathway has been renamed the
soil exposure and subsurface intrusion pathway to reflect both
components of

the new pathway. No changes are included in the other three HRS
pathways, with the exception of the use of a reference concentration
instead of a reference dose to determine a hazardous substance's
health-based benchmark in the air migration pathway. See Figure 4 for a
depiction of how the promulgated addition fits into the HRS structure.
[GRAPHIC] [TIFF OMITTED] TR09JA17.061

    As explained in the preamble to the proposed HRS SsI addition, the
subsurface intrusion component is added as a new component of the soil
exposure and subsurface intrusion pathway. The soil exposure pathway
included in the 1990 HRS is retained as one component of the Soil
Exposure and Subsurface Intrusion pathway. The scoring of the soil
exposure component remains unaltered, but the score is assigned as the
soil exposure component score, not the pathway score. (See section 5.1
of the HRS). As discussed in greater detail below, the SsI component
has the same basic structure, scoring, and weighting as other parts of
the HRS.
    The score for the soil exposure and subsurface intrusion pathway is
based on a combination of the two component scores--soil exposure and
subsurface intrusion but the pathway score is capped at the same value
as other HRS pathways. The soil exposure component score is added to
the subsurface intrusion component score to determine the pathway
score. The two component scores are additive to reflect that
populations may be exposed 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 same 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. SsI Component Addition

    The structure of the HRS is fundamentally the same for all
individual pathways, components, and/or threats. 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 associated with a target from the releases,

     For the three migration pathways, the probability that the
release will reach a target and the expected change in the
concentration of hazardous substances during the movement from the
location of the contamination to the targets. For the exposure pathway,
the probability a receptor will be exposed at the target location,
     The expected dose to the receptor, and
     The expected number and type 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 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.
1. New Definitions--See Section 1.1 of the HRS \2\
---------------------------------------------------------------------------

    \2\ For references to a specific section of the HRS addition,
please refer to the regulatory text of the rulemaking.
---------------------------------------------------------------------------

    EPA has added 15 new definitions to the HRS, section 1.1, along
with updated nomenclature to existing definitions. EPA received no
comments on the 14 proposed new definitions to the rule; therefore, EPA
is finalizing the new definitions as proposed with the following
change: The term surficial ground water has been changed to shallow
ground water for clarity. In addition, EPA has added the term non-
aqueous phase liquid (NAPL) to the definition section because EPA added
consideration of NAPLs to the assignment of degradation factor values
and the weighting of targets in the area of subsurface contamination
(ASC).
2. Delineation of Areas of Subsurface Intrusion--See Section 5.2.0 of
the HRS
    EPA has included 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 shallow 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.
a. Area of Observed Exposure (AOE) (See Section 5.2.0 of the HRS)
    An area (or areas) of observed exposure at a site is identified
based on the location of regularly occupied structures with a
documented significant increase in hazardous substance concentrations
above background levels resulting at least in part 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) are also inferred to be in the AOE unless
available information indicates otherwise.
b. Area of Subsurface Contamination (ASC)--See Section 5.2.0 of the HRS
    An area (or areas) of subsurface contamination is identified as an
area outside that of the AOE, at 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 in subslab or semi-enclosed or
enclosed crawl space samples or in a subsurface sample. (See section
2.3 of the HRS for observed exposure criteria.) In addition, EPA is
limiting the delineation of an ASC to be based on the location of
subsurface contamination meeting the criteria for observed exposure or
observed release and has a vapor pressure greater than or equal to one
torr or a Henry's constant greater than or equal to 10-5
atm-m\3\/mol. 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, the sample media, and the presence of a non-aqueous phase
liquid (NAPL).
3. Likelihood of Exposure--See Section <IP_ADDRESS> of the HRS
    A key factor considered in the HRS relative risk ranking is whether
any exposure to a hazardous substance via subsurface intrusion has
occurred, or if not, whether there is a probability that exposure could
occur in a regularly occupied structure. This is termed the likelihood
of exposure for the subsurface intrusion component.
a. Observed Exposure--See Section <IP_ADDRESS>.1 of the HRS
    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.
b. Potential for Exposure--See Section <IP_ADDRESS>.2 of the HRS
    When an observed exposure has not been established, the potential
for exposure can be determined for any regularly occupied structure
located in an ASC.
    The evaluation of the potential for exposure for the subsurface
intrusion component uses the same concept and framework used to
estimate the potential to release in other pathways. This involves
predicting the probability of exposure in an area of subsurface
contamination based on structural containment features of the regularly
occupied structure and a hazardous substance's physical and chemical
properties and the physical subsurface properties that influence the
probability that intrusion is occurring. These factor values include:

 Structure Containment
 Depth to Contamination
 Vertical Migration
 Vapor Migration Potential

    Consistent with potential to release determinations in the HRS, the
potential for exposure for this component is calculated by summing
depth to contamination, vertical migration and vapor migration
potential 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 HRS
    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.

4. Waste Characteristics--See Section <IP_ADDRESS> of the HRS
    The waste characteristics factor category is based on factors that
are related to the relative risk considerations included in the basic
HRS structure. 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.
a. Toxicity/Degradation--See Section <IP_ADDRESS>.1 of the HRS
    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 source, release or exposure at a site, and is
assigned the same factor value for all the pathways and components in
the HRS. 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 degradation factor represents the possibility for a substance
to degrade in the subsurface prior to intruding into a regularly
occupied structure. The subsurface intrusion component evaluates
degradation based on the substance being evaluated, the depth to
contamination, and the presence of a NAPL. It also assumes the presence
of biologically active soil unless information indicates otherwise. 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. Additionally, any eligible hazardous
substance present in the subsurface below an AOE or ASC as a NAPL at
depth less than 30 feet is assigned a degradation value to reflect the
likelihood that the substance will not significantly degrade in the
subsurface environment.
    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 HRS
    The waste quantity factor value for this component reflects only
the amount of hazardous substances that people are exposed to, that is,
the amount in regularly occupied structures. EPA has retained a four-
tiered hierarchical approach consistent with the HRS as well as minimum
waste quantity factors. The estimation of waste quantity for the
subsurface intrusion component considers the regularly occupied
structures located within the AOE and ASC. For sites at which the
component waste quantity (the sum waste quantities for all regularly
occupied structures in the AOE and ASC) is below 10, a minimum factor
of 10 would apply, the same as in other pathways and components. The
minimum waste quantity factors are included because of insufficient
information at many sites to adequately estimate waste quantity with
confidence.
c. Calculation of the Waste Characteristics Factor Category Value--See
Section <IP_ADDRESS>.3 of the HRS
    As in all HRS pathways and components, the waste characteristics
category value is the product of the waste characteristics factor
values (e.g., toxicity/degradation factor value) for the SsI component
and the hazardous waste quantity factor value, all of which are 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 1990
HRS structure.
5. Targets--See Section <IP_ADDRESS> of the HRS
    The targets factor is based upon estimates of the expected dose to
each receptor associated with a target and the number and type of
receptors present at each target. In assessing human risk, it is
critical to understand the nature and extent of exposure to
individuals, populations, and resources.
a. Identification of Eligible Targets--See Section <IP_ADDRESS> of the HRS
    The soil exposure and subsurface intrusion pathway uses the same
target categories used in the HRS soil exposure pathway, including
exposed individual, resident populations, workers, and resources.
However, unlike the HRS soil exposure pathway, workers are 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.
b. Exposed Individual and Levels of Exposure--See Section <IP_ADDRESS>.1 of
the HRS
i. Identifying Levels of Exposure and Benchmarks for Subsurface
Intrusion
    In the SsI component, 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.
    The targets within an ASC are categorized based on the type of
sample (e.g., gas, soil, water), the distance of the sample from the
targets (e.g., the depth of the sample below the structure), and
whether a NAPL is present. Weighting factors ranging from 0.1 to 0.9
are then assigned accordingly.
ii. Exposed Individual--See Section <IP_ADDRESS>.1 of the HRS
    The evaluation of exposed individuals in the SsI component includes
individuals living, attending school or day care, or working in a
regularly occupied structure. Individuals in the eligible target
population are expected to be exposed to the highest concentration of
the hazardous substance in question for a significant time.
c. Population--See Section <IP_ADDRESS>.2 of the HRS
    The population factor for the SsI component includes all
populations qualifying as exposed individuals, including residents,
students, workers, and those attending day care. Workers are weighted
slightly differently than other exposed individuals to reflect that a
worker's exposure is limited to the time present in a workplace. The
number of workers present in a structure or subunit is adjusted by an
appropriate factor reflecting whether or not they are a full-time or
part-time worker.
i. Weighting of Targets in the Area of Observed Exposure (AOE)--See
Sections <IP_ADDRESS>.2.1 and <IP_ADDRESS>.2.2 of the HRS
    Consistent with the weighting of populations throughout the HRS,
the subsurface intrusion component will weight targets in an AOE
subject to Level I contaminant concentrations by a factor of 10 and
weight targets subject to Level II contaminant concentrations by a
factor of 1. Eligible populations include individuals living, working,
and

attending school or day care in regularly occupied structures.
    Within the AOE, those populations in regularly occupied structures
for which observed exposures have not been established but the
structures are surrounded by regularly occupied structures in which
observed exposures have been identified, are also considered as
actually contaminated unless evidence indicates otherwise. Targets
inferred to be exposed to this contamination will be weighted as Level
II as there are no actual sample results to compare against benchmarks.
    In the case of multi-story/multi-subunit structures, all regularly
occupied subunits on a level with an observed exposure and all levels
below are considered to be within an AOE, unless available information
indicates otherwise. For multi-story/multi-subunit structures located
within an AOE, but where an observed exposure has not been documented,
only those regularly occupied spaces on the lowest level are considered
to be within an AOE, unless available information indicates otherwise.
ii. Weighting of Targets in the Area of Subsurface Contamination
(ASC)--See Section <IP_ADDRESS>.2.3 of the HRS
    Due to the variability in subsurface intrusion rates, the potential
weighting factor values for targets within an ASC range from 0.1 to 0.9
and depend on where the subsurface contamination has been found and
whether a NAPL is present.
    Potential targets are weighted to reflect the distance to or the
depth at which contamination is found and whether a NAPL is present.
The weighting factors applied to populations being evaluated based on
the presence of subsurface contamination containing a NAPL reflects
greater subsurface source concentrations and an increased probability
that contaminant intrusion into a regularly occupied structure from the
subsurface will result in a concentration significantly above
background levels for the site. In the case of multi-story/multi-
subunit structures, all regularly occupied subunits on a level above
one where an observed exposure has been documented or inferred, or
where a gaseous indoor air sample meeting observed release criteria is
present, are considered to be located within an ASC, unless available
information indicates otherwise. For multi-story/multi-subunit
structures located only within an ASC, only those regularly occupied
subunits within the lowest level are considered in an HRS evaluation.
    Eligible populations in an ASC include individuals living in,
attending school or day care, and working in regularly occupied
structures. However, the number of workers is adjusted to reflect that
their exposure is limited to the time they are in a workplace.
d. Resources--See Section <IP_ADDRESS>.3 of the HRS
    Resources for this component include regularly occupied structures
that are located within a defined AOE or ASC and in which populations
may be exposed to contamination due to subsurface intrusion. Libraries,
recreational facilities, and religious or tribal structures used by
individuals may qualify as eligible resources.
e. Calculation of the Targets Factor Category Value--See Section
<IP_ADDRESS>.4 of the HRS
    The Target Factor Category Value is the sum of all the Target
Factor values.
6. Calculation and Incorporation of the SsI Component Score Into the
HRS Site Score
    The following subsections summarize the calculation of the
subsurface intrusion component score, how the component score is used
in the calculation of the soil exposure and subsurface intrusion
pathway score, and how, in turn, the pathway score is subsequently
incorporated into the HRS site score.
a. Calculation of the SsI Component Score--See Section 5.2.2 of the HRS
    The SsI Component score is the product of the likelihood of
exposure factor category value, the waste characteristics factor
category value, and the targets factor category value; that value is
divided by a weighting factor so that it has equal magnitude to other
component scores (subject to a maximum value).
b. Incorporation of the SsI Component Score into the Soil Exposure and
Subsurface Intrusion Pathway Score--See Section 5.3 of the HRS
    The Soil Exposure and Subsurface Intrusion pathway score is a
combination of the two component scores.
c. Incorporation of the Soil Exposure and Subsurface Intrusion Pathway
Score Into a Site Score--See Section 2.1.1 of the HRS
    EPA did not change the methodology used to assign an overall site
score due to the addition of the subsurface intrusion component to the
soil exposure pathway and renaming that pathway the soil exposure and
subsurface intrusion pathway. The overall site score remains a function
of four pathway scores and the same weighting is given to each pathway
score as in the 1990 HRS.

C. Testing the SsI Component

    The SsI component was tested extensively throughout the development
of this rule, using multiple methods. The main goals of testing the
component included:
     Ensuring the addition of the SsI component to the soil
exposure pathway did not change relative contribution to the site score
as the other HRS pathways and maintained the same relative risk of a
site with a similar threshold for qualifying for the NPL.
     Ensuring the number of targets subject to actual
contamination needed to achieve a site score sufficient for NPL
proposal remained consistent across pathways.
     Ensuring that applying the SsI component as part of an HRS
evaluation would not result in identification of sites with a low level
of risk or would not identify sites with a high level of risk.
    These goals were met by using conceptual simulations to project the
effectiveness and appropriateness for factor values, by developing and
testing numerous example site scenarios to refine the model and by
applying the model to test sites to determine its efficacy. The
following information provides details on the approaches used to test
the SsI component.
1. Conceptual Site Model/Sensitivity Analysis
    Sensitivity analyses were performed during development of the rule
to test the SsI component and identify and assign the relative
magnitude of the factors having the greatest impact on the HRS site
score. The analyses illustrated the types of sites that would qualify
for the NPL considering subsurface intrusion contamination, and sites
that would qualify for the NPL considering the contribution of
subsurface intrusion contamination to other pathways. The scenarios
illustrate different site characteristics and different factor value
weightings. An initial conceptual site scenario evaluation was
developed with varying likelihood of intrusion levels, zone of
contamination, waste characteristics and levels of contamination. The
conceptual site scenario evaluation was varied to reflect possible
ranges in the factors considered in the HRS evaluation.
    The first phase of testing estimated site scores based on options
considered

for identifying eligible targets and delineating target areas. The
testing was conducted using factor values, factor category values, and
scoring algorithms consistent with other parts of the HRS. This ensured
relative risk was evaluated and consistently weighted among pathways. A
second phase was conducted for identifying target areas delineated by
AOEs and ASCs of various site scenarios to test the HRS addition and to
illustrate the features of sites that would qualify for the NPL
considering vapor intrusion contamination. To illustrate the subsurface
intrusion component and contribution of weighting of factor values,
three comprehensive site scoring scenarios were evaluated: A site would
not qualify for placement on the NPL (score below 28.50), a site would
marginally qualify for the NPL (score of or about 28.50), and a site
would exceed the scoring criterion for the NPL (site score considerably
above 28.50). Based on this final rule, the results revealed that sites
without areas of observed exposures and a typical waste characteristic
value would require a minimum of 685 receptors living, working or
attending school or daycare above an area of subsurface contamination
to receive a score of 28.50 based on shallow subsurface sampling. Sites
with documented subsurface intrusion into an occupied structure, a
typical waste characteristic value and indoor air samples below health-
based benchmarks would require a minimum of 223 receptors to receive a
score of 28.50. This illustrates that this final rule will not result
in a large number of sites qualifying for the NPL as it is unlikely
this number of receptors in an area of subsurface contamination will
commonly occur. This is the similar number of receptors needed for a
site to qualify for the NPL in other pathways.
2. Test Sites (Tier 1)
    To support the final rulemaking, EPA conducted a screening-level
assessment of sites with identified subsurface intrusion threats. As a
first step in collecting the list of sites potentially affected by the
final rule, EPA consulted with site assessment experts that work in
Superfund to identify potential site candidates. EPA also reached out
to state counterparts, in particular to state programs that were known
to have taken a more thorough investigation of the subsurface intrusion
pathway at sites. Through this process, EPA identified approximately
1,073 sites. These sites are not currently on the NPL, and all have a
potential or identified SsI threat. Within the group of sites
potentially affected by the HRS SsI Addition, EPA defined four
categories:
    1. Tier 4: Sites identified as having a suspected SsI threat based
on EPA's Superfund database and Agency for Toxic Substances and Disease
Registry keyword searches, as well as EPA or state self-identification,
but for which no sampling data were obtained;
    2. Tier 3: Sites identified as having characteristics or evidence
that indicate SsI may have occurred or will occur;
    3. Tier 2: Sites identified as having an SsI threat documented by
subslab, crawl space, or indoor air samples, but insufficient HRS-
required evaluation factors to qualify for the NPL; and
    4. Tier 1: Sites identified as having an SsI threat with documented
actual exposure of a sufficient number of targets with enough other
HRS-required evaluation factors to suggest the site may qualify for the
NPL.
    EPA selected the Tier 1 sites for use in testing the SsI component
evaluation process. The 11 Test Sites had documentation of indoor
contamination due to subsurface intrusion based on actual sampling data
and other typically HRS-required data. Of the 11 sites scored, 9 were
projected to score 28.50 or higher using only the SsI component. 1 site
was projected to score 28.50 or higher only by including both the
scores from the SsI component evaluation and the ground water migration
pathway evaluation in the site score. It was unknown whether these
sites would qualify for the NPL when they were chosen as Test Sites, as
the SsI scoring process had not been developed. The Test Site with a
projected score below 28.50 did not qualify for the NPL even though the
site was located in a mixed-used residential and industrial area,
illustrating that not all sites in an urban area will qualify for the
NPL.
    That 10 of the 11 Test Sites have a projected HRS site score of
28.50 or greater using the SsI component is not an indication that the
addition of the SsI component will result in a large number of SsI
sites qualifying for the NPL; this would be a possible projection if
the Test Sites were chosen randomly so as to represent a typical SsI
site. The Test Sites were not randomly chosen, but instead were
specifically chosen because they have a documented subsurface intrusion
threats at the sites and sufficient available data to test all parts of
the SsI component. The Test Sites all had areas of observed exposure;
most had more than 38 structures at the site (some with hundreds of
structures), and all but two Test Sites had at least 50 targets (more
than half had over 100 targets). Each site was also associated with
volatile hazardous substances that are considered hazardous to human
health at low concentrations. Appendix B of the Technical Support
Document (TSD) for this final rulemaking provides a summary of these
scoring evaluations.
3. Pilot Study
    The main purpose of the Pilot Study was to identify sites currently
being evaluated for SsI by the EPA regions with a suspected subsurface
intrusion threat and determine whether an SI would provide enough
information to score a site under the new component. Additional goals
of the Pilot Study were to gather data and determine if design of the
SsI model is practical and gives expected results; identify a range for
the cost of a projected SsI site assessment; and assist in developing
future guidelines for SsI assessments. A total of 10 sites were
identified across 5 of the 10 EPA Regions. The pilot studies were not
intended to identify sites for placement on the NPL, and not all sites
considered for the pilot studies achieved an HRS score greater than (or
equal to) 28.50. However, collecting actual data for the purposes of
generating an SsI component score, ensured the HRS was considering
subsurface intrusion threats appropriately. Ultimately, the pilot
studies were used to proof the concept and validate the SsI component
in terms of the application of selected weighting factor values and the
efficacy for accurately identifying sites with significant relative
risk.

IV. Summary of Changes to the HRS

    Comments on the Proposed Rule were received from 15 organizations/
individuals. The commenters included state and federal agencies,
industry associations, community groups, consultants, and private
citizens. No major conceptual or structural changes were necessary
based on comments received during the public comment period. While many
of the comments focused on the structure of the SsI component, there
was not sufficient rationale for making major changes to the basic
structure of the SsI component. There were minor revisions made based
on comments, which help refine the mechanics of assigning an HRS site
score. As a result, the SsI component better reflects current science
and better aligns with underlying concepts in the OSWER Technical Guide
for Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Sources to Indoor Air (VI Guide). These changes had no
impact on the overall structure of the SsI component and do not impact
the relative weighting among the HRS

pathways or the level of risk required to qualify for the NPL.

A. Changes Since Proposal

1. Consideration of Contaminated Ground Water Intrusion
    Section 5.2 was revised to clarify that areas of subsurface
contamination are only delineated based on the presence of hazardous
substances meeting the criteria for observed exposure or observed
release and have a vapor pressure greater than or equal to one torr or
a Henry's constant greater than or equal to 10-5 atm-m\3\/
mol. However, if samples indicate intrusion of liquids containing
hazardous substances has occurred into regularly occupied structures,
the samples of that liquid are still used in delineating an Area of
Observed Exposure to reflect the threat to targets. These revisions
were made to correct a seeming inconsistency in wording between the
discussion in the preamble to the proposed rule and the proposed
regulatory language.
2. Consideration of Non-Aqueous Phase Liquids (NAPLs) in Weighting of
Targets in an ASC
    Table 5-21, Weighting Factor Values for Populations within an Area
of Subsurface Contamination, of the HRS was revised to include
consideration of the presence of NAPLs identified in an area of
subsurface contamination. These additions increase the weighting of the
population in an area of subsurface contamination to the SsI component
score. These revisions were in response to comments that the proposed
addition did not reflect the magnitude of contaminant concentrations in
the evaluation of targets in the area of subsurface contamination.
While EPA considers it unlikely that the actual aerial distribution and
magnitude of contaminant concentrations can be determined in an area of
observed contamination during a site inspection, if NAPLs are
identified as present, EPA agrees that there is a greater risk to
receptors than if no NAPL is present.
3. Modifications to the Determination of Degradation Factor Values
    Section <IP_ADDRESS>.1.2 of the HRS was revised to make it easier for
the reader to determine degradation factor values and to add
consideration of the presence of NAPLs. Commenters asserted that the
text was difficult to follow and that the presence of NAPLs was a major
factor in the impact of degradation. A new table, Table 5-18 of the
HRS, simplifying the assignment of degradation factor values based on
the depth to contamination and a substance's half-life was inserted to
replace proposed text. Additionally, if no half-life information is
available for a hazardous substance and the substance is not already
assigned a degradation factor value of 1, a value of 1 will be
assigned. This modification further simplifies the degradation
evaluation and is also protective of human health, for if no half-life
information is available for a hazardous substance, EPA cannot assume
that degradation will occur. In addition, parent-daughter relationships
between substances are no longer considered in the assignment of the
degradation factor value, in part to simplify the assignment and in
part to reflect the variation in rates of degradation due to site-
specific subsurface conditions. Even if degradation occurs, if a
contaminant is at high enough concentration to exist as a NAPL at
depths less than or equal to 30 feet, it is more likely to pose a
threat to populations in overlying structure.
4. Modifications Made to Section <IP_ADDRESS>.2.1, Structure Containment and
Table 5-12
    Section <IP_ADDRESS>.2.1 and Table 5-12 of the HRS were revised in
response to comments on the rationale for assigning containment values
to individual structures. The assignment of a structure containment
factor value assigned to structures in Table 5-12 with vapor mitigation
systems or other response actions was revised. These revisions were
made in response to a comment questioning why response actions taken by
federal, state, and tribal authorities are treated differently than
those taken by private entities in determining containment for a
structure. The language regarding treatment of removals by federal,
state, and tribal authorities has been removed from Table 5-12 and the
corresponding containment value was assigned a 1. This change allows a
consideration of public and private removal actions to be evaluated in
a consistent manner.
    Section <IP_ADDRESS>.2.1 and Table 5-12 of the HRS was also revised to
remove from the table the direction of the assignment of a structure
containment value for a regularly occupied structure with unknown
containment features. This direction, which assigns a value of
``greater than zero'' to this situation, was moved to the text in
section <IP_ADDRESS>.2.1 of the HRS. This revision was made in response to a
comment questioning the rationale for the various containment values
and was made to improve the continuity of the table, which directs the
assignment of values when containment features of the structure are
known. A structure with a containment factor value of greater than zero
cannot be used in assigning a potential for exposure factor value. EPA
considers it appropriate that the potential for exposure factor value
should be based on actual field observations. However a structure with
a structure containment value of greater than zero allows the structure
to be evaluated for assigning waste characteristics values (e.g., a
hazardous waste quantity factor value) and for assigning target factor
values. EPA considers the inclusion of structures with unknown
containment features in the calculation of waste characteristics and
targets values appropriate as it reflects that very few structures are
built to be sufficiently air tight to prevent subsurface intrusion.
5. Consideration of Hydraulic Conductivity in Vertical Migration
    Table 5-14 of the HRS was revised to allow assignment of an
effective porosity/permeability factor value based on site-specific
measurements of hydraulic conductivity, if known. This addition was
made in response to a comment suggesting the rule be modified to allow
use of site-specific information for this purpose when available.
6. Changes to Definitions
    The term surficial ground water was re-named shallow ground water
and was changed to be consistent with current EPA usage.
    EPA has added the term non-aqueous phase liquid (NAPL) to the
definition section. EPA added consideration of the identification of
concentrations of hazardous substances high enough to indicate the
presence of NAPLs in the subsurface during a site inspection to the
assignment of degradation factor values and the weighting of targets in
the ASC. The presence of NAPLs in the subsurface demonstrates the
hazardous substances will be present at high concentrations for a
significant time period at that location and the high concentration is
not a transient situation.

B. Summary of Updates to the HRS (Sections 2, 5, 6, and 7)

1. Addition of an SsI Component to the HRS (Sections 2, 5, and 7)
    a. The addition of a subsurface intrusion component is added to the
1990 Soil Exposure pathway as section 5.2 in Chapter 5 of the 2016
Revised HRS. The new pathway name is the soil exposure and subsurface
intrusion pathway. The existing method for evaluating the soil exposure
threat will remain unchanged.
    b. Chapter 2: Evaluations Common to All Pathways is updated to
reflect the

addition of the subsurface intrusion component to the renamed the soil
exposure and subsurface intrusion pathway. The evaluations for the
migration pathways and the soil exposure component remain unchanged. A
parallel structure was added for the subsurface intrusion component.
    c. Chapter 7: Sites Containing Radioactive Substances is updated to
reflect how radioactive substances are evaluated using the added
subsurface intrusion component.
2. Terminology Updates Affecting Specific Sections of the HRS (Sections
2, 5 & 6)
    The following terms are updated to reflect current terminology and
procedures used by EPA in performing risk assessments.
    a. Ambient Water Quality Criteria: Ambient Water Quality Criteria
(AWQC) are now identified also as National Recommended Water Quality
Criteria (NRWQC). In addition, the acute AWQC are now identified as the
Criterion Maximum Concentration (CMC) and the chronic criteria are
referred to as the Criterion Continuous Concentration (CCC). (See
section 1.1 of the HRS.) These criteria are used to determine the level
of threat to environmental targets.
    b. Reference Concentrations: For inhalation exposures, EPA is
adopting the use of Reference Concentrations (RfCs) instead of
Reference Doses (RfDs) when determining non-cancer-related risk levels.
RfCs are used in determining the level of threat to human targets due
to possible inhalation and when determining the toxicity of the
substances.
    c. Cancer Unit Risk: For inhalation exposures, EPA is adopting the
use of Inhalation Unit Risk (IUR) instead of cancer slope factors in
determining cancer-related risk levels. IURs are used in determining
the level of threat to human targets due to possible inhalation and
when determining the toxicity of the substances.
    d. Weight-of-Evidence Groupings: The 2005 EPA weight-of-evidence
groupings supporting the designation of a substance as a human
carcinogen have been incorporated into the HRS algorithm for assigning
the toxicity factor value. (The former EPA weight-of-evidence
categories included as part of the 1990 HRS have been retained as EPA
has not yet completed assigning all substances to the revised
categories and are doing so at the time the EPA substance literature
reviews are updated.)

V. Discussion of Major Comments

    Comments on the Proposed Rule were received from 15 organizations/
individuals. The commenters included state and federal agencies,
industry associations, community groups, consultants, and private
citizens. This section discusses the major issues raised by commenters,
which are summarized, and EPA's summary of responses. In addition, EPA
solicited and received input from commenters on three technical
questions posed in the Preamble to the Proposed Rule.
    A support document, Response to Comments on the 2016 Revisions to
the Hazard Ranking System (HRS), that includes all issues raised during
the public comment period, comments received on the questions posed in
the preamble to the proposed rule and EPA's more comprehensive response
to each issue, is available in the docket for this rulemaking.

A. Responses to Comments on EPA Questions Posed in the Proposed Rule

    Question 1: 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 received multiple comments in response to this question. One
commenter suggested that this activity is beyond the scope of the site
assessment process, while another commenter suggested that EPA consider
measuring specific compounds or other factors reflecting biological
activity when conducting soil vapor analysis. A third commenter
remarked that half-lives faster than 100 days are presumably due to
aerobic biodegradation and that most vadose zone soils that are not
grossly impacted are considered biologically active. A commenter also
suggested using soil characteristics reflected in soil surveys to
reflect the possibility that biologically active soil could be present.
No commenter suggested practical methods to determine site-specific
biological activity throughout a site or over time.
    The HRS SsI addition was revised to clarify the assumption of the
presence of biologically soil in evaluating the degradation factor
unless evidence indicates otherwise (see section <IP_ADDRESS>.1.2 of the
HRS).
    Question 2: How could EPA further take into account the difference
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?
    EPA received multiple comments in response to this question. One
commenter suggested developing intrusion screening values based on
exposure scenarios for ``most sensitive individual'' and ``industrial''
models. One commenter indicated that there is not a dependable way to
account for the differences between large commercial/industrial
structures and smaller residential/commercial structures. Another
commenter noted that there are several parameters (e.g., building
energy efficiency) that would impact the differences in dilution and
air exchange rates and which are generally unavailable during an
initial assessment. A commenter discussed developing a sliding scale
based on the size of the building and the building's general use to
account for the differences in contaminant clearance rates.
    EPA did not make any changes to the final rule based on the
comments received as the type of information requested in these
responses is generally not available during a typical site inspection.
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 site-specific risk.
    Question 3: The HRS SsI addition considers source strength in
delineating ASCs and AOEs, in scoring in 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?
    EPA received multiple comments in response to this question. One
commenter suggested that EPA assign a higher score when the contaminant
concentration is high (e.g., when a non-aqueous phase liquid is
present) to account for source strength. Comments were also received
that reflected the difficulty of accessing large low concentration
sources and how to account for that in considering source strength.
Another commenter remarked that there may be a large ground water plume
without a discrete source that would cause an increased risk of vapor
intrusion; and that a large diffuse source is different from having a
concentrated discrete source. One commenter

provided a copy of the proposed rule with their suggested edits
reflecting the evaluation of source strength in assigning HRS specific
factors.
    The assignment of a degradation factor value (see section
<IP_ADDRESS>.1.2 of the HRS) and the weighting factors for targets in an
area of subsurface contamination (see Table 5-21 of the HRS) were
revised to include consideration of source strength; specifically in
the situation where NAPLs are present.

B. Major Comment Theme Summaries and Responses

Statutory Authority and Rationale for the Proposed HRS Addition
Justification for Revising the HRS
    EPA received comments suggesting that sufficient justification or
rationale for the need to revise the HRS has not been provided and that
a revision to the HRS is unnecessary because the 1990 HRS adequately
evaluates the relative risk posed by a site and identifies those
priority sites for further investigation.
    The rationale for revising the HRS to add a subsurface intrusion
component is EPA's statutory authority. Specifically, CERCLA
105(a)(8)(A), requires EPA to amend the HRS ``to assure to the maximum
extent feasible, that the HRS accurately assess the relative degree of
risk to human health and the environment posed by sites and facilities
subject to review.'' Contamination due to subsurface intrusion is a
known risk to human health and the ability to evaluate those risks is
consistent with the CERCLA 105 mandate. The 1990 HRS did not evaluate
the risk posed by subsurface intrusion when evaluating sites for the
NPL. As part of the development of this rule, EPA identified high
priority sites with significant contamination due to SsI that could not
be evaluated using the 1990 HRS for possible placement on the NPL. With
the addition of the SsI component to the HRS, sites can now be
evaluated more comprehensively to consider the relative risk posed by a
site.
Priority for Drinking Water Sites
    EPA received comments suggesting that the proposed HRS SsI addition
conflicts with CERCLA's statutory mandate regarding prioritizing
drinking water sites.
    The revision to the HRS to add a subsurface intrusion component is
not in conflict with the CERCLA 105 mandate to prioritize drinking
water sites. The priority given by EPA under CERCLA to sites with a
high risk of populations exposed to hazardous substances in drinking
water has not decreased with the addition of a subsurface intrusion
component to the HRS. In fact, the score for some sites with
contaminated drinking water supplies may increase because sites with
contaminated drinking water may also be associated with subsurface
intrusion contamination and the combination of the ground water
migration pathway score and the SsI component score may increase the
overall site score. Furthermore, EPA notes that drinking water is a
priority identified by CERCLA, but it is not the only priority
identified in CERCLA 105, which also mandates the prioritization of
dangers of direct human contact, for which SsI is one example.
    The addition of the SsI component does not change the priority
given to drinking water sites. It does not change the scoring of
contaminated drinking water supplies under the HRS, reduce in anyway
the overall HRS score for any site based on drinking water
contamination (or any other threat due to exposure to released
hazardous substances in the HRS), or change the site score of 28.50
being the HRS score that qualifies sites for placement on the NPL. If a
site qualifies for placement on the NPL based on its HRS score
reflecting drinking water contamination prior to the addition of the
SsI component, it will continue to do so. Adding an evaluation of the
SsI component can only increase an overall site score. The algorithm
used to combine pathways scores to obtain an overall site score results
in an increase in the overall site score with the evaluation of
additional pathways, components and threats scored. In fact, the SsI
addition may raise the overall site score at some sites with ground
water drinking water contamination from below the 28.50 cut-off score
to above it. This may occur because, as stated above, a site's HRS
score can increase with the scoring of additional threats. Sites with
ground water contaminated by volatile substances and used for drinking
water are also sites at which the ground water contamination may
volatilize and intrude into overlying regularly occupied structures.
Thus, a site at which ground water contamination has occurred but does
not have an HRS score above 28.50 based only on the ground water
threat, may have an overall HRS site score above 28.50 based on the
combination of the scores for the contaminated drinking water and SsI
threats.
    Furthermore, EPA notes that CERCLA 118 refers to CERCLA sections104
and 108, which address activities that occur pre- or post-NPL-listing,
and not to the section of CERCLA that addresses site ranking using the
HRS, which is addressed in CERCLA section 105. CERCLA Section 105 and
specifically 105(a)(8)(A) requires EPA to prioritize sites based on
``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.'' Since subsurface intrusion
contamination is a direct human contact threat, the addition of a
subsurface intrusion component, which addresses this threat, is
mandated by CERCLA.
Resource Impacts of the Proposed HRS Addition
Increased Cost and Level of Effort
    EPA received comments suggesting that contrary to EPA's suggestion
that the HRS SsI addition may not result in more site assessments per
year and only minimal cost increases, commenters claimed that there
will be substantial increases in cost and level of effort for states
and federal agencies, due to the complexity in assessing subsurface
intrusion sites.
    EPA acknowledges that in some cases the scope of a typical site
inspection (SI) may need to be expanded to collect the information
necessary to evaluate the SsI threat present at a site. EPA also
acknowledges that sites that did not qualify previously for the NPL,
may now do so. The number of samples and level of effort required to
evaluate a site using the 1990 HRS pathways or components already
varies on a site-by-site basis depending on the size and extent of
contamination at the site. Therefore, it cannot be predicted with
certainty that there will be an overall increase in cost or level of
effort for any particular site due to the HRS SsI addition. However,
the overall budget for performing site assessments per year is not
expected to change significantly. EPA's budget for site assessment is
dependent on Congressional appropriation and EPA does not expect the
rulemaking to impact the appropriation. EPA's budget for site
assessment has remained relatively constant for the last several years.
Hence, EPA expects that the allocation of available resources may be
changed to reflect this rulemaking but will continue to be optimized by
EPA, its state and tribal partners, and with other federal agencies to
evaluate priority sites. However, the number of site assessments or NPL
proposals conducted each year will not significantly increase.

Potential Limitations With Implementing the HRS SsI Addition
Scope of Site Inspection
    EPA received comments stating that the type and amount of
information available for collection during a time-limited site
inspection would be insufficient to properly evaluate a site using the
HRS SsI addition and would be beyond the scope of site evaluations
typically conducted at the preliminary assessment or site inspection
stage.
    During development of the HRS SsI addition EPA considered the type
of information that could be collected during a time-limited site
inspection when selecting the factors to include in an evaluation of
the subsurface intrusion component. The purpose of the site inspection
(NCP 300.420(c)) is to determine if a release of a hazardous substance
poses an actual or potential threat to human health or the environment,
to determine if there is an immediate threat to people or the
environment, and to collect sufficient data to enable the site to be
scored using the HRS. EPA also notes that neither the NCP nor the HRS
requires a certain number of samples be collected during an SI, because
the number of samples required to evaluate a site varies on a site-by-
site basis and the possible risk pathways being evaluated. However, to
properly evaluate the subsurface intrusion component, additional
information may be required beyond that collected during a typical
current site inspection may be required; this is consistent with the
need to collect data on the threat posed by a different pathway. In
these instances, as stated in EPA's Guidance for Performing Site
Inspections under CERCLA (September 1992), an expanded site inspection
(ESI) may be required. The objective of the ESI is to collect data that
was not collected during an initial site inspection. Furthermore, EPA
found that information required for an SsI evaluation was available
based on a pilot study which included several candidate NPL sites. The
pilot study was performed in part to demonstrate the availability of
the necessary data from screening level investigations. Therefore, EPA
considers that the information required to properly evaluate the
subsurface intrusion component can be obtained during the site
assessment process.
Need for Guidance
    EPA received comments questioning or requesting additional
information or guidance regarding the type and amount of data to
collect, data collection methods, and how to apply the subsurface
intrusion component to a site. Commenters also suggested it was
difficult to properly evaluate and comment on the proposed HRS SsI
addition without a thorough understanding of how the SsI component
would be implemented and that promulgation should be delayed until
guidance is developed.
    The HRS does not provide prescriptive methods for performing site
investigations for any HRS pathway evaluation because the methods used
during the collection and analysis of environmental samples depend on
site conditions and could not be written to cover all possible
situations and could also become outdated in the future. Additionally,
it is outside the scope of the HRS to identify and describe methods for
conducting a subsurface intrusion screening for HRS purposes. The
sampling and data collection information in the EPA OSWER VI Guide,
(particularly in section 6 of the guide) are an appropriate resource
for gathering data for HRS purposes. For example, Section 6.4 of the
guide identifies basic principles, methods and procedures for indoor
air sampling. In addition, states, federal agencies, and private
contractors have considerable experience in VI investigations and
collecting VI-related data. Guidance on implementation of the proposed
SsI addition is not necessary for evaluating the SsI component, which
is a scoring mechanism not procedures for data collection. Any guidance
developed will provide details on collecting data to support an HRS SsI
evaluation. EPA also notes that to delay addressing sites that may pose
a significant human health risk until all necessary guidance documents
have been developed would not be consistent with EPA's mandate to
protect human health. Therefore, EPA does not agree that promulgation
of the HRS SsI addition needs to be delayed until guidance documents
related to its implementation have been developed.
Roles of the HRS SsI Addition and the 2015 OSWER VI Guide
    EPA received comments suggesting that the HRS SsI addition is not
consistent with the VI Guide, published in June 2015 and will create
confusion when evaluating sites for SsI.
    The VI Guide and HRS SsI rule work in concert to establish national
consistency in the evaluation of SsI threats. The HRS SsI addition and
the OSWER VI Guide both address the threat posed by vapor intrusion and
use the same principles, sampling procedures and concepts to
characterize the threat posed by vapor intrusion as the sites. However,
the HRS SsI addition and the OSWER VI Guide serve different purposes
and support different phases of EPA's site remediation process with
different data quality requirements and different enabling
legislations.
    The purpose of the OSWER VI Guide is to guide the investigation and
assessment of the threat posed by vapor intrusion into structures from
all sources under all Office of Land and Emergency Management (OLEM,
formerly OSWER) programs, particularly actions taken under CERCLA and
RCRA. This guidance is used to support decisions by EPA on whether
vapor intrusion is posing an unacceptable risk to human health based on
sufficient site specific data. It contains principles for making such a
decision, as well as procedures and guidance for collecting the
information necessary to make these decisions.
    The HRS and the SsI addition is part of the NCP, (the regulations
implementing CERCLA) required by CERCLA to identify priority sites for
further investigation based on screening level information (Such sites
are identified for the public by placing the sites on the NPL, a
separate rulemaking process). This prioritization is based on the
possible cumulative relative risk amongst all candidate sites posed by
releases of hazardous substances to human health and the environment by
either migration to receptors or by direct contact with the
contamination, such as by subsurface intrusion. The HRS is only a
method for assigning a relative score to candidate sites. It is not a
method for determining site specific risk. The HRS SsI addition is not
guidance. The HRS SsI addition does not address such subjects as data
collection and sampling procedures: Many of the procedures and many of
the guidelines in the OSWER VI Guide are also applicable for HRS
purposes if they can be implemented as part of a screening level
assessment.
    Given that the purposes for the two documents are considerably
different and based on different levels of information, it is not an
issue that decision criteria are different in the two documents. It is
certainly possible that, based on an HRS evaluation, EPA may determine
a site warrants further investigation, and that after further
investigation is performed EPA may decide no remediation is necessary.
However until further information is collected during a remedial
investigation, such an outcome cannot be predicted. Furthermore, such a
situation is not an indication the results of the HRS evaluation was
incorrect.

Application of HRS SsI Component
Inferring Contamination
    EPA received comments suggesting that by inferring contamination
between sampling locations, the extent of the risk is overstated. The
commenters considered identifying targets as actually or potentially
exposed based on inference to inflate the HRS site score. It was also
suggested that this method conflicts with the other HRS pathways.
    The HRS is not a quantitative risk assessment. Instead, the HRS SsI
addition score reflects the possible threat posed by subsurface
intrusion at one site relative to other sites. By inferring
contamination in an AOE or an ASC between sampling locations, it is not
assumed that all populations within the two areas are exposed to
contamination from the subsurface. Inferring contamination also allows
sites with large populations within the two areas to be ranked higher
than sites with smaller populations. If the HRS scoring required
sampling every structure a sufficient number of times to assure that
all exposed targets were accounted for, the scope of the sampling
effort would be beyond that of a screening tool and more consistent
with the scope of a remedial investigation.
    Inference of contamination between sampling locations is also
assumed in other HRS pathways. The other pathways allow the inference
of contamination based on the location of samples documenting the
presence of contamination attributable to the site being investigated.
For example, in the soil exposure component, inference of contamination
is done by drawing AOC boundaries based on sample locations and
inferring that those targets associated with the properties within the
boundaries are actually exposed.
    In the SsI component, unless site-specific information indicates
otherwise, when delineating an AOE or an ASC, populations in occupied
structures within an AOE are inferred to be actually exposed, and,
populations in occupied structures within an ASC are inferred to likely
be exposed to contamination.
Purpose of Hazardous Waste Quantity
    Commenters noted that as explained in the TSD for the proposed HRS
SsI Addition, the hazardous waste quantity factor serves as a surrogate
for the contaminant dose that populations may be exposed to. Commenters
asserted that the hazardous waste quantity factor is not adequately
reflective of this dose to be used as a surrogate.
    The commenters appear to be confusing consideration of waste
quantity as a surrogate for dose in an HRS evaluation with the
calculation of a site-specific risk level based on the ratio of waste
quantity to receptors. EPA is not projecting a specific risk level
based on the waste quantity alone when it performs an HRS evaluation.
Other HRS factors such as the population associated with the
structures, the probability of a release into the occupied structures,
the possibility of degradation, and the toxicity of the substances are
also considered,
    The decision to include waste quantity as a surrogate for dose in
all pathways and components in the HRS algorithm was made when the HRS
was last revised in 1990 (see Section V.3 of the proposed 1988 HRS, 53
FR 51692, December 23, 1988; Section III.C of the 1990 HRS, 55 FR
51542, December 14, 1990). The decision was based on the concept that
determining an accurate dose that receptors would be exposed to was
beyond the scope of information available after a site inspection. It
is not possible to accurately predict the hazardous substance
concentration that receptors would be exposed to over a representative
exposure period based on information collected during a site inspection
due to the variability in exposure levels over time and space. Instead,
hazardous waste quantity is used as a surrogate for dose in the sense
that the quantity of the hazardous substances is at least qualitatively
correlated to the magnitude of the exposure. If there is no waste
quantity, there will be no exposure; as the waste quantity increases,
the greater the possibility of exposure to hazardous substances that a
receptor may come in contact with. EPA agrees this is not a perfect
correlation, and has built into the HRS four order of magnitude ranges
for assigning factor values that reflect the imperfection of this
correlation.
    In addition, the inclusion of hazardous waste quantity in the
subsurface intrusion component is consistent with its inclusion in all
the other existing HRS pathway evaluations and is consistent with the
goal that the scoring of the new component not impact the balance built
into overall HRS site scoring algorithm among the HRS pathways.
    Furthermore, for determining waste quantity for the SsI component,
EPA made a specific alteration to how waste quantity is calculated as
compared to other HRS pathway. EPA decided to only include the amount
of hazardous substance that actually enters into or that could enter
into occupied structures, not the total amount in the release to the
environment, based on the rationale that at least some of the original
release in the subsurface would vent directly to the atmosphere.
Therefore only the amount of hazardous substances that has entered into
occupied structures or the amount located under structures is reflected
in the estimate. This was achieved by not estimating the waste quantity
based on the area or the volume of the contaminated media in the
subsurface, but instead on the volume of the structures, or the basal
area if the volume cannot be determined.
    Finally, no comments were received that provided a viable
alternative to the proposed method of estimating hazardous waste
quantity. Commenters stated the amount of exposure was overestimated
for large buildings because in general larger buildings have lower air
exchange rates and suggested that this consideration be built into the
estimation methods for all structures. However, the commenters did not
present data to document this generality nor suggest how to determine
the air exchange rate for all structures if it is not provided by the
building owner. EPA notes that if air exchange rates are available, the
present estimation method (which has not changed since proposal) allows
for a hazardous waste quantity estimate using that information (see,
HRS section <IP_ADDRESS>.2 Tier B, hazardous wastestream quantity).
    While some commenters suggested procedures for determining a more
accurate hazardous waste quantity for specific situations they did not
suggest how the hazardous waste quantity calculated for these
situations could be relatively ranked against sites where equivalent
information was not available. When developing a hazardous waste
quantity factor in 1988, EPA performed studies that showed this level
of information was not available at all sites, and was not likely to be
collectible during a limited screening assessment. Therefore, EPA
considers it inappropriate to incorporate the suggested procedures into
the HRS.
    In addition, EPA proposed the present hazardous waste quantity
estimation process as part of the revision of the HRS in 1988. At that
time EPA requested the Science Advisory Board's (SAB's) assistance on
the use of concentration data in determining the hazardous waste
quantity factor as part of the overall SAB peer review of the HRS
changes. The current method for use of concentration data in
determining the hazardous waste quantity factor is based on the SAB's
recommendation.
Establishment of Attribution
    Commenters noted that establishing that indoor air contamination is

attributable to subsurface intrusion will be very complex to
demonstrate given all other possible origins of the indoor
contamination (e.g., outdoor air, consumer products).
    The HRS SsI addition, just as in other HRS pathways and components,
does not require absolute proof that the significant increase in indoor
contaminant concentrations is due to subsurface intrusion. It only
requires at least part of the significant increase be attributable to
subsurface intrusion. EPA expects to use multiple lines-of-evidence in
meeting the attribution requirement as discussed in various comments.
The VI Guide outlines use of multiple lines-of-evidence and provides
guidance on how to distinguish subsurface intrusion from other sources
of vapor intrusion. As is done for other HRS pathways and components,
the HRS standard for establishing attribution is to establish a
reasoned explanation that is not shown to be incorrect during public
review of placement of a proposed site on the NPL.
Establishing Observed Exposure
    EPA received comments suggesting that the criteria for establishing
background for the SsI component is too complex given the variability
in sampling for SsI and that a significant difference between the
background level and release concentration is not an adequate measure
for establishing an observed exposure in a regularly occupied
structure.
    EPA agrees that establishing a background level for indoor air can
be difficult. However, it does not mean that the HRS criteria for
establishing actual exposure should not be used. Methods for
establishing background levels are too site-specific to be discussed in
the HRS regulation, which is a scoring methodology. Instead, as
occurred after the 1990 HRS was promulgated, criteria for establishing
background was refined based on actual experience gained as sites were
being scored. EPA expects the same to occur for the HRS SsI component.
    Comparison of background levels and indoor air concentrations are
used only to establish that the contaminant level in a structure is
elevated (i.e., significantly different). This is only the first step
in establishing observed exposure. The second step is to attribute at
least a part of the significant increase to subsurface intrusion.
    The argument that vapor intrusion rates are too variable to justify
the use of the same procedure for establishing observed releases or
exposures as in other parts of the HRS is invalid. Hazardous substance
concentrations are unpredictably variable temporally and spatially for
all HRS pathways and SsI variability is no different in that regard.
For example, in the surface water migration pathway overland flow
threat, the hazardous substance may only be entering surface water via
runoff due to rain events. No runoff occurs if it is not raining. The
amount entering surface water in this situation has been shown to vary
with the length of time between rains, which impacts the amount of
material deposited and available for entrainment into the runoff.
Runoff also varies with the portion of each rain cycle whether the
sample is collected at the beginning, middle or end of a rain event. At
the beginning of a rain event all erodible materials are present and
available. During the middle or during a high intensity period of rain,
the force of the rain drops can dislodge and entrain hazardous
substances at greater rates that during low intensity periods. At the
end of a rain event, it may be that much of the hazardous substances
have already been washed away. In continuous air releases, the
contaminant concentration can vary by order of magnitudes with distance
from the source, with wind direction and wind speed all of which can
cause differences in concentrations spatially due to the three
dimensionality of the atmosphere, and cannot be predicted or accounted
for based on a screening assessment. Even in ground water
contamination, the contaminant plume's concentration can vary spatially
depending on the rate of ground water movement from the original spill
concentrations. It is not possible to account for these factors that
can drastically impact the contaminant concentration at a sampling
location, based on screening level information.
    For example, variation in the occurrence of releases is no greater
in the SsI component than would be expected in point-source air
releases or spills to surface water.
Degradation
    Commenters suggested changes in how the degradation factor value
for the subsurface intrusion component is assigned. Other comments
dealt with conditions associated with assigning different degradation
factor values based on the depth of biologically active soil and the
half-lives of individual hazardous substances. In addition, commenters
suggested moving the consideration of degradation from the waste
characteristics factor category value calculations to the likelihood of
exposure factor category value calculations.
    After evaluation of the comments, EPA modified the assignment of
the degradation factor to simplify the evaluation and to consider the
presence of non-aqueous phase liquids (NAPLs); other changes suggested
by commenters were not implemented. Some changes were not made because
a sufficient rationale was not provided to justify a change. Regarding
the placement of the degradation factor in the HRS equation, the
consideration of an individual substance's characteristics in the waste
characteristics factor category is consistent with other HRS pathways
and components. Furthermore, whether the degradation factor is put in
the likelihood of release or waste characteristic factor category, the
impact of the factor on the score would be similar.
Targets
    EPA received comments on the weightings assigned to targets in both
the AOE and ASC. Commenters suggested that the weightings reflect the
strength of the attribution argument that the significant increase in
indoor air concentrations is due to subsurface intrusion and also
reflect the concentration of the contaminants in the subsurface.
    After consideration of these comments, EPA has changed the
weightings of targets in the ASC to reflect the presence of NAPLs
(i.e., to reflect contaminant concentrations in the subsurface). EPA
did not incorporate any changes into the weightings of targets based on
the strength of attribution or concentration of contaminants in the
subsurface. Regarding the strength of an attribution argument, the HRS
does not recognize gradations of attribution in any other pathway or
component and therefore for consistency, will not in this component.
EPA notes that with the limited sampling that occurs during an SI, it
is not reasonable to project the concentration of contaminants in the
subsurface over time or distance.

VI. Statutory and Executive Order Reviews

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

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

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

legal or policy issues arising out of legal mandates, the President's
priorities, or the principles set forth in the EO. Any changes made in
response to OMB recommendations have been documented in the docket.
    EPA prepared an analysis of the potential costs and benefits
associated with this action. This analysis, Addition of a Subsurface
Intrusion (SsI) Component to the Hazard Ranking System (HRS):
Regulatory Impact Analysis is available in the docket for this action.

B. Paperwork Reduction Act (PRA)

    This action does not impose any new information collection burden
under the PRA. OMB has previously approved the information collection
activities contained in the existing regulations and has assigned OMB
control number 2050-0095.
    This regulatory change will only affect how EPA and organizations
performing work on behalf of EPA (state or tribal partners) conduct
site assessments and HRS scoring at sites where certain environmental
conditions exist. This regulatory change will result in data collection
at these types of sites to allow evaluation under the HRS. EPA expects
that the total number of site assessments performed and the number of
sites added to the NPL per year will not increase, but rather expects
that there will be a realignment and reprioritization of its internal
resources and state cooperative agreement funding.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. This
regulatory change enables the HRS evaluation to directly consider human
exposure to hazardous substances that enter building structures through
subsurface intrusion. This addition to the HRS would not impose direct
impacts on any other entities. For additional discussion on this
subject, see section 4.9 of the Regulatory Impact Analysis (see the
docket for this action).

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect
small governments. The action imposes no enforceable duty on any state,
local, or tribal governments or the private sector.

E. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.

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

    This action does not have tribal implications, as specified in
Executive Order 13175. EPA's evaluation of a site using the HRS does
not impose any costs on a tribe (except those already in a cooperative
agreement relationship with EPA). Thus, Executive Order 13175 does not
apply to this action.
    Although Executive Order 13175 does not apply to this action, EPA
consulted with tribal officials through meetings and correspondence,
including a letter sent to all federally recognized tribes asking for
comment on the ``Notice of Opportunity for Public Input'' that was
published in the Federal Register on January 31, 2011 (76 FR 5370), and
public listening sessions regarding the decision to proceed with the
development of this action. All tribal comments indicated support for
this action.

G. Executive Order 13045: Protection of Children From Environmental
Health Risks and Safety Risks

    EPA interprets Executive Order 13045 as applying only to those
regulatory actions that concern environmental health or safety risks
that EPA has reason to believe may disproportionately affect children,
per the definition of ``covered regulatory action'' in section 2-202 of
the Executive Order. This action is not subject to Executive Order
13045 because it does not concern an environmental health risk or
safety risk.

H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The site assessment activities affected
by this rule are limited in scope and number and rely on existing
energy distribution systems. Further, we have concluded that this rule
would not significantly expand the energy demand for site assessments,
and would not require an entity to conduct any action that would
require significant energy use, that would significantly affect energy
supply, distribution, or usage. Thus, Executive Order 13211 does not
apply to this action.

I. National Technology Transfer and Advancement Act

    This rulemaking does not involve technical standards.

J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations

    The EPA believes the human health or environmental or environmental
risk addressed by this action will not have potential
disproportionately high and adverse human health or environmental
effects on, low-income or indigenous populations. The results of this
evaluation are contained in section 4.3 (and all subsections) of the
Regulatory Impact Analysis for this rulemaking. A copy of the Addition
of a Subsurface Intrusion (SsI) Component to the Hazard Ranking System
(HRS): Regulatory Impact Analysis is available in the docket for this
action.

K. Executive Order 12580--Superfund Implementation

    Executive Order 12580, section 1(d), states that revisions to the
NCP shall be made in consultation with members of the National Response
Team (NRT) prior to publication for notice and comment. Revisions shall
also be made in consultation with the Director of the Federal Emergency
Management Agency (FEMA) and the Nuclear Regulatory Commission (NRC) to
avoid inconsistent or duplicative requirements in the emergency
planning responsibilities of those agencies. Executive Order 12580
delegates responsibility for revision of the NCP to EPA.
    The agency has complied with Executive Order 12580 to the extent
that it is related to the addition of a new component to the HRS,
through consultation with members of the NRT.

L. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule
report to each House of the Congress and to the Comptroller General of
the United States. This action is not a ``major rule'' as defined by 5
U.S.C. 804(2).

List of Subjects in 40 CFR Part 300

    Environmental protection, Air pollution control, Chemicals,
Hazardous substances, Hazardous waste, Intergovernmental relations,
Natural resources, Oil pollution, Penalties, Reporting and
recordkeeping

requirements, Superfund, Water pollution control, Water supply.

    Dated: December 7, 2016.
Gina McCarthy,
Administrator.

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

PART 300--NATIONAL OIL AND HAZARDOUS SUBSTANCES POLLUTION
CONTINGENCY PLAN

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

    Authority: 33 U.S.C. 1321(d); 42 U.S.C. 9601-9657; E.O. 13626,
77 FR 56749, 3 CFR, 2013 Comp., p. 306; E.O. 12777, 56 FR 54757, 3
CFR, 1991 Comp., p. 351; E.O. 12580, 52 FR 2923, 3 CFR, 1987 Comp.,
p. 193.

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2. Amend Appendix A to Part 300:
0
a. In section 1.1 by:
0
i. Removing the definition heading ``Ambient Water Quality Criteria
(AWQC) and adding ``Ambient Water Quality Criteria (AWQC)/National
Recommended Water Quality Criteria'', in its place; and removing the
text ``maximum acute or chronic toxicity'' and adding ``maximum acute
(Criteria Maximum Concentration or CMC) or chronic (Criterion
Continuous Concentration or CCC) toxicity.'' in its place;
0
ii. Adding in alphabetical order the definition ``Channelized flow'';
0
iii. Revising the definition ``Chronic toxicity'';
0
iv. Adding in alphabetical order the definition ``Crawl space'';
0
v. Revising the definitions ``Distance weight'' and ``Half-life'';
0
vi. Amending the definition ``HRS pathway'' by removing the word
``soil,'' and adding ``soil exposure and subsurface intrusion,'' in its
place;
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vii. Adding in alphabetical order the definitions ``Indoor air'',
``Inhalation Unit Risk (IUR)'', ``Non-Aqueous Phase Liquid (NAPL)'',
``Preferential subsurface intrusion pathways'', and ``Reference
concentration (RfC)'';
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viii. Revising the definition ``Reference dose (RfD)'';
0
ix. Adding in alphabetical order the definition ``Regularly occupied
structures'';
0
x. Revising the definition ``Screening concentration'';
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xi. Adding in alphabetical order the definition ``Shallow ground
water'';
0
xii. Revising the definition ``Slope factor (also referred to as cancer
potency factor)'';
0
xiii. Adding in alphabetical order the definitions ``Soil gas'', ``Soil
porosity''; ``Subslab'', ``Subsurface intrusion'', ``Unit risk'', and
``Unsaturated zone''; and
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xiv. Revising the definition ``Weight-of-evidence''.
0
b. Revising section 2.0;
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c. Revising section 5.0;
0
d. In section 6.0 by revising Table 6-14; and
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e. In section 7.0 by:
0
i. Revising Table 7-1;
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ii. Under Table 7-1, the second undesignated paragraph, revising the
third sentence;
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iii. Revising sections 7.1, 7.1.1, and 7.1.2; 7.2.1; 7.2.3; 7.2.4;
<IP_ADDRESS>, <IP_ADDRESS>.1 through <IP_ADDRESS>.3; <IP_ADDRESS>; <IP_ADDRESS>; 7.3, 7.3.1, and
7.3.2; and
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iv. Adding section 7.3.3.
    The revisions and additions read as follows:

Appendix A to Part 300--Hazard Ranking System

* * * * *

1.1 Definitions

* * * * *
    Channelized flow: Natural geological or manmade features such as
karst, fractures, lava tubes, and utility conduits (e.g., sewer
lines), which allow ground water and/or soil gas to move through the
subsurface environment more easily.
    Chronic toxicity: Measure of toxicological responses that result
from repeated exposure to a substance over an extended period of
time (typically 3 months or longer). Such responses may persist
beyond the exposure or may not appear until much later in time than
the exposure. HRS measures of chronic toxicity include Reference
Dose (RfD) and Reference Concentration (RfC) values.
* * * * *
    Crawl space: The enclosed or semi-enclosed area between a
regularly occupied structure's foundation (e.g., pier and beam
construction) and the ground surface. Crawl space samples are
collected to determine the concentration of hazardous substances in
the air beneath a regularly occupied structure.
* * * * *
    Distance weight: Parameter in the HRS air migration pathway,
ground water migration pathway, and the soil exposure component of
the soil exposure and subsurface intrusion pathway that reduces the
point value assigned to targets as their distance increases from the
site. [unitless].
* * * * *
    Half-life: Length of time required for an initial concentration
of a substance to be halved as a result of loss through decay. The
HRS considers five decay processes for assigning surface water
persistence: Biodegradation, hydrolysis, photolysis, radioactive
decay, and volatilization. The HRS considers two decay processes for
assigning subsurface intrusion degradation: Biodegradation and
hydrolysis.
* * * * *
    Indoor air: The air present within a structure.
    Inhalation Unit Risk (IUR): The upper-bound excess lifetime
cancer risk estimated to result from continuous exposure to an agent
(i.e., hazardous substance) at a concentration of 1[mu]g/m\3\ in
air.
* * * * *
    Non-Aqueous Phase Liquid (NAPL): Contaminants and substances
that are water-immiscible liquids composed of constituents with
varying degrees of water solubility.
* * * * *
    Preferential subsurface intrusion pathways: Subsurface features
such as animal burrows, cracks in walls, spaces around utility
lines, or drains through which a hazardous substance moves more
easily into a regularly occupied structure.
* * * * *
    Reference concentration (RfC): An estimate of a continuous
inhalation exposure to the human population that is likely to be
without an appreciable risk of deleterious effects during a
lifetime.
    Reference dose (RfD): An estimate of a daily oral exposure to
the human population that is likely to be without an appreciable
risk of deleterious effects during a lifetime.
    Regularly occupied structures: Structures with enclosed air