[Federal Register Volume 84, Number 170 (Tuesday, September 3, 2019)]
[Proposed Rules]
[Pages 46138-46182]
[FR Doc No: 2019-17223]

Vol. 84

Tuesday,

No. 170

September 3, 2019

Part III

 Environmental Protection Agency

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40 CFR Part 63

 National Emission Standards for Hazardous Air Pollutants: Site
Remediation Residual Risk and Technology Review; Proposed Rule

Federal Register / Vol. 84 , No. 170 / Tuesday, September 3, 2019 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2018-0833; FRL-9998-13-OAR]
RIN 2060-AU19

National Emission Standards for Hazardous Air Pollutants: Site
Remediation Residual Risk and Technology Review

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is proposing
amendments to the National Emission Standards for Hazardous Air
Pollutants (NESHAP) for the Site Remediation source category. This
proposal presents the results of the residual risk and technology
review (RTR) conducted as required under the Clean Air Act (CAA). Based
on the results of the residual risk review, the EPA is proposing that
risks due to emissions of air toxics are acceptable and that no
revision to the standards is required to provide an ample margin of
safety to protect public health. Based on the technology review, we are
proposing to amend the requirements for leak detection and repair
(LDAR). In addition, the EPA is proposing amendments to revise
regulatory provisions pertaining to emissions during periods of
startup, shutdown and malfunction (SSM), including adding requirements
for pressure relief devices; to add requirements for electronic
submittal of semiannual reports and performance test results; to
clarify provisions pertaining to open-ended valves and lines; and to
make minor clarifications and corrections. The proposed revisions to
the rule would increase the level of emissions control and
environmental protection provided by the Site Remediation NESHAP. We
are also requesting additional comment related to subcategorization of
sources relating to certain exemption provisions of the original rule
that were proposed for removal in 2016.

DATES:
    Comments. Comments must be received on or before October 18, 2019.
Under the Paperwork Reduction Act (PRA), comments on the information
collection provisions are best assured of consideration if the Office
of Management and Budget (OMB) receives a copy of your comments on or
before October 3, 2019.
    Public hearing. If anyone contacts us requesting a public hearing
on or before September 9, 2019, we will hold a hearing. Additional
information about the hearing, if requested, will be published in a
subsequent Federal Register document and posted at https://www.epa.gov/stationary-sources-air-pollution/site-remediation-national-emission-standards-hazardous-air. See SUPPLEMENTARY INFORMATION for information
on requesting and registering for a public hearing.

ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-
OAR-2018-0833, by any of the following methods:
     Federal eRulemaking Portal: https://www.regulations.gov/
(our preferred method). Follow the online instructions for submitting
comments.
     Email<EMAIL_ADDRESS>Include Docket ID No. EPA-
HQ-OAR-2018-0833 in the subject line of the message.
     Fax:<PHONE_NUMBER>. Attention Docket ID No. EPA-HQ-OAR-
2018-0833.
     Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Docket ID No. EPA-HQ-OAR-2018-0833, Mail Code 28221T, 1200
Pennsylvania Avenue NW, Washington, DC 20460.
     Hand/Courier Delivery: EPA Docket Center, WJC West
Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004.
The Docket Center's hours of operation are 8:30 a.m.-4:30 p.m., Monday-
Friday (except Federal holidays).
    Instructions: All submissions received must include the Docket ID
No. for this rulemaking. Comments received may be posted without change
to https://www.regulations.gov/, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the SUPPLEMENTARY
INFORMATION section of this document.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Matthew Witosky, Sector Policies and Programs Division
(E143-05), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711; telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>;
and email address<EMAIL_ADDRESS>For specific information
regarding the risk modeling methodology, contact Matthew Woody, Health
and Environmental Impacts Division (C539-02), Office of Air Quality
Planning and Standards, U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina 27711; telephone number:<PHONE_NUMBER>;
fax number:<PHONE_NUMBER>; and email address<EMAIL_ADDRESS>
For questions about monitoring and testing requirements, contact
Theresa Lowe, Sector Policies and Programs Division (D143-05), Office
of Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina 27711; telephone number
(919) 541-4786; fax number:<PHONE_NUMBER>; and email address:
<EMAIL_ADDRESS>For information about the applicability of the
NESHAP to a particular entity, contact Marcia Mia, Office of
Enforcement and Compliance Assurance, U.S. Environmental Protection
Agency, WJC South Building (Mail Code 2227A), 1200 Pennsylvania Avenue
NW, Washington, DC 20460; telephone number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    Public hearing. Please contact Virginia Hunt at<PHONE_NUMBER> or
by email at<EMAIL_ADDRESS>to request a public hearing, to
register to speak at the public hearing, or to inquire as to whether a
public hearing will be held.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2018-0833. All documents in the docket are
listed in Regulations.gov. Although listed, some information is not
publicly available, e.g., CBI (Confidential Business Information) 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. Publicly
available docket materials are available either electronically in
Regulations.gov or in hard copy at the EPA Docket Center, Room 3334,
WJC West Building, 1301 Constitution Avenue NW, Washington, DC. The
Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Public
Reading Room is<PHONE_NUMBER>, and the telephone number for the EPA
Docket Center is<PHONE_NUMBER>.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2018-0833. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at https://www.regulations.gov/, including any personal
information provided, unless the comment includes information claimed
to be CBI or other information whose disclosure is restricted by
statute.

Do not submit information that you consider to be CBI or otherwise
protected through https://www.regulations.gov/ or email. This type of
information should be submitted by mail as discussed below.
    The EPA may publish any comment received to its public docket.
Multimedia submissions (audio, video, etc.) must be accompanied by a
written comment. The written comment is considered the official comment
and should include discussion of all points you wish to make. The EPA
will generally not consider comments or comment contents located
outside of the primary submission (i.e., on the Web, cloud, or other
file sharing system). For additional submission methods, the full EPA
public comment policy, information about CBI or multimedia submissions,
and general guidance on making effective comments, please visit https://www2.epa.gov/dockets/commenting-epa-dockets.
    The https://www.regulations.gov/ website allows you to submit your
comment anonymously, which means the EPA will not know your identity or
contact information unless you provide it in the body of your comment.
If you send an email comment directly to the EPA without going through
https://www.regulations.gov/, your email address will be automatically
captured and included as part of the comment that is placed in the
public docket and made available on the internet. If you submit an
electronic comment, the EPA recommends that you include your name and
other contact information in the body of your comment and with any
digital storage media you submit. If the EPA cannot read your comment
due to technical difficulties and cannot contact you for clarification,
the EPA may not be able to consider your comment. Electronic files
should not include special characters or any form of encryption and be
free of any defects or viruses. For additional information about the
EPA's public docket, visit the EPA Docket Center homepage at https://www.epa.gov/dockets.
    The EPA is soliciting comment on numerous aspects of the proposed
rule. The EPA has indexed each comment solicitation with an alpha-
numeric identifier (e.g., ``C-1,'' ``C-2,'' ``C-3'') to provide a
consistent framework for effective and efficient provision of comments.
Accordingly, the EPA asks that commenters include the identifier in
either a heading, or within the text of each comment (e.g., ``In
response to solicitation of comment C-1, . . .'') to make clear which
comment solicitation is being addressed. The EPA emphasizes that the
Agency is not limiting comment to these identified areas and encourages
provision of any other comments on topics within the scope of this
proposal.
    Submitting CBI. Do not submit information containing CBI to the EPA
through https://www.regulations.gov/ or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
any digital storage media that you mail to the EPA, mark the outside of
the digital storage media as CBI and then identify electronically
within the digital storage media the specific information that is
claimed as CBI. In addition to one complete version of the comments
that includes information claimed as CBI, you must submit a copy of the
comments that does not contain the information claimed as CBI directly
to the public docket through the procedures outlined in Instructions
above. If you submit any digital storage media that does not contain
CBI, mark the outside of the digital storage media clearly that it does
not contain CBI. Information not marked as CBI will be included in the
public docket and the EPA's electronic public docket without prior
notice. Information marked as CBI will not be disclosed except in
accordance with procedures set forth in 40 Code of Federal Regulations
(CFR) part 2. Send or deliver information identified as CBI only to the
following address: OAQPS Document Control Officer (C404-02), OAQPS,
U.S. Environmental Protection Agency, Research Triangle Park, North
Carolina 27711, Attention Docket ID No. EPA-HQ-OAR-2018-0833.
    Preamble acronyms and abbreviations. We use multiple acronyms and
terms in this preamble. While this list may not be exhaustive, to ease
the reading of this preamble and for reference purposes, the EPA
defines the following terms and acronyms here:

AEGL acute exposure guideline level
AERMOD air dispersion model used by the HEM-3 model
BACT best available control technology
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CFR Code of Federal Regulations
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guideline
ERT Electronic Reporting Tool
GACT generally achievable control technology
HAP hazardous air pollutant(s)
HCl hydrochloric acid
HEM-3 Human Exposure Model
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
IRIS Integrated Risk Information System
km kilometer
LAER lowest achievable emission rate
MACT maximum achievable control technology
mg/kg-day milligrams per kilogram per day
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NESHAP national emission standards for hazardous air pollutants
NRC National Research Council
NSR New Source Review
NTTAA National Technology Transfer and Advancement Act
OAQPS Office of Air Quality Planning and Standards
OECA Office of Enforcement and Compliance Assurance
OMB Office of Management and Budget
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PM particulate matter
POM polycyclic organic matter
ppm parts per million
RACT reasonably available control technology
RBLC RACT/BACT/LAER clearinghouse
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SIC Standard Industrial Classification
SSM startup, shutdown, and malfunction
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure model
UF uncertainty factor
[micro]g/m\3\ microgram per cubic meter
UMRA Unfunded Mandates Reform Act
URE unit risk estimate
VCS voluntary consensus standards

    Organization of this document. The information in this preamble is
organized as follows:

I. General Information
    A. Does this action apply to me?
    B. Where can I get a copy of this document and other related
information?
II. Background
    A. What is the statutory authority for this action?
    B. What is this source category?
    C. What data collection activities were conducted to support
this action?
    D. What other relevant background information and data are
available?
III. Analytical Procedures and Decision-Making
    A. How do we consider risk in our decision-making?
    B. How do we perform the technology review?
    C. How do we estimate post-MACT risk posed by the source
category?
IV. Analytical Results and Proposed Decisions

    A. What actions are we taking pursuant to CAA sections 112(d)(2)
and 112(d)(3)?
    B. What are the results of the risk assessment and analyses for
affected sources?
    C. What are our proposed decisions regarding risk acceptability,
ample margin of safety, and adverse environmental effect?
    D. Additional Modeling for Site Remediation
    E. What are the results and proposed decisions based on our
technology review?
    F. What other actions are we proposing?
    G. What compliance dates are we proposing?
V. Summary of Cost, Environmental, and Economic Impacts
    A. What are the affected sources?
    B. What are the air quality impacts?
    C. What are the cost impacts?
    D. What are the economic impacts?
    E. What are the benefits?
VI. Request for Comment
    A. Request for Comment Regarding CERCLA/RCRA Exempt Sources
    B. Request for Comment on all Aspects of the Risk and Technology
Review
VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulation and Controlling
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
    I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    J. National Technology Transfer and Advancement Act (NTTAA) and
1 CFR Part 51
    K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations

I. General Information

A. Does this action apply to me?

    Table 1 of this preamble lists the NESHAP and associated regulated
industrial source category that is the subject of this proposal. Table
1 is not intended to be exhaustive, but rather provides a guide for
readers regarding the entities that this proposed action is likely to
affect. The proposed standards, once promulgated, will be directly
applicable to the affected sources. Federal, state, local, and tribal
government entities conducting site remediations subject to the Site
Remediation NESHAP may be affected by this proposed action. As defined
in the Initial List of Categories of Sources Under Section 112(c)(1) of
the Clean Air Act Amendments of 1990 (see 57 FR 31576, July 16, 1992)
and Documentation for Developing the Initial Source Category List,
Final Report (see EPA-450/3-91-030, July 1992), the Site Remediation
source category is any facility engaged in the cleanup of sites that
possess contaminated media. Sites undergoing remediation of
contaminated media include, but are not limited to, any facility at
which organic materials currently are or have been in the past stored,
processed, treated, or otherwise managed at the facility. These
facilities include organic liquid storage terminals, petroleum
refineries, chemical manufacturing facilities, and other manufacturing
facilities with collocated site remediation activities. Units requiring
cleanup can include hazardous waste dumps, industrial surface
impoundments, leaking tanks, and municipal, industrial, and combined
landfills. Site remediation includes, but is not limited to, the
following activities: Contaminated soils cleaning; soil vapor
extraction (SVE); groundwater cleanup; oil recovery from below ground;
surface flow control; waste material removal from the site; treatment
of waste material after removal; and cleansing of water mains, sewers,
wetlands, and water bodies that have been contaminated by wastes. Site
remediation does not include the installation of controls to municipal
solid waste landfills to comply with the new source performance
standards or Clean Air Act (CAA) section 111(d) emission guidelines.

    Table 1--NESHAP and Industrial Source Categories Affected by This
                             Proposed Action
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        Source category               NESHAP           NAICS code \1\
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Industry......................  40 CFR part 63,    325211, 325192,
                                 subpart GGGGG.     325188, 32411,
                                                    49311, 49319, 48611,
                                                    42269, 42271.
Federal Government............  .................  Federal agency
                                                    facilities that
                                                    conduct site
                                                    remediation
                                                    activities.
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\1\ North American Industry Classification System.

B. Where can I get a copy of this document and other related
information?

    In addition to being available in the docket, an electronic copy of
this action is available on the internet. Following signature by the
EPA Administrator, the EPA will post a copy of this proposed action at
https://www.epa.gov/stationary-sources-air-pollution/site-remediation-national-emission-standards-hazardous-air. Following publication in the
Federal Register, the EPA will post the Federal Register version of the
proposal and key technical documents at this same website. Information
on the overall RTR program is available at https://www3.epa.gov/ttn/atw/rrisk/rtpg.html.
    A redline version of the regulatory language that incorporates the
proposed changes in this action is available in the docket for this
action (Docket ID No. EPA-HQ-OAR-2018-0833).

II. Background

A. What is the statutory authority for this action?

    The statutory authority for this action is provided by sections 112
and 301 of the CAA, as amended (42 U.S.C. 7401 et seq.). Section 112 of
the CAA establishes a two-stage regulatory process to develop standards
for emissions of hazardous air pollutants (HAP) from stationary
sources. Generally, the first stage involves establishing technology-
based standards and the second stage involves evaluating those
standards that are based on maximum achievable control technology
(MACT) to determine whether additional standards are needed to address
any remaining risk associated with HAP emissions. This second stage is
commonly referred to as the ``residual risk review.'' In addition to
the residual risk review, the CAA also requires the EPA to review
standards set under CAA section 112 every 8 years to determine if there
are ``developments in practices, processes, or control technologies''
that may be appropriate

to incorporate into the standards. This review is commonly referred to
as the ``technology review.'' When the two reviews are combined into a
single rulemaking, it is commonly referred to as the ``risk and
technology review.'' The discussion that follows identifies the most
relevant statutory sections and briefly explains the contours of the
methodology used to implement these statutory requirements. A more
comprehensive discussion appears in the document titled CAA Section 112
Risk and Technology Reviews: Statutory Authority and Methodology, in
the docket for this rulemaking.
    In the first stage of the CAA section 112 standard setting process,
the EPA promulgates technology-based standards under CAA section 112(d)
for categories of sources identified as emitting one or more of the HAP
listed in CAA section 112(b). Sources of HAP emissions are either major
sources or area sources, and CAA section 112 establishes different
requirements for major source standards and area source standards.
``Major sources'' are those that emit or have the potential to emit 10
tons per year (tpy) or more of a single HAP or 25 tpy or more of any
combination of HAP. All other sources are ``area sources.'' For major
sources, CAA section 112(d)(2) provides that the technology-based
NESHAP must reflect the maximum degree of emission reductions of HAP
achievable (after considering cost, energy requirements, and non-air
quality health and environmental impacts). These standards are commonly
referred to as MACT standards. CAA section 112(d)(3) also establishes a
minimum control level for MACT standards, known as the MACT ``floor.''
The EPA must also consider control options that are more stringent than
the floor. Standards more stringent than the floor are commonly
referred to as beyond-the-floor standards. In certain instances, as
provided in CAA section 112(h), the EPA may set work practice standards
where it is not feasible to prescribe or enforce a numerical emission
standard. For area sources, CAA section 112(d)(5) gives the EPA
discretion to set standards based on generally available control
technologies or management practices (GACT standards) in lieu of MACT
standards.
    The second stage in standard-setting focuses on identifying and
addressing any remaining (i.e., ``residual'') risk according to CAA
section 112(f). For source categories subject to MACT standards,
section 112(f)(2) of the CAA requires the EPA to determine whether
promulgation of additional standards is needed to provide an ample
margin of safety to protect public health or to prevent an adverse
environmental effect. Section 112(d)(5) of the CAA provides that this
residual risk review is not required for categories of area sources
subject to GACT standards. Section 112(f)(2)(B) of the CAA further
expressly preserves the EPA's use of the two-step approach for
developing standards to address any residual risk and the Agency's
interpretation of ``ample margin of safety'' developed in the National
Emissions Standards for Hazardous Air Pollutants: Benzene Emissions
from Maleic Anhydride Plants, Ethylbenzene/Styrene Plants, Benzene
Storage Vessels, Benzene Equipment Leaks, and Coke By-Product Recovery
Plants (Benzene NESHAP) (54 FR 38044, September 14, 1989). The EPA
notified Congress in the Risk Report that the Agency intended to use
the Benzene NESHAP approach in making CAA section 112(f) residual risk
determinations (EPA-453/R-99-001, p. ES-11). The EPA subsequently
adopted this approach in its residual risk determinations and the
United States Court of Appeals for the District of Columbia Circuit
(the Court) upheld the EPA's interpretation that CAA section 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See
Natural Resources Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1083
(D.C. Cir. 2008).
    The approach incorporated into the CAA and used by the EPA to
evaluate residual risk and to develop standards under CAA section
112(f)(2) is a two-step approach. In the first step, the EPA determines
whether risks are acceptable. This determination ``considers all health
information, including risk estimation uncertainty, and includes a
presumptive limit on maximum individual lifetime [cancer] risk (MIR)
\1\ of approximately 1 in 10 thousand.'' 54 FR 38045, September 14,
1989. If risks are unacceptable, the EPA must determine the emissions
standards necessary to reduce risk to an acceptable level without
considering costs. In the second step of the approach, the EPA
considers whether the emissions standards provide an ample margin of
safety to protect public health ``in consideration of all health
information, including the number of persons at risk levels higher than
approximately 1 in 1 million, as well as other relevant factors,
including costs and economic impacts, technological feasibility, and
other factors relevant to each particular decision.'' Id. The EPA must
promulgate emission standards necessary to provide an ample margin of
safety to protect public health. After conducting the ample margin of
safety analysis, we consider whether a more stringent standard is
necessary to prevent, taking into consideration costs, energy, safety,
and other relevant factors, an adverse environmental effect.
---------------------------------------------------------------------------

    \1\ Although defined as ``maximum individual risk,'' MIR refers
only to cancer risk. MIR, one metric for assessing cancer risk, is
the estimated risk if an individual were exposed to the maximum
level of a pollutant for a lifetime.
---------------------------------------------------------------------------

    CAA section 112(d)(6) separately requires the EPA to review
standards promulgated under CAA section 112 and revise them ``as
necessary (taking into account developments in practices, processes,
and control technologies)'' no less often than every 8 years. In
conducting this review, which we call the ``technology review,'' the
EPA is not required to recalculate the MACT floor. NRDC v. EPA, 529
F.3d 1077, 1084 (D.C. Cir. 2008). Association of Battery Recyclers,
Inc. v. EPA, 716 F.3d 667 (D.C. Cir. 2013). The EPA may consider cost
in deciding whether to revise the standards pursuant to CAA section
112(d)(6).

B. What is this source category?

    The EPA promulgated the final Site Remediation NESHAP at 40 CFR
part 63, subpart GGGGG on October 8, 2003. The NESHAP applies to
``remediation material.'' Site remediation means one or more activities
or processes used to remove, destroy, degrade, transform, immobilize,
or otherwise manage remediation material. Monitoring or measuring of
contamination levels in media, whether by using wells, sampling, or
other means, is not considered to be a site remediation. The rule
applies only to active remedial operations at sites that are major
sources with affected facilities subject to another MACT standard. The
Site Remediation NESHAP applies to various types of affected sources
including process vents, remediation material management units, and
equipment leaks. The affected source for process vents is the entire
group of process vents associated with the in-situ and ex-situ
remediation processes used at the site to remove, destroy, degrade,
transform, or immobilize hazardous substances in the remediation
material. Examples of process vents for in-situ remediation processes
include the discharge vents to the atmosphere used for SVE and
underground bioremediation processes. Examples of process vents for ex-
situ remediation processes include vents for thermal desorption,
bioremediation, and stripping processes (air or steam stripping). The
affected source for remediation material management units is the entire
group of tanks, surface

impoundments, containers, oil-water separators, and transfer systems
used for the site remediation activities involving clean-up of
remediation material. The affected source for equipment leaks is the
entire group of remediation equipment components (pumps, valves, etc.)
that is intended to operate for 300 hours or more during a calendar
year in remediation material service and that contains or contacts
remediation material having a concentration of regulated HAP equal to
or greater than 10 percent by weight.
    The Site Remediation MACT standards include a combination of
equipment standards, work practice standards, operational standards,
and performance standards for each of the affected emission sources
noted above.

C. What data collection activities were conducted to support this
action?

    The primary sources of data for the risk assessment are EPA
databases. These include the EPA's Enforcement and Compliance History
Online (ECHO) database, which was queried to identify facilities
potentially subject to the Site Remediation NESHAP. Information from
this search was then used in a query of the EPA's National Emissions
Inventory (NEI) to identify site remediation emission sources,
quantities of emissions, and emissions release characteristics. The EPA
also reviewed the Toxic Release Inventory to determine whether that
data would be useful in supplementing the information extracted from
the NEI.
    We reviewed a variety of data sources in our investigation of
potential practices, processes, or controls to consider in the
technology review and to provide further information for the risk
assessment. These included the Reasonably Available Control Technology
(RACT)/Best Available Control Technology (BACT)/Lowest Achievable
Emission Rate (LAER) Clearinghouse (RBLC), NESHAP for various
industries that were promulgated since the Site Remediation NESHAP was
promulgated, major source operating permits, minor and synthetic minor
source operating permits, and academic and trade literature.
    The RBLC provides a central database of air pollution control
technology information and can help identify appropriate technologies
to mitigate most air pollutant emission streams: https://www.epa.gov/catc/ractbactlaer-clearinghouse-rblc-basic-information. As site
remediation may include sources from any industrial activity, we
searched the RBLC with a focus on control of off-gasses in disparate
applications, including processes in three broad categories:
Miscellaneous Combustion, Waste Combustion and Waste Disposal, and
Other Waste Processing and Disposal. Each of these three categories was
further searched more specifically. For Miscellaneous Combustion, the
EPA searched emission control afterburners and incinerators, digester
and landfill gas flares, and other miscellaneous combustion. For Waste
Combustion and Waste Disposal categories, the search included mixed/
other waste combustion/incineration. Finally, the search under Other
Waste Processing and Disposal included contaminated soil treatment,
hazardous waste treatment, storage, and disposal facilities, and other
waste processing and disposal facilities.
    The EPA also reviewed the NESHAP for various industries that were
promulgated since the Site Remediation NESHAP was promulgated. We
reviewed the regulatory requirements and/or technical analyses
associated with these regulatory actions to identify any practices,
processes, and control technologies considered in these efforts that
could be applied to emission sources in the Site Remediation source
category, as well as the costs, non-air impacts, and energy
implications associated with the use of these technologies.
    The EPA searched available state databases for minor source permits
and synthetic minor source permits of facilities performing
remediation. The Technology Review memorandum in the docket lists the
permits reviewed and summarizes key findings about the remediation
projects and emissions controls in use. Other scientific literature was
reviewed for new and novel control technologies in use at site
remediation sources and similar sources to control volatile organic
compounds (VOC) and HAP air emissions. Literature for controls in use
for land farming applications and material extraction activities was
also reviewed. For a list of material reviewed, see the memorandum, CAA
section 112(d)(6) Technology Review for the Site Remediation Source
Category, which is available in the docket for this action.

D. What other relevant background information and data are available?

    Documents from previous rulemakings for the Site Remediation source
category can be found in the docket under Docket ID No. EPA-HQ-OAR-
2002-0021.

III. Analytical Procedures and Decision-Making

    In this section, we describe the analyses performed to support the
proposed decisions for the RTR and other issues addressed in this
proposal.

A. How do we consider risk in our decision-making?

    As discussed in section II.A of this preamble and in the Benzene
NESHAP, in evaluating and developing standards under CAA section
112(f)(2), we apply a two-step approach to determine whether or not
risks are acceptable and to determine if the standards provide an ample
margin of safety to protect public health. As explained in the Benzene
NESHAP, ``the first step judgment on acceptability cannot be reduced to
any single factor'' and, thus, ``[t]he Administrator believes that the
acceptability of risk under section 112 is best judged on the basis of
a broad set of health risk measures and information.'' 54 FR 38046,
September 14, 1989. Similarly, with regard to the ample margin of
safety determination, ``the Agency again considers all of the health
risk and other health information considered in the first step. Beyond
that information, additional factors relating to the appropriate level
of control will also be considered, including cost and economic impacts
of controls, technological feasibility, uncertainties, and any other
relevant factors.'' Id.
    The Benzene NESHAP approach provides flexibility regarding factors
the EPA may consider in making determinations and how the EPA may weigh
those factors for each source category. The EPA conducts a risk
assessment that provides estimates of the MIR posed by the HAP
emissions from each source in the source category, the hazard index
(HI) for chronic exposures to HAP with the potential to cause noncancer
health effects, and the hazard quotient (HQ) for acute exposures to HAP
with the potential to cause noncancer health effects.\2\ The assessment
also provides estimates of the distribution of cancer risk within the
exposed populations, cancer incidence, and an evaluation of the
potential for an adverse environmental effect. The scope of the EPA's
risk analysis is consistent with the EPA's response to comments on our
policy under the Benzene NESHAP where the EPA explained that:
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    \2\ The MIR is defined as the cancer risk associated with a
lifetime of exposure at the highest concentration of HAP where
people are likely to live. The HQ is the ratio of the potential
exposure concentration to the noncancer dose-response value; the HI
is the sum of HQs for HAP that affect the same target organ or organ
system.

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[t]he policy chosen by the Administrator permits consideration of
multiple measures of health risk. Not only can the MIR figure be
considered, but also incidence, the presence of noncancer health
effects, and the uncertainties of the risk estimates. In this way,
the effect on the most exposed individuals can be reviewed as well
as the impact on the general public. These factors can then be
weighed in each individual case. This approach complies with the
Vinyl Chloride mandate that the Administrator ascertain an
acceptable level of risk to the public by employing his expertise to
assess available data. It also complies with the Congressional
intent behind the CAA, which did not exclude the use of any
particular measure of public health risk from the EPA's
consideration with respect to CAA section 112 regulations, and
thereby implicitly permits consideration of any and all measures of
health risk which the Administrator, in his judgment, believes are
appropriate to determining what will `protect the public health'.

See 54 FR 38057, September 14, 1989. Thus, the level of the MIR is only
one factor to be weighed in determining acceptability of risk. The
Benzene NESHAP explained that ``an MIR of approximately one in 10
thousand should ordinarily be the upper end of the range of
acceptability. As risks increase above this benchmark, they become
presumptively less acceptable under CAA section 112, and would be
weighed with the other health risk measures and information in making
an overall judgment on acceptability. Or, the Agency may find, in a
particular case, that a risk that includes an MIR less than the
presumptively acceptable level is unacceptable in the light of other
health risk factors.'' Id. at 38045. Similarly, with regard to the
ample margin of safety analysis, the EPA stated in the Benzene NESHAP
that: ``EPA believes the relative weight of the many factors that can
be considered in selecting an ample margin of safety can only be
determined for each specific source category. This occurs mainly
because technological and economic factors (along with the health-
related factors) vary from source category to source category.'' Id. at
38061. We also consider the uncertainties associated with the various
risk analyses, as discussed earlier in this preamble, in our
determinations of acceptability and ample margin of safety.
    The EPA notes that it has not considered certain health information
to date in making residual risk determinations. At this time, we do not
attempt to quantify the HAP risk that may be associated with emissions
from other facilities that do not include the source category under
review, mobile source emissions, natural source emissions, persistent
environmental pollution, or atmospheric transformation in the vicinity
of the sources in the category.
    The EPA understands the potential importance of considering an
individual's total exposure to HAP in addition to considering exposure
to HAP emissions from the source category and facility. We recognize
that such consideration may be particularly important when assessing
noncancer risk, where pollutant-specific exposure health reference
levels (e.g., reference concentrations (RfCs)) are based on the
assumption that thresholds exist for adverse health effects. For
example, the EPA recognizes that, although exposures attributable to
emissions from a source category or facility alone may not indicate the
potential for increased risk of adverse noncancer health effects in a
population, the exposures resulting from emissions from the facility in
combination with emissions from all of the other sources (e.g., other
facilities) to which an individual is exposed may be sufficient to
result in an increased risk of adverse noncancer health effects. In May
2010, the Science Advisory Board (SAB) advised the EPA ``that RTR
assessments will be most useful to decision makers and communities if
results are presented in the broader context of aggregate and
cumulative risks, including background concentrations and contributions
from other sources in the area.'' \3\
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    \3\ Recommendations of the SAB Risk and Technology Review Panel
are provided in their report, which is available at: https://
yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
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    In response to the SAB recommendations, the EPA incorporates
cumulative risk analyses into its RTR risk assessments, including those
reflected in this proposal. The Agency (1) conducts facility-wide
assessments, which include source category emission points, as well as
other emission points within the facilities; (2) combines exposures
from multiple sources in the same category that could affect the same
individuals; and (3) for some persistent and bioaccumulative
pollutants, analyzes the ingestion route of exposure. In addition, the
RTR risk assessments consider aggregate cancer risk from all
carcinogens and aggregated noncancer HQs for all noncarcinogens
affecting the same target organ or target organ system.
    Although we are interested in placing source category and facility-
wide HAP risk in the context of total HAP risk from all sources
combined in the vicinity of each source, we are concerned about the
uncertainties of doing so. Estimates of total HAP risk from emission
sources other than those that we have studied in depth during this RTR
review would have significantly greater associated uncertainties than
the source category or facility-wide estimates. Such aggregate or
cumulative assessments would compound those uncertainties, making the
assessments too unreliable.

B. How do we perform the technology review?

    Our technology review focuses on the identification and evaluation
of developments in practices, processes, and control technologies that
have occurred since the MACT standards were promulgated. Where we
identify such developments, we analyze their technical feasibility,
estimated costs, energy implications, and non-air environmental
impacts. We also consider the emission reductions associated with
applying each development. This analysis informs our decision of
whether it is ``necessary'' to revise the emissions standards. In
addition, we consider the appropriateness of applying controls to new
sources versus retrofitting existing sources. For this exercise, we
consider any of the following to be a ``development'':
     Any add-on control technology or other equipment that was
not identified and considered during development of the original MACT
standards;
     Any improvements in add-on control technology or other
equipment (that were identified and considered during development of
the original MACT standards) that could result in additional emissions
reduction;
     Any work practice or operational procedure that was not
identified or considered during development of the original MACT
standards;
     Any process change or pollution prevention alternative
that could be broadly applied to the industry and that was not
identified or considered during development of the original MACT
standards; and
     Any significant changes in the cost (including cost
effectiveness) of applying controls (including controls the EPA
considered during the development of the original MACT standards).
    In addition to reviewing the practices, processes, and control
technologies that were considered at the time we last updated the
NESHAP, we review a variety of data sources in our investigation of
potential practices, processes, or controls to consider. See sections
II.C and II.D of this preamble for information on the specific data

sources that were reviewed as part of the technology review.

C. How do we estimate post-MACT risk posed by the source category?

    In this section, we provide a complete description of the types of
analyses that we generally perform during the risk assessment process.
In some cases, we do not perform a specific analysis because it is not
relevant. For example, in the absence of emissions of HAP known to be
persistent and bioaccumulative in the environment (PB-HAP), we would
not perform a multipathway exposure assessment. Where we do not perform
an analysis, we state that we do not and provide the reason. While we
present all of our risk assessment methods, we only present risk
assessment results for the analyses actually conducted (see section
IV.B of this preamble).
    The EPA conducts a risk assessment that provides estimates of the
MIR for cancer posed by the HAP emissions from each source in the
source category, the HI for chronic exposures to HAP with the potential
to cause noncancer health effects, and the HQ for acute exposures to
HAP with the potential to cause noncancer health effects. The
assessment also provides estimates of the distribution of cancer risk
within the exposed populations, cancer incidence, and an evaluation of
the potential for an adverse environmental effect. The eight sections
that follow this paragraph describe how we estimated emissions and
conducted the risk assessment. The docket for this rulemaking contains
the following document which provides more information on the risk
assessment inputs and models: Residual Risk Assessment for the Site
Remediation Source Category in Support of the 2019 Risk and Technology
Review Proposed Rule. The methods used to assess risk (as described in
the eight primary steps below) are consistent with those described by
the EPA in the document reviewed by a panel of the EPA's SAB in 2009;
\4\ and described in the SAB review report issued in 2010. They are
also consistent with the key recommendations contained in that report.
---------------------------------------------------------------------------

    \4\ U.S. EPA. Risk and Technology Review (RTR) Risk Assessment
Methodologies: For Review by the EPA's Science Advisory Board with
Case Studies--MACT I Petroleum Refining Sources and Portland Cement
Manufacturing. (EPA-452/R-09-006; June 2009.) https://www3.epa.gov/airtoxics/rrisk/rtrpg.html.
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1. How did we estimate actual emissions and identify the emissions
release characteristics?
a. Sources Subject to the Site Remediation NESHAP
    The EPA began compiling the list of facilities for the risk review
by searching for facilities identified as being subject to the Site
Remediation NESHAP in the EPA's ECHO database. There are currently 102
facilities identified as ``subject to NESHAP GGGGG'' in ECHO. This list
of facilities was used as the basis for a query into the NEI to obtain
facility and emissions data for the 2014 reporting year.
    Of the 102 facilities with data retrieved from the 2014 NEI, six
facilities reported emissions under the Source Classification Code
(SCC) for site remediation, and 96 reported emissions only from their
primary activity and did not report any emissions for remediation
activities. We attribute the absence of site remediation data for these
96 facilities to either the facilities' completion of site remediation
activities or reporting of site remediation emissions data under other
SCCs in the NEI. The EPA chose to model all 102 facilities rather than
only the six for which remediation data was reported, in order to take
the broadest possible approach to the risk assessment. For example,
while a remediation may not have been occuring in 2014 that would be
reported in the 2014 inventory, the EPA assumed that a remediation
would have taken place at some point at all 102 facilities since
adoption of the Site Remediation NESHAP. By including all 102
facilities, the EPA attempted to estimate the risk of anyone who may
have been exposed to risk from a remediation at an affected source,
regardless of the current (as of 2014) status of a specific remediation
action.
    To address the lack of apparent site remediation emissions data for
these 96 facilities, the EPA developed a profile of site remediation
emissions for each facility based on the facility's primary processes.
Since site remediation projects occur at many different types of
industrial facilities, ranging from petroleum refineries to federal
facilities, and the emissions from the site remediation are likely a
subset of HAP emitted by the facility, this emissions profile approach
was used to account for the disparate nature of sources with site
remediation activities. To develop the emission profiles for each
facility, the EPA used the six facilities that reported HAP emissions
both from their remediation activities and from their whole facility in
the NEI and determined the proportion of remediation HAP emissions to
facility-wide HAP emissions for each facility. Of the six facilities,
the highest proportion of remediation to whole-facility HAP emissions
was 0.79 percent. For the other 96 facilities, the EPA used this
proportion to assign 0.79 percent of the total amount of each HAP
reported in the NEI for the whole facility to the Site Remediation
source category for each facility, arriving at a unique profile of site
remediation emissions for each facility.
    With respect to the risk analysis, the EPA considers this to be a
conservative approach to addressing the lack of remediation emissions
reported in the NEI. First, the data show that remediation emissions
are generally small compared to major source emissions at affected
facilities, and the highest proportion of remediation emissions from
the six facilities was chosen for the remediation emissions profiles.
Second, all process pollutants emitted by a facility were included as
the universe of potential pollutants emitted during remediation. While
site remediation projects likely emit only a subset of the HAP emitted
by the facility, this assumption was made to ensure no specific
pollutant was excluded that could represent risk from that facility.
For several facilities, we found that the emissions profile approach
had resulted in estimated site remediation emissions that included
ethylene oxide. These ethylene oxide emissions were removed from the
source category risk analysis because the EPA considered that ethylene
oxide would be unlikely to persist in contaminated media long enough to
be emitted during a site remediation. Additional details on this
determination can be found in the Residual Risk Assessment for the Site
Remediation Source Category in Support of the 2019 Risk and Technology
Proposed Rule, which is available in the docket for this action. The
EPA requests comment on this model plant approach to address data gaps
in the RTR, and HAP emissions from the Site Remediation source
category. (C-1)
b. Sources Exempt From the Site Remediation NESHAP
    The Site Remediation NESHAP currently exempts site remediation
activities conducted under federal oversight authority under the
Comprehensive Environmental Response, Compensation, and Liability Act
(CERCLA) or Resource Conservation and Recovery Act (RCRA) corrective
action or other required RCRA order (see 40 CFR 63.7881(b)(3)). In
2016, in response to a petition for

reconsideration regarding this exemption and other aspects of the
NESHAP, the EPA proposed to revise the NESHAP to remove the exemption
for site remediation activities conducted under the authority of CERCLA
or RCRA (81 FR 29821, May 13, 2016). At proposal, the EPA developed a
list of 125 facilities that could potentially become subject to the
rule upon promulgation if the exemption for remediation projects
subject to RCRA or CERCLA standards was removed.\5\ Although exempt
from the regulatory requirements of the Site Remediation NESHAP, these
facilities are part of the Site Remediation source category. To
understand both the risks from the facilities already subject to the
Site Remediation NESHAP requirements and the risks from the facilities
exempt from the Site Remediation NESHAP requirements, these groups of
facilities were kept separate for the purposes of the risk assessment.
---------------------------------------------------------------------------

    \5\ Stobert, L. EC/R Inc. to Hirtz, J., EPA/OAQPS. National
Impacts Associated with the Proposed Amendments to Remove the
Exemption for Facilities Performing Site Remediations under CERCLA
or RCRA in the NESHAP for Site Remediation. February 4, 2016. EPA
Docket Item No. EPA-HQ-OAR-2002-0021-0055. The EPA estimated in 2016
that of the 125 facilities listed, only 69 would likely become
subject to the rule. For the purpose of the risk review, the EPA
modeled the 118 facilities that could be identified in the NEI.
---------------------------------------------------------------------------

    A process similar to that used to estimate emissions from affected
facilities was used for the exempt facilities. The EPA began with the
list of 125 facilities previously developed and queried the NEI to
obtain facility and emissions data for the 2014 reporting-year.
Information was available in the NEI for 118 of these facilities.\6\ Of
the 118 facilities with data retrieved from the NEI, 10 facilities
reported emissions under the SCC for site remediation, and 108 reported
emissions only from their production activity and did not report any
emissions for remediation activities. For these 108 facilities, the EPA
applied the same site remediation emissions ratio as that used for
affected sources to the whole-facility HAP emissions to arrive at a
unique site remediation emission profile for each facility. For these
facilities, we used the same assumptions with respect to ethylene oxide
emissions as were made in the affected facility modeling.
---------------------------------------------------------------------------

    \6\ Seven of the 125 facilities were unable to be clearly
identified in the NEI and were not modeled.
---------------------------------------------------------------------------

2. How did we estimate MACT-allowable emissions?
    The available emissions data in the RTR emissions dataset include
estimates of the mass of HAP emitted during a specified annual time
period. These ``actual'' emission levels are often lower than the
emission levels allowed under the requirements of the current MACT
standards. The emissions allowed under the MACT standards are referred
to as the ``MACT-allowable'' emissions. We discussed the consideration
of both MACT-allowable and actual emissions in the final Coke Oven
Batteries RTR (70 FR 19998-19999, April 15, 2005) and in the proposed
and final Hazardous Organic NESHAP RTR (71 FR 34428, June 14, 2006, and
71 FR 76609, December 21, 2006, respectively). In those actions, we
noted that assessing the risk at the MACT-allowable level is inherently
reasonable since that risk reflects the maximum level facilities could
emit and still comply with national emission standards. We also
explained that it is reasonable to consider actual emissions, where
such data are available, in both steps of the risk analysis, in
accordance with the Benzene NESHAP approach. (54 FR 38044, September
14, 1989.)
    For the Site Remediation source category, the EPA treated actual
emissions as allowable emissions. Allowable emissions under typical
MACT standards are the emissions that would occur under full-capacity
potential operating conditions and as allowed under the applicable MACT
standards. These are the conditions included in the title V permit for
the facility. In the case of site remediation, most remediation
projects do not appear in the title V permit or appear there for
approximately the duration of the remediation and are then removed.
Since most facilities performing remediation have the incentive to
conclude remediation expeditiously, the EPA assumed that actual
emissions would equal allowed emissions under a facility permit. Where
no permit condition was available, the EPA assumed the remediation was
being conducted at full capacity to complete the remediation as soon as
possible. Based on the NEI data available and the relatively little
information found in title V permits for remediation projects, the EPA
modeled actual emissions as allowable emissions.
3. How do we conduct dispersion modeling, determine inhalation
exposures, and estimate individual and population inhalation risk?
    Both long-term and short-term inhalation exposure concentrations
and health risk from the source category addressed in this proposal
were estimated using the Human Exposure Model (HEM-3).\7\ The HEM-3
performs three primary risk assessment activities: (1) Conducting
dispersion modeling to estimate the concentrations of HAP in ambient
air, (2) estimating long-term and short-term inhalation exposures to
individuals residing within 50 kilometers (km) of the modeled sources,
and (3) estimating individual and population-level inhalation risk
using the exposure estimates and quantitative dose-response
information.
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    \7\ For more information about HEM-3, go to https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.
---------------------------------------------------------------------------

a. Dispersion Modeling
    The air dispersion model AERMOD, used by the HEM-3 model, is one of
the EPA's preferred models for assessing air pollutant concentrations
from industrial facilities.\8\ To perform the dispersion modeling and
to develop the preliminary risk estimates, HEM-3 draws on three data
libraries. The first is a library of meteorological data, which is used
for dispersion calculations. This library includes 1 year (2016) of
hourly surface and upper air observations from 824 meteorological
stations, selected to provide coverage of the United States and Puerto
Rico. A second library of United States Census Bureau census block \9\
internal point locations and populations provides the basis of human
exposure calculations (U.S. Census, 2010). In addition, for each census
block, the census library includes the elevation and controlling hill
height, which are also used in dispersion calculations. A third library
of pollutant-specific dose-response values is used to estimate health
risk. These are discussed below.
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    \8\ U.S. EPA. Revision to the Guideline on Air Quality Models:
Adoption of a Preferred General Purpose (Flat and Complex Terrain)
Dispersion Model and Other Revisions (70 FR 68218, November 9,
2005).
    \9\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

b. Risk From Chronic Exposure to HAP
    In developing the risk assessment for chronic exposures, we use the
estimated annual average ambient air concentrations of each HAP emitted
by each source in the source category. The HAP air concentrations at
each nearby census block centroid located within 50 km of the facility
are a surrogate for the chronic inhalation exposure concentration for
all the people who reside in that census block. A distance of 50 km is
consistent with both the analysis supporting the 1989 Benzene NESHAP
(54 FR 38044, September 14, 1989) and the limitations of Gaussian
dispersion models, including AERMOD.

    For each facility, we calculate the MIR as the cancer risk
associated with a continuous lifetime (24 hours per day, 7 days per
week, 52 weeks per year, 70 years) exposure to the maximum
concentration at the centroid of each inhabited census block. We
calculate individual cancer risk by multiplying the estimated lifetime
exposure to the ambient concentration of each HAP (in micrograms per
cubic meter ([mu]g/m \3\)) by its unit risk estimate (URE). The URE is
an upper-bound estimate of an individual's incremental risk of
contracting cancer over a lifetime of exposure to a concentration of 1
microgram of the pollutant per cubic meter of air. For residual risk
assessments, we generally use UREs from the EPA's Integrated Risk
Information System (IRIS). For carcinogenic pollutants without IRIS
values, we look to other reputable sources of cancer dose-response
values, often using California EPA (CalEPA) UREs, where available. In
cases where new, scientifically credible dose-response values have been
developed in a manner consistent with EPA guidelines and have undergone
a peer review process similar to that used by the EPA, we may use such
dose-response values in place of, or in addition to, other values, if
appropriate. The pollutant-specific dose-response values used to
estimate health risk are available at https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.
    To estimate individual lifetime cancer risks associated with
exposure to HAP emissions from each facility in the source category, we
sum the risks for each of the carcinogenic HAP \10\ emitted by the
modeled facility. We estimate cancer risk at every census block within
50 km of every facility in the source category. The MIR is the highest
individual lifetime cancer risk estimated for any of those census
blocks. In addition to calculating the MIR, we estimate the
distribution of individual cancer risks for the source category by
summing the number of individuals within 50 km of the sources whose
estimated risk falls within a specified risk range. We also estimate
annual cancer incidence by multiplying the estimated lifetime cancer
risk at each census block by the number of people residing in that
block, summing results for all of the census blocks, and then dividing
this result by a 70-year lifetime.
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    \10\ The EPA's 2005 Guidelines for Carcinogen Risk Assessment
classifies carcinogens as: ``carcinogenic to humans,'' ``likely to
be carcinogenic to humans,'' and ``suggestive evidence of
carcinogenic potential.'' These classifications also coincide with
the terms ``known carcinogen, probable carcinogen, and possible
carcinogen,'' respectively, which are the terms advocated in the
EPA's Guidelines for Carcinogen Risk Assessment, published in 1986
(51 FR 33992, September 24, 1986). In August 2000, the document,
Supplemental Guidance for Conducting Health Risk Assessment of
Chemical Mixtures (EPA/630/R-00/002), was published as a supplement
to the 1986 document. Copies of both documents can be obtained from
https://cfpub.epa.gov/ncea/risk/recorddisplay.cfm?deid=20533&CFID=70315376&CFTOKEN=71597944. Summing
the risk of these individual compounds to obtain the cumulative
cancer risk is an approach that was recommended by the EPA's SAB in
their 2002 peer review of the EPA's National Air Toxics Assessment
(NATA) titled, NATA--Evaluating the National-scale Air Toxics
Assessment 1996 Data--an SAB Advisory, available at https://
yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
---------------------------------------------------------------------------

    To assess the risk of noncancer health effects from chronic
exposure to HAP, we calculate either an HQ or a target organ-specific
hazard index (TOSHI). We calculate an HQ when a single noncancer HAP is
emitted. Where more than one noncancer HAP is emitted, we sum the HQ
for each of the HAP that affects a common target organ or target organ
system to obtain a TOSHI. The HQ is the estimated exposure divided by
the chronic noncancer dose-response value, which is a value selected
from one of several sources. The preferred chronic noncancer dose-
response value is the EPA RfC, defined as ``an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure to the human population (including sensitive
subgroups) that is likely to be without an appreciable risk of
deleterious effects during a lifetime'' (https://iaspub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary). In cases where an RfC
from the EPA's IRIS is not available or where the EPA determines that
using a value other than the RfC is appropriate, the chronic noncancer
dose-response value can be a value from the following prioritized
sources, which define their dose-response values similarly to the EPA:
(1) The Agency for Toxic Substances and Disease Registry (ATSDR)
Minimum Risk Level (https://www.atsdr.cdc.gov/mrls/index.asp); (2) the
CalEPA Chronic Reference Exposure Level (REL) (https://oehha.ca.gov/air/crnr/notice-adoption-air-toxics-hot-spots-program-guidance-manual-preparation-health-risk-0); or (3), as noted above, a scientifically
credible dose-response value that has been developed in a manner
consistent with the EPA guidelines and has undergone a peer review
process similar to that used by the EPA. The pollutant-specific dose-
response values used to estimate health risks are available at https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.
c. Risk From Acute Exposure to HAP That May Cause Health Effects Other
Than Cancer
    For each HAP for which appropriate acute inhalation dose-response
values are available, the EPA also assesses the potential health risks
due to acute exposure. For these assessments, the EPA makes
conservative assumptions about emission rates, meteorology, and
exposure location. In this proposed rulemaking, as part of our efforts
to continually improve our methodologies to evaluate the risks that HAP
emitted from categories of industrial sources pose to human health and
the environment,\11\ we are revising our treatment of meteorological
data to use reasonable worst-case air dispersion conditions in our
acute risk screening assessments instead of worst-case air dispersion
conditions. This revised treatment of meteorological data and the
supporting rationale are described in more detail in Residual Risk
Assessment for the Site Remediation Source Category in Support of the
2019 Risk and Technology Review Proposed Rule and in Appendix 5 of the
report: Technical Support Document for Acute Risk Screening Assessment.
We will be applying this revision in RTR rulemakings proposed on or
after June 3, 2019.
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    \11\ U.S. EPA. Screening Methodologies to Support Risk and
Technology Reviews (RTR): A Case Study Analysis (Draft Report, May
2017. https://www3.epa.gov/ttn/atw/rrisk/rtrpg.html).
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    To assess the potential acute risk to the maximally exposed
individual, we use the peak hourly emission rate for each emission
point,\12\ reasonable worst-case air dispersion conditions (i.e., 99th
percentile), and the point of highest off-site exposure. Specifically,
we assume that peak emissions from the source category and reasonable
worst-case air dispersion conditions co-occur and that a person is
present at the point of maximum exposure.
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    \12\ In the absence of hourly emission data, we develop
estimates of maximum hourly emission rates by multiplying the
average actual annual emissions rates by a factor (either a
category-specific factor or a default factor of 10) to account for
variability. This is documented in Residual Risk Assessment for Site
Remediation Source Category in Support of the 2019 Risk and
Technology Review Proposed Rule and in Appendix 5 of the report:
Technical Support Document for Acute Risk Screening Assessment. Both
are available in the docket for this rulemaking.
---------------------------------------------------------------------------

    To characterize the potential health risks associated with
estimated acute

inhalation exposures to a HAP, we generally use multiple acute dose-
response values, including acute RELs, acute exposure guideline levels
(AEGLs), and emergency response planning guidelines (ERPG) for 1-hour
exposure durations), if available, to calculate acute HQs. The acute HQ
is calculated by dividing the estimated acute exposure concentration by
the acute dose-response value. For each HAP for which acute dose-
response values are available, the EPA calculates acute HQs.
    An acute REL is defined as ``the concentration level at or below
which no adverse health effects are anticipated for a specified
exposure duration.'' \13\ Acute RELs are based on the most sensitive,
relevant, adverse health effect reported in the peer-reviewed medical
and toxicological literature. They are designed to protect the most
sensitive individuals in the population through the inclusion of
margins of safety. Because margins of safety are incorporated to
address data gaps and uncertainties, exceeding the REL does not
automatically indicate an adverse health impact. AEGLs represent
threshold exposure limits for the general public and are applicable to
emergency exposures ranging from 10 minutes to 8 hours.\14\ They are
guideline levels for ``once-in-a-lifetime, short-term exposures to
airborne concentrations of acutely toxic, high-priority chemicals.''
Id. at 21. The AEGL-1 is specifically defined as ``the airborne
concentration (expressed as ppm (parts per million) or mg/m\3\
(milligrams per cubic meter)) of a substance above which it is
predicted that the general population, including susceptible
individuals, could experience notable discomfort, irritation, or
certain asymptomatic nonsensory effects. However, the effects are not
disabling and are transient and reversible upon cessation of
exposure.'' The document also notes that ``Airborne concentrations
below AEGL-1 represent exposure levels that can produce mild and
progressively increasing but transient and nondisabling odor, taste,
and sensory irritation or certain asymptomatic, nonsensory effects.''
Id. AEGL-2 are defined as ``the airborne concentration (expressed as
parts per million or milligrams per cubic meter) of a substance above
which it is predicted that the general population, including
susceptible individuals, could experience irreversible or other
serious, long-lasting adverse health effects or an impaired ability to
escape.'' Id.
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    \13\ CalEPA issues acute RELs as part of its Air Toxics Hot
Spots Program, and the 1-hour and 8-hour values are documented in
Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The
Determination of Acute Reference Exposure Levels for Airborne
Toxicants, which is available at https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary.
    \14\ National Academy of Sciences, 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, page 2. Available at https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf. Note that the
National Advisory Committee for Acute Exposure Guideline Levels for
Hazardous Substances ended in October 2011, but the AEGL program
continues to operate at the EPA and works with the National
Academies to publish final AEGLs, (https://www.epa.gov/aegl).
---------------------------------------------------------------------------

    ERPGs are ``developed for emergency planning and are intended as
health-based guideline concentrations for single exposures to
chemicals.'' \15\ Id. at 1. The ERPG-1 is defined as ``the maximum
airborne concentration below which it is believed that nearly all
individuals could be exposed for up to 1 hour without experiencing
other than mild transient adverse health effects or without perceiving
a clearly defined, objectionable odor.'' Id. at 2. Similarly, the ERPG-
2 is defined as ``the maximum airborne concentration below which it is
believed that nearly all individuals could be exposed for up to one
hour without experiencing or developing irreversible or other serious
health effects or symptoms which could impair an individual's ability
to take protective action.'' Id. at 1.
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    \15\ American Industrial Hygiene Association. ERPGS Procedures
and Responsibilities. March 2014. Available at: https://www.aiha.org/get-involved/AIHAGuidelineFoundation/EmergencyResponsePlanningGuidelines/Documents/ERPG%20Committee%20Standard%20Operating%20Procedures%20%20-%20March%202014%20Revision%20%28Updated%2010-2-2014%29.pdf.
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    An acute REL for 1-hour exposure durations is typically lower than
its corresponding AEGL-1 and ERPG-1. Even though their definitions are
slightly different, AEGL-1s are often the same as the corresponding
ERPG-1s, and AEGL-2s are often equal to ERPG-2s. The maximum HQs from
our acute inhalation screening risk assessment typically result when we
use the acute REL for a HAP. In cases where the maximum acute HQ
exceeds 1, we also report the HQ based on the next highest acute dose-
response value (usually the AEGL-1 and/or the ERPG-1).
    For this source category, we used a default acute emissions
multiplier of 10 as hourly emissions data from site remediation
activities were generally not available.
    In our acute inhalation screening risk assessment, acute impacts
are deemed negligible for HAP for which acute HQs are less than or
equal to 1, and no further analysis is performed for these HAP. In
cases where an acute HQ from the screening step is greater than 1, we
assess the site-specific data to ensure the acute HQ is an off-site
location.
4. How do we conduct the multipathway exposure and risk screening
assessment?
    The EPA conducts a tiered screening assessment examining the
potential for significant human health risks due to exposures via
routes other than inhalation (i.e., ingestion). We first determine
whether any sources in the source category emit any PB-HAP, as
identified in the EPA's Air Toxics Risk Assessment Library (see Volume
1, Appendix D, at https://www.epa.gov/sites/production/files/2013-08/documents/volume_1_reflibrary.pdf).
    For the Site Remediation source category, we identified PB-HAP
emissions of arsenic compounds, cadmium compounds, mercury compounds,
polycyclic organic matter (POM), and lead compounds, so we proceeded to
the next step of the evaluation. In this step, we determine whether the
facility-specific emission rates of the emitted PB-HAP are large enough
to create the potential for significant human health risk through
ingestion exposure under reasonable worst-case conditions. To
facilitate this step, we use previously developed screening threshold
emission rates for several PB-HAP that are based on a hypothetical
upper-end screening exposure scenario developed for use in conjunction
with the EPA's Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure (TRIM.FaTE) model. The PB-HAP with screening
threshold emission rates are arsenic compounds, cadmium compounds,
chlorinated dibenzodioxins and furans, mercury compounds, and POM.
Based on the EPA estimates of toxicity and bioaccumulation potential,
the pollutants above represent a conservative list for inclusion in
multipathway risk assessments for RTR rules. (See Volume 1, Appendix D
at https://www.epa.gov/sites/production/files/201308/documents/volume_1_reflibrary.pdf.) In this assessment, we compare the facility-
specific emission rates of these PB-HAP to the screening threshold
emission rates for each PB-HAP to assess the potential for significant
human health risks via the ingestion pathway (combined ingestion rates
for a fisher and farmer scenario). We call this application of the
TRIM.FaTE model the Tier 1 screening assessment. The ratio of a
facility's actual emission rate to the Tier 1 screening threshold
emission rate is a ``screening value.''

    We derive the Tier 1 screening threshold emission rates for these
PB-HAP (other than lead compounds) to correspond to a maximum excess
lifetime cancer risk of 1-in-1 million (i.e., for arsenic compounds,
polychlorinated dibenzodioxins and furans and POM) or, for HAP that
cause noncancer health effects (i.e., cadmium compounds and mercury
compounds), a maximum HQ of 1. If the emission rate of any one PB-HAP
or combination of carcinogenic PB-HAP in the Tier 1 screening
assessment exceeds the Tier 1 screening threshold emission rate for any
facility (i.e., the screening value is greater than 1), we conduct a
second screening assessment, which we call the Tier 2 screening
assessment (ingestion rates are decoupled into separate upper-bound
ingestion rates for the fisher, farmer, and gardener scenarios).
    In the Tier 2 screening assessment, the location of each facility
that exceeds a Tier 1 screening threshold emission rate is used to
refine the assumptions associated with the Tier 1 fisher and farmer
exposure scenarios at that facility. A key assumption in the Tier 1
screening assessment is that a lake and/or farm is located near the
facility. As part of the Tier 2 screening assessment for the fisher
scenario, we use a U.S. Geological Survey (USGS) database to identify
actual waterbodies within 50 km of each facility and assume the fisher
only consumes fish from lakes within that 50 km zone. For the Tier 2
farmer scenario, we assume the farmer consumes meat, eggs, vegetables,
and fruit grown near the facility. If further Tier 2 screening is
necessary for the farmer scenario, we may also assess the gardener
scenario. For the gardener scenario, we assume the gardener only grows
and consumes eggs, vegetables, and fruit at the same ingestion rate as
the farmer. For Tier 2, we replace the meteorology used in the Tier 1
screening assessment with the local meteorology near each facility. We
then adjust the previously-developed Tier 1 screening threshold
emission rates for each PB-HAP for each facility based on an
understanding of how exposure concentrations estimated for the
screening scenario change with the use of local meteorology and USGS
waterbody data. If the PB-HAP emission rates for a facility exceed the
Tier 2 screening threshold emission rates and data are available, we
may conduct a Tier 3 screening assessment, or if the screening values
are excessively high, go straight to a site-specific assessment
utilizing TRIM FaTE. If PB-HAP emission rates do not exceed a Tier 2
screening value of 1, we consider those PB-HAP emissions to pose risks
below a level of concern.
    There are several analyses that can be included in a Tier 3
screening assessment, depending upon the extent of refinement
warranted, including validating that the lakes are fishable,
considering plume-rise to estimate emissions lost above the mixing
layer, and considering hourly effects of meteorology and plume rise on
chemical fate and transport. If the Tier 3 screening assessment
indicates that risks above levels of concern cannot be ruled out, the
EPA may further refine the screening assessment through a site-specific
assessment.
    For further information on the multipathway assessment approach,
see the Residual Risk Assessment for the Site Remediation Source
Category in Support of the Risk and Technology Review 2019 Proposed
Rule, which is available in the docket for this action.
5. How do we assess risks considering emissions control options?
    In addition to assessing baseline inhalation risks and screening
for potential multipathway risks, we also estimate risks considering
the potential emission reductions that would be achieved by the control
options under consideration. In these cases, the expected emission
reductions are applied to the specific HAP and emission points in the
RTR emissions dataset to develop corresponding estimates of risk and
incremental risk reductions.
6. How do we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect, Environmental HAP, and Ecological
Benchmarks
    The EPA conducts a screening assessment to examine the potential
for an adverse environmental effect as required under section
112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA defines ``adverse
environmental effect'' as ``any significant and widespread adverse
effect, which may reasonably be anticipated, to wildlife, aquatic life,
or other natural resources, including adverse impacts on populations of
endangered or threatened species or significant degradation of
environmental quality over broad areas.''
    The EPA focuses on eight HAP, which are referred to as
``environmental HAP,'' in its screening assessment: Six PB-HAP and two
acid gases. The PB-HAP included in the screening assessment are arsenic
compounds, cadmium compounds, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), and lead compounds. The acid
gases included in the screening assessment are hydrochloric acid (HCl)
and hydrogen fluoride (HF).
    HAP that persist and bioaccumulate are of particular environmental
concern because they accumulate in the soil, sediment, and water. The
acid gases, HCl and HF, are included due to their well-documented
potential to cause direct damage to terrestrial plants. In the
environmental risk screening assessment, we evaluate the following four
exposure media: Terrestrial soils, surface water bodies (includes
water-column and benthic sediments), fish consumed by wildlife, and
air. Within these four exposure media, we evaluate nine ecological
assessment endpoints, which are defined by the ecological entity and
its attributes. For PB-HAP (other than lead), both community-level and
population-level endpoints are included. For acid gases, the ecological
assessment evaluated is terrestrial plant communities.
    An ecological benchmark represents a concentration of HAP that has
been linked to a particular environmental effect level. For each
environmental HAP, we identified the available ecological benchmarks
for each assessment endpoint. We identified, where possible, ecological
benchmarks at the following effect levels: Probable effect levels,
lowest-observed-adverse-effect level, and no-observed-adverse-effect
level. In cases where multiple effect levels were available for a
particular PB-HAP and assessment endpoint, we use all of the available
effect levels to help us to determine whether ecological risks exist
and, if so, whether the risks could be considered significant and
widespread.
    For further information on how the environmental risk screening
assessment was conducted, including a discussion of the risk metrics
used, how the environmental HAP were identified, and how the ecological
benchmarks were selected, see Appendix 9 of the Residual Risk
Assessment for the Site Remediation Source Category in Support of the
Risk and Technology Review 2019 Proposed Rule, which is available in
the docket for this action.
b. Environmental Risk Screening Methodology
    For the environmental risk screening assessment, the EPA first
determined whether any facilities in the Site Remediation source
category emitted any of the environmental HAP. For the Site Remediation
source category, we identified emissions of arsenic

compounds, cadmium compounds, mercury compounds, POM, HCl, and
hydrofluoric acid. Because one or more of the environmental HAP
evaluated (arsenic compounds, cadmium compounds, mercury compounds,
POM, lead compounds, and HCl, and hydrofluoric acid) are emitted by at
least one facility in the source category, we proceeded to the second
step of the evaluation.\16\
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    \16\ The environmental HAP emitted by facilities modeled were
not attributed to Site Remediation source category emissions, but
rather were emitted from other emission points at the facility.
These pollutants were profiled as part of model plant emissions
because the facility otherwise emits environmental HAP.
---------------------------------------------------------------------------

c. PB-HAP Methodology
    The environmental screening assessment includes six PB-HAP, arsenic
compounds, cadmium compounds, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), and lead compounds. With the
exception of lead, the environmental risk screening assessment for PB-
HAP consists of three tiers. The first tier of the environmental risk
screening assessment uses the same health-protective conceptual model
that is used for the Tier 1 human health screening assessment.
TRIM.FaTE model simulations were used to back-calculate Tier 1
screening threshold emission rates. The screening threshold emission
rates represent the emission rate in tons of pollutant per year that
results in media concentrations at the facility that equal the relevant
ecological benchmark. To assess emissions from each facility in the
category, the reported emission rate for each PB-HAP was compared to
the Tier 1 screening threshold emission rate for that PB-HAP for each
assessment endpoint and effect level. If emissions from a facility do
not exceed the Tier 1 screening threshold emission rate, the facility
``passes'' the screening assessment, and, therefore, is not evaluated
further under the screening approach. If emissions from a facility
exceed the Tier 1 screening threshold emission rate, we evaluate the
facility further in Tier 2.
    In Tier 2 of the environmental screening assessment, the screening
threshold emission rates are adjusted to account for local meteorology
and the actual location of lakes in the vicinity of facilities that did
not pass the Tier 1 screening assessment. For soils, we evaluate the
average soil concentration for all soil parcels within a 7.5-km radius
for each facility and PB-HAP. For the water, sediment, and fish tissue
concentrations, the highest value for each facility for each pollutant
is used. If emission concentrations from a facility do not exceed the
Tier 2 screening threshold emission rate, the facility ``passes'' the
screening assessment and typically is not evaluated further. If
emissions from a facility exceed the Tier 2 screening threshold
emission rate, we evaluate the facility further in Tier 3.
    As in the multipathway human health risk assessment, in Tier 3 of
the environmental screening assessment, we examine the suitability of
the lakes around the facilities to support life and remove those that
are not suitable (e.g., lakes that have been filled in or are
industrial ponds), adjust emissions for plume-rise, and conduct hour-
by-hour time-series assessments. If these Tier 3 adjustments to the
screening threshold emission rates still indicate the potential for an
adverse environmental effect (i.e., facility emission rate exceeds the
screening threshold emission rate), we may elect to conduct a more
refined assessment using more site-specific information. If, after
additional refinement, the facility emission rate still exceeds the
screening threshold emission rate, the facility may have the potential
to cause an adverse environmental effect.
    To evaluate the potential for an adverse environmental effect from
lead, we compared the average modeled air concentrations (from HEM-3)
of lead around each facility in the source category to the level of the
secondary National Ambient Air Quality Standards (NAAQS) for lead. The
secondary lead NAAQS is a reasonable means of evaluating environmental
risk because it is set to provide substantial protection against
adverse welfare effects which can include ``effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and on
personal comfort and well-being.''
d. Acid Gas Environmental Risk Methodology
    The environmental screening assessment for acid gases evaluates the
potential phytotoxicity and reduced productivity of plants due to
chronic exposure to HF and HCl. The environmental risk screening
methodology for acid gases is a single-tier screening assessment that
compares modeled ambient air concentrations (from AERMOD) to the
ecological benchmarks for each acid gas. To identify a potential
adverse environmental effect (as defined in section 112(a)(7) of the
CAA) from emissions of HF and HCl, we evaluate the following metrics:
the size of the modeled area around each facility that exceeds the
ecological benchmark for each acid gas, in acres and km\2\; the
percentage of the modeled area around each facility that exceeds the
ecological benchmark for each acid gas; and the area-weighted average
screening value around each facility (Calculated by dividing the area-
weighted average concentration over the 50-km modeling domain by the
ecological benchmark for each acid gas). For further information on the
environmental screening assessment approach, see Appendix 9 of the
Residual Risk Assessment for the Site Remediation Source Category in
Support of the Risk and Technology Review 2019 Proposed Rule, which is
available in the docket for this action.
7. How do we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine the HAP emissions not only from the
source category emission points of interest, but also emissions of HAP
from all other emission sources at the facility for which we have data.
    For this source category, we conducted the facility-wide assessment
using a dataset that the EPA compiled from the 2014 NEI. We used the
NEI data for the facility and did not adjust any category or ``non-
category'' data. Therefore, there could be differences in the dataset
from that used for the source category assessments described in this
preamble. We analyzed risks due to the inhalation of HAP that are
emitted ``facility-wide'' for the populations residing within 50 km of
each facility, consistent with the methods used for the source category
analysis described above. For these facility-wide risk analyses, we
made a reasonable attempt to identify the source category risks, and
these risks were compared to the facility-wide risks to determine the
portion of facility-wide risks that could be attributed to the source
category addressed in this proposal. We also specifically examined the
facility that was associated with the highest estimate of risk and
determined the percentage of that risk attributable to the source
category of interest. The Residual Risk Assessment for the Site
Remediation Source Category in Support of the Risk and Technology
Review 2019 Proposed Rule, available through the docket for this
action, provides the methodology and results of the facility-wide
analyses,

including all facility-wide risks and the percentage of source category
contribution to facility-wide risks.
8. How do we consider uncertainties in risk assessment?
    Uncertainty and the potential for bias are inherent in all risk
assessments, including those performed for this proposal. Although
uncertainty exists, we believe that our approach, which used
conservative tools and assumptions, ensures that our decisions are
health and environmentally protective. A brief discussion of the
uncertainties in the RTR emissions dataset, dispersion modeling,
inhalation exposure estimates, and dose-response relationships follows
below. Also included are those uncertainties specific to our acute
screening assessments, multipathway screening assessments, and our
environmental risk screening assessments. A more thorough discussion of
these uncertainties is included in the Residual Risk Assessment for the
Site Remediation Source Category in Support of the Risk and Technology
Review 2019 Proposed Rule, which is available in the docket for this
action. If a multipathway site-specific assessment was performed for
this source category, a full discussion of the uncertainties associated
with that assessment can be found in Appendix 11 of that document,
Site-Specific Human Health Multipathway Residual Risk Assessment
Report.
a. Uncertainties in the RTR Emissions Dataset
    Although the development of the RTR emissions dataset involved
quality assurance/quality control processes, the accuracy of emissions
values will vary depending on the source of the data, the degree to
which data are incomplete or missing, the degree to which assumptions
made to complete the datasets are accurate, errors in emission
estimates, and other factors. The emission estimates considered in this
analysis generally are annual totals for certain years, and they do not
reflect short-term fluctuations during the course of a year or
variations from year to year. The estimates of peak hourly emission
rates for the acute effects screening assessment were based on an
emission adjustment factor applied to the average annual hourly
emission rates, which are intended to account for emission fluctuations
due to normal facility operations.
b. Uncertainties in Dispersion Modeling
    We recognize there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, AERMOD. In using a model to estimate
ambient pollutant concentrations, the user chooses certain options to
apply. For RTR assessments, we select some model options that have the
potential to overestimate ambient air concentrations (e.g., not
including plume depletion or pollutant transformation). We select other
model options that have the potential to underestimate ambient impacts
(e.g., not including building downwash). Other options that we select
have the potential to either under- or overestimate ambient levels
(e.g., meteorology and receptor locations). On balance, considering the
directional nature of the uncertainties commonly present in ambient
concentrations estimated by dispersion models, the approach we apply in
the RTR assessments should yield unbiased estimates of ambient HAP
concentrations. We also note that the selection of meteorology dataset
location could have an impact on the risk estimates. As we continue to
update and expand our library of meteorological station data used in
our risk assessments, we expect to reduce this variability.
c. Uncertainties in Inhalation Exposure Assessment
    Although every effort is made to identify all of the relevant
facilities and emission points, as well as to develop accurate
estimates of the annual emission rates for all relevant HAP, the
uncertainties in our emission inventory likely dominate the
uncertainties in the exposure assessment. Some uncertainties in our
exposure assessment include human mobility, using the centroid of each
census block, assuming lifetime exposure, and assuming only outdoor
exposures. For most of these factors, there is neither an under nor
overestimate when looking at the maximum individual risk or the
incidence, but the shape of the distribution of risks may be affected.
With respect to outdoor exposures, actual exposures may not be as high
if people spend time indoors, especially for very reactive pollutants
or larger particles. For all factors, we reduce uncertainty when
possible. For example, with respect to census-block centroids, we
analyze large blocks using aerial imagery and adjust locations of the
block centroids to better represent the population in the blocks. We
also add additional receptor locations where the population of a block
is not well represented by a single location.
d. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and noncancer effects from both chronic and acute
exposures. Some uncertainties are generally expressed quantitatively,
and others are generally expressed in qualitative terms. We note, as a
preface to this discussion, a point on dose-response uncertainty that
is stated in the EPA's 2005 Guidelines for Carcinogen Risk Assessment;
namely, that ``the primary goal of EPA actions is protection of human
health; accordingly, as an Agency policy, risk assessment procedures,
including default options that are used in the absence of scientific
data to the contrary, should be health protective'' (the EPA's 2005
Guidelines for Carcinogen Risk Assessment, page 1-7). This is the
approach followed here as summarized in the next paragraphs.
    Cancer UREs used in our risk assessments are those that have been
developed to generally provide an upper bound estimate of risk.\17\
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit). In some circumstances, the true risk could be as low
as zero; however, in other circumstances the risk could be greater.\18\
Chronic noncancer RfC and reference dose (RfD) values represent chronic
exposure levels that are intended to be health-protective levels. To
derive dose-response values that are intended to be ``without
appreciable risk,'' the methodology relies upon an uncertainty factor
(UF) approach,\19\ which considers uncertainty, variability, and gaps
in the available data. The UFs are applied to derive dose-response
values that are intended to protect against appreciable risk of
deleterious effects.
---------------------------------------------------------------------------

    \17\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \18\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
    \19\ See A Review of the Reference Dose and Reference
Concentration Processes, U.S. EPA, December 2002, and Methods for
Derivation of Inhalation Reference Concentrations and Application of
Inhalation Dosimetry, U.S. EPA, 1994.
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    Many of the UFs used to account for variability and uncertainty in
the development of acute dose-response values are quite similar to
those

developed for chronic durations. Additional adjustments are often
applied to account for uncertainty in extrapolation from observations
at one exposure duration (e.g., 4 hours) to derive an acute dose-
response value at another exposure duration (e.g., 1 hour). Not all
acute dose-response values are developed for the same purpose, and care
must be taken when interpreting the results of an acute assessment of
human health effects relative to the dose-response value or values
being exceeded. Where relevant to the estimated exposures, the lack of
acute dose-response values at different levels of severity should be
factored into the risk characterization as potential uncertainties.
    Uncertainty also exists in the selection of ecological benchmarks
for the environmental risk screening assessment. We established a
hierarchy of preferred benchmark sources to allow selection of
benchmarks for each environmental HAP at each ecological assessment
endpoint. We searched for benchmarks for three effect levels (i.e., no-
effects level, threshold-effect level, and probable effect level), but
not all combinations of ecological assessment/environmental HAP had
benchmarks for all three effect levels. Where multiple effect levels
were available for a particular HAP and assessment endpoint, we used
all of the available effect levels to help us determine whether risk
exists and whether the risk could be considered significant and
widespread.
    Although we make every effort to identify appropriate human health
effect dose-response values for all pollutants emitted by the sources
in this risk assessment, some HAP emitted by this source category are
lacking dose-response assessments. Accordingly, these pollutants cannot
be included in the quantitative risk assessment, which could result in
quantitative estimates understating HAP risk. To help to alleviate this
potential underestimate, where we conclude similarity with a HAP for
which a dose-response value is available, we use that value as a
surrogate for the assessment of the HAP for which no value is
available. To the extent use of surrogates indicates appreciable risk,
we may identify a need to increase priority for an IRIS assessment for
that substance. We additionally note that, generally speaking, HAP of
greatest concern due to environmental exposures and hazard are those
for which dose-response assessments have been performed, reducing the
likelihood of understating risk. Further, HAP not included in the
quantitative assessment are assessed qualitatively and considered in
the risk characterization that informs the risk management decisions,
including consideration of HAP reductions achieved by various control
options.
    For a group of compounds that are unspeciated (e.g., glycol
ethers), we conservatively use the most protective dose-response value
of an individual compound in that group to estimate risk. Similarly,
for an individual compound in a group (e.g., ethylene glycol diethyl
ether) that does not have a specified dose-response value, we also
apply the most protective dose-response value from the other compounds
in the group to estimate risk.
e. Uncertainties in Acute Inhalation Screening Assessments
    In addition to the uncertainties highlighted above, there are
several factors specific to the acute exposure assessment that the EPA
conducts as part of the risk review under section 112 of the CAA. The
accuracy of an acute inhalation exposure assessment depends on the
simultaneous occurrence of independent factors that may vary greatly,
such as hourly emissions rates, meteorology, and the presence of a
person. In the acute screening assessment that we conduct under the RTR
program, we assume that peak emissions from the source category and
reasonable worst-case air dispersion conditions (i.e., 99th percentile)
co-occur. We then include the additional assumption that a person is
located at this point at the same time. For this source category,
together, these assumptions represent a reasonable worst-case exposure
scenario. In most cases, it is unlikely that a person would be located
at the point of maximum exposure during the time when peak emissions
and reasonable worst-case air dispersion conditions occur
simultaneously.
f. Uncertainties in the Multipathway and Environmental Risk Screening
Assessments
    For each source category, we generally rely on site-specific levels
of PB-HAP or environmental HAP emissions to determine whether a refined
assessment of the impacts from multipathway exposures is necessary or
whether it is necessary to perform an environmental screening
assessment. This determination is based on the results of a three-
tiered screening assessment that relies on the outputs from models--
TRIM.FaTE and AERMOD--that estimate environmental pollutant
concentrations and human exposures for five PB-HAP (dioxins, POM,
mercury, cadmium, and arsenic) and two acid gases (HF and hydrogen
chloride). For lead, we use AERMOD to determine ambient air
concentrations, which are then compared to the secondary NAAQS standard
for lead. Two important types of uncertainty associated with the use of
these models in RTR risk assessments and inherent to any assessment
that relies on environmental modeling are model uncertainty and input
uncertainty.\20\
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    \20\ In the context of this discussion, the term ``uncertainty''
as it pertains to exposure and risk encompasses both variability in
the range of expected inputs and screening results due to existing
spatial, temporal, and other factors, as well as uncertainty in
being able to accurately estimate the true result.
---------------------------------------------------------------------------

    Model uncertainty concerns whether the model adequately represents
the actual processes (e.g., movement and accumulation) that might occur
in the environment. For example, does the model adequately describe the
movement of a pollutant through the soil? This type of uncertainty is
difficult to quantify. However, based on feedback received from
previous EPA SAB reviews and other reviews, we are confident that the
models used in the screening assessments are appropriate and state-of-
the-art for the multipathway and environmental screening risk
assessments conducted in support of RTR.
    Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
1 of the multipathway and environmental screening assessments, we
configured the models to avoid underestimating exposure and risk. This
was accomplished by selecting upper-end values from nationally
representative datasets for the more influential parameters in the
environmental model, including selection and spatial configuration of
the area of interest, lake location and size, meteorology, surface
water, soil characteristics, and structure of the aquatic food web. We
also assume an ingestion exposure scenario and values for human
exposure factors that represent reasonable maximum exposures.
    In Tier 2 of the multipathway and environmental screening
assessments, we refine the model inputs to account for meteorological
patterns in the vicinity of the facility versus using upper-end
national values, and we identify the actual location of lakes near the
facility rather than the default lake location that we apply in Tier 1.
By refining the screening approach in Tier 2 to account for local
geographical and meteorological data, we decrease the likelihood that
concentrations in

environmental media are overestimated, thereby increasing the
usefulness of the screening assessment. In Tier 3 of the screening
assessments, we refine the model inputs again to account for hour-by-
hour plume rise and the height of the mixing layer. We can also use
those hour-by-hour meteorological data in a TRIM.FaTE run using the
screening configuration corresponding to the lake location. These
refinements produce a more accurate estimate of chemical concentrations
in the media of interest, thereby reducing the uncertainty with those
estimates. The assumptions and the associated uncertainties regarding
the selected ingestion exposure scenario are the same for all three
tiers.
    For the environmental screening assessment for acid gases, we
employ a single-tiered approach. We use the modeled air concentrations
and compare those with ecological benchmarks.
    For all tiers of the multipathway and environmental screening
assessments, our approach to addressing model input uncertainty is
generally cautious. We choose model inputs from the upper end of the
range of possible values for the influential parameters used in the
models, and we assume that the exposed individual exhibits ingestion
behavior that would lead to a high total exposure. This approach
reduces the likelihood of not identifying high risks for adverse
impacts.
    Despite the uncertainties, when individual pollutants or facilities
do not exceed screening threshold emission rates (i.e., screen out), we
are confident that the potential for adverse multipathway impacts on
human health is very low. On the other hand, when individual pollutants
or facilities do exceed screening threshold emission rates, it does not
mean that impacts are significant, only that we cannot rule out that
possibility and that a refined assessment for the site might be
necessary to obtain a more accurate risk characterization for the
source category.
    The EPA evaluates the following HAP in the multipathway and/or
environmental risk screening assessments, where applicable: Arsenic,
cadmium, dioxins/furans, lead, mercury (both inorganic and methyl
mercury), POM, HCl, and HF. These HAP represent pollutants that can
cause adverse impacts either through direct exposure to HAP in the air
or through exposure to HAP that are deposited from the air onto soils
and surface waters and then through the environment into the food web.
These HAP represent those HAP for which we can conduct a meaningful
multipathway or environmental screening risk assessment. For other HAP
not included in our screening assessments, the model has not been
parameterized such that it can be used for that purpose. In some cases,
depending on the HAP, we may not have appropriate multipathway models
that allow us to predict the concentration of that pollutant. The EPA
acknowledges that other HAP beyond these that we are evaluating may
have the potential to cause adverse effects and, therefore, the EPA may
evaluate other relevant HAP in the future, as modeling science and
resources allow.

IV. Analytical Results and Proposed Decisions

A. What actions are we taking pursuant to CAA sections 112(d)(2) and
112(d)(3)?

    To ensure that CAA section 112 standards apply at all times, the
EPA is proposing to add provisions for pressure relief device (PRD)
releases and for bypass lines on closed vent systems. The results and
proposed decisions based on the analyses performed pursuant to CAA
section 112(d)(2) and (3) are presented below.
    The acronym ``PRD'' means pressure relief device and is common
vernacular to describe the variety of devices regulated as PRDs or
valves (see the end of this section for our proposed addition of the
definition for ``pressure relief device'' or ``valve,'' to provide
clarity). PRDs are designed to remain closed during normal operation,
but they may ``actuate'' (e.g., the valve seat opens or a rupture disk
ruptures) in the event of an overpressure in the system caused by
operator error, a malfunction such as a power failure or equipment
failure, or other unexpected cause that results in immediate venting of
gas from process equipment in order to avoid safety hazards or
equipment damage. For the Site Remediation source category, emissions
vented directly to the atmosphere from a PRD actuation in remediation
material service may contain HAP that would have been subject to
control under the Site Remediation NESHAP, if the PRD actuation had not
occurred (e.g., through a process vent standard). However, the EPA
recognizes that the characteristics of a release from a PRD may be
different from HAP emission generated from remediation processes under
typical operating conditions (i.e., non malfunction) and which are
routed through a process vent.
    The Site Remediation NESHAP currently regulates fugitive emissions
from PRDs, when they are seated, through the equipment leak provisions.
The equipment leak provisions also require that the PRD be returned to
a condition of no detectable emissions, after a pressure release;
however, these equipment leak provisions do not establish a standard
for emissions releases from a PRD when the PRD actuates. In addition,
the current Site Remediation NESHAP follows the EPA's previous practice
of exempting SSM events from otherwise applicable emission standards.
Consequently, with emissions releases from a PRD release actuation
event treated as a type of malfunction, the Site Remediation NESHAP did
not restrict emissions releases from a PRD actuation event to the
atmosphere (i.e., they were exempt from the otherwise applicable
emission standards). In Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir.
2008), the Court determined SSM exemptions in section 112 standards
violate the CAA. Section IV.E.2 of this preamble contains additional
discussions on the removal of the SSM exemptions for this source
category.
    We evaluated the Site Remediation NESHAP provisions for PRDs to
ensure a standard continuously applies during malfunctions that result
in an emissions release from a PRD actuation event, consistent with the
Sierra Club v. EPA decision. First, we determined that emissions
releases from PRD actuation events that vent to a closed vent system
and control device are appropriately regulated. We are proposing at 40
CFR 63.7923 that emissions releases from a PRD actuation event routed
through a closed vent system to a control device or to a process, fuel
gas system, or drain system must meet the requirements at 40 CFR
63.7925 to 40 CFR 63.7928 for the applicable control system.
    Second, the EPA determined that emissions from PRD actuation events
that vent directly to the atmosphere as the result of a malfunction may
not meet an applicable emission standard for this source category.
Therefore, we examined whether it would be feasible to establish a
numeric emission standard for emissions releases from PRD actuation
events that vent directly to the atmosphere.
    As detailed here, we determined it is not feasible to regulate
emissions from PRD actuation events through a numeric emission
standard, and, therefore, it is more appropriate to regulate emissions
releases from PRD events that vent to the atmosphere through work
practice standards under CAA section 112(h), established consistent
with CAA section 112(d)(2) and (3). The EPA is proposing work practice
standards at 40 CFR 63.7923 that are intended to reduce the number of
emissions releases from PRD actuation events and will incentivize

owners or operators to eliminate the causes of emissions releases from
PRD actuation events that vent directly to the atmosphere.
    When the EPA initially promulgated the Site Remediation NESHAP, it
did not consider malfunction events when establishing emissions
standards for the various emissions sources at site remediation
facilities. In undertaking that consideration now, we propose that it
is not feasible to regulate emissions releases from PRD actuation
events that vent to the atmosphere using numeric emission limits due to
technological and economic limitations that make it impracticable to
measure emissions from PRDs which have actuated. CAA section 112(h)(1)
states that the EPA may prescribe a work practice standard or other
requirement, consistent with the provisions of CAA sections 112(d) or
(f), in those cases where, in the judgment of the Administrator, it is
not feasible to enforce an emission standard. CAA section 112(h)(2)(B)
further defines the term ``not feasible'' in this context as meaning
that ``the application of measurement technology to a particular class
of sources is not practicable due to technological and economic
limitations.'' We consider it appropriate to establish a work practice
standard for emissions releases from PRD actuation events that vent to
the atmosphere as provided in CAA section 112(h), because the
application of a measurement methodology for emissions releases from
PRD actuation events that vent directly to the atmosphere is not
practicable due to technological and economic limitations. As discussed
previously, PRDs are designed to remain closed during normal operations
and release emissions only during nonroutine and unplanned events, and
the venting time can be very short and may vary widely in emissions
composition and flow rate.
    Additionally, it would be economically prohibitive to construct an
appropriate conveyance and install and operate continuous monitoring
systems for each individual PRD that vents directly to the atmosphere
in order to attempt to quantitatively measure an actuation release
event that may occur infrequently. See U.S. v. Sugar Corp., 830 F.3d
579, 664-67 (D.C. Cir. 2016). Further, we have not identified any
available, technically feasible continuous emission monitoring system
that can accurately determine a mass release quantity of HAP given the
flow, composition, and compositional variability of potential PRD
releases that vent directly to the atmosphere from remediation units.
Rather, we have identified only monitoring systems capable of alerting
an owner or operator when an emissions release from a PRD actuation
event occurs. Consequently, we propose that it is appropriate to
establish a work practice standard for emissions releases from PRD
actuation event that vent directly to the atmosphere as provided in CAA
section 112(h).
    We next reviewed information about site remediation facilities to
determine how the best performers are minimizing emissions releases
from PRD actuation events that vent directly to the atmosphere. A
review of the title V operating permits for facilities subject to the
Site Remediation NESHAP indicated that many facilities are subject to
the Chemical Accident Prevention Provisions (CAP) rule (40 CFR 68.215
requires permits to list 40 CFR part 68 as an applicable requirement,
if subject) for at least some portion of the facility. As a result, we
further reviewed this rule for consideration in developing a PRD work
practice standard.
    The CAP rule requires facilities to develop a Risk Management Plan
that includes a hazard assessment, an accident prevention program and
an emergency response program. The CAP rule includes three program
levels which dictate the requirements for the hazard assessment,
accident prevention program and emergency response program based on the
types of chemicals and processes used at a facility. If the
applicability of the CAP rule extends to site remediation affected
facilities, the facilities would fall under either prevention program
level 1 or 3 (depending on a facility's NAICS code). We evaluated
program 3, which is more stringent, because based on a review of the
rule's applicability requirements and preamble rationale, it is our
understanding that site remediation facilities may not be subject to
the program 1 criteria. We also chose to evaluate program 3 because if
any facility is subject to program 3 and the Site Remediation NESHAP,
those sources would be the best performers in the source category,
requisite for a MACT determination. The program 3 prevention program
includes: Documentation of process safety information, conducting a
hazard analysis, documentation of operating procedures, employee
training, on-going maintenance, and incident investigations. The
process safety information documented must include information
pertaining to the hazards of the regulated substances in the process,
the technology of the process, and the process equipment (including
relief valves). When conducting the hazard analysis, facilities must
identify, evaluate, and control the hazards in the process. Facilities
that use controls may consider the application of detection
methodologies (e.g., process monitoring and control instrumentation) to
provide early warning of releases. The operating procedures must
address multiple operating scenarios (e.g., normal operations, startup,
emergency shutdown) and provide instructions for safely conducting
process activities. The acts of conducting the hazard analysis and
documenting operating procedures are similar to prevention measures,
discussed below, though we note a specific number of measures or
controls is not specified for the program 3 prevention program.
Incident investigations must document the factors that contributed to
an incident and any resolutions and corrective actions (incident
investigations are consistent with analysis of the cause of the release
and corrective measures, discussed below). Facilities are also required
to document this information in a Risk Management Plan that must be
updated at least every 5 years.
    Next, we considered that some companies operating site remediation
facilities also own and operate petroleum refineries or chemical
production facilities and may have established company-wide best
practices as a result of specific state and Federal requirements. For
example, petroleum refineries located in certain counties in California
are subject to and complying with specific requirements for PRDs such
as the Bay Area Air Quality Management District (BAAQMD) Rule 8-28-304
and South Coast Air Quality Management District (SCAQMD) Rule 1173.
These rules also formed the basis of the work practice standards
promulgated for emissions releases from PRD actuation events at
petroleum refineries in the recent Petroleum Refinery Sector RTR
performed by the EPA (80 FR 75178, December 1, 2015).
    Considering our review of the EPA's Chemical Accident Prevention
Provisions and company-wide best practices that site remediation
facilities may have implemented, we expect that the best performing
site remediation facilities have implemented a program for emissions
releases from PRD actuation events that vent directly to the atmosphere
that consists of conducting an analysis of the cause of the PRD
actuation event and the implementation of corrective measures. We used
this information as the basis of the work practice standards that we
are proposing at 40 CFR 63.7923.
    Specifically, we are proposing a limit on the number of emissions
releases

from PRD actuation events that if exceeded, would result in a violation
to the work practice standard for emissions releases for PRD actuation
events that vent directly to the atmosphere. We believe setting
criteria to determine a deviation is necessary for the work practice to
be effective. We considered limits on the number of emissions releases
from PRD actuation events over a 3-year period. Based on a Monte Carlo
analysis of random rare events (conducted for the Petroleum Refinery
Sector MACT), we note that a facility is likely to have two or three
events in an average 5-year period when a long time-horizon (e.g., 20
years) is considered. Therefore, we are proposing to limit the number
of emissions releases from a PRD actuation event from a single PRD to
either two or three (depending on the PRD release actuation event
cause) in a 3-year period as the basis of a deviation of the work
practice standard. We considered it reasonable to use a 3-year period
rather than a 5-year period given that company-wide best practices
forming the basis of the work practice standards promulgated for
emissions releases from PRD actuation events at petroleum refineries
are also our underlying basis for the proposed work practice standards
at site remediation facilities. We are proposing that it is a deviation
of the work practice standard if a single PRD that vents emissions from
an actuation event directly to the atmosphere has two releases within a
3-year period due to the same cause. We believe this provision will
help ensure that analyses and corrective actions are conducted
effectively. Otherwise, we are proposing that it is a deviation of the
work practice standard if a single PRD that vents emissions from an
actuation event directly to the atmosphere has three releases within a
3-year period for any reason. In addition, we are proposing that any
emissions release directly to the atmosphere from a PRD actuation event
for which the cause was determined to be operator error or poor
maintenance is a violation of the work practice standard. We are
proposing that ``force majeure'' events would not be included when
counting the number of releases. We are proposing to define ``Force
majeure'' as including events resulting from natural disasters, acts of
war or terrorism, or external power curtailment beyond the facility's
control. These types of events are beyond the control of the owner or
operator. We are providing that these events should not be included in
the event count, but that they would be subject to the PRD actuation
event cause analysis in order to confirm or determine whether the
release was due to a force majeure event.
    In addition, consistent with our treatment of site remediation
process vents (in general, an open PRD is essentially the same as a
site remediation process vent that is vented directly to the
atmosphere), we believe it is appropriate to exclude certain types of
PRDs that have very low potential to emit based on their type of
service, size, and/or pressure from the proposed work practice standard
for PRD releases that vent directly to the atmosphere. Both the CAP and
the California petroleum refinery PRD rules also exempt or impose
simpler requirements for certain PRDs. We are proposing at 40 CFR
63.7923 that the following types of PRDs would not be subject to the
work practice standard for PRDs that vent directly to the atmosphere:
(1) PRDs in heavy liquid service; (2) PRDs that are designed solely to
release due to liquid thermal expansion; and (3) pilot-operated and
balanced bellows PRDs if the primary release valve associated with the
PRD is vented through a control system. With regard to PRDs in heavy
liquid service and thermal relief valves, any release of HAP to the
atmosphere from a PRD in heavy liquid service would be expected to be
small. We are also proposing that pilot-operated PRDs (where emissions
from actuation events can be released to the atmosphere through a pilot
discharge vent) and balanced bellow PRDs (where emissions can be
released to the atmosphere through a bonnet vent) are not subject to
the work practice standard, if the primary release valve associated
with the PRD is vented through a control system. Due to its design,
which includes a bellows to shield the pressure relief stem and top
portion of the valve seat from the discharge vent pressure, a balanced
bellows PRD will not discharge gas to the atmosphere during a pressure