[Federal Register Volume 84, Number 71 (Friday, April 12, 2019)]
[Proposed Rules]
[Pages 15046-15077]
[FR Doc No: 2019-07024]

Vol. 84

Friday,

No. 71

April 12, 2019

Part II

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Stationary
Combustion Turbines Residual Risk and Technology Review; Proposed Rule

Federal Register / Vol. 84 , No. 71 / Friday, April 12, 2019 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2017-0688; FRL-9991-97-OAR]
RIN 2060-AT00

National Emission Standards for Hazardous Air Pollutants:
Stationary Combustion Turbines Residual Risk and Technology Review

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing
amendments to the National Emission Standards for Hazardous Air
Pollutants (NESHAP) for Stationary Combustion Turbines to address the
results of the residual risk and technology review (RTR) the EPA is
required to conduct in accordance with the Clean Air Act (CAA). The EPA
is proposing to find that the risks from this source category due to
emissions of air toxics are acceptable and that the existing NESHAP
provides an ample margin of safety to protect public health. The EPA
identified no new cost-effective controls under the technology review
that would achieve further emissions reductions from the source
category. The EPA is also proposing to amend provisions addressing
periods of startup, shutdown, and malfunction (SSM) and to require
electronic reporting. In addition, the EPA is proposing to remove the
stay of the effectiveness of the standards for new lean premix and
diffusion flame gas-fired turbines that was promulgated in 2004.

DATES:
    Comments. Comments must be received on or before May 28, 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 May 13, 2019.
    Public Hearing. If anyone contacts us requesting a public hearing
on or before April 17, 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/stationary-combustion-turbines-national-emission-standards. See SUPPLEMENTARY INFORMATION for
information on requesting and registering for a public hearing.

ADDRESSES: Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2017-0688, at https://www.regulations.gov. Follow the online
instructions for submitting comments. Once submitted, comments cannot
be edited or removed from Regulations.gov. See SUPPLEMENTARY
INFORMATION for detail about how the EPA treats submitted comments.
Regulations.gov is our preferred method of receiving comments. However,
the following other submission methods are also accepted:
     Email: [email protected]. Include Docket ID No. EPA-
HQ-OAR-2017-0688 in the subject line of the message.
     Fax:<PHONE_NUMBER>. Attention Docket ID No. EPA-HQ-OAR-
2017-0688.
     Mail: To ship or send mail via the United States Postal
Service, use the following address: U.S. Environmental Protection
Agency, EPA Docket Center, Docket ID No. EPA-HQ-OAR-2017-0688, Mail
Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.
     Hand/Courier Delivery: Use the following Docket Center
address if you are using express mail, commercial delivery, hand
delivery, or courier: EPA Docket Center, EPA WJC West Building, Room
3334, 1301 Constitution Avenue NW, Washington, DC 20004. Delivery
verification signatures will be available only during regular business
hours.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Melanie King, Sector Policies and Programs Division
(D243-01), 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 protected]. For specific information
regarding the risk modeling methodology, contact Mark Morris, 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>;
email address: [email protected]. For information about the
applicability of the NESHAP to a particular entity, contact Sara Ayres,
Office of Enforcement and Compliance Assurance, U.S. Environmental
Protection Agency, 77 West Jackson Boulevard (Mail Code E-19J),
Chicago, Illinois 60604; telephone number:<PHONE_NUMBER>; and email
address: [email protected].

SUPPLEMENTARY INFORMATION:
    Public hearing. Please contact Adrian Gates at<PHONE_NUMBER> or by
email at [email protected] 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-2017-0688. All documents in the docket are
listed in Regulations.gov. Although listed, some information is not
publicly available, e.g., Confidential Business Information (CBI) or
other information whose disclosure is restricted by statute. Certain
other material, such as copyrighted material, is not placed on the
internet and will be publicly available only in hard copy. Publicly
available docket materials are available either electronically in
Regulations.gov or in hard copy at the EPA Docket Center, Room 3334,
EPA 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-
2017-0688. 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://www.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.
    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-2017-0688.
    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
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CFR Code of Federal Regulations
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guideline
ERT Electronic Reporting Tool
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
MACT maximum achievable control technology
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NATA National Air Toxics Assessment
NEI National Emissions Inventory
NESHAP national emission standards for hazardous air pollutants
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
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PDF portable document format
PM particulate matter
POM polycyclic organic matter
ppbvd parts per billion by volume, dry basis
ppm parts per million
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
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
[mu]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 and how does the current NESHAP
regulate its HAP emissions?
    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 are the results of the risk assessment and analyses?
    B. What are our proposed decisions regarding risk acceptability,
ample margin of safety, and adverse environmental effect?
    C. What are the results and proposed decisions based on our
technology review?
    D. What other actions are we proposing?
    E. 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 Comments
VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulations 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 categories that are 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 would be affected by this proposed action only if
they own or operate stationary combustion turbines at major sources of
hazardous air pollutants (HAP). 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), the Stationary Turbines source category is any
stationary combustion turbine used by electric and gas utilities,
industrial establishments, and commercial/institutional operations to
provide electricity, gas compression, or other functions. Included in
the category are turbines fired by fuel oil, natural gas, and mixed or
other fuel. The Stationary Turbine source category includes simple
cycle and regenerative cycle turbines and the turbine portion of a
combined cycle steam/electric generating system.

    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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Stationary Turbines.........  Stationary            2211, 486210,
                               Combustion Turbines.  211111, 211112,
                                                     221.
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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/stationary-combustion-turbines-national-emission-standards. 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/rtrpg.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-2017-0688).

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 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, which is available 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
Residual Risk Report to Congress 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 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.
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    \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.
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    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. Natural Resources
Defense Council (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 and how does the current NESHAP
regulate its HAP emissions?

    The source category for Stationary Combustion Turbines is all
equipment including, but not limited to, the turbine, the fuel, air,
lubrication and exhaust gas systems, control systems (except emissions
control equipment), and any ancillary components and subcomponents
comprising any simple cycle stationary combustion turbine, any
regenerative/recuperative cycle stationary combustion turbine, or the
combustion turbine portion of any stationary combined cycle steam/
electric generating system. Stationary means that the combustion
turbine is not self-propelled or intended to be propelled while
performing its function. A stationary combustion turbine may, however,
be mounted on a vehicle for portability or transportability. The source
category does not include stationary combustion turbines located at a
research or laboratory facility, if research is conducted on the
turbine itself and the turbine is not being used to power other
applications at the research or laboratory facility. This NESHAP, 40
CFR part 63, subpart YYYY, only applies to stationary combustion
turbines located at major sources of HAP.
    Stationary combustion turbines have been divided into the following
eight subcategories: (1) Emergency stationary combustion turbines, (2)
stationary combustion turbines which burn landfill or digester gas
equivalent to 10 percent or more of the gross heat input on an annual
basis or where gasified municipal solid waste is used to generate 10
percent or more of the gross heat input to the stationary combustion
turbine on an annual basis, (3) stationary combustion turbines of less
than 1 megawatt rated peak power output, (4) stationary lean premix
combustion turbines when firing gas and when firing oil at sites where
all turbines fire oil no more than an aggregate total of 1,000 hours
annually (also referred to herein as ``lean premix gas-fired
turbines''), (5) stationary lean premix combustion turbines when firing
oil at sites where all turbines fire oil more than an aggregate total
of 1,000 hours annually (also referred to herein as ``lean premix oil-
fired turbines''), (6) stationary diffusion flame combustion turbines
when firing gas and when firing oil at sites where all turbines fire
oil no more than an aggregate total of 1,000 hours annually (also
referred to herein as ``diffusion flame gas-fired turbines''), (7)
stationary diffusion flame combustion turbines when firing oil at sites
where all turbines fire oil more than an aggregate total of 1,000 hours
annually (also referred to herein as ``diffusion flame oil-fired
turbines''), and (8) stationary combustion turbines operated on the
North Slope of Alaska (defined as the area north of the Arctic Circle
(latitude 66.5[deg] North)).
    The sources of emissions are the exhaust gases from combustion of
gaseous and liquid fuels in a stationary combustion turbine. The HAP
that are present in the exhaust gases from stationary combustion
turbines include formaldehyde, toluene, benzene, and acetaldehyde.
Metallic HAP are present in the exhaust from distillate oil-fired
turbines; these metallic HAP are generally carried over from the fuel
constituents.
    The NESHAP requires new or reconstructed stationary combustion
turbines in the lean premix gas-fired, lean premix oil-fired, diffusion
flame gas-fired, and diffusion flame oil-fired subcategories to meet a
formaldehyde limit of 91 parts per billion by volume, dry basis (ppbvd)
at 15-percent oxygen (O2). Compliance is demonstrated
through initial and annual performance testing and continuous
monitoring of operating parameters.
    During the original Stationary Combustion Turbine NESHAP
rulemaking, the EPA received a petition from the Gas Turbine
Association to delist two subcategories of stationary combustion
turbines under CAA section 112(c)(9). The subcategories were lean
premix firing natural gas with limited oil backup and a low-risk
subcategory where facilities would make site-specific demonstrations
regarding risk levels. On April 7, 2004, the EPA proposed to delist
lean premix gas-fired turbines as well as three additional
subcategories: Diffusion flame gas-fired, emergency, and turbines
located on the North Slope of Alaska. At the same time, the EPA
proposed to stay the effectiveness of the NESHAP for new lean premix
gas-fired and diffusion flame gas-fired turbines. On August 18, 2004,
the EPA finalized the stay of the effectiveness of the NESHAP for new
lean premix gas-fired and diffusion flame gas-fired turbines, pending
the outcome of the proposed delisting. As discussed further in section
IV.D.3 of this preamble, the EPA is proposing to lift the stay as part
of this action.

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

    The EPA used several means to collect the information necessary to
conduct the RTR for the Stationary Combustion Turbine source category.
Where possible, the EPA used data from the 2014 National Emissions
Inventory (NEI) to estimate HAP emissions from affected facilities and
turbines. More information about the sources of data used to estimate
HAP emissions is provided in section III.C.1 of this preamble. The list
of facilities potentially subject to the NESHAP was initially developed
using the EPA's Enforcement and Compliance History Online database.\2\
To confirm whether facilities identified as potentially subject to the
NESHAP were in fact subject to the standards, the EPA asked state and
local air pollution control agencies and EPA Regional offices to review
our draft list of affected facilities and turbines and revise it as
necessary. The EPA also shared the draft list with a number of industry
trade groups, including the American Petroleum Institute, Interstate
Natural Gas Association of America, Council of Industrial Boiler
Owners, National Waste & Recycling Association, American Public Power
Association, National Rural Electric Cooperative Association, Utility
Air Regulatory Group, Edison Electric Institute, and American Chemistry
Council, and asked member companies to review and revise the list. The
EPA also posted the draft list on the EPA website for the Stationary
Combustion Turbine NESHAP so that other stakeholders could provide
input on the list. The EPA also reviewed air permits for each facility
to ensure the accuracy of our information. The facility-specific
information from state and local agencies and companies with affected
facilities provided support for this action's risk and technology
reviews. No formal information collection request was performed.
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    \2\ https://echo.epa.gov/.
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D. What other relevant background information and data are available?

    In order to determine whether there have been any developments in
practices, processes, or control technologies since promulgation of the
original NESHAP, the EPA reviewed several sources of information,
including the EPA's Reasonably Available Control Technology/Best
Available Control Technology/Lowest Achievable Emission Rate
Clearinghouse,\3\ construction and operating permits for stationary
combustion turbines, information provided by industry trade groups
representing owners and operators of stationary combustion turbines,
and manufacturers of emission control technologies and emission testing
equipment. Additional details of the technology review can be found in
the Technology Review for Stationary Combustion Turbines Risk and
Technology Review (RTR) memorandum, which is available in the docket
for this action. The EPA also reviewed the stationary combustion
turbine performance test data that were collected for the original
NESHAP rulemaking, as well as new HAP emissions data from tests of
stationary combustion turbines conducted in recent years that were
primarily provided by state and local air pollution control agencies.
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    \3\ https://cfpub.epa.gov/rblc/.
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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.\4\ 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:
---------------------------------------------------------------------------

    \4\ 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 to the HAP to the level at or below which no adverse
chronic noncancer effects are expected; the HI is the sum of HQs for
HAP that affect the same target organ or organ system.

[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 non-cancer 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.'' \5\
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    \5\ Recommendations of the SAB RTR 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.
---------------------------------------------------------------------------

    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 originally developed
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.A).
    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 seven 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 Stationary
Combustion Turbines Source Category in Support of the 2019 Risk and
Technology Review Proposed Rule (risk document). The methods used to
assess risk (as described in the seven 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; \6\ and described in the SAB review
report issued in 2010. They are also consistent with the key
recommendations contained in that report.
---------------------------------------------------------------------------

    \6\ 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, June 2009. EPA-452/R-09-006. https://www3.epa.gov/airtoxics/rrisk/rtrpg.html.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    For each stationary combustion turbine that was determined to be
subject to 40 CFR part 63, subpart YYYY, we gathered data for emissions
of particulate matter (PM), volatile organic compounds (VOC), and HAP
from Version 1 of the 2014 NEI. If a turbine had multiple processes
reported in NEI, the emissions associated with each process were summed
for a total emissions value for the turbine. The following HAP, which
account for 98-99 percent of the HAP emissions from turbines subject to
40 CFR part 63, subpart YYYY, regardless of fuel type, were modeled
with the available NEI data per the applicable fuel types.

                                                      Table 2--HAP Modeled for Residual Risk Review
--------------------------------------------------------------------------------------------------------------------------------------------------------
               HAP                      Natural gas           Distillate oil           Landfill gas              Jet fuel               Process gas
--------------------------------------------------------------------------------------------------------------------------------------------------------
Formaldehyde....................  Yes                     Yes                     Yes                     Yes                     Yes
Toluene.........................  Yes                     Yes                     Yes
Xylenes (Mixed Isomers).........  Yes                     Yes                     Yes
Acetaldehyde....................  Yes                     Yes
Ethylbenzene....................  Yes                     Yes                     Yes
Propylene Oxide.................  Yes                     Yes
Benzene.........................  Yes                     Yes                     Yes                     ......................  Yes
Hexane..........................  Yes                     Yes                     Yes
Hydrochloric Acid...............  Yes                     Yes                     Yes
Acrolein........................  Yes                     Yes
Manganese Compounds.............  ......................  Yes                     ......................  Yes
Nickel Compounds................  ......................  Yes                     ......................  Yes                     Yes
Lead Compounds..................  ......................  Yes                     ......................  Yes                     Yes
Arsenic Compounds...............  ......................  Yes                     ......................  ......................  Yes
Chromium Compounds..............  ......................  Yes                     ......................  ......................  Yes
Cadmium Compounds...............  ......................  Yes                     ......................  ......................  Yes
Mercury Compounds...............  ......................  Yes                     ......................  Yes
Selenium Compounds..............  ......................  Yes
Cobalt Compounds................  ......................  Yes
Beryllium Compounds.............  ......................  Yes                     ......................  Yes
Antimony Compounds..............  ......................  Yes
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Whenever possible, the 2014 NEI HAP emissions values were used for
each turbine unit included in the inputs for the residual risk modeling
documented in section III.C.3 of this preamble, hereafter referred to
as the modeling file. However, many of the turbine units used in the
modeling file either were not included in the 2014 NEI or did not have
reported emissions values for one or more of the expected HAP (see
Table 2). For units with emissions values that were missing, a three-
tiered approach was developed for filling in emissions. In Tier 1,
emissions were estimated using the NEI-reported VOC and/or PM of 10
micrometers or less (PM10) emission values and the developed
HAP emission factor speciation profiles per fuel type. For units that
did not have a NEI-reported VOC and/or PM10 value available,
or were not included in the 2014 NEI, the Tier 2 calculation
methodology was used to estimate HAP emissions. In Tier 2, emissions
were calculated using the design capacity (million British thermal
units per hour) of each unit and developed HAP emission factor
speciation profiles per fuel type. Tier 3 was used for estimating
emissions for those units that did not have a design capacity value
available. In Tier 3, emissions were conservatively estimated using the
maximum HAP emission value reported to NEI for any turbine unit for the
applicable fuel type. A more detailed discussion regarding the
methodology for estimating actual emissions is provided in the
Emissions Data Used for Stationary Combustion Turbines Risk and
Technology Review (RTR) Modeling Files memorandum in the rulemaking
docket.
    Stack parameters (height, diameter, temperature, exit velocity, and
flow rate) and stack locations (latitudes and longitudes) were taken
from the 2014 NEI when reported. For those units that did not have 2014
NEI stack parameters, three sets of default stack parameters were
developed based on the unit design capacity. The default parameters
were created by averaging the NEI-reported values for each parameter in
each data set.
    The modeling file input values were reviewed for completeness and
accuracy. Data quality checks included reviewing turbine latitudes and
longitudes using mapping tools and correcting as needed, performing
statistical analysis of modeling inputs to flag outliers for review,
and identifying and correcting stack parameters that were missing or
outside of standard industry range.
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 this source category, allowable emissions were determined using
the emission limitations currently included in 40 CFR part 63, subpart
YYYY. There are no current emission limits for existing source
stationary combustion turbines in the rule. As such, allowable
emissions have been set equal to the actual emissions for existing
sources. For new or reconstructed gas-fired and oil-fired stationary
combustion turbines where construction/reconstruction commenced after
January 14, 2003, a formaldehyde emission limit of 91 ppbvd at 15-
percent O2 is established in 40 CFR part 63, subpart YYYY.
However, the emission limits for new or reconstructed stationary
combustion turbines that are lean premix gas-fired or diffusion flame
gas-fired were stayed by the EPA. Therefore, as no emissions
limitations currently apply to gas-fired turbine units, the allowable
emissions have been set equal to the actual emissions for natural gas
units constructed after January 14, 2003. For all new oil-fired units
subject to the current emission limitation in 40 CFR part 63, subpart
YYYY, allowable annual emissions were estimated using the 91 ppbvd
formaldehyde limit and the NEI-reported operating hours.
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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    \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/recordisplay.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 screening-level risk assessments, the
EPA makes conservative assumptions about emission rates, meteorology,
and exposure location. We use the peak hourly emission rate,\11\ worst-
case dispersion conditions, and, in accordance with our mandate under
section 112 of the CAA, the point of highest off-site exposure to
assess the potential risk to the maximally exposed individual.
---------------------------------------------------------------------------

    \11\ 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 the risk document and in Appendix
5 of the report: Analysis of Data on Short-term Emission Rates
Relative to Long-term Emission Rates. 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 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.'' \12\ 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.\13\ 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.
---------------------------------------------------------------------------

    \12\ 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.
    \13\ 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.'' \14\ 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.
---------------------------------------------------------------------------

    \14\ ERPGS Procedures and Responsibilities. March 2014. American
Industrial Hygiene Association. 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.
---------------------------------------------------------------------------

    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).
    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 (even under the conservative assumptions of the screening
assessment), and no further analysis is performed for these HAP. In
cases where an acute HQ from the screening step is greater than 1, we
often consider additional site-specific data if available to develop a
more refined estimate of the potential for acute exposures of concern.
For this source category, we did not have short-term emissions data;
therefore, we used the default multiplication factor of 10. The acute
assessment methods are discussed more fully in the risk document, which
is available in the docket for this action.
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 HAP known to be PB-
HAP, as identified in the EPA's Air Toxics Risk Assessment Library (See
Volume 1, Appendix D, at https://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library.
    For the Stationary Combustion Turbine source category, we
identified PB-HAP emissions of arsenic, cadmium, lead, and mercury, 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 polycyclic organic matter (POM). Based on 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. 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.
    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, we use a U.S.
Geological Survey (USGS) database to identify actual waterbodies within
50 km of each facility. We also examine the differences between local
meteorology near the facility and the meteorology used in the Tier 1
screening assessment. 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. 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.
    In evaluating the potential multipathway risk from emissions of
lead compounds, rather than developing a screening threshold emission
rate, we compare maximum estimated chronic inhalation exposure
concentrations to the level of the current National Ambient Air Quality
Standard (NAAQS) for lead.\15\ Values below the level of the primary
(health-based) lead NAAQS are considered to have a low potential for
multipathway risk.
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    \15\ In doing so, the EPA notes that the legal standard for a
primary NAAQS--that a standard is requisite to protect public health
and provide an adequate margin of safety (CAA section 109(b))--
differs from the CAA section 112(f) standard (requiring, among other
things, that the standard provide an ``ample margin of safety to
protect public health''). However, the primary lead NAAQS is a
reasonable measure of determining risk acceptability (i.e., the
first step of the Benzene NESHAP analysis) since it is designed to
protect the most susceptible group in the human population--
children, including children living near major lead emitting
sources. 73 FR 67002/3; 73 FR 67000/3; 73 FR 67005/1. In addition,
applying the level of the primary lead NAAQS at the risk
acceptability step is conservative, since that primary lead NAAQS
reflects an adequate margin of safety.
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    For further information on the multipathway assessment approach,
see the risk document, which is available in the docket for this
action.

5. 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 risk document, 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 Stationary Combustion Turbine
source category emitted any of the environmental HAP, and we identified
emissions of arsenic, cadmium, mercury, lead, and HCl. Because one or
more of the environmental HAP evaluated are emitted by at least one
facility in the source category, we proceeded to the second step of the
evaluation.
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 tpy 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 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 risk
document, which is available in the docket for this action.
6. 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 risk document, 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.
7. 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 risk document, 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.\16\
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.\17\
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 \18\ 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.
---------------------------------------------------------------------------

    \16\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \17\ 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.
    \18\ 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.
---------------------------------------------------------------------------

    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 humans
at the location of the maximum concentration. In the acute screening
assessment that we conduct under the RTR program, we assume that peak
emissions from the source category and worst-case meteorological
conditions co-occur, thus, resulting in maximum ambient concentrations.
These two events are unlikely to occur at the same time, making these
assumptions conservative. We then include the additional assumption
that a person is located at this point during this same time period.
For this source category, these assumptions would tend to be worst-case
actual exposures, as it is unlikely that a person would be located at
the point of maximum exposure during the time when peak emissions and
worst-case meteorological 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.\19\
---------------------------------------------------------------------------

    \19\ 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 are the results of the risk assessment and analyses?

    As described above, for the Stationary Combustion Turbines source
category, we conducted an inhalation risk assessment for all HAP
emitted and we also conducted multipathway and environmental risk
screening assessments on the PB-HAP emitted. We present results of the
risk assessment briefly below and in more detail in the risk document.
Note that risk modeling was conducted for 253 facilities. Additional
information obtained after the risk modeling was completed was used to
refine our estimate of facilities in the source category to 242. The
risk assessment results presented in this preamble and in the risk
document are shown for the 253 facilities modeled.
1. Inhalation Risk Assessment Results
    Table 3 of this preamble provides a summary of the results of the
inhalation risk assessment for the source category. More detailed
information on the risk assessment can be found in the risk document,
available in the docket for this action.

                                       Table 3--Stationary Combustion Turbines Inhalation Risk Assessment Results
--------------------------------------------------------------------------------------------------------------------------------------------------------
                Maximum individual      Population at increased   Annual cancer incidence       Maximum chronic       Maximum  screening acute noncancer
                 cancer risk (in 1      risk of cancer >=1-in-1      (cases per year)        noncancer  TOSHI \3\                   HQ \4\
                   million) \2\                 million         ----------------------------------------------------------------------------------------
 Number of  ----------------------------------------------------      Based on . . .            Based on . . .
 facilities       Based on . . .            Based on . . .      ----------------------------------------------------
    \1\     ----------------------------------------------------
                Actual     Allowable      Actual     Allowable      Actual     Allowable      Actual     Allowable     Based on actual emissions level
              emissions    emissions    emissions    emissions    emissions    emissions    emissions    emissions
                level        level        level        level        level        level        level        level
--------------------------------------------------------------------------------------------------------------------------------------------------------
       253            3            3       42,000       42,000         0.04         0.04         0.04         0.04   HQREL = 2 (acrolein), HQAEGL-1 =
                                                                                0.07.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Number of facilities evaluated in the risk analysis.
\2\ Maximum individual excess lifetime cancer risk due to HAP emissions from the source category.
\3\ Maximum TOSHI. The target organ system with the highest TOSHI for the source category is respiratory. The respiratory TOSHI was calculated using the
  CalEPA chronic REL for acrolein. The EPA is in the process of updating the IRIS RfC for acrolein. If the RfC is updated prior to signature of the
  final rule, we will use it in the assessment.
\4\ The maximum estimated acute exposure concentration was divided by available short-term threshold values to develop an array of HQ values. HQ values
  shown use the lowest available acute threshold value, which in most cases is the REL. When an HQ exceeds 1, we also show the HQ using the next lowest
  available acute dose-response value.

    As shown in Table 3, based on actual and allowable emissions, the
estimated cancer MIR is 3-in-1 million, and formaldehyde emissions are
the major contributor to the risk. The total estimated cancer incidence
from this source category is 0.04 excess cancer cases per year, or one
excess case in every 25 years. Approximately 42,000 people are
estimated to have cancer risks at or above 1-in-1 million from HAP
emitted from the facilities in this source category. The estimated
maximum chronic noncancer TOSHI for the source category is 0.04
(respiratory), which is driven by emissions of formaldehyde. No
individuals are exposed to TOSHI levels above 1.
2. Acute Risk Results
    Table 3 provides the worst-case acute HQ (based on the REL) of 2,
driven by actual emissions of acrolein. Only one facility has an HQ
(REL) that exceeds 1. To better characterize the potential health risks
associated with estimated worst-case acute exposures to HAP, and in
response to a key recommendation from the SAB's peer review of the
EPA's RTR risk assessment methodologies, we examine a wider range of
available acute health metrics than we do for our chronic risk
assessments. This is in acknowledgement that there are generally more
data gaps and uncertainties in acute reference values than there are in
chronic reference values. By definition, the acute REL represents a
health-protective level of exposure, with effects not anticipated below
those levels, even for repeated exposures; however, the level of
exposure that would cause health effects is not specifically known.
Therefore, when an REL is exceeded and an AEGL-1 or ERPG-1 level is
available (i.e., levels at which mild, reversible effects are
anticipated in the general public for a single exposure), we typically
use them as an additional comparative measure, as they provide an upper
bound for exposure levels above which exposed individuals could
experience effects. As the exposure concentration increases above the
acute REL, the potential for effects increases.
    The worst-case maximum estimated 1-hour exposure to acrolein
outside the facility fence line is 0.004 mg/m\3\. This estimated worst-
case exposure exceeds the 1-hour REL by a factor of 2 (HQ=2) and is
less than 10 percent of the 1-hour AEGL-1 and ERPG-1. For more detailed
acute risk results, refer to the risk document.
3. Multipathway Risk Screening Results
    Potential multipathway health risks under a fisher and gardener
scenario were evaluated using a three-tier screening assessment of the
PB-HAP emitted by facilities in this source category. Of the 253
facilities modeled, 35 facilities have reported emissions of
carcinogenic PB-HAP (arsenic) that exceed a Tier 1 cancer screening
value of 1, and 15 facilities have reported emissions of non-
carcinogenic PB-HAP (mercury and/or cadmium) that exceed a Tier 1
noncancer screening value of 1. For facilities that exceeded a Tier 1
multipathway screening value of 1, we used additional facility-specific
information to perform an assessment through Tiers 2 and 3, as
necessary, to determine the maximum chronic cancer and noncancer
multipathway health risks for the source category. For cancer, the
highest Tier 2 screening value was 20 and there were 17 facilities with
Tier 2 screening values greater than 1. This highest screening value
was reduced to 4 after Tier 3. For noncancer, the highest Tier 2
screening value was 4 (for mercury), and there were 3 facilities with
Tier 2 screening values greater than 1. After Tier 3, the highest
screening value was 1.
    An exceedance of a screening value in any of the tiers cannot be
equated with a risk value or an HQ (or HI). Rather, it represents a
high-end estimate of what the risk or hazard may be. For example, a
screening value of 2 for a non-carcinogen can be interpreted to mean
that we are confident that the HQ would be lower than 2. Similarly, a
screening value of 30 for a carcinogen means that we are confident that
the risk is lower than 30-in-1 million. Our confidence comes from the
conservative, or health-protective, assumptions encompassed in the
screening tiers: We choose inputs from the upper end of the range of
possible values for the influential parameters used in the screening
tiers; and we assume that the exposed individual exhibits ingestion
behavior that would lead to a high total exposure.
    In evaluating the potential for multipathway effects from emissions
of lead, we compared modeled annual lead concentrations to the primary
NAAQS for lead (0.15 [micro]g/m\3\). The highest annual lead
concentration of 0.0003 [micro]g/m\3\ is well below the NAAQS for lead,
indicating a low potential for multipathway impacts of concern due to
lead.
4. Environmental Risk Screening Results
    As described in section III.C.5 of this document, we conducted an
environmental risk screening assessment for the Stationary Combustion
Turbine source category for the following pollutants: Arsenic, cadmium,
mercury, lead, and HCl.
    In the Tier 1 screening analysis for PB-HAP (other than lead, which
was evaluated differently), arsenic had no exceedances of any of the
ecological benchmarks evaluated. Divalent mercury and methyl mercury
emissions had Tier 1 exceedances for surface soil benchmarks. Cadmium
emissions had Tier 1 exceedances for surface soil and fish benchmarks.
    A Tier 2 screening analysis was performed for cadmium, divalent
mercury, and methyl mercury emissions. In the Tier 2 screening
analysis, there were no exceedances of

any of the ecological benchmarks evaluated for any of the pollutants.
    For lead, we did not estimate any exceedances of the secondary lead
NAAQS. For HCl, the average modeled concentration around each facility
(i.e., the average concentration of all off-site data points in the
modeling domain) did not exceed any ecological benchmark. In addition,
each individual modeled concentration of HCl (i.e., each off-site data
point in the modeling domain) was below the ecological benchmarks for
all facilities.
    Based on the results of the environmental risk screening analysis,
we do not expect an adverse environmental effect as a result of HAP
emissions from this source category.
5. Facility-Wide Risk Results
    Based on facility-wide emissions, the estimated cancer MIR is
2,000-in-1 million, and ethylene oxide from chemical manufacturing is
the major contributor to the risk. The total estimated cancer incidence
based on facility-wide emissions is 0.7 excess cancer cases per year,
or one excess case in every 1 to 2 years. Approximately 2.8 million
people are estimated to have cancer risks at or above 1-in-1 million.
The estimated maximum chronic noncancer TOSHI based on facility-wide
emissions is 4 (respiratory), driven by emissions of chlorine from
chemical manufacturing, and approximately 360 people are exposed to a
TOSHI above 1.
6. What demographic groups might benefit from this regulation?
    To examine the potential for any environmental justice issues that
might be associated with the source category, we performed a
demographic analysis, which is an assessment of risk to individual
demographic groups of the populations living within 5 km and within 50
km of the facilities. In the analysis, we evaluated the distribution of
HAP-related cancer and noncancer risk from the Stationary Combustion
Turbines source category across different demographic groups within the
populations living near facilities.\20\
---------------------------------------------------------------------------

    \20\ Demographic groups included in the analysis are: White,
African American, Native American, other races and multiracial,
Hispanic or Latino, children 17 years of age and under, adults 18 to
64 years of age, adults 65 years of age and over, adults without a
high school diploma, people living below the poverty level, people
living two times the poverty level, and linguistically isolated
people.
---------------------------------------------------------------------------

    The results of the demographic analysis are summarized in Table 4
below. These results, for various demographic groups, are based on the
estimated risk from actual emissions levels for the population living
within 50 km of the facilities.

                    Table 4--Stationary Combustion Turbines Demographic Risk Analysis Results
----------------------------------------------------------------------------------------------------------------
                                                                                Source category
                                                               -------------------------------------------------
                                                  Nationwide     Population with cancer
                                                                  risk greater than or    Population with hazard
                                                                equal to 1-in-1 million    index greater than 1
----------------------------------------------------------------------------------------------------------------
     Stationary Combustion Turbines Source Category: Demographic Assessment Results--50 km Study Area Radius
----------------------------------------------------------------------------------------------------------------
Total Population..............................     317,746,049                   42,191                        0
----------------------------------------------------------------------------------------------------------------
                                          White and Minority by Percent
----------------------------------------------------------------------------------------------------------------
White.........................................              62                       52                        0
Minority......................................              38                       48                        0
----------------------------------------------------------------------------------------------------------------
                                               Minority by Percent
----------------------------------------------------------------------------------------------------------------
African American..............................              12                       11                        0
Native American...............................             0.8                      0.1                        0
Hispanic or Latino (includes white and                      18                       31                        0
 nonwhite)....................................
Other and Multiracial.........................               7                        6                        0
----------------------------------------------------------------------------------------------------------------
                                                Income by Percent
----------------------------------------------------------------------------------------------------------------
Below Poverty Level...........................              14                       19                        0
Above Poverty Level...........................              86                       81                        0
----------------------------------------------------------------------------------------------------------------
                                              Education by Percent
----------------------------------------------------------------------------------------------------------------
Over 25 and without a High School Diploma.....              14                       13                        0
Over 25 and with a High School Diploma........              86                       87                        0
----------------------------------------------------------------------------------------------------------------
                                       Linguistically Isolated by Percent
----------------------------------------------------------------------------------------------------------------
Linguistically Isolated.......................               6                        9                        0
----------------------------------------------------------------------------------------------------------------

    The results of the Stationary Combustion Turbines source category
demographic analysis indicate that emissions from the source category
expose approximately 42,000 people to a cancer risk at or above 1-in-1
million and no people to a chronic noncancer TOSHI greater than 1.
Regarding cancer risk, the specific demographic results indicate that
the percentage of the population potentially impacted by Stationary
Combustion Turbine emissions is greater than its corresponding
nationwide percentage for the following demographics: Hispanic or
Latino (31 percent for the source category compared to 18 percent
nationwide), minority (48 percent for

the source category compared to 38 percent nationwide), age 18 to 64
(69 percent for the source category compared to 63 percent nationwide),
below the poverty level (19 percent for the source category compared to
14 percent nationwide), and linguistically isolated (9 percent for the
source category compared to 6 percent nationwide). The remaining
demographic group percentages are the same as or less than the
corresponding nationwide percentages.
    The methodology and the results of the demographic analysis are
presented in a technical report, Risk and Technology Review--Analysis
of Demographic Factors for Populations Living Near Stationary
Combustion Turbines Source Category Operations, available in the docket
for this action.

B. What are our proposed decisions regarding risk acceptability, ample
margin of safety, and adverse environmental effect?

1. Risk Acceptability
    As noted in section III of this preamble, the EPA sets standards
under CAA section 112(f)(2) using ``a two-step standard-setting
approach, with an analytical first step to determine an `acceptable
risk' that considers all health information, including risk estimation
uncertainty, and includes a presumptive limit on MIR of approximately
1-in-10 thousand'' (54 FR 38045, September 14, 1989). In this proposal,
the EPA estimated risks based on actual and allowable emissions from
stationary combustion turbines located at major sources of HAP, and we
considered these in determining acceptability.
    The estimated inhalation cancer risk to the individual most exposed
to actual or allowable emissions from the source category is 3-in-1
million. The estimated incidence of cancer due to inhalation exposures
is 0.04 excess cancer cases per year, or one excess case every 25
years. Approximately 42,000 people face an increased cancer risk at or
above 1-in-1 million due to inhalation exposure to actual or allowable
HAP emissions from this source category. The estimated maximum chronic
noncancer TOSHI from inhalation exposure for this source category is
0.04. The screening assessment of worst-case inhalation impacts
indicates a worst-case maximum acute HQ of 2 for acrolein based on the
1-hour REL and concentrations that are less than 10 percent of the 1-
hour AEGL-1 and ERPG-1. Only one facility has an HQ (REL) that exceeds
1.
    Potential multipathway human health risks were estimated using a
three-tier screening assessment of the PB-HAP emitted by facilities in
this source category. The only pollutants with elevated Tier 1 and Tier
2 screening values are arsenic (cancer), cadmium (noncancer), and
mercury (noncancer). The Tier 3 screening values for these pollutants
are low. For cancer, the Tier 3 screening value for arsenic is 4. For
noncancer, the Tier 3 screening value for cadmium is less than 1, and
the screening value for mercury is 1.
    In determining whether risks are acceptable for this source
category, the EPA considered all available health information and risk
estimation uncertainty as described above. The risk results indicate
that both the actual and allowable inhalation cancer risks to the
individual most exposed are well below 100-in-1 million, which is the
presumptive limit of acceptability. In addition, the highest chronic
noncancer TOSHI is well below 1, indicating low likelihood of adverse
noncancer effects from inhalation exposures. There are also low
estimated risks associated with ingestion, with the highest cancer risk
being 4-in-1 million and the highest noncancer HI being 1, based on a
Tier 3 multipathway screening assessment.
    The acute screening analysis results in a maximum acute noncancer
HQ of 2 based on the acute REL for acrolein. This occurs at only one
facility of the 253 that were modeled. For acute screening analyses, to
better characterize the potential health risks associated with
estimated worst-case acute exposures to HAP, we examine a wider range
of available acute health metrics than we do for our chronic risk
assessments. This is in acknowledgement that there are generally more
data gaps and uncertainties in acute reference values than there are in
chronic reference values. By definition, the acute REL represents a
health-protective level of exposure, with effects not anticipated below
those levels, even for repeated exposures; however, the level of
exposure that would cause health effects is not specifically known. As
the exposure concentration increases above the acute REL, the potential
for effects increases. Therefore, when an REL is exceeded and an AEGL-1
or ERPG-1 level is available (i.e., levels at which mild, reversible
effects are anticipated in the general population for a single