[Federal Register Volume 81, Number 248 (Tuesday, December 27, 2016)]
[Proposed Rules]
[Pages 95352-95396]
[FR Doc No: 2016-30471]

Vol. 81

Tuesday,

No. 248

December 27, 2016

Part III

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Publicly
Owned Treatment Works; Proposed Rule

Federal Register / Vol. 81 , No. 248 / Tuesday, December 27, 2016 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2016-0490; FRL-9956-87-OAR]
RIN 2060-AS85

National Emission Standards for Hazardous Air Pollutants:
Publicly Owned Treatment Works

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 Publicly Owned Treatment Works (POTW) to
address the results of the residual risk and technology review (RTR)
conducted under the Clean Air Act (CAA). As a result of our review, we
are proposing to include pretreatment requirements to limit emissions
from collection systems and the POTW treatment plant; requirements for
existing, new, or reconstructed industrial (Group 1) POTW to comply
with both the requirements in this rule and those in the applicable
NESHAP for which they act as control; and hazardous air pollutants
(HAP) emission limits for existing, non-industrial (Group 2) POTW. In
addition, the EPA is proposing to revise the applicability criteria,
revise the names and definitions of the industrial (Group 1) and non-
industrial (Group 2) subcategories, revise regulatory provisions
pertaining to emissions during periods of startup, shutdown, and
malfunction, add requirements for electronic reporting, and make other
miscellaneous edits and technical corrections.

DATES: Comments. Comments must be received on or before February 27,
2017. 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 January 26, 2017.
    Public Hearing. A public hearing will be held on January 11, 2017,
if requested by January 3, 2017.

ADDRESSES: Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2016-0490, at http://www.regulations.gov. Follow the online
instructions for submitting comments. Once submitted, comments cannot
be edited or removed from http://www.regulations.gov. The EPA may
publish any comment received to its public docket. Do not submit
electronically any information you consider to be Confidential Business
Information (CBI) or other information whose disclosure is restricted
by statute. Multimedia submissions (audio, video, etc.) must be
accompanied by a written comment. The written comment is considered the
official comment and should include discussion of all points you wish
to make. The EPA will generally not consider comments or comment
contents located outside of the primary submission (i.e., on the Web,
cloud, or other file sharing system). For additional submission
methods, the full EPA public comment policy, information about CBI or
multimedia submissions, and general guidance on making effective
comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Karen Marsh, Sector Policies and Programs Division
(E143-05), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711; telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>;
and email address<EMAIL_ADDRESS>For specific information
regarding the risk modeling methodology, contact Michael Stewart,
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: (919)
541-7524; fax number:<PHONE_NUMBER>; and email address:
<EMAIL_ADDRESS>For information about the applicability of the
NESHAP to a particular entity, contact Patrick Yellin, Office of
Enforcement and Compliance Assurance, U.S. Environmental Protection
Agency, EPA WJC South Building, Mail Code 2227A, 1200 Pennsylvania
Avenue NW., Washington DC 20460; telephone number:<PHONE_NUMBER>; fax
number:<PHONE_NUMBER>; and email address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2016-0490. All documents in the docket are
listed in the Regulations.gov index. Although listed in the index, some
information is not publicly available, e.g., CBI or other information
whose disclosure is restricted by statute. Certain other material, such
as copyrighted material, is not placed on the Internet and will be
publicly available only in hard copy. 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
(202) 566-1744, and the telephone number for the EPA Docket Center is
(202) 566-1742.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2016-0490. 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 http://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 http://www.regulations.gov or email. The
http://www.regulations.gov Web site is an ``anonymous access'' system,
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 http://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 disk or
CD-ROM 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 http://www.epa.gov/dockets.
    Public Hearing. A public hearing will be held, if requested by
January 3, 2017, to accept oral comments on this proposed action. If a
hearing is requested, it will be held at the EPA's Washington, DC
campus located at 1201 Constitution Avenue NW., Washington, DC. The
hearing, if requested, will begin at 9:00 a.m. (local time) and will
conclude at 4:00 p.m. (local time) on January 11, 2017. To request a
hearing,

to register to speak at a hearing, or to inquire if a hearing will be
held, please contact Aimee St. Clair at<PHONE_NUMBER> or by email at
<EMAIL_ADDRESS>The last day to pre-register to speak at a
hearing, if one is held, will be January 9, 2017. Additionally,
requests to speak will be taken the day of the hearing at the hearing
registration desk, although preferences on speaking times may not be
able to be fulfilled. Please note that registration requests received
before the hearing will be confirmed by the EPA via email.
    The EPA will make every effort to accommodate all speakers who
arrive and register. Because the hearing will be held at a U.S.
governmental facility, individuals planning to attend the hearing
should be prepared to show valid picture identification to the security
staff in order to gain access to the meeting room. Please note that the
REAL ID Act, passed by Congress in 2005, established new requirements
for entering federal facilities. If your driver's license is issued by
Alaska, American Samoa, Arizona, Kentucky, Louisiana, Maine,
Massachusetts, Minnesota, Montana, New York, Oklahoma or the state of
Washington, you must present an additional form of identification to
enter the federal building. Acceptable alternative forms of
identification include: Federal employee badges, passports, enhanced
driver's licenses and military identification cards. In addition, you
will need to obtain a property pass for any personal belongings you
bring with you. Upon leaving the building, you will be required to
return this property pass to the security desk. No large signs will be
allowed in the building, cameras may only be used outside of the
building and demonstrations will not be allowed on federal property for
security reasons.
    Please note that any updates made to any aspect of the hearing,
including whether or not a hearing will be held, will be posted online
at https://www.epa.gov/stationary-sources-air-pollution/publicly-owned-treatment-works-potw-national-emission-standards. We ask that you
contact Aimee St. Clair at<PHONE_NUMBER> or by email at
<EMAIL_ADDRESS>or monitor our Web site to determine if a hearing
will be held. The EPA does not intend to publish a notice in the
Federal Register announcing any such updates. Please go to https://www.epa.gov/stationary-sources-air-pollution/publicly-owned-treatment-works-potw-national-emission-standards for more information on the
public hearing.
    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 levels
AERMOD Air dispersion model used by the HEM-3 model
ATSDR Agency for Toxic Substances and Disease Registry
BACT Best available control technology
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
CWA Clean Water Act
ECHO Enforcement and Compliance History Online
EJ Environmental justice
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT Electronic Reporting Tool
FR Federal Register
HAP Hazardous air pollutants
HCl Hydrochloric acid
HEM-3 Human Exposure Model, Version 1.1.0
HF Hydrogen fluoride
HI Hazard index
HQ Hazard quotient
ICR Information collection request
IRIS Integrated Risk Information System
km Kilometer
LAER Lowest achievable emission rate
LOAEL Lowest-observed-adverse-effect level
MACT Maximum achievable control technology
MGD Million gallons per day
mg/kg-day Milligrams per kilogram per day
mg/m\3\ Milligrams per cubic meter
MIR Maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NAS National Academy of Sciences
NATA National Air Toxics Assessment
NEI National Emissions Inventory
NESHAP National emissions standards for hazardous air pollutants
NOAA National Oceanic and Atmospheric Administration
NOAEL No-observed-adverse-effect levels
NRC National Research Council
NSR New source review
NTTAA National Technology Transfer and Advancement Act
OAQPS Office of Air Quality Planning and Standards
OMB Office of Management and Budget
PAH polycyclic aromatic hydrocarbons
PB-HAP Hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PEL Probable effect level
POM Polycyclic organic matter
POTW Publicly owned treatment works
ppm Parts per million
PRA Paperwork Reduction Act
RACT Reasonably available control technology
REL Reference exposure level
RFA Regulatory Flexibility Act
RfC Reference concentration
RfD Reference dose
RTR Residual risk and technology review
SAB Science Advisory Board
SOP Standard operating procedure
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?

    Table 1 of this preamble lists the NESHAP and associated regulated
industrial source category that is the subject of this proposal. Table
1 is not intended to be exhaustive, but rather provides a guide for
readers regarding the entities that this proposed action is likely to
affect. The proposed standards, once promulgated, will be directly
applicable to the affected sources. Federal, state, local, and tribal
governments would be affected as discussed below. By definition, a POTW
is owned by a municipality, state, intermunicipal or interstate agency,
or any department, agency, or instrumentality of the federal government
(See 40 CFR 63.1595 of subpart VVV). If a POTW has a design capacity to
treat at least 5 million gallons per day (MGD) of wastewater, receives
wastewater from industrial users, and is either a major source of HAP
emissions or treats wastewater to comply with requirements of another
NESHAP, then the POTW is affected by these standards. (Note, these
applicability criteria represent proposed revisions to the current
criteria and are discussed further in section IV.D.1 of this document.)
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), the POTW source category includes emissions from
wastewaters that are treated at a POTW. These wastewaters are generated
by industrial, commercial, and domestic sources, although only
industrial and commercial dischargers might consistently discharge HAP
in quantities high enough to potentially result in an exceedance of the
major source emission threshold at the POTW. Emissions from these
wastewaters can

occur within the collection system (sewers) as well as during treatment
at the POTW. Control options include, but are not limited to, reduction
of HAP at the industrial discharger before wastewater enters the
collection systems, add-on emission controls on the collection system
and at the POTW, and/or treatment process modifications/substitutions.

    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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Sewage Treatment Facilities......  Subpart VVV.........          221320
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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. 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-2016-0490). 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/publicly-owned-treatment-works-potw-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 Web site. Information
on the overall residual risk and technology review (RTR) program is
available at http://www3.epa.gov/ttn/atw/rrisk/rtrpg.html.

C. What should I consider as I prepare my comments for the EPA?

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM 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 for inclusion in the public docket. If
you submit a CD-ROM or disk that does not contain CBI, mark the outside
of the disk or CD-ROM 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 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-2016-
0490.

II. Background

A. What is the statutory authority for this action?

    Section 112 of the CAA establishes a two-stage regulatory process
to address emissions of HAP from stationary sources. In the first
stage, after the EPA has identified categories of sources emitting one
or more of the HAP listed in CAA section 112(b), CAA section 112(d)
requires us to promulgate technology-based NESHAP for those sources.
``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. For major sources, 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) and are commonly referred to as
maximum achievable control technology (MACT) standards.
    MACT standards must reflect the maximum degree of emissions
reduction achievable through the application of measures, processes,
methods, systems, or techniques, including, but not limited to,
measures that (1) Reduce the volume of or eliminate pollutants through
process changes, substitution of materials or other modifications; (2)
enclose systems or processes to eliminate emissions; (3) capture or
treat pollutants when released from a process, stack, storage, or
fugitive emissions point; (4) are design, equipment, work practice, or
operational standards (including requirements for operator training or
certification); or (5) are a combination of the above. CAA section
112(d)(2)(A)-(E). The MACT standards may take the form of design,
equipment, work practice, or operational standards where the EPA first
determines either that (1) a pollutant cannot be emitted through a
conveyance designed and constructed to emit or capture the pollutant,
or that any requirement for, or use of, such a conveyance would be
inconsistent with law; or (2) the application of measurement
methodology to a particular class of sources is not practicable due to
technological and economic limitations. CAA section 112(h)(1)-(2).
    The MACT ``floor'' is the minimum control level allowed for MACT
standards promulgated under CAA section 112(d)(3) and may not be based
on cost considerations. For new sources, the MACT floor cannot be less
stringent than the emissions control that is achieved in practice by
the best-controlled similar source. The MACT floor for existing sources
can be less stringent than floors for new sources, but not less
stringent than the average emissions limitation achieved by the best-
performing 12 percent of existing sources in the category or
subcategory (or the best-performing five sources for categories or
subcategories with fewer than 30 sources). In developing MACT
standards, the EPA must also consider control options that are more
stringent than the floor. We may establish standards more stringent
than the floor based on considerations of the cost of achieving the
emission reductions, any non-air quality health and environmental
impacts, and energy requirements.
    The EPA is then required to review these technology-based standards
and revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every 8 years. CAA section 112(d)(6). In conducting this 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 second stage in standard-setting focuses on reducing any
remaining (i.e.,

``residual'') risk according to CAA section 112(f). CAA section
112(f)(1) requires that the EPA prepare a report to Congress discussing
(among other things) methods of calculating the risks posed (or
potentially posed) by sources after implementation of the MACT
standards, the public health significance of those risks, and the EPA's
recommendations as to legislation regarding such remaining risk. The
EPA prepared and submitted the Residual Risk Report to Congress, EPA-
453/R-99-001 (Risk Report) in March 1999. CAA section 112(f)(2) then
provides that if Congress does not act on any recommendation in the
Risk Report, the EPA must analyze and address residual risk for each
category or subcategory of sources 8 years after promulgation of such
standards pursuant to CAA section 112(d).
    Section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of
the two-step process for developing standards to address any residual
risk and the Agency's interpretation of ``ample margin of safety''
developed in the National Emissions Standards for Hazardous Air
Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the Agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable 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) (``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also, A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety, and other relevant factors,
an adverse environmental effect.
1. Step 1--Determination of Acceptability
    The Agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Benzene NESHAP
at 54 FR 38046, September 14, 1989. The determination of what
represents an ``acceptable'' risk is based on a judgment of ``what
risks are acceptable in the world in which we live'' (Risk Report at
178, quoting NRDC v. EPA, 824 F. 2d 1146, 1165 (D.C. Cir. 1987) (en
banc) (``Vinyl Chloride''), recognizing that our world is not risk-
free.
    In the Benzene NESHAP, we stated that ``EPA will generally presume
that if the risk to [the maximum exposed] individual is no higher than
approximately one in 10 thousand, that risk level is considered
acceptable.'' 54 FR at 38045, September 14, 1989. We discussed the
maximum individual lifetime cancer risk (or maximum individual risk
(MIR)) as being ``the estimated risk that a person living near a plant
would have if he or she were exposed to the maximum pollutant
concentrations for 70 years.'' Id. We explained that this measure of
risk ``is an estimate of the upper bound of risk based on conservative
assumptions, such as continuous exposure for 24 hours per day for 70
years.'' Id. We acknowledged that maximum individual lifetime cancer
risk ``does not necessarily reflect the true risk, but displays a
conservative risk level which is an upper-bound that is unlikely to be
exceeded.'' Id.
    Understanding that there are both benefits and limitations to using
the MIR as a metric for determining acceptability, we acknowledged in
the Benzene NESHAP that ``consideration of maximum individual risk * *
* must take into account the strengths and weaknesses of this measure
of risk.'' Id. Consequently, the presumptive risk level of 100-in-1
million (1-in-10 thousand) provides a benchmark for judging the
acceptability of maximum individual lifetime cancer risk, but does not
constitute a rigid line for making that determination. Further, in the
Benzene NESHAP, we noted that:

[p]articular attention will also be accorded to the weight of
evidence presented in the risk assessment of potential
carcinogenicity or other health effects of a pollutant. While the
same numerical risk may be estimated for an exposure to a pollutant
judged to be a known human carcinogen, and to a pollutant considered
a possible human carcinogen based on limited animal test data, the
same weight cannot be accorded to both estimates. In considering the
potential public health effects of the two pollutants, the Agency's
judgment on acceptability, including the MIR, will be influenced by
the greater weight of evidence for the known human carcinogen.

Id. at 38046. The Agency also explained in the Benzene NESHAP that:

[i]n establishing a presumption for MIR, rather than a rigid line
for acceptability, the Agency intends to weigh it with a series of
other health measures and factors. These include the overall
incidence of cancer or other serious health effects within the
exposed population, the numbers of persons exposed within each
individual lifetime risk range and associated incidence within,
typically, a 50 km exposure radius around facilities, the science
policy assumptions and estimation uncertainties associated with the
risk measures, weight of the scientific evidence for human health
effects, other quantified or unquantified health effects, effects
due to co-location of facilities, and co-emission of pollutants.

Id. at 38045. In some cases, these health measures and factors taken
together may provide a more realistic description of the magnitude of
risk in the exposed population than that provided by maximum individual
lifetime cancer risk alone.
    As noted earlier, in NRDC v. EPA, the court held that CAA section
112(f)(2) ``incorporates the EPA's interpretation of the Clean Air Act
from the Benzene Standard.'' The court further held that Congress'
incorporation of the Benzene standard applies equally to carcinogens
and non-carcinogens. 529 F.3d at 1081-82. Accordingly, we also consider
non-cancer risk metrics in our determination of risk acceptability and
ample margin of safety.
2. Step 2--Determination of Ample Margin of Safety
    CAA section 112(f)(2) requires the EPA to determine, for source
categories subject to MACT standards, whether those standards provide
an ample margin of safety to protect public health.

As explained in the Benzene NESHAP, ``the second step of the inquiry,
determining an `ample margin of safety,' again includes consideration
of all of the health factors, and whether to reduce the risks even
further. . . . Beyond that information, additional factors relating to
the appropriate level of control will also be considered, including
costs and economic impacts of controls, technological feasibility,
uncertainties, and any other relevant factors. Considering all of these
factors, the Agency will establish the standard at a level that
provides an ample margin of safety to protect the public health, as
required by section 112.'' 54 FR 38046, September 14, 1989.
    According to CAA section 112(f)(2)(A), if the MACT standards for
HAP ``classified as a known, probable, or possible human carcinogen do
not reduce lifetime excess cancer risks to the individual most exposed
to emissions from a source in the category or subcategory to less than
one in one million,'' the EPA must promulgate residual risk standards
for the source category (or subcategory), as necessary to provide an
ample margin of safety to protect public health. In doing so, the EPA
may adopt standards equal to existing MACT standards if the EPA
determines that the existing standards (i.e., the MACT standards) are
sufficiently protective. NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir.
2008) (``If EPA determines that the existing technology-based standards
provide an `ample margin of safety,' then the Agency is free to readopt
those standards during the residual risk rulemaking.'') The EPA must
also adopt more stringent standards, if necessary, to prevent an
adverse environmental effect,\1\ but must consider cost, energy,
safety, and other relevant factors in doing so.
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    \1\``Adverse environmental effect'' is defined as any
significant and widespread adverse effect, which may be reasonably
anticipated to wildlife, aquatic life, or natural resources,
including adverse impacts on populations of endangered or threatened
species or significant degradation of environmental qualities over
broad areas. CAA section 112(a)(7).
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    The CAA does not specifically define the terms ``individual most
exposed,'' ``acceptable level,'' and ``ample margin of safety.'' In the
Benzene NESHAP, 54 FR at 38044-38045, September 14, 1989, we stated as
an overall objective:

    In protecting public health with an ample margin of safety under
section 112, EPA strives to provide maximum feasible protection
against risks to health from hazardous air pollutants by (1)
protecting the greatest number of persons possible to an individual
lifetime risk level no higher than approximately 1-in-1 million and
(2) limiting to no higher than approximately 1-in-10 thousand [i.e.,
100-in-1 million] the estimated risk that a person living near a
plant would have if he or she were exposed to the maximum pollutant
concentrations for 70 years.

The Agency further stated that ``[t]he EPA also considers incidence
(the number of persons estimated to suffer cancer or other serious
health effects as a result of exposure to a pollutant) to be an
important measure of the health risk to the exposed population.
Incidence measures the extent of health risks to the exposed population
as a whole, by providing an estimate of the occurrence of cancer or
other serious health effects in the exposed population.'' Id. at 38045.
    In the ample margin of safety decision process, the Agency again
considers all of the health risks and other health information
considered in the first step, including the incremental risk reduction
associated with standards more stringent than the MACT standard or a
more stringent standard that the EPA has determined is necessary to
ensure risk is acceptable. In the ample margin of safety analysis, the
Agency considers additional factors, including costs and economic
impacts of controls, technological feasibility, uncertainties, and any
other relevant factors. Considering all of these factors, the Agency
will establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by CAA section 112(f).
54 FR 38046, September 14, 1989.

B. What is this source category and how does the 2002 NESHAP regulate
its HAP emissions?

1. Definition of the POTW Source Category and the Affected Source
    The NESHAP for the POTW source category (henceforth referred to as
the ``POTW NESHAP'') was promulgated on October 26, 1999 (64 FR 57572)
and codified at 40 CFR part 63, subpart VVV. The POTW NESHAP was
amended on October 21, 2002 (67 FR 64742). As amended in 2002, the POTW
NESHAP applies to new and existing POTW treatment plants that are
located at a POTW that is a major source of HAP emissions and that is
required to develop and implement a pretreatment program as defined by
40 CFR 403.8 under the Clean Water Act. Emissions from a POTW originate
from wastewaters that are treated at a POTW. These wastewaters are
generated by industrial, commercial, and domestic sources, although
only industrial and commercial dischargers might consistently discharge
HAP in quantities high enough to potentially result in an exceedance of
the major source emission threshold at the POTW. Emissions from these
wastewaters can occur within the collection system (sewers) as well as
during treatment at the POTW treatment plant. Control options include,
but are not limited to, reduction of HAP at the source before they
enter the collection system, add-on emission controls on the collection
system and at the POTW, and/or treatment process modifications/
substitutions.
    The POTW NESHAP (40 CFR 63.1595) defines ``POTW'' as ``a treatment
works, as that term is defined by section 112(e)(5) of the Clean Air
Act, which is owned by a municipality (as defined by section 502(4) of
the Clean Water Act),\2\ a state, an intermunicipal or interstate
agency, or any department, agency, or instrumentality of the federal
government. This definition includes any intercepting sewers, outfall
sewers, sewage collection systems, pumping, power, and other equipment.
The wastewater treated by these facilities is generated by industrial,
commercial, and domestic sources. As used in this regulation, the term
POTW refers to both any publicly owned treatment works which is owned
by a state, municipality, or intermunicipal or interstate agency and
therefore eligible to receive grant assistance under the Subchapter II
of the Clean Water Act, and any federally owned treatment works as that
term is described in section 3023 of the Solid Waste Disposal Act.''
The ``affected source'' regulated by the 2002 POTW NESHAP is defined in
40 CFR 63.1595 of the POTW NESHAP as the ``group of all equipment that
comprise the POTW treatment plant.'' The ``POTW treatment plant'' is
defined as the ``portion of the POTW which is designed to provide
treatment (including recycling and reclamation) of municipal sewage and
industrial waste.'' The 2002 POTW NESHAP excludes collection systems,
including sewers, pump stations, and other conveyance equipment located
outside the POTW treatment plant from the definition of affected
source.
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    \2\ CAA section 112(e)(5) adopts the definition of ``treatment
works'' from Clean Water Act (CWA) section 212(2), 33 U.S.C.
1292(2).
---------------------------------------------------------------------------

2. Applicability of the 2002 NESHAP: Industrial (Group 1) and Non-
Industrial (Group 2) Subcategories
    The 2002 POTW NESHAP set air pollution control requirements or
emission limits on existing, new, and reconstructed POTW. Briefly, a
POTW

is subject to the POTW NESHAP if: (1) The POTW is required to establish
and implement a pretreatment program per the requirements in 40 CFR
403.8 under the CWA. Pretreatment programs are required for POTW with a
design capacity of greater than 5 MGD and that receive wastewater from
an ``industrial user'' that contains pollutants which pass through or
interfere with the operation of the POTW. Pollutants that pass through
are those that remain in the wastewater and are not removed during
treatment operations at the POTW; and (2) either of the following:
     The POTW accepts waste streams regulated by another NESHAP
and provides treatment and controls as an agent for the industrial
facility. The industrial facility complies with its NESHAP requirements
specific to that wastewater stream by using the treatment and controls
located at the POTW; or
     The POTW is a major source of HAP emissions.
    Accordingly, POTW that are area sources are not subject to the
requirements in the 2002 rule unless they receive wastewater that is
subject to control under another NESHAP.
    Today we estimate that six facilities are subject to the POTW
NESHAP. A complete list of facilities subject to the POTW NESHAP is
available in the POTW RTR database, which is available for review in
the docket for this proposed rulemaking. The EPA recognizes that there
are approximately 16,000 POTW in the U.S.; however, most of these are
small municipalities that do not treat wastewater from industrial
users, and therefore, would not be subject to this regulation.
Additionally, POTW that do treat wastewater from industrial users are
generally required to develop and implement a pretreatment program that
limits the concentration of pollutants in wastewaters received at the
POTW, thus reducing the potential emissions of HAP so that they are
below major source thresholds. The EPA requests comment specifically
identifying other POTW that are subject to the POTW NESHAP.
    In the 2002 NESHAP, the source category is subcategorized based on
the way in which the POTW is providing treatment for wastewaters
received from an industrial source. The 2002 POTW NESHAP defines (40
CFR 63.1595) an ``industrial POTW'' as ``a POTW that accepts a waste
stream regulated by another NESHAP and provides treatment and controls
as an agent for the industrial discharger. The industrial discharger
complies with its NESHAP by using the treatment and controls located at
the POTW. For example, an industry discharges its benzene-containing
waste stream to the POTW for treatment to comply with 40 CFR part 61,
subpart FF--National Emission Standard for Benzene Waste Operations.
This definition does not include POTW treating waste streams not
specifically regulated under another NESHAP.'' In other words, if a
POTW is used as the control method by which an industrial source meets
the wastewater requirements in their source category NESHAP, then the
POTW is considered an ``industrial POTW treatment plant.'' An
``industrial POTW treatment plant'' is affected by the 2002 POTW NESHAP
regardless of the HAP emissions (i.e., does not have to be a major
source).
    In contrast, under the 2002 NESHAP, a ``non-industrial POTW'' is
defined (40 CFR 63.1595) as ``a POTW that does not meet the definition
of an industrial POTW as defined above.'' If a POTW treats wastewater
from industrial users, but does not treat industrial wastewaters
subject to control requirements in another NESHAP, then the POTW is a
``non-industrial POTW treatment plant.'' See section IV.D.2 of this
preamble for a discussion on proposed changes to these subcategories,
including proposed changes to the names for these subcategories (i.e.,
Group 1 and Group 2).
3. HAP Emission Points
    The amount and type of HAP emitted from a POTW is dependent on the
composition of the wastewater streams discharged to a POTW by
industrial users. Because HAP are not typically used in large
quantities by domestic dischargers, we do not expect domestic
dischargers to consistently or frequently contribute HAP constituents
to the wastewater and any domestic discharges of HAP are trivial in
comparison to industrial dischargers. An industrial user is defined in
the 2002 regulation to include both industrial and commercial
facilities that discharge wastewaters to the POTW. The primary HAP
emitted from the POTW that were identified as subject to the 2002
NESHAP include acetaldehyde, acetonitrile, chloroform, ethylene glycol,
formaldehyde, methanol, methylene chloride, tetrachloroethylene,
toluene, and xylenes. HAP present in wastewater entering POTW can
biodegrade, adhere to sewage sludge, volatilize to the air, or pass
through (remain in the wastewater discharge) to receiving waters.
Within the POTW source category, wastewater treatment units are the
most likely source for HAP emissions, but wastewater collection
systems, including sewers and other transport systems, may also have
significant emissions in cases where the systems transport industrial
wastewater. In addition to the wastewater treatment processes at a
POTW, other sources of HAP emissions, such as sewage sludge
incinerators, may be collocated at the same site. Sewage sludge
incineration is regulated under section 129 of the CAA and is not a
part of the POTW source category regulated under the POTW NESHAP as
discussed in this preamble. However, HAP emissions from any collocated
sources must be included when determining whether a source is a major
source of HAP.
4. Regulation of HAP Emissions in the 2002 POTW NESHAP
    The POTW NESHAP specifies requirements for both subcategories.
Under the POTW NESHAP, an existing, industrial (Group 1) POTW must meet
the requirements of the industrial source's NESHAP. For example, a POTW
that accepts and treats wastewater for a pulp and paper facility in
order to meet the wastewater requirements in 40 CFR part 63, subpart S
is subject to the specific requirements found in subpart S, instead of
requirements found in 40 CFR part 63, subpart VVV. A new or
reconstructed, industrial (Group 1) POTW must meet the requirements of
the industrial source's NESHAP or the requirements for new or
reconstructed, non-industrial (Group 2) POTW, whichever is more
stringent.
    There are no control requirements in the 2002 NESHAP for existing,
non-industrial (Group 2) POTW. However, new or reconstructed, non-
industrial (Group 2) POTW must equip each treatment unit up to, but not
including, the secondary influent pumping station, with a cover. The
affected emission points at new or reconstructed non-industrial (Group
2) POTW include, but are not limited to, influent waste stream
conveyance channels, bar screens, grit chambers, grinders, pump
stations, aerated feeder channels, primary clarifiers, primary effluent
channels, and primary screening stations. In addition, all covered
units, except the primary clarifiers, must have the air in the
headspace ducted to a control device in accordance with 40 CFR 63.693,
the standards for closed-vent systems and control devices found in
subpart DD of this part. As an alternative to these requirements, a new
or reconstructed, non-industrial (Group 2) POTW can demonstrate, for
all units up to the secondary influent pumping station or the secondary
treatment units, that the HAP fraction emitted does not exceed 0.014.
This is demonstrated by dividing the sum of all HAP emissions

from the primary treatment units by the sum of all HAP mass loadings
(i.e., the concentration of all HAP in the influent wastewater) on an
annual rolling average. The POTW is allowed to use any combination of
pretreatment, wastewater treatment plant modifications, and control
devices to achieve this performance standard.

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

    In October 2015, the EPA issued an information collection request
(ICR), pursuant to CAA section 114, to nine POTW (covering a total of
18 facilities) that were known to, or thought to potentially, own and
operate a POTW subject to the POTW NESHAP. EPA requested information on
the treatment units that are subject to requirements in the POTW NESHAP
(primary treatment units), as well as information on pretreatment
programs, collection sewers, and secondary treatment units. EPA also
requested information on control devices and location coordinates
(latitude and longitude) of the individual treatment units (if fugitive
sources) and emission points (if point sources). The ICR requested
information on any HAP-containing chemicals used as part of the
wastewater treatment process, point and fugitive HAP emissions,
practices used to control HAP emissions, and other aspects of facility
operations. The respondents to the ICR provided information on a total
of five facilities subject to the POTW NESHAP and 12 synthetic area \3\
or area source facilities not subject to the POTW NESHAP. Only the POTW
subject to the NESHAP were included in the risk modeling analysis. One
facility did not provide a response and it is unknown if this POTW is
subject to the POTW NESHAP. We received emissions data directly from
each POTW subject to the POTW NESHAP that responded to the survey in
the form of ToxChem+ or WATER9 modeling results. Following the initial
response, one POTW that was previously thought to be subject to the
POTW NESHAP submitted correspondence from their state, which defines
the POTW as an area source of HAP emissions, therefore, not subject to
the POTW NESHAP.\4\ Thus, we identified a total of four POTW subject to
the POTW NESHAP through the 2015 ICR.
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    \3\ A synthetic area facility installs controls in order to
reduce HAP emissions below major source thresholds prior to the
initial compliance date of the NESHAP.
    \4\ See Letter from State of Missouri regarding Bissell Point,
2016. While the agency no longer considers this POTW to be a major
source or subject to the POTW NESHAP, the POTW is still included in
discussions in supporting materials and risk modeling.
---------------------------------------------------------------------------

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

    The 2011 National Emissions Inventory (NEI version 2) provided
supplemental information for this RTR. The NEI is a database that
contains information about sources that emit criteria air pollutants,
their precursors, and HAP. The database includes estimates of annual
air pollutant emissions from point, nonpoint, and mobile sources in the
50 states, the District of Columbia, Puerto Rico, and the Virgin
Islands. The EPA collects this information and releases an updated
version of the NEI database every 3 years. The NEI includes information
necessary for conducting risk modeling, including annual HAP emissions
estimates from individual emission points at facilities and the related
emissions release parameters.
    For each emission record needed for the model input file for the
risk assessment (hereafter referred to as the ``RTR emissions
dataset'') that was not available from the 2015 ICR responses, the EPA
used available data in the 2011 NEI as the first alternative.\5\ The
2011 NEI was used to identify an additional two POTW that are subject
to the POTW NESHAP that had not received the ICR. For the six sources
found subject to the POTW NESHAP (the four POTW identified in the ICR
responses and the two POTW identified from the NEI), the 2011 NEI
provided emissions estimates for co-located emission points that are
not part of the POTW source category. These data include emissions from
boilers, engines, and sewage sludge incinerators that are located at
the POTW, but are not in the POTW source category. These data were
incorporated into the RTR emissions dataset to determine the whole
facility risk.
---------------------------------------------------------------------------

    \5\ See Inputs to the Publicly Owned Treatment Works March 2016
Residual Risk Modeling, June 2016, located in docket number EPA-HQ-
OAR-2016-0490.
---------------------------------------------------------------------------

    The EPA's Enforcement Compliance History Online (ECHO) database was
also used as a tool to identify which POTW were potentially subject to
the POTW NESHAP and provided a list of sources to consider for the 2015
ICR. ECHO provides integrated compliance and enforcement information
for approximately 800,000 regulated facilities nationwide. Using the
search feature in ECHO, the EPA identified twenty POTW that could
potentially be subject to the POTW NESHAP. The EPA then searched state
Web sites for operating permits for these 20 POTW to determine whether
the permits stated the POTW was subject to the rule. The four POTW
identified as subject to the POTW NESHAP through the ICR were
identified in the list of potential sources found in the ECHO database
and subsequent permit search.
    The EPA searched for Reasonably Available Control Technology
(RACT), Best Available Control Technology (BACT), and Lowest Achievable
Emission Rate (LAER) determinations in the RACT/BACT/LAER
Clearinghouse. This is a database that contains case-specific
information of air pollution technologies that have been required to
reduce the emissions of air pollutants from stationary sources. Under
the EPA's New Source Review (NSR) program, if a facility is planning
new construction or a modification that will increase the air emissions
by a large amount, an NSR permit must be obtained. This central
database promotes the sharing of information among permitting agencies
and aids in case-by-case determinations for NSR permits. We examined
information contained in the RACT/BACT/LAER Clearinghouse to determine
what technologies are currently used at POTW to reduce air emissions.

III. Analytical Procedures

    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 did we estimate post-MACT risks posed by the source category?

    The EPA conducted 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 non-cancer health effects, and the hazard quotient (HQ) for
acute exposures to HAP with the potential to cause non-cancer health
effects. The assessment also provides estimates of the distribution of
cancer risks within the exposed populations, cancer incidence, and an
evaluation of the potential for adverse environmental effects. 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 Publicly Owned Treatment Works Source Category in
Support of the December 2016 Risk and Technology Review Proposed Rule
(hereafter ``Residual Risk Report''). The

methods used to assess risks (as described in the seven primary steps
below) are consistent with the methods that were peer-reviewed by a
panel of the EPA's Science Advisory Board (SAB) in 2009 and described
in their peer review report issued in 2010.\6\ The methods used here
are also consistent with the key recommendations contained in that
report.
---------------------------------------------------------------------------

    \6\ U.S. EPA SAB. 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, May 2010.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    Data for seven POTW were used to create the RTR emissions dataset,
as described in section II.C of this preamble. As stated in section
II.C of this preamble, we evaluated the risk associated with emissions
from seven POTW, even though one POTW was later determined to be an
area source of HAP emissions. The emissions sources included in the RTR
emissions dataset include the following types of emission sources
currently regulated by the POTW NESHAP: Primary treatment units
including, lift stations, bar screens, grit chambers, grinders,
Parshall flumes, denitrification, primary clarifiers, primary settling
basins, and primary effluent channels. The RTR emissions dataset also
includes the following types of emission sources not currently
regulated by the POTW NESHAP: Secondary treatment units, including
secondary clarifiers, aeration tanks, trickling filters, UNOX systems,
and open lagoons; tertiary treatment units, including chlorine sumps,
splitter boxes, and chlorine contact tanks; and gravity thickeners for
sludge handling. For both emissions sources that are and those that are
not currently regulated by the POTW NESHAP, the dataset includes both
fugitive emissions and stack emissions. This RTR emissions dataset is
based primarily on data gathered through the 2015 ICR and supplemented
with data from 2011 NEI, 2011 NATA, and ECHO, as described in sections
II.C and II.D of this preamble. These data sources provided all of the
emissions data in the RTR emissions dataset and nearly all of the
facility-specific data needed to conduct the risk modeling analysis.
However, there were limited instances where default values were used to
fill gaps in the facility-specific data used in the risk modeling
analysis. For example, default values were used for stack and fugitive
release parameters. Use of defaults are discussed in detail in the
memorandum, Inputs to the Publicly Owned Treatment Works March 2016
Residual Risk Modeling, June 2016 (Modeling Inputs Memo), available in
the docket for this action.
    The RTR emissions dataset was refined following an extensive
quality assurance check of source locations, emission release
characteristics, and annual emission estimates. We checked the
coordinates of each emission source in the dataset using ArcGIS to
ensure the emission point locations were correct. For further
information on the EPA's quality assurance review, see the Modeling
Inputs Memo available in the docket for this action.
    A list of the six POTW and additional information used to develop
the RTR emissions dataset are available in the POTW RTR database
itself, and additional documentation on the development of this
database is provided in the Modeling Inputs Memo, both of which are
available in the docket for this action.
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 the specified annual time
period. In some cases, these ``actual'' emission levels are lower than
the emission levels required to comply with the current MACT standards.
The emissions level allowed to be emitted by the MACT standards is
referred to as the ``MACT-allowable'' emissions level. We discussed the
use 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 RTRs (71 FR 34428, June 14, 2006,
and 71 FR 76609, December 21, 2006, respectively). In those actions, we
noted that assessing the risks at the MACT-allowable level is
inherently reasonable since these risks reflect 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.)
    We used the RTR emissions dataset to estimate MACT-allowable
emissions levels. POTW were asked to provide their design capacity and
their average treatment capacity as part of the 2015 ICR. In
discussions with the POTW that responded, EPA noted that most POTW
operate below their design capacity. To be conservative, the EPA
estimated that the reported emissions were for operations at half
capacity. Therefore, the EPA chose to use a single multiplier of 2.0 to
scale the actual annual emissions to allowable annual emissions. The
docket for this rulemaking contains information on the development of
estimated MACT-allowable emissions in the Modeling Inputs Memo.
3. How did we conduct dispersion modeling, determine inhalation
exposures, and estimate individual and population inhalation risks?
    Both long-term and short-term inhalation exposure concentrations
and health risks from the source category addressed in this proposal
were estimated using the Human Exposure Model (Community and Sector
HEM-3 version 1.1.0). 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,\7\ and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
---------------------------------------------------------------------------

    \7\ This metric comes from the Benzene NESHAP. See 54 FR 38046.
---------------------------------------------------------------------------

    The air dispersion model used by the HEM-3 model (AERMOD) is one of
the EPA's preferred models for assessing 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 (2011) of
hourly surface and upper air observations for more than 800
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 unit risk factors and other
health benchmarks is used to estimate health risks. These risk factors
and

health benchmarks are the latest values recommended by the EPA for HAP
and other toxic air pollutants. These values are available at https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants and are discussed in more
detail later in this section.
---------------------------------------------------------------------------

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

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentrations of each HAP
emitted by each source for which we have emissions data in the source
category. The air concentrations at each nearby census block centroid
were used as a surrogate for the chronic inhalation exposure
concentration for all the people who reside in that census block. We
calculated the MIR for each facility as the cancer risk associated with
a continuous lifetime (24 hours per day, 7 days per week, and 52 weeks
per year for a 70-year period) exposure to the maximum concentration at
the centroid of inhabited census blocks. Individual cancer risks were
calculated by multiplying the estimated lifetime exposure to the
ambient concentration of each of the 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 probability 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 URE values 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) URE values, where available. In cases where
new, scientifically credible dose response values have been developed
in a manner consistent with the 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 EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source category as
the sum of the risks for each of the carcinogenic HAP (including those
classified as carcinogenic to humans, likely to be carcinogenic to
humans, and suggestive evidence of carcinogenic potential) \10\ emitted
by the modeled sources. Cancer incidence and the distribution of
individual cancer risks for the population within 50 km of the sources
were also estimated for the source category as part of this assessment
by summing individual risks. 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.
---------------------------------------------------------------------------

    \10\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen, and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks 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 http://yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
---------------------------------------------------------------------------

    To assess the risk of non-cancer health effects from chronic
exposures, we summed the HQ for each of the HAP that affects a common
target organ system to obtain the HI for that target organ system (or
target organ-specific HI, TOSHI). The HQ is the estimated exposure
divided by the chronic reference value, which is a value selected from
one of several sources. First, the chronic reference level can be the
EPA reference concentration (RfC) (https://iaspub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary), 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.'' Alternatively, in cases
where an RfC from the EPA's IRIS database is not available or where the
EPA determines that using a value other than the RfC is appropriate,
the chronic reference level can be a value from the following
prioritized sources: (1) The Agency for Toxic Substances and Disease
Registry (ATSDR) Minimum Risk Level (http://www.atsdr.cdc.gov/mrls/index.asp), which is defined as ``an estimate of daily human exposure
to a hazardous substance that is likely to be without an appreciable
risk of adverse non-cancer health effects (other than cancer) over a
specified duration of exposure''; (2) the CalEPA Chronic Reference
Exposure Level (REL) (http://oehha.ca.gov/air/crnr/notice-adoption-air-toxics-hot-spots-program-guidance-manual-preparation-health-risk-0),
which is defined as ``the concentration level (that is expressed in
units of micrograms per cubic meter ([mu]g/m\3\) for inhalation
exposure and in a dose expressed in units of milligram per kilogram-day
(mg/kg-day) for oral exposures), at or below which no adverse health
effects are anticipated for a specified exposure duration''; 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, in
place of or in concert with other values.
    As mentioned above, in order to characterize non-cancer chronic
effects, and in response to key recommendations from the SAB, the EPA
selects dose-response values that reflect the best available science
for all HAP included in RTR risk assessments.\11\ More specifically,
for a given HAP, the EPA examines the availability of inhalation
reference values from the sources included in our tiered approach
(e.g., IRIS first, ATSDR second, CalEPA third) and determines which
inhalation reference value represents the best available science. Thus,
as new inhalation reference values become available, the EPA will
typically evaluate them and determine whether they should be given
preference over those currently being used in RTR risk assessments.
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    \11\ The SAB peer review of RTR Risk Assessment Methodologies is
available at http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
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    The EPA also evaluated screening estimates of acute exposures and
risks for each of the HAP (for which appropriate acute dose-response
values are available) at the point of highest potential off-site
exposure for each facility. To do this, the EPA estimated the risks
when both the peak hourly emissions rate and worst-case dispersion
conditions occur. We also assume that a person is located at the point
of highest impact during that same time. In accordance with our mandate
in section 112 of the CAA, we use the point of highest off-site
exposure to assess the potential risk to the maximally exposed
individual. The acute HQ is the estimated acute exposure divided by the
acute dose-response value. In each case, the EPA calculated acute HQ
values using best available, short-term dose-response values. These
acute dose-response values, which are described below, include the
acute REL, acute exposure guideline levels (AEGL) and emergency
response planning guidelines (ERPG) for 1-hour exposure durations. As
discussed below, we used conservative

assumptions for emissions rates, meteorology, and exposure location.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary) is defined as ``the concentration level at
or below which no adverse health effects are anticipated for a
specified exposure duration.'' Id. at page 2. Acute REL values are
based on the most sensitive, relevant, adverse health effect reported
in the peer-reviewed medical and toxicological literature. Acute REL
values 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.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). The National Advisory Committee (NAC)
for the Development of Acute Exposure Guideline Levels for Hazardous
Substances, usually referred to as the AEGL Committee or the NAC/AEGL
committee, developed AEGL values for at least 273 of the 329 chemicals
on the AEGL priority chemical list. The last meeting of the NAC/AEGL
Committee was in April 2010, and its charter expired in October 2011.
The NAC/AEGL Committee 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).
    As described in Standing Operating Procedures (SOP) of the National
Advisory Committee on Acute Exposure Guideline Levels for Hazardous
Chemicals (https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf),\12\ ``the
NRC's previous name for acute exposure levels--community emergency
exposure levels was replaced by the term AEGL to reflect the broad
application of these values to planning, response, and prevention in
the community, the workplace, transportation, the military, and the
remediation of Superfund sites.'' Id. at 2. This document also states
that AEGL values ``represent threshold exposure limits for the general
public and are applicable to emergency exposures ranging from 10
minutes to eight hours.'' Id. at 2.
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    \12\ National Academy of Sciences (NAS), 2001. Standing
Operating Procedures for Developing Acute Exposure Levels for
Hazardous Chemicals, page 2.
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    The document lays out the purpose and objectives of AEGL by stating
that ``the primary purpose of the AEGL program and the National
Advisory Committee for Acute Exposure Guideline Levels for Hazardous
Substances is to develop guideline levels for once-in-a-lifetime,
short-term exposures to airborne concentrations of acutely toxic, high-
priority chemicals.'' Id. at 21. In detailing the intended application
of AEGL values, the document states that ``[i]t is anticipated that the
AEGL values will be used for regulatory and nonregulatory purposes by
U.S. Federal and state agencies and possibly the international
community in conjunction with chemical emergency response, planning,
and prevention programs. More specifically, the AEGL values will be
used for conducting various risk assessments to aid in the development
of emergency preparedness and prevention plans, as well as real-time
emergency response actions, for accidental chemical releases at fixed
facilities and from transport carriers.'' Id. at 31.
    The AEGL-1 value is then 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.'' Id. at 3. 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. Similarly, the document defines AEGL-2 values 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.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's Emergency Response
Planning (ERP) Committee document titled, ERPGS Procedures and
Responsibilities (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), which
states that, ``Emergency Response Planning Guidelines were developed
for emergency planning and are intended as health based guideline
concentrations for single exposures to chemicals.'' \13\ Id. at 1. The
ERPG-1 value 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 value is defined as ``the
maximum airborne concentration below which it is believed that nearly
all individuals could be exposed for up to one hour without
experiencing or developing irreversible or other serious health effects
or symptoms which could impair an individual's ability to take
protective action.'' Id. at 1.
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    \13\ ERP Committee Procedures and Responsibilities. November 1,
2006. American Industrial Hygiene Association.
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    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly-defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been developed
because the types of effects for these chemicals are not consistent
with the AEGL-1/ERPG-1 definitions; in these instances, we compare
higher severity level AEGL-2 or ERPG-2 values to our modeled exposure
levels to screen for potential acute concerns. When AEGL-1/ERPG-1
values are available, they are used in our acute risk assessments.
    Acute REL values for 1-hour exposure durations are typically lower
than their corresponding AEGL-1 and ERPG-1 values. Even though their
definitions are slightly different, AEGL-1 values are often the same as
the corresponding ERPG-1 values, and AEGL-2 values are often equal to
ERPG-2 values. Maximum HQ values from our acute screening risk
assessments typically result when basing them on the acute REL value
for a particular pollutant. In cases where our maximum acute HQ value
exceeds 1, we also report the HQ value based on the next highest acute
dose-response value (usually the AEGL-1 and/or the ERPG-1 value).
    To develop screening estimates of acute exposures in the absence of
hourly

emissions data, generally we first develop estimates of maximum hourly
emissions rates by multiplying the average actual annual hourly
emissions rates by a default factor to cover routinely variable
emissions. We choose the factor to use partially based on process
knowledge and engineering judgment. The factor chosen also reflects a
Texas study of short-term emissions variability, which showed that most
peak emission events in a heavily-industrialized four-county area
(Harris, Galveston, Chambers, and Brazoria Counties, Texas) were less
than twice the annual average hourly emissions rate. The highest peak
emissions event was 74 times the annual average hourly emissions rate,
and the 99th percentile ratio of peak hourly emissions rate to the
annual average hourly emissions rate was 9.\14\ Considering this
analysis, to account for more than 99 percent of the peak hourly
emissions, we apply a conservative screening multiplication factor of
10 to the average annual hourly emissions rate in our acute exposure
screening assessments as our default approach. However, we use a factor
other than 10 if we have information that indicates that a different
factor is appropriate for a particular source category. For this source
category, the default factor of 10 was used.
---------------------------------------------------------------------------

    \14\ Allen, et al., 2004. Variable Industrial VOC Emissions and
their impact on ozone formation in the Houston Galveston Area. Texas
Environmental Research Consortium. https://www.researchgate.net/publication/237593060_Variable_Industrial_VOC_Emissions
and_their_Impact_on_Ozone_Formation_in_the_Houston_Galveston_Area.
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    As part of our acute risk assessment process, for cases where acute
HQ values from the screening step were less than or equal to 1 (even
under the conservative assumptions of the screening analysis), acute
impacts were deemed negligible and no further analysis was performed
for these HAP. In cases where an acute HQ from the screening step was
greater than 1, additional site-specific data were considered to
develop a more refined estimate of the potential for acute impacts of
concern. Ideally, we would prefer to have continuous measurements over
time to see how the emissions vary by each hour over an entire year.
Having a frequency distribution of hourly emissions rates over a year
would allow us to perform a probabilistic analysis to estimate
potential threshold exceedances and their frequency of occurrence. Such
an evaluation could include a more complete statistical treatment of
the key parameters and elements adopted in this screening analysis.
Recognizing that this level of data is rarely available, we instead
rely on the multiplier approach. To better characterize the potential
health risks associated with estimated 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,\15\ we generally examine a
wider range of available acute health metrics (e.g., RELs, AEGLs) than
we do for our chronic risk assessments. This is in response to the
SAB's acknowledgement that there are generally more data gaps and
inconsistencies in acute reference values than there are in chronic
reference values. In some cases, when Reference Value Arrays \16\ for
HAP have been developed, we consider additional acute values (i.e.,
occupational and international values) to provide a more complete risk
characterization.
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    \15\ The SAB peer review of RTR Risk Assessment Methodologies is
available at http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
    \16\ U.S. EPA. Chapter 2.9, Chemical Specific Reference Values
for Formaldehyde in Graphical Arrays of Chemical-Specific Health
Effect Reference Values for Inhalation Exposures (Final Report).
U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-09/
061, 2009, and available online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
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4. How did we conduct the multipathway exposure and risk screening?
    The EPA conducted a screening analysis examining the potential for
significant human health risks due to exposures via routes other than
inhalation (i.e., ingestion). We first determined whether any sources
in the source category emitted any HAP known to be persistent and
bioaccumulative in the environment (PB-HAP). The PB-HAP compounds or
compound classes are identified for the screening from the EPA's Air
Toxics Risk Assessment Library (available at http://www2.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library).
    For the POTW source category, we identified emissions of a single
polycyclic organic matter (POM) species, specifically 2-
methylnaphthalene. Because one or more of these PB-HAP are emitted by
at least one facility in the POTW source category, we proceeded to the
next step of the evaluation. In this step, we determined whether the
facility-specific emissions rates of the emitted PB-HAP were large
enough to create the potential for significant non-inhalation human
health risks under reasonable worst-case conditions. To facilitate this
step, we developed emissions rate screening levels for several PB-HAP
using 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 emissions rate screening levels are: Lead, cadmium,
chlorinated dibenzodioxins and furans, mercury compounds, and POM. We
conducted a sensitivity analysis on the screening scenario to ensure
that its key design parameters would represent the upper end of the
range of possible values, such that it would represent a conservative,
but not impossible scenario. The facility-specific emissions rates of
these PB-HAP were compared to the emission rate screening levels for
these PB-HAP to assess the potential for significant human health risks
via non-inhalation pathways. We call this application of the TRIM.FaTE
model the Tier 1 TRIM-screen or Tier 1 screen.
    For the purpose of developing emissions rates for our Tier 1 TRIM-
screen, we derived emission levels for these PB-HAP (other than lead
compounds) at which the maximum excess lifetime cancer risk would be 1-
in-1 million (i.e., for polychlorinated dibenzodioxins and furans and
POM) or, for HAP that cause non-cancer health effects (i.e., cadmium
compounds and mercury compounds), the maximum HQ would be 1. If the
emissions rate of any PB-HAP included in the Tier 1 screen exceeds the
Tier 1 screening emissions rate for any facility, we conduct a second
screen, which we call the Tier 2 TRIM-screen or Tier 2 screen.
    In the Tier 2 screen, the location of each facility that exceeded
the Tier 1 emission rate is used to refine the assumptions associated
with the environmental scenario while maintaining the exposure scenario
assumptions. A key assumption that is part of the Tier 1 screen is that
a lake is located near the facility; we confirm the existence of lakes
near the facility as part of the Tier 2 screen. We then adjust the
risk-based Tier 1 screening level for each PB-HAP for each facility
based on an understanding of how exposure concentrations estimated for
the screening scenario change with meteorology and environmental
assumptions. PB-HAP emissions that do not exceed these new Tier 2
screening levels are considered to pose no unacceptable risks. If the
PB-HAP emissions for a facility exceed the Tier 2 screening emissions
rate and data are available, we may decide to conduct a more refined
Tier 3 multipathway

assessment. There are several analyses that can be included in a Tier 3
screen depending upon the extent of refinement warranted, including
validating that the lake is fishable and considering plume-rise to
estimate emissions lost above the mixing layer. If the Tier 3 screen is
exceeded, the EPA may further refine the assessment. Notably, for the
POTW source category, emissions of POM did not exceed the Tier 1
screening level. Therefore, the Tier 2 and 3 screening scenarios were
not necessary.
    For further information on the multipathway analysis approach, see
the Residual Risk Report, which is available in the docket for this
action.
5. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
    The EPA conducts a screening assessment to examine the potential
for adverse environmental effects 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.''
b. Environmental HAP
    The EPA focuses on seven HAP, which we refer to as ``environmental
HAP,'' in its screening analysis: Five PB-HAP and two acid gases. The
five PB-HAP are cadmium, dioxins/furans, POM, mercury (both inorganic
mercury and methyl mercury), and lead compounds. The two acid gases are
hydrogen chloride (HCl) and hydrogen fluoride (HF). The rationale for
including these seven HAP in the environmental risk screening analysis
is presented below.
    HAP that persist and bioaccumulate are of particular environmental
concern because they accumulate in the soil, sediment, and water. The
PB-HAP are taken up, through sediment, soil, water, and/or ingestion of
other organisms, by plants or animals (e.g., small fish) at the bottom
of the food chain. As larger and larger predators consume these
organisms, concentrations of the PB-HAP in the animal tissues increases
as does the potential for adverse effects. The five PB-HAP we evaluate
as part of our screening analysis account for 99.8 percent of all PB-
HAP emissions nationally from stationary sources (on a mass basis from
the 2005 EPA NEI).
    In addition to accounting for almost all of the mass of PB-HAP
emitted, we note that the TRIM.FaTE model that we use to evaluate
multipathway risk allows us to estimate concentrations of cadmium
compounds, dioxins/furans, POM, and mercury in soil, sediment, and
water. For lead compounds, we currently do not have the ability to
calculate these concentrations using the TRIM.FaTE model. Therefore, to
evaluate the potential for adverse environmental effects from lead
compounds, we compare the estimated HEM-modeled exposures from the
source category emissions of lead with the level of the secondary NAAQS
for lead.\17\ We consider values below the level of the secondary lead
NAAQS to be unlikely to cause adverse environmental effects.
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    \17\ The Secondary Lead NAAQS is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``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.''
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    Due to their well-documented potential to cause direct damage to
terrestrial plants, we include two acid gases, HCl, and HF in the
environmental screening analysis. According to the 2005 NEI, HCl and HF
account for about 99 percent (on a mass basis) of the total acid gas
HAP emitted by stationary sources in the U.S. In addition to the
potential to cause direct damage to plants, high concentrations of HF
in the air have been linked to fluorosis in livestock. Air
concentrations of these HAP are already calculated as part of the human
multipathway exposure and risk screening analysis using the HEM3-AERMOD
air dispersion model, and we are able to use the air dispersion
modeling results to estimate the potential for an adverse environmental
effect.
    The EPA acknowledges that other HAP beyond the seven HAP discussed
above may have the potential to cause adverse environmental effects.
Therefore, the EPA may include other relevant HAP in its environmental
risk screening in the future, as modeling science and resources allow.
The EPA invites comment on the extent to which other HAP emitted by the
source category may cause adverse environmental effects. Such
information should include references to peer-reviewed ecological
effects benchmarks that are of sufficient quality for making regulatory
decisions, as well as information on the presence of organisms located
near facilities within the source category that such benchmarks
indicate could be adversely affected.
c. Ecological Assessment Endpoints and Benchmarks for PB-HAP
    An important consideration in the development of the EPA's
screening methodology is the selection of ecological assessment
endpoints and benchmarks. Ecological assessment endpoints are defined
by the ecological entity (e.g., aquatic communities, including fish and
plankton) and its attributes (e.g., frequency of mortality). Ecological
assessment endpoints can be established for organisms, populations,
communities or assemblages, and ecosystems.
    For PB-HAP (other than lead compounds), we evaluated the following
community-level ecological assessment endpoints to screen for organisms
directly exposed to HAP in soils, sediment, and water:
     Local terrestrial communities (i.e., soil invertebrates,
plants) and populations of small birds and mammals that consume soil
invertebrates exposed to PB-HAP in the surface soil;
     Local benthic (i.e., bottom sediment dwelling insects,
amphipods, isopods, and crayfish) communities exposed to PB-HAP in
sediment in nearby water bodies; and
     Local aquatic (water-column) communities (including fish
and plankton) exposed to PB-HAP in nearby surface waters.
    For PB-HAP (other than lead compounds), we also evaluated the
following population-level ecological assessment endpoint to screen for
indirect HAP exposures of top consumers via the bioaccumulation of HAP
in food chains:
     Piscivorous (i.e., fish-eating) wildlife consuming PB-HAP-
contaminated fish from nearby water bodies.
    For cadmium compounds, dioxins/furans, POM, and mercury, we
identified the available ecological benchmarks for each assessment
endpoint. An ecological benchmark represents a concentration of HAP
(e.g., 0.77 ug of HAP per liter of water) that has been linked to a
particular environmental effect level through scientific study. For PB-
HAP we identified, where possible, ecological benchmarks at the
following effect levels:
     Probable effect levels (PEL): Level above which adverse
effects are expected to occur frequently;

     Lowest-observed-adverse-effect level (LOAEL): The lowest
exposure level tested at which there are biologically significant
increases in frequency or severity of adverse effects; and
     No-observed-adverse-effect levels (NOAEL): The highest
exposure level tested at which there are no biologically significant
increases in the frequency or severity of adverse effect.
    We established a hierarchy of preferred benchmark sources to allow
selection of benchmarks for each environmental HAP at each ecological
assessment endpoint. In general, the EPA sources that are used at a
programmatic level (e.g., Office of Water, Superfund Program) were used
in the analysis, if available. If not, the EPA benchmarks used in
regional programs (e.g., Superfund) were used. If benchmarks were not
available at a programmatic or regional level, we used benchmarks
developed by other federal agencies (e.g., National Oceanic and
Atmospheric Administration (NOAA)) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. 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.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the following
ecological assessment endpoint:
     Local terrestrial plant communities with foliage exposed
to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available chronic benchmarks). For HCl, the EPA
identified chronic benchmark concentrations. We note that the benchmark
for chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where the EPA includes regulatory requirements to prevent an
exceedance of the reference concentration for human health, additional
analyses for adverse environmental effects of HCl would not be
necessary.
    For HF, the EPA identified chronic benchmark concentrations for
plants and evaluated chronic exposures to plants in the screening
analysis. High concentrations of HF in the air have also been linked to
fluorosis in livestock. However, the HF concentrations at which
fluorosis in livestock occur are higher than those at which plant
damage begins. Therefore, the benchmarks for plants are protective of
both plants and livestock.
e. Screening Methodology
    For the environmental risk screening analysis, the EPA first
determined whether any facilities in the POTW source category emitted
any of the seven environmental HAP. For the POTW source category, we
identified emissions of a single POM species, specifically 2-
methylnaphthalene.
    Because one or more of the seven environmental HAP evaluated are
emitted by at least one facility in the source category, we proceeded
to the second step of the evaluation.
f. PB-HAP Methodology
    For cadmium, mercury, POM, and dioxins/furans, the environmental
screening analysis consists of two tiers, while lead compounds are
analyzed differently as discussed earlier. In the first tier, we
determined whether the maximum facility-specific emission rates of each
of the emitted environmental HAP were large enough to create the
potential for adverse environmental effects under reasonable worst-case
environmental conditions. These are the same environmental conditions
used in the human multipathway exposure and risk screening analysis.
    To facilitate this step, TRIM.FaTE was run for each PB-HAP under
hypothetical environmental conditions designed to provide
conservatively high HAP concentrations. The model was set to maximize
runoff from terrestrial parcels into the modeled lake, which in turn,
maximized the chemical concentrations in the water, the sediments, and
the fish. The resulting media concentrations were then used to back-
calculate a screening level emission rate that corresponded to the
relevant exposure benchmark concentration value for each assessment
endpoint. To assess emissions from a facility, the reported emission
rate for each PB-HAP was compared to the screening level emission rate
for that PB-HAP for each assessment endpoint. If emissions from a
facility do not exceed the Tier 1 screening level, the facility
``passes'' the screen, and, therefore, is not evaluated further under
the screening approach. If emissions from a facility exceed the Tier 1
screening level, we evaluate the facility further in Tier 2.
    In Tier 2 of the environmental screening analysis, the emission
rate screening levels 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 screen. The modeling domain for each facility in the
Tier 2 analysis consists of 8 octants. Each octant contains 5 modeled
soil concentrations at various distances from the facility (5 soil
concentrations x 8 octants = total of 40 soil concentrations per
facility) and one lake with modeled concentrations for water, sediment,
and fish tissue. In the Tier 2 environmental risk screening analysis,
the 40 soil concentration points are averaged to obtain an average soil
concentration for each facility for each 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 level, the facility passes
the screen, and typically is not evaluated further. If emissions from a
facility exceed the Tier 2 screening level, the facility does not pass
the screen and, therefore, may have the potential to cause adverse
environmental effects. Such facilities are evaluated further to
investigate factors such as the magnitude and characteristics of the
area of exceedance. Notably, for the POTW source category, emissions of
POM did not exceed the Tier 1 ecological screening level. Therefore,
the Tier 2 screen was not necessary.
    For further information on the environmental screening analysis
approach, see the Residual Risk Report, which is available in the
docket for this action.
6. How did 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 from all other
emission sources at the facility for which we have data. Using the most
current available NEI data at the time of the analysis, the EPA
developed ``facility-wide'' emissions estimates. For this category, the
latest available version of the NEI was the 2011 NEI Version 2. It is
important to note that the NEI

facility-wide inventory may not always reflect the level of detail or
be representative of the same temporal period that is found in the
source category specific inventory. Further information on the NEI,
which is developed from state/local/tribal submitted data, can be found
on the EPA's Web site at: https://www.epa.gov/air-emissions-inventories/national-emissions-inventory.
    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, the
modeled source category 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
specifically examined the facility that was associated with the highest
estimate of risk and determined the percentage of that risk
attributable to the source category of interest. The Residual Risk
Report, 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 did we consider uncertainties in risk assessment?
    In the Benzene NESHAP, we concluded that risk estimation
uncertainty should be considered in our decision-making under the ample
margin of safety framework. 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 protective 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. A more thorough discussion of these
uncertainties is included in the Residual Risk Report, which is
available in the docket for this action.
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.
c. Uncertainties in Inhalation Exposure
    The EPA did not include the effects of human mobility on exposures
in the assessment. Specifically, short-term mobility and long-term
mobility between census blocks in the modeling domain were not
considered.\18\ The approach of not considering short or long-term
population mobility does not bias the estimate of the theoretical MIR
(by definition), nor does it affect the estimate of cancer incidence
because the total population number remains the same. It does, however,
affect the shape of the distribution of individual risks across the
affected population, shifting it toward higher estimated individual
risks at the upper end and reducing the number of people estimated to
be at lower risks, thereby increasing the estimated number of people at
specific high risk levels (e.g., 1-in-10 thousand or 1-in-1 million).
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    \18\ Short-term mobility is movement from one micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
---------------------------------------------------------------------------

    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live farther from the
facility and under-predict exposures for people in the census block who
live closer to the facility. Thus, using the census block centroid to
predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence. We reduce this uncertainty by analyzing
large census blocks near facilities using aerial imagery and adjusting
the location of the block centroid to better represent the population
in the block, as well as adding additional receptor locations where the
block population is not well represented by a single location.
    The assessment evaluates the cancer inhalation risks associated
with pollutant exposures over a 70-year period, which is the assumed
lifetime of an individual. In reality, both the length of time that
modeled emission sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
domestic facilities) will influence the future risks posed by a given
source or source category. Depending on the characteristics of the
industry, these factors will, in most cases, result in an overestimate
both in individual risk levels and in the total estimated number of
cancer cases. However, in the unlikely scenario where a facility
maintains, or even increases, its emissions levels over a period of
more than 70 years, residents live beyond 70 years at the same
location, and the residents spend most of their days at that location,
then the cancer inhalation risks could potentially be underestimated.
However, annual cancer incidence estimates from exposures to emissions
from these sources would not be affected by the length of time an
emissions source operates.
    The exposure estimates used in these analyses assume chronic
exposures to ambient (outdoor) levels of pollutants. Because most
people spend the majority

of their time indoors, actual exposures may not be as high, depending
on the characteristics of the pollutants modeled. For many of the HAP,
indoor levels are roughly equivalent to ambient levels, but for very
reactive pollutants or larger particles, indoor levels are typically
lower. This factor has the potential to result in an overestimate of 25
to 30 percent of exposures.\19\
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    \19\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
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    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 that
should be highlighted. 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.
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 non-cancer effects from both chronic and acute
exposures. Some uncertainties may be considered quantitatively, and
others generally are expressed in qualitative terms. We note as a
preface to this discussion a point on dose-response uncertainty that is
brought out in the EPA's 2005 Cancer Guidelines; 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'' (EPA's 2005 Cancer Guidelines,
pages 1-7). This is the approach followed here as summarized in the
next several paragraphs. A complete detailed discussion of
uncertainties and variability in dose-response relationships is given
in the Residual Risk Report, which is available in the docket for this
action.
    Cancer URE values used in our risk assessments are those that have
been developed to generally provide an upper bound estimate of risk.
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).\20\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances the risk could be
greater.\21\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for other uses
(e.g., priority-setting or expected benefits analysis).
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    \20\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \21\ 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.
---------------------------------------------------------------------------

    Chronic non-cancer RfC and reference dose (RfD) values represent
chronic exposure levels that are intended to be health-protective
levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure (RfC) or a daily oral exposure (RfD) to the human
population (including sensitive subgroups) that is likely to be without
an appreciable risk of deleterious effects during a lifetime. To derive
values that are intended to be ``without appreciable risk,'' the
methodology relies upon an uncertainty factor (UF) approach (U.S. EPA,
1993 and 1994) which considers uncertainty, variability, and gaps in
the available data. The UF are applied to derive reference values that
are intended to protect against appreciable risk of deleterious
effects. The UF are commonly default values,\22\ e.g., factors of 10 or
3, used in the absence of compound-specific data; where data are
available, UF may also be developed using compound-specific
information. When data are limited, more assumptions are needed and
more UF are used. Thus, there may be a greater tendency to overestimate
risk in the sense that further study might support development of
reference values that are higher (i.e., less potent) because fewer
default assumptions are needed. However, for some pollutants, it is
possible that risks may be underestimated.
---------------------------------------------------------------------------

    \22\ According to the NRC report, Science and Judgment in Risk
Assessment (NRC, 1994) ``[Default] options are generic approaches,
based on general scientific knowledge and policy judgment, that are
applied to various elements of the risk assessment process when the
correct scientific model is unknown or uncertain.'' The 1983 NRC
report, Risk Assessment in the Federal Government: Managing the
Process, defined default option as ``the option chosen on the basis
of risk assessment policy that appears to be the best choice in the
absence of data to the contrary'' (NRC, 1983a, p. 63). Therefore,
default options are not rules that bind the Agency; rather, the
Agency may depart from them in evaluating the risks posed by a
specific substance when it believes this to be appropriate. In
keeping with the EPA's goal of protecting public health and the
environment, default assumptions are used to ensure that risk to
chemicals is not underestimated (although defaults are not intended
to overtly overestimate risk). See EPA, An Examination of EPA Risk
Assessment Principles and Practices, EPA/100/B-04/001, 2004,
available at https://nctc.fws.gov/resources/course-resources/pesticides/Risk%20Assessment/Risk%20Assessment%20Principles%20and%20Practices.pdf.
---------------------------------------------------------------------------

    While collectively termed ``UF,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies differences);
(3) uncertainty in extrapolating from data obtained in a study with
less-than-lifetime exposure (i.e., extrapolating from sub-chronic to
chronic exposure); (4) uncertainty in extrapolating the observed data
to obtain an estimate of the exposure associated with no adverse
effects; and (5) uncertainty when the database is incomplete or there
are problems with the applicability of available studies.
    Many of the UF used to account for variability and uncertainty in
the development of acute reference values are quite similar to those
developed for chronic durations, but they more often use individual UF
values that may be less than 10. The UF are applied based on chemical-
specific or health effect-specific information (e.g., simple irritation
effects do not vary appreciably between human individuals, hence a
value of 3 is typically used), or based on

the purpose for the reference value (see the following paragraph). The
UF applied in acute reference value derivation include: (1)
Heterogeneity among humans; (2) uncertainty in extrapolating from
animals to humans; (3) uncertainty in lowest observed adverse effect
(exposure) level to no observed adverse effect (exposure) level
adjustments; and (4) uncertainty in accounting for an incomplete
database on toxic effects of potential concern. 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 reference value at another exposure duration (e.g., 1 hour).
    Not all acute reference 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 reference value or
values being exceeded. Where relevant to the estimated exposures, the
lack of short-term dose-response values at different levels of severity
should be factored into the risk characterization as potential
uncertainties.
    Although every effort is made to identify appropriate human health
effect dose-response assessment 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 assessment 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 new IRIS assessment of 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 with regard to 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 reference 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 reference value, we also apply the most
protective reference value from the other compounds in the group to
estimate risk.
e. Uncertainties in the Multipathway Assessment
    For each source category, we generally rely on site-specific levels
of PB-HAP emissions to determine whether a refined assessment of the
impacts from multipathway exposures is necessary. This determination is
based on the results of a three-tiered screening analysis that relies
on the outputs from models that estimate environmental pollutant
concentrations and human exposures for four PB-HAP. 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.\23\
---------------------------------------------------------------------------

    \23\ 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 selected models are
appropriate for the assessment being conducted and whether they
adequately represent the actual processes that might occur for that
situation. An example of model uncertainty is the question of whether
the model adequately describes 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 screen are appropriate and
state-of-the-art for the multipathway 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 screen, 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 and 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 assessment, 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 screen. The
assumptions and the associated uncertainties regarding the selected
ingestion exposure scenario are the same for Tier 1 and Tier 2.
    For both Tiers 1 and 2 of the multipathway assessment, 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 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 not screen out, it does not
mean that multipathway impacts are significant, only that we cannot
rule out that possibility and that a refined multipathway analysis for
the site might be necessary to obtain a more accurate risk
characterization for the source category.
    For further information on uncertainties and the Tier 1 and 2
screening methods, refer to the risk document, Appendix 2, Technical
Support Document for TRIM-Based Multipathway Tiered Screening
Methodology for RTR: Summary and Evaluation.
f. Uncertainties in the Environmental Risk Screening Assessment
    For each source category, we generally rely on site-specific levels
of environmental HAP emissions to perform an environmental screening
assessment. The environmental screening assessment is based on the
outputs from models that estimate environmental HAP concentrations. The
same models, specifically the

TRIM.FaTE multipathway model and the AERMOD air dispersion model, are
used to estimate environmental HAP concentrations for both the human
multipathway screening analysis and for the environmental screening
analysis. Therefore, both screening assessments have similar modeling
uncertainties.
    Two important types of uncertainty associated with the use of these
models in RTR environmental screening assessments (and inherent to any
assessment that relies on environmental modeling) are model uncertainty
and input uncertainty.\24\
---------------------------------------------------------------------------

    \24\ In the context of this discussion, the term
``uncertainty,'' as it pertains to exposure and risk assessment,
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 selected models are
appropriate for the assessment being conducted and whether they
adequately represent the movement and accumulation of environmental HAP
emissions 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 screen are appropriate and state-
of-the-art for the environmental risk assessments conducted in support
of our RTR analyses.
    Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
1 of the environmental screen for PB-HAP, we configured the models to
avoid underestimating exposure and risk to reduce the likelihood that
the results indicate the risks are lower than they actually are. 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, the location and size of any bodies of water,
meteorology, surface water and soil characteristics, and structure of
the aquatic food web. In Tier 1, we used the maximum facility-specific
emissions for the PB-HAP (other than lead compounds, which were
evaluated by comparison to the secondary lead NAAQS) that were included
in the environmental screening assessment and each of the media when
comparing to ecological benchmarks. This is consistent with the
conservative design of Tier 1 of the screen. In Tier 2 of the
environmental screening analysis for PB-HAP, 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 locations
of water bodies near the facility location. 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
screen. To better represent widespread impacts, the modeled soil
concentrations are averaged in Tier 2 to obtain one average soil
concentration value for each facility and for each PB-HAP. For PB-HAP
concentrations in water, sediment, and fish tissue, the highest value
for each facility for each pollutant is used.
    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 both Tiers 1 and 2 of the environmental screening assessment,
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 potential risks for adverse environmental
impacts.
    Uncertainty also exists in the ecological benchmarks for the
environmental risk screening analysis. We established a hierarchy of
preferred benchmark sources to allow selection of benchmarks for each
environmental HAP at each ecological assessment endpoint. In general,
EPA benchmarks used at a programmatic level (e.g., Office of Water,
Superfund Program) were used if available. If not, we used EPA
benchmarks used in regional programs (e.g., Superfund Program). If