National Emissions Standards for Hazardous Air Pollutants: Primary Aluminum Reduction Plants

Federal RegisterDec 8, 2014

Ask Donna

What actually matters in this document.

Text

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2011-0797; FRL-9917-44-OAR]

RIN 2060-AQ92

National Emissions Standards for Hazardous Air Pollutants: Primary Aluminum Reduction Plants

AGENCY:

Environmental Protection Agency.

ACTION:

Supplemental proposed rulemaking.

SUMMARY:

This action supplements our proposed amendments to the national emission standards for hazardous air pollutants (NESHAP) for the Primary Aluminum Production source category published in the

Federal Register

on December 6, 2011. In that action, the Environmental Protection Agency (EPA) proposed amendments based on the initial residual risk and technology reviews (RTR) for this source category, and also proposed certain emission limits reflecting performance of Maximum Achievable Control Technology (MACT). Today's action reflects a revised technology review and a revised residual risk analysis for the Primary Aluminum Production source category and proposes new and revised emission standards based on those analyses, newly obtained emissions test data, and comments we received in response to the 2011 proposal, including certain revisions to the technology-based standards reflecting performance of MACT. This action also proposes new compliance requirements to meet the revised standards. This action, if adopted, will provide improved environmental protection regarding potential emissions of hazardous air pollutant (HAP) emissions from primary aluminum production facilities.

DATES:

Comments.

Comments must be received on or before January 22, 2015. A copy of comments on the information collection provisions should be submitted to the Office of Management and Budget (OMB) on or before January 7, 2015.

Public Hearing.

If anyone contacts the EPA requesting to speak at a public hearing by December 15, 2014, a public hearing will be held on December 23, 2014 at the U.S. EPA building at 109 T.W. Alexander Drive, Research Triangle Park, NC 27711. If you are interested in requesting a public hearing or attending the public hearing, contact Ms. Virginia Hunt at (919) 541-0832 or at

hunt.virginia@epa.gov.

If the EPA holds a public hearing, the EPA will keep the record of the hearing open for 30 days after completion of the hearing to provide an opportunity for submission of rebuttal and supplementary information.

ADDRESSES:

Comments.

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2011-0797, by one of the following methods:

•

Federal eRulemaking Portal: http://www.regulations.gov.

Follow the online instructions for submitting comments.

•

Email: A-and-R-docket@epa.gov.

Include Attention Docket ID No. EPA-HQ-OAR-2011-0797 in the subject line of the message.

•

Fax:

(202) 566-9744. Attention Docket ID No. EPA-HQ-OAR-2011-0797.

•

Mail:

Environmental Protection Agency, EPA Docket Center (EPA/DC), Mail Code: 28221T, Attention Docket ID No. EPA-HQ-OAR-2011-0797, 1200 Pennsylvania Avenue NW., Washington, DC 20460. Please mail a copy of your comments on the information collection provisions to the Office of Information and Regulatory Affairs, Office of Management and Budget (OMB), Attn: Desk Officer for EPA, 725 17th Street NW., Washington, DC 20503.

•

Hand/Courier Delivery:

EPA Docket Center, Room 3334, EPA WJC West Building, 1301 Constitution Avenue NW., Washington, DC 20004, Attention Docket ID No. EPA-HQ-OAR-2011-0797. Such deliveries are only accepted during the Docket's normal hours of operation, and special arrangements should be made for deliveries of boxed information.

Instructions.

Direct your comments to Docket ID No. EPA-HQ-OAR-2011-0797. 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 confidential business information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or otherwise protected through

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.

Docket.

The EPA has established a docket for this rulemaking under Docket ID No. EPA-HQ-OAR-2011-0797. 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.

Public Hearing:

If anyone contacts the EPA requesting a public hearing by December 15, 2014, the public hearing will be held on December 23, 2014 at the EPA's campus at 109 T.W. Alexander Drive, Research Triangle Park, North Carolina. The hearing will begin at 10:00 a.m. (Eastern Standard Time) and conclude at 5:00 p.m. (Eastern Standard Time). There will be a lunch break from 12:00 p.m. to 1:00 p.m. Please contact Ms. Virginia Hunt at 919-541-0832 or at

hunt.virginia@epa.gov

to register to speak at the hearing or to inquire as to whether or not a hearing will be held. The last day to pre-register in advance to speak at the hearing will be December 22, 2014. 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 accommodated. If you require the service of a translator or

special accommodations such as audio description, please let us know at the time of registration. If you require an accommodation, we ask that you pre-register for the hearing, as we may not be able to arrange such accommodations without advance notice. The hearing will provide interested parties the opportunity to present data, views or arguments concerning the proposed action. The EPA will make every effort to accommodate all speakers who arrive and register. Because these hearing are being held at U.S. government facilities, 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. The EPA may ask clarifying questions during the oral presentations, but will not respond to the presentations at that time. Written statements and supporting information submitted during the comment period will be considered with the same weight as oral comments and supporting information presented at the public hearing.

Docket:

The EPA has established a docket for this rulemaking under Docket ID No. EPA-HQ-OAR-2011-0797. All documents in the docket are listed in the

www.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, will be publicly available only in hard copy. Publicly available docket materials are available either electronically in

www.regulations.gov

or in hard copy at the EPA Docket Center, EPA WJC West Building, Room 3334, 1301 Constitution Ave. 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 Air Docket is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT:

For questions about this proposed action, contact Mr. David Putney, Sector Policies and Programs Division (D243-02), Office of Air Quality Planning and Standards, Environmental Protection Agency, Research Triangle Park, NC 27711; telephone (919) 541-2016; fax number: (919) 541-3207; and email address:

putney.david@epa.gov.

For specific information regarding the risk modeling methodology, contact Mr. Jim Hirtz, Health and Environmental Impacts Division (C539-02), Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711; telephone number: (919) 541-0881; fax number: (919) 541-0840; and email address:

hirtz.james@epa.gov.

For information about the applicability of the NESHAP to a particular entity, contact Mr. Patrick Yellin, Office of Enforcement and Compliance Assurance, U.S. Environmental Protection Agency, EPA WJC West Building, Mail Code 2227A, 1200 Pennsylvania Avenue NW., Washington, DC 20460; telephone number: (202) 564-2970 and email address:

yellin.patrick@epa.gov.

SUPPLEMENTARY INFORMATION:

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:

As arsenic

ADAF age-dependent adjustment factor

AEGL acute exposure guideline levels

AERMOD air dispersion model used by the HEM-3 model

ATSDR Agency for Toxic Substances and Disease Registry

BLDS bag leak detection system

BTF beyond-the-floor

CAA Clean Air Act

CalEPA California EPA

CBI Confidential Business Information

Cd cadmium

CE Cost Effectiveness

CFR Code of Federal Regulations

COS carbonyl sulfide

Cr chromium

trivalent chromium

hexavalent chromium

CWPB1 center-worked prebake one

CWPB2 center-worked prebake two

CWPB3 center-worked prebake three

D/Fs polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans

EF Emission Factors

EJ environmental justice

EPA Environmental Protection Agency

ERPG Emergency Response Planning Guidelines

ERT Electronic Reporting Tool

Federal Register

HAP hazardous air pollutants

HEM-3 Human Exposure Model, Version 1.1.0

HF hydrogen fluoride

Hg mercury

HI Hazard Index

HQ Hazard Quotient

HSS horizontal stud Soderberg

IRIS Integrated Risk Information System

km kilometer

LOAEL lowest-observed-adverse-effect level

LOEL lowest-observed-effect level

MACT maximum achievable control technology

MCEM methylene chloride extractable matter

mg/dscm milligrams per dry standard cubic meter

mg/kg-day milligrams per kilogram-day

mg/m

milligrams per cubic meter

MIR maximum individual risk

Mn manganese

MRL Minimal Risk Level

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

Ni nickel

NOAEL no-observed-adverse-effect level

NRC National Research Council

NTTAA National Technology Transfer and Advancement Act

OAQPS Office of Air Quality Planning and Standards

OECA Office of Enforcement and Compliance Assurance

OMB Office of Management and Budget

PAH polycyclic aromatic hydrocarbons

Pb lead

PB-HAP hazardous air pollutants known to be persistent and bio-accumulative in the environment

PCB polychlorinated biphenyls

PEL probable effect level

PM particulate matter

POM polycyclic organic matter

ppm parts per million

RDL representative method detection level

REL reference exposure level

RFA Regulatory Flexibility Act

RfC reference concentration

RfD reference dose

RTR residual risk and technology review

SAB Science Advisory Board

SBA Small Business Administration

SSM startup, shutdown and malfunction

SWPB side-worked prebake

TF total fluorides

TOSHI target organ-specific hazard index

TPY tons per year

TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and Ecological Exposure model

TTN echnology Transfer Network

UF uncertainty factor

μg/dscm micrograms per dry standard cubic meter

μg/m

micrograms per cubic meter

UMRA Unfunded Mandates Reform Act

UPL Upper Prediction Limit

URE unit risk estimate

VCS voluntary consensus standards

VSS1 vertical stud Soderberg one

VSS2 vertical stud Soderberg two

Organization of this Document.

The information in this preamble is organized as follows:

I. General Information

A. Does this action apply to me?

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

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

II. Background Information

A. What is the statutory authority for this action?

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

C. What is the history of the Primary Aluminum Production source category risk and technology review?

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

III. Analytical Procedures

A. For purposes of this supplemental proposal, how did we estimate the post-MACT risks posed by the Primary Aluminum Production source category?

B. How did we consider the risk results in making decisions for this supplemental proposal?

C. How did we perform the technology review?

IV. Revised Analytical Results and Proposed Decisions for the Primary Aluminum Production Source Category

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

B. What are the results of the risk assessment and analyses?

C. What are our proposed decisions regarding risk acceptability, ample margin of safety and adverse environmental effects based on our revised analyses?

D. What are the results and proposed decisions based on our technology review?

E. What other actions are we proposing?

F. What compliance dates are we proposing?

V. Summary of the Revised Cost, Environmental and Economic Impacts

A. What are the affected sources?

B. What are the air quality impacts?

C. What are the cost impacts?

D. What are the economic impacts?

E. What are the benefits?

VI. Request for Comments

VII. Submitting Data Corrections

VIII. Statutory and Executive Order Reviews

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

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

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

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

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

I. National Technology Transfer and Advancement Act

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

I. General Information

A. Does this action apply to me?

Table 1 of this preamble lists the industrial source category that is the subject of this supplemental proposal. Table 1 is not intended to be exhaustive but rather to provide a guide for readers regarding the entities that this proposed action is likely to affect. The proposed standards, once promulgated, will be directly applicable to the affected sources. Federal, state, local and tribal government entities would not be affected by this proposed action. As defined in the “Initial List of Categories of Sources Under Section 112(c)(1) of the Clean Air Act Amendments of 1990” (see 57 FR 31576, July 16, 1992), the “Primary Aluminum Production” source category is any facility which produces primary aluminum by the electrolytic reduction process.

U.S. EPA. Documentation for Developing the Initial Source Category List—Final Report, EPA/OAQPS, EPA-450/3-91-030, July, 1992.

Table 1—NESHAP and Industrial Source Categories Affected by This Proposed Action

Source category

NESHAP

NAICS code

Primary Aluminum Production

Primary Aluminum Reduction Plants

33131

2012 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 through EPA's Technology Transfer Network (TTN) Web site, a forum for information and technology exchange in various areas of air pollution control. Following signature by the EPA Administrator, the EPA will post a copy of this proposed action at:

http://www.epa.gov/ttn/atw/alum/alumpg.html.

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 RTR program is available at the following Web site:

http://www.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 Code of Federal Regulations (CFR) part 2. Send or deliver information identified as CBI

only to the following address: Roberto Morales, 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-2011-0797.

II. Background Information

A. What is the statutory authority for this action?

Section 112 of the Clean Air Act (CAA) establishes a two-stage regulatory process to address emissions of HAPs 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 HAPs. For major sources, the technology-based NESHAP must reflect the maximum degree of emission reductions of HAPs achievable (after considering cost, energy requirements and non-air quality health and environmental impacts) and are commonly referred to as 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) through (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) and (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)

EPA,

529 F.3d 1077, 1084 (D.C. Cir. 2008).

Association of Battery Recyclers, Inc.

EPA,

716 F.3d 667, 672-73 (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) required 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

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 38046. 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

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 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

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

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,

but must consider cost, energy, safety and other relevant factors in doing so.

“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).

The CAA does not specifically define the terms “individual most exposed,” “acceptable level” and “ample margin of safety.” In the Benzene NESHAP, 54 FR 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 current NESHAP regulate its HAP emissions?

The NESHAP for Primary Aluminum Reduction Plants were promulgated on October 7, 1997 (62 FR 52407), codified at 40 CFR part 63, subpart LL (referred to as subpart LL or MACT rule in the remainder of this preamble), and amended on November 2, 2005 (70 FR 66285). The MACT rule is applicable to facilities with affected sources associated with the production of aluminum by electrolytic reduction. These facilities are described in the following paragraph and collectively comprise what is commonly known as the Primary Aluminum Production source category.

Aluminum is produced from refined bauxite ore (also known as alumina), using an electrolytic reduction process in a series of cells called a “potline.” The raw materials include alumina, petroleum coke, pitch and fluoride salts. According to information available on the Web site of The Aluminum Association, Inc. (

http://www.aluminum.org

), approximately 40 percent of the aluminum produced in the U.S. comes from primary aluminum facilities. The two main potline types are prebake (a newer, higher efficiency, lower-emitting technology) and Soderberg (an older, lower efficiency, higher-emitting technology). There are currently 13 facilities located in the United States that are subject to the requirements of this NESHAP: 12 primary aluminum production plants and one carbon-only prebake anode production facility. These 12 primary aluminum production plants have approximately 45 potlines that produce aluminum. Ten primary aluminum production plants have a paste production operation, and 10 of the 12 primary aluminum production plants have anode bake furnaces. Eleven of the 12 primary aluminum facilities use prebake potlines; the other plant uses Soderberg potlines. Due to a decrease in demand for aluminum, four of the facilities are currently idle, including the Soderberg facility. The major HAPs emitted by these facilities are carbonyl sulfide (COS), hydrogen fluoride (HF), particulate HAP metals and polycyclic organic matter (POM), specifically polycyclic aromatic hydrocarbons (PAH).

The standards promulgated in 1997 and 2005 apply to emissions of HF, measured using total fluorides (TF) as a surrogate, from all potlines and anode bake furnaces and POM (as measured by methylene chloride extractables) from Soderberg potlines, anode bake furnaces, paste production plants and pitch storage tanks associated with primary aluminum production. Affected sources under the rules are each potline, each anode bake furnace (except for one that is located at a facility that only produces anodes for use off-site), each paste production plant and each new pitch storage tank.

The NESHAP designated seven subcategories of existing potlines based primarily on differences in the process operation and configuration. The control of primary emissions from the reduction process is typically achieved by a dry alumina scrubber (with a baghouse to collect the alumina and other particulate matter (PM)). The control technology typically used for anode bake furnaces is a dry alumina scrubber. A capture system vented to a dry coke scrubber is used for control of paste production plants. See Tables 2 and 3 for the applicable emission limits established under the 1997 NESHAP and the 2005 Amendments.

Table 2—Summary of Current MACT Emission Limits for Existing Sources Under the 1997 NESHAP, and the 2005 Amendments

Source

Pollutant

Emission limit

Potlines

CWPB1 potlines

0.95 kg/Mg (1.9 lb/ton) of aluminum produced.

CWPB2 potlines

1.5 kg/Mg (3.0 lb/ton) of aluminum produced.

CWPB3 potlines

1.25 kg/Mg (2.5 lb/ton) of aluminum produced.

SWPB potlines

0.8 kg/Mg (1.6 lb/ton) of aluminum produced.

VSS1 potlines

1.1 kg/Mg (2.2 lb/ton) of aluminum produced.

POM

1.2 kg/Mg (2.4 lb/ton) of aluminum produced.

VSS2 potlines

1.35 kg/Mg (2.7 lb/ton) of aluminum produced.

POM

2.85 kg/Mg (5.7 lb/ton) of aluminum produced.

HSS potlines

1.35 kg/Mg (2.7 lb/ton) of aluminum produced.

POM

2.35 kg/Mg (4.7 lb/ton) of aluminum produced.

Paste Production

POM

Install, operate and maintain equipment for capture of emissions and vent to a dry coke scrubber.

Anode Bake Furnace (collocated with a primary aluminum plant)

0.10 kg/Mg (0.20 lb/ton) of green anode.

POM

0.09 kg/Mg (0.18 lb/ton) of green anode.

CWPB1 = Center-worked prebake potline with the most modern reduction cells; includes all center-worked prebake potlines not specifically identified as CWPB2 or CWPB3.

CWPB2 = Center-worked prebake potlines located at Alcoa in Rockdale, Texas; Kaiser Aluminum in Mead, Washington; Ormet Corporation in Hannibal, Ohio; Ravenswood Aluminum in Ravenswood, West Virginia; Reynolds Metals in Troutdale, Oregon; and Vanalco Aluminum in Vancouver, Washington.

CWPB3 = Center-worked prebake potline that produces very high purity aluminum, has wet scrubbers as the primary control system and is located at the Century Aluminum primary aluminum plant in Kentucky.

HSS = Horizontal stud Soderberg potline (no facilities remain in the U.S.).

SWPB = Side-worked prebake potline.

VSS1 = Vertical stud Soderberg potline (no facilities remain in the U.S.).

VSS2 = Vertical stud Soderberg potlines (located at an idle facility known as Columbia Falls Aluminum in Columbia Falls, Montana).

Table 3—Summary of Current MACT Emission Limits for New Sources Under the 1997 NESHAP, and the 2005 Amendments

Source

Pollutant

Emission limit

All Potlines

0.6 kg/Mg (1.2 lb/ton) of aluminum produced.

VSS1, VSS2 and HSS potlines

POM

0.32 kg/Mg (0.63 lb/ton) of aluminum produced.

Paste Production

POM

Install, operate and maintain equipment for capture of emissions and vent to a dry coke scrubber.

Anode Bake Furnace (collocated with a primary aluminum plant)

0.01 kg/Mg (0.020 lb/ton) of green anode.

POM

0.025 kg/Mg (0.05 lb/ton) of green anode.

Pitch storage tanks

POM

Emission control system designed and operated to reduce inlet POM emissions by 95 percent or greater.

The 1997 NESHAP for primary aluminum reduction plants incorporates new source performance standards for potroom groups. These emission limits are listed in Table 3. The limits for new Soderberg facilities apply to any Soderberg facility that adds a new potroom group to an existing potline or is associated with a potroom group that meets the definition of a modified or reconstructed potroom group. Since these POM limits are very stringent, they effectively preclude the operation of any new Soderberg potlines. We expect any new potline would need to be a prebake potline to comply with the new source limits in the NESHAP.

Compliance with the emission limits in the current rule is demonstrated by performance testing which can be addressed individually for each affected source or according to emissions averaging provisions. Monitoring requirements include monthly measurements of TF secondary emissions, quarterly measurement of POM secondary emissions and annual measurement of primary emissions, continuous parametric monitoring for each emission control device, a monitoring device to track daily weight of aluminum produced and daily inspection for visible emissions. Recordkeeping for the rule is consistent with the General Provisions requirements with the addition of recordkeeping for daily production of aluminum, records supporting emissions averaging and records documenting the portion of TF measured as PM or gaseous form.

C. What is the history of the Primary Aluminum Production source category risk and technology review?

Pursuant to section 112(f)(2) of the CAA, in 2011 we conducted an initial evaluation of the residual risk associated with the NESHAP for Primary Aluminum Reduction Plants. At that time, we also conducted an initial technology review pursuant to section 112(d)(6) of the CAA. Finally, we also reviewed the 2005 MACT rule to determine whether other amendments were appropriate. Based on the results of that initial RTR, and the MACT rule review, we proposed amendments to the NESHAP (also known as subpart LL) on December 6, 2011 (76 FR 76260) (referred to as the 2011 proposal in the remainder of this FR document). The proposed amendments in the 2011 proposal which we are revisiting in today's supplemental proposal include the following:

• Proposed emission limits for POM from prebake potlines;

• Amendments to the monitoring, notification, recordkeeping and testing requirements; and

• Proposed provisions establishing an affirmative defense to civil penalties for violations caused by malfunctions.

As explained below, we are also proposing provisions which have no analogue in the 2011 proposal.

The comment period for the December 2011 proposal opened on December 6, 2011, and ended on February 1, 2012. We received significant comments from industry representatives, environmental organizations and state regulatory agencies. After reviewing the comments, and after consideration of additional data and information received since the 2011 proposal, we determined it is appropriate to revise some of our analyses and to publish a supplemental proposal. After collecting and reviewing additional data, we are proposing technology-based emission limits pursuant to CAA sections 112(d)(2) and (3) for PM, as a surrogate for particulate HAP metals, for new and existing potlines, anode bake furnaces and paste plants. We are also proposing revised technology-based emissions limits for POM emissions from prebake potlines and amendments to the monitoring, notification, recordkeeping and testing requirements to implement these emission limits. Pursuant to CAA section 112(f)(2), we are also proposing risk-based emission standards for POM, nickel (Ni) and arsenic (As) emissions from potlines in the VSS2 subcategory and proposing testing and monitoring requirements to demonstrate compliance with the standards for Ni and As. We are also proposing revisions to the testing and compliance requirements for COS emissions.

In addition, we are withdrawing our 2011 proposal to include an affirmative defense in this rule in light of a recent court decision vacating an affirmative defense in one of the EPA's CAA section 112(d) regulations.

NRDC

EPA,

749 F. 3d 1055 (D.C. Cir. 2014) (vacating affirmative defense provisions in CAA section 112(d) rule establishing emission standards for Portland cement kilns).

Today's supplemental proposed rulemaking will allow the public an opportunity to review and comment on the revised analyses and revised proposed amendments described above.

However, we also proposed other requirements in the 2011 proposal (listed below) for which we have made no revisions to the analyses, are not proposing any changes and are not reopening for public comment. These are:

• POM standards for existing pitch storage tanks and related monitoring, reporting and testing requirements;

• Emissions limits for COS from potlines;

• Elimination of startup, shutdown and malfunction (SSM) exemptions; and

• Electronic reporting.

The comment period for the December 2011 proposal opened on December 6, 2011, and ended on February 1, 2012. We will address the comments we received during the public comment period for the 2011 proposal at the time we publish final RTR amendments for the Primary Aluminum Production source category based on the 2011 proposal and today's supplemental proposal.

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

The 2011 risk assessment was based on estimates of PAH emissions derived from test measurements conducted in the 1990's on facilities that may not have been representative of current operating practices and using test methods that were inferior to those currently available. In addition, data available to estimate emissions of HAP metals from potlines were very limited, and no data were available to estimate HAP metals emissions from anode bake furnaces and paste plants. Furthermore, no data were available to estimate dioxin/furan (D/F) and polychlorinated biphenyl (PCB) emissions from potlines, anode bake furnaces and paste plants.

The proposed emission limits for POM from prebake potlines included in the 2011 proposal were based on extremely limited data. Also lacking were reliable data on which to base MACT standards for PM (as a surrogate for HAP metals) emissions from potlines, anode bake furnaces and paste plants.

Therefore, in March 2013 we sent an information request to the primary aluminum companies pursuant to section 114 of the CAA to gather additional relevant emissions test data. In response to this request, selected facilities provided the following data:

• Additional emission test data for POM emissions from prebake potlines;

• Additional emission test data for PM emissions from prebake potlines, Soderberg potlines (which have subsequently shut down), anode bake furnaces and paste plants;

• Additional emission test data for speciated PAH, speciated HAP metals, speciated PCBs and speciated polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans from potlines, anode bake furnaces and paste plants.

III. Analytical Procedures

A. For purposes of this supplemental proposal, how did we estimate the post-MACT risks posed by the Primary Aluminum Production 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 noncancer health effects and the hazard quotient (HQ) for acute exposures to HAP with the potential to cause noncancer 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 eight sections that follow this paragraph describe how we estimated emissions and conducted the risk assessment. The docket for this rulemaking contains the following document which provides more information on the risk assessment inputs and models:

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal.

The methods used to assess risks (as described in the eight primary steps below) are consistent with those 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;

they are also consistent with the key recommendations contained in that report.

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?

Using the test reports from the 2013 information request we calculated annual emission rates of PAHs, D/Fs, PCBs and HAP metals from primary and secondary potline exhausts, anode bake furnace exhausts and paste plant exhausts. Where no test data were available we calculated and applied emission factors (EF) for these pollutants and emission points based on average emission rates from similarly operated sources to estimate emissions. However, it is important to note that only two facilities tested for D/F and PCBs. Furthermore, many of the test results for D/Fs and PCBs were below detection limits. More than half of the mercury (Hg) emissions tests results were also below detection limit. Therefore, there are greater uncertainties regarding D/F, PCB and Hg emissions compared to the other HAP. To estimate emissions in cases where some, but not all, data were below the detection limit, we assumed the undetected emissions were equal to one-half the detection limit, which is the established approach for dealing with non-detects in the EPA's RTR program when developing emissions estimates for input to the risk assessments. Subsequently, we developed EF based on these limited data to estimate emissions at the other facilities. We believe the emissions estimates for D/F and PCBs are quite conservative (

i.e.,

more likely to be overestimated rather than underestimated) because we assumed undetected emissions were equal to one half the detection limit. We note that EPA may, but is not obligated to amend MACT standards. In the case of D/F, Hg and PCB, where many of the emissions tests were below detection limit, and given the uncertainties and limitations of the data (for example, we have test data for D/F and PCBs for only one of the 11 prebake facilities), the EPA is choosing not to propose standards for these HAP at this time.

We also obtained test data from recent compliance tests for TF and estimated HF emissions from primary and secondary potline exhausts and anode bake furnace exhausts. We estimated COS emissions as described in the 2011 risk assessment. We did not receive any additional test data for COS. Thus, the emissions estimates for COS have not changed since the 2011 proposal. As noted above, we are not accepting further comment on either this analysis or the proposed emission limit for COS.

We also verified information regarding emissions release characteristics such as stack heights, stack gas exit velocities, stack temperatures and source locations. In addition to the quality assurance (QA) of the source data for the facilities contained in the dataset, we also checked the coordinates of every emission source in the dataset using tools such as Google Earth. Where coordinates used in the 2011 risk assessment were found to be incorrect, we identified and corrected them. We also performed a QA assessment of the emissions data and release characteristics to ensure the data were reliable and that there were no outliers. The emissions data and the methods used to estimate emissions from all the various emissions sources are described in more detail in the technical document:

Revised Draft Development of the RTR Emissions Dataset for the Primary Aluminum Production Source Category,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

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 residual risk rule (70 FR 19998-19999, April 15, 2005) and in the proposed and final Hazardous Organic NESHAP residual risk rules (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.

For this supplemental proposal, we evaluated allowable emissions based on responses to the information request. We estimated that allowable emissions for the currently regulated HAP (

i.e.,

PAHs and HF) were generally about 1.5 times higher than the actual emissions. Therefore, to calculate allowable emissions of PAHs and HF, we assumed that allowable emissions were 1.5 times the actual emissions for all facilities except for one idle Soderberg facility (Columbia Falls). For Columbia Falls, which has the highest potential for emissions of all the facilities, we evaluated site-specific data and estimated that allowable emissions for the regulated HAP (

i.e.,

PAHs and HF) were about 1.9 times higher than estimated actual emissions when the plant is operating. Regarding unregulated HAP, the NESHAP currently includes no standards for COS, PCB, D/F and HAP metal emissions. Since there is no standard in place for these HAP and, therefore, no defined level of “MACT allowable” emissions levels, we assumed that allowable emissions for COS, PCB, D/F and HAP metal emissions were equal to estimated actual emissions. Further explanation is provided in the technical document:

Revised Draft Development of the RTR Emissions Dataset for the Primary Aluminum Production Source Category,

which is available in the docket (Docket ID No. EPA-HQ-OAR-2011-0797).

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,

and (3) estimating individual and population-level inhalation risks using the exposure estimates and quantitative dose-response information.

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.

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

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

http://www2.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants

and are discussed in more detail later in this section.

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).

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 (μg/m

)) 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 EPA 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.

In the case of Ni compounds, to provide a health-protective estimate of potential cancer risks, we used the IRIS URE value for Ni subsulfide in the assessment for the 2011 proposed rule for the Primary Aluminum Production source category. Based on past scientific and technical considerations, the determination of the percent of Ni subsulfide was considered a major factor for estimating the extent and magnitude of the risks of cancer due to Ni-containing emissions. Nickel speciation information for some of the largest Ni-emitting sources (including oil combustion, coal combustion and others) suggested that at least 35 percent of total Ni emissions may be soluble compounds and that the URE for the mixture of inhaled Ni compounds (based on Ni subsulfide, and representative of pure insoluble crystalline Ni) could be derived to reflect the assumption that 65 percent of the total mass of Ni may be carcinogenic.

Based on consistent views of major scientific bodies (

i.e.,

National Toxicology Program (NTP) in their 12th Report of the Carcinogens (ROC),

International Agency for Research on

Cancer (IARC)

and other international agencies)

that consider all Ni compounds to be carcinogenic, we currently consider all Ni compounds to have the potential of being carcinogenic to humans. The

12th Report of the Carcinogens

states that the “combined results of epidemiological studies, mechanistic studies, and carcinogenic studies in rodents support the concept that Ni compounds generate Ni ions in target cells at sites critical for carcinogenesis, thus allowing consideration and evaluation of these compounds as a single group.” Although the precise Ni compound (or compounds) responsible for the carcinogenic effects in humans is not always clear, studies indicate that Ni sulfate and the combinations of Ni sulfides and oxides encountered in the Ni refining industries cause cancer in humans (these studies are summarized in a review by Grimsrud et al., 2010

). The major scientific bodies mentioned above have also recognized that there are differences in toxicity and/or carcinogenic potential across the different Ni compounds.

National Toxicology Program (NTP), 2011.

Report on Carcinogens.

12th ed. Research Triangle Park, NC: US Department of Health and Human Services (DHHS), Public Health Service. Available online at

http://ntp.niehs.nih.gov/ntp/roc/twelfth/roc12.pdf.

International Agency for Research on Cancer (IARC), 1990.

IARC Monographs on the Evaluation of Carcinogenic Risks to Humans.

Chromium, nickel, and welding. Vol. 49. Lyons, France: International Agency for Research on Cancer, World Health Organization Vol. 49:256.

World Health Organization (WHO, 1991) and the European Union's Scientific Committee on Health and Environmental Risks (SCHER, 2006).

Grimsrud TK and Andersen A.

Evidence of Carcinogenicity in Humans of Water-soluble Nickel Salts.

J Occup Med Toxicol 2010, 5:1-7. Available online at

http://www.ossup-med.com/content/5/1/7.

In the inhalation risk assessment for this supplemental proposal, we chose to take a conservative approach: we considered all Ni compounds to be as carcinogenic as Ni subsulfide and applied the IRIS URE for Ni subsulfide without a factor to reflect the assumption that 100 percent of the total mass of Ni may be as carcinogenic as pure Ni subsulfide. However, given that there are two additional URE values

derived for exposure to mixtures of Ni compounds, as a group, that are 2-3 fold lower than the IRIS URE for Ni subsulfide, the EPA also considers it reasonable to use a value that is 50 percent of the IRIS URE for Ni subsulfide for providing an estimate of the lower end of the plausible range of cancer potency values for different mixtures of Ni compounds.

Two UREs (other than the current IRIS values) have been derived for nickel compounds as a group: One developed by the California Department of Health Services (

http://www.arb.ca.gov/toxics/id/summary/nickel_tech_b.pdf

) and the other by the Texas Commission on Environmental Quality (

http://www.epa.gov/ttn/atw/nata1999/99pdfs/healtheffectsinfo.pdf

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

) 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.

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) (

http://www.epa.gov/riskassessment/glossary.htm

), 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 (MRL) (

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) over a specified duration of exposure”; (2) the CalEPA Chronic Reference Exposure Level (REL) (

http://www.oehha.ca.gov/air/hot_spots/pdf/HRAguidefinal.pdf

), which is defined as “the concentration level (that is expressed in units of micrograms per cubic meter (μg/m

) 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.

POM, a carcinogenic HAP with a mutagenic mode of action, is emitted by the facilities in this source category.

For this compound group,

the EPA's analysis applies the age-dependent adjustment factors (ADAF) described in the EPA's

Supplemental Guidance for Assessing Susceptibility from Early-Life Exposure to Carcinogens.

This adjustment has the effect of increasing the estimated lifetime risks for POM by a factor of 1.6. In addition, although primary aluminum facilities reported most of their total POM emissions as individual compounds, the EPA expresses carcinogenic potency for compounds in this group in terms of benzo[a]pyrene equivalence, based on evidence that carcinogenic POM has the same mutagenic mechanism of action as benzo[a]pyrene. For this reason, the EPA's Science Policy Council

recommends applying the

Supplemental Guidance

to all carcinogenic PAH for which risk estimates are based on relative potency. Accordingly, we have applied the ADAF to the benzo[a]pyrene equivalent portion of all POM mixtures.

U.S. EPA. Performing risk assessments that include carcinogens described in the

Supplemental Guidance

as having a mutagenic mode of action.

Science Policy Council Cancer Guidelines Implementation Work Group Communication II:

Memo from W.H. Farland, dated October 4, 2005.

See the

Risk Assessment for Source Categories

document available in the docket for a list of HAP with a mutagenic mode of action.

U.S. EPA.

Supplemental Guidance for Assessing Early-Life Exposure to Carcinogens.

EPA/630/R-03/003F, 2005.

http://www.epa.gov/ttn/atw/childrens_supplement_final.pdf.

U.S. EPA.

Science Policy Council Cancer Guidelines Implementation Workgroup Communication II:

Memo from W.H. Farland, dated June 14, 2006.

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.

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.

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.

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 the mandate of section 112(f)(2) 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://www.oehha.ca.gov/air/pdf/acuterel.pdf

) 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). As described in

Standing Operating Procedures (SOP) of the National Advisory Committee on Acute Exposure Guideline Levels for Hazardous Substances

(

http://www.epa.gov/oppt/aegl/pubs/sop.pdf

“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.

National Academy of Sciences (NAS), 2001.

Standing Operating Procedures for Developing Acute Exposure Levels for Hazardous Chemicals,

page 2.

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

(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 non-sensory 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, non-sensory 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/ERP-SOPs2006.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.”

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.

ERP Committee Procedures and Responsibilities.

November 1, 2006. American Industrial Hygiene Association.

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 reflecting, where appropriate, circumstances of the particular source category at issue. 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.

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.

See

http://www.tceq.state.tx.us/compliance/field_ops/eer/index.html

or the docket to access the source of these data.

For the Primary Aluminum Production source category, information was available to determine process-specific factors. The processes in this source category are typically equipped with controls which will not allow startup of the emission source until the associated control device is operating and will automatically shut down the emission source if the associated controls malfunction. Further, some processes, for example, the potlines, operate continuously so there are no significant spikes in emissions. We, thus, believe emissions from the potlines are relatively consistent over time with minimal fluctuation. However, we realize that emissions vary over time. Furthermore, as described above, we estimate the maximum allowable emissions for this source category are about 1.5 times higher than the average long-term actual emissions for these sources. Therefore, we assume that hourly emissions rates from potlines could occasionally increase by a factor of up to 1.5 times the average hourly emissions, which, for the reasons stated above, we believe is a valid multiplier to estimate maximum acute emissions from potlines. Other processes, for example paste production and anode baking, may have specific cycles, with peak emissions occurring for a part of that cycle. We assume these peak emissions could be as high as 2 times the average emissions for paste plants and bake furnaces. As discussed in sections II.D and III.A.1 of this preamble, above, we collected data regarding the emissions from these processes. Those emissions data represent emissions during periods of normal operations (as opposed to during periods of peak emissions).

Therefore, based on the modes of operation and other factors described above, we applied an acute emissions multiplier of 1.5 to all potline emissions for input to the acute risk assessment, and for paste production and anode baking we applied an acute emissions multiplier of 2. We regard these factors as conservative (

i.e.,

they are designed not to underestimate variability). Even with data available to develop process-specific factors, our assessment of acute risk reflects conservative assumptions, in particular in its assumptions that every potline operates at the same hour and that every potline has emissions 1.5 times higher than the average at the same hour, that this is the same hour as the worst-case dispersion conditions, and that a person is at the location of maximum concentration during that hour. This results in a conservative exposure scenario.

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 for modeled HAPs (even under the conservative assumptions of the screening analysis), acute impacts were deemed negligible and no further analysis was performed for these HAPs. In cases where an acute HQ from the screening step was greater than 1, for some modeled HAPs additional site-specific data were considered to develop a more refined estimate of the potential for acute impacts of concern. These refinements are discussed more fully in the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797). 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.

As noted above, the agency may choose to refine the acute screen by also assessing the exposure that may occur at a centroid of census block. For this source category we first used conservative assumptions for emissions rates, meteorology and exposure location for our acute analysis. We then refined the acute assessment by also estimating the HQ for As at centroids of census blocks.

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,

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

for HAP have

been developed, we consider additional acute values (

i.e.,

occupational and international values) to provide a more complete risk characterization.

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.

U.S. EPA. (2009) 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, and available online at

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.

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 Primary Aluminum Production source category, we identified emissions of cadmium (Cd) compounds, D/F, POM, divalent Hg compounds and HF. However, as we explained in section III.A.1 of this preamble, many of the emissions tests for mercury and D/F were below detection limit or detection limit limited. Nevertheless, we estimated emissions of these HAP based on the conservative assumption that undetected emissions were equal to one half the detection limit. Therefore, we consider the estimates for D/F and Hg to be conservative (

i.e.,

more likely to be overestimated rather than underestimated).

Because one or more of the PB-HAP are emitted by at least one facility in the Primary Aluminum Production 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: Cd, lead, D/F, Hg 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 (Pb) compounds) at which the maximum excess lifetime cancer risk would be 1-in-1 million (

i.e.,

for D/F and POM) or, for HAP that cause non-cancer health effects (

i.e.,

Cd compounds and Hg 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 scenarios for the subsistence fisher and the subsistence farmer 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. When facilities exceed the Tier 2 screening levels, it does not mean that multipathway impacts are significant, only that we cannot rule out that possibility based on the results of the screen.

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. For this source category, we conducted 3 Tier 3 screening assessments at Alcoa (Ferndale, WA), Alumax (Goose Creek, SC) and Reynolds Metals (Massena, NY). The Reynolds Metals facility is a Soderberg facility which was operating at the time we sent out the information request and when we collected the emissions data and initiated the modeling assessment. However, recently this facility permanently shut down all their Soderberg potline operations. It is our understanding that this facility will either convert to a prebake facility or remain permanently shut down. A detailed discussion of the approach for this multipathway risk assessment can be found in Appendix 9 (

Technical Support Document: Human Health Multipathway Residual Risk Screening Assessment for the Primary Aluminum Production Source Category

) of the risk assessment document.

In evaluating the potential multipathway risk from emissions of Pb compounds, rather than developing a screening emissions rate for them, we compared maximum estimated chronic inhalation exposures with the level of the current National Ambient Air Quality Standard (NAAQS) for Pb.

Values below the level of the primary (health-based) Pb NAAQS were considered to have a low potential for multipathway risk.

In doing so, the EPA notes that the legal standard for a primary NAAQS—that a standard is requisite to protect public health and provide an adequate margin of safety (CAA section 109(b))—differs from the CAA section 112(f) standard (requiring among other things that the standard provide an “ample margin of safety”). However, the lead NAAQS is a reasonable measure of determining risk acceptability (

i.e.,

the first step of the Benzene NESHAP analysis) since it is designed to protect the most susceptible group in the human population—children, including children living near major lead emitting sources. 73 FR 67002/3; 73 FR 67000/3; 73 FR 67005/1. In addition, applying the level of the primary lead NAAQS at the risk acceptability step is conservative, since the primary lead NAAQS reflects an adequate margin of safety.

For further information on the multipathway analysis approach, see the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

5. How did we assess risks considering the revised emissions control options?

In addition to assessing baseline inhalation risks and potential multipathway risks, we also estimated risks considering the emission

reductions that would be achieved by the control options under consideration in this supplemental proposal (

i.e.,

emission reductions reflecting the proposed standards reflecting MACT). In these cases, the expected emission reductions were applied to the specific HAP and emission points in the RTR emissions dataset to develop corresponding estimates of risk that would exist after implementation of the proposed amendments in today's action.

6. 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 Cd, D/F, POM, Hg (both inorganic Hg and methylmercury) and Pb compounds. The two acid gases are hydrogen chloride (HCl) and HF. We have no data indicating primary aluminum plants emit HCl. Therefore, our analysis for this source category does not reflect HCl emissions. The rationale for including the remaining six HAP in the environmental risk screening analysis is presented below.

The 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 increase 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 National Emissions Inventory).

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 Cd compounds, D/F, POM and Hg in soil, sediment and water. For Pb 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 Pb compounds, we compare the estimated HEM-3 modeled exposures from the source category emissions of Pb with the level of the secondary NAAQS for Pb.

We consider values below the level of the secondary Pb NAAQS as unlikely to cause adverse environmental effects.

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.”

Due to its well-documented potential to cause direct damage to terrestrial plants, we include the acid gas HF emitted by primary aluminum sources, in the environmental screening analysis. 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 Pb 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 Pb 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 Cd compounds, D/F, POM and Hg, 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 Primary Aluminum Production source category emitted any of the seven environmental HAP. For the Primary Aluminum Production source category, we identified emissions of five of the PB-HAP (Cd, Hg, Pb, D/F and POM) and one acid gas (HF).

Because one or more of the seven environmental HAP evaluated are emitted by the facilities in the source category, we proceeded to the second step of the evaluation.

f. PB-HAP Methodology

For Cd, Hg, POM and D/F, the environmental screening analysis consists of two tiers, while Pb compounds are analyzed differently as discussed earlier. However, as we explained in section III.A.1 above, there are greater uncertainties in the emissions estimates for Hg or D/F because of the limitations in the available data and because a large portion of emissions tests results were below the detection limit for those HAP. Nevertheless, to be conservative (

i.e.,

more likely to overestimate risks rather than underestimate risks), we have included emissions estimates of Hg and D/F in the PB-HAP risk screen based on conservative assumptions (

i.e.,

emissions of one half the detection limit were assumed for those tests where no pollutants were detected).

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 eight octants. Each octant contains 5 modeled soil concentrations at various distances from the facility (5 soil concentrations × 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 is typically 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.

g. Acid Gas Methodology

The environmental screening analysis evaluates the potential phytotoxicity and reduced productivity of plants due to chronic exposure to HF (we have no data regarding HCl emissions from primary aluminum facilities and, therefore, HCl was not analyzed). The environmental risk screening methodology for HF is a single-tier screen that compares the average off-site ambient air concentration over the modeling domain to ecological benchmarks for each of the acid gases. Because air concentrations are compared directly to the ecological benchmarks, emission-based screening levels are not calculated for HF as they

are in the ecological risk screening methodology for PB-HAPs.

For purposes of ecological risk screening, the EPA identifies a potential for adverse environmental effects to plant communities from exposure to acid gases when the average concentration of the HAP around a facility exceeds the LOAEL ecological benchmark. In such cases, we further investigate factors such as the magnitude and characteristics of the area of exceedance (

e.g.,

land use of exceedance area, size of exceedance area) to determine if there is an adverse environmental effect.

For further information on the environmental screening analysis approach, see the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

7. 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 of interest, but also emissions of HAP from all other emissions sources at the facility for which we have data. 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. The

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

available through the docket for this action, provides the methodology and results of the facility-wide analyses, including all facility-wide risks.

8. 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

Revised Draft Development of the RTR Emissions Dataset for the Primary Aluminum Production Source Category,

and the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

a. Uncertainties in the RTR Emissions Dataset

Although the development of the RTR emissions dataset involved QA/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 for each emission process group and applied to the average annual hourly emission rates, which are intended to account for emission fluctuations due to normal facility operations.

As described above and in the

Revised Draft Development of the RTR Emissions Dataset for the Primary Aluminum Production Source Category,

we gathered a substantial amount of emissions test data from currently operating facilities (plus test data from a then-operating, now closed Soderberg facility). Required testing under the CAA section 114 request included measurements of HAP metal emissions from primary and secondary potline exhausts at seven facilities, as well as measurements of HAP metal emissions from three anode bake furnace exhausts and three paste plant exhausts. We also received additional POM emissions data from eight facilities. Furthermore, we received speciated PAH, PCB and D/F emissions data from primary and secondary exhausts of two potlines (one Soderberg potline and one prebake potline), as well as exhausts from one anode bake furnace and one paste plant. We used these data to estimate emissions from emission points for which we had no emissions test data. Also, there is additional uncertainty concerning the estimated emissions of Hg and D/F since, as discussed in sections III.A.1 and IV.A of this preamble, a substantial portion of the emissions test results for those HAP were reported as below laboratory detection limits. Finally, we received hexavalent chromium (Cr

) emissions stack test data from primary and secondary potline exhausts at two facilities and an anode bake furnace and a paste plant at one facility. We used the average results from these tests to apportion emissions of Cr

and trivalent chromium (Cr

) for the remaining facilities that did not test. Therefore, there are some uncertainties regarding the split between Cr

and Cr

for these remaining facilities. Nevertheless, we believe the test data we used are representative. Thus, the uncertainties are not significant. Furthermore, since we used the average results of the available tests, the values we used as input for the risk assessment are equally likely to be overestimates or underestimates of the actual speciated emissions.

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.

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).

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.

U.S. EPA.

National-Scale Air Toxics Assessment for 1996.

(EPA 453/R-01-003; January 2001; page 85.)

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(f) 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 the primary aluminum source category, these assumptions would tend to be conservative 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.

For the primary aluminum source category, we refined the acute exposure assessment by estimating the HQ at a centroid of a census block. This reduces the uncertainty in the assessment because we are evaluating the potential for exposures to occur at locations where people could actually live, rather than at the point of maximum off-site concentration.

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

Guidelines for Carcinogen Risk Assessment

(EPA/630/P-03/001B, March 2005); 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” (

Guidelines for Carcinogen Risk Assessment,

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 Assessment for the Primary Aluminum Production Source Category in Support of the November 2014 Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

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).

In some circumstances, the true risk could be as low as zero; however, in other circumstances the risk could be greater. 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).

IRIS glossary (

http://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary

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, 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,

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.

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, 2004,

An Examination of EPA Risk Assessment Principles and Practices,

EPA/100/B-04/001 available at:

http://www.epa.gov/osa/pdfs/ratf-final.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.

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.

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. The multipathway screens include some hypothetical elements, namely the hypothetical farmer and fisher scenarios. It is important to note that even though EPA conducted a multipathway assessment based on these scenarios, no data exist to verify the existence of either the farmer or fisher scenario outlined above.

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 5,

Technical Support Document for TRIM-Based Multipathway Tiered Screening Methodology for RTR.

We completed a Tier 3 multipathway screen for this supplemental proposal. This assessment contains less uncertainty compared to the Tier 1 and Tier 2 screens. The Tier 3 screen improves the lake characterization used in the Tier 2 analysis and improves the screen by adjusting for emissions lost to the upper air sink through plume-rise calculations. The Tier 3 screen reduces uncertainty through improved lake evaluations used in the Tier 2 screen and by calculating the amount of mass lost to the upper air sink through plume rise. Nevertheless, some uncertainties also exist here. The Tier 3 multipathway screen and related uncertainties are described in detail in section 4 of the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

which is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

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.

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 Pb compounds, which were evaluated by comparison to the secondary Pb 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 benchmarks were not available at a programmatic or regional level, we used benchmarks developed by other agencies (

e.g.,

NOAA) or by state agencies.

In all cases (except for Pb compounds, which were evaluated through a comparison to the NAAQS for Pb and its compounds), we searched for benchmarks at the following three effect levels, as described in section III.A.6 of this preamble:

1. A no-effect level (

i.e.,

NOAEL).

2. Threshold-effect level (

i.e.,

LOAEL).

3. Probable effect level (

i.e.,

PEL).

For some ecological assessment endpoint/environmental HAP combinations, we could identify benchmarks for all three effect levels, but for most, we could not. In one case, where different agencies derived significantly different numbers to represent a threshold for effect, we included both. In several cases, only a single benchmark was available. In cases where multiple effect levels were available for a particular PB-HAP and assessment endpoint, we used all of the available effect levels to help us to determine whether risk exists and if the risks could be considered significant and widespread.

The EPA evaluates the following seven HAP in the environmental risk screening assessment: Cd, D/F, POM, Hg (both inorganic Hg and methylmercury), Pb compounds, HCl

and HF, where applicable. These seven HAP represent pollutants that can cause adverse impacts for plants and animals either through direct exposure to HAP in the air or through exposure to HAP that is deposited from the air onto soils and surface waters. These seven HAP also represent those HAP for which we can conduct a meaningful environmental risk screening assessment. For other HAP not included in our screening assessment, the model has not been parameterized such that it can be used for that purpose. In some cases, depending on the HAP, we may not have appropriate multipathway models that allow us to predict the concentration of that pollutant. The EPA acknowledges that other HAP beyond the seven HAP that we are evaluating may have the potential to cause adverse environmental effects and, therefore, the EPA may evaluate other relevant HAP in the future, as modeling science and resources allow.

As noted above, we have no data regarding HCl emissions from primary aluminum plants so the EPA did not evaluate HCl in this screening assessment for this proposal.

Further information on uncertainties and the Tier 1 and 2 screening methods is provided in Appendix 5 of the document “Technical Support Document for TRIM-Based Multipathway Tiered Screening Methodology for RTR: Summary of Approach and Evaluation.” Also, see the

Residual Risk Assessment for the Primary Aluminum Production Source Category in Support of the 2014 Supplemental Proposal,

available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797).

B. How did we consider the risk results in making decisions for this supplemental proposal?

As discussed in section II.A of this preamble, in evaluating and developing standards under CAA section 112(f)(2), we apply a two-step process to address residual risk. In the first step, the EPA determines whether risks are acceptable. This determination “considers all health information, including risk estimation uncertainty, and includes a presumptive limit on maximum individual lifetime [cancer] risk (MIR)

of approximately [1-in-10 thousand] [

i.e.,

100-in-1 million].” 54 FR 38045, September 14, 1989. If risks are unacceptable, the EPA must determine the emissions standards necessary to bring risks to an acceptable level without considering costs. In the second step of the process, the EPA considers whether the emissions standards provide an ample margin of safety “in consideration of all health information, including the number of persons at risk levels higher than approximately 1-in-1 million, as well as other relevant factors, including costs and economic impacts, technological feasibility, and other factors relevant to each particular decision.”

Id.

The EPA must promulgate emission standards necessary to provide an ample margin of safety.

Although defined as “maximum individual risk,” MIR refers only to cancer risk. MIR, one metric for assessing cancer risk, is the estimated risk were an individual exposed to the maximum level of a pollutant for a lifetime.

In past residual risk actions, the EPA considered a number of human health risk metrics associated with emissions from the categories under review, including the MIR, the number of persons in various risk ranges, cancer incidence, the maximum non-cancer HI and the maximum acute non-cancer hazard. See,

e.g.,

72 FR 25138, May 3, 2007; 71 FR 42724, July 27, 2006. The EPA considered this health information for both actual and allowable emissions. See,

e.g.,

75 FR 65068, October 21, 2010; 75 FR 80220, December 21, 2010; 76 FR 29032, May 19, 2011. The EPA also discussed risk estimation uncertainties and considered the uncertainties in the determination of acceptable risk and ample margin of safety in these past actions. The EPA considered this same type of information in support of this action.

The agency is considering these various measures of health information to inform our determinations of risk acceptability and ample margin of safety under CAA section 112(f). As explained in the Benzene NESHAP, “the first step judgment on acceptability cannot be reduced to any single factor” and, thus, “[t]he Administrator believes that the acceptability of risk under [previous] section 112 is best judged on the basis of a broad set of health risk measures and information.” 54 FR 38046, September 14, 1989. Similarly, with regard to the ample margin of safety determination, “the Agency again considers all of the health risk and other health information considered in the first step. Beyond that information, additional factors relating to the appropriate level of control will also be considered, including cost and economic impacts of controls, technological feasibility, uncertainties, and any other relevant factors.”

Id.

The Benzene NESHAP approach provides flexibility regarding factors the EPA may consider in making determinations and how the EPA may weigh those factors for each source category. In responding to comment on our policy under the Benzene NESHAP, the EPA explained that:

“[t]he policy chosen by the Administrator permits consideration of multiple measures of health risk. Not only can the MIR figure be considered, but also incidence, the

presence of non-cancer health effects, and the uncertainties of the risk estimates. In this way, the effect on the most exposed individuals can be reviewed as well as the impact on the general public. These factors can then be weighed in each individual case. This approach complies with the

Vinyl Chloride

mandate that the Administrator ascertain an acceptable level of risk to the public by employing [her] expertise to assess available data. It also complies with the Congressional intent behind the CAA, which did not exclude the use of any particular measure of public health risk from the EPA's consideration with respect to CAA section 112 regulations, and thereby implicitly permits consideration of any and all measures of health risk which the Administrator, in [her] judgment, believes are appropriate to determining what will `protect the public health'.”

See 54 FR at 38057, September 14, 1989. Thus, the level of the MIR is only one factor to be weighed in determining acceptability of risks. The Benzene NESHAP explained that “an MIR of approximately one in 10 thousand should ordinarily be the upper end of the range of acceptability. As risks increase above this benchmark, they become presumptively less acceptable under CAA section 112, and would be weighed with the other health risk measures and information in making an overall judgment on acceptability. Or, the Agency may find, in a particular case, that a risk that includes MIR less than the presumptively acceptable level is unacceptable in the light of other health risk factors.”

Id.

at 38045. Similarly, with regard to the ample margin of safety analysis, the EPA stated in the Benzene NESHAP that: “EPA believes the relative weight of the many factors that can be considered in selecting an ample margin of safety can only be determined for each specific source category. This occurs mainly because technological and economic factors (along with the health-related factors) vary from source category to source category.”

Id.

at 38061. We also consider the uncertainties associated with the various risk analyses, as discussed earlier in this preamble, in our determinations of acceptability and ample margin of safety.

The EPA notes that it has not considered certain health information to date in making residual risk determinations. At this time, we do not attempt to quantify those HAP risks that may be associated with emissions from other facilities that do not include the source categories in question, mobile source emissions, natural source emissions, persistent environmental pollution or atmospheric transformation in the vicinity of the sources in these categories.

The agency understands the potential importance of considering an individual's total exposure to HAP in addition to considering exposure to HAP emissions from the source category and facility. We recognize that such consideration may be particularly important when assessing non-cancer risks, where pollutant-specific exposure health reference levels (

e.g.,

RfCs) are based on the assumption that thresholds exist for adverse health effects. For example, the agency recognizes that, although exposures attributable to emissions from a source category or facility alone may not indicate the potential for increased risk of adverse non-cancer health effects in a population, the exposures resulting from emissions from the facility in combination with emissions from all of the other sources (

e.g.,

other facilities) to which an individual is exposed may be sufficient to result in increased risk of adverse non-cancer health effects. In May 2010, the SAB advised the EPA “that RTR assessments will be most useful to decision makers and communities if results are presented in the broader context of aggregate and cumulative risks, including background concentrations and contributions from other sources in the area.”

The EPA's responses to this and all other key recommendations of the SAB's advisory on RTR risk assessment methodologies (which is available at:

http://yosemite.epa.gov/sab/sabproduct.nsf/4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf

) are outlined in a memo to this rulemaking docket from David Guinnup titled,

EPA's Actions in Response to the Key Recommendations of the SAB Review of RTR Risk Assessment Methodologies.

In response to the SAB recommendations, the EPA is incorporating cumulative risk analyses into its RTR risk assessments, including those reflected in this proposal. The agency is: (1) Conducting facility-wide assessments, which include source category emission points as well as other emission points within the facilities; (2) considering sources in the same category whose emissions result in exposures to the same individuals; and (3) for some persistent and bioaccumulative pollutants, analyzing the ingestion route of exposure. In addition, the RTR risk assessments have always considered aggregate cancer risk from all carcinogens and aggregate non-cancer hazard indices from all non-carcinogens affecting the same target organ system.

Although we are interested in placing source category and facility-wide HAP risks in the context of

total

HAP risks from all sources combined in the vicinity of each source, we are concerned about the uncertainties of doing so. Because of the contribution to total HAP risk from emission sources other than those that we have studied in depth during this RTR review, such estimates of total HAP risks would have significantly greater associated uncertainties than the source category or facility-wide estimates. Such aggregate or cumulative assessments would compound those uncertainties, making the assessments too unreliable.

As discussed in more detail below, based on the results of these risk analyses and evaluation of control options, we are proposing revised limits for emissions of POM from potlines, and first ever emissions limits for emissions of PM (as a surrogate for HAP metals) from potlines, anode bake furnaces and paste production plants and for emissions of Ni and As, from the VSS2 potline subcategory.

C. How did we perform the technology review?

Our technology review focused on the identification and evaluation of developments in practices, processes and control technologies that have occurred since the MACT standards were promulgated. Where we identified such developments, in order to inform our decision of whether it is “necessary” to revise the emissions standards, within the meaning of CAA section 112(d)(6), we analyzed the technical feasibility of applying these developments and the estimated costs, energy implications, non-air environmental impacts, as well as considering the emission reductions. We also considered the appropriateness of applying controls to new sources versus retrofitting existing sources.

Based on our analyses of the available data and information, we identified potential developments in practices, processes and control technologies. For this exercise, we considered any of the following to be a “development”:

• Any add-on control technology or other equipment that was not identified and considered during development of the original MACT standards.

• Any improvements in add-on control technology or other equipment (that were identified and considered during development of the original MACT standards) that could result in additional emissions reduction.

• Any work practice or operational procedure that was not identified or considered during development of the original MACT standards.

• Any process change or pollution prevention alternative that could be broadly applied to the industry and that was not identified or considered during

development of the original MACT standards.

• Any significant changes in the cost (including cost effectiveness) of applying controls (including controls the EPA considered during the development of the original MACT standards).

Since we are proposing some first-time MACT standards in this action, we considered the same factors with respect to these proposed MACT standards. In addition to reviewing the practices, processes and control technologies that were considered at the time we originally developed (or last updated) the NESHAP, we also reviewed a variety of data sources in our investigation of potential practices, processes or controls to consider. Among the sources we reviewed were the NESHAP for various industries that were promulgated since the MACT standards being reviewed in this action. We reviewed the regulatory requirements and/or technical analyses associated with these regulatory actions to identify any practices, processes and control technologies considered in these efforts that could be applied to emission sources in the Primary Aluminum Production source category, as well as the costs, non-air impacts and energy implications associated with the use of these technologies. Additionally, we requested information from facilities regarding developments in practices, processes or control technology. Finally, we reviewed information from other sources, such as state and/or local permitting agency databases and industry-supported databases.

For the 2011 proposal, our initial technology review focused on the identification and evaluation of developments in practices, processes and control technologies that have occurred since the EPA promulgated the 1997 NESHAP. We then made decisions on whether it is necessary to propose amendments to the 1997 NESHAP to require standards reflecting performance of the identified developments. Based on our analyses of the data and information collected and our general understanding of the industry and other available information on potential controls for this industry, we identified no developments in practices, processes and control technologies, other than the proposed startup work practices described in the December 2011 proposal (76 FR 76260).

Additional details regarding the previously conducted technology review can be found in the

Draft Technology Review for Primary Aluminum Reduction Plants

(Docket ID No. EPA-HQ-OAR-2011-0797-0149) and are discussed in the preamble to the December 2011 proposal (76 FR 76260). We conducted an additional review of developments in practices, processes and control technologies since the 2011 proposal and updated the technology review to reflect changes in the number and type of currently operating and idled facilities. As noted, this analysis indicates what developments may be possible assuming the EPA adopts the proposed amendments to the MACT standards discussed in the following section of this preamble. The

Revised Draft Technology Review for the Primary Aluminum Production Source Category

is available in the docket (Docket ID No. EPA-HQ-OAR-2011-0797).

IV. Revised Analytical Results and Proposed Decisions for the Primary Aluminum Production Source Category

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

As described previously, CAA section 112(d) requires the EPA to promulgate technology-based NESHAP for listed source categories, including this source category. The EPA did so in the 1997 primary aluminum NESHAP. As described above (in section II.B), the 1997 NESHAP included MACT standards for TF from all types of existing and new potlines and bake furnaces and MACT standards for POM from existing and new Soderberg potlines, paste plants, bake furnaces and new pitch storage tanks. In the 2011 proposal, we proposed emissions limits pursuant to CAA sections 112(d)(2) and (3) for a number of HAP or emissions points that were not previously covered by the NESHAP, including limits for POM from prebake potlines, COS from prebake and Soderberg potlines and POM from existing pitch storage tanks. After proposal, in response to the 2013 CAA section 114 information request, we received a substantial amount of additional data on POM emissions from prebake potlines and therefore we re-analyzed the proposed limits for emissions of POM from prebake potlines.

Based on those analyses we have determined it is appropriate to propose revised emission limits for POM from these existing potlines in these subcategories, and to propose different POM limits for new potlines.

As explained above, the EPA did not have POM emissions data for prebake potlines at the time of the December 2011 proposal. The EPA developed the POM emissions MACT floor limits for prebake potlines in that proposal by estimating POM emissions based on a ratio of POM emissions to TF emissions, an approach which found no support in the public comments. Today's proposal is based entirely on the new emission data obtained since the December 2011 proposal. See section II.D, above.

Additionally, after the 2011 proposal, in response to the 2013 CAA section 114 information request, we received data regarding PM and HAP metals emissions from potlines, anode bake furnaces and paste plants. These pollutants are not covered by the 1997 NESHAP. Based on those analyses, we have determined it is appropriate to propose emission limits for PM, as a surrogate for HAP metals, from existing potlines and new potlines, as well as from new and existing anode bake furnaces and new and existing paste plants. We have used PM as a surrogate for HAP metals in many other NESHAP (

e.g.,

secondary aluminum, see 65 FR 15692 (March 23, 2000), and Portland cement, 64 FR 31900 (June 14, 1999)). The agency believes PM is an appropriate surrogate for non-mercury HAP metals because those metals and particulate are captured indiscriminately by PM control technology. See

National Lime Ass'n

EPA,

233 F. 3d 625, 639 (D.C. Cir. 2000) (upholding use of PM as a surrogate for HAP metal for purposes of CAA section 112(d) MACT standard). We do not consider TF to be a suitable surrogate for HAP meta

This text is long and has been trimmed here. Open the source document for the complete record.

This is a copy of a public record, reproduced as it was published. It is not legal advice, and it may not be the version a court would rely on. Check the official source before you cite it.