[Federal Register Volume 83, Number 158 (Wednesday, August 15, 2018)]
[Proposed Rules]
[Pages 40487-40498]
[FR Doc No: 2018-17582]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R05-OAR-2015-0700; FRL-9982-28--Region 5]

Air Plan Approval; Indiana; Attainment Plan for Indianapolis,
Southwest Indiana, and Terre Haute SO2 Nonattainment Areas

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to
approve as a State Implementation Plan (SIP) revision an Indiana
submission to EPA dated October 2, 2015. The submission addresses
attainment of the 2010 sulfur dioxide (SO2) national ambient
air quality standard (NAAQS) for the Indianapolis (Marion County),
Southwest Indiana (Daviess and Pike Counties), and Terre Haute (Vigo
County) areas. Indiana also submitted a SIP revision request for the
Morgan County area. In this proposed action, EPA is not addressing the
Morgan County portion of the SIP revision request, and will address it
separately in a future action. This plan (herein called a
``nonattainment plan'') includes Indiana's attainment demonstration and
other elements required under the Clean Air Act (CAA). In addition to
an attainment demonstration, the nonattainment plan addresses the
requirement for meeting reasonable further progress (RFP) toward
attainment of the NAAQS, reasonably available control measures and
reasonably available control technology (RACM/RACT), base-year and
projection-year emission inventories, enforceable emissions limitations
and control measures, and contingency measures. EPA proposes to
conclude that Indiana has appropriately demonstrated that the plan
provisions provide for attainment of the 2010 SO2 NAAQS in
the Indianapolis, Southwest Indiana, and Terre Haute areas by the
applicable attainment date and that the plan meets the other applicable
requirements under the CAA.

DATES: Comments must be received on or before September 14, 2018.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R05-
OAR-2015-0700 at http://www.regulations.gov, or via email to
[email protected]. For comments submitted at Regulations.gov,
follow the online instructions for submitting comments. Once submitted,
comments cannot be edited or removed from Regulations.gov. For either
manner of submission, EPA may publish any comment received to its
public docket. Do not submit electronically any information you
consider to be Confidential Business Information (CBI) or other
information whose disclosure is restricted by statute. Multimedia
submissions (audio, video, etc.) must be accompanied by a written
comment. The written comment is considered the official comment and
should include discussion of all points you wish to make. EPA will
generally not consider comments or comment contents located outside of
the primary submission (i.e. on the web, cloud, or other file sharing
system). For additional submission methods, please contact the person
identified in the For Further Information Contact section. For the full
EPA public comment policy, information about CBI or multimedia
submissions, and general guidance on making effective comments, please
visit http://www2.epa.gov/dockets/commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT: Michelle Becker, Life Scientist,
Attainment Planning and Maintenance Section, Air Programs Branch (AR-
18J), Environmental Protection Agency, Region 5, 77 West Jackson
Boulevard, Chicago, Illinois 60604,<PHONE_NUMBER>,
[email protected].

SUPPLEMENTARY INFORMATION: Throughout this document whenever ``we,''
``us,'' or ``our'' is used, we mean EPA. The following outline is
provided to aid in locating information in this preamble.

Table of Contents

I. Why was Indiana required to submit an SO2 plan for
Indianapolis, Southwest Indiana, and Terre Haute?
II. Requirements for SO2 Nonattainment Area Plans
III. Requirements for Attainment Demonstrations and Longer-Term
Averaging
IV. Review of Indiana's Modeled Attainment Plans
    A. Model Selection
    B. Meteorological Data
    C. Emissions Data
    D. Emission Limits
    1. Enforceability
    2. Longer Term Average Limits
    E. Background Concentrations
    F. Comments Made During State Rulemaking
    G. Summary of Results
V. Review of Other Plan Requirements
    A. Emissions Inventory
    B. RACM/RACT
    C. New Source Review (NSR)
    D. RFP
    E. Contingency Measures
VI. EPA's Proposed Action
VII. Incorporation by Reference
VIII. Statutory and Executive Order Reviews

I. Why was Indiana required to submit an SO[bdi2] plan for
Indianapolis, Southwest Indiana, and Terre Haute?

    On June 22, 2010, EPA promulgated a new 1-hour primary
SO2 NAAQS of 75 parts per billion (ppb), which is met at an
ambient air quality monitoring site when the 3-year average of the
annual 99th percentile of daily maximum 1-hour average concentrations
does not exceed 75 ppb, as determined in accordance with appendix T of
40 CFR part 50. See 75 FR 35520, codified at 40 CFR 50.17(a)-(b). On
August 5, 2013, EPA designated a first set of 29 areas of the country
as nonattainment for the 2010 SO2 NAAQS, including the
Indianapolis (Marion County), Morgan County, Southwest Indiana (Daviess
and Pike Counties), and Terre Haute (Vigo County) areas within Indiana.
See 78 FR 47191, codified at 40 CFR part 81, subpart C. These area
designations were effective October 4, 2013. Section 191(a) of the CAA
directs states to submit SIPs for areas designated as nonattainment for
the SO2 NAAQS to EPA within 18 months of the effective date
of the designation, i.e., by no later than April 4, 2015 in this case.
Under CAA section 192(a), the states are required to

demonstrate that their respective areas will attain the NAAQS as
expeditiously as practicable, but no later than 5 years from the
effective date of designation, which is October 4, 2018.
    In response to the requirement for SO2 nonattainment
plan submittals, Indiana submitted nonattainment plans for the
Indianapolis, Morgan County, Southwest Indiana, and Terre Haute areas
on October 2, 2015. EPA will address the Morgan County portion of the
submittal in a future action. The remainder of this preamble describes
the requirements that such plans must meet in order to obtain EPA
approval, provides a review of the state's plans with respect to these
requirements, and describes EPA's proposed action on the plans.

II. Requirements for SO[bdi2] Nonattainment Area Plans

    Nonattainment SIPs must meet the applicable requirements of the
CAA, specifically CAA sections 110, 172, 191 and 192. EPA's regulations
governing nonattainment SIPs are set forth at 40 CFR part 51, with
specific procedural requirements and control strategy requirements
residing at subparts F and G, respectively. Soon after Congress enacted
the 1990 Amendments to the CAA, EPA issued comprehensive guidance on
SIPs, in a document entitled the ``General Preamble for the
Implementation of Title I of the Clean Air Act Amendments of 1990,''
published at 57 FR 13498 (April 16, 1992) (General Preamble). Among
other things, the General Preamble addressed SO2 SIPs and
fundamental principles for SIP control strategies. Id., at 57 FR 13545-
13549, 13567-13568. On April 23, 2014, EPA issued guidance for meeting
the statutory requirements in SO2 SIPs submitted under the
2010 NAAQS, in a document entitled, ``Guidance for 1-Hour
SO2 Nonattainment Area SIP Submissions,'' available at
https://www.epa.gov/sites/production/files/2016-06/documents/20140423guidance_nonattainment_sip.pdf. In this guidance EPA described
the statutory requirements for a complete nonattainment area
SO2 SIP, which includes: An accurate emissions inventory of
current emissions for all sources of SO2 within the
nonattainment area; an attainment demonstration; demonstration of RFP;
implementation of RACM (including RACT); new source review (NSR);
enforceable emissions limitations and control measures; and adequate
contingency measures for the affected area. A synopsis of these
requirements is also provided in the notice of proposed rulemaking on
the Illinois SO2 nonattainment plans, published on October
5, 2017 at 82 FR 46434.
    In order for EPA to fully approve a SIP as meeting the requirements
of CAA sections 110, 172 and 191-192 and EPA's regulations at 40 CFR
part 51, the SIP for the affected area needs to demonstrate to EPA's
satisfaction that each of the aforementioned requirements have been
met. Under CAA sections 110(l) and 193, EPA may not approve a SIP that
would interfere with any applicable requirement concerning NAAQS
attainment and RFP, or any other applicable requirement, and no
requirement in effect (or required to be adopted by an order,
settlement, agreement, or plan in effect before November 15, 1990) in
any area which is a nonattainment area for any air pollutant, may be
modified in any manner unless it ensures equivalent or greater emission
reductions of such air pollutant.

III. Requirements for Attainment Demonstrations and Longer-Term
Averaging

    CAA sections 172(c)(1), 172(c)(6) and 192(a) direct states with
SO2 areas designated as nonattainment to demonstrate that
the submitted plan provides for attainment of the NAAQS. 40 CFR part
51, subpart G further delineates the control strategy requirements that
SIPs must meet, and EPA has long required that all SIPs and control
strategies reflect four fundamental principles of quantification,
enforceability, replicability, and accountability. General Preamble, at
13567-68. SO2 attainment plans must consist of two
components: (1) Emission limits and other control measures that assure
implementation of permanent, enforceable and necessary emission
controls, and (2) a modeling analysis which meets the requirements of
40 CFR part 51, appendix W which demonstrates that these emission
limits and control measures provide for timely attainment of the
primary SO2 NAAQS as expeditiously as practicable, but by no
later than the attainment date for the affected area. In all cases, the
emission limits and control measures must be accompanied by appropriate
methods and conditions to determine compliance with the respective
emission limits and control measures and must be quantifiable (i.e., a
specific amount of emission reduction can be ascribed to the measures),
fully enforceable (specifying clear, unambiguous and measurable
requirements for which compliance can be practicably determined),
replicable (the procedures for determining compliance are sufficiently
specific and non-subjective so that two independent entities applying
the procedures would obtain the same result), and accountable (source
specific limits must be permanent and must reflect the assumptions used
in the SIP demonstrations).
    EPA's April 2014 guidance recommends that the emission limits be
expressed as short-term average limits (e.g., addressing emissions
averaged over one or three hours), but also describes the option to
utilize emission limits with longer averaging times of up to 30 days so
long as the state meets various suggested criteria. See 2014 guidance,
pp. 22 to 39. The guidance recommends that--should states and sources
utilize longer averaging times--the longer-term average limit should be
set at an adjusted level that reflects a stringency comparable to the
1-hour average limit at the critical emission value shown to provide
for attainment that the plan otherwise would have set.
    The April 2014 guidance provides an extensive discussion of EPA's
rationale for concluding that appropriately set comparably stringent
limitations based on averaging times as long as 30 days can be found to
provide for attainment of the 2010 SO2 NAAQS. In evaluating
this option, EPA considered the nature of the standard, conducted
detailed analyses of the impact of use of 30-day average limits on the
prospects for attaining the standard, and carefully reviewed how best
to achieve an appropriate balance among the various factors that
warrant consideration in judging whether a state's plan provides for
attainment. Id. at pp. 22 to 39. See also id. at Appendices B, C, and
D.
    As specified in 40 CFR 50.17(b), the 1-hour primary SO2
NAAQS is met at an ambient air quality monitoring site when the 3-year
average of the annual 99th percentile of daily maximum 1-hour average
concentrations is less than or equal to 75 parts per billion. In a year
with 365 days of valid monitoring data, the 99th percentile would be
the fourth highest daily maximum 1-hour value. The 2010 SO2
NAAQS, including this form of determining compliance with the standard,
was upheld by the U.S. Court of Appeals for the District of Columbia
Circuit in Nat'l Envt'l Dev. Ass'n's Clean Air Project v. EPA, 686 F.3d
803 (D.C. Cir. 2012). Because the standard has this form, a single
hourly exceedance of the 75 ppb level does not create a violation of
the standard. Instead, at issue is whether a source operating in
compliance with a properly set longer term average could cause hourly
exceedances, and if so the resulting frequency and magnitude of

such exceedances, and in particular whether EPA can have reasonable
confidence that a properly set longer term average limit will provide
that the three-year average of the annual fourth highest daily maximum
hourly value will be at or below 75 ppb. A synopsis of how EPA judges
whether such plans ``provide for attainment,'' based on modeling of
projected allowable emissions and in light of the NAAQS' form for
determining attainment at monitoring sites, follows.
    For plans for SO2 based on 1-hour emission limits, the
standard approach is to conduct modeling using fixed emission rates.
The maximum emission rate that would be modeled to result in attainment
(i.e., in an ``average year'' \1\ shows three, not four days with
maximum hourly levels exceeding 75 ppb) is labeled the ``critical
emission value.'' The modeling process for identifying this critical
emissions value inherently considers the numerous variables that affect
ambient concentrations of SO2, such as meteorological data,
background concentrations, and topography. In the standard approach,
the state would then provide for attainment by setting a continuously
applicable 1-hour emission limit at this critical emission value.
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    \1\ An ``average year'' is used to mean a year with average air
quality. While 40 CFR 50 appendix T provides for averaging three
years of 99th percentile daily maximum values (e.g., the fourth
highest maximum daily concentration in a year with 365 days with
valid data), this discussion and an example below uses a single
``average year'' in order to simplify the illustration of relevant
principles.
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    EPA recognizes that some sources have highly variable emissions,
for example due to variations in fuel sulfur content and operating
rate, that can make it extremely difficult, even with a well-designed
control strategy, to ensure in practice that emissions for any given
hour do not exceed the critical emission value. EPA also acknowledges
the concern that longer-term emission limits can allow short periods
with emissions above the ``critical emissions value,'' which, if
coincident with meteorological conditions conducive to high
SO2 concentrations, could in turn create the possibility of
a NAAQS exceedance occurring on a day when an exceedance would not have
occurred if emissions were continuously controlled at the level
corresponding to the critical emission value. However, for several
reasons, EPA believes that the approach recommended in its guidance
document suitably addresses this concern. First, from a practical
perspective, EPA expects the actual emission profile of a source
subject to an appropriately set longer term average limit to be similar
to the emission profile of a source subject to an analogous 1-hour
average limit. EPA expects this similarity because it has recommended
that the longer-term average limit be set at a level that is comparably
stringent to the otherwise applicable 1-hour limit (reflecting a
downward adjustment from the critical emissions value) and that takes
the source's emissions profile into account. As a result, EPA expects
either form of emission limit to yield comparable air quality.
    Second, from a more theoretical perspective, EPA has compared the
likely air quality with a source having maximum allowable emissions
under an appropriately set longer term limit, as compared to the likely
air quality with the source having maximum allowable emissions under
the comparable 1-hour limit. In this comparison, in the 1-hour average
limit scenario, the source is presumed at all times to emit at the
critical emission level, and in the longer-term average limit scenario,
the source is presumed occasionally to emit more than the critical
emission value but on average, and presumably at most times, to emit
well below the critical emission value. In an ``average year,''
compliance with the 1-hour limit is expected to result in three
exceedance days (i.e., three days with hourly values above 75 ppb) and
a fourth day with a maximum hourly value at 75 ppb. By comparison, with
the source complying with a longer-term limit, it is possible that
additional exceedances would occur that would not occur in the 1-hour
limit scenario (if emissions exceed the critical emission value at
times when meteorology is conducive to poor air quality). However, this
comparison must also factor in the likelihood that exceedances that
would be expected in the 1-hour limit scenario would not occur in the
longer-term limit scenario. This result arises because the longer-term
limit requires lower emissions most of the time (because the limit is
set well below the critical emission value), so a source complying with
an appropriately set longer term limit is likely to have lower
emissions at critical times than would be the case if the source were
emitting as allowed with a 1-hour limit.
    As a hypothetical example to illustrate these points, suppose a
source that always emits 1000 pounds of SO2 per hour, which
results in air quality at the level of the NAAQS (i.e., results in a
design value of 75 ppb). Suppose further that in an ``average year,''
these emissions cause the 5 highest maximum daily 1-hour average
concentrations to be 100 ppb, 90 ppb, 80 ppb, 75 ppb, and 70 ppb. Then
suppose that the source becomes subject to a 30-day average emission
limit of 700 pounds per hour (lbs/hour). It is theoretically possible
for a source meeting this limit to have emissions that occasionally
exceed 1000 lbs/hour, but with a typical emissions profile emissions
would much more commonly be between 600 and 800 lbs/hour. In this
simplified example, assume a zero background concentration, which
allows one to assume a linear relationship between emissions and air
quality. (A nonzero background concentration would make the mathematics
more difficult but would give similar results.) Air quality will depend
on what emissions happen on what critical hours, but suppose that
emissions at the relevant times on these 5 days are 800 pounds/hour,
1,100 lbs/hour, 500 lbs/hour, 900 lbs/hour, and 1,200 lbs/hour,
respectively. (This is a conservative example because the average of
these emissions, 900 lbs/hour, is well over the 30-day average emission
limit.) These emissions would result in daily maximum 1-hour
concentrations of 80 ppb, 99 ppb, 40 ppb, 67.5 ppb, and 84 ppb. In this
example, the fifth day would have an exceedance that would not
otherwise have occurred, but the third day would not have an exceedance
that otherwise would have occurred, and the fourth day would have had a
concentration below, rather than at 75 ppb. In this example, the fourth
highest maximum daily concentration under the 30-day average would be
67.5 ppb.
    This simplified example illustrates the findings of a more
complicated statistical analysis that EPA conducted using a range of
scenarios using actual plant data. As described in Appendix B of EPA's
April 2014 SO2 nonattainment planning guidance, EPA found
that the requirement for lower average emissions is highly likely to
yield better air quality than is required with a comparably stringent
1-hour limit. Based on analyses described in appendix B of its 2014
guidance, EPA expects that an emission profile with maximum allowable
emissions under an appropriately set, comparably stringent 30-day
average limit is likely to have the net effect of having a lower number
of exceedances and better air quality than an emission profile with
maximum allowable emissions under a 1-hour emission limit at the
critical emission value. This result provides a compelling policy
rationale for allowing the use of a longer averaging period, in
appropriate circumstances where the facts indicate this result can be
expected to occur.

    The question then becomes whether this approach--which is likely to
produce a lower number of overall exceedances even though it may
produce some unexpected exceedances above the critical emission value--
meets the requirement in sections 110(a)(1), 172(c)(1), 172(c)(6) and
192(a) for SIPs to contain emissions limitations and control measures
to ``provide for attainment'' of the NAAQS. For SO2, as for
other pollutants, it is generally impossible to design a nonattainment
plan in the present that will guarantee that attainment will occur in
the future. A variety of factors can cause a well-designed attainment
plan to fail and unexpectedly not result in attainment, for example if
meteorology occurs that is more conducive to poor air quality than was
anticipated in the plan. Therefore, in determining whether a plan meets
the requirement to provide for attainment, EPA's task is commonly to
judge not whether the plan provides absolute certainty that attainment
will in fact occur, but rather whether the plan provides an adequate
level of confidence of prospective NAAQS attainment. From this
perspective, in evaluating use of a 30-day average limit, EPA must
weigh the likely net effect on air quality. Such an evaluation must
consider the risk that occasions with meteorology conducive to high
concentrations will have elevated emissions leading to exceedances that
would not otherwise have occurred, and must also weigh the likelihood
that the requirement for lower emissions on average will result in days
not having exceedances that would have been expected with emissions at
the critical emissions value. Additional policy considerations, such as
in this case the desirability of accommodating real world emissions
variability without significant risk of violations, are also
appropriate factors for EPA to weigh in judging whether a plan provides
a reasonable degree of confidence that the plan will lead to
attainment. Based on these considerations, especially given the high
likelihood that a continuously enforceable limit averaged over as long
as 30 days, determined in accordance with EPA's guidance, will result
in attainment, EPA believes as a general matter that such limits, if
appropriately determined, can reasonably be considered to provide for
attainment of the 2010 SO2 NAAQS.
    The April 2014 guidance offers specific recommendations for
determining an appropriate longer-term average limit. The recommended
method starts with determination of the 1-hour emission limit that
would provide for attainment (i.e., the critical emission value), and
applies an adjustment factor to determine the (lower) level of the
longer-term average emission limit that would be estimated to have a
stringency comparable to the otherwise necessary 1-hour emission limit.
This method uses a database of continuous emission data reflecting the
type of control that the source will be using to comply with the SIP
emission limits, which (if compliance requires new controls) may
require use of an emission database from another source. The
recommended method involves using these data to compute a complete set
of emission averages, computed according to the averaging time and
averaging procedures of the prospective emission limitation. In this
recommended method, the ratio of the 99th percentile among these long
term averages to the 99th percentile of the 1-hour values represents an
adjustment factor that may be multiplied by the candidate 1-hour
emission limit to determine a longer term average emission limit that
may be considered comparably stringent.\2\ The guidance also addresses
a variety of related topics, such as the potential utility of setting
supplemental emission limits, such as mass-based limits, to reduce the
likelihood and/or magnitude of elevated emission levels that might
occur under the longer term emission rate limit.
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    \2\ For example, if the critical emission value is 1000 pounds
of SO2 per hour, and a suitable adjustment factor is
determined to be 70 percent, the recommended longer term average
limit would be 700 pounds per hour.
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    Preferred air quality models for use in regulatory applications are
described in Appendix A of EPA's Guideline on Air Quality Models (40
CFR part 51, appendix W).\3\ In 2005, EPA promulgated AERMOD as the
Agency's preferred near-field dispersion modeling for a wide range of
regulatory applications addressing stationary sources (for example in
estimating SO2 concentrations) in all types of terrain based
on extensive developmental and performance evaluation. Supplemental
guidance on modeling for purposes of demonstrating attainment of the
SO2 standard is provided in appendix A to the April 23, 2014
SO2 nonattainment area SIP guidance document referenced
above. Appendix A provides extensive guidance on the modeling domain,
the source inputs, assorted types of meteorological data, and
background concentrations. Consistency with the recommendations in this
guidance is generally necessary for the attainment demonstration to
offer adequately reliable assurance that the plan provides for
attainment.
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    \3\ EPA published revisions to the Guideline on Air Quality
Models (40 CFR part 51, appendix W) on January 17, 2017.
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    As stated previously, attainment demonstrations for the 2010
SO2 NAAQS must demonstrate future attainment and maintenance
of the NAAQS in the entire area designated as nonattainment (i.e., not
just at the violating monitor) by using air quality dispersion modeling
(see appendix W to 40 CFR part 51) to show that the mix of sources and
enforceable control measures and emission rates in an identified area
will not lead to a violation of the SO2 NAAQS. For a short-
term (i.e., 1-hour) standard, EPA believes that dispersion modeling,
using allowable emissions and addressing stationary sources in the
affected area (and in some cases those sources located outside the
nonattainment area which may affect attainment in the area) is
technically appropriate, efficient and effective in demonstrating
attainment in nonattainment areas because it takes into consideration
combinations of meteorological and emission source operating conditions
that may contribute to peak ground-level concentrations of
SO2.
    The meteorological data used in the analysis should generally be
processed with the most recent version of AERMET. Estimated
concentrations should include ambient background concentrations, should
follow the form of the standard, and should be calculated as described
in section <IP_ADDRESS> of the August 23, 2010 clarification memo on
``Applicability of Appendix W Modeling Guidance for the 1-hr
SO2 National Ambient Air Quality Standard'' (EPA, 2010a).

IV. Review of Indiana's Modeled Attainment Plans

    The following discussion evaluates various features of the modeling
that Indiana used in its attainment demonstrations.

A. Model Selection

    Indiana's attainment demonstrations used AERMOD, the preferred
model for these applications as identified in appendix W to CFR part
51. Indiana used version 14134 of this model, utilizing the regulatory
default mode for all air quality modeling runs. This version of AERMOD
was the most recent version at the time the state conducted its
nonattainment planning; and, in any case, the results of this version
are likely to be similar to those that more recent versions would
provide. Therefore, EPA finds the use of this version of AERMOD
acceptable.

    The receptor grids and modeling domain followed the recommended
approaches from appendix W, Guidelines on Air Quality Models. Receptor
spacing for each modeled facility fence line was every 50 meters with
100-meter spacing of receptors out to a distance of 500 meters beyond
each facility. The distances between modeled facilities contained
receptors which were spaced at 100-meter intervals. The 100-meter
spacing receptor grid contained in excess of several thousand receptors
for each modeled nonattainment area. The above receptor spacing and
facility fence line receptors brought the total modeled receptors for
Marion County to 17,925 receptors, including two additional receptors
placed at the Marion County SO2 monitor locations; Vigo
County to 7,111 receptors, including two receptors at each of the Vigo
County SO2 monitors; and Daviess and Pike to 5,354
receptors, including two located at Daviess and Pike County
SO2 monitors.
    Indiana did not assess impacts within any one facility's property
from the emissions from other facilities. EPA reviewed Indiana's
modeling results to assess whether any further modeling was warranted
to evaluate impacts within of other facilities on any plant's property.
For Southwest Indiana, peak impacts from the two facilities were well
off any plant property, and therefore insufficient to cause a violation
within each other's property. For the Terre Haute area, since the Duke
Wabash River Power Plant and sgSolutions sources were adjacent, EPA
conducted additional modeling that demonstrated that neither plant
contributed to a violation within the other plant's property. Finally,
in Indianapolis, EPA conducted additional modeling for the Vertellus
and Rolls Royce facilities due to their proximity to one another and
due to peak concentrations for both facilities occurring at their
property boundaries. The analysis showed that collective impacts at on-
property receptors from the other source and from other sources in
Marion County were below the NAAQS. Further description of EPA's review
is provided in the technical support document available in the docket
for this rulemaking.\4\ EPA finds that Indiana's receptor grids,
supplemented with the results of EPA's additional analysis, are
adequate for assessing whether the adopted limits provide for
attainment throughout the respective areas.
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    \4\ June 27, 2018 Technical Support Document--``Evaluation of
Concentrations on Facility Property Attributable to Nearby
Sources''.
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    The appropriate rural or urban land classifications were selected
by Indiana, with only the Indianapolis SO2 area being
classified as urban. The remaining 1-hour SO2 nonattainment
areas addressed in this action, in Southwest Indiana and Terre Haute,
were modeled as rural. While Indiana's submittal does not discuss the
rationale for these determinations, EPA agrees that these selections
appropriately characterize these areas. The Indianapolis area has
historically been modeled using ``urban dispersion.'' This combined
statistical area includes 2.3 million people, including Marion County,
with just under 1 million people. The population density for Marion
County is 917 people per square kilometer, and the modeled area is a
relatively urban portion of the county, thus meeting the criterion in
appendix W that areas with at least 750 people per square kilometer may
be treated as urban. Conversely, Vigo, Pike, and Daviess Counties have
population densities of 102, 13, and 42 people per square mile,
respectively. Examination of satellite imagery for these areas confirms
that a land use analysis of these areas would be expected to yield the
same character of Indianapolis as urban and the other areas as rural.
For Indianapolis, a population of 1,000,000 (reflecting the approximate
population of Marion County) was used in AERMOD to characterize the
strength of the urban heat island effect. The use of urban dispersion
with a 1,000,000 population is appropriate for this modeling. For these
reasons, EPA finds it appropriate to model these areas using the land
classifications identified by Indiana.

B. Meteorological Data

    Indiana used the Indianapolis National Weather Service (NWS)
surface data and the Lincoln, Illinois upper air station (WBAN#048233)
data for Indianapolis and Terre Haute, and the Evansville NWS for
surface data and the Lincoln upper air station data for Southwest
Indiana. These are the closest National Weather Service surface
stations to each respective area. The State determined these stations
to be the most representative for the respective modeling domains. The
upper air stations were chosen on the basis of regional
representativeness. EPA finds Indiana's choices of surface and upper
air meteorological stations appropriate based on: (1) The suitability
of meteorological data for the study area; and (2) the actual
similarity of surface conditions and surroundings at the emissions
source/receptor impact area compared to the locations of the
meteorological instrumentation towers.

C. Emissions Data

    Indiana modeled 14 sources in the three nonattainment areas of
Indianapolis (6 sources), Southwest Indiana (2 sources), and Terre
Haute (6 sources). The sources were physically located within the
nonattainment area; Indiana excluded facilities that emitted less than
ten tons per year, and Indiana found no sources outside the
nonattainment areas with sufficient likely concentration gradient in
the modeled area to warrant modeling explicitly. The emission limits
used for the model for 12 of the sources correspond to the revised
sulfur dioxide limitations on a 1-hour basis and are found in Indiana
Administrative Code (IAC) Part 326, Article 7, and have been included
by Indiana in this submission for SIP approval. The applicable emission
limits for sgSolutions in Vigo County (Terre Haute) and IPL--Petersburg
in Daviess County (Southwest Indiana) are established on a 30-day
average basis and are lower than the modeled 1-hour attainment emission
rates (the critical emission values) by virtue of application of
adjustment factors determined and applied in accordance with the 2014
SO2 Guidance. These limits are established and made
enforceable in 326 IAC 7. EPA finds Indiana's choice of included
sources appropriate, and finds that the modeled emission levels
appropriately correspond to the limits given in 326 IAC 7, in the case
of IPL--Petersburg and sgSolutions by modeling the 1-hour emission
level that corresponds (before adjustment) to the 30-day average limit
established in 326 IAC 7. Further discussion of the 30-day average
limits is provided below.

D. Emission Limits

    An important prerequisite for approval of an attainment plan is
that the emission limits that provide for attainment be quantifiable,
fully enforceable, replicable, and accountable. See General Preamble at
13567-68. Some of the limits that Indiana's plan relies on are
expressed as 30-day average limits. Therefore, part of the review of
Indiana's attainment plan must address the use of these limits, both
with respect to the general suitability of using such limits for this
purpose and with respect to whether the particular limits included in
the plan have been suitably demonstrated to provide for attainment. The
first subsection that follows addresses the enforceability of the
limits in the plan,

and the second subsection that follows addresses the 30-day average
limits.
1. Enforceability
    In preparing its plans, Indiana adopted revisions to a previously
approved state regulation governing emissions of SO2. These
rule revisions were adopted by the Indiana Environmental Rules Board
following established, appropriate public review procedures. In
addition, the rule revisions provide unambiguous, permanent emission
limits, expressed in lbs/hour of allowable SO2 emissions,
that, if exceeded by a source, would be clear grounds for an
enforcement action.
    The revised limits for significant contributing sources have a
compliance date of January 1, 2017 and are codified in 326 IAC 7,
titled ``Sulfur Dioxide Rules.'' Specifically, the list of rules is
``Compliance date'' (326 IAC 7-1.1-3), ``Reporting requirements;
methods to determine compliance'' (7-2-1), ``Marion County sulfur
dioxide emission limitations'' (7-4-2.1), ``Vigo County sulfur dioxide
emission limitations'' (7-4-3.1), and ``Pike County sulfur dioxide
emission limitations'' (7-4-15). The rules also include associated
monitoring, testing, and recordkeeping and reporting requirements. For
example, continuous emission monitoring will be conducted for assessing
compliance with the 30-day average limits. Specifically, 326 IAC 7-1-9
is being replaced by 7-4-2.1 for Marion County and 326 IAC 7-1-10.1 is
being replaced by 326 IAC 7-4-15 for Vigo County. EPA finds these
limits to be enforceable. A summary of the limits is shown in Table 1.
    As shown in this table, the emission limits for sgSolutions Tail
Gas Incinerator Stack EP1 and IPL-Petersburg Units 1-4 are expressed as
30-day average limits. Other limits in the rule are expressed as 1-hour
average limits. The limits are expressed as lbs/hour or pounds per
million British Thermal Units (MMBTU). EPA's review of Indiana's
nonattainment plan addresses the use of these limits, both with respect
to the general suitability of using such limits in attainment
demonstrations, and whether Indiana has demonstrated that the
particular limits included in the plan provide for attainment. EPA
addresses Indiana's use of a 30-day average emission limits below.

                               Table 1--Emission Limits in Submitted Indiana Rules
----------------------------------------------------------------------------------------------------------------
                                                                       Emission limit (lbs/hour)  Emission limit
                 Source                    Emission unit description     or other requirements      (lbs/MMBTU)
----------------------------------------------------------------------------------------------------------------
                        Marion County sulfur dioxide emission limitations 326 IAC 7-4-2.1
----------------------------------------------------------------------------------------------------------------
Citizens Thermal--Perry K Source ID No.   (A) Boiler 11.............  73.6......................             0.2
 00034.                                   (B) Boiler 13.............  80.6......................             0.2
                                          (C) Boiler 14.............  80.6......................             0.2
                                          (D) Boilers 12, 15, and 16  Burn natural gas..........  ..............
                                          (E) Boiler 17.............  72.6......................             0.3
                                          (F) Boiler 18.............  72.6......................             0.3
Belmont Advanced Wastewater Treatment     Incinerator 1, Incinerator  Comply with SO2 limit in    ..............
 Plant Source ID No. 00032.                2, Incinerator 3, and       40 CFR 60, subpart MMMM *
                                           Incinerator 4.              or 40 CFR 60, subpart
                                                                       LLLL *.
Rolls-Royce Source ID No. 00311.........  (A) Boiler 0070-58........  0.07......................          0.0015
                                          (B) Boiler 0070-59........  0.07......................          0.0015
                                          (C) Boiler 0070-62........  0.37......................          0.0015
                                          (D) Boiler 0070-63........  0.37......................          0.0015
                                          (E) Boilers 0070-64.......  Burn natural gas or                   0.01
                                                                       landfill gas.
                                          (F) Boiler 0070-65........  Burn natural gas or                   0.01
                                                                       landfill gas.
                                          (G) Generating Turbine      Burn natural gas or                   0.01
                                           0070-80.                    landfill gas.
                                          (H) 2 Gas Turbine Engines   ..........................             0.1
                                           0070-66.
                                          (I) 12 Gas Turbine Engines  ..........................            0.05
                                           0070-67.
                                          (J) 3 Gas Turbine Engines   ..........................            0.05
                                           0070-68c, 0070-68d, and
                                           0070-68e.
                                          (K) 2 Gas Turbine Engines   Burn natural gas..........  ..............
                                           0070-68a and 0070-68b.
                                          (L) 3 Gas Turbine Engines   ..........................            0.05
                                           0070-69.
                                          (M) Three Shack Heaters     Burn natural gas..........  ..............
                                           0070-70.
                                          (N) Rental Generators.....  ..........................          0.0015
                                          (O) Engine Test Cells       ..........................            0.05
                                           Plant 5.
                                          (P) Engine Test Cell Plant  ..........................             0.1
                                           8.
                                          (Q) Engine Test Cell N20..  18 foot vertical stack, if  ..............
                                                                       operating.
                                          (R) Engine Test Cell N21..  20 foot vertical stack, if  ..............
                                                                       operating.
                                          (S) Engine Test Cell N23..  30 foot vertical stack, if  ..............
                                                                       operating.
                                          (T) Engine Test Cell N24..  20 foot vertical stack, if  ..............
                                                                       operating.
Vertellus Agriculture and Nutrition       (A) 70K Boiler 70-2722W...  18.4......................            0.20
 Specialties Source ID No. 00315.         (B) 30K Boiler 30-2726S...  9.8.......................            0.25
                                          (C) 28K Boiler 28-186N....  9.9.......................            0.27
                                          (D) Boiler CB-70K.........  Burn natural gas..........  ..............
                                          (E) BM Furnace BM2724W....  1.1.......................            0.05
                                          (F) Box Furnace BX2707V...  0.8.......................            0.05
                                          (G) DAB Furnace 732714....  2.8.......................            0.05
                                          (H) Born Heater 722804....  0.34......................            0.05
                                          (I) Born Heater Furnace     0.3.......................            0.05
                                           BXS2706Q.
                                          (J) EP Furnace EP2729Q....  0.15......................            0.05
                                          (K) CB20 CB600-300 Boiler.  2.3.......................            0.09
                                          (L) 50K CN5-400 Boiler....  5.5.......................            0.09
                                          (M) BD Furnace BD2714V....  0.75......................            0.05

                                          (N) Heater BS2740Q........  0.3.......................            0.05
                                          (O) Heater BT2728S........  0.3.......................            0.05
                                          (P) Furnace HW-925.001....  12.25.....................            1.25
                                          (Q) CS Kettle Born Heater.  Burn natural gas..........  ..............
                                          (R) CS Still Born Heater..  Burn natural gas..........  ..............
                                          (S) Born Hot Oil Furnace    Burn natural gas..........  ..............
                                           (Process Heater) Unit
                                           2607T.
Quemetco Source ID No. 00079............  WESP Stack................  52.0......................  ..............
Indianapolis Power & Light Co.--Harding   (A) Boiler 9..............  Do not operate............  ..............
 Street Generating Station Source ID No.  (B) Boiler 10.............  Do not operate............  ..............
 00033.                                   (C) Boiler 50.............  Burn natural gas..........  ..............
                                          (D) Boiler 60.............  Burn natural gas..........  ..............
                                          (E) Boiler 70.............  Burn natural gas..........  ..............
                                          (F) Gas Turbine 1.........  29.9......................             0.1
                                          (G) Gas Turbine 2.........  29.9......................             0.1
                                          (H) Gas Turbine 4.........  87.5......................             0.1
                                          (I) Gas Turbine 5.........  86.7......................             0.1
                                          (J) Gas Turbine 6.........  Burn natural gas..........  ..............
                                          (K) Emergency Generator...  500 hour calendar year      ..............
                                                                       operating limit.
----------------------------------------------------------------------------------------------------------------
                            Vigo County sulfur dioxide limitations (326 IAC 7-4-3.1)
----------------------------------------------------------------------------------------------------------------
Wabash River Combined Cycle Source ID     Combustion Turbine Unit 1A  333.76....................           0.195
 No. 00147.
sgSolutions Source ID No. 00091.........  (A) Tail Gas Incinerator    230.6 *...................  ..............
                                           Stack EP1.
                                          (B) Process Flare Unit 2..  500 hour calendar year      ..............
                                                                       operating limit on coal/
                                                                       syngas.
SONY Digital Audio Disc Source ID No.     (A) #1 Kewanee Boiler.....  ..........................            0.05
 00032.                                   (B) #2 Kewanee Boiler.....  ..........................            0.05
                                          (C) Unit 3 Burnham Boiler.  ..........................            0.05
                                          (D) Unit 4 Burnham Boiler.  ..........................            0.05
                                          (E) Unit 5 Superior Boiler  ..........................            0.05
                                          (F) Unit 6 Superior Boiler  ..........................            0.05
                                          (G) Unit 18 Boiler........  ..........................            0.05
Taghleef Industries Source ID No. 00045.  (A) Clayton Boiler          0.03......................          0.0015
                                           (Standby).                 0.05......................          0.0015
                                          (B) Nebraska Boiler.......
                                          (C) Nebraska-D Boiler.....  Burn natural gas..........  ..............
Terre Haute Regional Hospital Source ID   (A) #1 Boiler.............  ..........................            0.45
 No. 00046.                               (B) New #2 Boiler.........  ..........................            0.45
Union Hospital Source ID No. 00047......  2 Keeler Boilers..........  ..........................            0.36
Duke Energy--Wabash River Generating      (A) Boiler 6..............  1,499.5...................             0.5
 Station Source ID No. 00021.             (B) Diesel Generators 7A,   500 hour calendar year                0.05
                                           7B, and 7C.                 operating limit (each).
----------------------------------------------------------------------------------------------------------------
                             Pike County sulfur dioxide limitations (326 IAC 7-4-15)
----------------------------------------------------------------------------------------------------------------
Hoosier Energy--Ratts Source ID No.       (A) Boiler 1..............  58........................            0.05
 00001.                                   (B) Boiler 2..............  58........................            0.05
                                          (C) No. 2 Auxiliary Boiler  1.0.......................            0.05
Indianapolis Power & Light--Petersburg    (A) Unit 1................  263.0 *...................          0.12 *
 Generating Station Source ID No. 00002.  (B) Unit 2................  495.4 *...................          0.12 *
                                          (C) Unit 3................  1,633.7 *.................          0.29 *
                                          (D) Unit 4................  1,548.2 *.................          0.28 *
                                          (E) Diesel Generators PB-   500 hour calendar year      ..............
                                           2, PB-3, and PB-4.          operating limit (each).
Indianapolis Power & Light--Petersburg    (A) Unit 1................  330.0.....................            0.15
 Generating Station Source ID No. 00002.  (B) Unit 2................  621.6.....................            0.15
                                          (C) Unit 3................  2,049.8...................            0.37
                                          (D) Unit 4................  1,942.5...................            0.35
                                          (E) Diesel Generators PB-   500 hour calendar year      ..............
                                           2, PB-3, and PB-4.          operating limit (each).
----------------------------------------------------------------------------------------------------------------
* Indicates emission limit for the unit is expressed as a 30-day average limit.

2. Longer Term Average Limits
    As noted above, the 2014 SO2 Guidance discusses the
option to establish limits with averaging times up to 30 days in length
that are comparably stringent to the 1-hour average limit that would
otherwise have been set, and recommends a detailed procedure for
determining such a comparably stringent limit. The Guidance also notes
that it might be appropriate to establish supplemental limits in order
to limit the

magnitude and/or frequency of elevated emissions, as a means of further
reducing the likelihood of elevated emissions occurring on those
occasions when the meteorology is conducive to high concentrations of
SO2.
    For both IPL-Petersburg and sgSolutions, Indiana closely followed
the six-step recommendation of the 2014 SO2 Guidance in
determining an appropriate level for the 30-day average limits. As a
first step in each case, Indiana conducted modeling which determined
the 1-hour emission limit that would provide for attainment. Indiana
conducted a series of modeling runs identifying baseline allowable air
quality (in absence of emission reductions), evaluating the air quality
consequences of feasible emission reductions, and ultimately
identifying a set of reduced allowable emission levels that would
provide for attainment. For IPL-Petersburg, these quantities were
expressed in lbs/MMBTU, and may be termed the critical emissions rates.
The critical emission rates were 0.15, 0.15, 0.37, and 0.35 lbs/MMBTU,
for IPL-Petersburg Units 1-4 respectively. For sgSolutions, Indiana
determined a critical emission level of 527 lbs/hour.
    For the second step of the process, for IPL-Petersburg, Indiana
compiled representative emissions data sets from the IPL-Petersburg
Unit 2 Flue Gas Desulfurization stack, which is the same control
technology IPL-Petersburg will use for Units 1,3, and 4 in order to
meet the emission limits associated with attaining the 2010
SO2 NAAQS. Indiana used data compiled from 2006-2010 for the
stack. For sgSolutions, Indiana used the data from the Tail Gas
Incinerator from 2009-2014 scaled to fewer operating hours to create
the emissions data set.
    The third step was calculating the 30-day rolling averages. The
analysis for IPL-Petersburg assessed the variability of the emission
rate. The 30-day average rate was calculated by summing the pounds
SO2 per hour values over the previous 720 hours (30 days)
and dividing by the sum of the MMBTU per hour over the past 720 hours,
yielding a separate 30-day average pounds of SO2 per MMBTU
for each successive ending hour. Using this calculation ensured that
any hours showing zero emissions did not affect the calculations. This
calculation is consistent with the procedures used in determining
compliance with the Mercury and Air Toxics Standard (MATS) rule, as
recommended in appendix C of the 2014 EPA SO2 Guidance. The
analysis for sgSolutions used statistics on the hourly mass emission
rate and the corresponding 720-hour average hourly emission rate.
    The fourth step determined 99th percentile values for the 1-hour
values and 30-day average values. The 1-hour values were determined by
compiling the values in step 2 over the five-year period. The result
for the 99th percentile 30-day average was determined from the
calculations in step 3. For IPL-Petersburg, the 99th percentile of 1-
hour values was 0.233 lbs/MMBTU, and the 99th percentile of 30-day
average values was 0.185 lbs/MMBTU. For sgSolutions, the 99th
percentile values were 139 and 60.7 lbs/hour among 1-hour and 30-day
average values, respectively. In the fifth step the ratio of the values
was calculated by dividing the 99th percentile values for the 30-day
rolling data and the 1-hour data identified in the fourth step. For
IPL-Petersburg the result was an adjustment factor of 79.7 percent, and
for sgSolutions the result was an adjustment factor of 43.6 percent.
The final step multiplied the modeled critical emissions values
calculated in the first step by the adjustment factors calculated in
the fifth step. This resulted in 30-day average limits of 0.12, 0.12,
0.29, and 0.35 lbs/MMBTU for IPL-Petersburg Units 1-4 respectively and
230.6 lbs/hr for sgSolutions.
    Based on a review of the state's submittal, these limits provide a
reasonable alternative to establishing a per hour 1-hour average
emission limit for this source. The state used an appropriate database
and then applied an appropriate adjustment, yielding an emission limit
that has comparable stringency to the 1-hour average limit that the
state determined would otherwise have been necessary to provide for
attainment. While the 30-day average limit allows for occasions in
which emissions are higher than the level that would be allowed under
the 1-hour limit, the state's limit compensates by requiring average
emissions to be lower than the level that would otherwise have been
required by a 1-hour average limit.
    As noted above, the April 2014 Guidance recommends that 30-day
average limits be accompanied by supplemental limits that help serve to
minimize the frequency and/or magnitude of occasions with elevated
emissions. Indiana did not use supplemental limits. Therefore, EPA
examined available emissions data at IPL-Petersburg and at sgSolutions
to evaluate the likely frequency and magnitude of spikes in emissions
above the critical emission value while nevertheless complying with the
30-day average limit. The most pertinent data for IPL-Petersburg are
for Unit 2, addressing a five-year time period before the relevant
limit became effective. Approximately seven percent of available 30-day
average values in this data set exceeded the 30-day average limit of
0.12 lbs/MMBTU. In this data set, approximately six percent of the
hourly emissions values exceeded the critical emission rate of 0.15
lbs/MMBTU; these elevated values on average were approximately 34
percent above 0.15 lbs/MMBTU. Reduction of emissions sufficient to meet
the 0.12 lbs/MMBTU limit consistently would reduce the frequency and
magnitude of hourly emissions values above the 0.15 lbs/MMBTU critical
emissions rate, although the precise levels are difficult to predict.
For sgSolutions, over a six-year period, in a data set with no
exceedances of the 30-day average limit of 230.6 lbs/hour (in which, in
fact, only one day had daily average emissions above 230.6 lbs/hour),
only seven hours (approximately 0.02 percent of the hours) exceeded the
critical emission value of 527 lbs/hour, and the magnitude of these
exceedances on average was only nine percent above the critical
emission value. Based on these data, EPA finds that the 30-day average
limit without supplemental limits should suffice in these cases to
provide adequate assurance of attainment.
    For IPL-Petersburg, Indiana's rule identifies both a set of 30-day
average limits and a corresponding set of 1-hour limits (the latter set
at the critical emission value) for the four units of this facility.
Indiana's rule specifies, ``Indianapolis Power & Light shall notify the
department prior to [January 1, 2017] to indicate if compliance . . .
will be determined using [the specified 1-hour limits or the specified
30-day average limits] and prior to switching [which set of limits
applies].'' Given this potential under Indiana's rules for IPL to
choose to switch back and forth between a set of 30-day average limits
and a set of 1-hour limits, EPA conducted additional review of the
enforceability of the limits and of whether the potential to switch
limits might adversely affect the degree to which these limits assure
attainment.
    Regarding enforceability, the primary question is whether at any
time the applicable requirements are unequivocally clear, such that the
occurrence of emissions above the specified level unquestionably
constitutes noncompliance. Since the limits themselves are clearly
specified in Indiana's rule, the pertinent question is whether the
choice of limits is clear, i.e. whether it is always clear whether the
30-day average limits or the 1-hour

limits apply. As noted above, Indiana's rule requires IPL-Petersburg to
notify the state of its initial choice of applicable limits and to
notify the state of any choice IPL makes to switch applicable limits.
Thus, pursuant to the requirements of the rule, the applicable set of
limits is always specified, Indiana always knows which set of limits
applies, and this information is available to EPA and any other
interested party upon request to Indiana.
    EPA also evaluated whether the option to switch applicable limits
might yield less air quality protection than permanently imposing 30-
day average limits or permanently imposing 1-hour limits. At any given
time, IPL is subject to a single set of limits; IPL cannot excuse
noncompliance with the applicable limits even if it is meeting the
alternative limits. Therefore, IPL does not have the option to choose
limits contemporaneously according to a short-term judgment as to which
set of limits is less stringent for that time period. Instead, IPL must
design its control strategy to meet the limits with the chosen
averaging time rather than to aim simply to meet whichever set of
limits might be less stringent for any particular period.
    A further question about switching limits is whether applying 1-
hour limits for part of a year and longer-term limits for another part
of the year provides as much air quality protection as applying a
single set of limits for the entire year. Use of long term average
limits creates the potential for periods with elevated emissions that
may yield additional, unmodeled exceedances (i.e., exceedances beyond
those identified in modeling of constant emissions), but also creates a
compensating likelihood of avoiding some of the modeled exceedances
because the downward adjusted long-term average limit requires
emissions to be lower most of the time. At issue here is the risk that
in a year when both types of limits apply, the periods subject to 30-
day average limits might have additional, unmodeled exceedances while
the periods subject to 1-hour limits might not avoid any of the
exceedances found in constant emissions modeling.
    For several reasons, EPA believes that this concern does not apply
in this case. Indiana's rule requires IPL to notify Indiana before any
change in limits and, in the case of a switch from 30-day average
limits to one-hour limits, to complete a 30-day period in compliance
with the 30-day average limits before the one-hour limits take effect.
IPL cannot change the applicable limits retroactively. While IPL may
change the prospective applicable set of limits if it anticipates
significant changes in operations, the experience to date is that IPL
has made no switches in the selection since electing the 30-day average
in January 2017, and nothing in the record suggests that IPL is likely
to switch which limits apply in the future. For these reasons, EPA
believes that Indiana's limits for IPL are an appropriate part of an
attainment plan for Southwest Indiana that provide for attainment, most
likely by requiring compliance with an appropriately adjusted set of
30-day average limits.
    The issue of switching limits does not apply to sgSolutions; this
source is permanently subject to a 30-day average limit. EPA believes
that the 30-day average limits for IPL-Petersburg and sgSolutions are
appropriate elements of Indiana's attainment plans for the applicable
areas.

E. Background Concentrations

    Indiana determined background concentrations by selecting the 99th
percentile of a monitoring data set that excluded values from emission
sources where the upwind SO2 concentration exceeded 10 ppb.
For Indianapolis, the background concentration was generated using the
hourly concentrations from the Harding Street monitor (18-097-0057). At
the time Indiana conducted its analysis this was the only suitable
background monitor. The monitor is sited about four kilometers
northeast of the Indianapolis Power and Light-Harding Station source.
For the determination of a background value Harding Station Power Plant
was considered a nearby source and was expressly included in the
modeling analysis, and so Indiana determined the Indianapolis
background concentration from a Harding Street data set that excluded
values during hours with winds from the south and southwest. The
resulting background concentration was 22.5 micrograms per cubic meter
([micro]g/m\3\) (8.6 ppb).
    In the Southwest Indiana area there are two monitors, one located
in each of Pike and Daviess counties. The monitor with the highest
background concentration is the Arda Lane monitor located in Pike
County (18-125-0005) with a value of 25.9 [micro]g/m\3\ (9.9 ppb). The
monitor is sited about 1 kilometer to the south of IPL-Petersburg
source and about 1.5 kilometers east of the Hoosier Plant. Indiana
considered these two sources nearby, and determined a background
concentration from a data set that excluded data when winds were from
the northwest. There are two monitors located in the Terre Haute
nonattainment area, both in Vigo County.
    For the Vigo County analysis, the controlling monitor (i.e.,
highest design value over the 2011-2013 period), Harrison Road monitor
(18-125-0005) was used. The monitor is sited approximately 2.5
kilometers southeast of the Duke Energy-Wabash River facility, which
Indiana considered nearby, so Indiana determined background
concentrations from a data set that excluded data when winds were from
the northwest. The result was a background concentration of 23.0
[micro]g/m\3\ (8.8 ppb). EPA has reviewed these background
concentrations and finds these values appropriate as model inputs.

F. Comments Made During State Rulemaking

    During the preparation of its nonattainment plans, Indiana received
and responded to a number of comments by, among others, EPA and the
Sierra Club that EPA believes warrant further discussion in this
action.
    The first comment from EPA to Indiana pertained to the IPL-
Petersburg facility having a choice between hourly and 30-day average
limits in the Pike county emission limit rules, and requesting that
Indiana assure clarity as to which limits apply, by including explicit
requirements for reporting and recordkeeping to which limits apply.
    Indiana responded to the comment by adding language at 326 IAC 7-4-
15(e) requiring the source to notify IDEM when switching from one set
of limits to the other. For any switch from the 1-hour limits to the
30-day average limits, IDEM's final rule requires compliance with the
1-hour limit until the first 30-day average emission rate is calculated
so that there is no gap in compliance. EPA agrees that this change in
the rulemaking ensures clear compliance requirements and establishes
the 30-day average limit (when applicable) in a manner (consistently
requiring a reduced level of emissions) that provides the full
protection against violations recommended in EPA's guidance.
    Sierra Club expressed concerns about the Duke Energy facility in
Gibson County (``Gibson''), commenting that Indiana should have modeled
Gibson explicitly. Indiana responded that emissions reductions from the
sources located within Pike and Daviess County nonattainment area were
the most responsible for bringing the area into attainment. Other
SO2 sources in surrounding counties are accounted for within
the representative 1-hour SO2

background concentration. EPA notes that the criterion recommended in
appendix W of 40 CFR 51 for sources to be modeled explicitly are those
nearby sources that are not adequately represented by ambient
monitoring data, such as sources that cause a significant concentration
gradient in the vicinity of the area of interest. Gibson is about 46
kilometers southwest of the Southwest Indiana nonattainment area. At
this distance, concentration gradients may be presumed to be quite
small, and the impacts of Gibson may reasonably be considered accounted
for in the background concentration for the Southwest Indiana
nonattainment area. Thus, EPA agrees with Indiana's conclusion that any
impact from Gibson on the Southwest Indiana nonattainment area is
appropriately captured in the background concentration for the
Southwest Indiana nonattainment area, such that explicit modeling of
this facility is unnecessary.
    In a related comment, Sierra Club commented that Indiana needed to
impose SO2 limits on the Duke Energy facility in order to
ensure that the Southwest Indiana nonattainment area (Daviess and Pike
counties) attained the standard. Indiana's attainment demonstration for
the Southwest Indiana nonattainment area did not depend on emission
limits for Gibson. Appendix W specifies the recommended consideration
of emission limits for sources that are required to be explicitly
modeled in the attainment demonstration. Sources such as Gibson that
are accounted for as part of the monitored background concentration
need not be modeled explicitly (as noted above) and in particular need
not be considered on the basis of allowable emissions. That is,
Appendix W advises consideration of distant sources such as Gibson on
the basis of available monitoring data, irrespective of any limits on
Gibson emissions that may apply. Indiana's modeling analysis, in
accordance with appendix W, demonstrates that the Southwest Indiana
nonattainment area can be expected to attain the standard without
regard to whether emission limits for Gibson are established. Thus,
Indiana's SIP submission is approvable without limits for Gibson.
    Also, several utility groups commented that Indiana should use a
compliance date of October 1, 2017, which would allow for twelve months
of data to demonstrate attainment of the standard prior to the October
2018 attainment deadline. Indiana chose instead to adopt its proposed
compliance date of January 1, 2017. This compliance date was
recommended in the 2014 EPA Guidance because monitoring site data are
certified annually on a calendar year, not a 12-month time span, so
compliance by January 1, 2017 is recommended to provide for a calendar
year of data for later informing whether timely attainment has
occurred. EPA supports the decision made by Indiana to require
compliance with the new limits by January 1, 2017.

G. Summary of Results

    The final dispersion modeling results submitted by Indiana show
design values, as provided in Table 2 below, that are less than 75 ppb.
Therefore, Indiana's modeling analysis demonstrates attainment of the
2010 SO2 NAAQS for the Indianapolis, Southwest Indiana, and
Terre Haute areas. EPA believes that Indiana's modeling appropriately
reflects allowable emissions in these areas, including, for sources
subject to 30-day average limits, the 1-hour emission rates that upon
appropriate adjustment correspond to the 30-day average limits that
Indiana has adopted. EPA has reviewed Indiana's attainment
demonstrations, agrees with Indiana's submitted results, and proposes
to determine that the enforceable measures in Indiana's plans provide
for attainment of the 2010 primary SO2 NAAQS in the
Indianapolis, Southwest Indiana, and Terre Haute nonattainment areas.

                                 Table 2--1-Hour SO2 Dispersion Modeling Results
----------------------------------------------------------------------------------------------------------------
                                                                                Southwest
                            Area name                              Indianapolis       Indiana       Terre Haute
----------------------------------------------------------------------------------------------------------------
Modeled Concentration (ppb).....................................            64.4            64.9            63.8
Background Concentration (ppb)..................................             8.6             9.9             8.8
                                                                 -----------------------------------------------
    Total Concentration (ppb)...................................              73            74.8            72.6
----------------------------------------------------------------------------------------------------------------

V. Review of Other Plan Requirements

A. Emissions Inventory

    The emissions inventory and source emission rate data for an area
serve as the foundation for air quality modeling and other analyses
that enable states to: (1) Estimate the degree to which different
sources within a nonattainment area contribute to violations within the
affected area; and (2) assess the expected improvement in air quality
within the nonattainment area due to the adoption and implementation of
control measures. As noted above, the state must develop and submit to
EPA a comprehensive, accurate and current inventory of actual emissions
from all sources of SO2 emissions in each nonattainment
area, as well as any sources located outside the nonattainment area
which may affect attainment in the area. See CAA section 172(c)(3).
    Indiana provided a comprehensive, accurate, and current inventory
of SO2 emissions for Marion (Indianapolis), Daviess and Pike
(Southwest Indiana), and Vigo counties (Terre Haute). The following
source categories were included: Electric-generating units (EGUs), non-
EGUs (point), non-point (area), non-road, and on-road sources of
SO2 and are summarized in Table 3. Indiana uploads point
source emissions to the National Emissions Inventory (NEI) annually.
For the 2011 base year inventory, emissions from EGU and non-EGUs are
actual reported emissions. Data for airport, area, non-road, and on-
road emissions were compiled from the EPA Emissions Modeling
Clearinghouse (SO2 NAAQS Emissions Modeling platform 2007/
2007v5) for the 2008 NEI and the 2018 projected inventory year. Data
were interpolated between 2008 and 2014 to determine the airport, area,
non-road, and on-road emissions 2011 inventory and between 2014-2020
for 2018. As noted above, these inventories addressed sources within
each nonattainment county and can be found in appendix H of the
submitted attainment demonstration. Indiana also provided modeling
inputs that include a listing of the individual sources with sufficient
proximity to and impact on the nonattainment areas to warrant being
explicitly included in the modeling analysis.

                                    Table 3--2011 Actual Emissions Inventory
----------------------------------------------------------------------------------------------------------------
                                                   Marion          Daviess
                                               (Indianapolis)     (southwest    Pike (southwest    Vigo (Haute
                                                   (tpy)        Indiana) (tpy)   Indiana) (tpy)    Terre) (tpy)
----------------------------------------------------------------------------------------------------------------
EGU.........................................        18,998.02                0        34,728.99        55,782.42
Point.......................................         4,582.46             8.39             2.74           102.79
Area........................................           193.21            55.63            13.60            32.51
Non-road....................................           125.37             1.23             1.38             9.42
On-road.....................................           121.88             3.14             1.85            13.72
----------------------------------------------------------------------------------------------------------------

    By providing a comprehensive, accurate, and current inventory of
SO2 emissions for Marion, Pike, Daviess, and Vigo counties,
Indiana has met the emission inventory requirement of CAA section
172(c)(3) for the Indianapolis, Southwest Indiana, and Terre Haute
areas. This inventory represents emissions in 2011, a time when the
areas were violating the standard. While section 172(c)(3) does not
have a formal requirement for an attainment year inventory, the state
did include allowable attainment year emissions in its modeling
analysis.

B. RACM/RACT

    In its submission, Indiana discusses its rationale for concluding
that the nonattainment plans meet the RACM/RACT requirements in
accordance with EPA guidance. For most criteria pollutants, RACT is
control technology as needed to meet the NAAQS that is reasonably
available considering technological and economic feasibility. However,
Indiana cites EPA guidance that the definition of RACT for
SO2 is, simply, ``that control technology which is necessary
to achieve the NAAQS (40 CFR 51.1 00(o))''. Indiana in fact requires
the control technology that modeling shows to be necessary to ensure
attainment of the SO2 NAAQS by the applicable attainment
date.
    Additionally, the Indiana submission includes limits for the
individual units in the nonattainment areas. The limits are established
in the attainment demonstration, and made permanent and enforceable in
SIP rule 326 IAC 7, Sulfur Dioxide Rules.
    Indiana has determined that these measures suffice to provide for
timely attainment. EPA concurs and proposes to conclude that the state
has satisfied the requirements in sections 172(c)(1) and (6) to adopt
and submit all RACT/RACM and emission limitations and control measures
as needed to attain the standards as expeditiously as practicable.

C. New Source Review (NSR)

    EPA approved Indiana's nonattainment new source review rules on
October 7, 1994 (94 FR 24838). These rules provide for appropriate new
source review for SO2 sources undergoing construction or
major modification in the Indianapolis, Southwest Indiana, and Terre
Haute without need for modification of the approved rules. Therefore,
EPA concludes that this requirement has already been met for these
areas.

D. RFP

    Indiana's adopted rules in 326 IAC 7 require that control measures
be implemented no later than January 1, 2017. Indiana has concluded
that this plan requires that affected sources implement appropriate
control measures as expeditiously as practicable in order to ensure
attainment of the standard by the applicable attainment date. Indiana
concludes that this plan therefore provides for RFP in accordance with
the approach to RFP described in EPA's guidance. EPA concurs and
proposes to conclude that the plan provides for RFP.

E. Contingency Measures

    In its November 15, 2017 clarification memo, Indiana explained its
rationale for concluding that the plans met the requirement for
contingency measures in accordance with EPA guidance. Specifically,
Indiana relies on EPA's guidance, noting the special circumstances that
apply to SO2 (as discussed above), and explaining on that
basis why the contingency requirement in CAA section 172(c)(9) is met
for SO2 by having a comprehensive program to identify
sources of violations of the SO2 NAAQS and to undertake an
aggressive follow-up for compliance and enforcement of applicable
emissions limitations. Indiana stated that it has such an enforcement
program as codified in Indiana Code Title 13, Articles 14 and 15,
identifying violators and taking prompt, appropriate enforcement
action. On this basis, EPA concludes that Indiana's nonattainment plans
satisfy contingency measure requirements for the Indianapolis,
Southwest Indiana, and Terre Haute nonattainment areas.
    Indiana's rules also provide for additional contingency measures as
necessary, following a review of any air quality problems that become
identified and following a review of options for mitigating the
problems that arise. However, Indiana is not relying on these
provisions to satisfy the requirements for contingency measures.

VI. EPA's Proposed Action

    EPA is proposing to approve Indiana's SIP submission, which the
state submitted to EPA on October 2, 2015, for attaining the 2010 1-
hour SO2 NAAQS for the Indianapolis, Southwest Indiana, and
Terre Haute areas.
    These SO2 nonattainment plans include Indiana's
attainment demonstration for the Indianapolis, Southwest Indiana, and
Terre Haute SO2 nonattainment areas. These nonattainment
plans also address requirements for emission inventories, RACT/RACM,
RFP, and contingency measures. Indiana has previously addressed
requirements regarding nonattainment area NSR. EPA has determined that
Indiana's SO2 nonattainment plans for Indianapolis,
Southwest Indiana, and Terre Haute meet the applicable requirements of
CAA sections 110, 172, 191, and 192. EPA is taking no action at this
time on Indiana's submittal with respect to Morgan County.
    EPA is taking public comments for thirty days following the
publication of this proposed action in the Federal Register. We will
take all comments into consideration in our final action.

VII. Incorporation by Reference

    In this rule, EPA is proposing to include in a final EPA rule
regulatory text that includes incorporation by reference. In accordance
with requirements of 1 CFR 51.5, EPA is proposing to incorporate by
reference Indiana Administrative Code, Title 326, Article 7,
``Compliance date'' (326 IAC 7-1.1-3), ``Reporting requirements;
methods to determine compliance'' (7-2-1), ``Marion County sulfur
dioxide emission limitations'' (7-4-2.1), ``Vigo County sulfur dioxide
emission

limitations'' (7-4-3.1), and ``Pike County sulfur dioxide emission
limitations'' (7-4-15), effective January 1, 2107. EPA has made, and
will continue to make, these documents generally available through
www.regulations.gov, and at the EPA Region 5 Office. (Please contact
the person identified in the For Further Information Contact section of
this preamble for more information.)

VIII. Statutory and Executive Order Reviews

    Under the CAA, the Administrator is required to approve a SIP
submission that complies with the provisions of the Act and applicable
Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in
reviewing SIP submissions, EPA's role is to approve state choices,
provided that they meet the criteria of the CAA. Accordingly, this
proposed action merely approves state law as meeting Federal
requirements and does not impose additional requirements beyond those
imposed by state law. For that reason, this proposed action:
     Is not a ``significant regulatory action'' subject to
review by the Office of Management and Budget under Executive Order
12866 58 FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21,
2011);
     Does not impose an information collection burden under the
provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
     Is certified as not having a significant economic impact
on a substantial number of small entities under the Regulatory
Flexibility Act (5 U.S.C. 601 et seq.);
     Does not contain any unfunded mandate or significantly or
uniquely affect small governments, as described in the Unfunded
Mandates Reform Act of 1995 (Pub. L. 104-4);
     Does not have Federalism implications as specified in
Executive Order 13132 (64 FR 43255, August 10, 1999);
     Is not an economically significant regulatory action based
on health or safety risks subject to Executive Order 13045 (62 FR
19885, April 23, 1997);
     Is not a significant regulatory action subject to
Executive Order 13211 (66 FR 28355, May 22, 2001);
     Is not subject to requirements of Section 12(d) of the
National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272
note) because application of those requirements would be inconsistent
with the CAA; and
     Does not provide EPA with the discretionary authority to
address, as appropriate, disproportionate human health or environmental
effects, using practicable and legally permissible methods, under
Executive Order 12898 (59 FR 7629, February 16, 1994).
    In addition, the SIP is not approved to apply on any Indian
reservation land or in any other area where EPA or an Indian tribe has
demonstrated that a tribe has jurisdiction. In those areas of Indian
country, the rule does not have tribal implications and will not impose
substantial direct costs on tribal governments or preempt tribal law as
specified by Executive Order 13175 (65 FR 67249, November 9, 2000).

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by
reference, Intergovernmental relations, Reporting and recordkeeping
requirements, Sulfur oxides.

    Dated: August 2, 2018.
Cathy Stepp,
Regional Administrator, Region 5.
[FR Doc. 2018-17582 Filed 8-14-18; 8:45 am]
 BILLING CODE 6560-50-P