[Federal Register Volume 83, Number 158 (Wednesday, August 15, 2018)]
[Proposed Rules]
[Pages 40487-40498]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[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, (312) 886-3901, 
[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

[[Page 40488]]

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

[[Page 40489]]

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.

[[Page 40490]]

    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 2.6.1.2 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.

[[Page 40491]]

    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,

[[Page 40492]]

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

[[Page 40493]]

 
                                          (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

[[Page 40494]]

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

[[Page 40495]]

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

[[Page 40496]]

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.

[[Page 40497]]



                                    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

[[Page 40498]]

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