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Federal Register / Vol. 86, No. 122 / Tuesday, June 29, 2021 / Proposed Rules
anticipates that the SO2 NAAQS will continue to be attained. See Table 1 for recent monitoring data trends at this monitor.
Additionally, the State entered into a Consent Agreement with Ameren Missouri Ameren, included as
Appendix B to the maintenance plan submission, limiting the SO2 emissions from three Ameren facilities. One facility, Ameren-Rush Island Energy Center Rush Island, is located within the nonattainment area boundary. The
other two facilities, Ameren Meramec Energy Center Meramec and Ameren Labadie Energy Center Labadie are located outside of the nonattainment area boundary. The Consent Agreement emission limits are provided in Table 2.

TABLE 2AMEREN/MISSOURI CONSENT AGREEMENT SO2 EMISSION LIMITS
Source
Emission limit per source pounds per hour
Averaging time
40,837
7,371
13,600

24-hr block average.
24-hr block average.
24-hr block average.

Labadie
Meramec
Rush Island

Because it is located inside of the Jefferson County SO2 nonattainment area, the State modeled Rush Island at a constant emission rate of 14,600 lbs SO2/hr for every hour of the year in all five years 20132017 of the modeling analysis. This modeled emission rate corresponds to the facilitys enforceable 24-hour block average limit for hourly SO2 emissions of 13,600 lbs SO2/hr when accounting for variability. The State modeled Meramec and Labadie as nearby sources in accordance with the code of federal regulations CFR 40 CFR
part 51, appendix W, Guideline on Air Quality Models. That is, the State modeled Meramec and Labadies permitted/allowable emission rate from the Consent Agreement with actual temporally varying heat input levels.
Please see the TSD for details of the modeling inputs and additional discussion of the air quality modeling.
The modeling results demonstrate attainment and project continued maintenance of the NAAQS, and the TSD also contains discussion of the EPAs review of the modeling.
Therefore, the EPA is proposing to find that the air quality improvement in the Jefferson County SO2 nonattainment area is due to permanent and enforceable reductions in emissions.

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Criterion 4The Jefferson County SO2
Nonattainment Area Has a Fully Approved Maintenance Plan Pursuant to Section 175A of the CAA
To redesignate a nonattainment area to attainment, the CAA requires the EPA
to determine that the area has a fully approved maintenance plan pursuant to section 175A of the CAA CAA section 107d3Eiv. In conjunction with its request to redesignate the Jefferson County SO2 nonattainment area to attainment for the 2010 1-hour SO2
NAAQS, the State submitted a SIP
revision to provide for the maintenance of the 2010 1-hour SO2 NAAQS for at
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least 10 years after the effective date of redesignation to attainment. The EPA is proposing to find that this maintenance plan for the area meets the requirements for approval under section 175A of the CAA.
a. What is required in a maintenance plan?
CAA section 175A sets forth the elements of a maintenance plan for areas seeking redesignation from nonattainment to attainment. Under section 175A, the plan must demonstrate continued attainment of the applicable NAAQS for at least 10
years after the Administrator approves a redesignation request to attainment.
Eight years after the redesignation, the State must submit a revised maintenance plan demonstrating that attainment will continue to be maintained for the 10 years following the initial 10-year period. To address the possibility of future NAAQS
violations, the maintenance plan must contain contingency measures as the EPA deems necessary to assure prompt correction of any future 2010 1-hour SO2 violations. The Calcagni Memo provides further guidance on the content of a maintenance plan, explaining that a maintenance plan should address five requirements: The attainment emissions inventory, maintenance demonstration, monitoring, verification of continued attainment, and a contingency plan. As is discussed more fully later in this section, the EPA is proposing to determine that Missouris maintenance plan includes all the necessary components and is thus proposing to approve it as a revision to the Missouri SIP.
b. Attainment Emissions Inventory As part of a States maintenance plan, the air agency should develop an attainment inventory to identify the level of emissions in the affected area
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which is enough to attain and maintain the SO2 NAAQS.28 The EPA is proposing to approve that Missouri has met this requirement through modeling of permanent and enforceable emissions limits that will result in continued attainment and maintenance of the NAAQS. Missouri also provided emissions inventories as part of the maintenance plan. Specifically, Missouri selected 2014 as the attainment emissions inventory year for developing an emissions inventory for SO2 in the nonattainment area through 2030. Please see the TSD included in the docket for this action for details of the base year, attainment year and future year emissions inventories and the EPAs review of these inventories.
The TSD also details the EPAs review of the modeling demonstration provided by Missouri which forms the basis for the EPAs approval of this maintenance plan requirement.
c. Maintenance Demonstration The Calcagni memo describes two ways for a State to demonstrate maintenance of the NAAQS for a period of at least 10 years following the redesignation of the area: 1 The State can show that future emissions of a pollutant will not exceed the level of the attainment inventory, or 2 the State can model to show that the future mix of sources and emission rates will not cause a violation of the standard. The memo goes on to say that areas that are required to model to demonstrate attainment of the standard should complete the same level of modeling to demonstrate that the permanent and enforceable emissions are enough to maintain the standard. The State performed several modeling iterations to demonstrate that the standard will be maintained. In its February 7, 2019, and February 25, 2019, supplemental 28 See
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2014 SO2 Guidance, at 66.

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