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Federal Register / Vol. 86, No. 122 / Tuesday, June 29, 2021 / Proposed Rules
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compiled annually.39 In addition, in compliance with the EPAs Air Emissions Reporting Requirements 80
FR 8787, Missouri develops a comprehensive emissions inventory of point, area, and mobile sources every 3
years. This triennial inventory compiled by the State is contained in the EPAs national emissions inventory NEI
which is made publicly available every 3 years. For these reasons, the EPA is proposing to find that Missouris maintenance plan meets the Verification of Continued Attainment requirement.
f. Contingency Measures in the Maintenance Plan Section 175A of the CAA requires that a maintenance plan include such contingency measures as the EPA deems necessary to assure that the State will promptly correct a violation of the NAAQS that occurs after redesignation.
The maintenance plan should identify the contingency measures to be adopted, a schedule and procedure for adoption and implementation, and a time limit for action by the State. A State should also identify specific indicators to be used to determine when the contingency measures need to be implemented. The maintenance plan must also include a requirement that a State will implement all measures with respect to control of the pollutant that were contained in the SIP before redesignation of the area to attainment in accordance with section 175Ad.
The contingency plan includes a triggering mechanism to determine when contingency measures are needed and a process of developing and implementing appropriate control measures. The triggering mechanisms contained in the maintenance plan and Consent Agreement are based on monitoring data from the Mott Street monitor and the industrial source monitors around the Ameren Rush Island facility. The EPA finds it appropriate to rely on monitoring data to trigger the contingency plan because the Mott Street monitor is being relied upon to demonstrate continued maintenance in the area as discussed in the Monitoring Network section of this document. Additionally, the industrial source monitors were sited consistent with relevant EPA guidance to capture maximum impacts from the Rush Island plant.40 Because the Rush Island plant 39 This information is available to the EPA or members of the public upon request from the State of Missouri.
40 See Missouris 2015 and 2016 annual monitoring network plans contained in the docket for this action for more information about the siting of the monitors around Rush Island.

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is the largest remaining source in the maintenance area, the EPA agrees that monitoring around the Rush Island plant would be the best indicator of any potential future air quality issues in the maintenance area and thereby represents a reasonable triggering mechanism for the States contingency plan.
The State listed two types of triggers of its contingency plan. The first, a warning level response, will be triggered by a 99th percentile of daily maximum 1-hour average SO2
concentrations greater than 79 ppb in a single calendar year in the Jefferson County maintenance area. The second, an action level response, will be triggered if a violation of the NAAQS is recorded in the Jefferson County maintenance area, specifically if the 3year average of annual 99th percentile daily maximum 1-hour concentrations is 76 ppb or higher.
If the warning level response is triggered, a study must be completed to determine whether the monitored SO2
value indicates a trend toward higher concentrations in the Jefferson County maintenance area. The study will evaluate whether the trend, if any, is likely to continue. The study shall be completed as expeditiously as possible, but no later than 24 months after the State has determined that a warning level response has been triggered. It should be noted that the EPA does not require a State to implement contingency measures when occasional exceedances are recorded.
If the action level response is triggered and is not found to be due to an exceptional event as defined at 40
CFR part 50.1j, measures to address the violation shall be implemented as expeditiously as possible, but no later than 24 months after quality assured ambient data that has been entered into the AQS database indicating that this trigger has occurred. If a new measure or control is already promulgated and scheduled to be implemented at the federal or State level, and that measure or control is determined to be enough to address the upward trend in ambient SO2 concentrations within the maintenance area, additional local measures may be unnecessary.
Furthermore, Missouri will submit to the EPA an analysis demonstrating the proposed action level response measures are adequate to return the area to attainment. Contingency measures considered will be based on an analysis of the cause of the elevated ambient SO2
concentrations from the entityies likely to be contributing to the elevated concentrations. Measures may include improvements to existing control
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devices, addition of secondary control devices or improvements in housekeeping and maintenance, among other measures. It is not possible to develop a comprehensive list of contingency measures that can address all possible violations until the cause of the elevated concentrations is known.
Any contingency measures implemented will require a compliance plan and expeditious compliance timeline from the entityies involved.
The EPA is proposing to find that Missouris maintenance plan meets the Contingency Measures requirement.
In addition to the contingency plan contained in the maintenance plan, the Consent Agreement contains specific contingency plan triggers and requirements for Ameren. Specifically, the Consent Agreement requires that Ameren perform an air quality analysis if any elevated monitoring values are recorded one occurrence of a measured SO2 concentration that exceeds 75 ppb for one hour at any of the three industrial source monitors. Ameren must submit this air quality analysis including the monitored information and any relevant operational information to Missouri within a specified time frame.
If through discussion of the air quality analysis, it is established that the elevated monitoring values were attributable to Ameren Rush Island, Ameren would provide the State with proposed potential mitigation measures, SO2 emissions limitations, and a compliance schedule.
The EPA proposes to conclude that the maintenance plan adequately addresses the five basic components of a maintenance plan: The attainment emissions inventory, maintenance demonstration, monitoring, verification of continued attainment, and a contingency plan. Therefore, the EPA
proposes to find that the maintenance plan SIP revision submitted by Missouri for the Jefferson County 2010 SO2
nonattainment area meets the requirements of section 175A of the CAA and proposes to approve the plan.
VI. What are the actions the EPA is proposing to take?
The EPA is proposing to approve the maintenance plan for the Jefferson County 2010 SO2 1-hour NAAQS
nonattainment area into the Missouri SIP as compliant with CAA section 175A. The maintenance plan demonstrates that the area will continue to maintain the 2010 1-hour SO2
NAAQS and includes a process to develop contingency measures to remedy any future violations of the 2010

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