SUPPLEMENTARY INFORMATION:

Throughout this document “we,” “us,” and “our” means the EPA.

I. Background

Section 110(a)(2)(D)(i)(I) of the CAA requires a State's SIP to include provisions prohibiting any source or other type of emission activity in the State from emitting any air pollutant in amounts that will contribute significantly to nonattainment, or interfere with maintenance, of the NAAQS in any other State. EPA has long interpreted this language to enact a “functional prohibition” on certain emissions from upwind States, necessitating the EPA's independent assessment whether those emissions will occur or have been adequately controlled in the State where they originate. The EPA often refers to these requirements as Prong 1 (significant contribution to nonattainment of the NAAQS) and Prong 2 (interference with maintenance of the NAAQS).

Additional background for this action is discussed in detail in the Federal Register proposal 91 FR 6575 (February 12, 2026). In that document we proposed approval of the portions of the infrastructure SIP submitted by the State of Oklahoma on May 28, 2021, certifying that its SIP meets the transport Prong 1 and 2 infrastructure obligations addressing interstate transport for the 2010 1-hour SO₂ NAAQS.

The EPA provided a 30-day public comment period for the February 12, 2026, proposal, which closed on March 16, 2026 (91 FR 6575, February 12, 2026). We received three comment submissions: one opposed, one in support, and a letter from the Quapaw Nation through its Environmental Office (QNEO). We summarize and respond to these comments in section II of this preamble. The full comments are available in the docket for this rulemaking.

II. Response to Comments

Comment: The commenter opposes EPA's proposed approval, asserting that the record does not sufficiently demonstrate that Oklahoma's emissions will not significantly contribute to nonattainment or interfere with maintenance in downwind States. The commenter argues EPA should not approve the SIP without more comprehensive analysis, given uncertainties in transport modeling and the health significance of 1-hour SO_2. They urge supplemental analysis with updated inventories, worst-case meteorology, and more comprehensive source evaluations.

Response: EPA considered this comment and continues to find that Oklahoma's SIP submission satisfies CAA section 110(a)(2)(D)(i)(I) for the 2010 1-hour SO₂ NAAQS. The EPA has consistently applied a case-specific weight-of-evidence (WOE) approach focused on near-field impacts for SO₂ . As explained in detail in the technical supporting document (TSD), because peak 1-hour SO₂ concentrations decrease with distance from the source and are more localized than ozone or PM_2.5 , EPA identifies an “urban scale” up to 50 km as appropriate for assessing transport and trends and relies on air quality modeling and/or available monitoring, Data Requirements Rule (DRR) ^[1] /designation analyses, and source-specific analysis where needed. Consistent with prior SO₂ transport evaluations, the proposal applies this 50-km focus and weight-of-evidence framework to Oklahoma.

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Regarding the data used, the EPA relied on the most recently available, complete, and quality-assured data/information at the time of the rulemaking. The EPA evaluated 2021-2023 certified design values (DVs) at monitors located within 50 km of Oklahoma's borders in adjacent States and found they are below the 75 ppb NAAQS. We also reviewed statewide and source-specific emissions trends and available modeling/DRR analyses. Regarding sources, EPA focused on Oklahoma point sources emitting ≥100 tpy within 50 km of a State line and found five facilities. The EPA then conducted source-specific evaluations of those five sources, using emissions, air quality, wind patterns, controls/retirements, and where available, past modeling analyses. The proposal and TSD summarize these evaluations and the basis for concluding that Oklahoma does not significantly contribute to nonattainment and does not interfere with maintenance in other States.

The record, as reviewed by EPA, shows no downwind receptors within the relevant 50-km ranges recording design values at or above the standard, and the facility-level analyses—including cases with prior modeling ( e.g., Choctaw/Kay Counties) and recent emissions reductions—do not indicate plausible cross-state contributions to nonattainment or maintenance issues in other States for this NAAQS under baseline or likely future scenarios.

In these circumstances, additional photochemical modeling or expanded “worst-case” air modeling, beyond the WOE approach, is not necessary to support the interstate transport determination for this NAAQS relative to other States, for the State of Oklahoma. Nor does commenter identify with specificity any particular source or emissions activity in Oklahoma that, contrary to EPA's analysis, commenter believes emits or will emit in violation of the interstate transport provision. Finally, EPA notes that this action does not prejudge any future determinations; should new certified data or other evidence arise, EPA can address it as appropriate under the Act ( e.g., future transport evaluations or CAA section 126 petitions).

Comment: A commenter supports the EPA's proposal to approve Oklahoma's SIP regarding the 2010 SO₂ NAAQS interstate transport provision, citing Oklahoma Department of Environmental Quality's (ODEQ's) efforts and noting “finding no issues within 50 km” indicates compliance with the interstate transport requirement, and urges EPA to move forward with the approval.

Response: The EPA acknowledges this comment. As the proposal explains, focusing on the 50-km distance reflects the appropriate “urban scale” for SO₂ as an element of EPA's weight-of-evidence (WOE) approach; but it is not a stand-alone test. EPA's evaluation considers multiple lines of evidence, including certified design values (DVs) at relevant monitors, emissions and trend data, and source-specific analyses for all Oklahoma SO₂ sources emitting ≥100 tons per year (tpy) within 50 km of State borders, as detailed in the technical support document (TSD). Those lines of evidence collectively support approval in this case.

Comment: The Quapaw Nation's Environmental Office (QNEO) requested clarification on whether “certain areas of Indian Country” that ODEQ is asserting regulatory authority over, include areas within the Quapaw Reservation, and how ODEQ would meet Tribal engagement/consultation requirements for monitoring/modeling or regulatory assumptions inside Indian country if relying on the Safe, Accountable, Flexible, Efficient Transportation Equity Act of 2005 (“SAFETEA”). QNEO also asked whether the EPA intends to provide guidance describing how interstate transport SIP rules will apply in Indian country under SAFETEA for this and future revisions, including the limits and safeguards necessary to prevent infringements on Tribal sovereignty. QNEO seeks greater transparency on the technical basis (modeling files, inventories, screening analysis) and clarification of any assumptions involving sources within or near the Quapaw Reservation. QNEO also requests additional information supporting EPA's statement that this action “poses no significant risk to human health and the environmental conditions in Oklahoma or its Tribal Nations” for any underlying analyses or regional background concentration data used to reach this conclusion.

Response: As explained in section IV, Impact on Areas of Indian Country, in the February 12, 2026, proposed action (91 FR 6575) and in section III of this final action, the EPA approved the State of Oklahoma request under section 10211(a) of SAFETEA on May 12, 2025, to administer the State's SIP in certain areas of Indian country including within the Quapaw Nation reservation. Neither the May 12, 2025, SAFETEA decision, nor the approval of the State's SIP under the CAA, requires the State to engage with affected Tribes.

However, consistent with longstanding Agency policy, the EPA encourages Tribes and States to coordinate on environmental matters of mutual concern and believes such coordination may be particularly useful in this case, where the State is administering a program in Indian country. The EPA notes that the Quapaw Nation has CAA Treatment as a State (TAS) approval under CAA sections 107(d)(3),^[2] 126,^[3] and 505(a)(2).^[4] The State is required to comply with affected-state status notifications to neighboring States, which includes the Nation.

Additionally, the EPA encourages the Quapaw Nation and other interested Tribes to coordinate with the State on issues relating to implementation of the SIP. EPA appreciates and respects the government-to-government relationship with the Nation. As such, the EPA will continue to engage with and be responsive to the Nation's inquiries and concerns about any program administered by EPA or the State. The EPA will continue to provide guidance, information, and support to Tribal governments and is committed to consultation as appropriate. Specific to this action, EPA is making the determination that the Oklahoma SIP contains adequate provisions to ensure that the air emissions in the State will not significantly contribute to nonattainment or interfere with maintenance of the 2010 SO₂ NAAQS in any other State, thereby protecting public health. In other words, the Oklahoma SIP includes sufficient measures to protect downwind communities in other States from SO₂ pollution. This action does not prejudge future determinations; should new certified monitoring data or other relevant information materially change conditions, EPA may consider and act on such information in a future action as appropriate. No changes have been made to our final rule because of this comment.

Regarding technical transparency, the docket contains our TSD, which provides the details and references for EPA's determination that Oklahoma SO₂ emissions do not significantly impact downwind States based on localized impact analyses. The Quapaw Nation ( printed page 64288) Reservation is located near the exterior northwestern boundaries of the State and therefore within the areas that EPA analyzed. Potential 1-hour SO₂ emissions impacts are typically found within 50 km of sources emitting ≥100 tpy. The only SO₂ source in Ottawa County, OK, is the Miami Municipal Airport with SO₂ emissions of 0.14 tpy reported in 2022. There are no other SO₂ sources identified within Ottawa County or the Quapaw Nation.

As the TSD identifies, currently all Oklahoma SO₂ monitors show levels well below the NAAQS. In addition, EPA determined that current SO₂ DVs in neighboring States in the relevant distance ranges are below the NAAQS. The TSD includes the list of the five SO₂ sources evaluated that meet EPA's distance and emissions quantity criteria, the monitoring and DVs reviewed, and source-specific analyses. The docket also identifies all data sources ( e.g., National Emissions Inventory (NEI), Emissions Inventory System (EIS)) used and includes a spreadsheet titled “Breakdown of 2020 NEI SO₂ Emissions from Oklahoma Point Sources Facilities” for an inclusive list. The proposed rule for this action (91 FR 6575; February 12, 2026) references SO₂ DRR and available designations modeling considered in the analysis. Supporting underlying data and other information is available in the docket.

III. Impact on Areas of Indian Country

As stated in the proposed action (91 FR 6575, February 12, 2026), the EPA approved Oklahoma's request to administer all the State's EPA-approved environmental regulatory programs, including the Oklahoma SIP, in the requested areas of Indian country pursuant to section 10211(a) of the Safe, Accountable, Flexible, Efficient Transportation Equity Act of 2005: A Legacy for Users, Public Law 109-59, 119 Stat. 1144, 1937 (August 10, 2005) (“SAFETEA”).

As requested by Oklahoma, the EPA's approval under SAFETEA does not include Indian country lands, including rights-of-way running through the same, that: (1) qualify as Indian allotments, the Indian titles to which have not been extinguished, under 18 U.S.C. 1151(c); (2) are held in trust by the United States on behalf of an individual Indian or Tribe; or (3) are owned in fee by a Tribe, if the Tribe (a) acquired that fee title to such land, or an area that included such land, in accordance with a treaty with the United States to which such Tribe was a party, and (b) never allotted the land to a member or citizen of the Tribe (collectively “excluded Indian country lands”). In addition, the State only sought approval to the extent that such approval is necessary for the State to administer a program in light of Oklahoma Dept. of Environmental Quality v. EPA, 740 F.3d 185 (D.C. Cir. 2014).

As explained earlier in this action, the EPA is addressing the portions of the Oklahoma SIP that address two of the interstate transport requirements for the 2010 1-hour SO₂ NAAQS, as these portions meet the requirements in CAA section 110, and specifically in CAA section 110(a)(2)(D)(i)(I). Consistent with the D.C. Circuit's decision in ODEQ v. EPA and with the EPA's SAFETEA approval, these SIP revisions will apply to areas of Indian country as follows: (1) pursuant to the SAFETEA approval, the SIP revisions will apply to all Indian country in the State of Oklahoma other than the excluded Indian country lands as described above; and (2) pursuant to the D.C. Circuit's decision in ODEQ v. EPA, the SIP revisions will also apply to any Indian allotments or dependent Indian communities that are located outside of any Indian reservation over which there has been no demonstration of Tribal authority.

IV. Final Action

The EPA approves the portions of the Oklahoma SIP submitted on May 28, 2021, that address two of the interstate transport requirements for the 2010 1-hour SO₂ NAAQS, as these portions meet the requirements in CAA section 110, and specifically in CAA section 110(a)(2)(D)(i)(I). EPA determines that the Oklahoma SIP contains adequate provisions to ensure that the air emissions in the State will not significantly contribute to nonattainment or interfere with maintenance of the 2010 SO₂ NAAQS in any other State. This action is being taken under section 110 of the Act.

V. Statutory and Executive Order Reviews

Under the Clean Air Act, the Administrator is required to approve 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, the EPA's role is to approve State choices, provided that they meet the criteria of the Clean Air Act. Accordingly, this 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 action:

• Is not a significant regulatory action subject to review by the Office of Management and Budget under Executive Orders 12866 (58 FR 51735, October 4, 1993);
• Is not anExecutive Order 14192 (90 FR 9065, February 6, 2025) regulatory action because this action is not significant under Executive Order 12866;
• 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 inExecutive Order 13132 (64 FR 43255, August 10, 1999);
• Is not subject toExecutive Order 13045 (62 FR 19885, April 23, 1997) because it approves a State program;
• Is not a significant regulatory action subject toExecutive Order 13211 (66 FR 28355, May 22, 2001); and
• 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 Clean Air Act.

This approval of the Oklahoma 2010 SO₂ Interstate Transport applies to certain areas of Indian county throughout Oklahoma as discussed in section III, and therefore has Tribal Implications as specified in E.O. 13175 (65 FR 67249, November 9, 2000). However, this action will neither impose substantial direct compliance costs on federally recognized Tribal governments, nor preempt Tribal law. This action will not impose substantial direct compliance costs on federally recognized Tribal governments because no actions will be required of Tribal governments. This action will also not preempt Tribal law as no Oklahoma Tribe implements a regulatory program under the CAA, and thus does not have applicable or related Tribal laws. Consistent with the EPA Policy on Consultation with Indian Tribes (December 7, 2023), the EPA offered consultation to Tribal governments on February 10, 2026. No Tribes engaged in consultation with EPA on this matter.

This action is subject to the Congressional Review Act, and the EPA will submit a rule report to each House of the Congress and to the Comptroller General of the United States. This action ( printed page 64289) is not a “major rule” as defined by 5 U.S.C. 804(2).

Under section 307(b)(1) of the Clean Air Act, petitions for judicial review of this action must be filed in the United States Court of Appeals for the appropriate circuit by December 7, 2026. Filing a petition for reconsideration by the Administrator of this final rule does not affect the finality of this action for the purposes of judicial review nor does it extend the time within which a petition for judicial review may be filed, and shall not postpone the effectiveness of such rule or action. This action may not be challenged later in proceedings to enforce its requirements. (See CAA section 307(b)(2).)

For the reasons stated in the preamble, the Environmental Protection Agency amends 40 CFR part 52 as follows:

91 FR 64286