The City of Dripping Springs applied to the Texas Commission on Environmental Quality (TCEQ) in 2015 for a permit to discharge up to 995,000 gallons per day of treated wastewater into Walnut Springs and then Onion Creek, because it was outgrowing its current land-application permit.¹ TCEQ's antidegradation rules require under Tier 1 that existing uses be maintained and, under Tier 2, bar discharges that cause "degradation," defined as "a lowering of water quality by more than a de minimis extent," in waters better than fishable and swimmable.² The City's modeling projected that dissolved oxygen (DO) in Onion Creek would fall from above 6.44 mg/L to at or just below the creek's site-specific 5.0 mg/L criterion at the discharge point, and TCEQ's staff required stricter effluent limits, including lower nutrient and higher DO limits, so that its own modeling predicted DO would not fall below 5.0 mg/L.³ The EPA withdrew its objections after reviewing the changes and said any change to the creek would be "de minimis," and the Executive Director approved the application in November 2017.⁴

Save Our Springs Alliance, Inc. (SOS) was the only protestant left after settlement agreements resolved the dispute with the others and reduced the maximum discharge to 822,500 gallons per day; the administrative law judge found the draft permit met all requirements, rejected SOS's "parameter-by-parameter" approach as a misreading of the rules, and found TCEQ's and the City's experts more compelling.¹ TCEQ issued a final order with 142 findings of fact and 22 conclusions of law, and the trial court reversed, holding a Tier 2 violation as a matter of law, but a divided Eighth Court of Appeals reversed and upheld the permit.² SOS petitioned for review on May 3, 2023, and TCEQ and the City responded on July 24, 2023; the Court requested merits briefing on September 29, 2023, granted the petition on June 14, 2024, heard argument on October 1, 2024, and decided the case on April 11, 2025.³

Petition for review from the Eighth Court of Appeals in a suit for judicial review of an agency order granting a wastewater discharge permit; argued October 1, 2024, and decided April 11, 2025.⁴

SOS presents whether the court of appeals erred in holding the final order's findings and conclusions adequate when TCEQ applied the wrong standard and made conclusory findings, whether it erred by not applying federal law and guidance, and whether it erred by not requiring TCEQ to follow the plain language of its own rules.¹ TCEQ presents whether the court of appeals correctly upheld a decision applying the plain language of a rule that the agency has long applied the same way and that is supported by substantial evidence, and the City presents whether the findings on dissolved oxygen are reasonably supported and whether TCEQ complied with its own regulations.² The Court decided TCEQ's Tier 2 methodology and whether the final order lacked a required statement of underlying facts.³

A reviewing court may not substitute its judgment for the agency's on the weight of the evidence but must reverse when the findings or decisions are erroneous as a matter of law, not reasonably supported by substantial evidence, or arbitrary or capricious; substantial evidence is a deferential reasonableness test, and the Court construes agency rules under ordinary statutory-construction principles, enforcing the text without adding to or subtracting from it.¹

Whether TCEQ's whole-water approach to Tier 2 follows its rules.

Petitioner (SOS): TCEQ delegated federal authority and must follow the plain language of its rules and federal requirements, which in SOS's view command a parameter-by-parameter approach, so that a drop in dissolved oxygen from at least 6.44 mg/L to 5.0 mg/L is degradation as a matter of law.¹

Respondents (TCEQ and the City): TCEQ followed the plain language of the rule, as it has for decades, based on substantial evidence in the record, and SOS asks the Court to reweigh the evidence; the City adds that SOS relies on EPA guidance that is not regulatory and that Tier 1 and Tier 2 reviews overlap.²

Whether the final order lacked a statement of underlying facts.

Petitioner (SOS): The final order's ultimate findings lack the required statement of underlying facts and rest on conclusory findings.³

Respondents (TCEQ and the City): Substantial evidence supports the findings, and findings of fact are not set out in statutory language, so no separate statement was required.⁴

SOS argued that the court of appeals failed to require TCEQ to follow the plain language of its own rules, approved inadequate and conclusory findings, and failed to apply controlling federal law, in a case concerning a permit to discharge treated wastewater into Onion Creek.¹ TCEQ responded that the Commission's rules require that existing uses be maintained and water quality not be degraded beyond a de minimis extent, and that following its long-held practice, which the EPA approved, it found the draft permit protective.²

• Findings. Asked if its point is just that the findings do not support the conclusion, SOS's counsel said the order never even quotes the rule, though SOS accepts the undisputed facts. (Oral Argument, 0:06:34)
• No finding needed. Asked about the 6.44-to-5.1 figures, TCEQ's counsel said no finding is required, because Section 2001.141(d) of the APA calls for underlying facts only when findings are in statutory language. (Oral Argument, 0:30:20)
• Degradation test. Asked whether a dissolved-oxygen drop alone could be degradation, the City's counsel said a drop leaving enough oxygen for existing uses is not, comparing it to blood oxygen. (Oral Argument, 0:36:16; 0:37:20)
• De minimis. Asked whether de minimis is a legal term or a margin of error for fluctuating oxygen levels, SOS's counsel disagreed, saying the science lets the agency predict effects. (Oral Argument, 0:13:06)
• EPA's withdrawal. Asked whether EPA withdrawing its objection matters, SOS's counsel said EPA has limited resources and withdrew before hearing any evidence or argument. (Oral Argument, 0:09:18; 0:10:52)
• Indigenous life. Asked whether Tier 1 requires protecting indigenous aquatic life, TCEQ's counsel pointed to the stream segment's designated uses in the appendix to Rule 307. (Oral Argument, 0:22:08; 0:23:12)
• Conflating the tiers. Asked whether the Commission conflated the Tier 1 and Tier 2 standards, TCEQ's counsel said no, because both tiers require maintaining existing uses under the same oxygen criteria. (Oral Argument, 0:25:52)
• Economic development. Asked why the case lacks a finding on necessary economic and social development, the City's counsel said there was no degradation, so the issue never arose. (Oral Argument, 0:38:27)
• Texas authority. Asked for Texas authority requiring parameter-by-parameter review, SOS's counsel cited the implementation procedures and EPA guidance, not a case, and said Robertson County requires comparing baseline with post-discharge. (Oral Argument, 0:47:22)

• Friends of the Brazos River, et al. filed an amicus curiae brief on January 11, 2024, which the opinion describes as supporting SOS.¹
• Bayou City Waterkeeper, et al. filed an amicus curiae brief on October 11, 2024, which the opinion also describes as supporting SOS.²
• Homebuilders Association of Greater Austin filed an amicus curiae brief on September 18, 2024, which the opinion describes as supporting TCEQ and the City.³
• TCEQ filed a response to the amici's brief on October 21, 2024.
• Ron Beal of Waco, Texas, filed amicus curiae letters on April 25, 2023, June 24, 2023, August 9, 2023, August 14, 2023, October 6, 2023.
• He filed further amicus curiae letters on November 16, 2023, January 29, 2024, October 2, 2024, October 3, 2024, October 16, 2024, April 11, 2025.

Holding

TCEQ's practice of assessing "degradation" under Tier 2 by whether overall water quality is lowered by more than a de minimis extent, and not by changes in any single parameter such as dissolved oxygen, conforms to its rules as written; substantial evidence supports TCEQ's findings that the discharge would not disturb existing uses or degrade water quality; and the final order did not need a separate statement of underlying facts.¹

Reasoning

Tier 1. SOS argued that one of the City's two DO models, which projected 4.87 mg/L, defeats Tier 1, but other modeling, including TCEQ's own, projected a minimum of 5.0 mg/L; TCEQ was not required to accept the lowest result, and all the modeling used the higher 995,000-gallon discharge, which makes it more conservative than the 822,500-gallon discharge the order authorizes.²

Tier 2. The text defines degradation as "a lowering of water quality," not a lowering of water-quality parameters, components, or constituents, so Tier 2's focus is on water quality as a whole and not on the DO level standing alone.³ TCEQ's implementation procedures also describe water quality as composed of several parameters of concern and say a parameter change is not the end of the matter, describing a qualitative whole-body assessment that begins by evaluating the individual parameters.⁴ The assimilative-capacity screening does not apply to DO, and the example SOS cites of when degradation "is likely," a 0.5 mg/L DO decrease over a substantial distance in a water body with exceptional aquatic life, is only a general guideline calling for further evaluation and does not describe Onion Creek, which has a high aquatic life use.⁵ TCEQ perhaps could have adopted a parameter-by-parameter approach but did not, and the Clean Water Act's goals and EPA guidance do not command one.⁶ SOS's argument that TCEQ "collapsed" Tier 1 and Tier 2 also fails because the findings use the words each standard employs and the order shows TCEQ evaluated both impacts on existing uses and the potential for degradation.⁷

Statement of underlying facts. Section 2001.141 requires an underlying-facts statement only for fact findings set forth in statutory language, and the final order separately states more than two dozen findings on the antidegradation review and adopts the Executive Director's responses to comments.⁸ SOS did not preserve the complaint because its rehearing motion did not identify the particular omitted findings, and in any event section 26.027 of the Water Code does not mandate criteria or findings, so TCEQ's rules are not statutory language requiring underlying findings.⁹

Disposition

The Court affirmed the court of appeals' judgment upholding the permit's issuance.¹⁰

Vote Breakdown

Majority Opinion by Justice Devine

Joined by Blacklock, C.J., and Bland, Boyd, Busby, Huddle, Lehrmann, Sullivan, and Young, JJ.