Antidegradation Review Explained: Why Some Texas Streams Trigger a Harder TCEQ Review Than the Standard Effluent Limit Process

What is antidegradation review in Texas?

It is the review TCEQ runs under the antidegradation policy in 30 TAC §307.5, part of the Texas Surface Water Quality Standards. The policy applies to actions that would increase pollution, including authorized wastewater discharges, TMDLs and waste load evaluations. It has three tiers:

• Tier 1: existing uses, and the water quality that protects them, must be maintained.
• Tier 2: no degradation of waters better than fishable and swimmable quality, unless the lowering is necessary for important economic or social development.
• Tier 3: outstanding national resource waters must be maintained and protected. TCEQ's 2010 procedures say none are designated in Texas, so Tier 3 rarely comes up.

For TPDES permits, §307.5(c)(1)(A) points to TCEQ's implementation procedures. The current version is the June 2010 Procedures to Implement the Texas Surface Water Quality Standards (RG-194). TCEQ's standards implementation page says EPA approved all of it except the sections on whole effluent toxicity, dechlorination for minor domestic plants and variances, where the 2003 procedures still apply. A revision has been in preparation since 2019, with no approval timeline yet.

How is it different from the standard effluent limit analysis?

Effluent limits keep the stream within its criteria. Tier 2 protects the margin above them. The usual analysis, covered in our post on how TCEQ sets effluent limits, asks whether dissolved oxygen, ammonia, bacteria and other parameters stay within the numeric and narrative criteria.

Tier 2 asks whether quality drops "by more than a de minimis extent" from baseline, even when every criterion is still met. Under §307.5(c)(2)(B), baseline is the highest water quality sustained since November 28, 1975. The procedures estimate it from existing conditions unless the water has degraded since then.

Which Texas streams trigger a Tier 2 review?

Waters whose quality is better than fishable and swimmable. The procedures say Tier 2 generally applies to water bodies with existing, designated or presumed primary or secondary contact recreation, and intermediate, high or exceptional aquatic life use. New permits and amendments that allow an increase in loading are reviewed.

Loading already allowed by a reviewed TMDL, waste load evaluation or general permit usually skips a separate Tier 2 review.

Some watersheds also carry their own discharge rules in 30 TAC Chapter 311, including Lakes Travis and Austin, Lake Houston, Clear Lake and parts of the Colorado River watershed. These can require phosphorus limits, advanced treatment, or irrigation instead of discharge.

How does TCEQ screen for degradation?

With a 10% screen first, then a judgment on the water body as a whole. The screen does not define degradation. It only shows when a load is small enough to skip more analysis:

• an existing discharge whose permitted loading rises less than 10% is usually not potential degradation, if standards are met, the aquatic community is not unusually sensitive and the discharge is not relatively large;
• a new discharge that uses less than 10% of the existing assimilative capacity at the edge of the mixing zone is usually not potential degradation, unless the aquatic community is unusually sensitive;
• 10% or more is not presumed to be degradation, but gets further evaluation.

The capacity screen does not apply to dissolved oxygen, pH or temperature. For oxygen demand, a sag of less than 0.5 mg/L below baseline is usually not degradation, unless the organisms are unusually sensitive. A drop of more than 0.5 mg/L for a substantial distance in an exceptional water with a sensitive community is a "likely" example.

In April 2025, the Texas Supreme Court upheld this whole-water-body approach in a challenge to a city's permit to discharge to Onion Creek. A projected change in one parameter, there dissolved oxygen falling from at least 6.44 to 5.0 mg/L, does not decide degradation by itself. The examples in the procedures are guidelines, and TCEQ weighs overall water quality.

What happens if Tier 2 screening says degradation is likely?

The burden shifts to the applicant. TCEQ notifies the applicant, who then provides:

• any more information about the discharge and the receiving water that could change the evaluation;
• an analysis of alternatives that could eliminate or reduce the degradation, with their cost and feasibility;
• an evaluation of whether the discharge provides important economic and social development, such as jobs, tax base, housing, or correcting a public health problem.

TCEQ then decides whether the lowering is necessary and whether reasonable alternatives are unavailable. It may refer questions to the State Office of Administrative Hearings, and any permit that allows degradation is subject to EPA review. Alternatives include another outfall, better treatment, reuse, or land application.

What if the receiving water is already impaired?

Then Tier 1 limits the listed pollutant directly. For a water on the 303(d) list, the procedures do not allow an increase in loading of a listed pollutant that will cause or contribute to a violation of the standards. That covers discharges directly to the listed water and those close enough to affect it.

For a dissolved oxygen listing, BOD loading may not increase unless the discharge is shown to attain the oxygen standard or not to lower oxygen where it is not being met. Check the 2026 Texas Integrated Report, whose 303(d) list EPA approved on August 5, 2026, for the outfall and the water just downstream.

Does antidegradation review change public notice?

Yes, it is written into the notice. Under §307.5(c)(2)(D), when degradation of a water better than fishable and swimmable quality is anticipated, the public notice must say the antidegradation policy applies. When none is anticipated, it must say so.

Comments and hearing evidence can then address existing uses, the degradation finding and the economic justification. Your alternatives analysis is the record you defend.

How should a developer or city plan for it?

Screen the receiving water during feasibility, before the outfall is fixed. Use this check. Every answer should be yes before the plant site is set; any no is an open schedule risk:

1. Do you know the receiving water's segment and its assigned uses?
2. Have you checked the Integrated Report for impairments at and below the outfall?
3. Have you checked whether a Chapter 311 watershed rule or the Edwards Aquifer rules apply?
4. Have you estimated how much assimilative capacity the new loading would use?
5. Do you know whether a TMDL or waste load evaluation already covers your pollutants?
6. Could you document at least one alternative: another outfall, better treatment, reuse or land application?

This is general education on 30 TAC §307.5 and TCEQ's 2010 procedures as published in October 2026. TCEQ makes each determination case by case, and a pending revision could change the procedures, so confirm the current rules with your engineer before you commit to an outfall.

Frequently asked questions

Does antidegradation review apply to an expanded existing discharge?

Yes. Permit amendments that allow an increase in loading get Tier 1 review, and Tier 2 review where the water qualifies. An increase under 10% over the existing permitted loading usually passes the screen, but repeated small increases can be screened together.

I'm planning a 0.5 MGD package plant for a Hill Country subdivision that would discharge to a creek with high aquatic life use. Will antidegradation review change my treatment design?

Plan on a Tier 2 review. If screening points to degradation, you may face tighter limits or an alternatives analysis. In the Onion Creek case, EPA pointed to "considerably more stringent" limits, including very low phosphorus and nitrogen limits. Check Chapter 311 and the Edwards Aquifer rules too.

If our receiving water is already impaired, is antidegradation review easier or harder?

Harder for the listed pollutant. The permit cannot allow an added load of that pollutant that causes or contributes to the violation, so you may need limits that hold the load flat.

I run a small city utility in Central Texas. Is land application a way around antidegradation review for our new plant?

It avoids the surface water Tier 2 procedure for TPDES permits, but §307.5(c)(1)(C) keeps other permitted activities that increase pollution under the policy, and land application has its own site rules. Treat it as a real alternative with its own constraints.

Related resources

• How to Read a TCEQ Effluent Limit Table: What the Numbers Actually Mean for Your Development
• Nutrient Trading and Offset Credits: An Emerging Texas Compliance Tool for Nitrogen and Phosphorus Limits
• Reuse vs. Discharge: Which Wastewater Strategy Fits Your Texas Development?
• The 330 Day Timeline: Why Texas Wastewater Permits Take Longer Than Developers Expect

Choosing an outfall before you know what the receiving water will allow?

We screen the receiving water, the impairments and the watershed rules at feasibility, and compare discharge and non-discharge options before the plant design is locked.

We specialize in:

• TCEQ wastewater discharge permitting
• Receiving water and antidegradation screening
• Alternatives analysis for discharge, reuse and land application
• Wastewater treatment plant design
• Site feasibility and due diligence
• Construction administration

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com