Antidegradation Policy in Colorado Water Permits: What It Means for New Discharge Permits

Colorado antidegradation review can give a new discharge tighter effluent limits than the stream standards alone would. Under Regulation 31, a new discharge to a stream the state calls "reviewable" may not cause significant degradation unless the Water Quality Control Division finds it necessary for important development and no reasonable, less degrading alternative exists. Outstanding waters get stricter protection. Use protected waters skip the review. Know your receiving segment's designation before you pick a site.

What does antidegradation mean in a Colorado discharge permit?

It is a second test on top of the water quality standards. Standards protect a stream's uses. Antidegradation limits how much of the stream's remaining clean capacity a new or bigger discharge may use.

The federal rule, 40 CFR 131.12, sets three levels of protection. Colorado's version is section 31.8 of Colorado Regulation 31 (5 CCR 1002-31):

• Every surface water keeps its existing classified uses and the quality needed to protect them (31.8(1)(c)).
• Reviewable waters keep their existing quality unless lowering it is necessary for important economic or social development (31.8(1)(b)).
• Outstanding waters keep their existing quality, with only short-term degradation allowed for a long-term ecological or water quality benefit or a clear public interest (31.8(1)(a)).

Under any designation, CDPHE's antidegradation overview says dischargers may not cause or contribute to a standards violation.

Which antidegradation designation applies to your receiving water?

Look it up. Every classified segment is outstanding, use protected or reviewable, and reviewable is the default (31.8(2)).

CDPHE's water quality planning targets guidance says the designation sits at the top right of the segment's table in the basin regulations, Regulations 32 through 38.

The Water Quality Control Commission sets designations in rulemaking. Use protected waters are generally warm water aquatic life class 2 segments, or segments already worse than the rule's reference values for at least three of 12 listed parameters (31.8(2)(b)). Outstanding waters must meet those values for all 12 parameters and be an outstanding natural resource, such as a significant attribute of a Gold Medal trout fishery, national park or wilderness area, or a designated wild river (31.8(2)(a)). CDPHE notes that many waters in Rocky Mountain National Park are outstanding.

When does a new discharge trigger antidegradation review?

When it is a regulated activity with new or increased impacts on a reviewable water.

Regulated activities are those that need a discharge permit or water quality certification under federal or state law, or fall under a state control regulation (31.8(3)(a)).

The Division makes the call. Its determinations are subject to de novo review by the Commission, including on petition by any interested person who submitted written comments during the Division's review (31.8(3)(b)).

How does the Division decide whether degradation is significant?

It runs the screening tests in section 31.8(3)(c). Pass, and the review stops there.

For all pollutants, a discharge is not significant if one of these holds:

• It is diluted 100 to 1 or more at low flow by water in the stream.
• After mixing, it consumes less than 15 percent of the baseline available increment, with all sources together capped at 15 percent.
• It causes only temporary or short-term changes, and long-term operation will not cause an adverse change.

Bioaccumulative toxic pollutants, those with a bioaccumulation factor of 1,000 or more, also face a load test: the new load must be under 10 percent of the existing total load.

The baseline available increment is the gap between the stream's low-flow concentration and the standard, over the reach from the outfall to the first major tributary by default. The baseline is water quality as of September 30, 2000, or when a use protected designation was removed. Net effect counts, so environmental benefits and mitigation built into the project help.

In practice, CDPHE's overview says new dischargers get fairly low antidegradation limits. Its planning guidance says 15 percent of the water quality standard gives a conservative estimate for antidegradation parameters, a cautious starting number for early plant sizing.

What happens if the Division finds the degradation significant?

You need a necessity of degradation determination under section 31.8(3)(d). Is the project important economic or social development, and is the degradation necessary to accommodate it?

The first question favors the applicant. If you submit evidence that the project is important development, it is presumed important unless contrary information comes in during public review. Local government and land use planning determinations get substantial weight.

The second question is where the work is. Degradation is necessary only if no economically, environmentally and technologically reasonable alternative would cause less degradation. If one exists, the Division requires it. Alternatives must serve the project's purpose with technology proven in similar field conditions. On cost, the Division may compare the alternative with the proposal, with user charges at similar utilities, and with similar dischargers on the segment.

In engineering terms, the alternatives are usually more treatment, a different outfall, or no discharge through land application or reuse. If you are already weighing reuse against direct discharge in Colorado, finish that comparison before the permit application. When degradation is found necessary, CDPHE calls the more lenient limit an antidegradation alternative limit, and each one goes through public comment.

What if the receiving water is already impaired?

Then antidegradation is not your main problem. The standards are.

The federal permit rule at 40 CFR 122.4(i) bars a permit for a new discharger that will cause or contribute to a standards violation. Where a load allocation exists, the new discharger must show enough remaining allocation and that existing dischargers are on compliance schedules. CDPHE's planning guidance says impairment status under Regulation 93 signals no assimilative capacity for those parameters.

How should antidegradation shape your site and treatment choices?

Treat it as a siting input, not a permit step. The receiving water you pick sets the limits you design to.

Dilution is often the biggest lever. A discharge diluted 100 to 1 at low flow passes the significance screen, apart from the separate load test for bioaccumulative toxics, and a larger stream leaves more room under the 15 percent increment. Two outfalls on different segments can produce very different limits, which makes outfall location a site feasibility question.

For domestic discharges, CDPHE says a new discharge under 1 MGD should seek general permit coverage, and 1 MGD or more needs an individual permit. Site location and design review under Regulation 22 are separate from the Regulation 61 discharge permit, so map the site application and the discharge permit application before you set a schedule.

Is your Colorado discharge ready for antidegradation review?

Answer yes or no to each item. Any no is your next task.

1. Do you know the receiving segment's designation from its basin regulation table?
2. Do you know the segment's low flow and whether you get 100 to 1 dilution?
3. Have you estimated limits at 15 percent of the standards for the antidegradation parameters?
4. Have you checked Regulation 93 and any TMDL for the segment?
5. Have you looked at a second outfall and a no discharge option?
6. Is local government support for the project on record?

Then ask your engineer which pollutants drive the limits and when design flow crosses 1 MGD.

Scope note: This is general education on Regulation 31 (effective December 31, 2024) and CDPHE guidance, not a determination for your site. Designations change in rulemaking, so confirm yours and your limits with the Division.

Frequently asked questions

What are the antidegradation tiers in Colorado?

There are three. Use protected waters get the minimum protection of existing uses. Reviewable waters get the significance and necessity review. Outstanding waters must keep their existing quality.

Is a temporary discharge exempt from Colorado antidegradation review?

Usually. Temporary or short-term changes are not significant unless long-term operation causes an adverse change (31.8(3)(c)(ii)(D)). CDPHE's overview says temporary dischargers, under two years, are excluded.

I manage a small sanitation district in Colorado and we plan a new plant to replace failing septic systems, discharging to a reviewable creek. Does the septic cleanup count in the antidegradation review?

It can. The significance test weighs net effect, including environmental benefits built into the project, so document the loads the septic systems add to the segment under review today. If the discharge is still significant, the project is presumed important once you submit evidence, unless public review shows otherwise.

I'm a developer in Colorado choosing between a small creek and a larger river for a new package plant outfall. Can antidegradation decide which one we use?

It often should. Dilution of 100 to 1 at low flow means no significant degradation, except that bioaccumulative toxics also face a load test. Compare each segment's designation, impairment status, low flow and likely limits before you fix the plant site.

What is the baseline date for Colorado antidegradation?

September 30, 2000, or the date a segment's use protected designation was removed (31.8(3)(c)(ii)(B)).

Related resources

• Colorado CDPHE Discharge Permit Process: Timeline, Requirements, and How It Differs from Texas
• Colorado Water Quality Control Commission Basin Specific Standards: Why the Same Discharge Limit Does Not Apply Statewide
• Public Notice and Citizen Petition in Colorado Water Quality Permitting: Preparing for Opposition
• Seasonal Flow and Snowmelt: Designing Colorado Discharge Permits for Extreme Hydrologic Variation

Picking a discharge point for a new Colorado plant?

The receiving water sets your limits, so settle antidegradation during due diligence, before the plant site is fixed. MES is licensed in Colorado, has an office in Golden, and handles the wastewater side from feasibility through construction.

We specialize in site feasibility and discharge due diligence, package wastewater treatment plants, collection systems and lift stations, reuse and land application alternatives, CDPHE site location and discharge permit applications, and construction administration.

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