Colorado Water Quality Control Commission Basin Specific Standards: Why the Same Discharge Limit Does Not Apply Statewide

Why doesn't one discharge limit apply across Colorado?

Limits protect the receiving water, and Colorado classifies its waters segment by segment. Regulation 31 (5 CCR 1002-31), the Basic Standards and Methodologies for Surface Water, sets basic standards, an antidegradation rule and the system for classifying waters and assigning standards (§31.1).

The Commission then applies that system to specific segments through public hearings. Two more rules in §31.3 matter for a new discharge:

• the Commission must consider downstream waters' classifications and standards, so a downstream use can drive an upstream limit;
• standards and classifications are reviewed at least once every three years, so a segment's standards can change.

Which limits do apply everywhere in Colorado?

Technology-based limits are the floor, and they apply statewide. Regulation 62 sets BOD5 and TSS limits of 30 mg/L as a 30-day average and 45 mg/L as a 7-day average, pH from 6.0 to 9.0 and 85 percent removal (§62.5). Section 62.4(3) says these give way when stricter limits are needed to meet water quality standards.

Regulation 85 adds nutrient limits. A new domestic treatment facility discharging to surface water gets 0.7 mg/L total phosphorus and 7 mg/L total inorganic nitrogen as rolling annual medians (§85.5(1)(b)).

Even those numbers depend on the stream. Section 85.5(3)(b) waives the Regulation 85 limits where the discharger shows its discharge is unlikely to push the receiving water past the nutrient values in §31.17.

How does a stream segment's classification change the limits?

The classifications say which uses must be protected, and each use carries its own standards. Section 31.13 sets the classes:

• recreation: E (existing primary contact), P (potential), N (not primary contact) and U (undetermined);
• agriculture;
• aquatic life: Class 1 cold water, Class 1 warm water, and Class 2;
• domestic water supply, with a direct use subclass for supply lakes and reservoirs;
• wetlands.

Qualifiers such as Goal, Seasonal and Interrupted Flow can be added (§31.13(2)). A cold-water Class 1 segment used for swimming and a water supply intake downstream will carry far tighter standards than a Class 2 warm-water segment with no water supply use.

What do the basin regulations add?

They assign the actual classifications and numbers to each segment. CDPHE's list of Commission regulations shows seven basin rules:

• Regulation 32, Arkansas River Basin;
• Regulation 33, Upper Colorado River Basin and North Platte River;
• Regulation 34, San Juan and Dolores River Basins;
• Regulation 35, Gunnison and Lower Dolores River Basins;
• Regulation 36, Rio Grande Basin;
• Regulation 37, Lower Colorado River Basin;
• Regulation 38, South Platte, Laramie, Republican and Smoky Hill River Basins.

Some watersheds have their own control regulation on top: Regulations 71 to 74 cover Dillon Reservoir, Cherry Creek Reservoir, Chatfield Reservoir and the Bear Creek watershed. A discharge in one of those areas has to meet that rule as well.

Why do ammonia and temperature limits vary so much?

Both are set from equations and fish communities, not single numbers. The chronic ammonia criteria in Regulation 31 Table II are equations that use the stream's pH and temperature, with one form when fish early life stages are present and another when they are absent. For warm-water segments, that switch is seasonal.

Temperature works the same way. Cold Stream Tier I criteria apply where cutthroat or brook trout are expected, and Tier II where other cold-water species are. Warm streams have three tiers based on which species are expected. A mountain trout stream and a plains creek can face very different ammonia and temperature limits for the same plant.

Where do nutrient standards stand?

They are being phased in, with a 2027 milestone. Section 31.17 holds total phosphorus, total nitrogen and chlorophyll a values that the Commission applies segment by segment. Before December 31, 2027, they are used only in limited cases, such as headwaters upstream of all permitted domestic plants discharging before May 31, 2012, or where Regulation 85 alone will not control nutrient pollution.

After December 31, 2027, the Commission will consider those values for any segment that does not yet have nutrient standards (§31.17(2)(b)). A plant designed today for a 20-year life should be checked against that change.

How does antidegradation change the answer?

It can hold a stream at its existing quality even when standards would allow more. Under §31.8, outstanding waters are maintained at existing quality. Undesignated, or reviewable, waters cannot be degraded without an antidegradation review. Use-protected waters, such as many warm-water Class 2 segments, skip that review and get only the protection of their standards.

A clean mountain segment is often where this bites hardest. Our post on antidegradation in Colorado water permits walks through the review.

How do you find the limits for a specific site?

Work from the receiving segment outward, before land or technology is locked in. Use this check. Any "no" is a gap to close first.

1. Have you identified the exact segment the discharge will reach, including any ditch or tributary in between?
2. Have you read that segment's row in the right basin regulation?
3. Do you know its classifications and qualifiers?
4. Do you know its antidegradation designation and existing quality?
5. Have you checked downstream segments, intakes and any reservoir control regulation?
6. Have you asked the Division for water quality planning targets or preliminary effluent limits?
7. Have you compared direct discharge with land application or a plan of reuse?

Then ask your wastewater engineer which parameter is likely to control the design, and what it would take to meet it.

This article is general education on Colorado rules as read in October 2026, including Regulation 31 as effective December 31, 2024. Standards change through regular hearings, and the Division sets each permit's limits, so confirm the current rules for your segment.

Frequently asked questions

If two projects use the same treatment technology, will they get the same discharge limits?

Not necessarily. They share the statewide floors in Regulations 62 and 85, but water quality-based limits come from each receiving segment's classifications, numeric standards, downstream uses and antidegradation status.

Are basin standards the same as Colorado's statewide basic standards?

No. Regulation 31 sets basic standards that apply to all state surface waters and the tables used to build segment standards. The basin regulations then assign classifications and numeric standards to specific segments.

How often do Colorado stream standards change?

Regulation 31 requires standards and classifications to be reviewed at least once every three years and revised where appropriate (§31.3). Check the current basin regulation before relying on an old permit or report.

I'm planning a mountain resort near a trout stream in Colorado. Will our discharge limits be stricter than a plains subdivision's?

Often, yes. Cold-water Class 1 segments with trout carry tighter temperature tiers, ammonia criteria tied to sensitive life stages, and frequently an antidegradation review. Confirm with the segment's basin regulation and the Division's planning targets.

I'm a Front Range developer and want to reuse the effluent limits from a neighboring plant's permit for our package plant design. Is that safe?

Only as a rough first look. That permit reflects its own segment, flow and the standards in force when it was written. Ask the Division for planning targets for your own discharge point and design flow before sizing treatment.

Related resources

• CDPHE Site Application vs. Discharge Permit Application: The Two-Step Process Colorado Developers Often Confuse Into One
• Colorado CDPHE Discharge Permit Process: Timeline, Requirements, and How It Differs from Texas
• Seasonal Flow and Snowmelt: Designing Colorado Discharge Permits for Extreme Hydrologic Variation
• Public Notice and Citizen Petition in Colorado Water Quality Permitting: Preparing for Opposition

Need to know which Colorado limit will control your plant?

We start with the receiving segment and its basin standards, then size treatment to the limit that actually applies.

We specialize in:

• Receiving water and discharge feasibility studies
• CDPHE site location applications and engineering reports
• Discharge permit applications
• Package and custom wastewater treatment plant design
• Land application and reuse alternatives
• Construction administration

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