Quick Answer
Colorado’s antidegradation policy, established under Regulation 31 and required by the federal Clean Water Act, protects existing water quality in Colorado streams, rivers, lakes, and reservoirs from degradation caused by new or increased discharges. The policy applies to all new discharge permit applications and to significant modifications to existing permits. Colorado implements antidegradation through three tiers. Tier I protects all water bodies from degradation below their designated uses. Tier II applies to water bodies with quality exceeding what is necessary to support designated uses and requires review of proposed new or increased discharges to ensure they are justified and that existing quality is maintained to the extent practicable. Tier III provides the highest level of protection to outstanding waters designated by the Water Quality Control Commission. Understanding which tier applies to your proposed discharge location before committing to a site is not optional. It is the foundational step in Colorado discharge permit planning.What Antidegradation Means and Why Colorado Takes It Seriously
Antidegradation policy is the regulatory expression of a principle that is both scientifically sound and legally required: water quality that currently meets or exceeds standards should not be allowed to deteriorate simply because a new discharger wants to use the receiving water as an effluent disposal pathway. In a state like Colorado, where mountain streams, high-altitude lakes, and tributary headwaters support exceptional biological communities, outstanding recreational resources, and downstream water supply systems, the protection of existing water quality against incremental degradation from new discharges is a central regulatory objective rather than a peripheral concern.
The federal Clean Water Act requires all states to adopt antidegradation policies as part of their water quality standards frameworks. Colorado’s implementation of that requirement goes further than many states in the specificity and rigor of the review process, particularly for Tier II waters where new discharges are proposed. The Colorado Water Quality Control Commission has established its antidegradation policy in Regulation 31, and the Water Quality Control Division implements that policy when reviewing new and modified discharge permit applications. For project teams accustomed to Texas TCEQ discharge permitting, where antidegradation review is a less prominent feature of the permitting process, encountering Colorado’s antidegradation requirements for the first time during permit application review is a common source of schedule disruption and documentation surprise.
Tier I: The Baseline Protection That Applies Everywhere
Tier I antidegradation protection applies to every classified water body in Colorado and establishes the floor below which water quality cannot be allowed to fall. Under Tier I, existing uses of the water body and the water quality conditions necessary to protect those uses must be maintained and protected. A new discharge permit cannot be written in a way that would cause the receiving water to fall below its designated use classifications, regardless of how small the discharge volume is or how high the treatment quality is. Tier I is not typically the antidegradation tier that creates major permitting challenges for well-designed treatment systems discharging to waters that have assimilative capacity to absorb a modest pollutant load without falling below the applicable standards. It is the minimum standard that every discharge permit must satisfy as a baseline condition.
The practical significance of Tier I for project planning is that it establishes the absolute floor for receiving water quality that the permit limits must protect. If a receiving water is already impaired and does not meet its designated use classification, Tier I requires that the new permit not worsen the existing impairment. For project teams proposing to discharge to a water body on the Colorado 303(d) impaired waters list, this Tier I requirement translates into a significant permitting challenge even before Tier II considerations are evaluated. Discharging to an already impaired water body requires demonstration that the new discharge will not prevent or delay attainment of the applicable water quality standards, which may require treatment performance that makes the proposed discharge infeasible from a cost or technology standpoint.
Tier II: The Review That Most New Discharges Face
Tier II antidegradation review is the component of Colorado’s antidegradation policy that most directly affects new discharge permit applications on receiving waters that currently meet their water quality standards. Tier II applies when the quality of the receiving water is better than what is necessary to support the designated uses, which in practice describes a significant portion of Colorado’s classified stream segments, particularly in mountain and foothill watersheds where natural water quality is high and existing anthropogenic pollutant loading is low. When a new or increased discharge is proposed to a Tier II water, the permitting process must include an antidegradation review that evaluates whether the proposed discharge is socially and economically justified, whether it represents the least environmentally damaging practicable alternative, and whether it is consistent with the highest statutory and regulatory requirements applicable to the discharger.
The social and economic justification analysis requires the project team to demonstrate that the proposed discharge serves a legitimate and documented public benefit. For a municipality extending sewer service to address failing septic systems in an unsewered area, the justification is relatively straightforward to document. For a private developer seeking to discharge treated wastewater from a new resort or subdivision development, the justification must address the economic development value, employment effects, public service improvements, and other benefits that the project provides to the surrounding community. This justification analysis is not a formality. CDPHE evaluates it as part of the Tier II review, and a poorly documented or unconvincing justification can result in requests for additional information that extend the permit timeline.
The alternatives analysis component of Tier II review requires evaluation of whether less environmentally damaging alternatives to the proposed discharge exist and are practicable. For a project where a Plan of Reuse is technically and economically feasible, the alternatives analysis must address why direct discharge to the receiving water is preferable to reuse. If the alternatives analysis cannot credibly demonstrate that reuse is not practicable, CDPHE may require the project to pursue reuse rather than discharge. This is not a hypothetical outcome. It has affected Colorado projects where the proximity of irrigated agricultural land or landscape irrigation opportunities made reuse a clearly feasible alternative that the project team had not adequately considered in the permit application.
Tier III: Outstanding Waters and the Highest Protective Standard
Tier III protection applies to waters designated as outstanding by the Colorado Water Quality Control Commission. These are water bodies that the Commission has identified as having exceptional ecological, recreational, or aesthetic value that warrants a higher level of protection than Tier II provides. Outstanding waters in Colorado include certain segments of wild and scenic rivers, high altitude lakes in wilderness areas, and other water bodies that the Commission has formally designated through its regulatory process. The practical effect of Tier III protection is that new or increased discharges that would cause any degradation of an outstanding water are generally not permissible. This is a very high standard that effectively prohibits new point source discharges to Tier III designated waters in most circumstances.
For project teams, the significance of Tier III is primarily as a site selection constraint. Before committing to a discharge location near or upstream of a Tier III designated water body, the project team needs to confirm whether the proposed discharge would affect the outstanding water and whether any pathway exists for permit issuance given that constraint. In most cases where the receiving water or a nearby water body carries Tier III designation, the appropriate response is to evaluate alternative disposal pathways including reuse, alternative discharge locations to waters without Tier III designation, or project design modifications that eliminate or substantially reduce the wastewater volume requiring disposal. Attempting to permit a discharge that affects a Tier III water without an extremely compelling justification and exhaustive alternatives analysis is an approach that will consume engineering and legal resources without a predictable outcome.
How Antidegradation Review Affects Permit Timeline and Documentation Requirements
The practical effect of antidegradation review on permit timeline is one of the most important pieces of information a Colorado project team can have before initiating a permit application. A new discharge permit application that triggers a full Tier II antidegradation review adds analytical steps, documentation requirements, and potentially a public comment and hearing process to the permit development timeline that are not present in permit applications where antidegradation review is not triggered or where it is resolved quickly. The additional documentation required for a Tier II review includes the social and economic justification analysis, the alternatives analysis, and the technical analysis demonstrating that the proposed discharge will not cause lowering of existing water quality beyond what is justified through the Tier II process.
CDPHE staff must review and evaluate all of this documentation before a draft permit can be issued, and that review takes time that is additive to the technical review of the application itself. For straightforward Tier II reviews where the justification is clear, the alternatives analysis is credible, and the technical documentation is complete, the antidegradation review may add two to four months to the permit development timeline. For complex projects where the justification requires detailed economic analysis, where the alternatives analysis involves multiple reuse or discharge scenarios, or where the proposed discharge is to a particularly sensitive water body, the antidegradation review component can add significantly more time and may generate public interest that extends the comment period and hearing process.
Pre-Application Strategy for Navigating Antidegradation Review
The most effective strategy for managing antidegradation review risk in Colorado is early engagement with CDPHE before the formal permit application is submitted. The Water Quality Control Division offers pre-application meetings that allow project teams to present the proposed project and receive Division feedback on whether antidegradation review will apply, what tier of review the Division anticipates, what documentation the Division will expect in the application package, and whether any receiving water concerns exist that should be addressed in the project design before the application is filed.
These pre application conversations are particularly valuable for projects where the receiving water classification is uncertain, where the boundary between Tier II and non Tier II applicability is not obvious from the regulatory record, or where the project team is considering multiple discharge location options and wants to understand the antidegradation implications of each before committing to a site. The pre-application meeting is not a formal step in the permit process and does not bind CDPHE to any particular outcome, but it consistently produces more complete initial applications that require fewer rounds of additional information requests during formal review, which translates directly into shorter permit timelines.
For project teams, the pre application process should be initiated after enough engineering work has been completed to describe the proposed discharge location, volume, treatment approach, and effluent quality meaningfully, but before significant capital investment has been committed to the treatment facility or the discharge infrastructure. The reason for that timing is practical. If the pre-application conversation reveals that the proposed discharge location triggers a Tier II review that requires alternatives analysis, and that reuse is the alternative that CDPHE considers most practicable, discovering that before designing a treatment plant optimized for surface discharge is far less costly than discovering it after.
Frequently Asked Questions
How do we know whether the receiving water we are considering for discharge is a Tier II water subject to antidegradation review?
The starting point is Colorado’s water quality standards database and the segment-specific classifications established under Regulation 31. CDPHE maintains a searchable inventory of classified water body segments with their designated uses and antidegradation tier assignments. An engineer or water quality professional familiar with the Colorado classification system can evaluate the specific stream segment at your proposed discharge location and determine the applicable tier. In some cases, the tier assignment for a specific segment is straightforward from the regulatory record. In others, the segment boundary locations and classification history require more detailed review. Confirming the antidegradation tier for your proposed discharge location before initiating a permit application is a basic due diligence step that takes days rather than months and can prevent a project strategy built on incorrect regulatory assumptions.
We are a municipality extending sewer service to an unsewered area that currently relies on failing septic systems. Does our new discharge still require Tier II review?
Potentially yes, depending on the quality of the receiving water you are proposing to discharge to. The trigger for Tier II review is the receiving water quality status, not the nature of the project proposing the discharge. However, municipal sewer extension projects replacing failing on-site sewage facilities generally have a strong social and economic justification that is straightforward to document. The environmental benefit of eliminating failing septic systems that are currently degrading groundwater and surface water quality in the area often supports the justification analysis for the proposed discharge, and in some cases the comparison between the existing condition and the proposed permitted discharge demonstrates a net water quality improvement that strengthens the antidegradation analysis rather than complicating it.
Our project is considering two potential discharge locations on different stream segments. Can antidegradation analysis help us choose between them?
Yes, and comparative antidegradation analysis between multiple potential discharge locations is exactly the kind of pre-design evaluation that produces better project outcomes. If one stream segment carries a Tier II designation that will require full antidegradation review and an alternatives analysis, while another segment in the same watershed has a different classification status that triggers a less intensive review, that regulatory difference is a meaningful factor in discharge location selection alongside the hydraulic, technical, and logistical considerations. Evaluating both locations for their antidegradation implications, their receiving water quality standards, and the likely permit limits each would produce gives the project team a complete picture of the regulatory cost and risk associated with each option before a commitment is made.
Evaluating Antidegradation Requirements for Your Colorado Discharge Permit?
MES is licensed in Colorado and works with developers, municipalities, water districts, and engineering teams to evaluate receiving water antidegradation status, prepare social and economic justification documentation, conduct alternatives analysis, and build permit applications that address Tier II review requirements completely and efficiently.
We specialize in:
- Antidegradation tier evaluation and receiving water classification review for Colorado discharge permit applications
- Tier II social and economic justification analysis and documentation preparation for new and modified CDPS permits
- Alternatives analysis for proposed Colorado discharges including reuse feasibility and alternative discharge location evaluation
- Pre application coordination with CDPHE Water Quality Control Division for antidegradation review strategy
- Discharge location comparative analysis for projects evaluating multiple potential receiving water options
- Colorado wastewater engineering and permit application preparation for municipalities, developers, and utility districts
