Learn how CDPHE develops permit limits for surface water discharges, from water quality standards and monitoring data to site-specific permit requirements.
Colorado regulates discharges to surface waters through a permitting framework built on two key laws: the federal Clean Water Act and the Colorado Water Quality Control Act. Through this framework, the state establishes water quality standards and issues permits designed to protect Colorado’s rivers, lakes, streams, and other surface waters.
These permits contain limits that help ensure discharges remain protective of water quality. Permit limits are not arbitrary numbers. Rather, they are developed using water quality standards, receiving water conditions, facility-specific monitoring data, and applicable state and federal requirements.
What Are Discharge Permits and Permit Limits?
A discharge permit is a legal authorization that allows a facility to discharge treated wastewater or stormwater into a surface water body under specific conditions. In Colorado, these permits are issued through the Colorado Discharge Permit System (CDPS) program, administered by the Colorado Department of Public Health and Environment (CDPHE) under the authority of the federal Clean Water Act and the Colorado Water Quality Control Act.
Many types of facilities may require discharge permits, including:
- Muncipal wastewater treatment plans
- Industrial and manufacturing facilites
- Mining operations
- Food and beverage processing facilities
- Landfills
- Construction sites and certain stormwater discharges
One of the most important components of a discharge permit is a permit limit, which establishes the maximum concentration of a pollutant that may be discharged. Some limits are based on the performance of available treatment technologies, while others are based on the ability of the receiving water to safely assimilate pollutants without violating water quality standards.
Water Quality Standards: The Foundation of Permit Limits
Colorado’s permit program is designed to protect designated uses of state waters, including aquatic life, recreation, agriculture, and drinking water supplies. Permit limits are developed using applicable water quality standards while also considering downstream water quality protection.
Colorado Regulation 31 is the state’s foundational regulation for surface water, establishing basic water quality standards, an antidegradation rule, and a classification system for state water bodies. Managed by CDPHE, Regulation 31 provides the framework used to protect public health, aquatic ecosystems, and other beneficial uses of Colorado waters.
These water quality standards serve as the regulatory benchmark from which permit limits are developed.
How Does CDPHE Determine Whether a Pollutant Requires a Permit Limit?
A pollutant is not automatically assigned a permit limit. Instead, CDPHE evaluates whether a discharge has “reasonable potential” to cause or contribute to an exceedance of water quality standards. This evaluation considers:
- Effluent monitoring data
- Facility operations
- Receiving water conditions
- Pollutant characteristics
- Professional judgement
Under CDPHE’s reasonable potential policy, permit writers typically evaluate representative effluent samples and use facility monitoring data, receiving water conditions, and statistical analyses to estimate whether a discharge could exceed applicable standards.
If the analysis demonstrates a reasonable potential to exceed water quality standards, the permit must include water quality-based effluent limits.
What Role Does Data Monitoring Play in Permit Development?
The Clean Water 1 (CW-1) Reasonable Potential Policy relies heavily on monitoring data to evaluate whether permit limits are necessary. CDPHE generally reviews:
- Sample size
- Data variability
- Maximum observed concentrations
- Statistical multipliers
- Pollutants of concern
Based on that analysis, CDPHE may require a permit limit, monitoring only, or no additional requirements for a particular pollutant.
When data is limited or insufficient, the division will often require additional monitoring and reporting until enough information is available to complete a full evaluation. Importantly, the absence of a pollutant-specific limit does not eliminate the requirement to maintain permit coverage.
Water Quality-Based Limits vs. Technology-Based Limits
Colorado permits often include two types of limits — technology-based effluent limits (TBELs) and water quality-based effluent limits, or WQBELs — to protect state surface waters.
Technology-based effluent limits are based on the performance of available treatment technologies and may incorporate federal Effluent Limitation Guidelines (ELGs) as well as state regulatory requirements. These limits establish a minimum level of pollutant control regardless of receiving water conditions.
Water quality-based effluent limits provide additional site-specific protections when technology-based limits alone are not sufficient to meet water quality standards. These limits are based on receiving water conditions, assimilative capacity, and applicable water quality standards established through Regulation 31 and basin-specific regulations.
WQBELs are added to a permit when a facility’s discharge has reasonable potential to cause or contribute to an exceedance of human health or aquatic life standards. The reasonable potential process combines quantitative analyses and professional judgment to determine whether additional water quality-based controls are necessary.
Why Does Antidegradation Matter?
Even when a discharge may comply with applicable water quality standards, Colorado’s antidegradation requirements can influence permit development. Antidegradation protections help ensure that existing water quality and beneficial uses are maintained whenever possible.
The highest level of protection applies to waters designated as outstanding state or national resources. In these waters, only limited and carefully justified degradation may be allowed.
For many other Colorado water bodies, permit applicants must demonstrate that a new or expanded discharge will not result in significant degradation of existing water quality. As part of this process, CDPHE may evaluate:
- Existing water quality
- Available assimilative capacity
- Potential water quality impacts
- Alternatives analysis
These reviews are particularly important for new discharges, facility expansions, and proposed increases in pollutant loading.
Are Permit Limits Site-Specific?
While Colorado’s permitting framework is built on statewide regulations and water quality standards, permit limits are ultimately developed using site-specific information about a facility and its receiving waters.
When evaluating a discharge, CDPHE considers a variety of factors, including:
- Characteristics of the receiving waterbody
- Existing water quality conditions
- Available assimilative capacity
- Facility monitoring data
- Pollutant sources
- Potential for discharge to affect designated waterway uses
These factors help CDPHE determine whether permit limits, additional monitoring, or other controls may be necessary.
A recent CDPS permit renewal for the City of Glenwood Springs’ South Canyon Landfill illustrates this process in action. In the permit fact sheet, CDPHE evaluated monitoring data, receiving stream conditions, applicable water quality standards, antidegradation requirements, and federal effluent limitation guidelines to determine which pollutants required limits and which required only monitoring.
Rather than applying the same requirements across all facilities, the division assessed each pollutant individually and evaluated whether site-specific conditions supported permit limits, additional monitoring, or no action.
The fact sheet also demonstrates how facility-specific conditions can influence permit requirements. For example, CDPHE considered factors like local stream-flow conditions, existing water quality, pollutant concentrations, and whether certain pollutants were likely to be present based on landfill operations and industry information. These considerations helped shape the final permit requirements and monitoring obligations, resulting in a permitting process tailored to the unique circumstances of each discharge.
Understanding these site-specific factors early in the permitting process can help facilities anticipate potential requirements and better prepare for permit renewals or modifications.
How Can My Facility Navigate Colorado’s Permitting Framework?
Understanding how CDPHE develops permit limits for surface water discharges can help facilities anticipate regulatory requirements, prepare for permit renewals, and proactively manage compliance risk. Because permit limits are developed using a combination of regulatory requirements, monitoring data, and site-specific conditions, understanding the process behind them can be just as important as understanding the limits themselves.
