5 Colo. Code Regs. § 1002-72.29
The provisions of 25-8-202(1)(c) and (2) and 25-8-205, C.R.S. provide the specific statutory authority for adoption of these regulatory amendments. The Commission also adopted, in compliance with 24-4-103(4) C.R.S., the following statement of basis and purpose.
BASIS AND PURPOSE
As a part of the triennial review outlined in section 72.10 of this regulation, this statement provides a narrow list of changes to section 72.7 and applicable definitions in 72.2 that are focused on stormwater management for Municipal Separate Storm Sewer System (MS4) permitted areas. This rulemaking is an effort to continue building on the previous rulemaking as the knowledge and experience gained in MS4 stormwater management has progressed, specifically as it relates to construction and post-construction. A further review of the remaining sections in this control regulation will be completed at a future rulemaking.
MS4 permits are issued as a part of the Colorado Discharge Permit System (CDPS), as directed by the Colorado Water Quality Control Act and WQCC Regulation 61, and incorporate applicable requirements established in this regulation. Since the last revision of section 72.7 of this regulation, MS4 permits have advanced beyond the previous control regulation requirements. The revisions to this control regulation promote modernization to the construction and post-construction sections and alignment with the MS4 permitting structure while, at a minimum, maintain water quality protections and, in some instances, improve water quality. The major substantive revisions include changes to better align with current versions of stormwater MS4 permits, removal of prescriptive lists of Best Management Practices (BMPs), and simplification of the development/redevelopment tier structures. The changes resulted from a stakeholder process that included MS4 permittees that are members of the Cherry Creek Basin Water Quality Authority, the Cherry Creek Basin Water Quality Authority, the division, and Colorado Parks and Wildlife.
72.2 Definitions
"Disturbed area," "individual home construction," and Water Quality Capture Volume" are terms used only within Section 72.7 . These terms were removed from the broader definitions section in section 72.2 and were replaced within section 72.7 where the terms are used. "Regulated Stormwater" was refined to clarify that the term applies to stormwater discharges to state waters that are required to be permitted under section 61.3 of Regulation 61. "Enhanced BMP" was deleted as the term is not used within the regulation with the exception of the definition. Reference to Denver Regional Council of Governments (DRCOG) was deleted due to the obsolescence of DRCOG and the Metro Vision Plan as entities currently involved in regional water quality matters.
Section 72.7 Stormwater Permit Requirements
The commission adopted changes to the construction and post-construction sections to allow for better alignment with current versions of stormwater MS4 permits, which have advanced beyond the previous requirements in Section 72.7 . This modernization decreases unintended conflicts between MS4 permits and Regulation 72.7 requirements, which then decreases the administrative burden for local governments and improves clarity for consultants and contractors working in the basin while protecting water quality. Regulation 72.7 establishes the minimum requirements for stormwater management in the basin that are then implemented and enforced through MS4 permits. The MS4 permits provide more detailed information related to implementation of these requirements. Additionally, MS4 permittees in the basin rely on more detailed engineering design criteria in local storm drainage criteria manuals, typically building on the Urban Storm Drainage Criteria Manual, Volume 3, published by the Mile High Flood District. Volume 3 is updated periodically to improve design criteria and methods for stormwater quality control measures based on advances in the engineering profession. This evolution of practice is a key reason that the 2022 update to Regulation 72.7 shifts to performance standards for stormwater control measures rather than prescriptive lists of control measures that can become outdated relative to advances in engineering practice.
The revised language in section 72.7 also creates consistency between this regulation and the various MS4 permits. Several new definitions were included to correspond with terminology and concepts in the various MS4 permits, including "Design Standard," which enables a reference to the full range of performance-based standards included in the MS4 permits. This revision provides clarity for options allowed in MS4 permits related to the Water Quality Capture Volume, runoff reduction standard, pollutant reduction standard, regional facilities and control measure, and others. Since 2016, MS4 permit renewals in the basin have been more stringent than permits in place than when section 72. 7 was last updated, particularly including "clear, specific and measurable" permit requirements and performance standards. Because four different MS4 permits authorize stormwater discharges in the basin and language among these permits varies slightly, design standards for each permit are not explicitly repeated in Regulation 72 itself. Due to the anti-backsliding provision of Section 61.10 of Regulation 61, requirements in future MS4 permits in the basin are required to be as stringent as current permits. Additionally, general reference in Regulation 72.7 to design standards allows technical innovation and advancement in stormwater management under these permit design standards without additional iterations to Regulation 72.7 language.
Several additional definitions and edits to existing definitions were added to section 72.7 to improve clarity in the regulation. Where appropriate, term definitions are consistent with definitions in MS4 permits. In a few cases such as Applicable MS4 Permit and Stream Restoration, term definitions were added at the request of parties to the rulemaking to further improve clarity.
Previously, Section 72.7 included prescriptive lists of BMPs for the construction and post-construction requirements. The revisions move from prescriptive BMP lists, which in many instances are out-of-date, to performance and process-based control measure requirements using modern terminology developed from years of research on control measure effectiveness and pollutant removal standards. This is a protective approach for stormwater quality that also allows for flexibility of control measure selection and implementation based on site-specific conditions. Under the 2022 revision, there is a greater emphasis on runoff reduction and green infrastructure, which broaden the regulation's previous emphasis on "concentration-based" pollutant reduction to recognize the benefit of volume reduction as an important component of reducing phosphorus pollutant loads, which is the focus of this control regulation. Volume reduction occurs through infiltration, evaporation and evapotranspiration processes present in stormwater control measures typically referred to as green infrastructure practices. Documentation explaining how several different approaches for meeting post-construction control measure design standards are protective of water quality is provided in fact sheets accompanying MS4 permits in the basin. For example, treatment of 100% of the Water Quality Capture Volume using an extended detention basin reduces pollutant loads primarily by decreasing pollutant concentrations through sedimentation; however, runoff reduction practices reduce stormwater pollutant loads by reducing both volume and pollutant concentrations. Thus, runoff reduction practices that reduce the volume component of the pollutant load by 60% can be as or more effective than practices that treat 100% of the Water Quality Capture Volume but typically reduce concentrations for pollutants such as phosphorus by less than 60%. Additional discussion and technical support of these concepts can be found in the Mile High Flood District's Urban Storm Drainage Criteria Manual, Volume 3.
For construction-phase stormwater requirements, the level of water quality protection for stormwater runoff from construction sites expected under Regulation 72 is maintained with the 2022 revisions to section 72.7 . Additionally, outdated, prescriptive lists of specific control measures that are redundant and/or inconsistent with MS4 construction-phase requirements and/or the statewide Construction General Permit have been removed.
Simplification of the Tier 2 and 3 development/redevelopment categories to better align with the MS4 permit coverage threshold is mainly an administrative change that simplifies administration of this regulation by including sites with 5,000 square feet or more of added impervious area but less that one acre of disturbed area and not part of a larger common plan of development or sale that disturbs one acre or more, as Tier 2, rather than Tier 3. (Note: Tier 3 is based on area of disturbance, whereas the Tier 2 threshold is based on added impervious area.) Analysis of development land use review submittals to the Authority showed that the land area associated with the proposed tier change for these development/redevelopment projects during 2019-2021 totaled only 0.11% of the land area for 730 land use reviews conducted by the Authority. Given that Tier 2 sites must still implement post-construction measures that include minimum performance standards and encourage green infrastructure practices, the administrative benefits of simplifying the tiers to align with the current design standards found in the MS4 permit outweigh the de minimis change in treatment requirements for a very small land area that shifts from Tier 3 to Tier 2 classification under this change.
Other changes in the regulation related to exclusions and exemptions increase consistency with MS4 permits and include reorganizing the location of these exclusions and exemptions in section 72.7 to improve clarity. For example, inclusion of the "R-factor waiver" for small construction sites requires approval by the division based on supporting documentation that the construction activity does not have potential to contribute pollutants to State waters. This waiver is currently included in MS4 permits and the statewide Construction General Permit.
Stream restoration has been added as an exclusion because properly designed stream restoration inherently improves stream conditions and pollutant loading; therefore, it does not require additional treatment by a stormwater control measure. Construction-phase control measures are still required for stream restoration projects.
5 CCR 1002-72.29