Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-93 - REGULATION NO. 93 - COLORADO'S SECTION 303(D) LIST OF IMPAIRED WATERS AND MONITORING AND EVALUATION LIST
Section 5 CCR 1002-93.20 - STATEMENT OF BASIS, SPECIFIC STATUTORY AUTHORITY AND PURPOSE; MAY 2023 RULEMAKING; FINAL ACTION JUNE 24, 2021; EFFECTIVE DATE OF SEPTEMBER 14, 2023

The "Section 303(d) Listing Methodology - 2024 Listing Cycle" contains a description of the listing process and the criteria for listing. This Listing Methodology sets forth the criteria that generally were used to make decisions regarding which waters to include on the 2024 Section 303(d) List and the 2024 Monitoring & Evaluation List (M&E List). This document was adopted by the commission in March of 2022. This methodology was not adopted as a rule. The commission, therefore, has the flexibility to take into account other appropriate factors in making site-specific listing decisions.

The commission has considered all existing and readily available information in developing the 2024 Section 303(d) List. In determining whether data and information are existing and readily available, the commission has taken into account data and information that the division has utilized in the preparation of those identification processes, calculations, and models referenced in 40 CFR §130.7(b)(5)(i), (ii) and (iv) and that credible data and information presented in a readily usable format and provided to the division as referenced in 40 CFR §130.7(b)(5)(iii). In addition, the commission accepted credible data and information that was submitted in accordance with the listing process schedule. The division continues to collect and analyze new data on a rotating basin basis and will utilize such data and information in making future listing determinations. Existing data that were not brought forward through one of the above mechanisms or otherwise presented to the commission in accordance with the schedule were not treated as "readily available" for purposes of making the 2024 listing decisions. Such information will be considered in the next listing cycle.

This Regulation #93 hearing cycle focuses on the Arkansas River and Rio Grande Basins (Regulations #32 and #36) and the Upper and Lower Colorado River Basins (Regulations #33 and #37). This approach follows the rotating basin structure described in the 2024 303(d) Listing Methodology.

Segment COUCEA09c (Mainstem of the Eagle River from a point immediately below the confluence with Rube Creek to the confluence with the Colorado River) was placed on the 303(d) List due to exceedances of the aquatic life use-based cold stream tier II temperature standards. Since the data record for this segment had 4-6 years of data, an allowance of one warming event in one season was permitted. Regulation 31.16, Table 1, Footnote 5(c)(ii). Even with the allowance of one warming event in 2018, temperatures exceeded standards again in 2021, so the commission determined that placing this segment on the 303(d) List for temperature was warranted.

The commission determined that the division's application of the warming event allowance for 303(d) temperature assessments is consistent with Regulation #31 and the framework outlined in the 303(d) Listing Methodology. Furthermore, the commission determined that the warming event methodology applied to the 2024 temperature assessments is not a new interpretation and that no changes have been made to the warming event language defined in Regulation #31 and the 303(d) Listing Methodology since these documents were revised by the commission in 2016 and 2018.

The commission considers an exceedance of the temperature standard as any DM/WAT above the temperature standard (chronic and acute). In addition, the commission considers the warming event to be the methodology for applying the 1-in-3-year exceedance frequency, by considering the extent of the allowable exceedances above the standard applied in units of degree-days. The recurrence frequency of these warming events is limited to once every 3 years.

The commission concluded that segment COSPUS16c (All tributaries to the South Platte River, including all wetlands, from the outlet of Chatfield Reservoir, to a point immediately below the confluence with Big Dry Creek) includes the tributaries to Second Creek and that the 303(d) listings for selenium and E. coli on these tributaries should be retained. The City and County of Denver suggested that the division's interpretation of the segmentation for segment COSPUS16c was incorrect and that the tributaries to Second Creek should instead be included in the mainstem segments for these waterbodies (COSPUS16d). However, because description of segment 16d in Regulation #38 does not have the language 'including all tributaries' in the description, the commission agreed that the division was required to implement the description of segment 16d as it was adopted in Regulation #38, without including the tributaries to Second Creek. The commission encouraged the City and County of Denver to work with the division on evaluating the segment descriptions for Second Creek as a part of the next Regulation #38 rulemaking hearing. The commission also encouraged the City and County of Denver to address concerns regarding the 303(d) listings for selenium and E. coli on the tributaries to Second Creek by providing evidence of standards attainment or other supporting information as a part of the next Regulation #93 rulemaking hearing that focuses on the South Platte River Basin.

The division had originally proposed to add segment COUCBL14 to the M&E List for sulfate based on aggregation of data from West Tenmile Creek and data from the bottom of the mainstem of Tenmile Creek. Given the variability in sulfate data in these two portions of the segment, the division reassessed segment COUCBL14 and calculated the water supply standard for sulfate to be the table value standard of 250 mg/L for West Tenmile Creek (and 337 mg/L for the bottom of the mainstem of Tenmile Creek. Segment COUCBL14 attained for sulfate when these values were used, and the data from these two portions were not aggregated. The commission did not include segment COUCBL14 on the M&E List for sulfate.

In establishing water quality concentrations that represent conditions from the year 2000, the division utilizes a historical database that is maintained and updated on a regular basis. Assessment and permitting actions utilize the best available data at the time of the action and are re-evaluated when water quality assessments are being conducted, resulting in potentially different values. Historical data may be aggregated at different geographic scales for different program purposes. However, while the numbers resulting for each purpose may be different, the historical database and the same general processes are used. The commission recognizes that assessment values for the secondary water supply standards that were utilized in this hearing for COUCBL14 may differ from values used in permitting and/or other contexts.