Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-38 - REGULATION NO. 38 - CLASSIFICATIONS AND NUMERIC STANDARDS SOUTH PLATTE RIVER BASIN LARAMIE RIVER BASIN REPUBLICAN RIVER BASIN SMOKY HILL RIVER BASIN
Section 5 CCR 1002-38.72 - STATEMENT OF BASIS, SPECIFIC STATUTORY AUTHORITY AND PURPOSE: MARCH 10, 2009 RULEMAKING REGARDING CHERRY CREEK RESERVOIR; FINAL ACTION AUGUST 10, 2009; EFFECTIVE DATE JANUARY 1, 2010

Current through Register Vol. 47, No. 5, March 10, 2024

The provisions of C.R S. 25-8-202(1) (b), (c) and (2); 25-8-204; 25-8-205 and 25-8-402; 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.

At the same time that these changes were adopted in Regulation #38, the Commission adopted consistent changes in Regulation #72, Cherry Creek Reservoir Control Regulation (5 CCR 1002-72).

BASIS AND PURPOSE

The classified uses for the Cherry Creek Reservoir (Reservoir) include warm water aquatic life class 1, recreation class E (formerly 1a), water supply, and agriculture. The Reservoir is mildly eutrophic and has limited releases given the primary role of Cherry Creek Dam as a flood control structure. As a result of the data and analyses brought forward as part of the March 2009 Rulemaking Hearing, the Commission adopted revisions to the water quality standard for chlorophyll a in the Reservoir. Specifically, the Commission has changed the seasonal chlorophyll a standard from15 µg/l to18 µg/l to be attained four out of five years. The Commission also adopted the "class E" recreation classification to replace the previous "class 1a" label, and replaced the fecal coliform numeric standard with an E. coli numeric standard of 126/100ml to be consistent with Regulation #31 (5 CCR 1007-31). In conjunction with the adoption of a revised chlorophyll a standard, the Commission also adopted a number of changes to Regulation #72 (5 CCR 1002-72).

Background and Overview

During the September 2000 Rulemaking Hearing, the Commission repealed the prior total phosphorus water quality standards for the Reservoir and adopted a chlorophyll a standard of 15 µg/l (previously a "goal"), to be measured in the upper three meters of the water column during July through September. At that time, the Commission recognized that further data was needed to establish a scientifically appropriate chlorophyll a standard. To this end, the Commission directed the Cherry Creek Basin Water Quality Authority (Authority), with oversight by the Water Quality Control Division (Division), to conduct a number of special studies.

In 2000, the Commission also retained a Total Maximum Annual Load (TMAL) of 14,270 pounds of total phosphorus to the Reservoir provided in Regulation #72, with a consideration that this be a "phased TMAL" while the Authority completed the requested studies. An in-lake phosphorus goal, which was a July through September seasonal average, was set at 40 µg/l based on the Division's 90% confidence level that this goal would result in the attainment of the chlorophyll a standard.

The Commission further recognized in the 2000 Rulemaking Hearing the uncertain relationship between chlorophyll a and total phosphorus, such that the correlation resulting from that Rulemaking Hearing could change based on the analysis of the additional data obtained by the identified studies. The Commission's recognition of that uncertainty and the data modeling information obtained since that time provide the basis for adjusting the chlorophyll a standard in this rulemaking.

Chlorophyll a Standard

Between 1999 and 2008, the Authority gathered data to expand the data set used by the Commission to reach its decisions in the September 2000 Rulemaking. The Authority used this data and previously collected data in its modeling efforts to evaluate whether a chlorophyll a standard of 15 µg/l could feasibly be attained nine out of ten years. The Authority concluded that such a standard could not be attained and set forth to identify a feasibility-based chlorophyll a standard.

The Current Standard is not Attainable. Based on the evidence presented in the prehearing filings and at the March 2009 rulemaking hearing, the Commission agrees that a chlorophyll a standard of 15 µg/l cannot be feasibly attained nine out of ten years.

The Commission reaches this conclusion, based on the evidence presented throughout this process, for three reasons. First, the current chlorophyll a standard was an admitted compromise between parties with competing interests, where scarce Reservoir data existed to support the parties' respective positions. Second, to achieve a chlorophyll a standard of 15 µg/l, the Reservoir's long-term seasonal (July to September) mean chlorophyll a concentration would need to be less than10 µg/l, a value not observed within the Reservoir since 1991. The only way to achieve a long-term average summer chlorophyll a concentration at that level would be to require the reduction of flow-weighted total phosphorus concentrations into the Reservoir by more than 30percent beyond the lowest value observed entering the Reservoir. Finally, even the background concentrations are substantially above the concentration needed to achieve a chlorophyll a concentration of 15 µg/l.

Current Water Quality Protection Efforts. The evidence indicates that although the Cherry Creek Basin has experienced unprecedented growth during the past 20 years, the Authority and its partners have succeeded in implementing nutrient controls to help maintain the Reservoir's water quality. The Commission acknowledges that the Authority, its member agencies, and partners have improved wastewater treatment and have installed best available technology, installed nonpoint source controls, and utilized its land use agency responsibilities to control phosphorus in the watershed and inflow to the Reservoir.

The evidence also indicates that the Authority considered additional watershed management practices that it could implement in the future. By reviewing the outcome of the Authority's analyses presented during this rulemaking process, the Commission concludes that the additional practices identified to date are not feasible at this time. Some practices would be exorbitantly expensive, and it is unclear when and to what extent additional nutrient reductions might be realized within the Reservoir. Nutrient reductions depend in part on future development and current economic conditions create uncertainty regarding the pace and scope of future development. In addition, access and liability-based legal issues may preclude the Authority from implementing certain future practices.

The Commission also acknowledges that the Authority and its member agencies are committed to continuing watershed improvements, understanding that the watershed conditions are expected to improve with time and effort. While the Commission acknowledges that the additional future watershed management practices considered by the Authority are infeasible at this time, the Commission expects current water quality management strategies to continue and, as necessary, become more aggressive over time to attain water quality objectives and protect the uses of the Reservoir.

Science and Policy Support the Commission's Adoption of a 18 µg/l Chlorophyll a Standard. Based on the evidence advanced by the parties to this rulemaking, the Commission concludes that the appropriate assessment period by which to measure attainment with the standard should be five years, with attainment expected in four out of five years. The Commission understands that an assessment period of five years will allow the Division and the Authority to respond more promptly to attainment issues, such that water quality can be managed more effectively. Moreover, the Commission acknowledges that this adjusted assessment period would also preclude longer periods of time during which the standard could be exceeded, which could have a greater adverse impact on the Reservoir's water quality.

Recognizing that the 15 µg/l standard is not attainable, the Commission has chosen to set a standard that provides protection of reservoir uses to the maximum degree practical, recognizing present uncertainty as to the specific chlorophyll a level that will prove to be attainable over time. The Commission has determined as a matter of policy that it would be premature to set a chlorophyll a standard based solely on the assumption that no additional improvement is feasible. The Commission believes that it is important to retain the goal of full protection of the Reservoir's uses.

The adopted standard (18 µg/L) was developed from a prediction of the "most likely" chlorophyll a concentration and a measured variability component. Prediction of the most likely chlorophyll a concentration (16.2 µg/L) was based on an equation, created through the reservoir modeling effort, relating chlorophyll to input, flow-weighted phosphorus concentration (0.177 mg/L). The most likely chlorophyll concentration represents a long-term mean, which is not the appropriate value for a standard to be attained 80% of the time (in four of five years). The 80th percentile value is calculated from the long-term mean and a measured variability component (standard deviation of 2.3 µg/L). The standard deviation used in this calculation is smaller than the one proposed by the CCBWQA, which had argued that it should be based on an 8-year record of "existing conditions." However, the Commission chose to base the standard on the most recent five years of data as a matter of policy, to better reflect optimism about what can be achieved for this reservoir. Because chlorophyll concentrations have been less variable over the last five years than over the last eight years, a smaller standard deviation is employed in development of the appropriate standard.

Future Watershed Practices

Given the Authority's duty to continue its water quality control management strategies, the Commission's adoption of a 18 µg/l chlorophyll a standard will not result in a reduction or change in the Authority's commitment to controlling chlorophyll a in the Reservoir. The Commission's adoption of revisions to Regulation #72 (5 CCR 1002-72) reflect the Authority's commitment to maintain current water quality management strategies and, as necessary, become more aggressive over time to attain water quality objectives and protect the uses of the Reservoir.

In accordance with statutory requirements, both the control regulation and the underlying standards will be revisited as efforts are implemented over time, such that more information is developed regarding influences on, and the attainability of, identified levels of Reservoir water quality.

PARTIES TO THE RULEMAKING