5 Colo. Code Regs. § 1002-38.69
The provisions of C.R S. 25-8-202(1)(a), (b) and (2); 25-8-203; 25-8-204; and 25-8-402; 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
The Commission revised the site-specific phosphorus standard and changed the chlorophyll goal to a standard for Chatfield Reservoir (Upper South Platte segment 6b) and revised the Chatfield Reservoir Control Regulation (Regulation No 73) to be consistent with these revised standards.
Current Review: The Commission directed the Division to undertake a technical review of the scientific basis for the Chatfield Reservoir phosphorus standard for the following reasons:
The technical review showed:
Revised Water Quality Standards for Chatfield Reservoir: With the benefit of the lengthy historical record now available, the Commission believes it is appropriate to set chlorophyll and phosphorus standards consistent with the trophic condition that has been maintained. The Commission adopted a chlorophyll standard of 10 µg/l and a phosphorus standard of 0.030 mg/L to preserve the intended trophic condition and protect uses. Each standard is to be attained in four of five years.
Because the phosphorus and chlorophyll standards are defined as seasonal averages, some additional guidance is required concerning timing and location of samples to be used in calculating the average. Samples are to be collected at a site near the dam and should be representative of conditions in the mixed layer. Past monitoring has resulted in 6 samples during the summer months (July, August, and September); it is anticipated that the same level of effort will be applied in the future. For assessment, the average (arithmetic mean) is calculated for the summer samples in each year.
Development of Assessment Thresholds: For Chatfield Reservoir, a distinction is made between the standard and an assessment threshold. The assessment threshold is designed to address the concern about the risk of incorrectly counting an exceedance when a high summer value is the result of natural variability, but does not indicate a substantive change in trophic condition. The approach is justified by the special nature of the pollutants (chlorophyll and phosphorus are not toxic) and the site-specific nature of the concern about false exceedances. Another reason for establishing an assessment threshold that is different than the standard is that the site-specific standard is derived from historical data, which creates the expectation that a number of exceedances will occur. Natural variability, especially for chlorophyll, is sufficient to produce much more uncertainty in the assessed value than in the standard, which was derived from the set of all summer averages. The Commission is establishing assessment thresholds for Chatfield Reservoir nutrient standards based on this unique combination of circumstances and does not intend this action to be a precedent for other standards and/or other segments. "Assessment thresholds" were developed by calculating the standard error of each summer average. A regression of the upper confidence limit on the average provides an equation that can be used to specify the upper confidence limit (90%) for any particular concentration (e.g., the standard). Assessment thresholds were added in section 38.6 ,(4) with a reference in the standards table "qualifier" column." The resulting assessment thresholds were chlorophyll = 11.2 µg/l, summer average, 1 in 5 year allowable exceedance frequency and phosphorus = 0.035 mg/l, summer average, 1 in 5 year allowable exceedance frequency.
At the same time that this change was adopted in Regulation No. 38, the Commission adopted changes in the Control Regulation for Chatfield Reservoir (Regulation No. 73) that are consistent with the revised standard.
PARTIES TO THE RULEMAKING
5 CCR 1002-38.69