5 Colo. Code Regs. § 1002-38.19
The provisions of 25 8 202(1)(b) and (2); and 25 8 204 C.R.S. provide the specific statutory authority for adding the numeric standards adopted by the Commission.
The standard of 2.7 mg/liter NO2 N is based upon an equation derived from published studies and a bioassay conducted by the Division on fathead minnows, representing the predominant family in these segments. The bioassay and the studies indicate that as the concentration of chloride increases, the toxicity of nitrite decreases. As a result of the bioassay the Division calculated a 96 hour LC 50 of 40.6 mg/l, based on the annual average chloride of 20 mg/l in Segments 11 and 12. The Division then determined the nitrite standard by dividing the calculated LC 50 value by an acute chronic ratio. A literature review of nitrite toxicity to fish, submitted to the Division by the City of Fort Collins, suggests that the acute chronic ratio is between 5 and 10 for salmonids and channel catfish. Based on this information, the Division determined that an acute chronic ratio of 15 for fathead minnows could be used to calculate the stream standard and still provide an adequate margin of safety for the aquatic life in the stream.
Based on these factors the equation is:
Nitrite Standard = (1.99 x 20 mg/l (Chloride Conc.) + 0.7258) 15 (acute chronic ratio)
This standard is determined to be protective of a balanced aquatic life population found in these segments in recognition of the cold water/warm water transitional characteristics of Segment 11 and the relative paucity of toxicity data on certain species found in the segment.
FISCAL IMPACT STATEMENT SEGMENTS 11 AND 12, CACHE LA POUDRE RIVER
The primary fiscal impact of these regulations can be summarized as a potential for decreased treatment costs to the rate payers associated with the Cities of Fort Collins, Windsor, and Greeley. Additionally, the treatment costs related to the Kodak wastewater discharge may be decreased.
These potentials for decreased costs are associated with no expected decrease in the quality of aquatic life protection in these segments, as recent scientific findings have indicated that the species present will tolerate higher levels of nitrite concentrations. Thus, no impact upon beneficial uses is expected.
5 CCR 1002-38.19