5 Colo. Code Regs. § 1002-32.32
The provisions of § 25-8-202(1)(a) and (b); § 25-8-204; and § 25-8-402 C.R.S. provide specific statutory authority for the amendments to this regulation adopted by the Commission. 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 Colorado Water Quality Control Commission established site-specific metals standards for Arequa Gulch, Segment 22a of the Upper Arkansas River Basin in 1998. In the September, 1998 Classification and Standards Rulemaking Hearing for the Arkansas River Basin, the Commission decided to continue the temporary modification of pH for Arequa Gulch of 5.5 to 9.0. The temporary modification was effective until November 30, 2000. This expiration date was later extended by the Commission to June 30, 2002. In the Statement of Basis, Specific Statutory Authority and Purpose, the Commission adopted this temporary modification "in order to provide time for additional and appropriate studies to be performed by CC&V to evaluate CC&V's claim that the 5.5 pH level in protective of the aquatic life use in Arequa Gulch".
Since the 1998 rulemaking, CC&V performed additional studies on Arequa Gulch including biological studies, a supplemental Use Attainability Analysis, and reviewed appropriate scientific literature. Based on the results of these studies and consideration of appropriate scientific literature, CC&V proposed to replace the existing temporary modification of 5.5 to 9.0 and underlying standard of 6.5 to 9.0 for pH in Arequa Gulch, and to add a site-specific standard of 5.5 to 9.0. Based on its review of the CC&V studies and scientific literature, the Division recommended an alternative proposal of removing the existing temporary modification and underlying standard, and adding a permanent site-specific standard of 6.0 to 9.0 with a new temporary modification of 5.6 to 9.0 that will be effective until December 31, 2007. This alternate proposal was accepted by CC&V. The Division's proposal for a new underlying standard of 6.0 to 9.0 was based upon the last 3 years of pH data because they have remained relatively constant over this period of time. The lower15th percentile of these data showed a value of 6.06 . Further, the Division's review of the scientific literature show that a pH of 6.0 to 9.0 would be protective of pH-sensitive species that would be expected to be found in Arequa Gulch. [Basic Standards, §31.7]. The Division's proposal for a temporary modification of 5.6 to 9.0 was based upon the lowest observed value in the last 3 years of data (5.6) and was intended to allow time to determine if the trend of improving pH will continue and thus resolve any uncertainty about the appropriateness of the underlying standard. [Basic Standards, §31.7(3)].
The Commission adopted the Division's alternative proposal and requested that CC&V continue its monitoring efforts during the term of the temporary modification. If, during this time the pH conditions in Arequa Gulch do not improve or show a declining trend, a rulemaking hearing to re-evaluate the temporary modification may be requested. CC&V committed to continue its efforts with regard to its operation of an experimental passive treatment system in Arequa Gulch at the upcoming Arkansas River basin-wide hearing in July of 2002.
PARTIES TO THE RULEMAKING HEARING
5 CCR 1002-32.32