5 Colo. Code Regs. § 1002-37.38
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
Pursuant to the requirements in the Basic Standards (at 31.7(3)), the commission reviewed the status of temporary modifications scheduled to expire before December 31, 2019 to determine whether the temporary modification should be modified, eliminated, or extended.
No action: The commission took no action on the temporary modifications on the following segments:
Lower Colorado Segment 4e: temporary modification of the copper standard (expires 12/31/2019). Tri-State Generation and Transmission Association, Inc. presented evidence that it is making progress on the temporary modification of the copper standard.
Site specific standards: The commission adopted site specific standards on the following segment:
Lower Colorado Segment 4e: Tri-State Generation and Transmission Association, Inc. (Tri-State) submitted sufficient data and justification to adequately characterize the highest attainable use in Segment 4e, and support adoption of site-specific ambient-based total recoverable iron standards for Segment 4e with two associated assessment locations. Based on this evidence, the commission determined that the highest attainable uses for Segment 4e are the existing classifications of Agriculture, Recreation N, and Aquatic Life Cold 2. The commission determined that there is no need to change the Aquatic Life Cold 2 use, which is appropriate for segments like Dry Creek that are not capable of sustaining a wide variety of cold water biota due to physical habitat, water flows or levels, or uncorrectable water quality conditions. To accurately represent the spatial and temporal variability in natural iron conditions, water quality data included were from samples taken at times when no discharge from the Rifle Station was occurring. These data demonstrated that natural sources within the Dry Creek watershed are solely driving elevated instream concentrations of total recoverable iron.
The commission adopted ambient-based site-specific iron standards of 3500 µg/L for the unnamed tributary (39.519572, -107.729424), and 5900 µg/Lforthe remaining portions of the segment including Dry Creek (39.523944, -107.73496) in accordance with 31.7(1)(b)(ii). Evidence submitted by Tri-State demonstrated that ambient iron concentrations are adequate to protect the highest attainable Aquatic Life use. Therefore, ambient iron concentrations provide a reasonable basis for site-specific standards, which protect the aquatic community, and the highest attainable uses for the segment.
Based on the evidence presented by Tri-State and in accordance with Section 31.3 of the Basic Standards and Methodologies for Surface Waters, the commission found that given the current discharge and environmental conditions, the ambient-based standards adopted in Segment 4e will not jeopardize downstream waters and that water quality classifications and standards of downstream waters will be attained and maintained.
The commission specified assessment locations for the two iron standards at section 37.6 to ensure that future assessment is consistent with the methods used to derive the standards.
The commission removed the temporary modification for iron of "current condition" that had previously been in place for Lower Colorado Segment 4e.
PARTIES TO THE RULEMAKING HEARING
5 CCR 1002-37.38