5 Colo. Code Regs. § 1002-37.43
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, 2022 to determine whether the temporary modification should be modified, eliminated, or extended.
Lower Colorado River Segment 4e (COLCLC04e) and Segment 4f (COLCLC04f): The commission extended the existing "current conditions" temporary modifications for acute and chronic copper on Segment 4e (from 6/30/2021 to 6/30/2023) and adopted new "current conditions" temporary modifications for acute and chronic copper on downstream Segment 4f. The expiration date for the temporary modifications was set at 6/30/2023 to target resolution of the uncertainty that is the basis for the temporary modifications on segments 4e and 4f in the December 2022 Temporary Modifications Hearing.
For Segment 4e, Tri-State Generation and Transmission Association, Inc. (Tri-State) provided an update to the commission regarding progress being made in implementing the existing plan to resolve uncertainty (PTRU) and demonstrating the ongoing need for the temporary modifications for acute and chronic copper. Tri-State has continued to make progress to resolve uncertainty, including collection of data to enable calculation of site-specific copper standards using the Biotic Ligand Model (BLM), but to date has only collected 23 of the 24 samples required by division BLM guidance. Tri-State demonstrated that there continues to be a water quality-based effluent limit (WQBEL) compliance problem and instream non-attainment of copper standards on Segment 4e (Tri-State Exhibits 9 and 10). The commission determined that the temporary modifications continue to be justified, as additional time is needed to resolve the uncertainty regarding the appropriate copper standards and complete an alternatives analysis to determine the extent to which copper in the discharge can be controlled.
In August 2020, a final renewal permit was issued by the division for the Rifle Station and included updated WQBELs to reflect copper standards in the downstream Segment 4f. This application of Segment 4f standards in Tri-State's discharge permit has resulted in a new WQBEL compliance problem and the need for copper temporary modifications on Segment 4f. Instream copper data are not available for Segment 4f to assess attainment of the underlying standards at this time, so Tri-State's proposal was based on predicted instream non-attainment; however, Tri-State plans to collect instream copper data in Segment 4f beginning in January 2021 (Tri-State Exhibit 12) with the intent to collect 24 data points over two years to enable calculation of criteria-based site-specific copper standards using the BLM. Based on this information, the commission determined that acute and chronic temporary modifications for copper on Segment 4f are justified based on predicted instream non-attainment, uncertainty regarding the underlying standard, and demonstrated WQBEL compliance problems.
Tri-State supported the proposal for the extended and new temporary modifications on segments 4e and 4f, respectively, by committing to a new PTRU (Tri-State Exhibit 12). In addition to detailing steps to resolve the uncertainty in the copper standard, the updated PTRU includes completion of an alternatives analysis by 2022 to resolve the uncertainty regarding the extent that copper in Rifle Station's effluent can be controlled. In addition, the PTRU includes a flow study to determine the connectivity of segments 4e and 4f. Information collected as part of the PTRU will eliminate the need for the temporary modifications by 6/30/2023.
The operative value of the temporary modifications for Segment 4e is the narrative "current conditions." In the 2019 basin hearing, the commission established baseline values to characterize "status quo" in the effluent and instream in Segment 4e (37.40(K)). A typo in the period of record used to establish the effluent status quo was identified in the previous Statement of Basis and Purpose language at 37.40(K) and has been corrected. In future reviews of these temporary modifications, the commission will use the following values to compare to the most recent five years of data to determine if water quality is maintained and ensure that the existing uses are protected. These values are for use by the commission in future reviews of the temporary modification and are not intended to direct implementation of "current condition" temporary modifications in permits:
Use of the ambient standards assessment methodology to compare the baseline period water quality to current water quality (11/2015-9/2020) indicates that the lower confidence limit of the effluent water quality and the 85th and 95th percentiles of instream water quality is not currently higher than the baseline. Based on this information, the commission finds "status quo" is currently being preserved instream and in effluent.
The operative value of the temporary modifications for Segment 4f is also the narrative "current conditions". In future reviews of these temporary modifications, the commission will use the following value to compare to the most recent five years of representative effluent data to determine if effluent quality is maintained and ensure that the existing uses are protected. This value is for use by the commission in future reviews of the temporary modification and is not intended to direct implementation of "current condition" temporary modifications in permits:
While there are sufficient data to represent the maximum 30-day average potentially dissolved copper effluent concentration, water quality data are not yet available from which to characterize the current conditions with 85th and 95th percentiles instream in Segment 4f. Data to characterize the status quo of the waterbody will be collected as part of the PTRU and it is the commission's expectation that representative numeric values to represent instream status quo will be determined as soon as possible for the commission's use in future reviews of this temporary modification.
5 CCR 1002-37.43