5 Colo. Code Regs. § 1002-37.37
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, 2018, to determine whether the temporary modifications should be modified, eliminated or extended.
Lower Colorado Segment 4e: temporary modifications of the copper (expire 6/30/2017) and iron (expirel2/31/2017) standards on Lower Colorado Segment 4e were reviewed. Tri-State Generation and Transmission Association, Inc. presented evidence that it is making progress on the plan for eliminating the need for need for the temporary modifications.
IRON
The commission considered the temporary modification for iron on Lower Colorado Segment 4e, Dry Creekand all tributaries upstream of the Last Chance Ditch. Tri-State Generation and Transmission Association, Inc., originally proposed ambient-based site-specific standards for iron in this rulemaking. Preliminary data shows an ambient standard for iron is likely appropriate because in-stream conditions in minimally impacted sites exceed the table value standards (TVS) for chronic iron. The data also indicate that natural or irreversible sources of ambient iron are present within the upper reaches of Dry Creek, which are driving the instream concentrations of total recoverable iron detected at sample locations downstream of the Tri-State Rifle Station. However, due to challenges in obtaining data because of the limited/intermittent flow in Segment 4e, the dataset for this segment is small. More data is needed to accurately characterize the ambient-condition in this segment. Therefore, the commission extended the iron temporary modification on Lower Colorado Segment 4e until December 31, 2018, to allow Tri-State an additional year to collect data to calculate appropriate ambient-based iron standards.
COPPER
The commission considered the temporary modification for copper for Lower Colorado Segment 4e. Tri-State proposed extending the temporary modification. Tri-State submitted evidence that it has been collecting data to determine whether copper standards may be based on the Biotic Ligand Model (BLM).
The division recommends that a minimum of 24 samples be collected over a two-year period in order to fully capture seasonality before implementation of any BLM-based site-specific standard. The temporary modification for copper was set to expire June 30, 2017, with the assumption that since water quality data collection began in 2015, this would provide sufficient time for data to be collected prior to expiration of the temporary modification. However, due to the extremely intermittent nature of the discharge and an ephemeral stream which flows only in response to precipitation or discharge events, it has been challenging to develop a database containing a sufficient number of samples. Based on the limited number of samples available at this time, the commission determined that additional time was necessary and extended the temporary modification expiration to December 31, 2019. The extended timeframe will allow additional collection of samples in this difficult environment of limited flows. The expiration date is coordinated with the June 2019 basin hearing.
ANTIDEGRADATION
The commission reviewed the antidegradation designation for Segment 4e. Based on available water quality data that meet the requirements of Section 31.8 , the commission determined that Segment 4e should retain the Use Protected designation.
SUMMARY
An extension of the iron temporary modification was adopted of December 31, 2018, in order to collect additional data to develop appropriate ambient-based iron standards. An extension of the copper temporary modification was adopted of December 31, 2019, in order to continue building the existing database. The commission retained the Use Protected designation based on Section 31.8(2)(b)(i)(B).
New Temporary Modifications of the Arsenic Standard:
Consistent with the actions taken in 2013, the commission adopted a temporary modification of the arsenic standard on segments on the following list, with an expiration date of 12/31/2021. At the April 8, 2013 rulemaking, the commission heard testimony that concurred with the finding from a December 13, 2011 hearing that an initial reasonable lower limit of treatment technology for arsenic is 3.0 µg/L, pending further investigation by the division, dischargers and stakeholders. The temporary modification was established by the commission to allow for a temporarily less stringent application of the chronic arsenic standard in control requirements for both existing discharges and new or increased discharges.
Lower Yampa Segment 9
Lower Yampa Segment 12a
Lower Yampa Segment 12b
Lower Yampa Segment 12c
Lower Yampa Segment 15
White River Segment 4b
White River Segment 14a
White River Segment 20
Lower Colorado Segment 17b
PARTIES TO THE RULEMAKING HEARING
5 CCR 1002-37.37