5 Colo. Code Regs. § 1002-37.47
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
In April 2013 (37.31) and subsequent rulemaking hearings (37.33, 37.37, and 37.40), the commission has adopted and extended temporary modifications for arsenic of "As(ch)=hybrid" (expiration date of 12/31/24) on many segments with the 0.02 µg/L Water + Fish numeric arsenic standard. The arsenic temporary modification recognizes existing and predicted compliance issues, instream nonattainment, and the uncertainty regarding the water quality standard necessary to protect current and/or future uses and the extent to which ambient concentrations of arsenic are natural or irreversible (31.7(3)). The division submitted a plan to resolve uncertainty in the 2019 Temporary Modifications rulemaking (37.41(B)).
The division plans to propose revised standards for arsenic as soon as possible following updated toxicological information from EPA's Integrated Risk Information System (IRIS) and completion of ongoing studies to better understand arsenic conditions in Colorado. Furthermore, per the conditions of the revised and extended temporary modification at 37.6(2)(c) (effective 6/30/2020 and expires 12/31/2024), and based on the widespread need to make progress to understand sources of arsenic and set forth processes for lowering arsenic in discharges, additional permit Terms and Conditions (T&Cs) are being implemented for facilities benefitting from the "current condition" temporary modification. These T&Cs may include requirements for additional monitoring, source identification, and characterization of source control and treatment options for reducing arsenic concentrations in effluent. The commission recognizes the need to resolve the uncertainty in the arsenic standards and ensure that human health is adequately protected.
The commission identified segments where an arsenic temporary modification had previously been inadvertently omitted. The commission adopted arsenic temporary modifications on the following segments:
Lower Yampa River/Green River: 19a (COLCLY19a) and 27 (COLCLY27)
White River: 3 (COLCWH03), 6 (COLCWH06), and 11 (COLCWH11)
Lower Colorado River: 20 (COLCLC20)
To remain consistent with the commission's decisions regarding arsenic in section 37.31, all existing temporary modifications for arsenic of "As(ch)=hybrid" (expiration date of 12/31/24), with the exception of those listed below, were retained.
Where evidence indicated the requirements to qualify for a temporary modification were not met, temporary modifications were deleted. The commission deleted chronic arsenic temporary modifications (expiring 12/31/2024) on several segments due to a lack of evidence of a demonstrated or predicted water quality-based effluent limit compliance problem for these segments. These segments have all been designated as Outstanding Waters, have no CDPS permitted dischargers with WQBELs for arsenic, and are without dischargers on upstream segments who may receive WQBELs based on protection of downstream uses. Temporary modifications for arsenic were deleted from the following segments:
Lower Yampa River/Green River: 12c (COLCLY12c)
White River: 4b (COLCWH04b)
5 CCR 1002-37.47