5 Colo. Code Regs. § 1002-36.50

Current through Register Vol. 47, No. 20, October 25, 2024

Section 5 CCR 1002-36.50 - STATEMENT OF BASIS, SPECIFIC STATUTORY AUTHORITY AND PURPOSE; OCTOBER 10, 2023 RULEMAKING; FINAL ACTION OCTOBER 10, 2023; EFFECTIVE DATE DECEMBER 31, 2023

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:

A. Site-specific Standards

Rio Grande segments 4a and 7 (CORGRG04a and CORGRG07): The commission continued the application of the current ambient quality-based site-specific standards (which are based on irreversibility) through December 31, 2028; delayed the effective dates of the Tier 1 and Tier 2 feasibility-based site-specific standards by five years; and replaced two Tier 1 feasibility-based site-specific standards on Segment 4a with the table value standards (TVS). The site-specific standards, assessment locations, and effective dates are included in Section 36.6(4)(b)-(c).

BACKGROUND

In the 2013 Temporary Modifications Hearing, the commission adopted the proposal of Rio Grande Silver, Inc. (RGS) to add two tiers of feasibility-based site-specific standards to segments 4a and 7 based on the feasibility of reversing historical sources of metals. See Section 36.35. These tiered feasibility-based site-specific standards would have delayed-effective dates and were based on improvements in water quality tied to future reopening of the Bulldog Mine. The Tier 1 standards represented predicted improvements in water quality in the Rio Grande and Willow Creek mainstem due to discharge of treated water from the Bulldog Mine during dewatering of the lower mine pool, and once effective the Tier 1 standards would be in place for two years. The Tier 2 standards reflected further water quality improvements predicted by a 90% reduction in flow and metals load from the Nelson Tunnel, after RGS had drawn down water levels in the Bulldog Mine to support operations, and would be pumping at a lower rate.

In the 2018 Rio Grande Basin Hearing, the commission replaced temporary modifications for multiple metals on segments 4a and 7 with interim ambient-quality based site-specific standards (based on irreversibility), which were to apply until the effective dates of the feasibility-based site-specific standards. See Section 36.42(K). Existing water quality did not attain TVS for several metals, but represented the highest attainable condition for the near future because no improvement is feasible in the absence of Nelson Tunnel remediation or the potential restart of the Bulldog Mine. Therefore, interim ambient-quality based site-specific standards were calculated using water quality data from 2012-2017. The commission also updated the Tier 1 and Tier 2 feasibility-based site-specific standards that are based on improvements in water quality contingent on the reopening of the Bulldog Mine, dewatering of the lower mine workings, and construction of a water treatment plant to treat this water.

REVISIONS TO SITE-SPECIFIC STANDARDS

In this 2023 Hearing, the commission adopted the proposal of RGS to:

a) replace the Segment 4a Tier 1 feasibility-based site-specific standards for low flow chronic cadmium and low flow chronic manganese with the underlying TVS;

b) delay the effective dates of the tiered feasibility-based site-specific standards by five years; and

c) extend the effective dates of the interim ambient-quality based site-specific standards by five years:

a) Changes to Segment 4a Tier 1 feasibility-based site-specific standards: Based on a review of recent water quality data, treatment feasibility investigations, modeling assumptions, and the commission's 2019 changes to the cadmium table value standards (Section 36.45), which result in slightly less stringent chronic cadmium standard at the water hardness in Segment 4a, water quality during Tier 1 is now anticipated to result in attainment of the numeric TVS applicable to

Segment 4a for chronic cadmium and chronic manganese during low flow conditions. The commission replaced the Segment 4a Tier 1 low flow chronic cadmium feasibility-based site-specific standard with the TVS, which the commission revised statewide in 2019 based on updated scientific information about the protection of aquatic life. See Regulation 31, Section 31.57. The commission replaced the Segment 4a Tier 1 low flow chronic manganese feasibility-based site-specific standard with the Water Supply (WS) standard based on the treatability study demonstrating this standard was feasible to meet. No other changes were made to the tiered feasibility-based site-specific standards.

b) Delay of feasibility-based site-specific standards on segments 4a and 7: Delaying the effective dates of the remaining Tier 1 and Tier 2 feasibility-based site-specific standards is appropriate because the underlying assumptions of these standards (i.e., dewatering of the lower mine workings, construction of a mine water treatment plant, and reopening of the Bulldog Mine) will not occur for several more years. RGS is actively exploring the potential for reopening of the Bulldog Mine, and additional time is needed to continue exploring the viability of the resource (including dewatering of the upper mine pool). The commission delayed the effective dates of the tiered standards by five years: the remaining Tier 1 feasibility-based site-specific standards will be effective from January 1, 2029, to December 31, 2030, and the Tier 2 feasibility-based site-specific standards will be effective starting January 1, 2031.

c) Extension of ambient quality-based site-specific standards on segments 4a and 7:Continuing the application of the interim ambient-quality based site-specific standards is appropriate. As previously described, existing water quality conditions are irreversible in the absence of Nelson Tunnel remediation or the potential restart of the Bulldog Mine. In addition, RGS compiled all recent water quality data collected from the assessment locations for segments 4a and 7 (See Section 36.6(4)(b)-(c)) and confirmed that existing water quality still does not attain TVS for several metals. Water quality data collected from 2018-2023 show that metals concentrations continue to vary seasonally and annually, and are comparable to concentrations observed in the 2012-2017 dataset used to calculate the standards. This is expected, as no activities impacting water quality have occurred in the watershed since the standards were adopted in 2018. In addition, the 2018-2023 dataset is less robust than the 2012-2017 dataset due to inconsistencies and changes in water quality sampling and flow monitoring; RGS's longevity plan outlines its strategy for ensuring a robust dataset will be available for the commission's next review of these site-specific standards. Therefore, no changes were made to the interim ambient-quality based site-specific standards at this time. The commission determined that the interim ambient-quality based site-specific standards continue to be justified unless and until RGS progresses with dewatering of the lower mine workings, construction of a water treatment plant, and reopening of the Bulldog Mine, or until the Nelson Tunnel is remediated. The commission extended the application of the interim ambient quality-based site-specific standards by five years, through December 31, 2028. The commission also revised Section 36.6(4)(b)-(c) to include alternative site names for the assessment locations used to implement the site-specific standards.

RGS' proposal was supported by a longevity plan, updated treatability review, manganese treatability study, and data collected from assessment locations by the division, RGS, and local non-profit organizations.

The commission will reevaluate the basis of and information used to develop these standards in or before 2028, before the Tier 1 feasibility-based site-specific standards go into effect January 1, 2029. The commission will not reevaluate these standards during the June 2024 Regulation No. 36 rulemaking hearing.

B.Temporary Modifications

In April 2013 (36.33) and subsequent rulemaking hearings (36.39 and 36.42), 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 (36.44).

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 36.6(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:

Alamosa River/La Jara Creek/Conejos River: 12 (CORGAL12)

Closed Basin-San Luis Valley River Basin: 13 (CORGCB13)

To remain consistent with the commission's decisions regarding arsenic in section 36.33, 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:

Rio Grande: 1 (CORGRG01)

5 CCR 1002-36.50

46 CR 21, November 10, 2023, effective 12/31/2023