5 Colo. Code Regs. § 1002-72.30
The provisions of Sections 25-8-202(1)(c), 25-8-202(1)(f), 25-8-202(2), and 25-8-205, C.R.S. provide the specific statutory authority for adoption of these regulatory amendments. The Commission also adopted, in compliance with Section 24-4-103(4) C.R.S., the following statement of basis and purpose.
BASIS AND PURPOSE
Background and Overview
Based on the 2000-2001 rulemaking hearings, the Commission established and maintained a phased phosphorus Total Maximum Annual Load (TMAL) of 14,270 pounds for the Cherry Creek Reservoir. Subsequently, the Cherry Creek Basin Water Quality Authority (Authority) and other interested parties performed several investigations-or "special studies" under Section 72.8(4) of Regulation #72-to evaluate the basis for the chlorophyll a standard in Regulation 38 and the phased TMAL requirements for phosphorus in Regulation 72. Based on the special studies, the Authority recommended that the Commission convert Regulation 72 from a mass-based TMAL to a concentration-based control strategy. The special studies did not consider the feasibility of construction dewatering complying with a 0.05 mg/l total phosphorus limit. The special studies also indicated that the treatment required to comply with a 0.05 mg/l limit is achieved by adding chemicals to precipitate dissolved phosphorus, which has the unintended consequence of possible impairment of water quality in Cherry Creek. Based on the special studies, in 2009 the Commission removed the TMAL requirements and adopted a concentration-based management strategy for phosphorus control in the Chery Creek basin. In 2011 and 2013 General Permits, the Division began requiring additional monitoring for phosphorus. This monitoring found that dewatering source water (groundwater) in the Cherry Creek Reservoir control area frequently contains phosphorus at concentrations above the applicable limitation of 0.05 mg/l. Thus, in 2020, the Division began implementing the 0.05 mg/l total phosphorus limit in dewatering general permits in the Cherry Creek basin.
Modified Effluent Limit Requirements for Construction Dewatering
Although the Commission has previously determined that construction dewatering is classified as an "industrial process wastewater source" for purposes of Regulation 72 and application of a 0.05 mg/l total phosphorus limit, 5 C.C.R. §§ 1002-72.26, 72.4, the 0.05 mg/l limit was not applied to construction dewatering until 2020, and the Commission has never considered the feasibility of construction dewatering operations complying with such limit. Proponents Parker Water and Sanitation District and the Town of Castle Rock presented significant evidence during this proceeding showing that a 0.05 mg/l total phosphorus limit is not feasible for construction dewatering based on present treatment alternatives, nor economically reasonable in light of the modeled total phosphorus loading associated with construction dewatering in the basin. The Commission finds the evidence persuasive and anticipates continuing to evaluate these issues. The 10-Year Water Quality Roadmap (WQCD Clean Water Policy 8) is committed to completing the state-wide phased implementation of nutrient standards in 2027, including adopting phosphorus standards for Cherry Creek Reservoir. As part of the Roadmap, the Division has previously committed to:
The Commission has determined that reasonable progress can be made towards reducing total phosphorus loading associated with construction dewatering through implementation of well-established practice-based effluent limits, otherwise known as "standards of performance," adopted as part of this proceeding, in addition to existing water quality-based and technology-based effluent limitations applied in accordance with Regulation 61. This is a protective approach that allows for flexibility of control measure selection and implementation based on site-specific conditions. The Commission anticipates it will be infeasible for very few construction dewatering permits to apply the practice-based effluent limits. In the rare event that any of the practice-based effluent limits are infeasible, it is the burden of the permittee to document why each practice-based effluent limit is infeasible.
PARTIES TO THE RULEMAKING
5 CCR 1002-72.30