5 Colo. Code Regs. § 1002-33.66
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 2010, the commission adopted the discharger specific variance (DSV) provisions at Regulation 31.7(4), which allow a temporary water quality standard to be adopted in cases where water quality-based effluent limits (WQBELs) are not feasible to achieve. A DSV is a hybrid standard that maintains the long-term water quality goal of fully protecting all designated uses, while temporarily authorizing an alternative effluent limit (AEL) to be developed for a specific pollutant and specific point source discharge where compliance with the WQBEL is not feasible.
Pursuant to 40 CFR 131.14(b)(1)(v)-(vi), the commission must re-evaluate every DSV with a duration longer than five years and provide EPA notice of the results within 30 days of the completion of the reevaluation process. If the commission does not complete this action, the federal regulation states that the DSV will no longer be the applicable water quality standard for purposes of the Clean Water Act. This reevaluation is consistent with commission Regulation 31.7(4), which requires that the commission reexamine all DSVs not less than once every three years.
The commission adopted a DSV for Yampa River Segment 7 (COUCYA07) for total inorganic nitrogen (TIN) that represents the highest degree of protection of the classified use that is economically feasible for the Town of Oak Creek. The AEL is an acute (1-day) concentration of 15 mg/L, which is to be achieved by the end of the variance through implementation of the selected alternatives. The DSV requires that the Town of Oak Creek's TIN effluent concentrations do not exceed the current condition at any time during the variance. To ensure that the requirements of the DSV do not result in any lowering of currently attained ambient water quality, the commission relies on the implementation of numeric initial effluent limits to be developed in a method consistent with the division's policy for current condition temporary modifications (Clean Water Policy 13). The DSV will expire on 6/30/2026.
A comprehensive alternatives analysis demonstrated that compliance with the TIN WQBEL would cause substantial and widespread adverse social and economic impacts in the area where the discharge is located. Treatment that would allow the Town of Oak Creek to meet the TIN WQBEL, such as replacing the lagoon with a mechanical plant, would result in user fees that exceed the community's ability to pay. The commission determined that any alternative that would result in user fees exceeding 1.5% of median household income for the Town of Oak Creek's residents was economically infeasible at this time, due to the current economic conditions in the Town of Oak Creek, including a high level of debt-per capita and a local median household income that is significantly lower than the State's average.
The commission adopted a DSV with an alternative effluent limit that is based upon the best feasible alternative identified for the Town of Oak Creek. This alternative includes modification of the operation of the plant to recycle a portion of the treated effluent from the moving bed bioreactor (MBBR) unit back to the anaerobic cell, where conditions favor denitrification. The five year term of this DSV provides time for the Town of Oak Creek to run a pilot test to evaluate the effectiveness of this approach. If recycling the effluent results in reductions in TIN concentrations, the recycle will be continued and optimized through the term of the variance. If no improvement is observed within the first two years, the remaining three years of this DSV will be used to modify the MBBR unit to perform both nitrification and denitrification.
Currently, there is significant seasonal variability in influent flows to the wastewater treatment plant that is believed to be due to groundwater inflow and residential sump pump contributions to the Town of Oak Creek's collection system. During the term of this variance, the Town of Oak Creek will be taking steps to reduce groundwater inflow, which will reduce influent volume.
At the re-evaluation of this DSV at the 2024 Upper Colorado Basin rulemaking hearing, the commission will review the Town of Oak Creek's progress towards achieving the alternative effluent limit, and will determine whether the requirements of the DSV continue to be the highest attainable condition. The requirements of the DSV will be reviewed during the re-evaluation rulemaking hearing, and will either remain as the AEL identified at the time of the adoption of the variance or be modified to reflect the highest attainable condition. Because there is significant uncertainty in the final effluent quality that will be achieved, the Town of Oak Creek will collect additional data to characterize the effectiveness of treatment and may request a hearing before the commission to modify the DSV before it is re-evaluated or expires. If it remains infeasible for the Town of Oak Creek to achieve TIN WQBELs at the end of the variance, a subsequent DSV may be appropriate.
5 CCR 1002-33.66