5 Colo. Code Regs. § 1002-31.9
Current through Register Vol. 47, No. 20, October 25, 2024
Section 5 CCR 1002-31.9 - IMPLEMENTATION OF STANDARDS(1)Low Flow Exceptions(a) Water quality standards shall apply at all times; provided, that in developing effluent limitations or other requirements for discharge permits, the Division shall normally define critical flow conditions using the following low-flow values: (i) Generally: the empirically based 30-day average low flow with an average 1-in-3 year recurrence interval (30E3) for chronic standards and the empirically based 1-day low flow with an average 1-in-3 year recurrence interval (1E3) for acute standards, or the equivalent statistically-based flow.(ii) Temperature limitations: the empirically based 7-day average low flow with an average 1-in-3 year recurrence interval (7E3), and the empirically based 1-day low flow with an average 1-in-3 year recurrence interval (1E3) for acute standards, or the equivalent statistically-based flow. (iii) Total phosphorus and total nitrogen limitations: the annual median of the daily average flows with a 1-in-5 year recurrence interval.(b)Data RequirementsThe period of record for determining low flows shall be based on a minimum of ten years of flow data, except that, when ten years of data is not available, low flows may be determined, on a case-by-case basis, using a period of record of less than ten years. If more than ten years of flow data is available, it may be more appropriate to establish low flow conditions based on a longer period of record to more accurately reflect site-specific conditions.
(c)Streams With Rapid Flow ChangesFor streams with seasonal rapidly rising or falling hydrographs, the Division shall use, if so requested by a discharger, the procedure set forth in subparagraphs (i) through (v) below for calculating 30E3 values for those transitional flow periods of the year. For certain substances such as ammonia, the low flow exceptions may be based on periodic or seasonal flows as determined on a case-by-case basis by the Division.
(i) Averaging Procedure - Calculation of 30-day Forward Moving Harmonic Means - Moving harmonic means shall first be calculated for each consecutive thirty-day period in the period of record being considered.(ii) Calculate Annual 30E3 Value - Determine the annual 30E3 value using the procedure set forth in Appendix A using(A) 30-day forward moving harmonic means, and(B) the excursion procedure for a 1-in-3 year recurrence interval.(iii) Assigning Harmonic Means - Each 30-day harmonic mean shall then be assigned to a month. A harmonic mean shall be assigned to a specific month only if the harmonic mean is calculated using data for 15 or more days from that month.(iv) Ranking of Harmonic Means - Harmonic means shall be ranked from the lowest to highest for each month of the year. The lowest harmonic mean for a month shall be used to establish the low flow value for that month using the procedure set forth in subparagraph (v) below.(v) Establishing Monthly 30E3 Low Flows - The low flow for a month shall be either the lowest harmonic mean assigned to that month (as determined in subparagraphs (iii) and (iv), above), or the annual low flow value (as determined in subparagraph (ii), above), whichever is greater.(d)Waters Not Yet ClassifiedDischarges to waters not presently classified must meet established effluent limitation regulations, the basic standards, antidegradation rule and control regulations. Effluent flows which reach a classified body of water, even though the discharge point is to a water not yet classified, must be of a quality which will not cause the standards of the classified body of water to be violated.
(2)Compliance SchedulesWhere the Commission has adopted new standards, temporary modifications or revised standards that have become more stringent, or where the Division has developed new interpretations of existing standards, including, but not limited to, implementation requirements through approved TMDLs and Wasteload Allocations, interim and final AELs for variances and antidegradation reviews; the Division may include schedules of compliance in Colorado Discharge Permit System (CDPS) permits when it determines such schedules to be necessary and appropriate.
(3)Temperature LimitsThe Division will determine whether temperature limits are to be included in permits utilizing the following approach.
(a) No temperature effluent limit will be applied if a discharge is to an effluent-dependent stream and there is no evidence that the aquatic life use may be negatively affected by the thermal component of the discharge. In implementing this provision, the Division will consider all readily available and pertinent evidence regarding the potential for the thermal properties of a discharge to affect aquatic life.(b) No temperature effluent limit will be applied to a discharge of water from a natural hot springs, so long as that water enters the receiving water in the vicinity of its natural outflow.(c) Where neither (a) nor (b) above apply to a discharge, the Division will determine whether a limitation for temperature is to be included in a permit consistent with procedures developed in accordance with Section 61.8 of the CDPS Regulations. Where there are not adequate data to determine reasonable potential, the Division may require the permittee to collect and submit temperature data.(d) At the time of permit renewal, where a site-specific recalculation procedure demonstrates that alternative numerical criteria are more appropriate for protection of aquatic life, these alternative criteria will be used for development of permit limits.(e) Consistent with section 316(a) of the federal Clean Water Act, and federal implementing regulations, the Division may impose alternate effluent limitations with respect to the thermal component of such discharge.(4)Temporary ModificationsWhere a temporary modification is adopted, permits for discharges to the segment in question:
(a) For existing discharges: (i) Will not include a compliance schedule to meet limits based on the underlying standard during the period that the temporary modification is in effect.(ii) Will, regardless of whether the operative value of the temporary modification is numeric or narrative, include permit effluent limits, where appropriate, that ensure that, at a minimum, status quo is maintained during the temporary modification.(iii) May include limitations or other conditions (e.g., source identification, pretreatment, and evaluation of other source control and treatment options) for the parameter(s) in question based on an assessment of the level of effluent quality reasonably achievable without requiring significant investment in facility infrastructure (e.g., based on past facility performance). Such limits (numeric or otherwise) may be at or below the level derived from the temporary modification, where such a requirement would not cause an undue economic burden, but not more restrictive than necessary to achieve the underlying standard.(b) For expanding discharges: Will include effluent limits that, at a minimum, do not pose an unreasonable risk to downstream uses and ensure status quo is maintained.(c) For new discharges: Will include effluent limits based on the underlying standard, rather than the temporary modification, unless the Commission has established a specific limit or value for new dischargers.(d) May include a permit condition requiring actions intended to eliminate the uncertainty regarding the appropriate underlying standard.39 CR 11, June 10, 2016, effective 6/30/201639 CR 17, September 10, 2016, effective 12/31/201640 CR 03, February 10, 2017, effective 3/2/201740 CR 23, December 10, 2017, effective 12/30/201741 CR 01, January 10, 2018, effective 1/31/201843 CR 03, February 10, 2020, effective 6/30/202043 CR 11, June 10, 2020, effective 6/30/202044 CR 17, September 10, 2021, effective 12/31/2021Renumbered from 5 CCR 1002-31.57 44 CR 17, September 10, 2021, effective 12/31/2021Renumbered to 5 CCR 1002-31.5844 CR 17, September 10, 2021, effective 12/31/202146 CR 10, May 25, 2023, effective 6/14/2023