North Carolina Administrative Code › Title 15A - Environmental Quality › Chapter 02 - Environmental Management › Subchapter B - SURFACE WATER AND WETLAND STANDARDS › Section .0200 - CLASSIFICATIONS AND WATER QUALITY STANDARDS APPLICABLE TO SURFACE WATERS AND WETLANDS OF NORTH CAROLINA › Section 02B .0206 - FLOW DESIGN CRITERIA FOR EFFLUENT LIMITATIONS

Current through Register Vol. 39, No. 2, July 15, 2024
(a)
				  Water quality based effluent limitations shall be developed to allow
				  appropriate frequency and duration of deviations from water quality standards
				  so that the designated uses of receiving waters are protected. There are water
				  quality standards for a number of categories of pollutants and to protect a
				  range of water uses. For this reason, the appropriate frequency and duration of
				  deviations from water quality standards shall not be the same for all
				  pollutants. A flow design criterion shall be used in the development of water
				  quality based effluent limitations as a simplified means of estimating the
				  acceptable frequency and duration of deviations. Effluent limitations shall be
				  developed using the following flow design criteria:
(1) All standards except toxic substances and
				  aesthetics shall be protected using the minimum average flow for a period of
				  seven consecutive days that has an average recurrence of once in ten years
				  (7Q10 flow). Other governing flow strategies, such as varying discharges with
				  the receiving waters ability to assimilate wastes, may be designated by the
				  Commission or its designee on a case-by-case basis if the discharger or permit
				  applicant provides evidence that establishes that the alternative flow
				  strategies will give equal or better protection for the water quality
				  standards. "Better protection for the water quality standards" means that
				  deviations from the standard would be expected less frequently than provided by
				  using the 7Q10 flow.
(2) Toxic
				  substance standards to protect aquatic life from chronic toxicity shall be
				  protected using the 7Q10 flow.
(3)
				  Toxic substance standards to protect aquatic life from acute toxicity shall be
				  protected using the 1Q10 flow.
(4)
				  Toxic substance standards to protect human health shall be the following:
(A) The 7Q10 flow for standards to protect
				  human health through the consumption of water, fish, and shellfish from
				  noncarcinogens; and
(B) The mean
				  annual flow to protect human health from carcinogens through the consumption of
				  water, fish, and shellfish unless site specific fish contamination concerns
				  necessitate the use of an alternative design flow;
(5) Aesthetic quality shall be protected
				  using the minimum average flow for a period of 30 consecutive days that has an
				  average recurrence of once in two years (30Q2 flow).
More complex modeling techniques may also be used to set
					 effluent limitations directly based on frequency and duration criteria
					 published by the U.S. Environmental Protection Agency, available free of charge
					 at
http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm
and incorporated by reference, including subsequent amendments and editions,
					 and the Commission or its designee has determined, on a case-by-case basis,
					 that the techniques will protect the designated uses of receiving
					 waters.