15A NCAC 02B .0206. FLOW DESIGN CRITERIA FOR EFFLUENT LIMITATIONS

(a)  Water quality based effluent limitations are 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 is not the same for all categories of standards.  A flow design criterion is used in the development of water quality based effluent limitations as a simplified means of estimating the acceptable frequency and duration of deviations.  More complex modeling techniques can also be used to set effluent limitations directly based on frequency and duration criteria published by the U.S. Environmental Protection Agency pursuant to Section 304(a) of the Federal Clean Water Act as amended.  Use of more complex modeling techniques to set water quality based effluent limitations will be approved by the Commission or its designee on a case‑by‑case basis.  Flow design criteria to calculate water quality based effluent limitations for categories of water quality standards are listed as follows:

(1)           All standards except toxic substances and aesthetics will 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 provide evidence which establishes to the satisfaction of the Director that the alternative flow strategies will give equal or better protection for the water quality standards.  Better protection for the 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 will be protected using the 7Q10 flow.

(3)           Toxic substance standards to protect human health will be:

(A)          The 7Q10 flow for standards to protect human health through the consumption of water, fish and shellfish from noncarcinogens;

(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;

(4)           Aesthetic quality will 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).

(b)  In cases where the stream flow is regulated, a minimum daily low flow may be used as a substitute for the 7Q10 flow except in cases where there are acute toxicity concerns for aquatic life.  In the cases where there are acute toxicity concerns, an alternative low flow such as the instantaneous minimum release may be used on a case‑by‑case basis.

(c)  Flow design criteria are used to develop water quality based effluent limitations and for the design of wastewater treatment facilities.  Deviations from a specific water quality standard resulting from discharges which are affirmatively demonstrated to be in compliance with water quality based effluent limitations for that standard will not be a violation pursuant to G.S. 143‑215.6 when the actual flow is significantly less than the design flow.

(d)  In cases where the 7Q10 flow of the receiving stream is estimated to be zero, water quality based effluent limitations will be assigned as follows:

(1)           Where the 30Q2 flow is estimated to be greater than zero, effluent limitations for new or expanded (additional) discharges of oxygen consuming waste will be set at BOD₅= 5 mg/l, NH₃‑N = 2 mg/l and DO = 6 mg/l, unless it is determined that these limitations will not protect water quality standards.  Requirements for existing discharges will be determined on a case‑by‑case basis by the Director.  More stringent limits will be applied in cases where violations of water quality standards are predicted to occur for a new or expanded discharge with the limits set pursuant to this Rule, or where existing limits are determined to be inadequate to protect water quality standards.

(2)           If the 30Q2 and 7Q10 flows are both estimated to be zero, no new or expanded (additional) discharge of oxygen consuming waste will be allowed.  Requirements for existing discharges to streams where the 30Q2 and 7Q10 flows are both estimated to be zero will be determined on a case‑by‑case basis.

(3)           Other water quality standards will be protected by requiring the discharge to meet the standards unless the alternative limitations are determined by the Director to protect the classified water uses.

(e)  Receiving water flow statistics will be estimated through consultation with the U.S. Geological Survey.  Estimates for any given location may be based on actual flow data, modeling analyses, or other methods determined to be appropriate by the Commission or its designee.

 

History Note:        Authority G.S. 143‑214.1; 143‑215.3(a)(1);

Eff. February 1, 1976;

Amended Eff. February 1, 1993; October 1, 1989; August 1, 1985; January 1, 1985.