1. (a) The ambient upstream flow used to calculate permit limits shall be as specified in (b) through (g), below.
2. (b) For tidal waters, the low flow condition shall be equivalent to the conditions that result in a dilution that is exceeded 99 percent of the time.
3. (c) For non-tidal rivers and streams, permit limits for all human health criteria for carcinogens shall be developed based on the long-term harmonic mean flow, which is the number of daily flow measurements divided by the sum of the reciprocals of the daily flows.

4. (d) Permit limits to meet nutrient criteria including, but not limited to, nitrogen and phosphorus species, shall be based on the following downstream ambient targets and flows:

   1. (1) The ambient nutrient target used in the reasonable potential analysis conducted pursuant to 40 CFR 122.44(d) shall be based on one of the following methods provided that existing and designated uses are fully protected:

      1. a. Site-specific criteria adopted pursuant to Env-Wq 1704;
      2. b. An EPA approved total maximum daily load (TMDL) study pursuant to 40 CFR 130.7; or

      3. c. One of the following federal requirements if deemed by the department to be protective of all existing and designated uses:

         1. 1. Criteria published by EPA pursuant to 33 U.S.C. 1314(a)(1); or
         2. 2. Permits issued pursuant to 40 CFR 122; and

   2. (2) The flows for nutrients used in the reasonable potential analysis shall be commensurate to, as applicable:

      1. a. Site-specific nutrient criteria adopted pursuant to Env-Wq 1704;
      2. b. Established conditions for the nutrient target in an EPA approved TMDL;
      3. c. Nutrient target used in criteria published by EPA pursuant to 33 U.S.C. 1314(a)(1); or
      4. d. Nutrient target used in permits issued pursuant to 40 CFR 122.
5. (e) For non-tidal rivers and streams, permit limits to prevent ammonia toxicity in aquatic life shall be based on a flow equal to the 7Q10 flow.
6. (f) Nutrient effluent permit limits shall be based on the 7Q10 flow if the nutrient limit is needed to achieve compliance with other water quality criteria that must have permit limits based on the 7Q10 flow in accordance with (g) below.
7. (g) For non-tidal rivers and streams, permit limits for all non-nutrient aquatic life criteria and human health criteria for non-carcinogens shall be based on the 7Q10 flow except as described in Env-Wq 1705.02(d)(2) through Env-Wq 1705.02(f), above.

8. (h) To the maximum extent practicable, data used for setting permit limits and calculating reasonable potential pursuant to 40 CFR 122.44(d)(1)(ii) shall be based upon:

   1. (1) Data, modeling, or reasonable estimates of the ambient condition representative in space and time of the limiting conditions as defined in (a) through (g) above, for a particular criterion; and
   2. (2) Data, modeling, or reasonable estimates of the ambient condition representative of the conditions on which a criterion is based.

Source. (See Revision Notes #1, #2, and #3 at chapter heading for Env-Wq 1700) #12042, eff 12-1-16 New. #14203, eff 2-26-25