(a)

1. (1) Where mixing zones are allowed, the effects of wastes on the receiving stream shall be determined after the wastes have been thoroughly mixed with the mixing zone volume.
2. (2) Outfall structures should be designed to minimize the extent of mixing zones to ensure rapid and complete mixing.

(b)

1. (1) For aquatic life toxic substances in larger streams (those with Q7-10 flows equal to or greater than one hundred cubic feet per second (≥100 cfs), the zone of mixing shall not exceed one-fourth (1/4) of the cross-sectional area and/or critical flow volume of the stream.
2. (2) The remaining three- fourths (3/4) of the stream shall be maintained as a zone of passage for swimming and drifting organisms, and shall remain of such quality that stream ecosystems are not significantly affected.

3. (3)

   1. (A) In the smaller streams (Q7-10 flows less than one hundred cubic feet per second (<100 cfs) because of varying local physical and chemical conditions and biological phenomena, a site-specific determination shall be made on the percentage of river width necessary to allow passage of critical free-swimming and drifting organisms so that negligible or no effects are produced on their populations.
   2. (B) As a guideline, no more than two-thirds (2/3) of the cross-sectional area and/or critical flow volume of smaller streams should be devoted to mixing zones thus leaving at least one-third (1/3) of the cross-sectional area free as a zone of passage.