Mixing zones and zones of passage

(a) Mixing zones. (1) In streams, the chronic regulatory mixing zone extends downstream a distance equivalent to thirteen (13) times the width of the water within the receiving stream at the point of effluent discharge and encompasses 25% of the total stream flow of the 7Q2 or 1 cfs, whichever is larger, immediately downstream of the point of effluent discharge. (2) The acute regulatory mixing zone is encompassed by the R = 0.01 (cfs-1) isopleth. R is the ratio of concentration to wasteload. (3) Acute toxicity within the mixing zone is prohibited. (4) Mixing zones in lakes shall be designated on a case-by-case basis. (5) The water quality in a portion of the mixing zone may be unsuitable for certain beneficial uses. (6) Where overlapping mixing zones occur because of multiple outfalls, the total length of the chronic regulatory mixing zone will extend thirteen (13) stream widths downstream from the downstream discharge point. (b) Zones of passage. (1) All discharges permitted for any criteria listed for protection of fish and wildlife propagation shall be regulated to ensure that a zone of passage shall be maintained within the stream at the outfall and adjacent to the mixing zone that shall be no less than seventy-five percent (75%) of the volume of flow. (2) Water quality standards shall be maintained throughout the zone of passage. (3) Zones of passage in lakes shall be designated on a case-by-case basis. 252:730-5-27 [RESERVED] 252:730-5-28 [RESERVED]