Illinois Administrative Code › Title 35 - ENVIRONMENTAL PROTECTION › Part 302 - WATER QUALITY STANDARDS › Subpart A - GENERAL WATER QUALITY PROVISIONS › Section 302.102 - Allowed Mixing, Mixing Zones, and ZIDs

Current through Register Vol. 48, No. 28, July 12, 2024
a) Whenever a water quality standard is more
				restrictive than its corresponding effluent standard, or where there is no
				corresponding effluent standard specified at 35 Ill. Adm. Code 304, an
				opportunity will be allowed for compliance with 35 Ill. Adm. Code
				304.105 by the mixture of an
				effluent with its receiving waters, provided the discharger has made every
				effort to comply with the requirements of 35 Ill. Adm. Code
				304.102.
b) The portion, volume, and area of any
				receiving waters within which mixing is allowed pursuant to subsection (a) must
				be limited by the following:
1) Mixing must be
				confined in an area or volume of the receiving water no larger than the area or
				volume that would result after incorporation of outfall design measures to
				attain optimal mixing efficiency of effluent and receiving waters. These
				measures may include the use of diffusers and engineered location and
				configuration of discharge points.
2) Mixing is not allowed in waters that
				include a tributary stream entrance if the mixing occludes the tributary mouth
				or otherwise restricts the movement of aquatic life into or out of the
				tributary.
3) Mixing is not allowed
				in water adjacent to bathing beaches, bank fishing areas, boat ramps or
				dockages or any other public access area.
4) Mixing is not allowed in waters containing
				mussel beds; endangered species habitat; fish spawning areas; areas of
				important aquatic life habitat; or any other natural features vital to the
				well-being of aquatic life in a manner that maintaining aquatic life in the
				body of water as a whole would be adversely affected.
5) Mixing is not allowed in waters that
				contain intake structures of public or food processing water supplies, points
				of withdrawal of water for irrigation, or watering areas accessed by wild or
				domestic animals.
6) Mixing must
				allow for a zone of passage for aquatic life in which water quality standards
				are met. However, a zone of passage is not required in receiving streams that
				have zero flow for at least seven consecutive days recurring on average in nine
				years out of 10.
7) The area and
				volume in which mixing occurs, alone or in combination with other areas and
				volumes of mixing, must not intersect any area of any body of water in such a
				manner that the maintenance of aquatic life in the body of water as a whole
				would be adversely affected.
8) The
				area and volume in which mixing occurs, alone or in combination with other
				areas and volumes of mixing must not contain more than 25% of the
				cross-sectional area or volume of flow of a stream except for those streams for
				which the dilution ratio is less than 3:1. In streams where the dilution ratio
				is less than 3:1, the volume in which mixing occurs, alone or in combination
				with other volumes of mixing, must not contain more than 50% of the volume flow
				unless an applicant for an NPDES permit demonstrates, pursuant to subsection
				(d), that an adequate zone of passage is provided for pursuant to subsection
				(b)(6).
9) No mixing is allowed
				when the water quality standard for the constituent in question is already
				violated in the receiving water.
10) No body of water may be used totally for
				mixing of single outfall or combination of outfalls, except as provided in
				subsection (b)(6).
11) Single
				sources of effluents that have more than one outfall must be limited to a total
				area and volume of mixing no larger than that allowable if a single outfall
				were used.
12) The area and volume
				in which mixing occurs must be as small as is practicable under the limitations
				prescribed in this subsection (b), and in no circumstances may the mixing
				encompass a surface area larger than 26 acres.