Authority: IC 13-14-8; IC 13-18-3

Affected: IC 13-18-4

Sec. 4. (a) All surface water quality standards in this rule, except those provided in section 6(a)(1) of this rule, are to be applied at a point outside of the mixing zone to allow for a reasonable admixture of waste effluents with the receiving waters.

(b) Due to varying physical, chemical, and biological conditions, no universal mixing zone may be prescribed. The commissioner shall determine the mixing zone upon application by the discharger. The applicability of the guideline set forth in subsection (c) will be on a case-by-case basis and any application to the commissioner shall contain the following information:

1. (1) The dilution ratio.
2. (2) The physical, chemical, and biological characteristics of the receiving body of water.
3. (3) The physical, chemical, and biological characteristics of the waste effluent.
4. (4) The present and anticipated uses of the receiving body of water.
5. (5) The measured or anticipated effect of the discharge on the quality of the receiving body of water.
6. (6) The existence of and impact upon any spawning or nursery areas of any indigenous aquatic species.
7. (7) Any obstruction of migratory routes of any indigenous aquatic species.
8. (8) The synergistic effects of overlapping mixing zones or the aggregate effects of adjacent mixing zones.

(c) The mixing zone should be limited to no more than one-fourth (1/4) (twenty-five percent (25%)) of the cross-sectional area and/or volume of flow of the stream, leaving at least three-fourths (3/4) (seventy-five percent (75%)) free as a zone of passage for aquatic biota nor should it extend over one-half (1/2) (fifty percent (50%)) of the width of the stream.

(d) Based on consideration of aquatic life or human health effects, the commissioner may deny a mixing zone for a discharge or certain substances in a discharge.

(e) Notwithstanding other subsections of this section, no mixing zone shall be allowed for discharges to lakes except for those consisting entirely of noncontact cooling water which meet the requirements set forth in Section 316(a) of the Federal Water Pollution Control Act of 1972.

(Water Pollution Control Division; 327 IAC 2-1-4; filed Sep 24, 1987, 3:00 p.m.: 11 IR 580; filed Feb 1, 1990, 4:30 p.m.: 13 IR 1020; readopted filed Oct 18, 2024, 1:57 p.m.: 20241113-IR-327230810RFA)