Ariz. Admin. Code § R18-11-206 - Mixing Zones

A. The Director may establish a mixing zone for a point source discharge to a non-WOTUS protected surface water as a condition of an individual AZPDES permit on a pollutant-by-pollutant basis. A mixing zone is prohibited where there is no water for dilution, or as prohibited pursuant to subsection (H).

B. The owner or operator of a point source seeking the establishment of a mixing zone shall submit a request to the Director for a mixing zone as part of an application for an AZPDES permit. The request shall include:

1. An identification of the pollutant for which the mixing zone is requested;

2. A proposed outfall design;

3. A definition of the boundary of the proposed mixing zone. For purposes of this subsection, the boundary of a mixing zone is where complete mixing occurs; and

4. A complete and detailed description of the existing physical, biological, and chemical conditions of the receiving water and the predicted impact of the proposed mixing zone on those conditions. The description shall also address the factors listed in subsection (D) that the Director must consider when deciding to grant or deny a request and shall address the mixing zone requirements in subsection (H).

C. The Director shall consider the following factors when deciding whether to grant or deny a request for a mixing zone:

1. The assimilative capacity of the receiving water;

2. The likelihood of adverse human health effects;

3. The location of drinking water plant intakes and public swimming areas;

4. The predicted exposure of biota and the likelihood that resident biota will be adversely affected;

5. Bioaccumulation;

6. Whether there will be acute toxicity in the mixing zone, and, if so, the size of the zone of initial dilution;

7. The known or predicted safe exposure levels for the pollutant for which the mixing zone is requested;

8. The size of the mixing zone;

9. The location of the mixing zone relative to biologically sensitive areas in the surface water;

10. The concentration gradient of the pollutant within the mixing zone;

11. Sediment deposition;

12. The potential for attracting aquatic life to the mixing zone; and

13. The cumulative impacts of other mixing zones and other discharges to the surface water.

D. Director determination.

1. The Director shall deny a request to establish a mixing zone if an applicable water quality standard will be violated outside the boundaries of the proposed mixing zone.

2. If the Director approves the request to establish a mixing zone, the Director shall establish the mixing zone as a condition of an AZPDES permit. The Director shall include any mixing zone condition in the AZPDES permit that is necessary to protect human health and the designated uses of the surface water.

E. Any person who is adversely affected by the Director's decision to grant or deny a request for a mixing zone may appeal the decision under A.R.S. § 49-321 et seq. and A.R.S. § 41-1092 et seq.

F. The Director shall reevaluate a mixing zone upon issuance, reissuance, or modification of the AZPDES permit for the point source or a modification of the outfall structure.

G. Mixing zone requirements.

1. A mixing zone shall be as small as practicable in that it shall not extend beyond the point in the waterbody at which complete mixing occurs under the critical flow conditions of the discharge and of the receiving water.

2. The total horizontal area allocated to all mixing zones on a lake shall not exceed 10 percent of the surface area of the lake.

3. Adjacent mixing zones in a lake shall not overlap or be located closer together than the greatest horizontal dimension of the largest mixing zone.

4. The design of any discharge outfall shall maximize initial dilution of the wastewater in a surface water.

5. The size of the zone of initial dilution in a mixing zone shall prevent lethality to organisms passing through the zone of initial dilution. The mixing zone shall prevent acute toxicity and lethality to organisms passing through the mixing zone.

H. The Director shall not establish a mixing zone in an AZPDES permit for the following persistent, bioaccumulative pollutants:

1. Chlordane,

2. DDT and its metabolites (DDD and DDE),

3. Dieldrin,

4. Dioxin,

5. Endrin,

6. Endrin aldehyde,

7. Heptachlor,

8. Heptachlor epoxide,

9. Lindane,

10. Mercury,

11. Polychlorinated biphenyls (PCBs), and

12. Toxaphene.

Notes

Ariz. Admin. Code § R18-11-206

Former Section R9-21-206 renumbered and amended as Section R9-21-204, new Section R9-21-206 adopted effective January 29, 1980 (Supp. 80-1). Amended by adding subsection (B) effective October 14, 1981 (Supp. 81-5). Amended subsection (B) and Table 1 effective January 29, 1982 (Supp. 82-1). Amended subsection (B) and Table 1 effective August 13, 1982 (Supp. 82-4). Former Section R9-21-206 renumbered as Section R9-21-205, former Section R9-21-207 renumbered as Section R9-21-206 and amended effective January 7, 1985 (Supp. 85-1). Former Section R9-21-206 renumbered and amended as Section R9-21-207, former Section R9-21-205 renumbered and amended as R9-21-206 effective August 12, 1986 (Supp. 86-4). Former Section R9-21-206 renumbered without change as Section R18-11-206 (Supp. 87-3). New Section made by final rulemaking at 29 A.A.R. 302, effective 2/20/2023.