Section 323.1082. Mixing zones.

(1) A mixing zone is that portion of a water body allocated by the department where a point source or venting groundwater discharge is mixed with the surface waters of the state. Exposure in mixing zones shall not result in deleterious effects to populations of aquatic life or wildlife. As a minimum restriction, the final acute value (FAV) for aquatic life shall not be exceeded when determining a wasteload allocation (WLA) for acute aquatic life protection, unless it is determined by the department that a higher level is acceptable or it can be demonstrated to the department that an acute mixing zone is acceptable consistent with subrule (7) of this rule. The mixing zone shall not prevent the passage of fish or fish food organisms in a manner that would result in adverse impacts on the immediate  or   future populations of the fish or fish food organisms. The area of mixing zones shall be minimized.  To this end, devices for rapid mixing, dilution, and dispersion are encouraged where practicable.A watercourse or portions of a watercourse that, without 1 or more point source discharges, would have no flow except during periods of surface runoff may be considered as a mixing zone for a point source discharge. A mixing zone established in this manner shall not apply to pollutants of initial focus specified in 40 C.F.R. §132 (1995) unless a site-specific determination under R 323.1057(2) has been conducted that shows that the existing and expected aquatic life in the watercourse will be adequately protected in  the absence of chronic aquatic life water quality values.

(2)      Unless otherwise stated in this rule, not more than 25% of the receiving water design flow for lotic systems, as stated in R 323.1090(2), shall be used when determining a whole effluent toxicity limit or a wasteload allocation for a toxic substance, in the absence of, or consistent with, a total maximum daily load, unless it can be demonstrated to the department that the use of a larger volume is acceptable consistent with subrule (7) of this rule.

(3)     For ammonia and substances not included in subrule (2) of this rule, the design flow for lotic systems, as stated in R 323.1090(2)(a) or (3), shall be used when determining WLAs if the provisions in subrule (1) of this rule are met, unless the department determines that a more restrictive volume is necessary.

(4)       For all substances, physical mixing zone boundaries may be established and shall be determined by the department on a case-by-case basis.

(5)         Mixing zones in the Great Lakes and inland lakes for the purpose of determining WLAs and WET limits shall assume no greater dilution than 1 part effluent to 10 parts receiving water, unless it can be demonstrated   to  the department that use of a larger volume is acceptable consistent with subrule

(7)  of this rule. Except for ammonia, a larger mixing zone shall not be granted if it exceeds the area where discharge-induced mixing occurs. Mixing zones established under this subrule for thermal discharges to meet the Great Lakes and inland lake requirements of R 323.1069, R 323.1070, R 323.1072, R 323.1073, and R 323.1075 shall  be determined  by the  department   on   a case-by-case basis.

(6)       In addition to subrules (1), (2), (4), and (5) of this rule, the following provisions are applicable to bioaccumulative chemicals of concern (BCCs) when establishing WLAs:

(a)       There shall be no mixing zones available for new discharges of BCCs to the surface waters of the state.

(b)     Mixing zones for BCCs may be allowed for existing discharges to the surface waters of the state through November 14, 2010, pursuant to  the provisions of this rule. After this date, except as provided in subdivisions (c) and (d) of this subrule, permits shall not authorize mixing zones for existing discharges of BCCs to the surface waters of the state, and WLAs for such discharges shall be set equal to the most stringent water quality value for that BCC.

(c)     The department may grant mixing zones for any existing discharge of BCCs to the surface waters of the state where it can be demonstrated, on a case-by-case basis, that failure to grant a mixing zone would preclude water conservation measures that would lead to overall load reductions in BCCs.

(d)     Upon the request of an existing discharger of a BCC to the surface waters of the state, the department may grant mixing zones beyond November 14, 2010, based upon technical and economic considerations, subject to all of the following provisions:

(i)     The department must determine that all of the following provisions are satisfied:

(A)       The discharger is in compliance with, and will continue to implement, all applicable technology-based treatment and pretreatment requirements of the clean water act of 1972, as amended, 33 U.S.C. §§301, 302, 304, 306, 307, 401, and 402, and is in compliance with its existing NPDES WQBELs, including those based on a mixing zone.

(B)     The discharger has reduced, and will continue to reduce, to the maximum extent possible, the loading of the BCC for which a mixing zone is requested, by the use of cost-effective controls or pollution-prevention alternatives that have been adequately demonstrated  and are reasonably available to the discharger.

(C)        The discharger has evaluated alternative means of reducing the BCC elsewhere in the watershed.

(ii)     In making the determination in paragraph (i) of this subdivision, the department shall consider all of the following factors:

(A)      The availability and feasibility, including cost effectiveness, of additional controls or pollution prevention measures for reducing and ultimately eliminating BCCs for the discharger, including additional controls or pollution prevention measures used by similar dischargers for reducing and ultimately eliminating BCCs.

(B)            Whether  the  discharger  or   affected  communities   will   suffer unreasonable economic effects if the mixing zone is eliminated.

(C)      The extent to which the discharger will  implement   an  ambient monitoring plan to ensure compliance with water quality values at the edge of any authorized mixing zone.

(D)    Other information the department deems appropriate.

(iii)           Any exceptions to the mixing zone elimination provision for existing discharges of BCCs granted pursuant to this subdivision shall comply with all of the following provisions:

(A)      Not result in any less stringent limitations than the limitations that existed on July 29, 1997.

(B)        Be limited to 1 permit term unless the department makes a new determination in accordance with this subrule for   each  successive  permit application in which a mixing zone for the BCC is sought.

(C)         Not likely jeopardize the continued existence of any  endangered   or threatened species listed or proposed under section 4 of the endangered species act or result in the destruction  or adverse modification  of  the species' critical habitat.

(iv)        For each draft NPDES permit that allows a mixing zone for a BCC after November 14, 2010, the NPDES fact sheet shall specify relevant information used to establish the mixing zone, including the mixing provisions used in calculating the permit limits and the identity of each BCC for which a mixing zone is proposed.

(7)     For purposes of establishing a mixing zone other than as specified in subrules (1), (2), and (5) of this rule, a mixing zone demonstration shall be submitted to the department for approval and all of the following provisions apply:

(a)    The mixing zone demonstration shall include all of the following:

(i)        A description of the amount of dilution occurring at the boundaries of the proposed mixing zone and the size, shape, and location of the area of mixing, including the manner in which diffusion and dispersion occur.

(ii)     For sources discharging to the Great Lakes and  inland  lakes,  a definition of the location at which discharge-induced mixing ceases.

(iii)     Documentation of the substrate character within the mixing zone.

(iv)        Confirmation that the mixing zone does not interfere with or block the passage of fish or aquatic life.

(v)     Confirmation that the mixing zone would not likely jeopardize the continued existence of any  endangered   or  threatened   species  listed  or proposed under section 4 of the endangered species act or result  in  the destruction or adverse modification of the species' critical habitat.

(vi)      Confirmation that the mixing zone does not extend to a public water supply source pursuant to R 323.1100(8).

(vii)     Confirmation that the mixing zone would  not  interfere  with  the designated or existing uses of the receiving water or downstream waters.

(viii)     Documentation of background water quality concentrations.

(ix)      Confirmation that the mixing zone does not promote undesirable aquatic life or result in a dominance of nuisance species.

(x)        Confirmation that, by allowing additional  mixing/dilution,  the following will not occur:

(A)    The formation of objectionable deposits.

(B)      The concentration of floating debris, oil, scum, and other matter in concentrations that form nuisances.

(C)    The production of objectionable color, odor, taste, or turbidity.

(b)     The mixing zone demonstration shall also address all of the following items:

(i)     Whether or not adjacent mixing zones overlap.

(ii)      Whether organisms would be attracted to the area of mixing  as  a result of the effluent character.

(iii)     Whether the habitat supports endemic or naturally occurring species.

(iv)     Why an increased mixing zone is necessary.

(v)     Describe any pollution prevention measures that were evaluated   to eliminate the need for an increased mixing zone.

(c)        The mixing zone demonstration shall be based on the assumption that environmental fate or other physical, chemical, or biological factors do not affect the concentration of the toxic substance in the water column, within the proposed mixing zone, unless both of the following occur:

(i)      Scientifically valid field studies or other relevant  information demonstrate that degradation of the toxic substance is expected to occur during typical environmental conditions expected to be encountered.

(ii)      Scientifically valid field studies or other relevant information address other factors that affect the level of toxic substances in the water column, including all of the following factors:

(A)    )  Sediment release or resuspension.

(B)    )  Chemical speciation.

(C)    Biological and chemical transformation.

History: 1979 AC; 1984 AACS; 1986 AACS; 1997 AACS; 2006 AACS.