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(1) No mixing zone will be granted if it would threaten or impair existing beneficial uses. Before any mixing zone is allowed, the applicant must provide information, as requested by the department, to determine whether a mixing zone will be allowed as well as the conditions which should be applied.

(2) In making its determination, the department will consider the following factors:

(a) Biologically important areas: the presence of fish spawning areas or shallow water nursery areas within the proposed mixing zone or a "shore hugging" effluent plume in an aquatic life segment will support a finding that the mixing zone may be inappropriate during the spawning or nursery periods.

(b) Drinking water or recreational activities: the existence of a drinking water intake, a zone of influence around a drinking water well or a well used for recreational purposes, or a recreational area within or immediately adjacent to the proposed mixing zone will support a finding that a mixing zone is not appropriate. For purposes of these rules, "recreational" refers to swimming and "recreational area" refers to a public beach or swimming area, including areas adjacent to streams or lakes.

(c) Attraction of aquatic life to the effluent plume: where currently available data support a conclusion that fish or other aquatic life would be attracted to the effluent plume, resulting in adverse effects such as acute or chronic toxicity, it may be appropriate to adjust a given mixing zone for substances believed to cause the toxic effects.

(d) Toxicity/persistence of the substance discharged: where a discharge of a parameter is at a concentration that is both toxic and persistent, it may be appropriate to deny a mixing zone. Toxicity and persistence will be given added weight to deny a mixing zone where the parameter is expected to remain biologically available and where a watershed-based solution has not been implemented. For ground water, this factor will also be considered in areas where the parameter may remain in the ground water for a period of years after the discharge ceases.

(e) Passage of aquatic organisms (including access to tributaries) : where currently available data indicate that a mixing zone would inhibit migration of fish or other aquatic species, no mixing zone may be allowed for the parameters that inhibit migration. In making this determination, the department will consider whether any parameter in the effluent plume will block migration into tributary segments.

(f) Cumulative effects of multiple mixing zones: in some cases, the existence of multiple or overlapping mixing zones may threaten or impair the existing uses of the receiving water, so that any additional mixing zone will be limited or denied for the parameter of concern.

(g) Aquifer characteristics: when currently available data indicate that the movement of ground water or pollutants within the subsurface cannot be accurately predicted, such as the movement of ground water through fractures, and also indicate that this unpredictability might result in adverse impacts due to a particular concentration of a parameter in the mixing zone, it may be appropriate to deny the mixing zone for the parameter of concern.

(h) Ground water discharges to surface water: In the case of a discharge to ground water which in turn discharges to surface water within a reasonably short time or distance, the mixing zone may extend into the surface water, and the same considerations which apply to setting mixing zones for direct discharges to surface water will apply in determining the allowability and extent of the mixing zone in the surface water.

(i) Discharges to intermittent and ephemeral streams: the "natural condition" of these waters during periods of no flow will be the average quality that occurs during periods when flow is present. If a proposed discharge occurs when there is no flow, the quality of the discharge must be at or better than this quality. If variations in seasonal stream flow are known and a mixing zone is limited to use during periods when dilution is available, such a mixing zone may be allowed by the department.

History: 75-5-301, MCA; IMP, 75-5-301, MCA; NEW, 1994 MAR p. 2136, Eff. 8/12/94; TRANS, from DHES, 1996 MAR p. 1499.

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