Administrative Rules of Montana › Department 17 - ENVIRONMENTAL QUALITY › Chapter 17.30 - WATER QUALITY › Subchapter 17.30.5 - Mixing Zones in Surface and Ground Water › Rule 17.30.506 - WATER QUALITY ASSESSMENT

Current through Register Vol. 13, July 5, 2024
(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.