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
Universal Citation: MT Admin Rules 17.30.506
Current through Register Vol. 6, March 22, 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.
75-5-301, MCA; IMP, 75-5-301, MCA;
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