(1) Mixing zones for surface waters are subject to the following water quality standards:
(a) narrative water quality standards, standards for harmful substances, numeric acute and chronic standards for aquatic life; standards in Department Circular DEQ-12A; and standards based on human health must not be exceeded beyond the boundaries of the surface water mixing zone;
(b) acute standards for aquatic life for any parameter may not be exceeded in any portion of a mixing zone, unless the department specifically finds that allowing minimal initial dilution will not threaten or impair existing beneficial uses.
(2) Discharges to wetlands (other than constructed wetlands) will not be granted a mixing zone for parameters for which the state has adopted numeric acute or chronic standards for aquatic life or for human health in the surface water quality standards, unless the following can be demonstrated to the satisfaction of the department:
(a) the standards referenced in (1) will not be exceeded beyond the boundaries of the mixing zone;
(b) existing beneficial uses will not be threatened or harmed; and
(c) the conditions in 75-5-303(3), MCA, are met.
(3) For discharges to surface water that first pass through the ground, such as discharges from infiltration systems or land application areas, the surface water mixing zone begins at the most upstream point of discharge into the receiving surface water. If the discharge continues to occur downstream beyond a distance equal to 10 times the stream width measured at the upstream discharge point at low flow, a standard mixing zone will not be granted.