(1) After an assessment of information received from the applicant concerning the biological, chemical, and physical characteristics of the receiving water, as specified in ARM 17.30.506 or as requested by the department, the department will determine the applicability of a mixing zone and, if applicable, its size, configuration, and location. In defining a mixing zone, the department will consider the following principles:
(a) Mixing zones may be granted for individual parameters present in a discharge.
(b) For new or increased sources, changes in water quality at the boundary of a mixing zone must be nonsignificant as defined by ARM Title 17, chapter 30, subchapter 7, unless the change in water quality is authorized by the department pursuant to 75-5-303 , MCA.
(c) For sources discharging under a permit issued by the department prior to April 29, 1993, any mixing zone allowed under the permit will remain in effect until renewal. Upon renewal, any previously allowed mixing zone will be designated in the renewed permit, unless there is evidence that the previously allowed mixing zone will impair existing or anticipated uses.
(d) In accordance with 75-5-306 , MCA, it is not necessary that industrial wastes, sewage, or other wastes, as defined in 75-5-103 , MCA, be treated to a purer condition than the natural condition of the receiving water as long as the minimum treatment requirements are met and provided all reasonable land, soil, and water conservation practices have been applied.
(e) Estimated parameter levels in the mixing zone area will be calculated, unless the department determines that monitoring is necessary due to the potential harm to the impacted water and its beneficial uses.
(f) In granting a mixing zone, the department may impose conditions as necessary to comply with the requirements of this subchapter.
(2) Where the department determines that allowing a mixing zone at a given level for a parameter would threaten or impair existing beneficial use pursuant to this subchapter, discharge limitations will be modified as necessary to prevent the interference with or threat to the beneficial use. If necessary, these modifications may require achieving applicable numeric water quality criteria at the end-of-pipe for the parameter so that no mixing zone will be necessary or granted.