Idaho Administrative Code › Title IDAPA 58 - Environmental Quality, Department of › Rule 58.01.02 - WATER QUALITY STANDARDS › Section 58.01.02.060 - MIXING ZONE POLICY

Current through August 31, 2023
01.
Mixing
			 Zones for Point Source Discharges
. Whether a mixing zone is authorized,
			 and its size, configuration and location, is determined by the Department on a
			 case-by-case basis. This determination is made in accordance with the
			 provisions of Section
			 060 at the time a permit is issued,
			 renewed, or materially modified and is in effect as long as the permit remains
			 in effect. Such an authorization is required before a mixing zone can be used
			 to determine the need for, or level of, effluent limits for a particular
			 pollutant. (3-31-22)
a. Mixing zones shall not
			 be authorized for a given pollutant when the receiving water does not meet
			 water quality criteria for that pollutant; provided, however, the Department
			 may authorize a mixing zone when the permitted discharge is consistent with an
			 approved TMDL allocation or other applicable plans or analyses (such as 4b
			 implementation plans, watershed loading analyses, or facility-specific water
			 quality pollutant management plans) that demonstrate that there is available
			 assimilative capacity and authorizing a mixing zone is consistent with
			 achieving compliance with water quality standards in the receiving water.
			 (3-31-22)
b. Water quality within
			 an authorized mixing zone is allowed to exceed chronic water quality criteria
			 for those parameters approved by the Department. If approved by the Department,
			 acute water quality criteria for one (1) or more parameters may be exceeded
			 within the zone of initial dilution inside the mixing zone. Narrative criteria
			 in Subsections 200.03 and
			 200.05 apply within the mixing
			 zone. All water quality criteria must be met at the boundary of any mixing zone
			 under its design conditions. (3-31-22)
c. The size of mixing zone(s) and the
			 concentration of pollutant(s) present shall be evaluated based on the permitted
			 design flow. The Department shall not authorize a mixing zone that is
			 determined to be larger than is necessary considering siting, technological,
			 and managerial options available to the discharger. (3-31-22)
d. Mixing zones, individually or in
			 combination with other mixing zones, shall not cause unreasonable interference
			 with, or danger to, beneficial uses. Unreasonable interference with, or danger
			 to, beneficial uses includes, but is not limited to, the following: (3-31-22)
i. Impairment to the integrity of the aquatic
			 community, including interfering with successful spawning, egg incubation,
			 rearing, or passage of aquatic life. (3-31-22)
ii. Heat in the discharge that causes thermal
			 shock, lethality, or loss of cold water refugia. (3-31-22)
iii. Bioaccumulation of pollutants (as
			 defined in Section 010)
			 resulting in tissue levels in aquatic organisms that exceed levels protective
			 of human health or aquatic life. (3-31-22)
iv. Lethality to aquatic life passing through
			 the mixing zone. (3-31-22)
v.
			 Concentrations of pollutants that exceed Maximum Contaminant Levels at drinking
			 water intake structures. (3-31-22)
vi. Conditions which impede or prohibit
			 recreation in or on the water body. Mixing zones shall not be authorized for
E. coli
. (3-31-22)
e. Multiple nested mixing zones may be
			 established for a single point of discharge, each being specific for one (1) or
			 more pollutants contained within the discharge. (3-31-22)
f. Multiple mixing zones may be established
			 for a single activity with multiple points of discharge. When these individual
			 mixing zones overlap or merge, their combined area and volume shall not exceed
			 that which would be allowed if there was a single point of discharge. When
			 these individual mixing zones do not overlap or merge, they may be authorized
			 as individual mixing zones. (3-31-22)
g. Adjacent mixing zones of independent
			 activities shall not overlap. (3-31-22)
h. Mixing zones shall meet the following
			 restrictions; provided, however, that the Department may authorize mixing zones
			 that vary from the restrictions under the circumstances set forth in Subsection
			 060.01.i. below: (3-31-22)
i. For flowing waters: (3-31-22)
(1) The width of a mixing zone is not to
			 exceed twenty-five percent (25%) of the stream width; and (3-31-22)
(2) The mixing zone shall not include more
			 than twenty-five percent (25%) of the low flow design discharge conditions as
			 set forth in Subsection
			 210.03.b. of these rules.
			 (3-31-22)
ii. For all new
			 discharges to nonflowing waters authorized after July 1, 2015: (3-31-22)
(1) The size of the mixing zone is not to
			 exceed five percent (5%) of the total open surface area of the water body or
			 one hundred (100) meters from the point of discharge, whichever is smaller;
			 (3-31-22)
(2) Shore-hugging plumes
			 are not allowed; and (3-31-22)
(3)
			 Diffusers shall be used. (3-31-22)
iii. For all existing discharges to
			 nonflowing waters authorized prior to July 1, 2015, the total horizontal area
			 allocated to the mixing zone is not to exceed ten percent (10%) of the surface
			 area of the lake. (3-31-22)
iv.
			 Lakes and reservoirs with a mean detention time of fifteen (15) days or greater
			 shall be considered nonflowing waters for this purpose. Detention time will be
			 calculated as the mean annual storage volume divided by the mean annual flow
			 rate out of the reservoir for the same time period.
			 (3-31-22)
i. The
			 Department may authorize a mixing zone that varies from the limits in
			 Subsection 060.01.h. if it is established
			 that: (3-31-22)
i. A smaller mixing zone is
			 needed to avoid an unreasonable interference with, or danger to, beneficial
			 uses as described in Subsection
			 060.01.d., and the mixing zone
			 meets the other requirements set forth in Section
			 060; or (3-31-22)
ii. A larger mixing zone is needed by the
			 discharger and does not cause an unreasonable interference with, or danger to,
			 beneficial uses as described in Subsection
			 060.01.d., and the mixing zone
			 meets the other requirements set forth in Section
			 060. The discharger shall provide
			 to the Department an analysis that demonstrates a larger mixing zone is needed
			 given siting, technological, and managerial options.
			 (3-31-22)
j. The following
			 elements shall be considered when designing an outfall: (3-31-22)
i. Encourage rapid mixing to the extent
			 possible. This may be done through careful location and design of the outfall;
			 and (3-31-22)
ii. Avoid
			 shore-hugging plumes in those water bodies where the littoral zone is a major
			 supply of food and cover for migrating fish and other aquatic life or where
			 recreational activities are impacted by the plume.
			 (3-31-22)