These standards, except water quality criteria associated with aesthetics (Chapter 4, 005) and recreation (Chapter 4, 002) will not apply within mixing zones unless specified below.
Mixing zones for the initial assimilation of effluents or wastewaters may be necessary where discharges that have received the applicable level of treatment or control still do not adequately protect the water quality of a receiving stream. Mixing zones are to be limited to as small an area and volume of a receiving stream as is practical to prevent interference with or impairment of any beneficial uses. The requirements of mixing zones for heat are to be defined on a site-specific basis, in a manner consistent with Section 316 of the Clean Water Act.
010.01 The Department determines the applicability of a mixing zone, and if applicable, the allowable size, location, water quality, and outfall design. The following requirements will be used in defining all mixing zones. These requirements are not intended to define each individual mixing zone, but represent maximum limits which will satisfy most biological, chemical, physical, and radiological considerations. A smaller mixing zone may be required or no zone at all allowed, as necessary, in order to meet these requirements.010.02 The appropriateness, if any, of establishing a mixing zone for a pollutant which may be bioaccumulative, persistent, carcinogenic, mutagenic, or teratogenic will be carefully evaluated by the Department. In such cases, effects such as potential ground water contamination, known or predicted safe exposure levels for human health, bioaccumulation in aquatic life, fish attraction, sediment deposition, and protection of downstream beneficial uses will be considered.010.03 Mixing zones established for dissolved oxygen are to take into account the delayed effects caused by oxidation of organic matter and ammonia inside and outside the mixing zone. One-day minimum dissolved oxygen criteria apply at the boundary of and beyond acute mixing zones, but not within acute mixing zones. All applicable dissolved oxygen criteria, including the one-day minimum criteria, are to be met at and beyond the mixing zone boundaries.010.04 Mixing zones established for discharges impacting agricultural water supply criteria are to be based on the restrictions established for chronic mixing zones ( 010.06).010.05 All mixing zone specifications are to be based on critical conditions of minimum dilution. Flow variable calculations that use real-time flows for a point source discharge and receiving stream may be allowed to determine critical conditions of minimum dilution. If flow variable critical conditions are not defined, critical conditions are to be determined as follows. The average dry weather or seasonal flow for a point source discharge will be used with the 7-day 10-year low flow of the receiving stream for application of all criteria with the exception of thirty-day average ammonia criteria and acute criteria for aquatic life. The 30-day 5-year low flow of the receiving stream will be used for application of thirty-day average ammonia criteria. The 1-day 10-year low flow of the receiving stream will be used for application of acute criteria.010.06 Chronic Mixing Zones. Chronic toxicity to aquatic life will not be allowed at any time outside of a chronic mixing zone.
010.06A The length of a chronic mixing zone is not to exceed the following distances based on designated aquatic life use classes.010.06A1 Chronic mixing zones in Coldwater Class A, Coldwater Class B, and Warmwater Class B streams are to be designed to not exceed 2,500 feet in length.010.06A2 Chronic mixing zones in Warmwater Class A streams are to be designed to not exceed 5,000 feet in length.010.06B Chronic mixing zones are to be located in a receiving stream in such a manner that the maintenance of aquatic life and other beneficial uses will not be adversely affected.010.06B1 A chronic mixing zone is not to overlap with any other mixing zone unless it is demonstrated to the satisfaction of the Department (e.g. aquatic field studies, bioassays in the site water using resident or acceptable nonresident aquatic species) that the overlapping of the mixing zones will not result in any adverse effects to aquatic life or other beneficial uses.010.06B2 Chronic mixing zones are not to at any time:010.06B2a Extend across public drinking water supply intakes.010.06B2b Extend across heavily-used or state designated recreation bathing areas.010.06B2c Extend into publicly owned lakes and reservoirs listed in Chapter 6.010.06B2d Significantly impact federally and/or state designated threatened or endangered aquatic species.010.06C Water quality of chronic mixing zones. The Department may suspend the applicability of all or part of the water quality criteria within a chronic mixing zone, except those criteria relating to aesthetics (Chapter 4, 005) and acute toxicity to aquatic life (Chapter 4, 003.01C). In streams designated a recreational use, criteria relating to recreation (Chapter 4, 002) also apply within a chronic mixing zone. Waters at and beyond chronic mixing zone boundaries are to meet all chronic water quality criteria associated with the receiving stream any time the receiving streamflow is equal to or greater than 0.1 cfs for streams assigned a Coldwater Class A, Coldwater Class B, or Warmwater Class A Aquatic Life use; 1.0 cfs for streams assigned the Warmwater Class B Aquatic Life use; or its 7-day 10-year low flow (30-day 5 year low flow in the case of thirty-day average ammonia criteria), whichever is greater. To prevent chronic toxicity in a stream, the following conditions are to be met.
010.06C1 The pollutant levels or concentrations of wastewaters which contain unknown or complex mixtures of potentially additive or synergistic toxic pollutants are not to exceed 1.0 chronic toxic units (TUc) based on chronic bioassays representing the effluent dilution received at the chronic mixing zone boundary.010.06C2 Where more than one wastewater discharge is located in a specific area and the potential exists for additive or synergistic effects, the pollutant levels or concentrations in water from a receiving stream outside any mixing zone are not to exceed 1.0 TUc based on chronic bioassays.010.06C3 Where a mixing zone is not allowed by the Department, the pollutant levels or concentrations of the wastewater in the outfall structure itself are not to exceed the No Observed Effect Level (NOEL) based on chronic bioassays of the undiluted effluent. NOEL is the threshold concentration of a substance which causes no observed adverse effects to bioassay test organisms under test conditions specified or approved by the Department.010.07 Acute Mixing Zones. Acute toxicity to aquatic life will not be allowed at any time outside of an acute mixing zone.
010.07A Acute mixing zones are to allow at all times for a continuous zone of passage in the receiving stream for the movement or drift of aquatic biota. To provide for a zone of passage, the width of an acute mixing zone at any transect of the receiving stream is not to exceed more than 1/2 of the stream width. Where more than one wastewater discharge is located in a specific area, acute mixing zones are to be located in such a manner as to provide for a continuous zone of passage of at least 1/2 the stream width.010.07B The length of an acute mixing zone is not to exceed the following distances based on designated aquatic life use classes. 010.07B1 Acute mixing zones in Coldwater Class A, Coldwater Class B, and Warmwater Class B streams are to be designed to not exceed 125 feet in length or 5 percent of the length of the chronic mixing zone whichever is more restrictive.010.07B2 Acute mixing zones in Warmwater Class A streams are to be designed to not exceed 250 feet in length or 5 percent of the length of the chronic mixing zone whichever is more restrictive.010.07C Acute mixing zones are to be located in a receiving stream in such a manner that the maintenance of aquatic life and other beneficial uses will not be adversely affected. Acute mixing zones are not to at any time: 010.07C1 Extend across public drinking water supply intakes.010.07C2 Extend across heavily-used or state designated recreation bathing areas.010.07C3 Extend into publicly owned lakes and reservoirs listed in Chapter 6.010.07C4 Significantly impact federally and/or state designated threatened or endangered aquatic species.010.07C5 Extend across the mouth of a classified tributary stream segment.010.07D Water quality of acute mixing zones. The Department may suspend the applicability of all or part of the water quality criteria within an acute mixing zone, except those criteria relating to aesthetics (Chapter 4, 005). In streams designated a recreational use, criteria relating to recreation (Chapter 4, 002) also apply within the acute mixing zone. Waters at and beyond acute mixing zone boundaries are to meet all acute water quality criteria associated with the receiving stream any time the receiving streamflow is equal to or greater than 0.1 cfs or its 1-day 10-year low flow.
010.07D1 The pollutant levels or concentrations of wastewaters which contain unknown or complex mixtures of potentially additive or synergistic toxic pollutants are not to exceed 0.3 acute toxic units (TUa) based on acute bioassays representing the effluent dilution received at the acute mixing zone boundary.010.07D2 Where more than one wastewater discharge is located in a specific area and the potential exists for additive or synergistic effects, the pollutant levels or concentrations in water from a receiving stream outside any acute mixing zone are not to exceed 0.3 TUa based on acute bioassays.010.07D3 Where a mixing zone is not allowed by the Department, the pollutant levels or concentration of the wastewater in the outfall structure itself are not to exceed 0.3 TUa based on acute bioassays of the undiluted effluent.010.08 Mixing Zones for Public Drinking Water Supply Criteria. In waters designated as Water Supplies for Public Drinking Water, the criteria for protection of public drinking water supplies are not to be exceeded at any time outside of a mixing zone for public drinking water supply criteria.
010.08A Mixing zones for public drinking water supply criteria are to be designed to not extend to within a 2 mile zone of influence from any public drinking water supply intake.010.08B Mixing zones for public drinking water supply criteria are to be located in a receiving stream in such a manner that other beneficial uses will not be adversely affected.010.08C Water quality of mixing zones for public drinking water supply criteria. The Department may suspend the applicability of all or part of the water quality criteria for the protection of public drinking water supplies within a mixing zone for public drinking water supply criteria. Waters at and beyond boundaries of mixing zones for public drinking water supply criteria are to meet all public drinking water supply criteria any time the receiving stream is flowing equal to or greater than its 7-day 10-year low flow.
010.09 Outfall Design. Prior to designating a mixing zone, the Department will first approve pursuant to Title 123 - Rules and Regulations for Design, Operation, and Maintenance of Wastewater Treatment Works that the best practical engineering design for the outfall structure and its location and placement in the receiving stream have been applied, as necessary, to meet all mixing zone requirements for size, location, and water quality.
010.09A The following are acceptable circumstances for modifying the existing design, location, or placement of an outfall structure in a stream: 010.09A1 Where high-rate diffusers or similar devices are required to: (1) minimize or prevent exposure of aquatic biota to acutely toxic conditions within an acute mixing zone,(2) minimize or prevent exposure of aquatic biota to possible irreversible chronic effects within a mixing zone where wastewaters tend to attract aquatic organisms, or(3) otherwise expedite mixing and dispersion of wastewaters in order to meet mixing zone requirements for size, location, and water quality.010.09A2 Where changes are required in the location of an outfall structure (e.g., upstream, downstream, or to the opposite stream bank) or its placement (e.g., water depth, direction in relation to the stream current) in order to meet mixing zone requirements for size, location, and water quality.010.09B Water turbulence created by high-rate diffusers or similar devices is not to be of such a magnitude that the movement or drift of aquatic biota within a zone of passage is interfered with or prevented.117 Neb. Admin. Code, ch. 2, § 010
Amended effective 6/24/2019