Nebraska Administrative Code
Topic - ENVIRONMENT AND ENERGY
Title 117 - NEBRASKA DEPARTMENT OF ENVIRONMENTAL QUALITY
Chapter 2 - APPLICATION OF STANDARDS
Section 117-2-010
Current through November 25, 2023
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.
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.
010.07 Acute Mixing Zones.
Acute toxicity to aquatic life will not be allowed at any time outside of an acute mixing zone.
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.
010.08 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.
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.
Amended effective 6/24/2019