Wastewater discharges from industrial, municipal, or other sources may be permitted so long as the quality of the water body receiving the discharges is still protected. However, it is not always necessary to meet water quality criteria within the discharge outfall to protect the integrity of the water body as a whole. Except when the receiving stream does not meet water quality criteria, it is usually appropriate to allow a mixing zone as an area of transition from a higher effluent concentration to a lower ambient concentration. Concentrations within this zone may exceed criteria so long as the area is kept relatively small.
A mixing zone is a defined area or volume of a receiving water body surrounded by or adjacent to a wastewater discharge where the receiving water, as a result of the discharge, may not meet all applicable water quality criteria. This zone is where wastewater mixes with receiving water for dilution and is not where effluents are treated.
Idaho's mixing zone policy states that after a biological, chemical, and physical appraisal of the receiving water and the proposed discharge, and after consultation with the persons responsible for the wastewater discharge, DEQ will consider the applicability of a mixing zone and, if applicable, its size, configuration, and location.
In defining a mixing zone, several guidelines should be followed:
- The mixing zone should not interfere with beneficial uses.
- Water quality within a mixing zone may exceed chronic water quality criteria so long as these criteria are met at the boundary of any approved mixing zone.
- Acute water quality criteria may be exceeded within a zone of initial dilution inside the mixing zone.
- The mixing zone may not be acutely toxic to biota significant to the receiving water's aquatic community. This determination usually involves considering how long a drifting organism could be exposed to acutely harmful conditions.
- The mixing zone should be limited to 25% of the width and volume of the stream to allow a zone of passage for aquatic life.
These guidelines are to be considered when DEQ determines the applicability of mixing zones and do not constitute stand-alone regulatory requirements. DEQ is not limited to considering these guidelines in its exercise of discretion implementing the mixing zone policy. For example, narrative requirements that waters be free from toxic substances in amounts that impair beneficial uses also apply in mixing zones.
DEQ's Mixing Zone Implementation Practices
It has been DEQ's practice that if an applicant requests a standard mixing zone that follows the principle limiting the mixing zone to 25% of the width and volume of the stream, DEQ may certify the mixing zone without requiring case-specific justifications from the applicant. If an applicant seeks to use greater than 25% of the width or volume of a receiving stream, a mixing zone analysis is required before DEQ will certify the mixing zone. DEQ should be consulted early in the planning of a mixing zone analysis so that agreement can be reached concerning the objectives of the analysis, adequacy of existing data, additional data needs, analysis methods, and schedule.
The scope and focus of a mixing zone analysis depends on case-specific factors. However, the following topics should be included:
- Background water chemical concentrations
- Biological conditions in the receiving waters
- The mixing zone's effect on beneficial uses of the receiving water or downstream water (i.e., demonstration that the mixing zone will not interfere with beneficial uses)
- Acute and chronic toxicity of the effluent
- The mixing zone's effect on fish passage or aquatic life in the stream (i.e., show that the mixing zone does not interfere with these)
- Risk of persistent bioconcentrating chemicals in the discharge
- Relative flows of effluents and receiving waters
- Physical boundaries at which acute and chronic water quality criteria will be met
- The mixing zone's and permitted pollutant's effect on threatened or endangered species or habitat (i.e., show that the size of the mixing zone and the resulting levels of permitted pollutant in the receiving water would not be likely to jeopardize these)
A technical procedures manual has been developed (see link at right) to guide DEQ staff, federal water quality permit writers, and regulated entities on conducting a biological, chemical, and physical appraisal of the receiving water, accounting for data limitations, determining the appropriate mixing zone model, and selecting model input parameters.
DEQ undertook a mixing zone analysis for the Thompson Creek mine that evaluated the issues listed above and serves as a good example for others to follow. This report includes a section on antidegradation with respect to special resource waters. Special resource waters were originally designed to be a designation of water bodies within the state that deserved an additional level of protection. With the initiation of antidegradation implementation rules in 2011, the designation “special resource waters” was removed from water quality standards. This report also includes an evaluation of the validity of whole effluent toxicity testing to evaluate compliance with Idaho narrative "free from toxicity" water quality standards.
- Evaluation of Proposed New Point Source Discharges to a Special Resource Water and Mixing Zone Determinations: Thompson Creek Mine facility, Upper Salmon River Subbasin, Idaho (December 2000)