From the 1984 USEPA "Water Quality Handbook" (bold mine), the (regulatory) mixing zone is defined as an "allocated impact zone" where numeric water quality criteria may be exceeded as long as acutely toxic conditions are prevent. A (regulatory) mixing zone can be thought of as a limited area or volume where the initial dilution of a discharge occurs. Water quality criteria apply at the boundary of the (regulatory) mixing zone, not within the mixing zone itself.
Furthermore, "the area or volume of an individual (regulatory) mixing zone or group of (regulatory) mixing zones be limited to an area or volume as small as practicable that with not interfere with the designated uses or the established community of aquatic life in the segment for which the uses are designated," and the shape be "a simple configuration that is easy to locate in the body of water and avoids impingement on biologically important areas", and the "shore hugging plumes should be avoided."
Within the (regulatory) mixing zone, USEPA requires "any (regulatory) mixing zone should be free from point or nonpoint source related:
- Material in concentrations that will cause acute toxicity to aquatic life;
- Materials in concentrations that settle to form objectionable deposits;
- Floating debris, oil scum and other matter in concentrations that form nuisances;
- Substances in concentrations that produce objectionable color, odor, taste or turbidity;
- Substances in concentrations which produce undesirable aquatic life or result in a dominance of nuisance species.