Mixing Zones and Regulatory Mixing Zones

A mixing zone is the region in which the initial dilution of a discharge occurs. Many hydrodynamic definitions of mixing zones include both near-field mixing as well as boundary interactions processes.

In contrast, the regulatory mixing zone is a definition which allows for the initial dilution of a discharge rather than imposing strict end-of-pipe concentration requirements for NPDES water quality permits for conventional and toxic discharges. It can occur in the near-field or in the far-field after boundary interaction occurs. In theory, the regulatory mixing zone may therefore allow for efficient natural pollutant assimilation. In practice they can be used as long as the integrity of a water body as a whole is not impaired.

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:

1. Material in concentrations that will cause acute toxicity to aquatic life;
2. Materials in concentrations that settle to form objectionable deposits;
3. Floating debris, oil scum and other matter in concentrations that form nuisances;
4. Substances in concentrations that produce objectionable color, odor, taste or turbidity;
5. Substances in concentrations which produce undesirable aquatic life or result in a dominance of nuisance species.

CMC is spatially more restrictive than the CCC. The CCC is often treated like a water quality standard, it must be met at the edge of the same regulatory mixing zone specified for conventional or toxic pollutants.

USEPA allows a toxic dilution zone within the regulatory mixing zone, but it must comply with one of four of the following criteria:

In practical application, the regulatory mixing zone may occur in the near-field or in the far-field after boundary interaction occurs. In practice, regulatory mixing zones may be specified by a length, area, or volume around the discharge source. These images are visualizations of both near-field and far-field regulatory mixing zones using the CorVue tool.

Within the regulatory mixing zone, criterion continuous concentration or CCC values may be specified for conventional and toxic discharge, while a more restrictive spatial region for toxic discharges called a toxic dilution zone (TDZ) with criterion maximum concentration or CMC values may be specified.