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Water: Mixing Zones

Basic Information

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What is a "Mixing Zone"?

According to EPA's Technical Support Document for Water Quality-based Toxics Control (TSD) (USEPA, 1991), "a mixing zone is an area where an effluent discharge undergoes initial dilution and is extended to cover the secondary mixing in the ambient waterbody. A mixing zone is an allocated impact zone where water quality criteria can be exceeded as long as acutely toxic conditions are prevented." (Water quality criteria must be met at the edge of a mixing zone.)

Mixing Zone Images

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Types of Mixing Zones Covered

Allocated impact zone (AIZ): According to "Allocated Impact Zones for Areas of Non-Compliance" (USEPA, 1995), an allocated impact zone is the same as a mixing zone. The term is also used in "Water Quality Standards Regulation; Proposed Rule" (USEPA, 1998).
Legal mixing zone (LMZ): Refers to a mixing zone in a regulatory sense (e.g., the dimensions of the zone as the state has defined them) as opposed to the mixing zone that naturally occurs in a stream. This term is used in "Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations" (USEPA, 1992) and "CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of Conventional and Toxic Multiport Diffuser Discharges (USEPA, 1991)."
Toxic dilution zone (TDZ): According to "Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations" (USEPA, 1992), the toxic dilution zone, which is a short distance from the outfall or in the pipe itself, is an additional subregion within the usual mixing zone. The TDZ is usually more restrictive than the legal mixing zone for conventional and nonconventional pollutants. This term is also used in "CORMIX2: An Expert System for Hydrodynamic Mixing Zone Analysis of Conventional and Toxic Multiport Diffuser Discharges" (USEPA, 1991).
Zone of initial dilution (ZID): According to "Technical Guidance Manual for Performing Waste Load Allocations, Book III: Estuaries. Part 3: Use of Mixing Zone Models in Estuarine Waste Load Allocations" (USEPA, 1992), the zone of initial dilution is a regularly shaped area (e.g., circular or rectangular) surrounding the discharge structure (e.g., submerged pipe or diffuser line) that encompasses the regions of high (exceeding standards) pollutant concentrations under design conditions. This term is also used in "Initial Mixing Characteristics of Municipal Ocean Discharges: Volume 1 Procedures and Applications." (USEPA, 1985).

Code of Federal Regulations (CFR) Language

Section 131.13 general policies. States may, at their discretion, include in their state standards, policies generally affecting their application and implementation, such as mixing zones, low flows and variances. Such policies are subject to EPA review and approval.

Types of Pollutants Addressed

Toxic pollutants: sometimes referred to as "priority pollutants." EPA identified 126 pollutants from the 65 families of pollutants specified in Section 307(a) of the Clean Water Act. These pollutants are listed at 40 CFR Part 423, Appendix A.
Conventional pollutants: the five pollutants as defined by Section 304(a)(4) of the Clean Water Act and listed at 40 CFR 401.16. Those are biochemical oxygen demand (BOD), total suspended solids (nonfilterable) (TSS), pH, fecal coliform, and oil and grease.
Nonconventional pollutants: any pollutant not already defined as a toxic or conventional pollutant.

Types of Concentrations Discussed

CMC (criterion maximum concentration): the EPA national water quality criteria recommendation for the highest instream concentration of a toxicant or an effluent to which organisms can be exposed for a brief period of time without causing an acute effect (USEPA, 1991). The term applies to all pollutant types.
CCC (criterion continuous concentration): the EPA national water quality criteria recommendation for the highest instream concentration of a toxicant or an effluent to which organisms can be exposed indefinitely without causing unacceptable effect (USEPA, 1991). The term applies to all pollutant types.
RAC (reference ambient concentration): the concentration of a chemical in water which will not cause adverse impacts to human health; RAC is expressed in units of mg/L (USEPA, 1991).

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