Mixing zones are areas where:
A mixing zone is not defined by a physical process, but is a regulatory construct designed to allow dilution of an effluent before regulatory standards have to be met. The size of the mixing zone is designed to minimize environmental impacts, but also allow for a reasonable amount of dilution. Mixing processes do not stop at this arbitrary boundary. The 'mixing zone' is simply the region within which a contaminant plume is legally allowed to interact with the surrounding water. The 'contaminant' may be heat, a chemical or any other quantity that presents a possible impact to the environment.
Maryland has regulated thermal discharges since 1978, but only recently has 'expert' analysis become available to PC users. If such a computer model could accurately predict the physical behavior of thermal plumes, the environmental effects of different discharge designs could be simulated, and energy producers could demonstrate compliance without expensive field surveys.
Within a mixing zone, the concentration of a contaminant is allowed to exceed certain regulatory limits, as long as it reaches those limits at the mixing zone boundary. States can set limits on the size and shape of mixing zones, the concentration of a contaminant at the mixing zone boundary, and the maximum acute level of the contaminant allowed within the mixing zone itself.
Mixing zones are used primarily at the state level to regulate local impacts. Maryland's criteria for mixing zones are described in its Water Quality Standards. The US Environmental Protection Agency (USEPA) sets mixing zone requirements in addition to inidividual state criteria. According to the USEPA requirements, mixing zones:
This list is not complete but provides a general sense of USEPA's requirements. Individual states are free to regulate mixing zone dimensions and other specific criteria.
More on Thermal Plumes
The Cornell Mixing Zone Expert System (CORMIX) is a PC-based software package for the analysis, prediction and design of chemical and thermal discharges into diverse water bodies. The model focuses on prediction of the geometry and dilution characteristics of the initial mixing zone so that compliance with regulatory constraints can be evaluated. CORMIX also predicts plume behavior at larger distances.
More on CORMIX
This study investigates the ability of the Cornell Mixing Zone Expert System (CORMIX) to simulate thermal discharges from four Maryland power plants.
Introduction and Methods for this study.
Results and Discussion for the Calvert Cliffs, Chalk Point, Dickerson and Wagner power plants.
Detailed results are available as a published technical report titled "Validation of the CORMIX Model Using Thermal Data from Four Maryland Power Plants"