Modeling Thermal Impacts in Rivers and Estuaries


Power plants take in cold surface water and return heated water to the environment. These discharges mix with receiving waters within designated 'mixing zones' - geographic areas within which water quality standards may be exceeded.  The purpose of this investigation was to test the mixing zone model CORMIX (CORnell MIXing Zone Expert System) using measured thermal plume data from four Maryland power plants (Calvert Cliffs, Chalk Point, Dickerson, and Wagner). These facilities were chosen to represent a range of discharge environments used by power plants in the state, including a large freshwater river (the Potomac), a narrow tidal estuary (the Patuxent), a large and wide tidal estuary (the Chesapeake Bay), and a wind-driven tidal estuary (Baltimore Harbor). The availability of extensive historical thermal plume data provided an excellent source for validating the model and demonstrating its utility and limitations in a variety of circumstances.

To learn more, either continue reading below or proceed to a specific section by using the following links:

What is a 'Mixing Zone'?
The CORMIX Model
The Maryland Study

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What is a 'Mixing Zone'?

Cross section of a mixing zone

Cross section of a mixing zone for a submerged discharge into a river channel.  Adapted from Jirka et al., 1996.

Mixing zones are areas where:

  • initial dilution of a discharge takes place
  • certain water quality standards may be exceeded

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.

Plan view of a mixing zone

Plan view of an idealized mixing zone for a surface coastal discharge.  Water quality standards are applied at the boundaries of the mixing zone.  Adapted from Jirka et al., 1996.

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:

  • cannot experience acutely toxic conditions
  • cannot contain contaminants in concentrations great enough to form surface scum or precipitate out of solution
  • cannot contain substances in concentrations that favor undesireable aquatic life or result in a dominance of 'nuisance species'
  • should avoid 'biologically important areas'
  • shore-hugging plumes in particular should be avoided

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 CORMIX Model

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

The Maryland Study

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"


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This page was updated on March 31, 2001.