Abstract:
CORMIX is a water quality modeling and decision support system designed for
environmental impact assessment of mixing zones resulting from wastewater
discharge from point sources. The system emphasizes the role of boundary
interaction to predict plume geometry and dilution in relation to regulatory
mixing zone requirements. As an expert system, CORMIX is a user-friendly
... application which guides the water quality analysts in simulating a
site-specific discharge configuration. To facilitate its use, ample
instructions are provided, suggestions for improving dilution characteristics
are included, and warning messages are displayed when undesirable or uncommon
flow conditions occur.
Cormix contains three major subsystems. The first subsystem, CORMIX1, is used
to predict and analyze environmental impacts of submerged single port
discharges to lakes, rivers, and estuaries. The second subsystem, CORMIX2, may
be used to predict plume characteristics of submerged multiport discharges. The
third subsystem, CORMIX3, is used to analyze positively and neutrally buoyant
surface discharges to lakes, rivers, and estuaries with a high degree of
accuracy.
A mixing zone is a limited area where initial dilution of a discharge takes
place and where numeric water quality criteria can be exceeded but acutely
toxic conditions are prevented.
From the August 1994 EPA publication "Water Quality Standards Handbook: Second
Edition" (EPA-823-B94-005a) (PDF format, 18MB) allowable mixing zone
characteristics should be established to ensure that:
-mixing zones do not impair the integrity of the water body as a whole,
-there is no lethality to organisms passing through the mixing zone, and
-there is no significant health risk considering likely pathways of
exposure.
Also mixing zones should not be permitted where they may endanger critical
areas (e.g. , drinking water supplies, recreational areas, breeding grounds,
areas with sensitive biota).
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.
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.
[Summary provided by the EPA.] 
