What is a "regulatory mixing zone"?
A mixing zone is an area where wastewater discharged from a
permitted facility enters and mixes with a stream or water body. A
mixing zone is an established area where water quality standards may
be exceeded as long as acutely toxic conditions are prevented and
all beneficial uses, such as drinking water, fish habitat,
recreation, and other uses are protected.
DEQ calculates mixing zones to be as small as feasible. The size
of the area or zone varies based on how concentrated the
wastewater discharge is, water quality standards, location of the
discharge in relation to critical habitat or drinking water intakes,
and size or flow of the waterbody. Not all permitted facilities have
mixing zones. Most mixing zones in Oregon vary in size from 5 to 300
feet from the point of discharge.
|Figure 1: Example of a Typical Mixing Zone
Mixing zones are designed to be protective
DEQ does not permit mixing zones in locations where there is
long-term (chronic) human exposure, such as wading beaches or
dinking water intakes. Furthermore, DEQs water quality rules do not
allow for the discharge of toxic pollutants in concentrations that
would be dangerous for people and wildlife if they swim or float
through a mixing zone. Figure 2 is an example of the type of
modeling DEQ uses to evaluate the mixing zones and more information
can be found at the
CORMIX Mixing Zone Model Home
Mixing Zones are just one part of the process DEQ uses to protect
The calculation and alignment of a mixing zone is an important
component of the overall permitting process that includes the following
- Review of the ambient water quality of the receiving water
- In-depth inspection and review of facilities requesting
permits or renewals
- Ongoing monitoring of all facility effluent
- Effluent sampling and characterization
- Evaluation of the receiving water body to prevent any
degradation of water quality
- Development of water effluent limits to ensure the
protection of public health and the environment
|Figure 2: Example of Water Quality Modeling
Are mixing zones legal?
Yes. Under federal rules, the use of mixing zones is approved in
most states, including Oregon. Mixing zones are required to protect
the beneficial uses of water such as drinking water, fish habitat,
recreation, and irrigation.
How are mixing zones regulated?
The federal Environmental Protection Agency (EPA) allows states to
adopt their own mixing zone regulations as part of the states water
quality standards (40 CFR 131.13). These state regulations are subject
to review and approval by EPA. Oregons mixing zone rule has been
approved by EPA and can be found at
Oregon Administrative Rule (OAR) 340-041-0053.
In addition, there is both federal and state guidance on when mixing
zones may be allowed. EPA provides guidance on mixing zones in
Water Quality Standards Handbook, August 1994 and
Technical Support Document for Water Quality-based Toxics Control,
March 1991 PDF (6.5MB). DEQ has recently developed guidance
documents to assist permit writers to properly size and locate the
zones so that they are more protective of the environment and public
health. These documents are located on the web site at:
Who is subject to mixing zone regulations?
The mixing zone regulations currently affect about 300 facilities
(including both municipal wastewater treatment plants and industries)
that operate under existing water quality permits. DEQ will review and,
if necessary, update each mixing zone every five-years during the
facilities permit renewal cycle. Any permit applicants requesting a
mixing zone would also need to conform to the mixing zone regulations.
Where are mixing zones located?
Information about specific mixing zones is available by:
your regional DEQ office and ask for the Water Quality Program.
The staff will help in accessing public records which describe
permitted facilities and the location of their associated mixing
Access DEQs Wastewater Permits Database. The database can be
queried based upon county, city, zip code or watershed name, and
will report location by address and geographic coordinates.
- DEQ has developed maps of the location of major sewage treatment
plants and industrial facilities with mixing zones on the Willamette
River. The maps show the approximate river mile location of the
facilities and contain information from the permit and related
How do I get information on regulatory mixing zones in my area?
For more detailed on a particular permit or mixing zone,
contact the nearest DEQ office to schedule an appointment to
look at permit files. The files will include specific information about
the mixing zone, including its size and location. There is also a public
comment and review process associated with each new permit and permit
Available education resources
DEQ recognizes that the subject of mixing zones can be complex and
would like to recommend the following education references:
DEQ staff is also available to provide informational presentations
for interested parties concerning mixing zones and their calculation.
For more information, please contact:
- Sonja Biorn-Hansen
Surface Water Management Section, Water Quality Division, (503)