A mixing zone is an area of a lake or river where pollutants from a point source discharge are mixed, usually by natural means, with cleaner water. In the mixing zone, the level of toxic pollutants is allowed to be higher than the acceptable concentration for the general water body. The mixing zone is an area where the higher concentration is diluted to legal limits for water quality. Outside the mixing zone, the pollutant levels must meet water quality standards. A typical mixing zone consists of two parts: the zone of initial dilution (ZID), near the outfall, and the chronic mixing zone from the ZID out to where water quality criteria are met. The discharge into the mixing zone may be effluent from water treatment plants, chemicals, or hot water from cooling towers.
The U.S. Environmental Protection Agency (EPA) is taking steps to ban the use of mixing zones for toxic chemicals. The Great Lakes Initiative (2000) also bans the discharge of twenty-two chemicals considered to be bioaccumulative. Bioaccumulative chemicals (BCCs) are those that become more concentrated as they move up through the food chain, for instance, from aquatic insects to fish to humans. As the release of BCCs into water bodies is phased out, industries will need to treat the discharge at the source.
"Identification of Approved and Disapproved Elements of the Great Lakes Guidance Submissions From the States of Michigan, Ohio, Indiana, and Illinois, and Final Rule." (2000). In Federal Register 65:151.
Great Lakes Initiative Fact Sheet. Available from http://www.epa.gov/ost/GLI/mixingzones/finalfact.html .