A mixing zone is a designated area of a stream or river into which wastewater is permitted to empty.
Water quality standards may be exceeded in a mixing zone, but only if doing so poses no risk of degrading natural habitat, public health or recreation.
In New Zealand, regional councils are responsible for managing the mixing zone consent process.
During this process, measurements are made of the ecological effects of the discharge on the receiving water, based on the concentration of pollutant compared to the physical characteristics of the body of water.
NIWA