We can define a mixing zone as an explicitly defined area around an effluent discharge where some, or all, water quality objectives may not be met.
Under some circumstances, it is an accepted practice to apply the concept of a mixing zone. As a consequence, some community values of the water body may not be protected.
The size of a mixing zone — and the environmental conditions within it — are important concerns, particularly because degraded areas around effluent discharges compromise community values.
How to apply a mixing zone
If mixing zones are to be applied, then they should be as small as practicable. Water quality management should ensure that the agreed and designated community values and management goals of the broader waterway outside the mixing zone are not compromised.
If a mixing zone is applied to a river or estuarine waterway, then it is essential to ensure it does not extend across the entire width of the waterway and impact on biota migration pathways.
Mixing zones are regulated at the state or territory level in Australia and the regional council level in New Zealand. You should seek specific guidance on mixing zones from your relevant local jurisdiction.
Example guidance from some jurisdictions:
- Guidance for the Determination and Assessment of Mixing Zones, EPA Victoria
- Mixing Zones—Environmental Guidelines, Northern Territory EPA
- Wastewater Release to Queensland Waters—ESR/2015/1654, Queensland Department of Environment and Heritage Protection
- TR2010/046 A Review of Definitions of “Mixing Zones” and “Reasonable Mixing” in Receiving Waters, Auckland Council