Under EPA regulation 40 CFR 131.13, states “may, at their discretion, include in their State standards, policies generally affecting their application and implementation, such as mixing zones, low flows and variances. such policies are subject to EPA review and approval.”
Mixing zones are defined as “a limited area or volume of water where initial dilution of a discharge takes place and where numeric water quality criteria can be exceeded but acutely toxic conditions are prevented from occurring.”
Many states – including California – allow mixing zones. If you are applying for a permit to discharge your waterwater directly to the navigable waters of the United States, you might want to seriously consider including a mixing zone in your application. The benefits to you can be enormous.
Once you obtain a mixing zone for a particular wastewater parameter, you will only need to meet that parameter’s water quality standard at the edge of the mixing zone. Without a mixing zone, you will have to meet the water quality standard at the end of your discharge pipe. Mixing zones usually carry a dilution ratio of 100:1. That means you only need to treat your waste parameter down to 100 times the concentration of its water quality standard at the end of your pipe. The dilution effect of the mixing zone will bring the concentration down to the water quality standard level at the edge of the mixing zone.
To get a mixing zone, you will need to do computer modelling of your discharge plume taking into consideration the depth, water quality and flow conditions of your receiving water. You must follow your state agency’s guidelines on mixing zones.