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Mixing Zone Studies Mixing zones are often used in discharge situations where effluent quality does not meet surface water quality standards and where state regulations allow for additional effluent mixing in the receiving water body. These mixing zone studies are typically developed for effluent parameters such as metals, toxic compounds and temperature. The Clean Water Act (CWA) allows mixing zones with specific regulatory requirements defined by the states. A mixing zone is defined as a limited area in a waterbody where ambient concentrations may exceed acute or chronic surface water quality standards but acutely toxic conditions are prevented. Mixing zones are typically comprised of two parts: an acute and chronic mixing zone. The acute mixing zone is the area of initial dilution, sometimes referred to as the zone of initial dilution (ZID), where acute criteria are met at the edge of this zone. Beyond the acute mixing zone and of a larger area is the chronic mixing zone where chronic criteria must be met.
HydroQual’s mixing zone or initial dilution analysis capabilities are outlined as follows:
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Chronic Mixing Zone Acute Mixing Zone |
Water Quality and Ecosystem Modeling | Water Supply | Watershed/Sewershed Modeling
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Estimating initial dilution can either be accomplished through mathematical modeling (initial dilution models) or through dye studies, where the latter is only valid for existing discharges. HydroQual has extensive experience with the EPA supported near-field modeling packages that are available for use such as CORMIX and PLUMES. In addition, HydroQual has employed various other modeling approaches to analyze initial dilution for mixing zones and these studies have used steady-state and time-variable far-field (hydrodynamic) models. HydroQual has completed numerous mixing zones studies around the country and overseas for single- and multi-port submerged diffusers, surface (bank) discharges for assessing dilution and developing mixing zones for existing discharges and also for proposed discharges. These studies have been completed for conventional and toxic substances and also for thermal discharges. In many studies, HydroQual has also been involved in the design of proposed diffusers that included iterations between dilution and diffuser hydraulics for determining additional pumping requirements at treatment facilities, as well as in the design of follow-up dye studies to verify outfall dilution.
