Mixing-Zone Modeling for Toxic Waste-Load Allocations

Abstract

Although some state regulations allow a mixing zone near a discharge outfall, many National Pollutant Discharge Elimination System (NPDES) permits still require that ambient water quality criteria be met at the end of the pipe. This paper presents a conservative approach to analyzing mixing zones for discharges with minimum initial dilution. An application of this approach to the effluent discharged from the Falling Creek wastewater-treatment plant to the James Estuary is presented. Model results have been approved by the regulatory agencies to lift an overly stringent effluent-toxicity limit while preventing adverse ecological impact on the receiving water.