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WASTE DISCHARGE REQWMENTS ORDER NO. 5-00-031 -5- CITY OF LODI WHITE SLOUGH WATER POLLUTION CONTROL PLANT SAN JOAQUIN COUNTY Standards Handbook and the Technical Support Document for Water Quality-based Toxics Control..." The Board finds that, based on water quality evidence and the applicable procedures guidelines currently available, provisions for a mixing zone by allowing compliance with water quality objectives to be determined at the confluence with Bishop Cut and White Slough should not be continued. Dredger Cut is a dead end slough, with minimal dilution in the vicinity of the discharge. Results of a study conducted over the previous permit term have shown that the average dilution is 2:1 over a tidal cycle at a location approximately 300 meters downstream of the outfall. This mixing zone covers the cross-sectional area of Dredger Cut. Because available dilution within the mixing zone is negligible, extending for at least 300 meters, the Board will not designate any mixing zone within which water quality objectives will not apply. The elimination of the mixing zone within Dredger Cut requires that this permit apply water quality objectives to Dredger Cut which have never been applied to this specific area of the water body. Effluent limitations for lead, zinc, and cyanide, as well as BOD (for compliance with the dissolved oxygen objective) and temperature have been included in this permit, as explained below, based on the application of water quality objectives within Dredger Cut. The Discharger is not currently capable of meeting these effluent limitations. This order provides for a time schedule for meeting the effluent limitations. The time schedule is authorized to be included in this order based on 40 CFR §122.47. The Board considers the elimination of the mixing zone in Dredger Cut for compliance with water quality objectives as a new interpretation of the Basin Plan. 23. The Basin Plan water quality objective for cyanide is a maximum concentration of 10 µg/1. EPA's ambient water quality criteria for protection of aquatic life are a continuous concentration of 5.2 µg/1 (30-day average, chronic criteria), and a maximum concentration of 22 µJ1 (1-hour average, acute criteria). Effluent sampling results submitted as part of the application, in studies, and as directed by monitoring and reporting programs show cyanide concentrations have exceeded the EPA acute and chronic criteria, as well as the Basin Plan objective. Background receiving water concentrations of cyanide are unknown, and are presumed to be non-detectable. The Board finds that cyanide is or may be discharged at a level that will cause or have the reasonable potential to cause, or contribute to, an in-stream excursion above the Basin Plan water quality objective. Effluent limitations for cyanide have been included in this Order based on the Basin Plan objective and the chronic criteria, with no mixing zone designated. The Discharger is not currently capable of meeting this limit. 24. The Basin Plan's numerical water quality objective for dissolved zinc is a maximum concentration of 100 µg/1, which is equivalent to a total recoverable concentration of 101 µ.g/1. EPA's ambient water quality criteria for protection of freshwater aquatic life are a continuous concentration of 133 µg/l (30-day average, chronic criteria), and a maximum concentration of 133 µg/1 (1-hour average, acute criteria), based on a water hardness of 113 mg/1. These criteria were originally developed using metals concentrations expressed as total recoverable metals, but have been converted to be expressed as dissolved metals. For zinc, the conversion factor (CF)