SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET ORDER NO. 97-115 (revised) 5 CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUIN COUNTY PREDICTION OF THE WORST CASE MIXING ZONE Manteca examined a number of"worst case" conditions, looking for the combination of discharge and receiving water conditions which would produce the maximum impact op the river. The worst case mixing zone was considered to occur during drought conditions with high plant effluent flow. During these conditions, the river flow is at its lowest and tidal influences are at the maximum. At 641 cfs (summer) river flow, tidal conditions strongly impacted the mixing zone analysis. Periods of rising tides may persist up to four hours. During these periods, river flow reversal will occur. Low tides may persist for up to three hours. During low tides, the river is at continuous slack conditions, and river velocities are less than 0.56 ft/sec (summer). A qualitative mixing analysis showed that at low river flows the effluent migrates across the River channel perpendicular to the River bank. At low tide periods (when river velocity is less than 0.56 ft/sec.), a 3-hour volume of undiluted effluent would potentially impact the entire 300+ foot width of channel. The acute and chronic toxicity concentration limits for ammonia would be exceeded across the river and 23 feet above and below the discharge location. The Discharger determined that this level of impact was unacceptable and proposes to mitigate this situation using existing storage ponds to store the treated effluent during rising tide periods when slack flow and flow reversals occur. Included in the analysis of the worst case mixing zone was an assumption that 2.5 million gallons of treated effluent would be stored for 8 hours per day. Worst Case Mixing Zone—Summer Conditions (June—September) River flow was assumed to be 641 cfs. Acute Toxicity Mixing Zone Acute toxicity calculations used a river pH of 8.5, and a background river ammonia concentration of 0.2 mg/l. Plant effluent discharge pH was 7.5, and effluent ammonia concentration was 31 mg N/1. The acute mixing zone was predicted to be: • Cigar-shaped plumes emanating from each diffuser port, extending a maximum of 40 feet down river. The plumes gradually widen as the effluent is mixed with river water to a maximum of 2.0 feet in diameter. The ammonia concentration decreases along the length of the plume. (Attachment D) • The cumulative cross sectional area of the 12 plumes is*30 square feet, and would cover a maximum of 1.41% of the river's cross sectional area.