SITE INFORMATION AND CORRESPONDENCE_1985-2004

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REVISED WASTE DISCHARGE REQUIREMENTS ORDER NO. 97-115 3 CITY OF MANTECA AND CITY OF LATHROP WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY 12. The current configuration of the Discharger's side bank outfall likely causes adverse toxicological effects to the benthic organisms along the river bottom. A mixing zone using the current outfall design will continue to adversely affect benthic organisms, and could also impact fish habitat on the riverbank. Revisions to this permit specify the construction of an effluent discharge diffuser to mitigate the existing effects of the discharge on benthic organisms. If it is found that the discharge continues to affect benthic organisms with the diffuser in place, this Order will be reopened to consider further mitigation measures. 13. A 12-port diffuser will be installed to replace the existing side bank outfall. The diffuser design will mitigate the effluent's affect on benthic organisms and fish habitat. The conceptual design of the diffuser is shown in Attachment C. The twelve 8-inch diameter discharge ports are angled 15 degrees from horizontal and are located three feet above the river bottom. The ports are at eight-foot centers and are grouped 105 feet from both river banks to prevent toxic concentrations of ammonia from contacting the river banks. 14. Acute and chronic mixing zones for ammonia toxicity were predicted for the diffuser discharge using the Cornel Mixing Zone Expert System (CORMIX) computer model, which is one of US EPA's approved mixing zone models for near-field analysis. 15. 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, 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. The Discharger will mitigate this situation by discharging to the river only at high river velocities. 16. Revisions to this permit prohibit discharge to the river when the river velocity is less than 0.56 ft/sec from June to September and 0.69 ft/sec from October to May. The Discharger proposes to utilize existing effluent storage ponds sized to hold the volume of effluent discharged in 4 continuous hours at peak design dry weather flow conditions. The Discharger must provide design calculations showing the ponds will always have at least 2 feet of freeboard plus 2.5 million gallons additional capacity for the required storage. 17. The Discharger will provide process control criteria and a process schematic showing how and when plant effluent will be diverted to the storage ponds and how and when effluent will be discharged from the ponds to the river. 18. The CORMIX model's ammonia mixing zone prediction for the worst case mixing zone was considered to occur during drought conditions with high plant effluent flow (8.11 mgd). At drought conditions (assumed to occur for one month every 35 years), the river flow is at its lowest (641 cfs, 0.56 fps) and tidal influences are at the maximum. An