SITE INFORMATION AND CORRESPONDENCE_1985-2004

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Staff Report • • -4- City 4- City of Manteca and City of Lathrop Wastewater Quality Control Facility The Discharger modeled the mixing zone using the CORMIX computer model. The longest mixing zone is predicted to occur during the summer months at low river flow periods (641 cfs). For these conditions, the acute toxicity plumes are predicted to be 40 feet long. The cumulative cross sectional area of the plumes at the widest point would cover 1.4%of the river channel. The chronic toxicity plumes are predicted to be 51 feet long. The cumulative cross sectional area of the plumes at the widest point would cover 3.16%of the river channel. The Discharger has proposed different discharge conditions in summer and non-summer periods because of the historically lower ammonia effluent concentrations in the summer. PERMIT REVISIONS INCLUDE: • An ammonia limit of 31 mg N/1 during summer(June— September) and 35 mg N/1 during non-summer months (October—May)was added as a maximum discharge limitation. • Survival of aquatic organisms in 96-hour bioassays, diluted 2.8 parts River water to 1 part waste water effluent, shall be no less than: Minimum for any one bioassay- - - - - - - - - 70% Median for any three or more consecutive bioassays - - - - 90% • Discharges to the San Joaquin River is restricted to when river flows assure continuous mixing of the effluent with river water to limit the zone of acute and chronic toxicity to a minimal area. These restrictive periods were modeled under worst case conditions and found for summer or non-summer conditions to be when river flow velocity are less than 0.56 to 0.69 feet per second(ft/sec), respectively. This permit restricts discharges to when river flows are less than 0.56 ft/sec from June to September and 0.69 ft/sec from October to May. The lower river velocity during the summer reflects the tighter ammonia effluent limit in effect during the summer. • Revisions to this permit specify the construction of an effluent discharge diffuser to mitigate the effects of the discharge on benthic organisms. If it is found that the discharge affects benthic organisms with the diffuser in place, this Order will be reopened to consider further mitigation measures. • Revisions to this permit specify the use of existing effluent storage ponds 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. The Discharger must also provide the operational criteria to be used for diversion of plant effluent to the storage ponds and for discharge from the ponds to the river. • To confirm the boundary conditions modeled in support of the permit revisions and to demonstrate the discharge does not cause acute or chronic toxicity outside the specified