SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET•ORDER NO. 97-115 (revised) -8- CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUIN COUNTY 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. • 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 worse 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. • 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 mixing zones, revisions to this permit require monitoring of the river for ammonia downstream of the acute and chronic mixing zones. If acute toxicity is found 40 feet beyond the discharge point or chronic toxicity is found 51 feet beyond the discharge point, then this Order will be reopened and effluent limitations added or amended for ammonia toxicity. • This permit allows a change in Effluent Bioassay testing. This permit allows dilution of the effluent with San Joaquin River water, in performing the bioassay testing, to better simulate the actual toxicity occurring in the receiving water. The City has found through studies that fish survive the 96-hour static bioassay and they are able to achieve full compliance with the acute toxicity requirements of their permit when their effluent ammonia concentrations are maintained at 10 mg./1 or less. In order to maintain continuous ammonia level of 10 mg/I or