SITE INFORMATION AND CORRESPONDENCE_1985-2004

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• INFORMATION SHEET ORDER NO. 97-115 (revised) -9- CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUINCOUNTY less, the plant effluent must be diluted with River water at a ratio of 2.8 parts of river water to 1 part of plant effluent. This change in their bioassay effluent limitation testing protocol is proposed in the revised permit. DELTA-WIDE IMPACTS The San Joaquin River near Stockton is on the 303(d) list for recurrent, seasonal low dissolved oxygen. Recent studies indicate that ammonia concentrations in the San Joaquin River, and particularly in the Stockton Deep Water Ship Channel, are a major source of oxygen demand in the water body. The ammonia concentrations are also sufficiently high to be toxic to aquatic life. Studies are not complete on the sources and impacts of ammonia discharges within and outside of the Delta. The most severe low dissolved oxygen conditions near Stockton occur when the net flow of river flow is extremely low. Major causes of the low net flow are low flows in the San Joaquin River at Vernalis and high pumping rates from the export pumps near Tracy, the net effect of which is that most of the water from the San Joaquin Valley flows along Old River,rather than past Stockton. As Manteca's discharge is up-river of the intersection of the San Joaquin River and Old River, it is likely that, at times when dissolved oxygen levels are most critical, Manteca's effluent would flow down Old River and not toward Stockton. Therefore, qualitatively, it.appears that ammonia from Manteca's treatment plant would not contribute directly to the dissolved oxygen problems near Stockton. Studies being conducted through the Dissolved Oxygen TMDL will confirm or refute this, as well as study the possible impact of Manteca's ammonia discharge as a source of nutrient stimulation to algae growth, which also impacts the dissolved oxygen conditions. EFFLUENT AND RECEIVING WATER LIMITATIONS (contained in Order No. 97-115) The "Water Quality Objectives" for the San Joaquin Delta Basin, contained in the Basin Plan, were reviewed. Compliance with effluent limits and requirements established in this permit will meet those objectives. The effluent limits for BODS and suspended solids have been established at 20 mg/l. The limits are set to protect against the potential for adverse impact by the discharge on dissolved oxygen in the San Joaquin River during low River flows. Flows in the San Joaquin River are subject to influences from local irrigation, municipal and industrial demand, water withdrawals from Clifton Court Forebay by Federal and State agencies, and tidal actions. In addition, there are proposals to modify the flow regime in the Delta to provide a more complete mix of San Joaquin and Sacramento River waters in the Delta prior to transport of these waters south. These proposals may significantly alter the quantity and quality of water in the San Joaquin River.