SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET ORIft NO. RS-2004-0028 • 2 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY 2.1 Available Receiving Water Flow Data DWR collects daily average flow data for the San Joaquin River near Vernalis at station RSAN112. Evaluation of this data for the period 1980 to 2002 provided a 1Q10 value of 567 cfs, a 7Q10 value of 620 cfs, and a 30Q10 value of 680 cfs. This period was selected because all current flow control structures on the San Joaquin River and its tributaries were in place by 1980. However, the data set may not accurately represent historical critical low flow periods. Table 2 summarizes,the flow data and calculations. Stage data collected at the Vernalis station does not indicate any tidal flows this far upstream. Downstream, DWR collects stage data near Mossdale at station RSAN087, near the Manteca outfall. Stage data fluctuated about 0.5 feet daily implying that the tidal flow is present. Under critical low flow conditions, upstream flows occur on the flood tide,no flow during the slack tide, and downstream flows during the ebb tide. Multiple dosing of the receiving water with effluent may occur as the tide moves the water column upstream and downstream past the outfall. The complex dynamics of the stream flow, the tidal flows, and the intermittent side bank discharges from the City of Manteca and the Brown Sand impoundment must be considered in an evaluation of the available dilution in the immediate area of these side-bank outfalls. 2.2 Available Hydrodynamic/Water Quality Models Hydrodynamic and water quality models were utilized for the analysis of the water quality impacts of the proposed expansion of the City of Manteca wastewater discharge to the San Joaquin River. Resource Management Associates (RMA)performed the modeling that was published in the Analysis of the Fate and Water Quality Impacts of the City of Manteca Discharge, Resource Management Associates, October 10, 2000. Larry Walker Associates utilized the modeling data developed by RMA to generate the Water Quality Analysis of Surface Water Discharge, Larry Walker Associates, October 2000. Both of these documents are included in the appendices of the Draft Environmental Impact Report for the Manteca WQCF Phase 11111VExpansion Project, October 2000. The near-field analysis was performed using the RMA-10 model which performed the hydrodynamic simulation and the temperature and ammonia evaluations. The near-field analysis was based on the assumptions that: 1. Minimum daily flows in the San Joaquin River at Vernalis since 1983 were used. 2. Discharge to the river would be only during the out-going tide. 3. Ambient water conditions for temperature and ammonia were based on the DWR-D-1485 site at Mossdale. The far-field water quality analysis was performed using a link-node hydrodynamic model of the San Joaquin River and Delta. The link-node tidally averaged water quality model simulates the long-term fate and transport of a discharge to the Delta. A total of three Delta configurations were considered for the parameters of dissolved oxygen, total organic carbon, and total dissolved solids. A tracer simulation was utilized to determine the potential influence of the treated effluent on downstream intakes. The model predicts very small changes to downstream locations as a result of the discharge.