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REGIONAL BOARD RESPONSE (SWRCB/OCC FILE A-1483) -4- PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY Regional Board in Cease and Desist Order No. R5-2002-0084, as suggested by the State Board in Order WQ 96-09. Stockton has filed a petition for review of these Orders, and the State Board again must determine if the permit limitations and Cease and Desist Order are appropriate. FIRST BASIS: DENIAL OF DILUTION CREDIT/DENIAL OF MIXING ZONE Stockton requests that the State Board direct the Regional Board to allow dilution credits and mixing zones and rescind Cease and Desist Order No. R5-2002-0084. Dilution is a key issue. Without dilution credits, Stockton must meet stringent effluent limitations to protect the beneficial uses of the SJR. Despite more than a decade of study, there is no agreementon how to determine the dilution available for Stockton's discharge. The SJR system is complex. Channels have been rebuilt. Natural flows are stored and diverted. Pumping in the South Delta by the Central Valley Project and State Water Project actually can make the net river flow go backwards. Tides flow in and out, often reversing the direction of the river four times daily so wastes are dumped into the same parcel of water as it moves back and forth, sometimes for days and weeks at a time. Dilution may exist for many pollutants much of the time. However, pollutants combine together in complex ways to increase environmental impact over the impact found for single pollutants. The assimilative capacity of the lower SJR is already diminished due to the stress of low oxygen,high temperature, and pollutants from other sources. Based on a thorough review of actual receiving water flow data, there are critical times when net flows are minimal resulting in little or no dilution. The available flow data are from wet years, so the flows and resulting dilution would be less during critically dry years, for which the NPDES permit must provide protection. For these reasons, the Regional Board determined that no dilution credit be granted for almost all constituents. The City's petition contains substantial information regarding their evaluation of dilution, including water quality models. The Regional Board considered this information and concluded that actual flow data,with reasonable assumptions for critical dry rainfall years, should be used in determining dilution rather than the water quality models prepared by the City. This Response contains a summary of the technical bases for the Regional Board's conclusions, and the administrative record, including the permit, fact sheet, and response to comments contain much more detail explaining the Regional Board's conclusions. The 1994 Permit allowed a long-term average dilution credit of 4:1 based on model predictions, since site-specific river flow data were not available at the time. In August 1995, the US Geological Survey (USGS) installed a flow monitoring station near Stockton's outfall and began collecting flow data. Since installation of the flow monitoring station, a critically dry rainfall year has not occurred. In fact, the years of record were above average wet years for the San Joaquin Basin. Extensive flow data are available from a USGS flow monitoring station upstream at Vernalis. That flow data is helpful, however, because a large fraction of the SJR flow measured at Vernalis is diverted into Old River and other Delta channels, the SJR flow at Stockton is much less than at Vemalis. Table 1 shows the actual lowest flow measurements at Stockton during specific averaging periods, as well as the corresponding flow at Vernalis.