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REGIONAL BOARD RESPONSE (SY3)RCB/0CC FILE A-1483) 6_ PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY record at Stockton, as listed in Table 1. Additionally, there is no guarantee that a minimum flow of 1000 cfs will be maintained at Vernalis. Therefore, the Regional Board found that there is likely to be minimal net flow at Stockton during a critically dry year, resulting in minimal to no available dilution. D-1641 recognized the SJR Agreement and approved, for a period of twelve years, the implementation of the Vernalis Adaptive Management Plan (VAMP). The VAMP is an experiment to determine the relative impact of flow in the SJR and exports in the Delta on Chinook salmon in the lower SJR. D-1641 assigns responsibility to the U.S. Bureau of Reclamation for SJR flow objectives for all months when there are objectives (February to June) except for a 30-day period in April and May for the implementation of the VAMP. The SJR Agreement also provides water for the October river flow objective for salmon attraction and additional water to be used as needed by the U.S. Department of Interior. The SJR Agreement would not provide water for any other potential responsibilities of parties in the San Joaquin Basin to meet water quality objectives. D-1641 approved the petitioned water right changes needed to conduct the VAMP. The VAMP requires conditions through a commitment of the U.S. Bureau of Reclamation to control exports and releases from New Melones Reservoir, and to operate the head of Old River barrier as needed for the experiment. Increased flows in the lower SJR, due to the VAMP, will not affect the critical low flow at Stockton. The VAMP flow objectives are to be imposed during April and May, which is not the period when critical minimum flows occur in the lower SJR. Jones & Stokes prepared the report for the City entitled,Evaluation of San Joaquin River Flows at Stockton, dated April 2001 (Administrative Record, Book 1, Item 4), to predict the critical monthly SJR flow at Stockton. The report describes a model used to predict the monthly average Stockton/Vernalis flow ratios by analyzing SJR flow data at Stockton, SJR flow data at Vernalis, export pumping data, and barrier operations. The Regional Board had several concerns with the model and has not used the model results in the dilution evaluation. Among the concerns, the model contains several coefficients that are subjective. Minor, reasonable modifications of the coefficients produce wide variations in output, including predictions of significantly lower flows than Jones and Stokes contended. In addition, the model has not been calibrated or validated, and a technical peer review has not been performed to ensure accuracy. Detailed comments regarding the flow model, including a demonstration as to the significant variability of model results with slight modifications of coefficients, are described in Attachment A of the Regional Board's 23 April 2002 Response to Comments (Administrative Record, Binder 1, Item 9). The predicted likely minimum monthly flow from the flow model discussed above was used in the April 2001 Jones & Stokes report for the City entitled, Tidal Dilution of the Stockton Regional Wastewater Control Facility Discharge into the San Joaquin River(Administrative Record,Book 1, Item 2), to estimate the minimum dilution. The report describes a dilution model,which simulates the mixing and dilution of the receiving water using a series of water segments or boxes that move upstream and downstream past the discharge with the tidal flow. The boxes are dosed with effluent as they move past the discharge. The model utilizes USGS SJR flow data at Stockton, which have been converted to the estimated critical monthly flow predicted in the Flow Report. The dilution model was not calibrated or validated, and a technical peer review was not performed. Staff found several inconsistencies and flaws in the documentation that made the results unreliable. Detailed comments on the dilution model are Evaluation of San Joaquin River Flows at Stockton, dated April 2001,Jones&Stokes(Administrative Record, Book 1,Item 4),