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Attachment B Comments on Tidal Dilution of the Stockton Regional Wastewater Control Facility Discharge Into the San Joaquin River April 2001 he Jones & Stokes report, Tidal Dilution of the Stockton Regional Wastewater Control Facility Discharge Into the San Joaquin River, dated April 2001 (Report) was prepared for the City of Stockton and describes a water quality model used to estimate mixing zones and dilution for the Regional Wastewater Control Facility discharge. The Report describes a water quality 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 actual river flow data, which have been converted to the estimated critical monthly flow-predicted in the Jones & Stokes report,Evaluation of San Joaquin River Flows at Stockton, dated April 2001 (Flow Report). A supplement to the Report with additional evaluations was submitted in June 2001. The Regional Board staff have many concerns with the model, including that the model is poorly documented, does not appear to have been calibrated and validated, and no scientific peer review has been performed. It is very difficult to verify the validity of the model based on the limited information provided in the Report. Specific comments regarding the Report are listed below: 1. The Report assumes the effluent plume only extends halfway across the river based on a near-field dye study performed by Systech Engineering in 1992. The dye study was incomplete and it is not clear if the plume reaches the other side of the river. The dye study was only performed during specific tidal conditions; ebb, low, and flood. Dye dosing was not continuous during the test with the dosing stopped for approximately two hours between tides. The dosing concentration was increased progressively from one tidal condition to the next to minimize possible interference of dye introduced earlier. These modifications make it difficult to identify the actual mixing and dilution over multiple tidal cycles, as would be necessary to assess a continuous discharge. The dye study should only be used to estimate mixing of short duration pulse discharges, such as chlorine, and is only applicable when there is a strong tidal current, not at slack tide. 2. The Report contains inconsistencies in the development of the lateral mixing rate, which are not explained: a. The lateral dispersion coefficient is calculated using a depth of 15 feet, which is greater than the average depth. Using 12 feet (0 ft mean sea level), the lateral dispersion coefficient is calculated as 0.6 times the tidal velocity as compared to 0.3 times the tidal velocity used in the Report. b. The equation for the lateral dispersion coefficient, found in the US EPA Technical Support Docum o for Water Quality-based Tot's Control, can be plus or mii .s 50% of the calc,:iated value. This information . pitted from th Report.