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Attachment B - 3- Comments 3_Comments on Tidal Dilution of the Stockton Regional Wastewater Control Facility Discharge Into the San Joaquin River April 2001 5. The Report uses September 1999 data from the ultrasonic velocity meter located near the RWCF discharge to simulate the tidal fluctuations. The September 1999 monthly ave .tie net flow was 916 cfs. The Report adju-rs the monthly average net flow to 100 cfs. It is not clear where the 100 cfs river flow originates. Jones & Stokes states in the Flow Report that the minimum monthly average flow at Stockton is 150 cfs. However, in this Report they use the flow passing Stockton as 100 cfs river flow plus 50 cfs RWCF effluent. This is not explained in the Report. 6. The method used in the Report to convert the flow to dry conditions is questionable. None of the daily net flows are negative, due to the unsupported assumption that daily net flow reversals do not exist in the river. Department of Water Resources models suggest that net flow reversals occur during low flow conditions at Vernalis. 7. The evaluation did not use data from other months of record to determine the affect on model results. It is possible that after converting the flow to dry conditions, other months may have extended periods of low flow. The Report makes the assumption that all months will be the same. The Report also does not evaluate overlap between months. The critical 30-day period may not occur in a single month. 8. The box model is composed of water segments with a constant volume of 150,000 cubic feet. At an average monthly flow of 150 cfs, 43 segments are filled per day. The Report states that downstream segments are dropped from the model at the end of each day to avoid having to track so many segments. The Report states, "These downstream segments do not influence the model results because they have been placed far downstream from the discharge and lateral mixing is complete by this time." The Report then contradicts itself in the next paragraph by stating, "During periods of low net river flows, tidal flows generally move the water past the RWCF effluent for about 5-7 days. During this time, the water may have effluent added more than 20 discrete times (i.e., during ebb and flood periods of more than 10 tidal cycles)." During low net river flows or reverse flows, it would be inaccurate to drop water segments at the end of each day. 9. The Report does not define the initial conditions of the box model. The river should have some concentration of RWCF effluent to begin with. The initial conditions and the specific steps used to formulate them should be described in the Report. 10. The study estimates the initial jet dilution as 7:1. However, the study does not consider the affect of recirculation of the effluent. Due to the tidal influences, the river is dosed multiple times. The study states, "...the water may have effluent added more than 20 discrete times..." There may be a 7:1 initial jet dilution with river r iowever, a substantia' portion of the river water maybe previously discharged ;, it low, or no flow ca:iditions.