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�r ,,.0v <br /> STAFF REPORT -4- <br /> STOCKTON REGIONAL WASTEWATER CONTROL FACILITY <br /> SAN JOAQUIN COUNTY <br /> 1. Mixing Zone <br /> The issue of a mixing zone allowance must be determined. Two types of modeling scenarios <br /> will be considered, far-field modeling and near-field modeling. Far-field modeling will <br /> consider how persistent pollutants, such as ammonia,BOD, pathogens, and salinity are <br /> assimilated, degraded or diluted by the receiving water. Near-field modeling will evaluate <br /> impacts by pollutants, such as chlorine residual, cyanide, and high ammonia concentrations, <br /> which may cause acute toxicity over short time periods and/or minimal distances from the <br /> effluent outfall. Delta flow modeling is complex and is influenced by a number of parameters <br /> including upstream and downstream water withdrawals, channel depth, residence time in the <br /> Deep Water Channel, precipitation events,flow, and tidal fluctuations. <br /> 2. Federal 303(d) Listing,Impaired Water Body <br /> The San Joaquin River and the Sacramento-San Joaquin Delta are listed on the Federal 303(d) <br /> list for various compounds which have been identified as causing impairment. The listing for <br /> DO deficiencies, electrical conductivity, organo-phosphate pesticides (diazinon and <br /> chlorpyrifos), organo-chlorine pesticides (DDT), mercury, and unknown toxicity require <br /> review and assessment to determine applicable effluent limitations. Staff must consider how <br /> the discharge will affect ambient surface water conditions with respect to the listed <br /> compounds, and whether or not a mixing zone should be allowed under any conditions. <br /> 3. Disinfection <br /> The DHS completed its technical review of the Health Risk Assessment prepared by Stockton <br /> as required by a provision of the existing report. The DHS submitted their comments to the <br /> Regional Board suggesting that a health risk to the public exists, and that more stringent <br /> disinfection requirements should be required of the discharger. The Discharger is in the <br /> process of responding to the DHS comments. The Regional Board, at a later meeting, may be <br /> asked to consider requiring Stockton to maximize the use of its filters pending resolution of <br /> the disinfection issues in the permit renewal. Board staff will evaluate the need to require the <br /> Discharger to upgrade its disinfection capabilities under a time schedule within the renewed <br /> permit. <br /> 4. Dissolved Oxygen <br /> As previously mentioned,DO deficiencies within the San Joaquin River have,in part, placed <br /> this water body on the Federal 303(d) listing as an impaired water body of the State. Staff will <br /> be required to evaluate ambient DO conditions and the impacts of the discharge to the <br /> receiving water. More stringent seasonal BOD/CBOD and ammonia limitations may be <br /> required to help attain the receiving water Basin Plan DO objectives. <br /> The Discharger completed a river model in 1993 assessing the impact of the RWCF on the <br /> receiving water DO quality. The model suggested that the RWCF discharge is a significant <br /> portion of the oxygen demand during periods of critically low DO and that the San Joaquin <br /> River would not meet the DO objectives even if the entire discharge was removed from the <br /> river. The Stockton RWCF refined the river model in 1997 and found that the principal <br /> factors controlling in-stream oxygen concentrations are temperature, flow, upstream algal <br />