ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Assessment of Water Quality Impacts water years. Estimated far-field water quality impacts are then evaluated as the change in concentration of a parameter at a Delta location of interest due to an increase in WQCF effluent The WQCF and collection system serves commercial and residential uses within the City of flowrate from the permitted 9.87 MGD(ADWF)to the proposed 27 MGD(ADWF). The Delta Manteca,a portion of the City of Lathrop,and one frozen food packager(Eckert Cold Storage). monitoring locations for which far-field water quality impacts were modeled are shown in The WQCF is permitted for treatment and discharge of 9.87 MGD(ADWF)of wastewater,with Figure 3. 8.42 MGD(or 85.3 percent)capacity allocated to the City of Manteca and 1.45 MGD(or 14.7 percent)allocated to the City of Lathrop. The City leases 150 acres of land from Dutra Farms (Assessor's parcel Nos.241-320-01 and 241-320-02)for land application of treated wastewater. Land disposal of treated effluent is maximized by discharging effluent at agronomic rates seasonally to these leased parcels as well as to existing City-owned property(2 10 acres). Excess flow of treated municipal wastewater is discharged via a side-bank outfall to the San Joaquin River approximately one mile upstream of the Interstate 5 Mossdale Bridge at the point latitude 37°,46',45"(deg,min,sec)and longitude 121°,18',00"(deg,min,sec). Current WQCF capacity will increase from 6.95 MGD(ADWF)to 9.87 MGD(ADWF) following completion of the Phase III—Schedule D expansion project in August 2007. The accommodation of such flows will require that WQCF effluent be discharged to the San Joaquin River only when downstream flows meet specific instantaneous flowrate criteria to avoid discharging during incoming or flood times. This capacity is anticipated to support the City's wastewater needs for approximately 5—10 years. Beyond the Phase III expansion,the City has identified the need to program future facilities to accommodate effluent flows(ADWF)up to 27 MGD(Nolte,2007). The existing WQCF outfall to the San Joaquin River will reach capacity following completion of the Phase III expansion project. Any expansion beyond a capacity of 9.87 MGD(ADWF)will require the construction of a second outfall pipeline. Additionally, replacing the side bank discharge with an in-stream diffuser outfall is currently under evaluation as a means of ameliorating the thermal plume caused by the side bank discharge. The near-field water quality impacts assessment evaluates the effects of increasing WQCF discharge,from the permitted 9.87 MGD(ADWF)to a proposed 27 MGD(ADWF)effluent flowrate. Near-field effects on San Joaquin River water quality will occur between the point of discharge and WQCF monitoring location R-3(approximately 1-mile downstream of the City's discharge)where advanced treatment effluent and ambient river water are well-mixed. Near- field water quality impacts are estimated using(1)projected WQCF effluent quality after the Phase III expansion;(2)ambient river concentrations calculated from dry/below normal water years,where possible;(3)current permitted and proposed future WQCF effluent flowrates;and (4)average late summer/early fall San Joaquin River flows observed during historic critical and dry water years. Estimated water quality conditions are then compared to existing water quality objectives or often used criteria to assess the impact of the proposed WQCF increased discharge on San Joaquin River water quality. San Joaquin River monitoring locations providing data used in the near-field water quality impacts analysis are presented in Figure 2. The far-field water quality impacts assessment evaluates the effects of increasing WQCF discharge,from the permitted 9.87 MGD(ADWF)to a proposed 27 MGD(ADWF)effluent flowrate,on specific Delta locations where surface water is diverted for eventual use as drinking water and in the DWSC. Far-field water quality impacts are estimated using(1)historic WQCF effluent quality,(2)projected WQCF effluent quality after Phase III expansion,(3)current permitted and proposed future WQCF effluent flowrates;and(4)modeled percent contribution of WQCF effluent at select Delta locations under representative critical and dry/below normal City of Manteca Antidegradation Analysis 19 June 2007 City of Manteca Antidegradation Analysis 20 June 2007