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Arsenic Evaluation: The incremental change in dissolved arsenic concentration in the San Joaquin <br /> River due to an increase in WQCF effluent discharged from the current permitted rate(9.87 <br /> Data Availability: Manteca WQCF NPDES self-monitoring data from the San Joaquin River at MGD(ADWF))to the proposed rate(27 MGD(ADWF))is slight and below the magnitude of <br /> R-1(just upstream of the WQCF discharge)corresponding to dry/below normal water years were change that could be reliably measured in the field. Similarly,the projected increase in dissolved <br /> used to calculate an estimated impact of WQCF effluent dissolved arsenic in the San Joaquin arsenic mass loading to the river is also slight. A reduction in historic effluent arsenic levels <br /> River under critical(600 cfs)and dry/below normal(1250 cfs)river flows at a permitted should occur as groundwater treatment is implemented at City wells as part of the WQCF <br /> discharge of 9.87 MGD(ADWF)and at proposed discharges of 17.5 MGD(ADWF)and 27 upgrade. Source water improvements should produce an average total arsenic effluent <br /> MGD(ADWF). Improved WQCF treatment processes are projected to produce treated effluent concentration of 8 pg/L when the water treatment target is met. Additionally,projected,median <br /> having an average total arsenic concentration of 8 pg/L. For the purpose of the current analysis, dissolved arsenic concentrations in the San Joaquin River are well below the Basin Plan <br /> and to be conservative,all arsenic present in WQCF treated effluent is assumed to be in the objective of 10 pg/L. <br /> dissolved form. <br /> Results: The effect of an increase in WQCF discharge from 9.87 MGD(ADWF)to 27 MGD Table 18: Estimated Impact of Dissolved Arsenic from WQCF Discharge in the San Joaquin River <br /> (ADWF)is appropriately addressed in the receiving water at well-mixed conditions downstream at WQCF R3 <br /> of the discharge. During critical and dry/below normal San Joaquin River flow conditions,an Arsenic,Dissolved San Joaquin River Manteca WQCF Effluent <br /> increase in WQCF effluent discharge will slightly increase dissolved arsenic concentration in the m o/ Flowrate(cfs) Flowrate(MGD ADWF) <br /> San Joaquin River,relative to its Basin Plan objective,downstream of the discharge as shown in R-1 50 concert.(pg/L)* 1.92 <br /> Figure 12 and Table 18. A slight increase in dissolved arsenic mass loading to the river is also Projected effluent concen.(pg/L) 8t1l 600 1250 9.87 17.5 27 <br /> projected. Est.mass loading(lbs/day) 8 18 0.7 1.2 1.8 <br /> Est.downstream R-3 river concentration(pg/!_)at 600 cfs 2.07 2.18 2.32 <br /> Arsenic,Dissolved Est.downstream R-3 river concentration(pg/L)at 1250 cfs 1.99 2.05 2.12 <br /> *501"percentile statistic calculated using the following data set: <br /> Data Period:February 2002—December 2002;Sample Size,n=11;Percent Detected Data=91 <br /> 12 (1)The projected effluent concentration is provided as total arsenic;however,it is assumed that all arsenic present in WQCF treated <br /> J effluent exists in the dissolved form. <br /> rn 10 SJR Flows <br /> c 8 0 600 cfs <br /> o <br /> 6 1250 cfs <br /> 4 —WQO <br /> c 2 <br /> o <br /> U 0 <br /> 9.87 17.5 27 <br /> WQCF Effluent Flow Rate(MGD) <br /> Figure 12: Projected Change in San Joaquin River Dissolved Arsenic Concentration at WQCF R3 <br /> with increasing WQCF Effluent Flowrate <br /> Comparison to Water Ouality Obiective: The Basin Plan objective for dissolved arsenic in <br /> the Delta is 10 pg/L,expressed as the dissolved fraction(Basin Plan,Table 111-1). Estimated <br /> concentrations of dissolved arsenic in the San Joaquin River under critical and dry/below normal <br /> flow conditions show a slight increase,relative to the Basin Plan objective,with an increase in <br /> WQCF effluent discharge from 9.87 MGD(ADWF)to 27 MGD(ADWF). The incremental <br /> change in dissolved arsenic concentration in the river is slight when increasing the WQCF <br /> discharge from the current permitted discharge of 9.87 MGD(ADWF)to the proposed 27 MGD <br /> (ADWF). Projected,median dissolved arsenic concentrations in the San Joaquin River are well <br /> below the Basin Plan objective. <br /> City of Manteca Antidegradation Analysis 47 June 2007 City of Manteca Antidegradation Analysis 48 June 2007 <br />