ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Total Aluminum 4.9-6 Concentrations in Receiving Waters. The proposed WQCF upgrades would be designed and operated to comply with the NPDES permit effluent limits for total aluminum.Available science indicates that effluent limits substantially greater than the proposed discharge concentrations under the project would be sufficiently protective of the beneficial uses of the San Joaquin River. Because the project's discharge concentrations of aluminum would be substantially below discharge concentrations shown by available scientific studies to be protective of water quality, the project would result in less-than-significant aluminum impacts. Manteca WQCF NPDES self-monitoring data from the San Joaquin River were analyzed from monitoring location R-1 just upstream of the WQCF discharge(see Exhibit 4.9-1). Data corresponding to dry/below normal water years were used to calculate the estimated water quality effect of WQCF effluent total aluminum discharges to the San Joaquin River under critical(600 cfs)and dry/below normal(1,250 cfs)river flows at the existing permitted discharge of 9.87 mgd and at proposed discharges of 17.5 mgd and 27 mgd. The current NPDES effluent limitation for total aluminum is 71 ug/l based on a pH of 8.4 and a temperature of 26°C(CVRWQCB 2004) on a monthly average,which is below the U.S. EPA chronic ambient water quality criterion for total aluminum(87µg/1). This criterion is often used by the RWQCB to interpret the narrative toxicity objective contained in the Basin Plan. The daily maximum NPDES effluent limitation for aluminum is 140µg/l. Past water quality data collected from the WQCF indicate that aluminum concentrations in the existing effluent– and the San Joaquin River–exceed these limitations. Therefore,the City is exploring various compliance options to meet these effluent limits, and has been granted the opportunity by the RWQCB to assess compliance with the aluminum effluent limits using the acid soluble method for aluminum analysis and/or by conducting a Water Effects Ratio(WER)to develop a site specific objective(SSO). Based on the data collected to date,the acid- soluble measurement does not appear to be useful or applicable to compliance measurement because the analysis of the acid-soluble portion of a total metal commonly does not result in significantly lower values of the total metal when compared to the analysis of the metal without acidification. In addition,the RWQCB does not allow the use of dissolved aluminum for compliance purposes. The proposed project is projected to produce treated effluent with an average daily total aluminum concentration of 150µg/1 at the point of discharge under buildout (27 mgd) conditions. During critical and dry/below normal San Joaquin River flow conditions, an increase in WQCF effluent discharge would slightly decrease total aluminum concentrations in the San Joaquin River downstream of the discharge compared to permitted discharge rates (9.87 mgd)as shown in Table 4.9-13. Further,proposed discharge concentrations(743.7 to 757.6µg/l)would be below existing measured aluminum concentrations(785.1 µg/1 at R-1, 50%concentration) and would have a diluting effect in river concentrations. A slight increase in total aluminum mass loading to the river is also projected. While the modeling indicates that estimated concentrations of total aluminum in the river exceed EPA chronic ambient water quality criterion(87µg/l) for the metal(Table 4.9-13),the current U.S. EPA chronic water quality criterion for total aluminum as applied to the San Joaquin River is believed to be over restrictive and over protective of beneficial uses because the criterion does not take into account the local water quality characteristics (high pH and high hardness)that reduce the toxicity of aluminum to aquatic life. The 87µg/L U.S. EPA chronic criterion is based on toxicity tests using striped bass and brook trout in water with pH between 6.5 and 6.6 and hardness concentrations (as CaCO3)of less than 10 mg/L (striped bass)and 12.3 mg/L(brook trout). WER studies conducted since this time and anecdotal evidence support the conclusion that aluminum is substantially less toxic at the higher pH and hardness values that are typical in the San Joaquin River. (LWA 2007). DER EDAW City of Manteca 4.9-35 Hydrology and Water Quality