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INFORMATION SHEET ORI*NO. R5-2004-0028 �. 18 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY The Clean Water Act requires publicly owned treatment works to comply with the secondary treatment and applicable water quality standards existing prior to 1 July 1977. USEPA's regulations state that any NPDES compliance schedule may not extend beyond an applicable Clean Water Act statutory deadline. Therefore, a compliance schedule that extends the date for compliance with water quality standards that existed prior to 1 July 1977 may not be included in the Order. 11 Priority Pollutants This section and its subsections discuss how priority pollutants are evaluated against criteria and how limitations and interim requirements are developed. For priority pollutants, guidance for determining reasonable potential, effluent limitations,and compliance schedules is provided by the SIP, adopted in March 2000 by the SWRCB. USEPA promulgated the numeric water quality criteria for priority pollutants with the adoption of the CTR in May 2000. Table 10 summarizes the priority pollutants of concern and their respective criteria. Priority pollutant constituents were analyzed in the effluent and the receiving water(location R-1) from January 2002 to December 2002. The results of these analyses were evaluated for their reasonable potential to exceed Basin Plan, CTR, or other applicable criteria. Section 1.3 of the SIP establishes the guidance for reasonable potential analysis. Table 10 summarizes the reasonable potential analysis of the detected constituents. 11.1 Inorganic Priority Pollutants The inorganic pollutants arsenic, copper and cyanide were found to have a reasonable potential to cause or contribute to an exceedance of the applicable Basin Plan objectives. Effluent limitations are therefore required for arsenic, copper, and cyanide. Based on the information received from the Discharger, the use of the steady-state model described in Section 1 AB of the SIP was utilized for calculating effluent limitations. Dilution credits are provided to the degree indicated in the dilution evaluation (see section 2). The acute and chronic criteria for . copper are a function of hardness. In general, lower hardness values provide more stringent criteria. The hardness value expected to occur at the point in the receiving water where the standard applies, is considered the design hardness. San Joaquin River hardness data is available at Vernalis,Mossdale, and at the Manteca outfall (R-1). The data sets have similar values. There is more river hardness data available over a longer period at Vemalis, therefore, the Vernalis data were used to evaluate receiving water hardness. In determining design hardness, the Regional Board analyzed the receiving water hardness measured at Vernalis during periods when critical low flow was probable (i.e. San Joaquin River flow at Vernalis ranging from 800 cfs to 1,200 cfs). The effluent hardness was also utilized for the acute criteria calculations where dilution is not available. Receiving water hardness is generally flow-related with lower flows providing higher hardness values. To determine the design hardness, receiving water hardness and flow data collected from the USGS monitoring station at Vernalis from 1950 through 1999 were evaluated. The dataset was filtered for