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WASTE DISCHARGE REQU&MENTS ORDER NO. R5-2004-002., -13- CITY OF MANTECA, CITYLATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY The overlap of the plumes from the City of Manteca and the Brown Sand impoundment will limit the extent of a mixing zone for arsenic, a constituent of mutual concern between these discharges. Additionally, the receiving water monitoring shows an average arsenic concentration of 3.0 ug/l, exceeding the USEPA recommended water quality criterion for protection of human health. The receiving water lacks assimilative capacity for arsenic. There is no dilution available for arsenic under these conditions. The assimilative capacity of the receiving water is dependent on the background concentration of the receiving water. Data collected in 2002 indicates that the receiving water has no assimilative capacity, and therefore no dilution can be granted for aluminum, electrical conductivity, iron, manganese, and mercury. Human health-based criteria that are based on safe-exposure levels for lifetime exposure (e.g., cancer risk estimates) utilize the harmonic mean flow to represent the receiving water flow. A steady state analysis utilizing the harmonic mean flow at Vemalis provides a dilution of 222:1. The Regional Board isnot required to grant a mixing zone or allocate the full assimilative capacity of the receiving water. For limitations based on these human health criteria, dilution is limited to the amount required to maintain compliance. Where the ambient background concentrations are lower than the applicable human health criterion, the dilution credits determined in Table 12 of the Information Sheet apply for the determination of effluent limitations for carcinogens. PRIORITY POLLUTANTS 32. Copper: The Report of Waste Discharge submitted by the Discharger indicates the presence of copper at levels that exceed the numeric water quality objective for copper contained in the Basin Plan (Table III-1). Based on twelve effluent samples, the maximum reported copper value is 13 ug/l,which is within a range that may cause the receiving stream to exceed the water quality objective for copper. Copper toxicity is hardness dependent and data submitted by the Discharger indicates a worst-case effluent hardness concentration of 170 mg/l as CaCO3. Based on a hardness of 170 mg/l, the calculated hardness dependent copper effluent limitations are 7.9 ug/1 as a monthly average and 10.4 ug/l as a daily maximum. Effluent limitations for copper are included in this Order for the protection of freshwater species, and are based on the Basin Plan objective. The determination of the final effluent limitations, which are hardness dependent, are summarized in Table 11 of the Information Sheet. 33. Cyanide: The Report of Waste Discharge submitted by the Discharger indicates the presence of cyanide at levels that exceed the water quality objective for cyanide contained in the Basin Plan (Table III-1). Based on twelve effluent samples,the maximum reported cyanide value is 31 ug/l, which may cause the receiving stream to exceed the Basin Plan objective of 0.01 mg/1. Effluent limitations for cyanide are included in this Order based on the Basin Plan objective and calculations outlined in the TSD. The calculated effluent limitations for cyanide are 3.7 ug/l as a monthly average and 10.0 ug/l as a daily maximum (see Table 11 of the Information Sheet).