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WASTE DISCHARGE REQUOMENTS ORDER NO. R5-2004-002 -12- CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY be utilized as representative of wastewater effluent sampling. The TSD recognizes that a minimum of ten data points is necessary to conduct a valid statistical analysis. The multipliers contained in Table 5-2 of the TSD are used to determine a maximum daily limitation based on a long-tern average objective. In this case, the long-term average objective is to maintain, at a minimum,the current plant performance level. Therefore,when there are less than ten sampling points for a constituent, interim limitations are based on 3.11 times the maximum observed sampling result to obtain the daily maximum interim limitation (TSD, Table 5-2). The Regional Board finds that the Discharger can undertake source control and treatment plant measures to maintain compliance with the interim limitations included in this Order. Interim limitations are established when compliance with NTR- and CTR-based effluent limitations cannot be achieved by the existing discharge. Discharge of constituents in concentrations in excess of the final effluent limitations,but in compliance with the interim effluent limitations, can significantly degrade water quality and adversely affect the beneficial uses of the receiving stream on a long- term basis. The interim limitations, however, establish an enforceable ceiling concentration until compliance with the effluent limitation can be achieved. 31. Dilution: As discussed in the information sheet, the Discharger developed a model to assess dilution and mixing zones. The accuracy of the model results are questionable due to a lack of site data to calibrate and validate the model, the lack of accounting for tidal cycles and recirculation, and the lack of accounting for the Brown Sand, Inc. discharge adjacent to the outfall. However, because there is no in-stream flow meter in the vicinity of the discharge to provide real-time data, this Order relies on flow information from the Vernalis monitoring station, as well as some of the model information as it is available. This Order also requires the Discharger to install a flow monitoring station in the vicinity of the outfall to provide real-time data to better assess available dilution. In the immediate vicinity of the outfall, little dilution is available for the side-bank discharge. hi addition,the dilution is reduced due to the added discharge from the Brown Sand impoundment immediately downstream. No dilution is available for the acute aquatic criteria due to the limited mixing of the side-bank discharge near the outfall, the commingling with an adjacent NPDES discharge, the 1-hour exposure interval that the acute criteria are intended to protect, and the periods of slack tide that can occur at low river flows. The SIP requires that a mixing zone not dominate or compromise the integrity of the entire water body and shall be as small as practicable. The thermal modeling,while not proven to be accurate, as discussed in the information sheet, presented a spatial definition to the changes in temperature that occur in the receiving water. This was used to define a mixing zone for constituents subject to chronic aquatic criteria and dilution to be determined at the edge of this mixing zone. As discussed further in the information sheet,the mixing zone will be restricted to the surface layer of the water column in a plume hugging the eastern shore of the river and extending to 450 feet downstream of the outfall. Temperature differences at the edge of this mixing zone indicate that a 4:1 dilution exists at the edge of this mixing zone. Therefore, for constituents subject to chronic aquatic criteria, a 4:1 dilution will be applied. This mixing zone will provide protection to the benthic community and minimize the impacts of the discharge to the river.