SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET ORIW NO. R5-2004-0028 • 4 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY water body except within any mixing zone granted by the Regional Board. The allowance of mixing zones is discretionary and shall be determined on a discharge-by-discharge basis. The Regional Board may consider allowing mixing zones and dilution credits only for discharges with a physically identifiable point of discharge that is regulated through an NPDES permit issued by the Regional Board." Section 1.4.2.1 of the SIP defines a dilution credit as, "a numerical value associated with the mixing zone that accounts for the receiving water entrained into the discharge. The dilution credit is a value used in the calculation of effluent limitations. Dilution credits may be limited or denied on a pollutant- by-pollutant basis, which may result in a dilution credit for all, some or no priority pollutants in a discharge." In allowing mixing zones for constituents governed by the SIP, a mixing zone shall be as small as practicable and shall not: ■ Compromise the integrity of the entire water body; ■ Cause acutely toxic conditions to aquatic life passing through the mixing zone; ■ Restrict the passage of aquatic life; ■ Adversely impact biologically sensitive or critical habitats, including, but not limited to, habitat of species listed under federal or State endangered species laws; ■ Produce undesirable or nuisance aquatic life; ■ Result in floating debris, oil, or scum; ■ Produce objectionable color, odor, taste, or turbidity; ■ Cause objectionable bottom deposits; ■ Cause nuisance; ■ Dominate the receiving water body or overlap a mixing zone from different outfalls; or ■ Be allowed at or near any drinking water intake. A mixing zone is not a source of drinking water. To the extent of any conflict between this determination and the Sources of Drinking Water Policy(SWRCB Resolution No. 88-63), this SIP supersedes the provisions of that policy. 2.4 Evaluation of Available Dilution for Acute Criteria The Technical Support Document (TSD) states that: "The CMC should be met within a distance offve times the local water depth in any horizontal direction from any discharge outlet. This restriction will prevent locating the discharge in very shallow environments or very close to shore, which would result in significant surface and bottom concentrations. " The outfall is located on the shore,which,by the TSD guidance,will greatly restrict the horizontal range that the acute criteria may be exceeded in the receiving water. Only a limited amount of water depth data was available around the outfall,but it appears to drop to about 4 feet within 10 feet of the bank. By the TSD, this provides about a 20-foot radius around the outfall for compliance with the CMC. The temperature modeling shows the discharge to remain concentrated on the surface of the receiving water and disperse horizontally and vertically as it moves downstream. With the spatial restrictions recommended by the TSD for compliance with acute criteria and the lack of dilution indicated by the temperature modeling at the outfall, no dilution is available for the acute aquatic criteria.