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INFORMATION SHEET ORDER NO.R5-2004-0028 3 INFORMATION SHEET ORDER NO.R5-2004-0028 4
<br /> CITY OF MANTECA,CITY OF LATHROP AND DUTRA FARMS CITY OF MANTECA,CITY OF LATHROP AND DUTRA FARMS
<br /> WASTEWATER QUALITY CONTROL FACILITY WASTEWATER QUALITY CONTROL FACILITY
<br /> SAN JOAQUIN COUNTY SAN JOAQUIN COUNTY
<br /> The FIR concludes that these small changes are insignificant. The EIR does not evaluate the water body except within any mixing zone granted by the Regional Board. The allowance of mixing
<br /> cumulative impacts of the Manteca discharge. zones is discretionary and shall be determined on a discharge-by-discharge basis. The Regional
<br /> Board may consider allowing mixing zones and dilution credits only for discharges with a physically
<br /> However,there are concerns about the accuracy of the modeling. The biggest concern is with the lack identifiable point of discharge that is regulated through an NPDES permit issued by the Regional
<br /> of a demonstrated calibration of the near-field RMA-10 modeling. Without comparison to field data Board."
<br /> (e.g.dye or temperature),there is no assurance that plume dimensions or in-stream dilutions are
<br /> accurate for the Manteca discharge. Dilution and plume dimensions were not determined under critical Section 1.4.2.1 of the SIP defines a dilution credit as,"a numerical value associated with the mixing
<br /> conditions that have occurred at the outfall. The timed discharge modeling did not appear to be run for zone that accounts for the receiving water entrained into the discharge. The dilution credit is a value
<br /> an adequate time period to allow the tidal cycles and their recirculation effects to be fully accounted for used in the calculation ofeffluent limitations. Dilution credits maybe limited or denied on a pollutant-
<br /> in the plume development. The Brown Sand,Inc.discharge was not taken into account to determine by pollutant basis,which may result in a dilution credit for all,some or no priority pollutants in a
<br /> its effects on plume development. discharge."
<br /> 2.3 Regulatory Guidance for Dilution Credits and Mixing Zones In allowing mixing zones for constituents governed by the SIP,a mixing zone shall be as small as
<br /> practicable and shall not:
<br /> The Clean Water Act directs states to adopt water quality standards to protect the quality of their Compromise the integrity of the entire water body;
<br /> waters. USEPA's current water quality standards regulation authorizes states to adopt general policies, Cause acutely toxic conditions to aquatic life passing through the mixing zone;
<br /> such as mixing zones,to implement state water quality standards(40 CFR§122.44 and§122.45). The Restrict the passage of aquatic life;
<br /> USEPA allows states to have broad flexibility in designing their mixing zone policies. Primary Adversely impact biologically sensitive or critical habitats,including,but not limited to,habitat
<br /> guidance on determining mixing zone and dilution credits is provided by the Policy for Implementation of species listed under federal or State endangered species laws;
<br /> of Toxics Standards for Inland Surface Waters,Enclosed Bays and Estuaries of California(State Produce undesirable or nuisance aquatic life;
<br /> Implementation Policy or SIP),the USEPA Technical Support Document for Water Quality-based Result in floating debris,oil,or scum;
<br /> Toxics Control(EPA/505/2-90-001)(TSD),and the Basin Plan. For NPDES permits in California,the Produce objectionable color,odor,taste,or turbidity;
<br /> SIP guidance supercedes the USEPA guidance for priority pollutants,to the extent that it addresses a Cause objectionable bottom deposits;
<br /> particular procedure. The SIP does not apply to non-priority pollutants,in which case the more Cause nuisance;
<br /> stringent of the Basin Plan or USEPA guidance applies. Dominate the receiving water body or overlap a mixing zone from different outfalls;or
<br /> • Be allowed at or near any drinking water intake. A mixing zone is not a source of drinking
<br /> The allowance of mixing zones by the Regional Board is discussed in the Basin Plan,Policy for water. To the extent of any conflict between this determination and the Sources of Drinking
<br /> Application of Water Quality Objectives,which states in part,"In conjunction with the issuance of Water Policy(SWRCB Resolution No.88-63),this SIP supersedes the provisions of that
<br /> NPDES and storm water permits,the Regional Board may designate mixing zones within which water policy.
<br /> quality objectives will not apply provided the discharger has demonstrated to the satisfaction of the
<br /> Regional Board that the mixing zone will not adversely impact beneficial uses. If allowed,different 2.4 Evaluation of Available Dilution for Acute Criteria
<br /> mixing zones may be designated for different types of objectives,including,but not limited to,acute
<br /> aquatic life objectives,chronic aquatic life objectives,human health objectives,and acute and chronic The Technical Support Document(TSD)states that: "The CMC should be met within a distance off ve
<br /> whole effluent toxicity objectives,depending in part on the averagingperiod over which the objectives times the local water depth in any horizontal direction from any discharge outlet. This restriction will
<br /> apply. In determining the size of such mixing zones,the Regional Board will consider the applicable prevent locating the discharge in very shallow environments or very close to shore,which would result
<br /> procedures and guidelines in the EPA's Water Quality Standards Handbook and the TSD. Pursuant to in significant surface and bottom concentrations." The outfall is located on the shore,which,by the
<br /> EPA guidelines,mixing zones designated for acute aquatic life objectives will generally be limited to a TSD guidance,will greatly restrict the horizontal range that the acute criteria may be exceeded in the
<br /> small zone of initial dilution in the immediate vicinity of the discharge." receiving water. Only a limited amount of water depth data was available around the outfall,but it
<br /> appears to drop to about 4 feet within 10 feet of the bank. By the TSD,this provides about a 20-foot
<br /> Section 1.4.2 of the SIP states that,"with the exception of effuent limitations derived f om TAMLs,in radius around the outfall for compliance with the CMC. The temperature modeling shows the
<br /> establishing and determining compliance with effluent limitations for applicable human health,acute discharge to remain concentrated on the surface of the receiving water and disperse horizontally and
<br /> aquatic life,or chronic aquatic life priority pollutant criteria/objectives or the toxicity objective for vertically as it moves downstream. With the spatial restrictions recommended by the TSD for
<br /> aquatic life protection in a basin plan,the Regional Board may grant mixing zones and dilution credits compliance with acute criteria and the lack of dilution indicated by the temperature modeling at the
<br /> to dischargers... The applicable priority pollutant criteria and objectives are to be met throughout a outfall,no dilution is available for the acute aquatic criteria.
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