SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET ORSR NO. R5-2004-0028 . 5 CITY OF MANTECA, CITYOF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY 2.5 Evaluation of Available Dilution for Chronic Criteria The TSD states that: "Concentrations above the chronic criteria are likely to prevent sensitive taxa from taking up long- term residence in.the mixing zone. In this regard, benthic organisms and territorial organisms are likely to be ofgreatest concern. The higher the concentration occurring within the isopleth, the more taxa are likely to be excluded, thereby affecting the structure and function of the ecological community. It is thus important to minimize the overall size of the mixing zone and the size of elevated concentration isopleths within the mixing zone. " The temperature model,while subject to the limitations discussed in section 2.2,provides information used as a basis to establish available dilution for compliance with chronic criteria to protect aquatic life. The model concludes that, for the timed discharge, the four degree F differential would reach a maximum area of 0.3 acre and would be contained in a shallow(less than one foot in depth)plume that hugs the east river bank until dissipating 450 feet downstream. The model also shows that significant vertical mixing does not occur until about 500 feet downstream at which point there will be contact with the benthic community. This is illustrated in Figure 8 of the Resource Management Associates, 2000, analysis. Using the conclusions of the temperature model, a 4 degree temperature differential downstream where the effluent and receiving water have a 15°F initial difference indicates that mixing in the near field is small and does not reach 4:1 until nearly 450 feet downstream and 15:1 at 1300 feet downstream. Complete mixing,which is defined in the SIP as not more than a 5 percent difference in the concentration of a pollutant across a transect of the water body,would not occur until over 1000 feet downstream. 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 presented a spatial definition to the changes in temperature that occur in the receiving water as discussed in the previous paragraph. This allowed a mixing zone to be defined and dilution to be determined at the edge of this mixing zone. 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. 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. 2.6 Evaluation of Available Dilution for Specific Constituents 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 at the 1-in- a-million risk level. Therefore, the receiving water lacks assimilative capacity for arsenic, and there is no dilution available. The assimilative capacity of the river 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.