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Staff Report • • -6 - <br /> City <br /> 6 -City of Manteca and City of Lathrop <br /> Wastewater Quality Control Facility <br /> POLICY ISSUES <br /> There are a number of significant policy issues concerning the proposed discharge: <br /> • Should the Board allow a mixing zone? The Basin Plan states "In conjunction with the <br /> issuance of NPDES and storm water permits,the Regional Water Board may designate <br /> mixing zones within which water quality objectives will not apply provided the discharger <br /> has demonstrated to the satisfaction of the Regional Water Board that the mixing zone will <br /> not adversely impact beneficial uses....Pursuant to EPA guidelines, mixing zones designated <br /> for acute aquatic life objectives will generally be limited to a small zone of initial dilution in <br /> the immediate vicinity of the discharge." [Chapter IV, Control Action Considerations of the <br /> Central Valley Regional Water Board,No. 8]. USEPA regulations and guidelines also allow <br /> for establishment of mixing zones,requiring that there be no lethality to aquatic organisms <br /> or impact on beneficial uses. <br /> Whether a particular chemical has an acute (death) or chronic (other than death) impact on <br /> an organism is a function of both the chemical's concentration and of the length of exposure <br /> to the chemical. The proposed action will allow Manteca to discharge effluent containing <br /> ammonia in concentrations that would be acutely and chronically toxic to fish IF the fish <br /> were to be exposed to that ammonia concentration for a sufficiently long period of time. <br /> Through several months of modeling and changes in the proposed design and operation of <br /> the discharge, Manteca has developed a proposal that should prevent lethality to aquatic <br /> organisms in either the water column or on the river bottom. The effluent will always need <br /> some level of dilution to achieve compliance with receiving water standards, so there will <br /> always be a mixing zone. Much of the time the mixing zone will be very small, in the <br /> immediate vicinity of the diffuser ports,but there will always be a zone in which"toxic" <br /> concentrations of ammonia exist. As river flows reduce,the volume and length of the <br /> mixing zone will increase. Additionally, the river is tidal, so the volume and length of the <br /> zone will vary through twice-daily cycles with the stage of the tide. <br /> The modeled river conditions are a worst-case event, with very low river flows and adverse <br /> tides. However, very similar mixing zones will exist any time river velocity drops to 0.7 <br /> feet per second, which will occur during each tidal cycle for weeks or months at a time <br /> every few years. Worst-case conditions will occur relatively rarely,but these are the same <br /> worst-case conditions in which river conditions are poorest for aquatic life. Worst-case river <br /> conditions(that is, lowest river flows), occur during extended droughts. During these <br /> periods the San Joaquin River is going to be predominantly agricultural and municipal <br /> wastewater, with the greatest chances of having other toxicants, high salts,warm water <br /> temperatures, and other conditions adverse to fish. <br /> This Board has never formally adopted a mixing zone in which receiving water standards <br /> are not fully protected,however compliance in many NPDES Permits requires dilution, so <br /> effectively a mixing zone was allowed. It must be recognized that a mixing zone for <br /> ammonia already exists for the Manteca discharge,predominantly hugging the bank where <br />