EXH0641024642

CC: Brian Pippen, Ken Ludwa, Paul Fen& SUBJECT: AlternativeDilutionModelingProceduresPotentiallyApplicabletoPortofSeattleStorm Water Ouffalls Summary This memo summarizes my review of the data and regulatory guidatlce do_urlcllts that ] was providedconcerningtheevaluationofthereasonablepotentialforcxceedenceofStatewaterquality sumdards(WQS) attheproposedstormwateroutfalls.Thisreviewidentifiedthreealternativem_thods thatcouldpotentiallybeusefultodeterminecompliance.The parametersandcalculationproceduresfor thesemethodsarelistedm theaccompanyingthreetablesshownasattachments. The firstmethodisbasedonaninternalEcologypositionpaper,and laterguidanceprovidedinEcology's PermitWriter'sManual 0astupdatedm July1998).Itisbasicallya singleevent(static)evaluationthat requiresdeterminationofparametersfromprobability-exceedeneerelationshipsthathavetobedetermined forthesite.The exceedencelevels,e.g.the95_ percentileeffluentpollutantconcentration,isstipulatedm the Permit Writer's Manual (Chapter VI Water Quality Based Effluent Limits for Surface Water). The steps that are required to complete this method are shown in Table 1. The HSPF simulation results that have been developed could be used to calculate the probabiliry-exceedence relationships needed for this method. A graphical plot of annual series data obtained from the output of ANNIE (or SWDM), the data - management program used with HSPF, is the easiest approach to obtain the values needed to calculate dilution factors. The dilution factors are used along with ambient and effluent pollutant concentrations to obtain an estimate of receiving water pollutant concentration to compare against the WQS. The second method is basically a dynamic (continuous) simulation approach. Ecology (Permit Writer's Manual) provides an option to use this approach. This is the approach that Bill Fox has used for wastewater dilution analyses to argue the single event method is too conservative. The continuous simulation approach that Brian Pippin and Lisa Martin have developed thus far considers the hourly hydrologic simulation results from the HSPF models. The steps required to complete this method are shown on Table 1. In taking this method to the next step, I would include the volumetric limitation to the…

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Call to order – Roll Call – Quorum Established – Land Acknowledgement We would like to acknowledge that our Agency operates in four present-day counties, which are the traditional lands of the Coast Salish people, past and present. This acknowledgement does not take the place of authentic relationships with indigenous communities in our jurisdiction and [...]