EXH0640024634

Subject: Sensitivity of Reasonable Potential Determination To Ambient Water Quality Contaminant Values cc: Linda Logan Ken Ludwa Brian Pippen Project Number. 55-2912-61-01 Project Name: Port of Seattle Storm Water Quality Plan _ Background You raised a question related to the effectiveness using mixing zones to achieve compliance with stormwater discharge standards for the new runway (and existing ones). The question is how sensitive is the reasonable potential (to exceed State water quality standards)? Recall from our discussion that the Permit Writer's Manual uses the following equation to characterize the reasonable potential: CP=(MEC+(MECB*DF-1))/DF Where CP = pollutant concentration MEC = effluent pollutant concentration MECB = background (ambient) pollutant concentration DF = Dilution Factor= ((Qa*vol_factor)+Qe)/Qe Qa = ambient flow rate Qe = effluent flow rate vol_factor = allowablevolumetric fraction of receiving body for mixing this equals 0.025 (2.5%) for the acute zone in rivers and streams and 0.10 (10%) for reservoirs and lakes If numerical modeling shows the expected dilution factor is smaller than that based on the volumetric limitation shown above, the smaller DF is used in the calculation (per Ecology guidance). obtained Ken's gap analysis data (see attachment 1) to understand the basis of the evaluation to date. It used a DF of about 1.0. During our discussion we pondered AR 024634 Paul Fendt August20, 1999 -- Page 2 of 5 whether there could be an increase in the dilution ratio such that it would more than offset increases in copper concentration with increased streamflow .during the =design" condition. Findings…

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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 [...]