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EXH AR024634: Memo — Sensitivity of Reasonable Potential Determination to Ambient Water Quality Contaminant Values

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Sea-Tac Airport
Third Runway
PCHB
Box 10

A 1999 technical memorandum from Parametrix, Inc. to the Port of Seattle analyzes the sensitivity of stormwater discharge compliance determinations for Sea-Tac Airport's runways, focusing on dissolved copper concentrations in Des Moines Creek. The memo evaluates whether mixing zones and dilution factors could offset elevated copper levels in stormwater runoff, finding that ambient copper concentrations already exceeded water quality criteria in 37–65% of samples. The analysis concludes that dilution factors based on streamflow alone are unlikely to be significantly greater than 1.0, though accounting for the Northwest Ponds' reservoir volume raises the estimated dilution factor to approximately 1.7.

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Parametrix, Inc. memorandum dated August 20, 1999 from Jim Dexter to Paul Fendt (cc: Linda Logan, Ken Ludwa, Brian Pippen), Project 55-2912-61-01, Port Of Seattle Storm Water Quality Plan. Analyzes sensitivity of reasonable potential (RP) determination to ambient water quality contaminant values for stormwater discharge from the new third runway at Sea-Tac Airport. Uses Permit Writer’s Manual equation CP=(MEC+(MECB*DF-1))/DF to evaluate dilution factors (DF) and dissolved copper concentrations against WAC 173-201A water quality criteria (WQC). Des Moines Creek Station DM1 ambient data analyzed; acute total dissolved copper criterion set at 4.8 ug/L based on minimum hardness. Finds 37–65 percent of ambient samples exceed WQC; evaluates NW Ponds as lake/reservoir under WAC 173-201A (10% volumetric mixing zone); HSPF model used for Log Pearson Type III flow probability distributions at SDS-3 outfall; Di Toro methodology applied; DF estimated at approximately 1.7 considering pond volume.

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