August 14, 2000August 2, 2026

Exhibit 725: Memorandum on Critical Condition for WER-Related Sampling, Port Of Seattle Third Runway Stormwater Discharge into Miller Creek

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Memorandum from Brian Pippin (Parametrix, Inc.) to Linda Logan (cc: John Brooker), dated August 14, 2000, Project Number 556-2912-001, Port of Seattle WERs. Discusses critical conditions for Water Effect Ratio (WER) sampling per the Permit Writers Manual for the third runway treated stormwater discharge into Miller Creek. Uses HSPF hindcasting over 49-year period to evaluate dilution scenarios; finds essentially zero probability of stormwater discharge during stream flows at 7Q10 or 30Q5 levels; predicts no third runway stormwater discharge below the 10th percentile flowrate; concludes critical conditions occur when both stream flow and effluent discharge exceed 80th percentile rates, driven by the same rainfall events. Handwritten notes reference first storm after summer low flow, water quality parameters including hardness, TSS, and TOC.

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As the source for federal transportation grants, the Puget Sound Regional Council was charged with developing a system to meet the growing need for commercial aviation. When the search for a second airport failed, they authorized the Port of Seattle to build the Third Runway, with a mandate to develop a noise abatement and mitigation program. In their effort to stop the Third Runway, the ACC argued over every detail of the Port's efforts--including property buyouts and sound insulation. The dispute was meant to be settled by a three member Expert Arbitration Panel. This is their final report. It finds 2-1, that the Port's program was insufficient in several respects. Despite that, funding for the Third Runway was approved by the PSRC, and the 'Port Package' program, proceeded largely unchanged. Expert Arbitration Panel's final decision finding that the Port of Seattle had not shown sufficient reduction in real on-the-ground noise impacts to satisfy noise reduction conditions required for approval of a third runway at Sea-Tac International Airport. The majority decision concluded the Port's noise abatement programs were insufficient despite being impressive in scope.

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