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EXH 271: Comprehensive Stormwater Management Plan — IWS Config & Overflow Analysis (Table 4-2)

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Sea-Tac Airport
Third Runway
PCHB
Box 08

This document describes the Integrated Water System (IWS) stormwater management configuration at Seattle-Tacoma International Airport (Sea-Tac), detailing storage lagoons totaling 76.9 million gallons and a treatment rate of 4.0 million gallons per day across approximately 435 acres. Simulations over a 50-year period confirmed zero overflow events under proposed conditions, with overflows only beginning to occur if treatment rates dropped below 3.1 million gallons per day. The system experienced only one historical overflow during an extreme rain-on-snow event in December 1996–January 1997, with no petroleum hydrocarbons detected downstream.

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Notes

Exhibit 271 (AR 019507–019508) from the STIA Master Plan Update Improvements Comprehensive Stormwater Management Plan, July 2001. Documents the Integrated Wetland System (IWS) configuration under proposed future conditions per Table 4-2, including land use (419 acres impervious, 16.53 acres till grass, 8.16 acres outwash grass), storage volumes (Lagoon 1: 1.6 mg, Lagoon 2: 3.3 mg, Lagoon 3: 72.0 mg, total 76.9 mg), outfall discharge capacity of 7.1 mgd, treatment rate of 4.0 mgd, and zero overflows in the 50-year KCRTS simulation. Analysis identifies treatment rates at which one overflow (3.1 mgd) and two overflows (2.4 mgd) occur. Notes that the only historical IWS overflow occurred during a December 1996–January 1997 rain-on-snow event releasing untreated water to Des Moines Creek, with no petroleum hydrocarbons detected downstream. Filed in PCHB Third Runway EIS administrative record.

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