Notes
Sea-Tac Airport Master Plan Update Final EIS Chapter IV, Section 10, covering water quality and hydrology impacts of Alternatives 2, 3, and 4 (“With Project” alternatives). Analyzes hydrologic impacts on Miller Creek and Des Moines Creek watersheds using HSP-F Version 10.0 continuous simulation model calibrated with 47 years of hourly precipitation data (1947–1994) and five years of King County Surface Water Management Division stream flow data. Quantifies detention storage requirements: 61 acre-feet for Miller Creek drainage areas and 31 acre-feet for Des Moines Creek drainage areas, sized to attenuate peak runoff for 100-year storm return periods. Documents impervious surface coverage (Miller Creek basin 1,224 acres; Des Moines Creek basin 1,202 acres), Class AA (extraordinary) water quality classification under WAC 173-201A, pollutant loading from urban stormwater runoff including nutrients, metals, fecal coliform, and oil and grease. Also addresses shallow groundwater contamination from leaking fuel distribution systems and underground storage tanks at Sea-Tac Airport. Exhibit identifier AR 022156.