Notes
Administrative Record document AR024680, filed in PCHB Third Runway EIS proceedings. Outlines Washington State Department of Ecology’s presumed position on stormwater mixing zones, covering: stormwater becoming water of the state upon entering natural or modified natural water bodies; design storm criteria for acute and chronic zones (2-year 72-hour, 2-year 2-hour, 3-year 96-hour storms); critical receiving water flow conditions including 1Q3 flow for acute zones and 5-year mean low flow (August through May) for chronic zones; 7Q10 flow as traditional standard for continuous discharges vs. stormwater; 90th percentile background concentration using all available receiving water data; mixing zone size criteria exceedances under WAC 173-201A-100(10)(c) and paragraph (12); and larger zone options including full stream width for chronic zones and chronic zone size limit for acute zones. Handwritten annotations query approximation of 3-year 96-hour storm, receiving water discharge flows for 2Y72H, 2Y2H, 3Y96H, 5Y72H storms, and approximation of 1Q3 and 5-year mean low flows.