TagWater Quality(478)

This technical document from a March 25 meeting discusses stormwater mixing zone analysis for proposed outfalls around Seattle-Tacoma International Airport (STIA). Because standard stream flow conditions (7Q10 and 30Q5) are extremely unlikely to coincide with stormwater discharge events, Parametrix proposed a site-specific approach using higher-probability rainfall-based discharge combinations instead. The analysis found that reasonable dilution of 14- to 24-fold could occur within about 15 to 20 meters downstream, though complete mixing requirements would exceed standard regulatory width and flow percentage criteria, prompting a request for exceptions under WAC 173-201A-100.

Handwritten notes outline eight key discussion points related to a mixing zone (MZ) study, including an examination of stream dynamics, regulatory questions about MZ implications under NPDES permitting, and how to weigh evidence for defensibility. The notes raise questions about whether mixing zones are appropriate at all, and whether existing regulations are flexible enough to accommodate site-specific MZ conditions. A final point flags that mixing zones near water quality listing designations will be a particularly difficult policy 'sell.'

This document outlines regulatory problems and compliance points for stormwater discharges into Washington State surface waters, focusing on where discharges must meet water quality standards. It identifies four key unresolved issues, including the lack of an approved design storm for mixing zones and undefined critical receiving water conditions. Various example situations are provided to help determine the exact location where stormwater discharge compliance must be measured.

This technical document describes a mixing zone analysis for stormwater discharge from the 3rd Runway North at Sea-Tac Airport into Miller Creek, designated as Basin SDW-1. The analysis uses design storm modeling with various rainfall scenarios (2-year/2-hour, 2-year/72-hour, and 3-year/72-hour events) to calculate effluent flow rates for both acute and chronic water quality criteria. Stormwater from the basin's approximately 128 acres of pervious and impervious area is routed through a detention pond before discharge, with outflow rates determined by a storage-discharge rating curve.

Letter and memorandum from EPA Assistant Administrator Robert Perciasepe transmitting the final Interim Permitting Approach for Water Quality-Based Effluent Limitations in Storm Water Permits to State Water Program Directors and EPA Water Management Division Directors. The policy addresses NPDES storm water permit effluent limitations, adopting best management practices (BMPs) in lieu of numeric water quality-based

This document is a fact sheet accompanying an NPDES (National Pollutant Discharge Elimination System) permit issued to the Port of Seattle for Seattle-Tacoma International Airport, governing how the airport manages and discharges treated industrial wastewater and stormwater. It covers discharge locations into Puget Sound, Des Moines Creek, Miller Creek, and the City of SeaTac storm sewer system, and establishes effluent limitations and monitoring requirements. The permit addresses water quality standards, pollution prevention plans, and compliance schedules to protect surrounding waterways from airport-related contaminants.

Attachment B table summarizing wetland acreage impacts by wetland function, associated with AR034994. Lists nine wetland functions including Resident/Anadromous Fish (8.6 acres), Passerine Birds (14.9 acres), Waterfowl (1.9 acres), Amphibians (9.8 acres), Small Mammals (13.2 acres), Exports Organic Matter (10.9 acres), Ground Water Exchange (13.0 acres), Flood Storage (4.6 acres), and Nutrient/Sediment Trapping (16.3 acres).

Resume of Joseph T. Brascher, Hydrologist at AQUA TERRA Consultants, Olympia, WA. Details expertise in hydrology, surface water modeling, computer programming, and web development. Experience with hydrologic modeling software packages including HSPF, SWMM, GENSCN, HEC-RAS, HYDRA, WATERWORKS, HYDRAIN, HY8, and MODFLOW. Representative assignments include Green Cove Creek Low Impact Development Study (City of Olympia), Tambark

This section of the Sea-Tac Airport Master Plan Update Environmental Impact Statement examines how proposed airport expansion alternatives would affect water quality and hydrology in the surrounding area, particularly Miller Creek and Des Moines Creek. The analysis finds that increased impervious surfaces would raise stormwater runoff volumes and peak flow rates, potentially causing downstream flooding and erosion, but that required detention storage facilities and mitigation measures would largely prevent significant water quality degradation. Both creeks, while classified as Class AA extraordinary waters, already show some pollution from urban stormwater runoff, though deeper regional groundwater used for drinking remains unaffected.

Fish biologist Donald E. Weitkamp submitted this declaration opposing a legal challenge to Sea-Tac Airport's expansion plans, arguing that the airport's stormwater controls and water quality measures adequately protect nearby streams. He concluded that the headwater portions of Miller, Walker, Des Moines, and Gilliam Creeks near the airport are unlikely to support chinook or chum salmon, as urbanization has altered those streams to favor warm-water species instead. The declaration supports the Port of Seattle's position that its Master Plan Update projects meet environmental standards and will not significantly harm local aquatic resources.