TagACC(1534)

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.

Exhibit 2084 from Administrative Record AR 033383.01 was not present at the time of scanning. This is a placeholder document indicating a missing exhibit. No substantive content is available.

This document, identified as AR 024333, has been officially withdrawn and contains no substantive content available for review. It is part of the administrative record but has been removed from public access, leaving only a placeholder page indicating its withdrawn status.

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.

Exhibit AR033807, WAC 173-340-747 soil concentration calculations for ground water and surface water protection under the Model Toxics Control Act (MTCA), SeaTac Third Runway Embankment Project. Tables derive cleanup levels for hazardous substances including antimony, arsenic, beryllium, cadmium, chromium +3, chromium +6, copper, mercury, nickel, selenium, silver, thallium, zinc, and lead using Equations 747-1 and

This draft document from Washington State's Department of Ecology outlines requirements for a Low Flow Technical Report and a Low Flow Operation Plan related to MPU development activities. The Technical Report must include all calculations, engineering analyses, and hydrological studies needed to address low flow impacts, and must be stamped by a licensed Washington civil engineer. The Operation Plan must detail how stormwater and groundwater will be managed to mitigate those impacts, including maintenance procedures, performance standards, and methods to ensure those standards are met.