Airport Stormwater Program

Stormwater at Seattle-Tacoma International Airport

Clean water is a priority for businesses and citizens in Seattle and surrounding communities. The importance of water quality encompasses agriculture, commerce, tourism, recreation, and wildlife, all key drivers of the regional economy. Water quality is influenced by stormwater runoff and management. SEA is responsible for 1,600 acres of drainage area that flow directly into three local streams and the Puget Sound.

Contamination of surface water can occur from the storage, use, handling, and disposal of fuel and hazardous products on airport property. Because a substantial amount of the airport property is impervious, rain that falls onto runways, ramp areas, and buildings onsite flows through drainage ditches and culverts and discharges into the stormwater drainage system. Aircraft fueling, maintenance activities as well as oil and fuel spills can contaminate stormwater runoff if appropriate measures are not addressed. SEA’s network of stormwater conveyance systems, retention ponds, stormwater best management practices (BMPs), and wastewater treatment facilities, effectively manage the risk to local waterways.

Stormwater runoff from SEA discharges to three local streams, Miller Creek, Des Moines Creek and Gilliam Creek, as well as directly to the Puget Sound. To protect these aquatic resources, the airport has implemented numerous BMPs to remove pollutants, reduce flooding and prevent spills from discharging into the environment.

Stormwater Management

Airport stormwater runoff is collected in two conveyance systems: Industrial Wastewater System (IWS) or Stormwater Drainage System (SDS) depending upon the activities that occur within each area. The IWS collects and treats stormwater from areas where aircraft or vehicle maintenance activities occur such as aircraft fueling and deicing. The IWS is permitted to discharge stormwater runoff to the Puget Sound via an outfall shared with the Midway Sewer District and to King County South Treatment Plant via Valley View Sewer District.

Stormwater runoff with low Biochemical Oxygen Demand (BOD) discharges to the Puget Sound under Ecology National Pollutant Discharge Elimination System (NPDES) Permit No. WA0024651. Stormwater runoff with high BOD discharges to King County South Treatment Plant for secondary treatment under King County Department of Natural Resources Permit No. 7810-03. Elevated BOD levels are typically associated with stormwater runoff that contains aircraft deicer fluid.

The SDS collects, detains and treats stormwater from the remaining portions of SEA including runways, expressways, the airport terminal, and service road areas. The SDS drains to Miller Creek, Des Moines Creek, and Gilliam Creek, as well as directly to the Puget Sound.

As part of the recently completed Master Plan improvements, the airport stormwater management systems have been completely retrofitted and upgraded to meet the most stringent environmental standards. With these improvements in place, runoff containing high concentrations of deicing fluids are captured and treated before being discharged to Puget Sound. Metals and other pollutants harmful to salmon and other aquatic organisms have been reduced to safe levels before entering local streams. Stormwater discharges from SEA are well below permit limits with low variability in comparison to other airports and industrial stormwater management systems.

SEA evaluates and implements new stormwater BMPs as part of its ongoing stormwater adaptive management program and Stormwater Pollution Prevention Plan (SWPPP) implementation. The adaptive management program takes a proactive approach and evaluates water quality monitoring results to determine if additional BMPs are necessary to prevent permit limit exceedances. If additional BMPs are needed, the program first evaluates if low impact development (LID) stormwater BMPs are feasible to meet permit limits followed by evaluation of traditional grey stormwater infrastructure. LID BMPs are evaluated on a project-by-project basis due to FAA restrictions related to compaction requirements and wildlife attractants, along with restrictions associated with industrial activities and structural limitations associated with infiltration in embankment areas.

SEA’s comprehensive Stormwater Pollution Prevention Plan includes a description of source control and treatment BMPs used at the airport.

SEA Stormwater Reports

Regular monitoring of all stormwater runoff generated at the airport is a critical component of our program. These testing requirements are the cornerstone of one of the most comprehensive NPDES Permits issued by the Washington State Department of Ecology for an industrial facility. In addition to testing and reporting, the permit requires airport stormwater quality to meet stringent effluent limits established to protect aquatic resources in our local streams as well as the Puget Sound.

The NPDES Permit under which the airport operates requires submitting monthly and/or quarterly Discharge Monitoring Reports that compare stormwater runoff quality against effluent limitation concentrations, and annual reports that summarize each year’s monitoring results as well as improvements to our treatment programs.

Airport Stormwater Program

Stormwater at Seattle-Tacoma International Airport

Stormwater Management

SEA Stormwater Reports

Construction Stormwater Monitoring Reports

Industrial Stormwater Monitoring Reports

Industrial Waste System Stormwater Monitoring Reports

Sanitary Sewer Monitoring Reports

IWS Discharge Request and Policy

NPDES Permit

NPDES Permit Fact Sheet

NPDES Permit Fact Sheet Addendum

IWS Discharge Request and Policy

NPDES Permit

NPDES Permit Fact Sheet

NPDES Permit Fact Sheet Addendum

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