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King County Washington home

Duwamish River Valley Groundwater Study

King County and the Port of Seattle are studying groundwater in Duwamish River Valley and Elliott Bay coastal area.  The study will help King County protect wastewater service, plan cost-effective investments, and prepare communities for flooding, high groundwater, and other changing conditions.

King County Wastewater Treatment Division (WTD) is partnering with the U.S. Geological Survey (USGS) and the Port of Seattle on a groundwater study in the Duwamish River Valley and Elliott Bay.

The study began in 2024 and is expected to be completed in early 2028. It will show how sea levels, tides, river flows, and rainfall affect groundwater today.

Understanding groundwater will help WTD make informed decisions about where and when to maintain, improve, or protect wastewater facilities. For ratepayers, better information supports cost-effective planning, helps reduce the risk of avoidable damage and emergency repairs, and protects reliable wastewater service over the long term. The findings will also provide a foundation for future work on how groundwater may change as the climate changes.

Why study groundwater?

WTD owns and operates pipes, pump stations, and treatment facilities in and near the Duwamish River Valley and Elliott Bay, including infrastructure in low-lying areas such as Interbay and West Point. These facilities help collect and treat wastewater for communities across King County. The Port of Seattle also owns and operates terminals, piers, and waterfront properties along the Duwamish River and Elliott Bay.

Heavy rain, high river flows, and high tides can raise groundwater levels. In some places, groundwater can reach or rise above the ground surface. Extra water can damage wastewater and seaport infrastructure. Salt water can also move farther upriver and inland, where it may damage concrete and steel, including pipes, building foundations, and other structures.

Planning before damage occurs can help WTD protect public investments, reduce risks to wastewater service, and make limited ratepayer dollars go further. The study will give WTD and its partners a stronger technical basis for future planning as sea levels and other climate conditions change.

Study area

A map of the greater Seattle area displaying which areas the Duwamish River Valley Groundwater Study includes. Areas such as Downtown Seattle, Tukwila, East Renton, Burien, Harbor Island, and Georgetown. Unincorporated areas include Skyway and White Center.

The study covers the Lower Duwamish River Valley and the area between Discovery Park and Madison Park. It includes flood-control and wastewater facilities, airports, businesses, parks, and neighborhoods. Some communities in and near the study area have experienced greater environmental and economic burdens. Better groundwater information can help WTD and its partners plan infrastructure that supports community resilience as climate conditions change.

Project update

September 2026

WTD continued its groundwater study with the U.S. Geological Survey (USGS). The team also partnered with the Port of Seattle and expanded the study to include Elliott Bay. The team finished collecting groundwater data in the Duwamish River Valley in 2025. Data collection in Elliott Bay began in 2026. USGS publishes all groundwater level data in its public database, Groundwater Levels In and Near the Duwamish River Valley, Washington

The study is also building a 3D groundwater model. The model shows how groundwater moves underground today.

Schedule

The study began in 2025 and will take about four and a half years to complete. We expect to publish the results in early 2028. The team will:

  • Gather existing groundwater data and identify wells and other data sources.
  • Measure groundwater levels and salt levels to understand current conditions.
  • Map how groundwater moves through the study area.
  • Build and evaluate a three-dimensional groundwater model.
  • Analyze the findings and publish a final report.
  • Use the results to inform WTD infrastructure planning, maintenance, and future climate-resilience work.

A timeline graphic showing the schedule for the Lower Duwamish River Groundwater Study. In 2024 well inventory and collecting data for the groundwater budget will begin and complete. From 2025 to 2026 monitoring network will begin and complete and throughout these three years hydrogeological model - construct 3D model will begin and complete.

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