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Project update

Spring 2023

Phase one of the Thornton Creek Basin Sewer Study and Upgrade Project is complete. See below for more information about the study’s findings.

Project need

Through all our projects, King County Wastewater Treatment Division works to protect our region’s environment, public health, and waterways by collecting and treating wastewater. The Thornton Creek Basin Sewer Study and Upgrade Project seeks to ensure that the Thornton Creek sewer pipe is able to carry wastewater flows so we can meet our clean water goals into the future.

The Thornton Creek sewer is an approximately 1.2-mile-long sewer pipe that collects wastewater from other pipes in our sewer system, serving 9.6 square miles of North Seattle and Shoreline. The pipe carries this wastewater to the Matthews Park Pump Station, where it is then conveyed and treated at the West Point Treatment Plant.

During heavy storms, the sewer pipe sometimes reaches its capacity. To avoid possible overflow, King County will need either to reduce the amount of stormwater and groundwater entering the system – known as infiltration and inflow (I/I) – or increase the size of the sewer pipe. Increasing the size of the sewer pipe may also require more costly upgrades to other parts of the regional wastewater system, including Matthews Park Pump Station.

Project benefits

This project can help provide benefits for:

  • Overall water quality in our region, by reducing wastewater overflows.
  • Local wastewater service reliability.
  • Resilience to climate change-related events.
  • Maintaining the regional wastewater system.

Project description

Why study Infiltration and Inflow (I/I)?

Sometimes during periods of heavy rainfall or major storms, additional water enters the sewer pipe, which can cause it to overflow. One reason for this problem is called infiltration and inflow (I/I), or excess water that enters sewer pipes from groundwater and stormwater.

  • Groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint failures, and faulty connections.
  • Stormwater (inflow) rapidly enters sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in maintenance hole covers.

House with tree and cutaway showing piping below ground and ways in which water can enter the sewer including roof drain connection, connected foundation drain, broken side sewer, faulty connections, cracked pipes, uncapped cleanout and root intrusion into side sewer.

What was the Infiltration and Inflow Study?

King County studied the ways that stormwater and groundwater enter the sewer system in the area served by the Thornton Creek sewer pipe. Our team looked at local and regional sewer pipes, maintenance hole covers, and drainage connections. We made progress in identifying the location and type of improvements that have the potential for the biggest impact to reduce I/I. The goal was to identify and evaluate ways to reduce I/I so that the Thornton Creek sewer pipe can continue to carry wastewater flows without needing a costly expansion to a larger sized pipe.

Study results

During this study, King County gathered data about the conditions of the sewer system in the Thornton Creek Trunk service area. We studied local and regional sewer pipes, maintenance hole covers and drainage connections to identify locations and types of improvements that have the potential for the largest reduction in I/I.

The community significantly supported our efforts in the following ways:

  • 700 people responded to the survey that was sent to property owners and residents in mid-2021.
  • 49 property owners allowed King County to investigate side sewers for possible cracks that were contributing to I/I.
  • 2,100 property owners cooperated as we carried out smoke testing that provided valuable information about the amount of I/I entering the local system from private property.

We learned valuable information, including:

  • Finding I/I issues throughout the area but we did not identify any single significant source.
  • Finding that a very large number of small problems would need to be fixed throughout the study area to reduce I/I peak flows enough to eliminate the need to increase the size of the Thornton Creek sewer pipe.
  • Finding many areas where small amounts of groundwater leak into the system. This is especially true where the privately-owned pipes from individual properties connect to the sewer main in the street. 
  • Finding many areas where stormwater sources from private property are tied directly into the sewer system, which contributes to I/I.

Before King County decides a path forward, we have more work to do.

  • We will look at the potential I/I reduction strategies that resulted from this study and compare them to options for increasing the size of the sewer pipe.
  • We will continue to discuss I/I reduction opportunities in the study area with the City of Seattle and the City of Shoreline.


Project timeline for the Infiltration and Inflow (I/I) Study. Spring 2021: identify project study area; data analysis. Summer 2021: community survey #1. Fall 2021 - Summer 2022: Field inspections. Spring 2022 - Fall 2022: identify I/I reduction alternatives. Winter 2023: identify preferred I/I reduction alternative.

Community engagement

In fall 2021, we surveyed residents in areas with high I/I to learn more about sewer and drainage conditions in their neighborhoods and properties. Thank you for taking the survey and sharing your feedback!

Read more about the engagement effort here .

In summer 2022, we conducted smoke testing in five small geographic areas. We were in the neighborhoods while the smoke testing was going on and able to respond to questions and concerns in real time. Thank you to the people who live in the impacted areas for your cooperation and patience while we conducted the tests and gathered data.


Contact us

Contact Bibiana Ocheke-Ameh, community services lead, at:

If you’d like to receive email or text updates on the Thornton Creek Basin sewer study and upgrade project:

Project location

The study area covers the entire area served by the Thornton Creek sewer pipe.

Map displays project area and highlights where smoke testing will occurProject map

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