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Groundwater on Vashon-Maury Island
Explore this page to learn about Vashon's groundwater, including where it comes from, data about water levels and water quality, and ways that everyone on the Island can care for this vital resource.
Vashon-Maury Island is a special place with a unique resource: safe, abundant fresh water that all begins as rain. On Vashon, rain feeds the streams and ponds and becomes groundwater. These fluid features make up a hydrologic system that supports the people, salmon, and wildlife that share the Island.
Groundwater is vital to life Vashon, so it is essential to understand, protect, and monitor this resource through education, action, and long-term scientific monitoring. Explore this page to dive into Vashon's precious liquid asset!
Understanding Vashon's water
All of Vashon’s fresh water comes from one place: the sky. Rain provides all the water that eventually flows through streams, wells, and taps to support people, wildlife, and plants across the Island. This fresh water is part of an interconnected system called a hydrologic cycle, or "water balance," and changes to one part of the system impact other parts. Learning how water moves across and through the Island makes it easier to understand why it's so important to protect this vital resource.
Vashon's Water Balance
Each year, about 46 inches of precipitation fall on Vashon. Some of that water evaporates or is used by plants in a process called “evapotranspiration” (20.7 inches, or 45% of annual precipitation). A smaller quantity of rainwater flows across the island and into Puget Sound as “runoff” (9.2 inches or 20%). The rest soaks into the ground to recharge Vashon’s complex groundwater system, as far as 300 feet below sea level (16.1 inches or 35%).
Once underground, some water seeps back into streams as “baseflow” (8.7 inches or 18.9%), or into Puget Sound as “outflow” (6.3 inches or 13.7%). A small amount comes back to the surface through local wells (1.1 inches or 2.4%), which provide all water for drinking, home and business use, and agriculture on the Island.
The graphic below illustrates these processes and relationships in a simplified way.
Protecting Vashon's fresh water
Vashon’s water is safe and abundant enough to meet the current needs of people and wildlife. Let’s keep it that way! Residents, business owners, and visitors can all help through good stewardship and collaboration with neighbors, local organizations, and King County.
- Testing and monitoring: Well owners, water purveyors, and King County scientists test and monitor well water and streams across the island to ensure drinking water is safe and to track environmental conditions.
- Citizen science: Locals volunteer and partner with the Vashon-Maury Island Groundwater Protection Committee, local organizations, and King County scientists to gather data and inform decision making.
- Act locally: Residents and business owners protect their water, their neighbors, and the environment by being mindful of water use, septic maintenance, and lawn and pet care.
There are many ways to help protect Vashon’s fresh water. Learn how you can do your part! [LINK GOES HERE]
Monitoring Vashon's freshwater resources
King County scientists monitor and test the water in wells and streams around Vashon and collect data from water purveyors and local volunteers.
King County's work includes:
- Measuring water levels at 25 wells (about 2.5% of Vashon's more than 1000 wells)
- Monitoring groundwater quality at 20 locations
- Monitoring flow and water quality at 6 streams
- Tracking rainfall at 7 sites
Together, this data helps us understand groundwater levels in different parts of the Island, and water quality in wells and streams. King County scientists analyze the data and share information that helps residents and policymakers understand status, trends, changes, and risks to Vashon's freshwater resources.
Learn more and get involved
Click below to learn more about key indicators of Vashon's water quantity and quality and discover different ways to protect Vashon’s liquid assets.
Does Vashon have enough water?
Is Vashon's water clean and safe to drink?
How can people keep Vashon's water abundant and safe for people and wildlife?
About groundwater protection in King County
King County staff oversee the Groundwater Protection Program and provide management, policy, and technical expertise to help protect groundwater and the people who depend on it. Learn more about the Groundwater Protection Program.
The Vashon-Maury Island Groundwater Protection Committee (GWPC) is responsible for Vashon's Groundwater Management Plan and Watershed Plan and strategizing, advising and sharing information about how to protect Vashon-Maury Island’s groundwater. Learn more about the Committee and how you can get involved!