Translate
Vashon's water quantity and quality
Vashon water quantity and water quality indicators
In 2005, the Vashon-Maury Island community helped develop the VMI Watershed Plan. In that plan, residents committed to use water sustainably so that Vashon's groundwater supply is "neither diminished in quality nor quantity" and to "preserve Vashon's natural hydrologic functions."
To track progress and identify areas for improvement, the Groundwater Protection Committee identified 11 indicators to help assess the state of ground, surface, and marine water resources. The target values for these indicators are intended to represent protective or sustainable conditions for water on Vashon. (Learn more about the Groundwater Protection Committee and the Watershed Plan or review download a full report on these indicators.)
For this year's reporting, four key indicators are highlighted because of their connection to human activities and risks to people, animals, and vegetation if conditions decline. Together, these indicators help us answer two important questions: Does Vashon have enough water? And is Vashon's water clean and safe to drink?
Does Vashon have enough water?
Yes, for now. Total island-wide water consumption is tracked and varies year to year. Total island-wide water consumption for 2022 is 501 million gallons per year (MGY). Additionally, the water levels of most monitored wells increased or remained steady between 2011 and 2023.
Climate shifts and future groundwater usage on the island could change this. The major use of water on Vashon-Maury Island is for municipal and domestic purposes; lesser uses are for agricultural and commercial purposes. Although Islanders have varying patterns of usage, it is common for increases in usage to occur during June through October. The annual total consumption during 2001-2022 ranged from 492 to 545 million gallons per year, with consumption increasing during periods of lower rainfall totals and decreasing during periods of higher rainfall totals.
Population increased 9% from 2000 to 2020 while the total usage from municipal users went down 4% in comparing the 2001-10 data to the 2011-22 data.
For more information about Island-wide water use, download a brief report.
Water quantity indicator: Groundwater levels
All of Vashon's springs, streams, and water for drinking, irrigation, fish and wildlife, and other uses come from rainwater alone. By tracking how much water falls on the island, how much gets stored underground, and how much is used in homes, businesses, and agriculture, King County can give people information to make decisions about personal water use and policies.
Groundwater levels are measured at numerous locations across the island within several different water-bearing zones. The frequency of this dataset varies from daily to annual to longer periods of time between measurements. When considering water levels on Vashon, the goal is to maintain or improve water levels over time.
- Water level data: Of the locations where water level data monitored by or reported to King County, data is available for 25 sites during both of two monitoring periods, 2001-2010 and 2011-2023.
- Water level status and trend: The water levels of most monitored wells increased or remained steady between 2011 and 2023.
- 11 wells had higher water levels
- 13 wells had unchanged water levels
- 1 well had a lower water level
Impacts on water levels
Many factors can influence water levels. These factors include weather, climate, and human activities:
- Changes to rainfall amounts: Vashon's total annual precipitation appears to be increasing over time, though rainfall is significantly lower in summer months.
- Water infiltration: Changes in the amount of water seeping into the ground to recharge groundwater systems can impact total available water. For example, impervious surfaces like streets and parking lots can lead to faster stormwater run-off and less infiltration.
- Groundwater extraction: The amount of water people use for their homes or businesses has a direct effect on groundwater levels.
Learn more about Vashon's water levels
For more information about water levels on Vashon, including monitoring locations, changes over time, and site data, download a brief report.
Is Vashon's water clean and safe to drink?
Yes, Vashon's water is clean and safe to drink!
Hundreds of parameters have been tested by King County and Vashon-Maury Island water purveyors and Vashon's water meets primary drinking water standards, except for a few wells where arsenic levels exceed the U.S. Environmental Protection Agency drinking water standard.
Three especially important indicators of groundwater quality indicators that King County monitors are:
- Nitrate+nitrite
- Arsenic
- Chloride
These indicators are highlighted below because they can cause harm to people and wildlife and are more likely to result of human activities.
Water quality indicator: Nitrate
In 2023, nitrate levels met water quality standards at all monitored sites.
High nitrate levels can be harmful to people, especially infants who may develop methemoglobinemia ("blue baby syndrome") if exposed to high levels of nitrates. High nitrate concentrations can also harm aquatic life in surface water.
- Nitrate data: King County has been monitoring nitrate concentrations annually on Vashon-Maury Island since 2001. Forty-four sites across the Island have sufficient data for analysis. Note: King County tests for nitrate and nitrite on Vashon, however nitrite has not been detected historically.
- Nitrate trends:
- 10 sites show increasing trends in nitrate levels
- 7 sites show decreasing trends in nitrate levels
- 27 sites show steady trends in nitrate levels
Where nitrate comes from
The most common sources of nitrate in water are:
- Runoff from fertilizer or manure
- Leaking or failing septic systems
Learn more about nitrate in Vashon's groundwater
For more information about nitrate on Vashon, including monitoring locations, changes over time, and site data, download a brief report.
Water quality indicator: Arsenic
In 2023, arsenic levels met water quality standards at 90% of monitored wells. In about 10% of the monitored wells, arsenic levels were above the standard of 10 parts per trillion.
Longterm exposure to arsenic can cause cancer and other health problems. Where necessary, public water systems dilute drinking water until it meets standards. Public water system results submitted to the Washington State Department of Health report no arsenic measurements above water quality standards on Vashon-Maury Island.
- Arsenic data: King County has been monitoring arsenic concentrations annually on Vashon-Maury Island since 2001. Forty-four sites across the Island have sufficient data for analysis. Note: King County tests for nitrate and nitrite on Vashon, however nitrite has not been detected historically.
- Arsenic status and trends:
- 6 sites show decreasing trends in arsenic levels
- 3 sites show increasing trends in arsenic levels
- 15 sites show steady trends in arsenic levels
Where arsenic comes from
Arsenic is a naturally occurring mineral, including in Vashon's aquifer system. Its presence in soil and water can also come from historic use of now-banned pesticides and herbicides and industrial activities, but soil contamination from the Asarco smelter in Tacoma is not known to impact Vashon's groundwater. Vashon wells with arsenic levels above allowable water standards pull water from deeper water-bearing zones that appear to have naturally occurring arsenic.
Learn more about arsenic in Vashon's groundwater
For more information about arsenic on Vashon, including monitoring locations, changes over time, and site data, download a brief report.
Water quality indicator: Chloride
In 2023, chloride levels met water quality standards at all monitored sites.
The presence of chloride, or salt, in freshwater can indicate saltwater intrusion from increased pumping, sea level rise, or both. Chloride negatively impacts the taste of water and can cause problems for people with sodium sensitivities. Chloride can also be corrosive and damage plants and crops.
- Chloride data: King County has been monitoring chloride concentrations annually on Vashon-Maury Island since 2001 and has monitoring data from 35 locations. Local and State Departments of Health require chloride testing of public water system sources. Department of Health reported chloride data from 55 public water sources.
- Chloride status and trend:
- 7 sites show decreasing trends chloride levels
- 3 sites show increasing trends in chloride levels
- 14 sites show steady trends in chloride levels
- Chloride measurements are extremely low relative to water quality standards
- Trends since 2001 indicate a slight improvement to already good conditions
Where chloride comes from
Pumping wells in aquifers that are hydraulically connected to the Puget Sound can cause saltwater intrusion into the aquifer. Chloride is also concentrated in animal urine and concentrations of animals (human or otherwise) have the potential to elevate the chloride levels in groundwater.
Learn more about chloride in Vashon's groundwater
For more information about chloride on Vashon, including monitoring locations, changes over time, and site data, download a brief report.