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Related Information

Vashon water and nitrate levels

Graph showing water table elevations from Vashon-Maury Island wells collected 2001-2006

About this indicator: King County has been tracking groundwater quality and quantity on Vashon-Maury Island since 2001. Nitrate is used to track groundwater quality because it is a good indicator of changes caused by human activities, such as land-use development. Water levels are tracked frequently in both volunteer and dedicated monitoring wells. King County's goal is to ensure sustainable water quantity through appropriate zoning regulations and high water quality through effective land-use and on-site septic regulations.

The groundwater quality indicator uses a nitrate index, defined as the maximum concentration of the annual sampling results divided by the maximum contaminant level (MCL) of Nitrate (10 mg/L). This method yields one number. The closer this index gets to 1 (or over 1) the greater concern. The nitrate index has been less than 0.5 since 2003.

Status: Of the 19 well/spring sites monitored, all have tested below the drinking water standard (Maximum Contaminant Level, MCL of 10 mg/L) and all have less than 5 mg per liter of nitrate present. Less than half the sites tested have seen above average nitrate increases since testing began.

(Decreases) Changes in water levels are thought to have been caused by (less) changes in precipitation/recharge to island aquifers.

Influencing factors: Poor drainage systems, improperly maintained septic systems and improper fertilizer use can increase nitrate levels. Changes in land use and/or vegetation, increases in groundwater withdrawals and climatic changes can adversely affect the quantity of groundwater.

Existing DNRP response: King County plans to continue monitoring Vashon's wells and springs annually for nitrate concentrations and water levels measurements.

Priority new actions: Additional locations have been sought to take water level measurements and increase our understanding of island aquifers. King County intends to produce Vashon-Maury Island-wide water table, contour maps with seasonal variability that will be reported every year.

Map of groundwater levels from well samples

Groundwater levels from well samples
2006 Findings
Click to download the PDF version.

Map of nitrates in groundwater from well samples

Nitrates in groundwater from well samples
2006 Findings
Click to download the PDF version.

Technical Notes

For definitions and more detail.

Technical notes for Groundwater: Vashon Water And Nitrate Levels

Data source: The data source for this indicator comes from the King County DNRP/WLRD Groundwater Protection Program.

Collection frequency: King County has been monitoring nitrate concentrations and water level measurements on Vashon-Maury Island since 2001. During 2006, 19 well/springs sites were sampled for nitrate concentration (mg/L) on an annual basis (once a year). Monthly water level measurements have been gathered since 2001 by five volunteers monitoring water levels in their own wells. A baseline was determined by averaging measurements taken between 2001 and 2002. The total number of measurements used to establish this baseline varied from 11 to 21. Measurements are typically taken once a month. Additional well locations are planned in the next year.

Methods for analysis: In previous indicator reports (Measuring for Results), for the time period of 2001- through 2005, four groundwater quality levels designations for nitrate concentrations existed and were defined as "above 5 mg/L", "1 to 5 mg/L", "0.1 to 1 mg/L" and "below 0.1 mg/L". Averages for nitrate concentrations were compared to previous years data and designated as "Above Average", "Same as Average" or "Below Average". The rating was based on a combination of average nitrate concentrations and the trend for the data. The groundwater quality indicator was redeveloped in 2006 to use a nitrate index, defined as the maximum concentration of the annual sampling results divided by the maximum contaminant level (MCL) of Nitrate (10 mg/L). This method yields one number. The closer this index gets to 1 (or over 1) the greater concern. Index values shown in the "Groundwater nitrate index" chart in the section above show newly calculated index values for years 2002-2006.

Water table elevations are calculated by subtracting the depth-to-water measurement from the elevation at the measuring point. This measurement "shows" the top of the water table (above sea level).

Data Reference

King County, Ambient Monitoring Report, 2001 - 2004. http://dnr.metrokc.gov/wlr/wq/ambient-groundwater-monitoring01-04.htm

King County, Water Resources Evaluation Project: 2005 Water Resources Data Report. ftp://dnr.metrokc.gov/dnr/library/2006/kcr1941.pdf

King County, Water Resources Evaluation Project: 2006 Water Resources Data Report.

King County DNRP/WLRD Groundwater: Vashon-Maury Island Water Resources Evaluation--Management Project Documents Web site http://dnr.metrokc.gov/wlr/wq/vashon-island/project-documents.htm

Ambient Groundwater Monitoring

We welcome your feedback and suggestions to improve this site, such as:

Other reliable environmental data sources for King County
Adjustments to the weightings for indicators and performance measures
Mistakes to fix

Share your thoughts by sending an e-mail to Richard Gelb, DNRP Performance Measurement Lead, at richard.gelb@kingcounty.gov so your input can be considered for subsequent updates.