Access To Potable Groundwater
Nitrates in Groundwater on Vashon-Maury Islands
About this indicator: King County has been tracking groundwater quality 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. King County's goal is to ensure 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.
Influencing factors: Poor drainage systems, improperly maintained septic systems and improper fertilizer use can increase nitrate levels.
Existing DNRP response: King County plans to continue monitoring Vashon's wells and springs annually for nitrate concentrations.
Priority new actions: Additional locations have been sought to 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.
Access To Potable Groundwater
2008 Findings
Click to download the PDF version.
Technical Notes
For definitions and more detail.
Technical notes for Nitrates in Groundwater on Vashon-Maury Islands
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 2008 19 well/springs sites were sampled for nitrate concentration (mg/L) on an annual basis (once a 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.
Data Reference:
Vashon-Maury Island Water Resources Evaluation (WRE) Project Documents Web Site
WRE 2007 Water Resources Data Report
WRE 2006 Water Resources Data Report
WRE 2005 Water Resources Data Report
King County Ambient Groundwater Monitoring Results, 2001-2004
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