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. Until 2009 the nitrate index had been less than 0.5 since 2003. The nitrate index for 2010 is below 0.5 with a value of 0.46.

Status: Of the 25 well/spring sites monitored, all have tested below the drinking water standard (Maximum Contaminant Level, MCL of 10 mg/L) and all are 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
2010 Findings
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