Indicators
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At Home At WorkRelated Information
- King County Watersheds
- King County Groundwater Management
- Interactive Groundwater Map
- A Survey of Ditches on County Roads For Their Potential to Affect Storm Runoff Water Quality
- On-Site Runoff Mitigation with Rooftop Rainwater Collection and Use
- Agricultural Waterways in King County
- Environmental Limitations to Vegetation Establishment and Growth in Vegetated Stormwater Biofilters
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.
Technical Notes
For definitions and more detail.