Groundwater

Most groundwater is low in nitrogen. However, in cases with a concentrated nearby source, groundwater can contain high concentrations of nitrogen. Groundwater interacts with surface waters and contributes to streamflow, especially during the drier summer months.

Nitrogen can leach into groundwater from the following sources:

  • agricultural and residential fertilizer application
  • livestock manure
  • on-site septic systems
  • pet waste
  • atmospheric deposition onto land surfaces
  • natural sources: wildlife and red alder trees

Groundwater nitrogen loads can enter Puget Sound through two pathways: direct discharges to marine waters along the shoreline, also called submarine groundwater discharge, and as baseflow in rivers and streams.

The amount of groundwater nitrogen entering Puget Sound varies depending on the proximity to the original source, local geology and soil properties, and the various transformations that happen in the sub-surface layers, as well as within rivers and streams. 

We do not yet have a good estimate of the magnitude of groundwater nitrogen loading to Puget Sound. A 2011 Ecology report estimated that the groundwater dissolved inorganic nitrogen load (DIN) entering Puget Sound through direct discharge is approximately 0.5 – 5% of the total freshwater dissolved inorganic nitrogen load to Puget Sound. Baseflow contributions of nitrogen from groundwater in rivers are included within our estimates of river loading (see ‘River’ tab above).

Groundwater nitrogen levels

This map is from a 2008 USGS study and shows the probability of detecting nitrate concentrations greater than 2 mg/l in groundwater at a depth of 145 feet below the land surface in the Puget Sound region. The map was developed using the modeling and statistical analysis. The map shows that groundwater in agricultural regions and urban areas are at the most risk of having elevated nitrate levels.

High levels of nitrogen in groundwater may be a problem for human health. Many people drink well water in the Puget Sound region, and one contaminant is nitrogen, generally in the form of nitrate. The drinking water standard is <10 mg/L for nitrate in groundwater to avoid blue-baby syndrome.

Nitrate in groundwater data & assessment has more information about nitrate contamination in groundwater and Protecting Washington’s Groundwater – The Nitrate Project, is another story map where you can find a map-based presentation about nitrate in groundwater in our state.