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Nitrogen in Stormwater Runoff

Once rainfall hits the ground it can follow several pathways:

  • infiltration to sub-surface layers
  • evaporation to the atmosphere
  • interception by vegetation
  • direct stormwater runoff

Stormwater runs off pavements, roofs, and other hard or impervious surfaces into the nearest waterway or storm drain. During this process, stormwater runoff can accumulate nitrogen as well as other pollutants.

Some stormwater can infiltrate into soil layers but still reach a stream or other waterbody where instream processes may transform some of the nitrogen into other forms, or assimilate it, before entering Puget Sound.

Nitrogen Concentrations in Stormwater

Nitrogen concentrations in stormwater vary between subbasins with different land-use types.

As part of Ecology’s Puget Sound Toxics Assessment project, Herrera (2011a) found that during storm events, median total nitrogen concentrations were higher in residential and agricultural subbasins (1.3 and 1.8 mg/L, respectively) relative to commercial/industrial and forested basins (0.3 and 0.4 mg/L, respectively).

Concentrations were found to be lower during baseflow events, with medians of 1.2 and 0.8 mg/L for residential and agricultural subbasins, respectively, and 0.7 and 0.2 mg/L for commercial/industrial and forested basins, respectively.

Nitrogen Loading from Stormwater

Ecology’s Puget Sound Toxics Assessment project also estimated unit-area loading rates of nitrate plus nitrite during storm events for subbasins in Puget Sound that have different dominant land use types.

Unit-area loading of nitrogen is higher during storm events relative to baseflow events, primarily as a result of higher flow volumes during storm events, but also due to some higher concentrations.

Similar to the patterns in nitrogen concentrations in stormwater from subbasins with different land uses, median unit-area loading rates during storm events were higher in residential and agricultural subbasins (308 and 374 kg/km2/yr, respectively) than in commercial/industrial and forested subbasins (94.5 and 144 kg/km2/yr, respectively).

Since these are aerial loading rates, they do not account for attenuation that occurs within watersheds and rivers before this nitrogen enters Puget Sound.

Source of loading rates: (Herrera (2011a)