Stormwater
Stormwater
Stormwater accumulates and delivers various land-based sources of nitrogen (and other pollutants) to the nearest waterway or storm drain during rain events.
Stormwater runs off pavements, roofs, and other hard or impervious surfaces. It can also run off agricultural fields and other surfaces when the intensity of rainfall exceeds the soil’s capacity to absorb it.
Some stormwater can infiltrate into soil layers but still reach a stream or other waterbody via groundwater. Once it enters rivers and streams, instream processes may transform some of the nitrogen into other forms, or assimilate it, before entering Puget Sound.
Stormwater accumulates and delivers various land-based sources of nitrogen (and other pollutants) to the nearest waterway or storm drain during rain events.
Stormwater runs off pavements, roofs, and other hard or impervious surfaces. It can also run off agricultural fields and other surfaces when the intensity of rainfall exceeds the soil’s capacity to absorb it.
Some stormwater can infiltrate into soil layers but still reach a stream or other waterbody via groundwater. Once it enters rivers and streams, instream processes may transform some of the nitrogen into other forms, or assimilate it, before entering Puget Sound.
Nitrogen in stormwater
Nitrogen concentrations and loading from stormwater runoff depend on land use. A 2011 report presented nitrogen loads in surface runoff and found that during storm events, the median total nitrogen concentrations were higher in residential and agricultural subbasins relative to commercial/industrial and forested basins.
The study also estimated the nitrogen contribution from a unit area of land with particular land use and found that:
- 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.
- Unit-area loading rates during storm events were higher in residential and agricultural sub-basins (308 and 374 kg/km2/yr, respectively) than in commercial/industrial and forested sub-basins (94.5 and 144 kg/km2/yr, respectively).
Since these are aerial loading rates, they do not account for processes within watersheds and rivers that will change the amount of stormwater nitrogen that eventually gets delivered to Puget Sound.