Nitrogen is a naturally occurring chemical element and essential nutrient that is required for the growth of plants, algae, and microorganisms. In the context of stormwater management, nitrogen is one of the primary nutrients of concern because excessive amounts can degrade water quality and contribute to problems such as eutrophication and harmful algal blooms.
Nitrogen exists in several forms in the environment, including organic nitrogen and inorganic forms such as ammonia, nitrate, and nitrite. These forms are interconverted through biological and chemical processes in soils, water, and the atmosphere, collectively referred to as the nitrogen cycle. The form of nitrogen present influences how it moves through stormwater systems and how readily it is taken up by plants or transported to receiving waters.
In stormwater, nitrogen originates from a wide range of natural and human-related sources. Common sources include fertilizers applied to lawns, agricultural fields, golf courses, and landscaped areas, which are often rich in nitrogen compounds designed to promote plant growth. Organic debris such as leaves, grass clippings, and other vegetation also contribute nitrogen as they decompose. Animal waste from pets, livestock, and wildlife is another significant source, particularly in urban and suburban environments. Atmospheric deposition, which occurs when nitrogen compounds from vehicle emissions, industrial processes, and power generation settle onto land or water surfaces, can also contribute measurable nitrogen loads. In addition, failing septic systems, sanitary sewer overflows, and illicit connections can introduce nitrogen-rich wastewater into stormwater systems.
Nitrogen enters stormwater primarily through runoff processes. During precipitation events, rainwater or snowmelt flows over impervious and pervious surfaces, picking up dissolved nitrogen and nitrogen attached to organic matter or soil particles. In urban areas, impervious surfaces such as roads, rooftops, and parking lots accelerate runoff and reduce opportunities for infiltration, increasing the transport of nitrogen to storm drains and receiving waters. In agricultural areas, excess fertilizer that is not taken up by crops can be mobilized by rainfall and carried into nearby ditches, streams, and rivers. Nitrogen can also enter stormwater through subsurface pathways, including leaching into groundwater that later discharges into surface waters, or through drainage systems such as tile drains.
Once in stormwater systems and receiving waters, nitrogen acts as a pollutant when present in excess concentrations. It promotes excessive growth of algae and aquatic plants, which can lead to eutrophication, reduced dissolved oxygen levels, and associated impacts on aquatic life. Effective stormwater management therefore focuses on reducing nitrogen inputs at the source, enhancing infiltration and biological uptake, and using treatment practices such as vegetated systems and retention or detention facilities to remove or transform nitrogen before it reaches downstream water bodies.