Nitrogen is an essential element found in all living organisms, forming a fundamental component of proteins and genetic material like DNA. It is naturally abundant, making up about 78% of the Earth’s atmosphere. While a certain amount of nitrogen is necessary for healthy ecosystems, an excess can disrupt natural balances, leading to significant environmental issues in aquatic environments. This overabundance, often termed nutrient pollution, can stimulate excessive algal growth, which then consumes oxygen when it decays, harming aquatic life. Understanding the pathways through which this excess nitrogen enters waterways is important for addressing its widespread impact.
Understanding Nitrogen’s Journey
Nitrogen naturally cycles through atmosphere, land, and water in the nitrogen cycle. Bacteria in soil and plant roots convert atmospheric nitrogen gas into forms plants can use, such as ammonia, called nitrogen fixation. Other bacteria transform ammonia into nitrates and nitrites, which plants absorb. Nitrogen returns to the atmosphere when other bacteria convert nitrates and nitrites back into nitrogen gas. This natural cycle maintains a balance, ensuring nitrogen is available for life without overwhelming ecosystems.
Human activities have altered this balance, introducing additional nitrogen into the environment at an accelerated rate. Industrial processes, such as the Haber-Bosch process, produce ammonia for fertilizers, significantly increasing reactive nitrogen. Land use changes, including extensive agriculture and urbanization, also contribute to this surplus. This increase disrupts natural cycling, leading to excess nitrogen that can leach into groundwater or run off into surface waters.
Agricultural Pathways to Waterways
Agriculture is a primary contributor to excess nitrogen in waterways, largely due to synthetic fertilizers. These fertilizers, rich in nitrates and ammonium, are applied to crops. Plants do not absorb all applied nitrogen, leaving a portion susceptible to loss. When rainfall or irrigation occurs, dissolved nitrates, highly soluble, can percolate through soil and enter groundwater. This groundwater then transports nitrogen to nearby streams, rivers, and larger bodies of water.
Animal waste, including manure, introduces organic nitrogen into the environment. If not properly managed, this waste can leach into soil or be carried by surface runoff during precipitation. Nitrogen in manure, often ammonia, can volatilize into the atmosphere or convert to nitrates. These then follow the same pathways of leaching and runoff into aquatic systems.
Surface runoff is a direct mechanism where nitrogen-laden soil particles and dissolved nitrogen wash from agricultural fields during heavy rains or irrigation. This runoff carries nutrients directly into ditches, streams, and rivers. This non-point source pollution is challenging to manage because it originates from diffuse areas rather than a single discharge point. Collectively, these agricultural practices release nitrogen at scales that overwhelm natural assimilation capacities, impacting water quality.
Urban and Industrial Discharges
Urban and industrial areas contribute to nitrogen pollution. Wastewater treatment plants, despite purifying sewage, often discharge treated effluent containing residual nitrogen, such as nitrates and ammonium. While these discharges meet regulatory standards, the sheer volume of treated wastewater released into rivers, lakes, and coastal waters means a continuous nitrogen input, acting as a point source.
Individual septic systems manage household wastewater in areas not served by centralized sewer systems. When old, poorly maintained, or improperly installed, nitrogen-rich wastewater can leach from drain fields into soil. This allows nitrogen to seep into groundwater, migrating into nearby surface waterways and contributing to localized nitrogen loading.
Urban stormwater runoff is another non-point source of nitrogen pollution. As rain falls on paved surfaces, it picks up nitrogen from urban sources, including pet waste, lawn fertilizers, and vehicle emissions. This nitrogen-laden runoff then flows into storm drains, which often empty directly into local waterways without treatment. Industrial facilities may also discharge nitrogen as a byproduct. Although regulations aim to control these releases, inadequate treatment or accidental spills can introduce concentrated nitrogen loads into water bodies.
Atmospheric Fallout into Water
Nitrogen can also enter waterways indirectly through atmospheric deposition, a process where airborne nitrogen compounds fall to Earth. Major sources of atmospheric nitrogen include burning fossil fuels in vehicles, power plants, and industrial activities, releasing nitrogen oxides (NOx). Additionally, ammonia volatilization from agricultural practices, like manure decomposition and fertilizer application, contributes to atmospheric nitrogen.
These airborne nitrogen compounds can then be deposited onto land and water surfaces through two primary mechanisms: wet and dry deposition. Wet deposition occurs when nitrogen compounds, such as nitrates and ammonium, dissolve in atmospheric moisture and fall as rain, snow, or fog. This precipitation can fall directly onto water bodies or onto land, running off into aquatic systems.
Dry deposition involves nitrogen-containing gases and fine particles settling directly onto water surfaces or land without precipitation. This continuous settling of nitrogen contributes to the overall nitrogen load in a watershed. While often less direct, atmospheric fallout is a pervasive and significant source of nitrogen to waterways, affecting even remote aquatic ecosystems.