Nutrient pollution occurs when excess nitrogen and phosphorus enter bodies of water, primarily streams, rivers, and oceans. These elements are fundamental for life, but human activities introduce them into aquatic ecosystems in concentrations far exceeding natural levels. This nutrient overload acts like an uncontrolled fertilizer, significantly altering the balance of water environments. This imbalance disrupts aquatic habitats, making it a widespread environmental problem.
Nonpoint Source Pollution: Agriculture
The largest contributor to nutrient pollution worldwide is agriculture, primarily through nonpoint source (NPS) pollution. NPS pollution is diffuse, meaning contamination comes from widespread runoff across large areas like farm fields, rather than a single pipe.
Farming relies heavily on synthetic fertilizers containing high levels of nitrogen and phosphorus. When these chemicals are applied in excess or when timing is poor, rain or irrigation washes unused nutrients off the fields. This runoff travels across the landscape into ditches, streams, and eventually larger waterways.
Animal feeding operations (AFOs) and concentrated animal feeding operations (CAFOs) also contribute significant nutrient loads through the manure they produce. Livestock waste is rich in nitrogen and phosphorus. If storage is inadequate or manure is spread improperly, runoff transports these nutrients into surface waters.
Poor soil management, such as excessive tilling, leads to significant soil erosion. Since nutrients, especially phosphorus, readily bind to soil particles, eroding topsoil transports attached nutrients into water bodies. This combination of fertilizer runoff, animal waste, and erosion establishes agriculture as the foremost challenge in controlling nutrient pollution.
Point Source Pollution: Wastewater Discharge
In contrast to agricultural runoff, point source (PS) pollution comes from easily identifiable, centralized locations, such as a discharge pipe. Municipal wastewater treatment facilities are a major PS contributor, processing billions of gallons of sewage daily. Wastewater contains high concentrations of nitrogen and phosphorus originating from human waste, food scraps, and certain soaps and detergents.
While these facilities remove many contaminants, many older plants lack the advanced technology to fully remove nitrogen and phosphorus before releasing the treated water, or effluent. Upgrading treatment systems to include tertiary nutrient removal is costly, leaving many municipalities to discharge water with elevated nutrient levels.
Individual septic systems, used by many homes, also pose a nutrient threat when they fail or are poorly maintained. A typical septic system allows wastewater to leach into the surrounding soil, where natural processes filter out pollutants.
Failing systems allow untreated effluent to leach directly into groundwater or surface water bodies, carrying nitrogen and phosphorus. Phosphorus tends to be retained by soil, making freshwater systems vulnerable to localized contamination. Conversely, nitrogen compounds, particularly nitrate, are highly mobile and can easily travel through the soil to contaminate both groundwater and surface water.
Diffuse Sources: Urban Runoff and Atmospheric Fallout
Nutrient pollution also enters waterways through other diffuse pathways, including urban stormwater runoff and atmospheric deposition. Urban runoff is generated when rain or snowmelt flows over impervious surfaces, such as roads, parking lots, and sidewalks. These hard surfaces prevent water from soaking into the ground, leading to rapid surface flow that picks up pollutants.
This runoff collects nutrients from several sources before entering storm drains that discharge directly into water bodies without treatment.
Sources of Urban Runoff Nutrients
- Fertilizers used on residential lawns.
- Decomposing organic debris like grass clippings and leaves.
- Pet waste left on streets and sidewalks.
- Vehicle exhaust deposits that settle on pavement.
The volume and speed of this runoff from paved areas make urban stormwater a significant source of nonpoint nutrient pollution.
Another pathway is atmospheric fallout, where nitrogen compounds released into the air eventually settle onto land and water. The combustion of fossil fuels in vehicles, power plants, and industrial facilities releases nitrogen oxides into the atmosphere. These compounds can travel long distances before being deposited as dry particles or dissolved in rain and snow. This process, known as atmospheric deposition, contributes to the overall nitrogen load in both terrestrial and aquatic ecosystems.