Phosphorus, an element present in all living organisms, is a fundamental nutrient required for growth and metabolism. In freshwater ecosystems, phosphorus, often in the form of phosphate, typically acts as the limiting nutrient, meaning its availability controls the overall productivity of the water body. When phosphate levels become elevated, it triggers a condition known as eutrophication, which is the over-enrichment of water with nutrients. This overabundance leads to excessive growth of algae and cyanobacteria, which can ultimately degrade water quality, deplete dissolved oxygen, and harm aquatic life.
Agricultural and Urban Land-Use Runoff
A significant source of high phosphate levels comes from non-point source pollution, which is diffuse contamination originating from broad areas rather than a single discharge point. Agricultural practices are major contributors, primarily through the application of synthetic fertilizers and animal manure to fields. Farmers often use compounds containing nitrogen, phosphorus, and potassium (N-P-K), and any phosphorus not fully absorbed by crops remains in the soil.
Rainfall and snowmelt wash excess phosphorus off the land surface and into adjacent streams and rivers, either attached to soil particles (particulate P) or dissolved in the runoff (soluble P). Manure application on fields, a common method for recycling nutrients, also introduces substantial amounts of phosphate easily transported during runoff events. Soluble phosphorus concentration increases directly with higher soil test phosphorus levels, making careful nutrient management essential for reducing this source.
Urban areas also contribute to non-point source pollution through stormwater runoff, particularly where impervious surfaces like roads and parking lots funnel water directly into drainage systems. Sources include residential lawn fertilizers, which may contain phosphates that wash away before absorption by turf. Pet waste left on sidewalks and lawns also contains phosphate compounds flushed into waterways by rain.
Municipal and Industrial Wastewater Discharge
In contrast to diffuse runoff, municipal and industrial discharges are point sources, originating from distinct, regulated outlets. Municipal sewage treatment plants (STPs) collect wastewater containing substantial phosphate loads from domestic sources. Human waste is a primary contributor, as a single person produces about 2 grams of phosphorus per day, primarily from urine and feces.
Another domestic source is the historical and continued use of phosphate-containing detergents and cleaning products, although regulations have reduced their use significantly. Modern STPs are often designed to remove phosphorus through processes like chemical precipitation or enhanced biological removal. However, the final treated effluent can still contain measurable levels, typically between 2 to 5 milligrams per liter of total phosphorus. Without specific removal measures, a high percentage of the original load can be released into receiving waters.
Industrial sources contribute to point source pollution by discharging wastewater that often contains high concentrations of phosphate compounds. Industries such as food processing, chemical manufacturing, and slaughterhouses use or produce phosphorus-rich compounds, which enter the municipal sewer system or are discharged directly. While pretreatment standards are mandated, the volume and concentration of these industrial effluents mean they remain a localized source of phosphate loading.
Natural Processes and Sediment Release
A constant, baseline source of phosphate in water bodies is the natural geological process of weathering. As phosphate-bearing rocks and soils break down, inorganic phosphate minerals are slowly released into the environment. This natural erosion provides a background level of phosphorus that all aquatic ecosystems possess, even without human activity.
A more complex and often self-perpetuating source is “internal loading,” which involves the release of phosphorus stored in the bottom sediments of a lake or reservoir. Under normal, oxygenated conditions (oxic), phosphate is chemically bound to metal compounds, particularly iron oxyhydroxides, forming a stable layer at the sediment surface. This binding acts as a natural cap, preventing the nutrient from mixing with the water column.
However, when the bottom layer of water becomes deprived of oxygen (anoxia), the chemical environment changes drastically. The iron compounds are chemically reduced, which breaks the bond holding the phosphate. This reaction causes soluble orthophosphate to be released from the sediment and diffuse back into the overlying water. This internal loading can sustain high phosphate levels and fuel algal blooms even years after external pollution sources have been reduced. The resulting high nutrient concentration drives more algal growth, and when the algae die, their decomposition consumes oxygen, reinforcing the anoxic conditions and creating a cyclical problem.