Phosphate, a form of the element phosphorus, is a fundamental nutrient required for all life, yet its presence in excess within water bodies poses a significant environmental risk. In aquatic systems, it is most commonly found as orthophosphate, the simple, bioavailable form that plants and algae can readily absorb. Because natural levels of this nutrient are typically very low in freshwater, phosphate often acts as the limiting factor for growth. When elevated concentrations of phosphate enter lakes, rivers, and streams, they can trigger rapid and excessive growth of algae, a process known as eutrophication.
Phosphate from Agricultural and Land Runoff
Agricultural practices represent the largest non-point source of phosphate pollution, with loss occurring through two primary mechanisms. The first is the runoff of dissolved phosphate, the water-soluble form coming directly from applied fertilizer or animal manure. This form is immediately available to aquatic organisms and is prone to flash losses if heavy rain occurs shortly after application.
The second, and often larger, contributor is particulate phosphate, which is phosphorus chemically bound to eroded soil particles. This occurs because phosphorus binds strongly to soil, and when topsoil is carried away by surface runoff, the attached phosphate travels with it. In runoff from cropland, where soil disturbance is common, particulate phosphate can account for a substantial portion of the total phosphorus load, often ranging from 75 to 90 percent.
Over-application of fertilizer or manure causes phosphate to accumulate in the soil. Once the soil’s capacity to bind the nutrient is saturated, additional phosphate remains highly mobile and susceptible to being washed away. This non-point source pollution is diffuse, making it challenging to control since it originates from the entire landscape rather than a single pipe.
Municipal Wastewater and Industrial Effluent
Centralized wastewater treatment plants (WWTPs) and industrial discharges are considered point sources, releasing effluent through a controlled, identifiable pipe or outfall. The phosphate load in municipal wastewater primarily originates from human waste (contributing an estimated 0.60 kg of phosphorus per person annually) and from household products like detergents and soaps. While many regions have phased out phosphate-based laundry detergents, the nutrient remains a significant component of the influent entering treatment facilities.
Conventional primary and secondary wastewater treatment processes remove solids and organic matter, but they are inefficient at removing dissolved nutrients like phosphate. Without an advanced third or fourth stage of treatment, a significant portion of the phosphate load, sometimes up to 75 percent, can pass through the plant and be discharged into receiving waters. To meet modern water quality standards, many WWTPs now employ tertiary treatment, such as chemical precipitation using metal salts, or enhanced biological removal, which can reduce effluent concentrations to less than 1.0 mg/L.
Industrial sources also contribute phosphate, often from processes that use phosphoric acid or phosphate compounds. Examples include chemical manufacturing facilities, metal finishing operations, and food and beverage processing plants. These discharges are strictly regulated and require permits, but the volume and concentration of their waste streams can make them locally important point sources of contamination.
Natural Weathering of Rocks and Sediment
Phosphate enters water naturally through a continuous, long-term geological process known as weathering. Phosphorus is locked within mineral forms, most notably in the calcium phosphate mineral apatite, which is ubiquitous in igneous, metamorphic, and sedimentary rocks. Over immense periods, the physical and chemical breakdown of these rocks releases phosphate ions into the soil and water.
Chemical weathering, often driven by the action of natural acids produced by microbes and plant roots, slowly dissolves these phosphate-bearing minerals. The phosphate is then made available to the surrounding environment and is transported by surface water runoff and shallow groundwater flow. However, the slow rate of this geological release means it is rarely the cause of the rapid water quality degradation seen today, which is predominantly driven by human activities.
Atmospheric Deposition and Decentralized Septic Systems
Two other pathways contribute to the overall phosphate load: atmospheric deposition and decentralized septic systems. Atmospheric deposition involves phosphate-containing particulate matter settling onto water surfaces or land, which is then washed into waterways by rain. Sources include wind-blown dust from arid regions or agricultural fields, and combustion byproducts from industrial activities and wildfires. Since phosphate does not exist in a stable gaseous form, its atmospheric transport relies entirely on these solid aerosol particles.
Decentralized septic systems, common in rural areas lacking municipal sewer connections, also serve as a source of phosphate. Wastewater from a home, containing phosphate from human waste and cleaners, flows into a septic tank and then into a drain field. Phosphate is removed primarily through chemical precipitation and sorption, binding to soil particles rich in aluminum, iron, or calcium. However, if the soil’s retention capacity is saturated over decades of use, or if the soil is sandy and low in binding minerals, phosphate can leach into the groundwater. This nutrient-rich groundwater then moves slowly toward surface water bodies, acting as a sustained, non-point source of pollution.