Phosphorus and Phosphate: Measuring Both and Their Role in the Environment

Phosphorus is one of the five essential elements of life, playing a critical role in organic processes. However, elemental phosphorus is rarely found in nature. Instead, it exists primarily in the form of phosphates, which are found in water, soils, sediments, and living organisms. These phosphates are essential for plant and animal life, and they also have various industrial and commercial applications.

Forms of Phosphorus in the Environment

Phosphorus occurs in nature mainly as phosphate. Within aquatic systems, phosphorus can be classified into two main forms:

• Organic Phosphate: This form is associated with carbon-based molecules and is found in living or decaying plant and animal matter.
• Inorganic Phosphate: This includes phosphate that is not bound to organic material, with orthophosphate (PO₄³⁻) being the most soluble and readily available form for uptake by photosynthetic organisms. However, orthophosphate is typically found in low concentrations in natural water bodies.

Other phosphate compounds exist within sediments and soils, where they can be released into the environment through natural processes or human activities.

Phosphorus in Human Applications

Because phosphorus is essential for photosynthesis, it is a key ingredient in fertilizers used to enhance plant growth. Beyond agriculture, phosphates are commonly used in various industries. They are added to foods, included in detergents for cleaning efficiency, and even in treated drinking water.

Phosphorus and Eutrophication

In freshwater ecosystems, phosphorus acts as a limiting nutrient for algae growth. When excess phosphorus enters a water body, it can cause eutrophication, where rapid algal growth, known as algal blooms, depletes oxygen levels. This lack of oxygen can lead to fish kills and other disruptions in the aquatic ecosystem. Some algal blooms can also produce toxins harmful to both humans and animals.

Sources of Phosphorus Pollution

Phosphorus in aquatic ecosystems comes from both natural and human sources. The most significant human-related contributors include:

• Fertilizer Runoff: Excess phosphorus from agricultural fields washes into nearby water bodies.
• Municipal and Industrial Wastewater: These facilities process wastewater, removing organic matter and solids, and are increasingly required to remove nutrients like phosphorus and nitrogen before discharging into freshwater systems.
• Animal Manure and Failing Septic Systems: Runoff from manure storage areas and malfunctioning septic systems introduces phosphorus into waterways.
• Disturbed Land and Urban Runoff: Construction sites, drained wetlands, and stormwater runoff carry phosphorus-laden sediments into aquatic environments.

Measuring Phosphate in Water

Phosphate levels can be measured using phosphate test strips, which specifically detect orthophosphate. These strips indicate phosphorus concentrations in parts per million (ppm). The color chart associated with the test includes values at 0, 10, 25, 50, and 100 ppm phosphorus. Since phosphorus exists as phosphate ions (PO₄³⁻), the phosphate concentration can be determined by multiplying the phosphorus value by three. For example:

• A test reading of 10 ppm phosphorus corresponds to 30 ppm phosphate (PO₄³⁻).
• A test reading of 100 ppm phosphorus corresponds to 300 ppm phosphate (PO₄³⁻).

These test strips are useful for measuring high levels of phosphorus, such as in detergent solutions, but they are not sensitive enough to detect the lower concentrations typically found in drinking water or clear freshwater streams.