Phosphorus and water quality
Phosphorus is one of the key elements necessary for growth of plants and animals. Phosphates
PO43- are formed from this element and can exist in three forms: orthophosphate,
metaphosphate (or polyphosphate), and organically bound phosphate. Each compound contains
phosphorous in a different chemical formula. Ortho forms are produced by natural processes and
are found in sewage. Poly forms are used for treating boiler waters and in detergents. In water,
this form is converted into the ortho form. Organic phosphates are important in nature and their occurrence
may result from the breakdown of organic pesticides which contain phosphates. They may exist
in solution, as particles, loose fragments or in the bodies of aquatic organisms.
Methodology: The analysis of phosphorus uses a spectrophotometer. Phosphorus is oxidized to
the phosphate ion (PO43- ) before reagent dye is added and the absorbance read.
Environmental Impact: Rainfall can cause varying amounts of phosphates to wash from farm
soils into nearby waterways. Phosphate will stimulate the growth of plankton and aquatic plants
which provide food for fish. This may cause an increase in the fish population and improve the
overall water quality. However, if an excess of phosphate enters the waterway, algae and aquatic
plants will grow wildly, choking up the waterway and using up large amounts of oxygen. This
condition is known as eutrophication or over-fertilization of receiving waters. This rapid growth
of aquatic vegetation eventually dies and, as it decays, uses up oxygen. This process in turn
causes the death of aquatic life because of the lowering of dissolved oxygen levels.
Phosphates are not toxic to people or animals unless they are present in very high levels.
Digestive problems could occur from extremely high levels of phosphate.
Criteria: There are no criteria for phosphorus content of water bodies. However, the following
criteria for total phosphorus have been proposed:
1. no more than 0.1 mg/L for streams which do not empty into reservoirs,
2. no more than 0.05 mg/L for streams discharging into reservoirs, and
3. no more than 0.025 mg/L for reservoirs.