Phosphates in the Environment

What are Phosphates?

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Phosphates enter waterways from human and animal waste, phosphorus rich bedrock, laundry, cleaning, industrial effluents, and fertilizer runoff. These phosphates become detrimental when they over fertilize aquatic plants and cause stepped up eutrophication.

Eutrophication is the natural aging process of a body of water such as a bay or lake. This process results from the increase of nutrients within the body of water which, in turn, create plant growth. The plants die more quickly than they can be decomposed. This dead plant matter builds up and together with sediment entering the water, fills in the bed of the bay or lake making it more shallow. Normally this process takes thousands of years.

Cultural eutrophication is an unnatural speeding up of this process because of man's addition of phosphates, nitrogen, and sediment to the water. Bodies of water are being aged at a much faster rate than geological forces can create new ones.

In testing for cultural eutrophication, one would expect to find an algal bloom or scum on the water accompanied by a fishy smell to the water and a low dissolved oxygen content. Do not expect to find a high phosphate reading if the algae are already blooming, as the phosphates will already be in the algae, not in the water. The algae bloom should start where running water enters the lake or bay, so test the water before the area where the bloom begins for high phosphate and nitrate levels.

Monitors should be aware that there are different kinds of phosphates in the water, but a total phosphate-phosphorous reading is all that is needed to calculate the water quality. Use the chart below to rate your water sample:

0.01 - 0.03 mg/L - the level in uncontaminated lakes

0.025 - 0.1 mg/L - level at which plant growth is stimulated

0.1 mg/L - maximum acceptable to avoid accelerated eutrophication

> 0.1 mg/L - accelerated growth and consequent problems

(some information from Water, Water Everywhere. HACH Company. Second Edition. 1983.)

How phosphorous affects aquatic life

If too much phosphate is present in the water the algae and weeds will grow rapidly, may choke the waterway, and use up large amounts of precious oxygen (in the absence of photosynthesis and as the algae and plants die and are consumed by aerobic bacteria.) The result may be the death of many fish and aquatic organisms.



Phosphorus and water quality

Phosphorus is one of the key elements necessary for the growth of plants and animals. Phosphates PO4--- are formed from this element. Phosphates 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, they change into the ortho form. Organic phosphates are important in nature. 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 (PO4---). 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, choke up the waterway and use 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 it 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: The following criteria for total phosphorus were recommended by US EPA (1986):

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.

For updated information on water quality criteria for nutrients, visit:

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