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The USGS Water Science School

Phosphorus and Water

Picture showing how soil erosion is a major contributor of phosphorus to streams. Photograph by Lisa Carter.Phosphorus is a common constituent of agricultural fertilizers, manure, and organic wastes in sewage and industrial effluent. It is an essential element for plant life, but when there is too much of it in water, it can speed up eutrophication (a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients) of rivers and lakes. Soil erosion is a major contributor of phosphorus to streams. Bank erosion occurring during floods can transport a lot of phosphorous from the river banks and adjacent land into a stream, as this picture of the Rio Chama near Chamita, New Mexico shows (photograph by Lisa Carter).

Phosphorus in surface and groundwater

Phosphorus gets into water in both urban and agricultural settings. Phosphorus tends to attach to soil particles and, thus, moves into surface-water bodies from runoff. A USGS study on Cape Cod, Massachusetts showed that phosphorus can also migrate with ground-water flows. Since groundwater often discharges into surface water, such as through streambanks into rivers, there is a concern about phosphorus concentrations in groundwater affecting the water quality of surface water.

Phosphorus is an essential element for plant life, but when there is too much of it in water, it can speed up eutrophication (a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients) of rivers and lakes. This has been a very serious problem in the Atlanta, Ga. area, as a major lake that receives Atlanta's waste water, West Point Lake, is south of the city. In metropolitan Atlanta, phosphorus coming into streams from point sources, primarily wastewater-treatment facilities, have caused West Point Lake to become highly eutrophic ("enriched"). A sign of this is excess algae in the lake. State laws to reduce phosphorus coming from wastewater-treatment facilities and to restrict the use of phosphorus detergents has caused large reductions in the amounts of phosphorus in the Chattahoochee River south of Atlanta, Georgia and in West Point Lake.

This chart shows the amount of phosphorus, in tons per year, upstream and downstream of the Chattahoochee River at Atlanta, which is a major source of the local water supply. The amounts of phosphorus downstream of the city have decreased about 77% from the highest levels in 1984 because of both voluntary and mandatory restrictions on phosphorus detergents in the city. As the bottom line shows, though, the total phosphorus load in the more agricultural area north of town continues to increase.

Graph of phosphorus loads before and after restrictions on  phosphorus usage

The chart below shows both the wastewater discharge and the amount of phosphorus discharged from Atlanta's wastewater-treatment plants. It makes sense that the total amount of wastewater would be going up as population increases, but the tonnage of phosphorus has been greatly reduced both by improvements in the treatment process and by restrictions on phosphate detergents.

Graph of phosphorus loads before and after restrictions on  phosphorus usage

Related topics:

Back to Nitrogen in water  Groundwater quality  Groundwater

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Page Last Modified: Monday, 17-Mar-2014 11:03:28 EDT