Orthophosphate (PO₄)

Status and Trends

Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical arrangement. These forms of phosphate occur in living and decaying plant and animal remains, as free ions or weakly chemically bounded in aqueous systems, chemically bounded to sediments and soils, or as mineralized compounds in soil, rocks, and sediments.

Natural processes produce orthophosphate, but major man-influenced sources include: partially treated and untreated sewage, runoff from agricultural sites, and application of some lawn fertilizers. Orthophosphate is readily available to the biological community and typically found in very low concentrations in unpolluted waters. Poly forms are used for treating boiler waters and in detergents. The DC Water and Sewer Authority (WASA) use orthophosphate compounds in drinking water to reduce lead leaching from the distribution system into the water supply. http://www.epa.gov/dclead/corrosion.htm . In water, these compounds are transformed into orthophosphate and available for plant uptake. Organic phosphate is typically estimated by testing for total phosphate. The organic phosphate is the phosphate that is bound or tied up in plant tissue, waste solids, or other organic material. After decomposition, this phosphate can be converted to orthosphate.

Excess phosphorus in surface water may contribute to eutrophication (nutrient enrichment) of surface water, particularly lakes and reservoirs. Nutrient enrichment can overstimulate algal production creating algal blooms that may reduce the aesthetic and recreational value of the water, create taste-and-odor problems in drinking water, and, in severe cases, stress or kill aquatic organisms as a result of dissolved oxygen depletion or the release of toxins when algal blooms die.

Median PO₄ concentration ranged from 0.006 mg/L on the North Branch Potomac River at MD 38 to 0.087 mg/L on Antietam Creek at Poffenberger Road. Orthophosphate median concentration was highest at 31% of all stations that are located in the agricultural regions of Maryland. Many of these were in the Potomac River and tributaries between Conococheague Creek and Seneca Creek. Stations with lowest PO₄ median concentration were 26% of all stations and occurred in the North Branch Potomac and tributaries, in Cabin John Branch, Gwynns Falls, Jones Falls and the lower two stations on the Gunpowder River. .

Increasing trends occurred at the station on the Casselman River, the Potomac at Paw Paw, the Patuxent below Duckett Reservoir, the Gunpowder River at Cromwell Bridge Road, the Susquehanna River and the Choptank River. Decreasing trends occurred at 18% of all stations and were more likely to occur at stations with the highest median concentration (11%, 5% and 0% respectively). Increasing trends occurred at 11% of the stations

Orthophosphate (PO₄) data for sampled locations are summarized in these plots.

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This page was updated on 12/5/2005