Total Phosphorus (TP)

Status and Trends

Phosphorus is usually present in river water as phosphates, and is in very low concentrations unless there has been human induced enrichment of the water. The natural scarcity of phosphorus can be explained by its attraction to organic matter in soil particles. Generally the lower the total phosphorus value in the water, the better. Total phosphorus includes organic and inorganic phosphate. Organic phosphate is a part of living plants and animals. Inorganic phosphates comprise the ions bonded to soil particles, and phosphates present in laundry detergents (as polyphosphates).

Phosphorus is considered to be a limiting factor in aquatic systems, as aquatic organisms quickly utilize PO₄. The amount of phosphates that water can hold without tipping the balance in the system toward eutrophication varies. In a river draining into a natural lake, the phosphate level should not exceed 0.05 mg/L. Natural lake level phosphates should not exceed 0.025 mg/L. A river that is not flowing into a lake should not exceed 0.1 mg/L (USEPA Quality Criteria for Water, 1986). Phosphorus is the major contributing factor in the process of euthrophication. Inputs of phosphorus come from erosion, fertilizers, detergents, and the draining of wetlands.

Median concentration of TP ranged from 0.005 mg/L on the North Branch Potomac at Kitzmiller, MD to 0.14 mg/L on Antietam Creek at Poffenberger Road below Funkstown, MD. Stations with highest median concentration of total phosphorus (24% of all stations) occurred in the highly agricultural region of eastern Upper Potomac basins, Seneca Creek and the lowest Patuxent station. Stations with lowest median concentration (42% of all stations) occurred in the Ohio drainage, western portion of the Upper Potomac basin, and in the urban/agricultural corridor of the Middle Potomac, Patuxent, Patapsco/Back and Upper Western Shore Basins.

Total phosphorus concentration decreased at 39% of all stations. Stations with decreasing trends were predominantly in the eastern Upper Potomac Basin and the urban/agricultural corridor north of Washington and Baltimore. Decreasing trends did not appear to be related to median concentrations. Total phosphorus concentration increased at the station on the Choptank River.

Total phosphorus was correlated with TN at Deer Creek, the North Branch Potomac at Old Town, the Little Youghiogheny and on the Patuxent at the Route 50 Bridge suggesting a common source for the two nutrients. TN and TP moved in opposite directions (i.e. TP was inversely correlated with TN) on the North Branch of the Patapsco at the MD 91 Bridge.
(For Total phosphorus correlation with Total Suspended Solids (TSS) see Figure 2 on the TSS page).

Total Phosphorus (TP) data for sampled locations are summarized in these plots.

• Land use/site map

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This page was updated on 12/5/2005