Sources of Phosphorus

Willaim P. Ruzzo, PE

Natural Processes

1. Precipitation. Rain and snow scour the air, accumulating airborne phosphorus (orthophosphate and particulate forms) and other pollutants, which are deposited in the watershed. Concentrations in precipitation range from 0.02 to 0.04 mg/l. The Authority reports the average annual load to the Reservoir from precipitation is 823 pounds.
2. Atmospheric fallout ("dry fall"). Wind currents deposit phosphorus (particulate form) and other pollutants attached to sediment carried from other areas. The total loading from dry fall can be in the same order as precipitation.
3. Oxidation (decomposition) of organic materials. Plants, atmospheric fallout, liquid and solid wastes accumulate in urban areas. These wastes are decomposed, producing phosphates (typically organic form). Novotny reports phosphate concentrations in street refuse from 1,300 to 3,900 mg/kg of solids.
4. Erosion of Watershed Soils. Measurements by Halepaska show that typical watershed soils have phosphorus (orthophosphate) concentrations of up to 3.9 mg/kg with an average of 1.5 mg/kg. The origin of all inorganic orthophosphate is the class of minerals known as apatites. As phosphorus moves through the system toward the Reservoir, the concentrations increase, a phenomenon known as pollutant enrichment . Other phosphorus measurements in Cherry Creek sediment by Halepaska show concentrations from 310- to 580-mg/kg, which indicates the extent of enrichment that takes place in the watershed.
5. Background. As the result of runoff and natural erosion, the background concentration of phosphorus (i.e.: for limited agriculture and urban land uses in Cherry Creek above Castlewood Canyon) is reported in the range of 0.02 to 0.03 mg/l. Measurements by Halepaska in Cherry Creek show concentrations around 0.2 mg/l. Measurements in the South Platte River at Littleton show mean concentrations around 0.1 mg/l.

Cherry Creek Phosphorus Loading Mass-Balance Schematic (2002)

Anthropogenic Processes

1. Commercial fertilizer. Commercial fertilizers typically contain some form of phosphates (P2O5). Application of fertilizers can add phosphorus to irrigation return flows and storm runoff, since it is well known that residents often over-water lawns.
2. Automotive products. Hydraulic fluids, fuels, tires, and rubber compounds deposited on impervious surfaces, all containing phosphorus, are washed into drainage ways and streams.
3. Detergents and water softeners. In the past, detergents contained phosphates, however, this source of phosphorus is likely small, as most detergent products are now phosphorus free.
4. Animal manure. Wastes from horses, cattle, and sheep, domestic and wild animals that is exposed to storm water produces organic phosphates.
5. Septic tanks and leach fields. The Authority’s monitoring of this potential source has not shown higher concentrations of phosphorus in Cherry Creek at this time. It is possible, that the upper watershed soils are immobilizing phosphorus due to the soils’ ability to sorb phosphorus onto small soil particles.
6. Agriculture Activities. Novotny reports that phosphorus concentrations in cropland runoff are in the range of 0.02 to 1.7 mg/l. Erosion and soil loss by surface runoff is the predominant source of pollution from croplands.

Transportation of Phosphorus

Water is the primary transport mechanism for phosphorus. For the Cherry Creek watershed, the following transport mechanisms are believed to be in relative order of importance:

1. Soil Erosion. During construction, soil erosion by wind and rain can be orders of magnitude higher than pre-or post-developed conditions.
2. Storm Runoff. A major factor contributing to the increase in phosphorus loads from urbanization over undeveloped land is the significant increase in storm runoff from impervious surfaces, estimated at from 10- to 15 times. Particulate phosphorus is only transported by surface runoff, while dissolved phosphorus can travel by surface and groundwater.
3. Groundwater movement. As phosphorus is moved through the system, the concentration in soils increases (i.e.: "enrichment factor"), which increases the phosphorus available for transport by groundwater movement. However, the ability of the soil to sorb phosphorus helps to immobilize phosphorus.
4. Wind. Local winds erode small soil particles (i.e.: a construction site) and redistribute them within the watershed, such as in a water body or stream. This action increases phosphorus loads in the water. Prevailing winds transport atmospheric particles (i.e.: smog) from other metropolitan areas to the watershed, where dry-fall takes place.

Phosphorus Cycle