Dissolved phosphorus in stormwater runoff - sources and management strategies

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Schematic showing analysis for different forms of phosphorus in water. Filtered phosphorus is considered to represent dissolved phosphorus, while unfiltered phosphorus represents all phosphorus. Particulate phosphorus is the difference between filtered and unfiltered.

This page provides a discussion of dissolved phosphorus in stormwater runoff, its sources, and strategies for managing dissolved phosphorus. While the focus is on urban runoff, the basic principles are applicable to agricultural runoff.

Definitions

Phosphorus in water is often classified as dissolved (soluble) or particulate (attached to or a component of particulate matter) phosphorus. This nomenclature is somewhat ambiguous, however, as dissolved phosphorus consists of multiple forms of phosphorus, including phosphorus attached to other materials.

• Dissolved phosphorus is typically identified as phosphorus passing through a 0.45 micron filter. It is this dissolved fraction that is considered to be most bioavailable and most difficult to treat.
• Reactive phosphorus is the phosphorus associated with the test for orthophosphate. It consists mostly of orthophosphate but includes a small fraction of other forms.
• Soluble reactive phosphorus is a measure of orthophosphate, the filterable (soluble, inorganic) fraction of phosphorus, the form directly taken up by plant cells.
• Bioavailable phosphorus is the sum of immediately available phosphorus, which can be transformed into an available form by naturally occurring processes.

References for phosphorus forms and testing includes the following.

• US EPA
• Lake Erie Algae
• City of Boulder, Colorado
• Water Research Center

Bioavailability of different forms of phosphorus

Dissolved phosphorus is considered to be more bioavailable than particulate forms of phosphorus. Below is a summary of some studies on bioavailability of phosphorus.

• About 95% of dissolved phosphorus transported to Lake Erie is bioavailable to algae, while only about 30% of the particulate phosphorus attached to eroded sediment is bioavailable (Lake Erie Algae).
• Ellison and Brett (2006) found on average only 20% of the particulate phosphorus transported in runoff from urban settings was biologically available.
• Abell and Hamilton (2012) found that about 25% of particulate phosphorus in a stream dominated by stormwater runoff was bioavailable.
• Prestigiacomo et al. found 10-20% of particulate phosphorus was bioavailable, compared to more than 90% of dissolved phosphorus being bioavailable. Bioavailable phosphorus in the particulate fraction increased somewhat with time after sampling, but never exceeded 30%.
• Uusitalo et al. (2003) found 6-10% of particulate phosphorus was bioavailable, but that 34-56% was redox-sensitive, meaning it could become bioavailable under anoxic (reducing) conditions. Other papers corroborate these findings, indicating that a significant portion of particulate phosphorus can become bioavailable under anoxic conditions ([1], [2], [3], [4])

Concentrations of dissolved phosphorus in runoff

There are insufficient data to support recommended event mean concentrations (emcs) of dissolved phosphorus for different land uses. The following table provides a summary of data we felt is appropriate for selecting an emc for dissolved phosphorus.

Summary of dissolved phosphorus event mean concentrations from various studies. There is inadequate information to provide recommended emcs for different land uses.
Link to this table

Study	Land cover/land use	Range (mg/L)	Mean	Median	Number of samples
Dallas-Fort Worth1	Commercial	0.01-0.47	0.09	0.06	42
Dallas-Fort Worth	Industrial	0.03-0.45	0.14	0.09	63
Dallas-Fort Worth	Residential	0.04-0.84	0.25	0.21	77
Forth Worth2	Transportation			0.11	28
Twin Cities3	Mixed	0.01-1.4	0.2	0.15	147
Madison4	Medium density residential		0.52	0.61	25
Madison4	Medium density residential		0.4	0.14	25
Madison4	Medium density residential		0.14	0.04	25
Madison4	Medium density residential		0.05	0.03	25
Madison4	Medium density residential		0.04	0.02	25
Madison4	Medium density residential		0.03	0.02	25
Madison4	Medium density residential		0.04	0.02	25
Madison4	Medium density residential		1.54	0.81	25
Madison4	Medium density residential		0.12	0.08	25
Madison4	Medium density residential		0.11	0.07	25
Madison4	Medium density residential		0.11	0.07	25
US EPA Nurp Study5	Residential			0.143
US EPA Nurp Study5	Mixed			0.056
US EPA Nurp Study5	Commercial			0.08
US EPA Nurp Study5	Open			0.026
New York6	Residential		0.20		738
New York6	Commercial		0.18		323
New York6	Industrial		0.16		325
New York6	Open		0.16		44
Capitol Region Watershed District7	Mixed	0.020 - 0.888	0.073	0.052	89
Capitol Region Watershed District7	Mixed	0.020 - 0.565	0.108	0.087	120
Capitol Region Watershed District7	Mixed	0.020 - 0.506	0.074	0.059	112
Capitol Region Watershed District7	Mixed	0.020 - 0.361	0.073	0.053	121
Capitol Region Watershed District7	Mixed	0.005 -- 0.182	0.019	0.012	195
Capitol Region Watershed District7	Mixed	0.020 - 0.758	0.102	0.072	69
Capitol Region Watershed District7	Mixed	0.020 - 1.10	0.072	0.053	115
Capitol Region Watershed District7	Mixed	0.020 - 0.60	0.099	0.057	113
Capitol Region Watershed District7	Mixed	0.020 - 0.499	0.071	0.046	138

¹Urban Stormwater Quality, Event-Mean Concentrations, and Estimates of Stormwater Pollutant Loads, Dallas-Fort Worth Area, Texas. 1992–93 Stanley Baldys III, T.H. Raines, B.L. Mansfield, and J.T. Sandlin U.S. Geological Survey Water-Resources Investigations Report 98–4158.
²Computed and Estimated Pollutant Loads, West Fork Trinity River, Fort Worth, Texas, 1997. United States Geological survey. Water Resources Investigations Report 01–4253
³Brezonik and stadelman. 2002. Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentrations from watersheds in the Twin Cities metropolitan area, Minnesota, USA. Water Research Volume 36, Issue 7, Pages 1743-1757
457.Waschbusch, R.J., W.R. Selbig, and R.T. Bannerman. 1999. Sources of phosphorus and street dirt from Two Urban Residential Basins in Madison, Wisconsin, 1994-95. USGS Water-Resources Investigation Report 99-4021
⁵U.S. EPA. Results of the Nationwide Urban Runoff Program. 1983. Volume I: Final Report. PB84-185552
⁶New York State Department of Environmental Conservation. August 2003. Stormwater Management Design Manual. Chapter 5 - Acceptable Stormwater Management Practices.
⁷Outfall monitoring data for Villa Park, Trout Brook East, Trout Brook West, Trout Brook Outlet, St. Anthony, Phalen Creek, Como 3, Como 7, and East Kittsendale

Ratios of dissolved to total phosphorus in runoff

Another consideration is the fraction or percent of total phosphorus in runoff that is in dissolved form. A more complete discussion of this is found here, including literature references. The data on phosphorus fractionation is limited, but the following general statements can be made.

• Dissolved phosphorus in urban runoff in Minnesota typically ranges from 25 to 50 percent of total phosphorus in runoff.
• The percent of phosphorus in dissolved form increases when phosphorus contributions from organic sources increase. For example, areas with higher tree canopy cover have a higher percent of dissolved phosphorus in runoff than areas with low canopy cover.
• Orthophosphate typically comprises about 60 to 80 percent of the dissolved phosphorus fraction.
• The percent of total phosphorus that is in dissolved form varies seasonally. Dissolved phosphorus comprises a higher fraction of total phosphorus when organic inputs are freshest, such as spring fruit drop from trees and during leaf drop in fall.

Management strategies for dissolved phosphorus

Effectiveness of stormwater BMPs in treating dissolved phosphorus (DP)
BMP	Effectiveness	Comment
Infiltration practices	Effective	DP may be transported to groundwater, but this generally represents a low risk to aquatic environments
Biofiltration (includes tree trenches)	Ineffective	Some DP removal occurs through plant uptake but phosphorus is typically released from the media
Enhanced biofiltration	Effective	In properly designed and maintained systems, iron, aluminum, and calcium adsorb DP
Swales designed for filtration	Ineffective	Addition of engineered media with a low phosphorus concentration may enhance removal through infiltration and biological uptake
Constructed ponds	Limited	Some biological uptake may occur, but as sediment builds in ponds, DP release may occur
Constructed wetlands	Limited	Some biological uptake and immobilization in sediment may occur
Green roofs	Ineffective	Typically leach DP from engineered media during the first several years after construction
Street sweeping	Effective	Most effective when done at times when coarse organic particles (e.g. from leaves) are targted
Pollution prevention	Effective	Focus on organic sources (e.g. yard debris), animal waste, detergents, fertilizer