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Minnesota Stormwater Manual

Dissolved phosphorus in stormwater runoff - sources and management strategies

image of phosphorus speciation
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.

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. The table does not include data for runoff from agricultural systems. Agricultural runoff is not the focus of this manual and the dynamics of phosphorus transport in agricultural systems are likely to vary widely with soil, crop, season, and phosphorus inputs. See the discussions below on dissolved phosphorus fractions in runoff and management strategies.

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


Sources of dissolved phosphorus in stormwater runoff

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.

  • Urban runoff
    • 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.
  • Agricultural runoff - for a good discussion of phosphorus in agricultural systems, link here.
    • Fertilizer and manure have highly soluble forms of phosphorus
    • Soluble phosphorus is slowly released from soil. However, if there are phosphorus inputs from fertilizer, soil will behave as a sink.
    • An important distinction from urban systems is that water has the opportunity to infiltrate in agricultural systems, while phosphorus delivered directly to connected impervious surfaces cannot be captured in soil.
    • Another important distinction between urban and agricultural systems is that infiltrated water in tile-drained systems may be captured by tiles and delivered directly to surface waters. Underdrained practices in urban areas may behave similarly, though the water from an underdrain is typically delivered back to the stormwater conveyance system.

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

The adjacent table describes the relative effectiveness of different stormwater runoff practices in treating dissolved phosphorus. Many of the practices shown in the adjacent table can be used in urban and agricultural settings. The following generalizations can be made.

  • Pollution prevention and source control are among the most effective ways to decrease phosphorus loads to receiving waters. In particular, the following practices are effective.
    • Residential practices, including composting organic material such as leaves (either on-site or off-site at a disposal facility), retaining water on site, removing leaves and grass clippings from impervious surfaces, avoiding use of detergents in vehicle washing, native landscaping, and lawn maintenance
    • Controlling pet waste and waste from other animal sources
    • Street sweeping using appropriate methods at appropriate times of the year
  • Infiltration practices move water from the land surface to shallow groundwater. Phosphorus is generally attenuated in soil, the vadose zone, and within the groundwater system, though in some situations shallow groundwater may discharge locally to surface water and increase phosphorus loading. Proper location of infiltration practices based on an understanding of the local hydrology is necessary if surface waters are at risk.
  • Filtration practices are generally ineffective unless they are designed to prevent loss of or retain dissolved phosphorus. Examples of designed systems include use treatment media with a low organic matter content, use of phosphorus-retaining amendments such as iron, and vegetated systems where the vegetation is removed annually. Filtration practices include systems having an underdrain, or tiled systems in agricultural settings.
  • Sedimentation practices are generally ineffective at retaining dissolved phosphorus unless amended with aluminum, spent lime, or other iron-sequestering chemicals.

Related pages

Each of these pages provides additional references and links that may be useful.

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This page was last edited on 20 February 2023, at 21:03.