Summary information - phosphorus concentrations in stormwater runoff
Land use
Recommended total phosphorus event mean concentration (mg/L)
Commercial
0.200
Industrial
0.235
Residential
0.325
Freeways/transportation
0.280
Mixed
0.290
Open space
0.190
Conventional roof
0.030

This page provides information on event mean concentrations for total phosphorus and dissolved phosphorus in urban stormwater runoff. For a discussion of phosphorus in stormwater runoff, including information on sources, fate, and water quality impacts, see this page. For a discussion of event mean concentrations, see Stormwater pollutant concentrations and event mean concentrations.

## Objective

Event mean concentrations (emcs) are used in models for predicting water quality impacts from stormwater runoff and stormwater treatment practices or pollution prevention practices. Pollutant loads, which are typically used to assess water quality impacts, including establishing (TMDLs), are a function of pollutant concentration and volume of runoff. It is therefore important to accurately determine appropriate event mean concentrations when assessing water quality impacts from stormwater runoff.

This page provides summary information that can be used for selecting or calculating appropriate emcs for total and dissolved phosphorus.

Information: Many factors affect phosphorus concentrations in stormwater. If you are unfamiliar with the concept of event mean concentrations, we recommend you first read Stormwater pollutant concentrations and event mean concentrations

## Methodology

We conducted a review of literature to develop the emcs shown on this page. Nearly all studies provided summary information; we therefore did not analyze raw data with the exception of data from Capitol Region Watershed District (see discussion below) and the National Stormwater Quality Database. We compiled the summary information into a spreadsheet and conducted simple statistical analysis of the information. Data from the following studies were compiled into the spreadsheet.

In addition to the above sources, we compiled water quality monitoring data from 10 storm sewer outfalls in the Capitol Region Watershed in Minnesota. The data period for each outlet varied but generally spanned the period from about 2005 to 2019. The following information was compiled for each monitoring location.

• Date
• Total phosphorus, dissolved phosphorus, and orthophosphate as P, all in mg/L.
• Sample type, which included runoff samples during precipitation events, snowmelt samples, and baseflow samples for those locations where groundwater contributed to flow.

We also downloaded the 2015 National Stormwater Quality Database. The dataset includes information from across the U.S. We selected only data from Region 1, which includes Minnesota, for analysis. Only data on total phosphorus was available. Four land uses included commercial, industrial, residential, and open space, with the number of samples for each land use varying.

For both of these data sets, we conducted simple statistical analyses.

## Recommended event mean concentrations (emc) for total phosphorus, by land use

Pollutant loads in stormwater runoff equal the pollutant concentration times the runoff volume. Thus, when calculating pollutant loading, it is necessary to consider not only the event mean concentration but factors affecting the volume of runoff. For most models and calculations, this requires adjusting curve numbers or runoff coefficients to account for differences in directly connected impervious surface between different land uses. There may be other adjustments to volume, such as accounting for interception by trees. See the discussion Accounting for differences in pollutant loading.

Emcs for total phosphorus vary by land use. This section provides recommended emcs for different land uses. A discussion of factors affecting emcs and potential adjustments to emcs are provided in separate sections below.

### Residential land use

Information: The recommended event mean concentration for total phosphorus in residential areas is 0.325 mg/L
Example of residential land use

Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of residential land use, typically low-, medium-, or high-density residential. Most studies do not define criteria for dividing residential land use into these subcategories. Various definitions can be found in the literature, including the following.

• Residential: "Residential land use means any real property or portion thereof which is used for housing human beings. This term includes property used for schools, day care centers, nursing homes, or other residential-style facilities or recreational areas." (Law Insider accessed December 31, 2019).
• High-density residential: More than 10 units per acre; can include multiple-occupant dwellings
• Medium-density residential: 1-10 dwellings per acre; can include multiple-occupant dwellings
• Low-density residential: one dwelling per 1-5 acres; can include multiple-occupant dwellings

Note that residential land uses can include other land uses, such as commercial and industrial. Many studies therefore classify land uses as mixed or urban, even though a specific land use may dominate a particular area.

Because of the variable and arbitrary manner in which residential land use is classified, we provide a single recommended value for event mean concentration in residential land uses. We provide information below for adjusting this recommended value depending on local conditions. We used the following references for generating a recommended value for residential land use.

• Baldys et al., (1998). 77 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.33 mg/L
• Yand and Toor, (2018). 35 data points from Florida. Median concentration of total phosphorus = 0.29 mg/L
• Waschbusch et al. (1999). 25 data points from Madison Wisconsin. Median concentration of total phosphorus = 0.17 mg/L
• National Urban Runoff Program (1983). Summary of data from 28 studies in the U.S. Median concentration of total phosphorus = 0.38 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.45 mg/L
• Capital Region Watershed District. Outfall data from 9 catchments in St. Paul, Minnesota. Median concentration of total phosphorus = 0.325 mg/L
• National Stormwater Quality database. Data from 249 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.27 mg/L

We chose these studies because they contained large amounts of data and they were located in humid and sub-humid areas of the U.S. The median of the above 7 values is 0.325 mg/L.

### Commercial land use emcs

Information: The recommended event mean concentration for total phosphorus in commercial areas is 0.200 mg/L
Example of commercial land use

"Commercial land use is the use of land for commercial purposes including building offices, shops, resorts and restaurants as opposed to construction of a residential house" (Reference, accessed December 24, 2019). Commercial areas considered in this analysis do not include areas used for commercial crop production.

We used the following studies in our analysis.

• Baldys et al., (1998). 42 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.14 mg/L
• National Urban Runoff Program (1983). Summary of data from 28 studies in the U.S. Median concentration of total phosphorus = 0.20 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.22 mg/L
• National Stormwater Quality database. Data from 165 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.11 mg/L
• van Guerard and Weiss (1995). Data from 7 samples in Colorado Springs. Median concentration of total phosphorus = 0.12 mg/L
• Wang et al. (2013). Data from 20 samples in Chongqing, China. Median concentration of total phosphorus = 0.12 mg/L

The median concentration from these studies is 0.17 mg/L. This value appears to be relatively low compared to other studies for which ranges were reported.

• Harper (1998) reported median concentrations of 0.15-0.43 mg/L in Florida
• Brezonik and Stadelmann (2001) reported concentrations of 0.22-0.77 mg/L in Minnesota

We recommend a value of 0.200 mg/L for stormwater runoff from commercial areas.

### Industrial land use emcs

Information: The recommended event mean concentration for total phosphorus in industrial areas is 0.235 mg/L
Example of an industrial area

Industrial land use includes "land used for commercial establishments, manufacturing plants, public utilities, mining, distribution of goods or services, administration of business activities, research and development facilities, warehousing, shipping, transporting, remanufacturing, stockpiling of raw materials, storage, repair and maintenance of commercial machinery or equipment, and waste management" (Law Insider; accessed December 31, 2019).

We used the following studies in our analysis.

• Baldys et al., (1998). 63 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.21 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.25 mg/L
• National Stormwater Quality database. Data from 84 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.235 mg/L
• Rhee et al. (2012), data from 24 locations Bundang-gu, Gyeonggi provinces, South Korea. Median concentration of total phosphorus = 0.11 mg/L
• Capital Region Watershed District, St. Paul, Minnesota, Hidden Falls outlet (which drains an industrial area). Median concentration of total phosphorus = 0.29 mg/L

The median TP concentration from these studies is 0.235 mg/L.

### Open space

Information: The recommended event mean concentration for total phosphorus in open space, urban parks, and urban recreations areas is 0.190 mg/L
Example of open space land use

Open space consists of land that is undeveloped. Typically it will not contain buildings or other built structures. Many open spaces are accessible to the public. Open space generally consists of green space (land that is partly or completely covered with grass, trees, shrubs, or other vegetation). Abandoned parcels lacking structures may be considered open space, but it is generally more accurate to include these areas in the land use that existed prior to the parcel being vacant, or including it in adjacent land use categories. The following references were used to generate a recommended TP emc for open space.

• National Urban Runoff Program, which consists of a nationwide study. Median concentration of total phosphorus = 0.121 mg/L
• The National Stormwater Quality Database (Region 1, which includes Minnesota). Median concentration of total phosphorus = 0.19 for 6 sites
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.22 mg/L

Parks and recreation areas are generally included in open space.

### Transportation corridors, highways, and freeways emcs

Information: The recommended event mean concentration for total phosphorus in transportation areas is 0.280 mg/L
Example of transportation land use

This land use includes major transportation corridors where the land use is exclusively transportation. These areas are typically highly impervious and may include only small vegetated areas consisting of swales or medians, and relatively small right-of-way areas. This land use does not include arterial streets in residential, commercial, and industrial areas. The following references were used to generate a recommended value a TP emc for open space.

• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.28 mg/L
• Baldys et al., (1998). 28 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.21 mg/L
• Wei et al. (2013). 12 data points from Handan, China. Median concentration of total phosphorus = 0.625 mg/L
• Roseen et al. (2009). Data from 15 events in New Hampshire. Median concentration of total phosphorus = 0.125 mg/L
• Schiff and Tiefenthaler. Data from 31 events in California. Median concentration of total phosphorus = 0.41 mg/L

The median value from these studies is 0.28 mg/L.

### Conventional roof emcs

Information: The recommended event mean concentration for total phosphorus in runoff from conventional (non-green) roofs is 0.030 mg/L

If roofs comprise a considerable portion of an area, it may be beneficial to include the contribution from roofs separate from other land uses. Note that the emcs for the above land uses generally consider the contribution from roofs.

Phosphorus concentrations from tradition (non-green) roofs is similar to concentrations in precipitation. Although concentrations vary, they are generally low and within the range of 0.01-0.05 mg/L (MPCA literature review). A value of 0.03 mg/L is therefore considered appropriate.

### Mixed land use emcs

In many cases, a specific land use will include multiple land uses. For these situations we recommend using the recommended value for mixed land uses (0.290 mg/L), adjusting this emc based on local data, or calculating the emc. An emc can be calculated if the total area of interest (Atotal), the area of each land use in the area of interest, and the emc for each land use in the area of interest are known.

Site emc = Σ1n ((AArea 1 * emcArea 1)/ (Atotal) + ... ((AArea n * emcArea n) / (Atotal)

where A = area in acres.

Example calculation

• 10 acres of residential; emc = 0.325 mg/L
• 10 acres of commercial; emc = 0.200 mg/L
• 10 acres of industrial; emc = 0.235 mg/L
• 1 acre of transportation; emc = 0.280 mg/L

Overall emc = ((0.325 * 10)/31) + ((10 * 0.200)/31) + ((10 * 0.235)/31) + ((1 * 0.280)/31) = 0.254 mg/L

### Agricultural land use emcs

We did not conduct a rigorous literature review of emcs for agricultural land uses. We briefly reviewed several papers and compiled the data into ranges. The following papers were reviewed.

Note that there is considerable variability in land uses described as agricultural (e.g. row crop, pasture, small grain, etc.) and in the practices employed within these land uses, particularly with the management of fertilizers.

To see the results of this literature review, see the table Event mean concentrations for total phosphorus.

### Summary table for event mean concentrations by land use

Event mean concentrations for total phosphorus.

Land cover/land use Range (mg/L) Recommended value (mg/L) Notes
Commercial 0.20 - 0.34 0.200 If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads
Industrial 0.23 - 0.55 0.235
• If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads
• Adjust upward if specific phosphorus sources exist
Residential 0.26 - 0.38 0.325 Concentrations vary widely depending on local conditions
High-density/Multi-family residential 0.28 - 0.40 Calculate1
• Insufficient information to recommend a specific emc
• Concentrations vary widely depending on local conditions
Medium density residential 0.18 - 0.40 Calculate1
• Insufficient information to recommend a specific emc
• Concentrations vary widely depending on local conditions
Low density residential 0.24 - 0.40 Calculate1
• Insufficient information to recommend a specific emc
• Concentrations vary widely depending on local conditions
Freeways/transportation 0.25 - 0.45 0.280
• Concentrations vary widely depending on inputs
• Adjust upward for areas receiving large inputs of road salt or sediment or having very heavy traffic loads
• Adjust downward for low traffic areas or areas with reduced inputs (e.g. little road salt application, limited truck traffic)
Mixed 0.16 - 0.84 0.290
• Residential land use was the primary land use in most studies that cited values for mixed land use
• If the study area can be delineated into specific land uses and impervious area for each land use is know, we recommend calculating the emc
Parks and recreation Use value for open space or calculate
• emc will be a function of vegetative cover
• Adjust upward if street tree canopy cover is high or pervious areas are primarily grass on compacted soils
Open space 0.12 - 0.31 0.190
Conventional roof 0.01 - 0.20 0.030
Institutional 0.14 - 0.422 See note
• Use low values in range (0.200 mg/L or less) for facilities such as campuses, where there is considerable pervious area
• Use high values in range (0.30 mg/L or greater) for areas with considerable impervious surface, such as sports facilities or facilities with large parking areas
Forest/shrub/grassland 0.03 - 0.45 0.090 Concentrations are likely to vary with season in areas with fall leaf drop
Open water and wetlands see Notes (next column)
• If data exist, use the phosphorus concentration for the water body of interest
• If data for a specific lake do not exist, use data from similar lakes in the area
• emcs for wetlands will typically be higher than for lakes in an area. Consider using a value equal to 2 times the value for lakes in an area.
Cropland (row crops) 0.126-1.348 2 Median from our review = 0.533
Pasture 0.35-0.45 2

1The link takes you to information on calculating event mean concentrations for areas with multiple land uses.
2Our literature review was not extensive enough to warrant a specific recommend emc for this land use

## Event mean concentrations for dissolved phosphorus

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.

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

## Factors affecting total phosphorus emcs in stormwater runoff

Several factors affect concentrations of total phosphorus in stormwater runoff. The following bullet list summarizes some of the most important factors.

• Emcs increase with tree canopy coverage over impermeable surfaces. Janke et al. (2017) observed a strong correlation between total phosphorus and street canopy cover. The observed relationship was linear, with concentrations ranging from about 0.2 mg/L to 0.45 mg/L at 40% street canopy coverage. Waschbusch et al. (1999) observed a similar relationship in two Madison, Wisconsin neighborhoods, with total phosphorus ranging from about 0.1 mg/L with 5 percent street canopy coverage to 0.40 mg/L with 40 percent street canopy coverage.
• Emcs are greatest during fall leaf drop and elevated during spring flower drop. Selbig (2016) measured monthly concentrations of total phosphorus in a residential area of Madison, Wisconsin having 17 percent street tree canopy coverage. Mean concentrations in spring were 0.67-0.74 mg/L, decreasing to 0.41-0.45 in summer, and rapidly increasing following Fall leaf drop, when concentrations increased to more than 2.5 mg/L. Janke et al. (2017) observed similar patterns in Minnesota. TP concentrations in spring (flowering) and fall (leaf drop) were correlated with street canopy coverage, while summer concentrations were not correlated with street canopy coverage.
• Emcs increase as the length of time between runoff events increases. Typically, street solids tend to build up over time and reach a maximum value within one or two weeks after the previous rainfall event. Yang and Toor (2018) observed higher fractions of particulate phosphorus compared to dissolved phosphorus in runoff as the period between runoff events increased. Schiff et al. (2011) observed similar patterns in California, where dry periods can be lengthy.
• Emcs may be greater during the initial period of a runoff event. This effect is often called "first flush" (Li et. al, 2015; Schiff et al., 2016; Stenstrom and Kayhanian, 2005).
• First flush is more pronounced in smaller watersheds.
• First flush is more pronounced when the length of time between runoff events increases.
• First flush is less pronounced when there is greater directly connected impervious surface, since runoff can reach a specific discharge point from greater distances.
• First flush is more pronounced with higher rainfall intensities in the early part of a runoff event.
• First flush is more pronounced when the majority of phosphorus is associated with particulate material.
• The fraction of total phosphorus in particulate form is greatest during first flush, with dissolved fraction increasing with time during a runoff event.
• Characteristics of permeable surfaces likely affect emcs. There are limited studies of runoff from pervious surfaces, but the following are likely.
• Emcs increase in areas with extensive lawns, particularly on compacted surfaces and where lawns are directly adjacent to impervious surfaces.
• Because of the statewide fertilizer ban, inputs from fertilizers should be minimal except in areas where exemptions exist (e.g. newly established lawns).
• Areas with organized leaf management and ordinances banning placement of leaves in streets should have lower emcs.
• Areas with extensive yard irrigation will likely have elevated emcs.
• The following operations can lead to elevated emcs.
• Emcs will be elevated in areas with heavy application of de-icers. There is wide variability in the phosphorus content in different de-icers. A study for the Minnesota Department of Transportation found a median total phosphorus concentration of 4.9 mg/kg (ppm) in sodium chloride deicers. A study in Mizzoula Montana showed that phosphorus from deicers accounted for less than 1 percent of annual phosphorus loading. Elevated phosphorus concentrations will be most prevalent in winter and early spring runoff events, with smaller contributions from residual deicer through spring and summer.
• Emcs will be elevated in areas with cleaning and washing operations
• Emcs increase in areas with heavy vehicle traffic
• Emcs may be elevated if specific industries such as food processing plants, meat packing plants and lockers, metal finishing facilities, and industries that generate or handle animal waste (including human sources) are present

Several factors affect emcs, as discussed above. Emcs can and should be adjusted when supporting data exist. Local monitoring data should be used to support different emcs than those recommended on this page, but the following guidelines may be used to adjust emcs. Below is a discussion of these factors and the effect on emcs for total phosphorus.

### Adjusting emcs for tree canopy coverage

Janke et al. (2017) established a linear relationship between total phosphorus in runoff (mg/L) versus street tree canopy coverage for canopy coverages up to about 45 percent. At zero canopy coverage, the phosphorus emc was 0.200 mg/L, with an increase of about 0.06 mg/L for each 10 percent increase in street tree canopy coverage.

We recommend adjusting the emc by 0.060 mg/L for each 10 percent increase or decrease in street canopy coverage for canopy coverages between 0 and 40 percent. There is limited data for calculating adjustments at canopy coverages greater than 40 percent. NOTE: The Janke et al. study was conducted in areas dominated by residential land use. The recommended TP concentration of 0.325 mg/L for residential areas, presented on this page, corresponds with a street tree canopy coverage of about 20-25 percent. This is slightly below the national urban tree cover of 27.1 percent (Deeproot (2010)).

Example calculation: Assuming an emc of 0.200 mg/L at zero percent canopy coverage, tree canopy coverage in two adjacent areas is 15 and 30 percent respectively. For the area with 15 percent canopy, the recommended emc is (0.200 + (0.060 * 15/10)) = 0.290 mg/L. For the area with 30 percent canopy coverage, the recommended emc is (0.200 + (0.060*30/10)) = 0.380 mg/L.

Studies show that phosphorus concentrations are generally greatest during fall leaf drop (Selbig, 2016; Janke et al., 2017; Yang, 2018). Concentrations may also be elevated during spring flowering and leaf-out. Data from Capitol Region Watershed District indicate that concentrations in winter may also be elevated. Despite these studies, there is insufficient data to establish specific relationships that allow for adjusting emcs based on season.

To date, Selbig's work (2016) is the most comprehensive study of seasonal effects on phosphorus concentration. His work, done in Madison Wisconsin, shows total phosphorus concentrations in the 2-3 mg/L range immediately following leaf-drop, compared to concentrations of 0.2-0.5 during most of the remainder of the year. Fifty-six percent of the annual total phosphorus load was due to leaf litter in the fall.

Additional studies on this topic are likely, including continued work by Selbig and the United States Geological Survey. Though specific quantitative adjustments cannot be recommended at this time, accurate estimates of loading in areas with significant leaf contributions likely depend on adjustments in total phosphorus emcs to account for inputs from leaves. Approaches to modeling this may include adjusting emcs upward or modeling loads for different seasons.

Phosphorus loading for 3 different total phosphorus emcs (0.20, 0.30, and 0.40 mg/L) anf five land uses (1 acre of impervious with no pervious, and 1 acre of impervious with 1 acre of pervious turf on either HSG A, B, C, or D).

To assess the effect of changing the phosphorus emc, we ran several scenarios using the Minimal Impact Design Standards Calculator. For each model run we assumed the 31.9 inches of precipitation annually. We varied the emc as follows

• 0.20 mg/L
• 0.30 mg/L (MIDS Calculator default)
• 0.40 mg/L

We varied land use as follows

• 1 acre of impervious
• 1 acre of impervious and 1 acre of turf on hydrologic group soil (HSG)A soil
• 1 acre of impervious and 1 acre of turf on B soil
• 1 acre of impervious and 1 acre of turf on C soil
• 1 acre of impervious and 1 acre of turf on D soil

The results, illustrated in the adjacent graph, indicate a small effect of soil. Changing the emc within a specific land use scenario, however, results in significant changes in loading. The change in loading is linear and equal to the following.

• impervious: 0.062 lbs/acre/yr increase in TP load for each 0.01 mg/L increase in TP
• A soil: 0.071 lbs/acre/yr increase in TP load for each 0.01 mg/L increase in TP
• B soil: 0.075 lbs/acre/yr increase in TP load for each 0.01 mg/L increase in TP
• C soil: 0.076 lbs/acre/yr increase in TP load for each 0.01 mg/L increase in TP
• D soil: 0.078 lbs/acre/yr increase in TP load for each 0.01 mg/L increase in TP

This exercise illustrates the importance of selecting an appropriate emc.

## Ratios of particulate to 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

Dissolved phosphorus (DP) 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. The adjacent table provides a summary of the effectiveness of stormwater BMPs in treating dissolved phosphorus. For a discussion of different forms of phosphorus in water, link here.

The Minimal Impact Design Standards (MIDS) Calculator assumes 55 percent of total phosphorus in stormwater runoff occurs as particulate phosphorus (PP) and 45 percent occurs as DP, for a PP:DP ratio of 55:45. This value is based on a nationwide study by Pitt and Maestre (2005).

The PP:DP ratio varies depending on several factors and assuming a 55:45 ratio when the actual value differs can lead to significant errors in estimating pollutant loading and treatment effectiveness. In particular, overestimating PP can result in reduced effectiveness of stormwater management for phosphorus since the most bioavailable forms are associated with the dissolved fraction. Similarly, attempting to reach a TP reduction goal by focusing on removing the PP fraction alone will likely be ineffective in attaining goals for receiving waters.

We conducted an analysis of data from the following studies to develop guidance for adjusting PP:DP ratios in models, calculators, and other pollutant loading tools.

Results of this analysis are provided in the adjacent table. The data suggest the ratio of 0.55:45 is low, but there is variability in the data, including the following observations.

• Particulate phosphorus (PP) fractions are greatest in spring and summer and lowest in fall and winter
• For the data from the National Stormwater Quality database, regressions were significant between total suspended solids and PP (p=1.21e-8; R2=0.185) and not significant between PP and precipitation (p=0.31) or 5 residential area (p=0.483)
 Particulate phosphorus (PP) as a fraction of total phosphorus. Dissolved phosphorus as a fraction would equal (1 - PP). Study Study description Particulate phosphorus fraction Pitt and Maestre (2005) National Stormwater Quality database 0.556 National Urban Runoff Program (NURP) 0.576 Nation Stormwater Quality database Nationwide data (n = 3312) 0.454 National Stormwater Quality database Rainfall Region 1 (includes Minnesota) 0.742 Capitol Region Watershed District All monitoring data from 9 subwatersheds 0.730 Capitol Region Watershed District Stormflow data from 9 subwatersheds 0.825 Capitol Region Watershed District Winter runoff data from subwatersheds 0.634 New York State Median for 4 land uses (residential, commercial, industrial, open space) 0.575

In addition to these data, we reviewed several additional papers addressing the speciation of phosphorus in urban stormwater runoff.

• Waschbusch at al. (1999) studied phosphorus in runoff from residential basins in Madison, Wisconsin. The primary sources of dissolved phosphorus are lawns and streets (45 and 39%, respectively, of DP inputs).
• DP as a fraction of TP increases during a typical runoff event (University of Wisconsin, Milwaukee, 2010).
• Organic-bound phosphorus typically comprises less than 10 percent of total soluble phosphorus (California Regional Water Quality Control Board. 2014; University of Wisconsin, Milwaukee, 2011). Data from Capitol Region Watershed District shows orthophosphate accounts for 71.2% of DP for storm events and 79.2% for winter runoff. Madge (2004) found 96% of dissolved phosphorus was bioavailable.
• PP fractions from open areas are greater than from developed (residential or commercial) areas (Nurp, 1983; Perry et al.)
• Results from testing of a residential site (Madge, 2004) found that the majority of phosphorus was attached to particles in the range of 5 to 20 um
• Several studies show that leaves are a source of soluble phosphorus.
• Riemersma et al. (2006) studied phosphorus sources and sinks in watersheds. Although their work focused primarily on agricultural watersheds, many of their observations are applicable to urban stormwater, including the following.
• Plant residues are an important source of dissolved (soluble) phosphorus
• Soil disturbance is an important source of dissolved phosphorus, which is released from organic matter
• Irrigation increases losses of dissolved phosphorus
• Wetlands may be an important source of dissolved phosphorus
• A high percentage of phosphorus from animal wastes is in dissolved form

There is insufficient information to provide specific recommended values for PP:DP. Appropriate monitoring would be required to establish specific values. However, the following recommendations may be useful when considering the PP:DP ratio used in a specific model or calculator.

• Consider lower PP:DP ratios under the following conditions
• Areas or periods of time where vegetative inputs are significant (e.g. leaf drop, fruit drop, grass clippings)
• Areas where inputs from animal waste are potentially important (e.g. areas with high bird densities)
• Areas or periods of time when winter runoff is important
• Areas with phosphorus fertilizer application (e.g. new residential developments)
• Consider higher PP:DP ratios under the following conditions
• Areas with low inputs from vegetative sources (e.g. ultra-urban settings)
• Areas with active construction and exposed soils
• Modeling or calculating loads for runoff associated with or dominated by first flush

Pollutant loads are a function of pollutant concentrations in runoff and the volume of runoff. Consequently, when calculating pollutant loads it is necessary to adjust both the emcs and volume of runoff. Volumes are typically calculated using curve numbers or runoff coefficients. The MPCA Simple Estimator, for example, employs a default runoff coefficient of 0.8 for commercial areas, compared to 0.44 for residential areas. The tables below may be used to determine the proper curve number or runoff coefficient. Percent impervious can be converted to a curve number using the following formula.

$$Curve number = (Impervious * 98) + ((1 - impervious) * (open space curve number in good condition for the specific soil))$$

where impervious is given as a fraction (not a percent).

For example, if an area on B soils is 50 percent impervious, the curve number is given as (0.5 * 98) + ((1 - 0.50)(61)) = 79.5.

Curve numbers for urban and agricultural areas. Source: USDA Urban Hydrology for Small Watersheds - TR-55.
Link to this Curve numbers for urban and agricultural areas

Cover type and hydrologic condition Soil Group A Soil Group B Soil Group C Soil Group D
Open space poor condition (<50% cover) 68 79 86 89
Open space average condition (50-75% cover) 49 69 79 84
Open space good condition (>75% cover) 39 61 74 80
Impervious surfaces 98 98 98 98
Commercial (85% impervious) 89 92 94 95
Industrial (72% impervious) 81 88 91 93
Residential (65% impervious) 77 85 90 92
Residential (30% impervious) 57 72 81 86
Residential (12% impervious) 46 65 77 82
Pervious, no vegetation (newly graded) 77 86 91 94
Fallow with residue cover 74-76 83-85 88-90 90-93
Row crop, no residue 67-72 78-81 85-88 89-91
Row crop with residue 64-71 75-80 82-87 85-90
Pasture, good condition 39 61 74 80
Pasture, poor condition 68 79 86 89
Woods, good condition 32 58 72 79
Woods, poor condition 57 73 82 86

Runoff coefficients for different soil groups and slopes. Coefficients are for recurrence intervals less than 25 years. Source: Hydrologic Analysis and Design (4th Edition) (McCuen.

Land use Soil Group A Soil Group B Soil Group C Soil Group D
0-2% 2-6% >6% 0-2% 2-6% >6% 0-2% 2-6% >6% 0-2% 2-6% >6%
Residential (65% impervious) 0.25 0.28 0.31 0.27 0.30 0.35 0.30 0.33 0.38 0.33 0.36 0.42
Residential (30% impervious) 0.19 0.23 0.26 0.22 0.26 0.30 0.25 0.29 0.34 0.28 0.32 0.39
Residential (12% impervious) 0.14 0.19 0.22 0.17 0.21 0.26 0.20 0.25 0.31 0.24 0.29 0.35
Commercial 0.71 0.71 0.72 0.71 0.72 0.72 0.72 0.72 0.72 0.72 0.72 0.72
Industrial 0.67 0.68 0.68 0.68 0.68 0.69 0.68 0.69 0.69 0.69 0.69 0.70
Streets 0.70 0.71 0.72 0.71 0.72 0.74 0.72 0.73 0.76 0.73 0.75 0.78
Parking 0.85 0.86 0.87 0.85 0.86 0.87 0.85 0.86 0.87 0.85 0.86 0.87
Open space 0.05 0.10 0.14 0.08 0.13 0.19 0.12 0.17 0.24 0.16 0.21 0.28
Cultivated land 0.08 0.13 0.16 0.11 0.15 0.21 0.14 0.19 0.26 0.18 0.23 0.31
Pasture 0.12 0.20 0.30 0.18 0.28 0.37 0.24 0.34 0.44 0.30 0.40 0.50
Meadow 0.10 0.16 0.25 0.14 0.22 0.30 0.20 0.28 0.36 0.24 0.30 0.40
Forest 0.05 0.08 0.11 0.08 0.11 0.14 0.10 0.13 0.16 0.12 0.16 0.20

There are numerous studies summarizing total phosphorus exports for different land uses. Examples include the following.

• Burton and Pitt (2002) reported export rates of 1.5 lb/ac/yr for commercial, 1.0 lb/ac/yr for high-density residential, 0.3 lb/ac/yr for medium density residential, and 0.03 lb/ac/yr for low density residential land use.
• Loehr et al. (1989) reported exported TP exports of 0.1-7.6 kg/ha/yr compared to 0.77-2.2 kg/ha/yr for residential land use.
• Baldys et al. (1998) reported TP export of 8.5 lb/mi2 for industrial land uses, compared to 6.23 lbs/mi2/yr for commercial land use and 4.96 lb/mi2/yr for residential land use in the Dallas-Fort Worth area

The studies illustrate the importance of estimating runoff volume, since loading from commercial areas, for example, is greater than from residential areas even though the emc for commercial areas is lower (0.20 mg/L compared to 0.325 mg/L for residential).

## Example using the Minimal Impact Design Standards (MIDS) Calculator

 Example MIDS Calculator with and without adjusted emcs Land use % impervious Impervious acres Pervious acres emc (mg/L) Total phosphorus load (lb/yr) TP export (lb/ac/yr) Residential (40% canopy) 30 0.90 2.1 0.44 3.65 1.22 Residential (10% canopy) 30 0.60 1.4 0.26 1.44 0.72 Commercial 85 0.85 0.15 0.20 1.09 1.09 Industrial 72 0.72 0.28 0.24 1.13 1.13 Open space 10 0.10 0.90 0.19 0.34 0.34 Total phosphorus load with adjusted emcs = 7.65 pounds/yr MIDS unadjusted 39.6 3.17 4.83 0.30 7.77 0.97

The following example illustrates how a variable land use setting may be modeled using the MIDS Calculator.

Site conditions.

• 31.9 inches annual precipitation
• B soils with turf
• 5 acres of residential consisting of the following
• 3 acres with 40 percent tree canopy and 30% impervious
• 2 acres with 10 percent tree canopy and 30% impervious
• 1 acre of commercial land and 85% impervious
• 1 acre if industrial land and 72% impervious
• 1 acre of open space and 10% impervious

EMCs are as follows.

• Residential with 40% canopy coverage = (0.200 + (0.060 * 40/10)) = 0.44 mg/L
• Residential with 10% canopy coverage = (0.200 + (0.060 * 10/10)) = 0.26 mg/L
• Commercial = 0.20 mg/L
• Industrial = 0.235 mg/L
• Open space = 0.19 mg/L

Total phosphorus loading for both scenarios (with adjusted emcs and without) are similar (7.65 lbs/yr for adjusted and 7.77 lbs/yr for unadjusted). However, the export, in lb/ac/yr,varies with land use, with the highest export being from residential with 40% canopy coverage (1.22 lbs/ac/yr), industrial (1.13 lb/ac/yr), and commercial (1.09 lb/ac/yr). Lower export is observed for open space (0.34 lb/ac/yr) and residential with 10% canopy (0.72 lb/ac/yr). Furthermore, dissolved phosphorus, as a fraction of total phosphorus, would be expected to be higher in the residential area with 40% canopy coverage. These results can be useful when targeting areas for treatment and identifying appropriate treatment practices.