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==Ratios of particulate to dissolved phosphorus== | ==Ratios of particulate to dissolved phosphorus== | ||
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| colspan="3" style="text-align: center;"| '''Effectiveness of stormwater BMPs in treating dissolved phosphorus''' | | colspan="3" style="text-align: center;"| '''Effectiveness of stormwater BMPs in treating dissolved phosphorus''' |
Land use | |
Commercial | |
Industrial | |
Residential | |
Freeways/transportation | |
Mixed | |
Open space | |
Conventional roof | |
|
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.
Event mean concentrations (emcs) are used in models for predicting water quality impacts from stormwater runoff and stormwater treatment practices. Pollutant loads, which are typically used to assess water quality impacts, including establishing total maximum daily loads (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.
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.
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.
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.
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.
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.
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 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.
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.
We recommend a value of 0.200 mg/L for stormwater runoff from commercial areas.
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.
The median TP concentration from these studies is 0.235 mg/L.
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.
Parks and recreation areas are generally included in open space.
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.
The median value from these studies is 0.28 mg/L.
If roof 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.
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
Overall emc = ((0.325 * 10)/31) + ((10 * 0.200)/31) + ((10 * 0.235)/31) + ((1 * 0.280)/31) = 0.254 mg/L
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.
Event mean concentrations for total phosphorus.
Link to this table
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 |
|
Residential | 0.26 - 0.38 | 0.325 | Concentrations vary widely depending on local conditions |
High-density/Multi-family residential | 0.28 - 0.40 | Calculate1 |
|
Medium density residential | 0.18 - 0.40 | Calculate1 |
|
Low density residential | 0.24 - 0.40 | Calculate1 |
|
Freeways/transportation | 0.25 - 0.45 | 0.280 |
|
Mixed | 0.16 - 0.84 | 0.290 |
|
Parks and recreation | Use value for open space or calculate |
|
|
Open space | 0.12 - 0.31 | 0.190 | |
Conventional roof | 0.01 - 0.20 | 0.030 | |
Institutional | 0.14 - 0.422 | See note |
|
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) |
|
|
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
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
Several factors affect concentrations of total phosphorus in stormwater runoff. The following bullet list summarizes some of the most important factors affecting total phosphorus concentrations in stormwater runoff.
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.
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.
Effectiveness of stormwater BMPs in treating dissolved phosphorus | ||
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) | Limited | Some DP removal occurs through plant uptake |
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 | Ineffective | Limited 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 |
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. 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 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.
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.
Pollutant loads are a function of pollutant concentrations in runoff and the volume of runoff. Consequently, when calculating pollutant loads
While trees contribute phosphorus to stormwater, resulting in high phosphorus concentrations during certain times of the year, trees also reduce total runoff. This occurs through interception by tree canopies and by reduced runoff from permeable surfaces. Adjusting TP emcs to account for inputs from leaves without adjusting runoff volumes will lead to overestimates of annual phosphorus loading.
Janke et al. (2017) observed that neither street canopy nor total vegetation were significant factors in nutrient loading. The researchers provide a discussion of the effect of street density, tree canopy coverage, and nutrient loading. Despite a relatively low sample size, they observed that nutrient loading decreased with increasing tree canopy at lower street densities, but as street density increased the opposite pattern occurred. They observed a threshold for TP loading at a street density of about 10 km/km2 (about 8-10 percent of an area consisting of streets, depending on street width). Below this threshold, reductions in runoff volume offset increased TP inputs from trees. Sanders (1986) modeled impacts of trees on runoff in Dayton, Ohio, and estimated under current conditions that tree canopy reduced runoff by 7 percent annually. Modest increases in canopy cover could increase this to 12 percent. Wang et al. (2008) developed a model to predict impacts of tree canopy on urban runoff in Maryland. Increasing the leaf area index from 3 to 6 increased tree interception by 2.7 percent, decreased runoff from pervious areas by 4.3 mm, and decreased runoff from directly connected impervious areas by 20.1 mm. Xiao and McPherson (2002) estimated interception accounted for 1.6 percent of annual precipitation, with seasonal differences and significantly greater interception by mature trees. Hathaway (2019) observed interception rates of 28-43 percent for three species in Tennessee.
Currently there is insufficient information to develop specific relationships between reduced loads associated with tree canopy
if the model or calculator adjusts for volume decreases associated with canopy coverage and varying canopy coverages, no adjustment in volume is needed to calculate the phosphorus load. For example, the Minimal Impact Design Standards (MIDS) calculator automatically adjusts volumes for canopy coverage.
Unless specific local data exist, we do not recommend adjusting the default emc of 0.20 mg/L. However, reported loads from commercial areas often exceed loads from residential areas due to greater impervious and directly connected impervious surface.
Adjustments may therefore be needed depending on the specific model being used. See the discussion on modeling adjustments below.
The above discussion primarily focuses on event mean concentrations for phosphorus. While estimating loads accurately requires
Some models may have curve number, runoff coefficient, or percent impervious as a model input. 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. USDA Urban Hydrology for Small Watersheds - TR-55].
Link to this table
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 |
Meadow | 30 | 58 | 71 | 78 |
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, 2017).
Link to this table
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 |