Phosphorus in stormwater

Photo of a eutrophic lake, a result of excessive phosphorus loading.

PAGE SUMMARY

The following list summarizes the important points on this page Phosphorus in stormwater occurs in dissolved and particulate forms. Dissolved forms typically are more than 90% bioavailable, while paarticulate forms are typically less than 25% bioavailable. For a discussion of dissolved and particulate fractions in stormwate rrunoff, see Ratios of particulate to dissolved phosphorus Typical concentrations of total phosphorus (TP) in stormwater vary with land use, ranging from about 0.19 mg/L from open space areas to about 0.32 mg/L for residential areas. TP export from urban land uses are typically 1-3 lb/ac/yr. Stormwater management should utilize a treatment train approach and focus on dissolved phosphorus in areas where contributions of dissolved phosphorus are significant. Infiltration, pollution prevention, and use of amendments are effective for dissolved phosphorus Erosion control, filtration, and sedimentation practices are effective for particulate phosphorus but less effective for dissolved phosphorus

The United States Geological Survey states: "Phosphorus is a common constituent of agricultural fertilizers, manure, [urban runoff], and organic wastes in sewage and industrial effluent. It is an essential element for plant life, but when there is too much of it in water, it can speed up eutrophication (a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients) of rivers and lakes." Phosphorus in stormwater runoff can generally be divided into the fraction associated with sediment, called particulate phosphorus, and the fraction dissolved in water, called dissolved or soluble phosphorus. Total phosphorus is the sum of particulate and dissolved phosphorus and includes the total amount of phosphorus in both organic and inorganic forms. Orthophosphate measures phosphorus that is most immediately biologically available. Most of the soluble phosphorus in stormwater is usually present in the orthophosphate form.

This article provides information on phosphorus in urban stormwater, including a discussion of sources of phosphorus and management strategies for minimizing phosphorus loading from urban stormwater runoff to surface water. For more information on phosphorus in water, click on these links: [1], [2], [3], [4], [5].

Information: Blue alert boxes are used throughout this page to identify best practices for minimizing dissolved phosphorus. Dissolved phosphorus has the greatest impact on receiving waters and minimizing or treating for dissolved phosphorus in runoff should be a priority for protecting receiving waters.

Forms of phosphorus found in stormwater and natural waters

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.

Phosphorus in water is often considered to occur 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. Understanding phosphorus behavior is further complicated by environmental conditions, particularly oxidation-reduction (redox) conditions, since a portion of particulate phosphorus will release phosphorus under anoxic (reducing) conditions.

Other terms encountered or forms of phosphorus discussed in the literature include the following.

• Total phosphorus (TP) is a measure of all the forms of phosphorus, dissolved or particulate, that are found in a sample.
• Inorganic phosphate is phosphate not associated with organic material. Types of inorganic phosphate include orthophosphate (PO₄^-2) and polyphosphates.
• Organic phosphate is phosphate that is bound to plant or animal tissue.
• Phosphate or orthophosphate refers to the phosphate molecule by itself.
• 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.

The following links provide discussion of phosphorus in stormwater.

• Event mean concentrations of total and dissolved phosphorus in stormwater runoff
• Ratios of particulate to dissolved phosphorus
• Source and concentrations of phosphorus in urban stormwater

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

An important consideration in treating stormwater runoff is the form of the phosphorus. As stated above, 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 ([6], [7], [8], [9])

Source and concentrations of phosphorus in urban stormwater

Sources of phosphorus in urban runoff include plant and leaf litter, soil particles, pet waste, road salt, fertilizer, and atmospheric deposition of particles. Lawns and roads account for the greatest loading. For example, Waschbusch et. al (1999) found that lawns and roads contributed about 80 percent of total and dissolved phosphorus loading. Land use affects the contribution from different sources, with lawns and leaf litter being more important in residential areas and roads being more important in commercial and industrial areas. Atmospheric sources of particles may derive from outside of the river basin (Hopke et al. 1980; Tipping et al., 2014).

The fraction of total phosphorus in dissolved form varies with the source of phosphorus, which in turn varies with season and land use. The dissolved fraction may exceed 50 percent when the source is plant litter, fertilizer, and animal waste, while the dissolved fraction may be as low as 25 percent when the source is predominantly sediment (Waschbusch et. al, 1999). For more information on phosphorus in urban stormwater, see the section on contribution of tree leaves, seeds, and flowers to phosphorus in urban runoff. For a detailed discussion of dissolved and particulate fractions in runoff, link here.

Concentrations of phosphorus in urban stormwater runoff are highly variable. The following table provides data on event mean concentrations of total phosphorus in stormwater, by land use. For more information on phosphorus concentrations in stormwater runoff, see Event mean concentrations of total and dissolved phosphorus in stormwater runoff.

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	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

Total phosphorus (TP) export varies with the amount of runoff and is calculated by multiplying the TP concentration by the volume of runoff. The Simple Method can be used to estimate TP loading as a function of different land uses. Export coefficients are presented in the literature for different land uses. The data are highly variable as a result of the differences in impervious surface, even within specific land uses. Typical annual TP export coefficients are shown below (data adapted from studies cited in Lin (2004) and Jeje (2006)).

Land use	Range (lb/ac/yr)	Recommended (lb/ac/yr)
Native grass	0.04-0.32	0.10
Forest	0.04-0.27	0.13
Pasture	0.27-0.89	0.70
Corn/soybean	1.8-3.4	2.2
Mixed agriculture	0.44-0.98	0.70
Low density residential		1.1
High density residential		1.3
Commercial		2.0
Highways		3.1

Stormwater management for phosphorus

Watershed scale stormwater management approach using a multi-BMP approach to managing the quantity and quality of stormwater runoff. The BMP sequence starts with pollution prevention and progresses through source control, on-site treatment, and regional treatment before the runoff water is discharged to a receiving water. On-site and regional practices treat stormwater runoff and can be incorporated into a stormwater treatment train.

Management of urban stormwater to control or reduce TP concentrations and loading should focus on identifying the most important sources and employing specific practices to address those sources. If significant reductions in TP loading are required or desired, a treatment train approach should be utilized. The treatment train approach for TP focuses on implementing the following hierarchy of practices:

• pollution prevention and source control
• pretreatment for structural stormwater BMPs
• infiltration
• sedimentation
• filtration

Construction stormwater

Data on TP loading from construction stormwater sites is limited. TSS concentrations in runoff from construction sites typically greatly exceed concentrations from other urban land uses and generally exceed 1000 milligrams per liter. Sediment export from construction sites typically ranges from 2000 to 16000 pounds per acre per year (Line et al., 2009; Line et al., 2011; Wolman and Schick, 1967). However, one study in Washington state showed TP concentrations ranged from 0.01 to 0.16 milligrams per liter in runoff from construction sites, with a median of 0.095 milligrams per liter. These are relatively low concentrations considering the amount of sediment leaving construction sites. Other potential sources of phosphorus on a construction site result from land treatment practices employed by the construction site personnel, such as fertilizers, tackifier, hydroseed, wood mulch, or other types of applications. These products could be evaluated for their phosphorus content. Erosion protection and sediment control practices described in this manual should be employed at construction sites.

Information: At most construction sites, most phosphorus will be in particulate form, which is typically easier to treat and less bioavailable than dissolved forms. To minimize dissolved phosphorus export, minimze the use of fertilizers, use slow release fertilizers, and avoid organic materials, such as compost, than can release soluble phosphorus.

Pollution prevention and source control

These practices reduce the amount of TP generated or remove TP prior to it being entrained in runoff. These are summarized below for residential, municipal, and industrial sources.

Information: Pollution prevention practices are among the most effective methods for minimizing dissolved phosphorus loads since these practices typically address important sources of dissolved phosphorus.

Prevention practices for residential areas

The following table summarizes residential prevention practices that are effective at reducing TP concentrations. The table indicates the relative effectiveness of each practice and provides a short description of the practice. TP removal efficiencies are not established for these BMPs.

Residential pollution prevention methods effective for controlling or reducing phosphorus.
Link to this table

Practice	Relative effectiveness	Method	Image1
Fertilizer and pesticide management	High	Reduce or eliminate the need for fertilizer and pesticides by practicing natural lawn care, planting native vegetation, and limiting chemical use; follow Minnesota Statutes Chapter 18C and federal regulatory requirements on fertilizer and pesticide storage and application if used.
Litter and animal waste control	High	Properly dispose of pet waste and litter in a timely manner and according to local ordinance requirements.
Yard Waste Management	High	Prevent yard waste from entering storm sewer systems and water bodies by either composting or using curbside pickup services and avoiding accumulation of yard waste on impervious surfaces; keep grass clippings and leaves out of the street.
Better Car and Equipment Washing	Moderate	Wash cars less often and on grassy areas using phosphorus free detergents and non-toxic cleaning products or use commercial car washes to prevent dirty wash water from flowing to storm sewer systems and water bodies.
Septic tank maintenance	High
Native Landscaping	High	Reduce turf areas by planting native species to reduce and filter pollutant-laden runoff and prevent the spread of invasive, non-native plant species into the storm sewer system.
Better Sidewalk and Driveway Deicing	Moderate	Reduce or eliminate the need for deicing products by manually clearing sidewalks and driveways prior to deicer use; use environmentally-friendly deicing products when possible, apply sparingly and store properly if used.
Exposed Soil Repair	High	Use native vegetation or grass to cover and stabilize exposed soil on lawns to prevent sediment wash off.
Healthy Lawns	Moderate	Maintain thick grass planted in organic-rich soil to a height of at least 3 inches to prevent soil erosion, filter stormwater contaminants, and absorb airborne pollutants; limit or eliminate chemical use and water and repair lawn as needed

¹ Photo credits

Prevention practices for municipalities

The following table summarizes municipal prevention practices that are effective at reducing TP concentrations. The table indicates the relative effectiveness of each practice and provides a short description of the practice. TP removal efficiencies are not established for these BMPs.

Municipal prevention practices for TP.
Link to this table

Practice	Relative effectiveness	Method	Image1
Proper Vehicle Management	High	Ensure that vehicles are fueled, maintained, washed and stored in a manner that prevents the release of harmful fluids, including oil, antifreeze, gasoline, battery acid, hydraulic and transmission fluids, and cleaning solutions.
Better Turf Management	High	Ensure that mowing, fertilization, pesticide application, and irrigation are completed in ways that will prevent or reduce grass clippings, sediment, and chemicals from entering storm sewer systems; use native vegetation where possible.
Sanitary Sewer Maintenance	High	Regularly inspect and flush sanitary pipes to ensure that there are no leaks in the system and that the system is properly functioning.
Litter and Animal Waste Control	High	Mandate litter and pet waste cleanup within the community and control waste-generating wildlife, such as geese; provide waste containers for litter and pet waste in public areas.
Temporary Construction Sediment Control	Moderate	Implement and encourage practices to retain sediment within construction project area; see Temporary Construction Erosion and Sediment Control Factsheets for additional information
Wind Erosion Control	Moderate	Institute a local program for wetting of open construction surfaces and other sources for windblown pollutants.
Streambank Stabilization2	Moderate	Repair erosion occurring on a streambank of lakeshore in a timely manner; inspect bank areas for ice damage in the spring.
Material Storage Control	Moderate	Reduce or eliminate spill and leakage loss by properly inspecting, containing, and storing hazardous materials and having a cleanup plan that can be quickly and efficiently implemented.
Dumpster and landfill management	Moderate	Ensure that contaminated material is contained to prevent solid and/or liquid waste from being washed into storm sewer systems or water bodies.
Better Street and Parking Lot Cleaning	Moderate	Maintain streets and parking lots frequently and especially in the spring by sweeping, picking up litter, and repairing deterioration; pressure wash pavement only as needed and avoid using cleaning agents.
Storm Sewer System Maintenance	Moderate	Regularly clean debris from storm sewer inlets, remove sediment from catch basin sumps, and remove any illicit connections to storm sewer systems.
Public Education	Moderate	Label storm drains to indicate that no dumping is allowed and institute pollution prevention programs to educate and implement needed community practices.
Staff, Employee, and Volunteer Education	Moderate	Provide internal training for staff and provide direction to hired employees or volunteers regarding pollution prevention techniques to be used during work activites.

¹ Photo credits
²Reductions in pollutant loading associated with this BMP are not eligible for credit toward NPDES permit requirements unless the stabilization is above the ordinary high water mark (i.e. the work is not completed within a Water of the State), prior to a permittee's discharge, and the load reduced from this action is included in a Wasteload Allocation in a U.S. EPA-approved TMDL.

Prevention practices for industrial sources

The following table summarizes industrial prevention practices that are effective at reducing TP concentrations. The table indicates the relative effectiveness of each practice and provides a short description of the practice. TP removal efficiencies are not established for these BMPs.

Industrial prevention practices for TP.
Link to this table

Practice	Relative effectiveness	Method	Image1
Better Turf Management	High	Ensure that mowing, fertilization, pesticide application, and irrigation are completed in ways that will prevent or reduce grass clippings, sediment, and chemicals from entering storm sewer systems; use native vegetation where possible.
Proper Vehicle Management	High	Ensure that vehicles are fueled, maintained, washed and stored in a manner that prevents the release of harmful fluids, including oil, antifreeze, gasoline, battery acid, hydraulic and transmission fluids, and cleaning solutions.
Sanitary Sewer Maintenance	High	Regularly inspect and flush sanitary pipes to ensure that there are no leaks in the system and that the system is properly functioning.
Temporary Construction Sediment Control	Moderate	Implement and encourage practices to retain sediment within construction project area; see Temporary Construction Erosion and Sediment Control Factsheets for additional information
Wind Erosion Control	Moderate	Institute a local program for wetting of open construction surfaces and other sources for windblown pollutants.
Material Storage Control	Moderate	Reduce or eliminate spill and leakage loss by properly inspecting, containing, and storing hazardous materials and having a cleanup plan that can be quickly and efficiently implemented.
Dumpster and landfill management	Moderate	Ensure that contaminated material is contained to prevent solid and/or liquid waste from being washed into storm sewer systems or water bodies.
Better Street and Parking Lot Cleaning	Moderate	Maintain streets and parking lots frequently and especially in the spring by sweeping, picking up litter, and repairing deterioration; pressure wash pavement only as needed and avoid using cleaning agents.
Storm Sewer System Maintenance	Moderate	Regularly clean debris from storm sewer inlets, remove sediment from catch basin sumps, and remove any illicit connections to storm sewer systems.

¹ Photo credits

Street sweeping

Information: The MPCA is currently working with the University of Minnesota to develop a street sweeping credit. Link to our Future Updates page for more information.

Street sweeping can be an effective practice for reducing sediment loading to surface water, which in turn reduces loading of particulate phosphorus. This Manual currently has a short page describing street sweeping practices as they relate to phosphorus management. The page includes a link to a calculator developed at the University of Minnesota. The calculator estimates reductions in wet and dry solids as a function of different management practices.

Several articles in the literature present results from street sweeping studies. Examples include the following.

• Selbig and Bannerman (2007) discuss changes in pollutant loading for regenerative-air, vacuum-assist, high-frequency broom, and low-frequency broom sweeping practices.
• Law et al. (2008) found for a given set of assumptions and sweeping frequencies, it is expected that the range in pollutant removal rates from street sweeping for total solids was 3 to 8 percent, with the lower end representing monthly street sweeping by a mechanical street sweeper and the upper end the pollutant removal efficiencies using regenerative air/vacuum street sweeper at weekly frequencies.
• Sutherland (2011) provides a comprehensive summary of street sweeping, including information on effectiveness of different sweepers and factors affecting the performance of street sweeping.

Information: Properly timed street sweeping (e.g. during fall leaf drop) can be a very effective method of reducing dissolved phosphorus loads since leaves are an important source of dissolved phosphorus

Pretreatment

Pretreatment is needed to protect infiltration and filtration BMPs from the build-up of trash, gross solids, and particulate matter. When the velocity of stormwater decreases, sediment and solids drop out. If pretreatment is not provided, this process will occur in the infiltration or filtration cell, resulting in long-term clogging and poor aesthetics. Therefore, pretreatment is a required part of the design for infiltration and filtration BMPs. There are three typical methods for pretreatment: Vegetated filter strips (VFS), forebays, and vegetated swales. These are discussed in the section on pretreatment.

Information: Pretreatment is generally ineffective for treating dissolved phosphorus, although devices that effectively screen coarse organic debris can be effective if cleaned during times when coarse organic loads are heavy (e.g. during leaf drop). Some pretreatment practices can be modified with an amendment to enhance removal of dissolved phosphorus.

Infiltration

Infiltration practices are structural Best Management Practices (BMPs) designed to capture stormwater runoff and allow the captured water to infiltrate into soils underlying the BMP. Infiltration BMPs are designed to capture a particular amount of runoff. For example, the construction stormwater permit requires that post-construction BMPs capture the first inch of runoff from new impervious surfaces, assuming there are no constraints to infiltration. BMPs designed to meet the construction stormwater permit are required to infiltrate captured water within 48 hours, with 24 hours recommended when discharges are to a trout stream.

TP removal is assumed to be 100 percent for all water that infiltrates. Any water bypassing the BMP does not receive treatment. Examples of infiltration BMPs, with links to appropriate sections in the Manual, include the following.

• Infiltration (infiltration basin, infiltration trench, dry well, underground infiltration)
• Bioinfiltration (rain garden or bioretention with no underdrain)
• Permeable pavement
• Tree trench/tree box
• Swales with a bioinfiltration base

Additional BMPs that result in infiltration include stormwater and rainwater harvest with irrigation and impervious surface disconnection. For these BMPs infiltration typically occurs into turf or other vegetated areas. Disconnection of impervious surface does not qualify for credits for meeting the Construction Stormwater permit. Harvest BMPs do qualify for credit because they capture an instantaneous volume of water.

The links above take you to the main page for each BMP. Each BMP section has a page on pollutant credits. These credit pages provide information on runoff volume and pollutant removal for the BMP, including credits that can be applied to meet a performance goal such as a Total Maximum Daily Load (TMDL).

In soils where there are constraints on infiltration, BMPs may be designed with underdrains. Unless the BMP is lined, some water will infiltrate through the bottom and sides of the BMP. TP removal for the portion of captured runoff that infiltrates is 100 percent. Water draining to the underdrain undergoes some treatment. These BMPs are discussed in more detail in the filtration section below.

Information: Infiltration practices are among the most effective treatment practices for removing dissolved phosphorus. Although infiltrating water may transport dissolved phosphorus to shallow groundwater, this is only a concern when infiltration practices are located adjacent to sensitive receiving waters that have a significant baseflow component

Settling practices

Information: Note that we refer to constructed ponds and constructed wetlands in this section. Natural ponds and wetlands are not stormwater treatment practices and are therefore not included in this discussion.

If prevention, source control and infiltration practices cannot fully meet protection or restoration targets for stormwater, settling and filtration practices may be used. Settling practices include constructed stormwater ponds, including variants, and constructed stormwater wetlands, including variants. Manufactured devices and forebays are both settling practices but are primarily used for pretreatment.

Constructed ponds are effective at removing particulate material, with TSS removal rates ranging from 60 to 90 percent depending on the design. Constructed wetlands are also very effective at removing TSS, with removal rates ranging from 39 to 81 percent depending on the design. Assuming particulate phosphorus accounts for about 55 percent of total phosphorus (TP), and no dissolved phosphorus is removed by constructed ponds or constructed wetlands, TP removal is approximately 50 percent in ponds and 40 percent in wetlands. Poorly designed or poorly functioning ponds and wetlands can export dissolved phosphorus.

Information on design, construction, operation and maintenance, credits, and other characteristics of these BMPs can be found on the main pages for constructed stormwater ponds and constructed stormwater wetlands.

Information: Constructed ponds and wetlands are generally not effective at removing dissolved phosphorus. Some uptake by plants may occur. Amendments such as aluminum can be added to retain phosphorus

Filtration practices

Filtration practices are typically used when infiltration practices are not feasible, such as areas with low infiltration soils or shallow bedrock (see section on infiltration constraints. Filtration practices include bioretention with underdrains, media filters, and swales. Vegetated filter strips are often used as a pretreatment practice.

Sand filters have removal rates of about 50 percent for TP. This is all considered to be particulate phosphorus. Amendments can be added to sand filters to remove dissolved phosphorus. This manual provides a credit of 60 percent removal of dissolved phosphorus, thus giving a TP removal of 77 percent for amended sand filters.

Filtration practices that utilize engineered media (bioretention, swales) have different removal efficiencies depending on the media and whether the media has been amended to remove dissolved phosphorus. Media that have high organic matter content will not effectively retain phosphorus and can leach phosphorus. These include Mixes A, B, E, and F. Mixes C and D have low organic matter contents and will attenuate phosphorus. The following sections in this manual are useful for determining the effectiveness of filtration practices with engineered media.

• Materials specifications - filter media
• Soil amendments to enhance phosphorus sorption
• Notes about soil phosphorus testing: applicability and interpretation
• Phosphorus credits for bioretention systems with an underdrain

Caution: Green roofs do not receive a phosphorus credit because they typically leach phosphorus in the first years after construction. They may retain phosphorus as they age.

Information on design, construction, operation and maintenance, credits, and other characteristics of these BMPs can be found on the main pages for media filters and swales, green roofs, and bioretention.

Information: Filtration practices that utilize engineered media leach phosphorus unless the organic matter content of the media is relatively low (30 mg-P/kg-media or lower). Both sand filters and biofiltration practices can be amended (e.g. iron) to retain dissolved phosphorus

Meeting TP water quality targets

Caution: Many phosphorus goals are expressed as goals for total phosphorus (TP). Many stormwater practitioners focus on achieving TP targets by utilizing practices that are effective for removing particulate phosphorus but ineffective for removing dissolved phosphorus. Studies show that dissolved phosphorus is highly bioavailable while particulate phosphorus is much less bioavailable. Consequently, achieving TP targets may have limited impacts on receiving waters. Practitioners should attempt to characterize runoff before selecting treatment practices.

Information on this page can be used to help meet water quality targets. Water quality targets are established for various purposes including meeting Clean Water Act (CWA) requirements, meeting local water quality goals or requirements, and meeting non-regulatory targets. CWA requirements include antidegradation, TMDL limits, and NPDES permit requirements. Each of these are described below.

Information: Note that information presented in the Stormwater Manual can be used to meet NPDES permit requirements. This includes information on all BMPs discussed in the Manual unless otherwise noted. Check with MPCA's Stormwater Program for applicability of information not contained in the Manual, including BMPs and BMP credits.

Antidegradation

Water quality standards include an antidegradation policy and implementation method. The water quality standards regulation requires States and Tribes to establish a three-tiered antidegradation program to protect existing water quality and water uses in receiving waters (see [10]).

Compliance with Minimum Control Measure (MCM) 5 of the MS4 permit constitutes compliance with antidegradation requirements. The permit requires no net increase in discharges of volume, total phosphorus (TP) and total suspended solids (TSS) for new development, while a reduction in these are required for redevelopment projects covered under the permit. Practices that infiltrate or capture and reuse stormwater runoff are typically used to meet these permit requirements because they help meet the volume requirements.

Total Maximum Daily Loads (TMDLs)

The 2016 impaired water list includes 358 lakes or reservoirs and 44 river or stream stretches impaired for nutrrient/eutrophication biological indicators. Phosphorus is likely to be the surrogate pollutant for most of these impairments. Other impairments, such as those for dissolved oxygen, may also be associated with excessive TP loads. Click here to link to MPCA's impaired waters website.

The MS4 permit requires permittees to demonstrate progress toward meeting applicable Wasteload Allocations in U.S. EPA-approved TMDLs. General information on meeting TMDL requirements in NPDES permits is found here, while reporting requirements are found here. Below is additional information that may be useful.

• Forms and guidance for TMDLs: includes information on completing the TMDL Annual Report form and other guidance documents.
• Information on modeling, including Available stormwater models and selecting a model and Detailed information on specific models.
• Information on pollutant removal by BMPs. Pollutant removal information is limited to structural BMPs. Each BMP included in the manual also has a section on credits, which provides useful information for determining pollutant removal for TP for that BMP.

Permittees with required reductions in TP loading should consider implementing a treatment train approach.

Stormwater design recommendations to enhance phosphorus removal

Summary of stormwater design recommendations to enhance phosphorus removal.
Link to this table

BMP Design	Design recommendations
Infiltration	Infiltration BMPs are preferred practices Provide a minimum 3-foot separation from the seasonally-high water table, bedrock or impervious soil layer
Filtration (includes practices with an underdrain)	Organic filters are a source of soluble phosphorus and should not be used Practices with engineered media are acceptable when the appropriate filter media is used Employ finer-grained media in the filter bed with a small diameter (15 microns), or provide a finer-grained layer at mid-depth in the filter profile The process for pretreatment and/or filtration should extend from 36 to 48 hours, where possible Filters should be oriented to provide maximum solar exposure Wet swales are not recommended because they may export phosphorus Open channels should be designed to promote maximum sediment retention
Stormwater ponds1	Pond aeration is encouraged Design wet ponds with a depth no greater than 10 feet to prevent stratification and potential release of phosphorus from bottom sediments Avoid the use of dry or dry extended detention ponds Designers should consider the snowmelt runoff volume and design ponds for seasonal operation Use a surface or mid-depth release from the pond Landscape pond to discourage geese Add shallow benches and wetland areas to enhance the plankton community Pond sediments may be amended to retain phosphorus (e.g. iron, aluminum)
Constructed Stormwater wetlands	Pond/ wetland system is the preferred wetland design Use a surface or mid-depth release from the wetland Maximize surface micro-topography Landscape wetland to discourage geese

¹The recommendations for constructed ponds are from the original Minnesota Stormwater Manual. MPCA anticipates updating this information in the near future.

References

• Abell, J.M., and D.P. Hamilton. 2012. Bioavailability of phosphorus transported during storm flow to a eutrophic, polymictic lake. New Zealand Journal of Marine and Freshwater Research. 47:4:481-489.
• Ellison, M.E,, and M.T. Brett. 2006. Particulate phosphorus bioavailability as a function of stream flow and land cover. Water Research. 40:1258 – 1268
• Hopke P.K., Lamb R.E., and F.S. Natusch. (1980) Multielemental characterization of urban roadway dust. Environ Sci Technol 14:164–172.
• Jeje, Yetunde. 2006. Export Coefficients for Total Phosphorus, Total Nitrogen and Total Suspended Solids in the Southern Alberta Region - A Review of Literature. Alberta Environment.
• Law, N.L., DiBlasi, K., and U. Ghosh 2008. Deriving Reliable Pollutant Removal Rates for Municipal Street Sweeping and Storm Drain Cleanout Programs in the Chesapeake Bay Basin. Center for Watershed Protection.
• Lin, J.P. 2004. Review of Published Export Coefficient and Event Mean Concentration (EMC) Data. WRAP Technical Notes Collection (ERDC TN-WRAP-04-3), U.S. Army Engineer Research and Development Center, Vicksburg, MS.
• Line, D.E., M. B. Shaffer, J. D. Blackwell. 2011. Sediment Export from a Highway Construction Site in Central North Carolina. Transactions of the ASABE. 54(1): 105-111.
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