Calculating credits for bioretention

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Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with permit requirements, including antidegradation (see [1]; [2]);
• meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how biofiltration practices can achieve stormwater credits.

Overview

Schematic illustrating the components and processes for a biofiltration system.

Biofiltration, commonly termed bioretention with underdrains, is primarily a stormwater quality control practice. Some water quantity reduction can be achieved through infiltration below the underdrain, particularly if the underdrain is raised above the bottom of the Best Management Practice (BMP), and through evapotranspiration.

Typically biofiltration consists of an engineered soil layer designed to treat stormwater runoff via filtration through plant and soil media, and evapotranspiration from plants. Pretreatment is REQUIRED for all biofiltration facilities to settle particulates before entering the BMP. Biofiltration includes an underdrain layer to collect the filtered runoff for downstream discharge. Other common components may include a stone aggregate layer to allow for increased retention storage and an impermeable liner on the bottom or sides of the facility if located near buildings, subgrade utilities, or in karst formations. Biofiltration is a versatile stormwater treatment method applicable to all types of settings such as landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage, and streetscapes.

Warning: The Construction Stormwater permit REQUIRES pretreatment for bioretention practices

Pollutant removal mechanisms

Biofiltration has one of the highest nutrient and pollutant removal efficiencies of any BMP (Mid-America Regional Council and American Public Works Association Manual of Best Management Practice BMPs for Stormwater Quality, 2012). Biofiltration pollutant removal primarily occurs through filtering by the engineered soil media and vegetation. Biofiltration also provides pollutant removal and volume reduction through evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption; the extent of these benefits is highly dependent on site specific conditions and design. In addition to phosphorus and total suspended solids (TSS), which are discussed in greater detail below, biofiltration treats a wide variety of other pollutants.

Removal of phosphorus is dependent on the engineered media. Media mixes with high organic matter content typically leach phosphorus and can therefore contribute to water quality degradation. The Manual provides a detailed discussion of media mixes, including information on phosphorus retention.

Location in the treatment train

Stormwater treatment trains are multiple Best Management Practice (BMPs) that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Under the Treatment Train approach, stormwater management begins with simple methods that prevent pollution from accumulating on the land surface, followed by methods that minimize the volume of runoff generated, and is completed by BMPs that reduce the pollutant concentration and/or volume of stormwater runoff. Biofiltration facilities are typically located in upland areas of the stormwater treatment train, controlling stormwater runoff close to the source.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Biofiltration generates credits for Total Suspended Solids (TSS) and Total Phosphorus (TP). Biofiltration does not substantially reduce the volume of runoff but may qualify for a partial volume credit as a result of evapotranspiration, infiltration occurring through the sidewalls above the underdrain, and infiltration below the underdrain piping. Biofiltration is also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and approach

Schematic illustrating the water quality volume (WQV) for a biofiltration BMP. The WQV equals the water ponded above the media and below the overflow point in the BMP.

In developing the credit calculations, it is assumed the bioretention practice is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the bioretention section of the Manual.

In the following discussion, the kerplunk method is assumed in calculating volume and pollutant reductions. This method assumes the water quality volume (WQV) is delivered instantaneously to the BMP. The WQV is stored as water ponded above the filter media and below the overflow point in the BMP. The WQV can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch off new impervious surface. For MIDS, the WQV is 1.1 inches.

In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The kerplunk method therefore may underestimate actual volume and pollutant losses.

Volume credit calculations

Schematic illustrating the different water loss terms for a biofiltration BMP with an underdrain at the bottom.

Schematic illustrating the different water loss terms for a biofiltration BMP with a raised underdrain.

Volume credits for biofiltration are available only if the BMP permanently removes a portion of the stormwater runoff via infiltration through sidewalls or beneath the underdrain piping, or through evapotranspiration. These credits are assumed to be instantaneous values based on the design capacity of the BMP for a specific storm event. Instantaneous volume reduction, also termed event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

Volume credits for biofiltration basins with underdrains are calculated by a combination of infiltration through the unlined sides and bottom of the basin, the volume loss through evapotranspiration (ET), and the retention volume below the underdrain, if applicable (this is based on the assumption that this stored water will infiltrate into the underlying soil). The main design variables impacting the volume credits include whether the underdrain is elevated above the native soils and if an impermeable liner on the sides or bottom of the basin is used. Other design variables include surface area at overflow, media top surface area, underdrain location, and basin bottom locations, total depth of media, soil water holding capacity and media porosity, and infiltration rate of underlying soils.

Information: For the following equations, units most commonly used in practice are given and unit correction factors are based on those units

The volume credit (V) for biofiltration basins with underdrains, in cubic feet, is given by

\( V = V_{inf_b} + V_{inf_s} + V_{ET} + V_U \)

where:

• V_infb = volume of infiltration through the bottom of the basin (cubic feet);
• V_infs = volume of infiltration through the sides of the basin (cubic feet);
• V_ET = volume reduction due to evapotranspiration (cubic feet); and
• V_U = volume of water stored beneath the underdrain that will infiltrate into the underlying soil (cubic feet).

Volume credits for infiltration through the bottom of the basin (V_infb) are accounted for only if the bottom of the basin is not lined. As long as water continues to draw down, some infiltration will occur through the bottom of the BMP. However, it is assumed that when an underdrain is included in the installation, the majority of water will be filtered through the media and exit through the underdrain. Because of this, the drawdown time is likely to be short. Volume credit for infiltration through the bottom of the basin is given by

\( V_{inf_B} = I_R/12 * A_B * DDT \)

where

• I_R = design infiltration rate of underlying soil (inches per hour);
• A_B = surface area at the bottom of the basin (square feet); and
• DDT = drawdown time for ponded water (hours).

If the kerplunk method is being applied and runoff water is delivered instantaneously to the BMP, the design infiltration rate (I_R) will equal the saturated hydraulic conductivity. The drawdown time is typically a maximum of 48 hours, which is designed to be protective of plants grown in the media. The Construction Stormwater permit requires drawdown within 48 hours and recommends 24 hours when discharges are to a trout stream. With a properly functioning underdrain, the drawdown time is likely to be considerably less than 48 hours.

Volume credit for infiltration through the sides of the basin is accounted for only if the sides of the basin are not lined with an impermeable liner. Volume credit for infiltration through the sides of the basin is given by

\( V_{inf_s} = I_R/12 * (A_O - A_U) * DDT \)

where

• A_O = the surface area at the overflow (square feet); and
• A_U = the surface area at the underdrain (square feet).

This equation assumes water will infiltrate through the entire sideslope area during the period when water is being drawn down. This is not the case, however, since the water level will decline in the BMP. The MIDS calculator assumes a linear drop in water level and thus divides the right hand term in the above equation by 2.

Volume credit for media storage capacity below the underdrain (V_U) is accounted for only if the underdrain is elevated above the native soils. Volume credit for media storage capacity below the underdrain is given by

\( V_U = (A_U + A_B)/2 * n * D_U \)

where

• A_B = surface area at the bottom of the media (square feet);
• n = media porosity (cubic feet per cubic foot); and
• D_U = the depth of media below the underdrain (feet).

Per the kerplunk method, this is an instantaneous volume. This will somewhat overestimate actual storage when the majority of water is being captured by the underdrains. This equation also assumes this stored water will infiltrate into the underlying soil. Thus, using the entire porosity will overestimate the actual drainage. The MIDS calculator assumes water between the soil porosity and field capacity will infiltrate into the underlying soil.

The volume of water lost through ET is assumed to be the smaller of two calculated values: potential ET and measured ET. Potential ET (ET_pot) is equal to the amount of water stored in the basin between field capacity and the wilting point. Measured ET (ET_mea) is the amount of water lost to ET as measured using available data and is assumed to be 0.2 inches/day. ET_mea is converted to ET by multiplying by a factor of 0.5. ET is considered to occur over a period equal to the drawdown time of the basin. Volume credit for evapotranspiration is given by the lesser of

\( ET_{mea} = 0.2/12 * A * 0.5 * t \) \( ET_{pot} = D * A * C_S \)

where

• t = time over which ET is occurring (days);
• D = depth being considered (feet);
• A = area being considered (square feet); and
• C_S = soil water available for ET, generally assumed to be the water between field capacity and wilting point.

Provided soil water content is greater than the wilting point, ET will continually occur during the non-frozen period. However, because the above volume calculations are event based, t will be equal to the time between rain events. In the MIDS calculator, a value of 3 days is used because this is the average number of days between precipitation events. ET will occur over the entire media depth. D may therefore be set equal to the media depth (D_M). In this case, the value for A would be the average area through the entire depth of the media. The MIDS calculator limits ET to the area above the underdrain. If infiltration is being computed through the bottom and sidewalls of the basin, then C_S would be field capacity minus the wilting point of soils (cubic feet per cubic foot) since water above the field capacity would infiltrate (or go to an underdrain).

The volume of water passing through underdrains can be determined by subtracting the volume loss (V) from the volume of water instantaneously captured by the BMP. No volume reduction credit is given for filtered stormwater that exits through the underdrain, but the volume of filtered water can be used in the calculation of pollutant removal credits through filtration.

The volume reduction credit (V) can be converted to an annual volume if desired. This conversion can be generated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the percentage annual volume reduction through performance curves developed from multiple modeling scenarios using the volume reduction capacity for biofiltration, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness.

Total suspended solids credit calculations

TSS reduction credits correspond with volume reduction through infiltration and filtration of water captured by the biofiltration basin and are given by

\( M_{TSS} = M_{TSS_i} + M_{TSS_f} \)

where

• M_TSS = TSS removal (pounds);
• M_{TSS_i} = TSS removal from infiltrated water (pounds); and
• M_{TSS_f} = TSS removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

\( M_{TSS_i} = 0.0000624 * (V_{inf_b} + V_{inf_s} + V_U) * EMC_{TSS} \)

where

• EMC_TSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMC_TSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

Removal for the filtered portion is less than 100 percent. The Stormwater Manual provides a recommended value of 85 percent removal for filtered water, while the MIDS calculator provides a value of 65 percent. Alternate justified percentages for TSS removal can be used if proven to be applicable to the BMP design.

The mass of pollutant removed through filtration, in pounds, is given by

\( M_{TSS_f} = 0.0000624 * (V_{total} - (V_{inf_b} + V_{inf_s} + V_U)) * EMC_{TSS} * R_{TSS} \)

where

• V_total is the total volume of water captured by the BMP (cubic feet); and
• R_TSS is the TSS pollutant removal percentage for filtered runoff. Alternate justified percentages for TSS removal, including direct monitoring data, can be used if proven to be applicable to BMP credit calculation.

The above calculations may be applied on an event or annual basis using the appropriate units.

Total phosphorus credit calculations

File:Bioretention example calculation 1.png

Example calculations of phosphorus removal using equations shown at left.

Total phosphorus (TP) reduction credits correspond with volume reduction through infiltration and filtration of water captured by the biofiltration basin and are given by

\( M_{TP} = M_{TP_i} + M_{TP_f} \)

where

• M_TP = TP removal (pounds);
• M_{TP_i} = TP removal from infiltrated water (pounds); and
• M_{TP_f} = TP removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

\( M_{TP_i} = 0.0000624 * (V_{inf_b} + V_{inf_s} + V_U) * EMC_{TP} \)

where

• EMC_TP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter).

The EMC_TP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

The filtration credit for TP in biofiltration with underdrains assumes removal rates based on the soil media mix used and the presence or absence of amendments. Soil mixes with more than 30 mg/kg phosphorus (P) content are likely to leach phosphorus and do not qualify for a water quality credit. If the soil phosphorus concentration is less than 30 mg/kg, the mass of phosphorus removed through filtration, in pounds, is given by

\( M_{TP_f} = 0.0000624 * (V_{total} - (V_{inf_b} + V_{inf_s} + V_U)) * EMC_{TP} * R_{TP} \)

Information: Soil mixes C and D are assumed to contain less than 30 mg/kg of phosphorus and therefore do not require testing

Again, assuming the phosphorus content in the media is less than 30 milligrams per kilogram, the removal efficiency (R_TP) provided in the Stormwater Manual is a function of the fraction of phosphorus that is in particulate or dissolved form, the depth of the media, and the presence or absence of soil amendments. For the purpose of calculating credits it can be assumed that TP in storm water runoff consists of 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). The removal efficiency for particulate phosphorus is 80 percent. The removal efficiency for dissolved phosphorus is 20 percent if the media depth is 2 feet or greater. The efficiency decreases by 1 percent for each 0.1 foot decrease in media thickness below 2 feet. If a soil amendment is added to the BMP design, an additional 40 percent credit is applied to dissolved phosphorus. Thus, the overall removal efficiency, (R_TP), expressed as a percent removal of total phosphorus, is given by

\( R_{TP} = [0.8 * 0.55] + [0.45 * ((0.2 * (D_{MU_{max=2}})/2) + 0.40_{if amendment is used})] * 100 \)

where

• the first term on the right side of the equation represents the removal of particulate phosphorus;
• the second term on the right side of the equation represents the removal of dissolved phosphorus; and
• D_MUmax=2 = the media depth above the underdrain, up to a maximum of 2 feet.

Methods for calculating credits

This section provides specific information on generating and calculating credits from biofiltration for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on accepted hydrologic models
2. The Simple Method and MPCA Estimator
3. MIDS Calculator
4. Quantifying volume and pollution reductions based on values reported in literature
5. Quantifying volume and pollution reductions based on field monitoring

Credits based on models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for biofiltration. The available models described below are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

• Model name and version
• Date of analysis
• Person or organization conducting analysis
• Detailed summary of input data
• Calibration and verification information
• Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for biofiltration BMPs.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Link to this table
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name	BMP Category						Assess TP removal?	Assess TSS removal?	Assess volume reduction?	Comments
	Constructed basin BMPs	Filter BMPs	Infiltrator BMPs	Swale or strip BMPs	Reuse	Manu- factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator	X	X	X		X		No	No	Yes	Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm							Yes	Yes	Yes	CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator	X		X		X		No	No	Yes	Primary purpose is to assess reductions in stormwater volume.
EPA SWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
HydroCAD	X		X	X			No	No	Yes	Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM	X	X	X	X			Yes	Yes	Yes
i-Tree-Hydro			X				No	No	Yes	Includes simple calculator for rain gardens.
i-Tree-Streets							No	No	Yes	Computes volume reduction for trees, only.
LSPC	X		X	X			Yes	Yes	Yes	Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed	X	X	X	X			Yes	Yes	Yes	Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator					X		Yes	No	Yes	Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet					X		No	No	Yes	Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator	X	X	X	X	X	X	Yes	Yes	Yes	Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE)	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
P8	X		X	X		X	Yes	Yes	Yes
PCSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.
PLOAD	X	X	X	X		X	Yes	Yes	No	User-defined practices with user-specified removal percentages.
PondNet	X						Yes	No	Yes	Flow and phosphorus routing in pond networks.
PondPack	X		[				No	No	Yes	PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA			X				No	No	Yes
SHSAM						X	No	Yes	No	Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN	X	X	X	X	X		Yes	Yes	Yes	Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT	X	X	X				Yes	Yes	Yes	Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method	X	X	X	X	X	X	Yes	No	Yes	Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF	X	X					Yes	Yes	Yes	Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF	X		X	X			Yes	Yes	Yes	USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM	X	X	X	X			Yes	Yes	Yes
XPSWMM	X		X		X		Yes	Yes	Yes	User defines parameter that can be used to simulate generalized constituents.

The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of mean pollutant concentrations and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or [The Simple Method for estimating phosphorus export].

Some simple stormwater calculators utilize the Simple Method (STEPL, Watershed Treatment Model. The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Warning: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

MIDS Calculator

Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including biofiltration. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the biofiltration device. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.

Designers may use the pollutant reduction values reported in this manual or may research values from other databases and published literature. Designers who opt for this approach should

• select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
• select a pollutant removal reduction from literature that studied a biofiltration device with site characteristics and climate similar to the device being considered for credits;
• review the article to determine that the design principles of the studied biofiltration are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
• give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed biofiltration device, considering such conditions as watershed characteristics, biofiltration sizing, soil infiltration rates, and climate factors.

• International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals
  • Compilation of BMP performance studies published through 2011
  • Provides values for TSS, Bacteria, Nutrients, and Metals
  • Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels
• Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon
  • Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations
  • Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics
  • Applicable to filters, wet ponds, porous pavements, soakage trenches, flow-through stormwater planters, infiltration stormwater planters, vegetated infiltration basins, swales, and treatment wetlands
• The Illinois Green Infrastructure Study
  • Figure ES-1 summarizes BMP effectiveness
  • Provides values for TN, TSS, peak flows / runoff volumes
  • Applicable to permeable pavements, constructed wetlands, infiltration, detention, filtration, and green roofs
• New Hampshire Stormwater Manual
  • Volume 2, Appendix B summarizes BMP effectiveness
  • Provides values for TSS, TN, and TP removal
  • Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
• Design Guidelines for Stormwater Bioretention Facilities. University of Wisconsin, Madison
  • Table 2-1 summarizes typical removal rates
  • Provides values for TSS, metals, TP, TKN, ammonium, organics, and bacteria
  • Applicable for bioretention
• BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
  • Appendix B provides pollutant removal performance curves
  • Provides values for TP, TSS, and zinc
  • Pollutant removal broken down according to land use
  • Applicable to infiltration trench, infiltration basin, bioretention, grass swale, wet pond, and porous pavement
• Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report
  • Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
  • Applicable to wet basins, stormwater wetlands, bioretention filter, sand filter, infiltration trench, and filter strips/grass swales

Credits based on field monitoring

Field monitoring may be used to calculate stormwater credits in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

• Establish the objectives and goals of the monitoring.
  • Which pollutants will be measured?
  • Will the monitoring study the performance of a single BMP or multiple BMPs?
  • Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
  • Will the results be compared to other BMP performance studies?
  • What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
• Plan the field activities. Field considerations include:
  • Equipment selection and placement
  • Sampling protocols including selection, storage, delivery to the laboratory
  • Laboratory services
  • Health and Safety plans for field personnel
  • Record keeping protocols and forms
  • Quality control and quality assurance protocols
• Execute the field monitoring
• Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

• Chapter 2: Designing the Program
• Chapters 3 & 4: Methods and Equipment
• Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
• Chapter 7: BMP Performance Analysis
• Chapters 8, 9, & 10: LID Monitoring

Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes the following chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP:

• Chapter 4: Stormwater Characterization
  • 4.2: General Characteristics and Pollutant Sources
  • 4.3: Sources of Stormwater Quality data
• Chapter 8: Performance Evaluation
  • 8.1: Methodology Options
  • 8.5: Evaluation of Quality Performance for Individual BMPs
  • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
• Chapter 10: Hydrologic Evaluation
  • 10.5: Performance Verification and Design Optimization

Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices.

In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The information contained in this White Paper would be of use to those considering the monitoring of a manufactured BMP. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include:

• Chapter 4: Monitoring Methods and Equipment
• Chapter 5: Analytical Methods and Laboratory Selection
• Chapter 6: Monitoring Site Selection
• Chapter 8: Equipment Installation and Maintenance
• Chapter 10: Pre-Storm Preparation
• Chapter 11: Sample Collection and Handling
• Chapter 12: Quality Assurance / Quality Control
• Chapter 13: Laboratory Reports and Data Review
• Chapter 15: Gross Solids Monitoring

Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice, involving:

• Level 1: Visual Inspection
• Level 2: Capacity Testing
• Level 3: Synthetic Runoff Testing
• Level 4: Monitoring
• Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

• Water Budget Measurement
• [Sampling Methods http://stormwaterbook.safl.umn.edu/content/sampling-methods]
• [Analysis of Water and Soils http://stormwaterbook.safl.umn.edu/content/analysis-water-and-soils]
• [Data Analysis for Monitoring http://stormwaterbook.safl.umn.edu/content/monitoring-data-analysis]

Other pollutants

In addition to TSS and phosphorus, constructed basins can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that biofiltration BMPs are effective at reducing concentrations of pollutants, including metals, and bacteria. A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Relative pollutant reduction from bioretention systems for metals, nitrogen, bacteria, and organics.
Link to this table

Pollutant	Constituent	Treatment capabilities1
Metals2	Cadmium, Chromium, Copper, Zinc, Lead	High
Nitrogen2	Total nitrogen, Total Kjeldahl nitrogen	Low/medium
Bacteria2	Fecal coliform, e. coli	High
Organics	Petroleum hydrocarbons3, Oil/grease4	High

¹ Low: < 30%; Medium: 30 to 65%; High: >65%
² International Stormwater Database, (2012)
³ LeFevre et al., (2012)
⁴ Hsieh and Davis (2005).
See Reference list

References

• Brown, Robert A., and William F. Hunt III. 2010. Impacts of media depth on effluent water quality and hydrologic performance of undersized bioretention cells. Journal of Irrigation and Drainage Engineering 137, no. 3: 132-143.
• Brown, R. A., and W. F. Hunt. 2011. Underdrain configuration to enhance bioretention exfiltration to reduce pollutant loads. Journal of Environmental Engineering 137, no. 11: 1082-1091.
• Bureau of Environmental Services. 2006. Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland. Oregon. Bureau of Environmental Services, Portland, Oregon.
• California Stormwater Quality Association. 2003. California Stormwater BMP Handbook-New Development and Redevelopment. California Stormwater Quality Association, Menlo Park, CA.
• Chris, Denich, Bradford Andrea, and Drake Jennifer. 2013. Bioretention: assessing effects of winter salt and aggregate application on plant health, media clogging and effluent quality. Water Quality Research Journal of Canada. 48(4):387.
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• Jurries, Dennis. 2003. Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal. Quality, State of Oregon, Department of Environmental Quality (Ed.).
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