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[[File:Pdf image.png|100px|thumb|left|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Calculating_credits_for_infiltration_-_Minnesota_Stormwater_Manual.pdf_May_2022.pdf Download pdf]</font size>]]
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[[File:Summary image.jpg|100px|left|thumb|alt=image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Credit_page_descriptions.mp4 Page video summary]</font size>]]
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[[File:Technical information page image.png|100px|left|alt=image]]
  
[http://stormwater.pca.state.mn.us/index.php/Overview_of_stormwater_credits Credit] refers to the quantity of stormwater or pollutant reduction achieved either by an individual [[Glossary#B|Best Management Practice]] (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in  
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{{alert|Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.|alert-danger}}
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{{alert|Infiltration practices can be an important tool for retention and detention of stormwater runoff and treatment of pollutants in stormwater runoff. If the practice utilizes vegetation, additional benefits may include cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.|alert-success}}
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[http://stormwater.pca.state.mn.us/index.php/Overview_of_stormwater_credits Credit] refers to the quantity of stormwater or pollutant reduction achieved either by an individual <span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practice'''</span> (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 [[Credits for Better Site design|preservation of natural areas and the reduction of the volume of stormwater]] runoff being conveyed to a best management practice (BMP);  
 
*providing incentives to site developers to encourage the [[Credits for Better Site design|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 [http://stormwater.pca.state.mn.us/index.php/Construction_stormwater_permit]; [http://stormwater.pca.state.mn.us/index.php/MS4_General_Permit]);
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*complying with permit requirements, including antidegradation (see [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction permit]; [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_Program_for_Municipal_Separate_Storm_Sewer_Systems_(MS4) Municipal (MS4) permit]);
 
*meeting the [http://stormwater.pca.state.mn.us/index.php/Performance_goals_for_new_development,_re-development_and_linear_projects MIDS performance goal]; or  
 
*meeting the [http://stormwater.pca.state.mn.us/index.php/Performance_goals_for_new_development,_re-development_and_linear_projects MIDS performance goal]; or  
*meeting or complying with water quality objectives, including [[Total Maximum Daily Loads (TMDLs)|Total Maximum Daily Load]] (TMDL) Wasteload Allocations (WLAs).
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*meeting or complying with water quality objectives, including <span title="The amount of a pollutant from both point and nonpoint sources that a waterbody can receive and still meet water quality standards"> [https://stormwater.pca.state.mn.us/index.php?title=Total_Maximum_Daily_Loads_(TMDLs) '''total maximum daily load''']</span> (TMDL) <span title="The portion of a receiving water's assimilative capacity that is allocated to one of its existing or future point sources of pollution"> '''wasteload allocations'''</span> (WLAs).
 
This page provides a discussion of how infiltration practices can achieve stormwater credits. Infiltration practices include infiltration basins, infiltration trenches (including dry wells), and underground infiltration systems. The discussion does not include [[Bioretention|bioinfiltration]] and [[Permeable pavement|permeable pavement]] systems, unless specifically mentioned. To view the credit articles for other BMPs, see the [[Calculating credits for infiltration basin#Related articles|Related pages]] section.
 
This page provides a discussion of how infiltration practices can achieve stormwater credits. Infiltration practices include infiltration basins, infiltration trenches (including dry wells), and underground infiltration systems. The discussion does not include [[Bioretention|bioinfiltration]] and [[Permeable pavement|permeable pavement]] systems, unless specifically mentioned. To view the credit articles for other BMPs, see the [[Calculating credits for infiltration basin#Related articles|Related pages]] section.
  
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[[File:Infiltration basin schematic..jpg|thumb|300px|alt=Infiltration Basin Detailed Cross Section|<font size=3>Schematic showing an infiltration basin. Note that inflow into the practice has undergone pretreatment. Once the infiltration basin is filled, water bypasses rather than enters the practice.</font size>]]
 
[[File:Infiltration basin schematic..jpg|thumb|300px|alt=Infiltration Basin Detailed Cross Section|<font size=3>Schematic showing an infiltration basin. Note that inflow into the practice has undergone pretreatment. Once the infiltration basin is filled, water bypasses rather than enters the practice.</font size>]]
  
Infiltration practices are designed to capture, store, and infiltrate stormwater runoff. They rely on naturally permeable soils to fully infiltrate the designed [[Glossary#W|water quality volume]] (V<sub>WQ</sub>). These are typically off-line practices utilizing an emergency spillway or outlet structure to capture the volume of stormwater runoff for which the practice is designed.  Volumes that exceed the rate or volume of the infiltration practice are allowed to bypass the BMP.
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Infiltration practices are designed to capture, store, and infiltrate stormwater runoff. They rely on naturally permeable soils to fully infiltrate the designed *<span title="The volume of water that is treated by a BMP."> [https://stormwater.pca.state.mn.us/index.php?title=Water_quality_criteria '''Water Quality Volume''']</span> (V<sub>WQ</sub>). These are typically off-line practices utilizing an emergency spillway or outlet structure to capture the volume of stormwater runoff for which the practice is designed.  Volumes that exceed the rate or volume of the infiltration practice are allowed to bypass the BMP.
  
 
===Pollutant removal mechanisms===
 
===Pollutant removal mechanisms===
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===Location in the treatment train===
 
===Location in the treatment train===
Stormwater [[Using the treatment train approach to BMP selection|Treatment Trains]] are comprised of multiple Best Management Practices 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. Because infiltration practices are designed to be off-line, they may either be located at the end of the treatment train, or used as off-line configurations to divert the [[Glossary#W|water quality volume]] from the on-line system.
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Stormwater <span title="Multiple BMPs that work together to remove pollutants utilizing combinations of hydraulic, physical, biological, and chemical methods"> [https://stormwater.pca.state.mn.us/index.php?title=Using_the_treatment_train_approach_to_BMP_selection '''treatment trains''']</span> are comprised of multiple Best Management Practices 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. Because infiltration practices are designed to be off-line, they may either be located at the end of the treatment train, or used as off-line configurations to divert the water quality volume from the on-line system.
  
 
==Methodology for calculating credits==
 
==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. Infiltration practices are 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 [[Calculating credits for infiltration basin#Other pollutants|references]] are provided that may be useful for calculating credits for other pollutants.  
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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. Infiltration practices are 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 [https://stormwater.pca.state.mn.us/index.php?title=Calculating_credits_for_infiltration#References_and_suggested_reading references] are provided that may be useful for calculating credits for other pollutants.  
  
 
===Assumptions and approach ===
 
===Assumptions and approach ===
 
In developing the credit calculations, it is assumed the infiltration 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 [[Infiltration basin|infiltration basin]] or [[Infiltration trench|infiltration trench]] sections of the Manual. Because of their high susceptibility of failure due to clogging, pretreatment is REQUIRED in all infiltration designs.
 
In developing the credit calculations, it is assumed the infiltration 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 [[Infiltration basin|infiltration basin]] or [[Infiltration trench|infiltration trench]] sections of the Manual. Because of their high susceptibility of failure due to clogging, pretreatment is REQUIRED in all infiltration designs.
  
{{alert|Pre-treatment is required for all infiltration practices|alert-danger}}
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{{alert|Pretreatment is required for all infiltration practices|alert-danger}}
 
   
 
   
In the following discussion, the water quality volume (V<sub>WQ</sub>) is delivered instantaneously to the BMP. V<sub>WQ</sub> is stored as water ponded above the soil or engineered media and below the overflow elevation. V<sub>WQ</sub> can vary depending on the stormwater management objective(s). For construction stormwater, V<sub>WQ</sub> is 1 inch off new impervious surface. For MIDS, V<sub>WQ</sub> is 1.1 inches.
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In the following discussion, the <span title="The volume of water that is treated by a BMP."> [https://stormwater.pca.state.mn.us/index.php?title=Water_quality_criteria '''Water Quality Volume''']</span> (V<sub>WQ</sub>) is delivered instantaneously to the BMP. V<sub>WQ</sub> is stored as water ponded above the soil or engineered media and below the overflow elevation. V<sub>WQ</sub> can vary depending on the stormwater management objective(s). For construction stormwater, V<sub>WQ</sub> is 1 inch off new impervious surface. For MIDS, V<sub>WQ</sub> is 1.1 inches.
  
 
In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The instantaneous volume method therefore may underestimate actual volume and pollutant losses.
 
In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The instantaneous volume method therefore may underestimate actual volume and pollutant losses.
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#Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
 
#Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
 
#The Simple Method and MPCA Estimator
 
#The Simple Method and MPCA Estimator
#[[Overview of Minimal Impact Design Standards (MIDS)|MIDS Calculator]]
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#MIDS Calculator
 
#Quantifying volume and pollution reductions based on values reported in literature
 
#Quantifying volume and pollution reductions based on values reported in literature
 
#Quantifying volume and pollution reductions based on field monitoring
 
#Quantifying volume and pollution reductions based on field monitoring
  
 
===Credits based on models===
 
===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 infiltration practices. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the [http://stormwater.pca.state.mn.us/index.php/Construction_stormwater_permit Construction Stormwater General permit] since the permit requires the water quality volume to be calculated as an instantaneous volume.
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{{alert|The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.|alert-danger}}
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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 infiltration practices. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction Stormwater General permit] since the permit requires the water quality volume to be calculated as an instantaneous volume.
  
 
Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:
 
Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:
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===The Simple Method and MPCA Estimator===
 
===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. [http://www.stormwatercenter.net/Library/Practice/13.pdf 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 [http://www.stormwatercenter.net/monitoring%20and%20assessment/simple%20meth/simple.htm The Simple method to Calculate Urban Stormwater Loads] or [[The Simple Method for estimating phosphorus export]].
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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 <span title="The average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentration'''</span> 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. [http://www.stormwatercenter.net/Library/Practice/13.pdf 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 [http://www.stormwatercenter.net/monitoring%20and%20assessment/simple%20meth/simple.htm The Simple method to Calculate Urban Stormwater Loads] or [[The Simple Method for estimating phosphorus export]].
  
Some simple stormwater calculators utilize the Simple Method ([http://it.tetratech-ffx.com/steplweb/models$docs.htm STEPL], [http://www.cwp.org/online-watershed-library/cat_view/65-tools/91-watershed-treatment-model Watershed Treatment Model]). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the [http://stormwater.pca.state.mn.us/index.php/Guidance_and_examples_for_using_the_MPCA_Estimator '''MPCA Estimator'''], this tool was developed specifically for complying with the [http://stormwater.pca.state.mn.us/index.php/MS4_PART_III.STORMWATER_POLLUTION_PREVENTION_PROGRAM_%28SWPPP%29#E._Discharges_to_Impaired_Waters_with_a_USEPA-Approved_TMDL_that_Includes_an_Applicable_WLA 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/trench, 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.
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Some simple stormwater calculators utilize the Simple Method ([https://www.epa.gov/nps/spreadsheet-tool-estimating-pollutant-loads-stepl EPA STEPL], [https://www.stormwatercenter.net/monitoring%20and%20assessment/watershed_treatment_model.htm Watershed Treatment Model]). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the [http://stormwater.pca.state.mn.us/index.php/Guidance_and_examples_for_using_the_MPCA_Estimator '''MPCA Estimator'''], this tool was developed specifically for complying with the [https://stormwater.pca.state.mn.us/index.php?title=Forms,_guidance,_and_resources_for_completing_the_TMDL_annual_report_form MS4 General Permit TMDL annual reporting requirement].  The MPCA Estimator provides default values for pollutant concentration, <span title="The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)."> [https://stormwater.pca.state.mn.us/index.php?title=Runoff_coefficients_for_5_to_10_year_storms '''runoff coefficients''']</span> 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.
  
{{alert|The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.|alert-danger}}
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{{alert|The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.|alert-warning}}
  
 
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.
 
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.
  
'''Download MPCA Estimator here: [[file:MPCA_Estimator.xlsx]]'''
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'''[https://stormwater.pca.state.mn.us/index.php?title=File:MPCA_simple_estimator_version_3.0_March_5_2021.xlsx Download MPCA Estimator here]'''
 
 
A quick guide for the estimator is available [[Quick Guide: MPCA Estimator tab]].
 
  
 
===MIDS Calculator===
 
===MIDS Calculator===
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Detailed [[Links to Manual pages that address the MIDS calculator|guidance]] has been developed for all BMPs in the calculator, including [[Requirements, recommendations and information for using infiltration basin/underground infiltration BMPs in the MIDS calculator|infiltration practices]]. An overview of individual input parameters and workflows is presented in the [http://stormwater.pca.state.mn.us/index.php/User%E2%80%99s_Guide MIDS Calculator User Documentation].
 
Detailed [[Links to Manual pages that address the MIDS calculator|guidance]] has been developed for all BMPs in the calculator, including [[Requirements, recommendations and information for using infiltration basin/underground infiltration BMPs in the MIDS calculator|infiltration practices]]. An overview of individual input parameters and workflows is presented in the [http://stormwater.pca.state.mn.us/index.php/User%E2%80%99s_Guide MIDS Calculator User Documentation].
  
===Credits Based on Reported Literature Values===
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===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 pond or wetland device.   A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page [[Information on pollutant removal by BMPs|here]]. Designers may use the pollutant reduction values or may research values from other databases and published literature.  Designers who opt for this approach should
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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 infiltrationtion device. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.
*select the median value from pollutant reduction databases that report a range of reductions, such as from the [http://bmpdatabase.org/index.htm International BMP Database];
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*select a pollutant removal reduction from literature that studied a bioretention device with site characteristics and climate similar to the device being considered for credits;
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Designers may use the pollutant reduction values [http://stormwater.pca.state.mn.us/index.php/Information_on_pollutant_removal_by_BMPs reported in this manual] or may research values from other databases and published literature.  Designers who opt for this approach should
*review the article to determine that the design principles of the studied bioretention are close to the design recommendations for Minnesota, as described in [http://stormwater.pca.state.mn.us/index.php/Design_criteria_for_bioretention this manual] and/or by a local permitting agency; and
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*select the median value from pollutant reduction databases that report a range of reductions, such as from the [https://bmpdatabase.org/ International BMP Database];
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*select a pollutant removal reduction from literature that studied an infiltration device with site characteristics and climate similar to the device being considered for credits;
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*review the article to determine that the design principles of the studied infiltration are close to the design recommendations for Minnesota, as described in [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_infiltration this manual] and/or by a local permitting agency; and
 
*give preference to literature that has been published in a peer-reviewed publication.
 
*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 stormwater pond, considering such conditions as watershed characteristics, pond sizing, and climate factors.  
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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 infiltration device, considering such conditions as watershed characteristics, infiltration sizing, soil infiltration rates, and climate factors.  
 
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*[https://bmpdatabase.org/ International Stormwater Best Management Practices (BMP) Database]
*[http://bmpdatabase.org/Docs/2012%20Water%20Quality%20Analysis%20Addendum/BMP%20Database%20Categorical_SummaryAddendumReport_Final.pdf International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals].
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**Compilation of BMP performance studies
**Compilation of BMP performance studies published through 2011.
 
 
**Provides values for TSS, Bacteria, Nutrients, and Metals
 
**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.
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**Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels
*[https://www.portlandoregon.gov/bes/article/133994 Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon].
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*[http://lshs.tamu.edu/docs/lshs/end-notes/updated%20bmp%20removal%20efficiencies%20from%20the%20national%20pollutant%20re-2854375963/updated%20bmp%20removal%20efficiencies%20from%20the%20national%20pollutant%20removal%20database.pdf Updated BMP Removal Efficiencies from the National Pollutant Removal Database (2007) & Acceptable BMP Table for Virginia]
**Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations.
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**Provides data for several 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.
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*[http://www.epa.state.il.us/green-infrastructure/docs/draft-final-report.pdf The Illinois Green Infrastructure Study]  
**Applicable to Filters, Wet Ponds, Porous Pavements, Soakage Trenches, Flow through Stormwater Planters, Infiltration Stormwater Planters, Vegetated Infiltration Basins, Swales, and Treatment Wetlands.
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**Figure ES-1 summarizes BMP effectiveness
*[http://www.epa.state.il.us/green-infrastructure/docs/draft-final-report.pdf The Illinois Green Infrastructure Study]
 
**Figure ES-1 summarizes BMP effectiveness
 
 
**Provides values for TN, TSS, peak flows / runoff volumes
 
**Provides values for TN, TSS, peak flows / runoff volumes
**Applicable to Permeable Pavements, Constructed Wetlands, Infiltration, Detention, Filtration, and Green Roofs
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**Applicable to permeable pavements, constructed wetlands, infiltration, detention, filtration, and green roofs
* [http://des.nh.gov/organization/divisions/water/stormwater/manual.htm New Hampshire Stormwater Manual].
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*[https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/wd-08-20b.pdf New Hampshire Stormwater Manual]
 
**Volume 2, Appendix B summarizes BMP effectiveness
 
**Volume 2, Appendix B summarizes BMP effectiveness
 
**Provides values for TSS, TN, and TP removal
 
**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
 
**Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
*[http://www.epa.gov/region1/npdes/stormwater/assets/pdfs/BMP-Performance-Analysis-Report.pdf BMP Performance Analysis].  Prepared for US EPA Region 1, Boston MA.  
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*[https://www.wri.wisc.edu/wp-content/uploads/FinalWR03R001.pdf Design Guidelines for Stormwater Bioretention Facilities].  University of Wisconsin, Madison
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**Table 2-1 summarizes typical removal rates
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**Provides values for TSS, metals, TP, TKN, ammonium, organics, and bacteria
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**Applicable for bioretention
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*[https://www3.epa.gov/region1/npdes/stormwater/tools/BMP-Performance-Analysis-Report.pdf BMP Performance Analysis].  Prepared for US EPA Region 1, Boston MA.  
 
**Appendix B provides pollutant removal performance curves
 
**Appendix B provides pollutant removal performance curves
**Provides values for TP, TSS, and Zn. 
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**Provides values for TP, TSS, and zinc
**Pollutant removal broken down according to land use.
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**Pollutant removal broken down according to land use
**Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement.
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**Applicable to infiltration trench, infiltration basin, bioretention, grass swale, wet pond, and porous pavement
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*Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. [http://www.lrrb.org/media/reports/200523.pdf The Cost and Effectiveness of Stormwater Management Practices: Final Report]
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**Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
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**Applicable to wet basins, stormwater wetlands, bioretention filter, sand filter, infiltration trench, and filter strips/grass swales
  
 
===Credits based on field monitoring===
 
===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.
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Field monitoring may be made 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.
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#Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
 
##Which pollutants will be measured?
 
##Which pollutants will be measured?
 
##Will the monitoring study the performance of a single BMP or multiple BMPs?
 
##Will the monitoring study the performance of a single BMP or multiple BMPs?
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##Will the results be compared to other BMP performance studies?
 
##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?
 
##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:
+
#Plan the field activities.  Field considerations include
##Equipment selection and placement
+
##equipment selection and placement;
##Sampling protocols including selection, storage, delivery to the laboratory
+
##sampling protocols including selection, storage, and delivery to the laboratory;
##Laboratory services
+
##laboratory services;
##Health and Safety plans for field personnel
+
##health and Safety plans for field personnel;
##Record keeping protocols and forms
+
##record keeping protocols and forms; and
##Quality control and quality assurance protocols
+
##quality control and quality assurance protocols
 
#Execute the field monitoring
 
#Execute the field monitoring
 
#Analyze the results
 
#Analyze the results
 +
 +
This manual contains the following guidance for monitoring.
 +
*[[Recommendations and guidance for utilizing monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing lake monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing stream monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing major stormwater outfall monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing stormwater best management practice monitoring to meet TMDL permit requirements]]
  
 
The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.
 
The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.
:[http://water.epa.gov/scitech/wastetech/guide/stormwater/monitor.cfm '''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.
+
:[https://www3.epa.gov/npdes/pubs/montcomplete.pdf '''Urban Stormwater BMP Performance Monitoring''']
Highlighted chapters in this manual include:
+
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
+
*Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
*Chapters 3 & 4: Methods and Equipment
+
*Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
*Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
+
*Chapter 4: Methods and equipment for water quality monitoring
 +
*Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
 
*Chapter 7: BMP Performance Analysis
 
*Chapter 7: BMP Performance Analysis
*Chapters 8, 9, & 10: LID Monitoring
+
*Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
  
 
:[http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_565.pdf '''Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)''']
 
:[http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_565.pdf '''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:
+
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 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
 
*Chapter 4: Stormwater Characterization
 
**4.2: General Characteristics and Pollutant Sources
 
**4.2: General Characteristics and Pollutant Sources
Line 219: Line 241:
 
**8.6: Overall Hydrologic and Water Quality Performance Evaluation
 
**8.6: Overall Hydrologic and Water Quality Performance Evaluation
 
*Chapter 10: Hydrologic Evaluation
 
*Chapter 10: Hydrologic Evaluation
**10.5: Performance Verification and Design Optimization  
+
**10.5: Performance Verification and Design Optimization
 +
 
 +
:[https://www.wef.org/globalassets/assets-wef/3---resources/topics/o-z/stormwater/stormwater-institute/wef-stepp-white-paper_final_02-06-14.pdf '''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 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).
  
:[http://www.wef.org/uploadedFiles/Access_Water_Knowledge/Stormwater_and_Wet_Weather/Stormwater_PDFs/WEF-STEPP-White%20Paper_Final_02-06-14%282%29.pdf '''Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices'''].
+
:'''Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)'''
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).
 
  
:[http://www.dot.ca.gov/hq/env/stormwater/pdf/CTSW_OT_13_999.pdf '''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.
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 4: Monitoring Methods and Equipment
 
*Chapter 5: Analytical Methods and Laboratory Selection
 
*Chapter 5: Analytical Methods and Laboratory Selection
Line 237: Line 260:
  
 
:[http://stormwaterbook.safl.umn.edu/ '''Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance''']
 
:[http://stormwaterbook.safl.umn.edu/ '''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
+
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.
*Level 2: Capacity Testing
+
*Level 1: [https://stormwaterbook.safl.umn.edu/assessment-programs/visual-inspection Visual Inspection]
*Level 3: Synthetic Runoff Testing
+
*Level 2: [https://stormwaterbook.safl.umn.edu/assessment-programs/capacity-testing Capacity Testing]
*Level 4: Monitoring
+
*Level 3: [http://stormwaterbook.safl.umn.edu/assessment-programs/synthetic-runoff-testing Synthetic Runoff Testing]
*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.
+
*Level 4: [https://stormwaterbook.safl.umn.edu/assessment-programs/monitoring 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:
 
Use these links to obtain detailed information on the following topics related to BMP performance monitoring:
*[http://stormwaterbook.safl.umn.edu/developing-assessment-program/water-budget-measurement Water Budget Measurement]
+
*[https://stormwaterbook.safl.umn.edu/water-budget-measurement Water Budget Measurement]
*[http://stormwaterbook.safl.umn.edu/developing-assessment-program/sampling-methods Sampling Methods]
+
*[https://stormwaterbook.safl.umn.edu/sampling-methods Sampling Methods]
*[http://stormwaterbook.safl.umn.edu/developing-assessment-program/analysis-water-and-soils Analysis of Water and Soils]
+
*[https://stormwaterbook.safl.umn.edu/analysis-water-and-soils Analysis of Water and Soils]
*[http://stormwaterbook.safl.umn.edu/assessment-programs/data-analysis Data Analysis for Monitoring]
+
*[https://stormwaterbook.safl.umn.edu/data-analysis Data Analysis for Monitoring]
  
 
==Other Pollutants==
 
==Other Pollutants==
In addition to TSS and phosphorus, infiltration practices can reduce loading of other pollutants.  According to the [http://www.bmpdatabase.org/ International Stormwater Database], studies have shown that infiltration practices are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD).  A compilation of the pollutant removal capabilities from a review of literature are summarized below.
+
In addition to TSS and phosphorus, infiltration practices can reduce loading of other pollutants.  According to the [https://bmpdatabase.org/ International Stormwater Database], studies have shown that infiltration practices are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD).  A compilation of the pollutant removal capabilities from a review of literature are summarized below.
  
 
{{:Infiltration Basin Pollutant Load Reduction}}
 
{{:Infiltration Basin Pollutant Load Reduction}}
Line 283: Line 308:
  
 
<noinclude>
 
<noinclude>
 +
 
==Related pages==
 
==Related pages==
 
*[http://stormwater.pca.state.mn.us/index.php/Infiltration Infiltration portal]
 
*[http://stormwater.pca.state.mn.us/index.php/Infiltration Infiltration portal]
Line 316: Line 342:
 
**[[Calculating credits for stormwater and rainwater harvest and use/reuse]]
 
**[[Calculating credits for stormwater and rainwater harvest and use/reuse]]
  
[[category:Calculating credits]]
+
[[Category:Level 3 - Best management practices/Guidance and information/Pollutant removal and credits]]
 +
[[Category:Level 2 - Pollutants/Pollutant removal]]
 
</noinclude>
 
</noinclude>

Latest revision as of 14:22, 26 July 2022

Recommended pollutant removal efficiencies, in percent, for infiltration BMPs. Sources.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
Pollutant removal is 100 percent for the volume that is captured and infiltrated
image
Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.
Green Infrastructure: Infiltration practices can be an important tool for retention and detention of stormwater runoff and treatment of pollutants in stormwater runoff. If the practice utilizes vegetation, additional benefits may include cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

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

This page provides a discussion of how infiltration practices can achieve stormwater credits. Infiltration practices include infiltration basins, infiltration trenches (including dry wells), and underground infiltration systems. The discussion does not include bioinfiltration and permeable pavement systems, unless specifically mentioned. To view the credit articles for other BMPs, see the Related pages section.

Overview

Infiltration Basin Detailed Cross Section
Schematic showing an infiltration basin. Note that inflow into the practice has undergone pretreatment. Once the infiltration basin is filled, water bypasses rather than enters the practice.

Infiltration practices are designed to capture, store, and infiltrate stormwater runoff. They rely on naturally permeable soils to fully infiltrate the designed * Water Quality Volume (VWQ). These are typically off-line practices utilizing an emergency spillway or outlet structure to capture the volume of stormwater runoff for which the practice is designed. Volumes that exceed the rate or volume of the infiltration practice are allowed to bypass the BMP.

Pollutant removal mechanisms

Infiltration practices reduce stormwater volume and pollutant loads through infiltration of the stormwater runoff into the native soil. Infiltration practices also can remove a wide variety of stormwater pollutants through secondary removal mechanisms including filtration, biological uptake, and soil adsorption through plantings and soil media (WEF Design of Urban Stormwater Controls, 2012). See Other Pollutants, for a complete list of other pollutants addressed by infiltration practices.

Location in the treatment train

Stormwater treatment trains are comprised of multiple Best Management Practices 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. Because infiltration practices are designed to be off-line, they may either be located at the end of the treatment train, or used as off-line configurations to divert the water quality volume from the on-line system.

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. Infiltration practices are 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

In developing the credit calculations, it is assumed the infiltration 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 infiltration basin or infiltration trench sections of the Manual. Because of their high susceptibility of failure due to clogging, pretreatment is REQUIRED in all infiltration designs.

Warning: Pretreatment is required for all infiltration practices

In the following discussion, the Water Quality Volume (VWQ) is delivered instantaneously to the BMP. VWQ is stored as water ponded above the soil or engineered media and below the overflow elevation. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch off new impervious surface. For MIDS, VWQ is 1.1 inches.

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

The approach in the following sections is based on the following general design considerations:

  • Credit calculations presented in this article are for both event and annual volume and pollutant load removals.
  • Stormwater volume credit equates to the volume of runoff that will ultimately be infiltrated into the soil subgrade.
  • TSS and TP credits are achieved for the volume of runoff that is infiltrated.

Volume Credit Calculations

Infiltration Basin Detailed Cross Section
Schematic showing terms used in calculating volume credits for an infiltration basin.

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil from the existing stormwater collection system. These credits are assumed to be instantaneous values entirely based on the capacity of the BMP to capture, store, and transmit water in any storm event. Because the volume is calculated as an instantaneous volume, the water quality volume (VWQ) is assumed to pond below the overflow elevation and above the bioretention media. This entire volume is assumed to infiltrate through the bottom of the BMP. The volume credit (Vinfb) for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by

\( V_{inf_b} = D_o\ (A_O + A_M)\ / 2 \)

where

AO is the overflow surface area of the bioretention system, in square feet;
AM is the area at the surface of the media, in square feet; and
Do is the ponded depth with the BMP, in feet.

If native soils are used rather than engineered media, the term AM may be substituted by AB, as shown in the above schematic and in the schematics for the MIDS calculator. To comply with the Construction Stormwater General Permit, VWQ must infiltrate within 48 hours (24 hours is recommended if discharges are to a trout stream).

Some of the VWQ will be lost to evapotranspiration rather than all being lost to infiltration. In terms of a water quantity credit, this differentiation is unimportant, but it may be important if attempting to calculate actual infiltration into the underlying soil.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


Total suspended solid (TSS) calculations

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

\( M_{TSS_i} = 0.0000624\ V_{inf_b}\ EMC_{TSS} \)

where

  • Vinfb is the volume of water infiltrated, in cubic feet; and
  • EMCTSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on EMC values for TSS, link here. The above calculation may be applied on an annual basis and is given by

\( M_{TSS_f} = 2.72\ F\ V_{annual}\ EMC_{TSS} \)

where

Vannual is the annual volume treated by the BMP, in acre-feet.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator.

Total phosphorus credit calculations

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

\( M_{TP_i} = 0.0000624\ V_{inf_b}\ EMC_{TP} \)

where

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

The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. The above calculation may be applied on an annual basis and is given by

\( M_{TP_f} = 2.72\ V_{annual}\ EMC_{TP} \)

where

Vannual is the annual volume treated by the BMP, in acre-feet.

Methods for calculating credits

This section provides specific information on generating and calculating credits from infiltration practices for volume, TSS and 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/hydraulic 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

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

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 infiltration practices. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

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

  1. Model name and version
  2. Date of analysis
  3. Person or organization conducting analysis
  4. Detailed summary of input data
  5. Calibration and verification information
  6. 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. Sort the table by Infiltrator BMPs to identify BMPs that may include infiltration practices.

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 event mean concentration 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 (EPA 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.

Caution: 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.

Download MPCA Estimator here

MIDS Calculator

mids logo
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 infiltration practices. 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 infiltrationtion 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 an infiltration 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 infiltration 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 infiltration device, considering such conditions as watershed characteristics, infiltration sizing, soil infiltration rates, and climate factors.

Credits based on field monitoring

Field monitoring may be made 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.

  1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
    1. Which pollutants will be measured?
    2. Will the monitoring study the performance of a single BMP or multiple BMPs?
    3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    4. Will the results be compared to other BMP performance studies?
    5. 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?
  2. Plan the field activities. Field considerations include
    1. equipment selection and placement;
    2. sampling protocols including selection, storage, and delivery to the laboratory;
    3. laboratory services;
    4. health and Safety plans for field personnel;
    5. record keeping protocols and forms; and
    6. quality control and quality assurance protocols
  3. Execute the field monitoring
  4. Analyze the results

This manual contains the following guidance for monitoring.

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: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
  • Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
  • Chapter 4: Methods and equipment for water quality monitoring
  • Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
  • Chapter 7: BMP Performance Analysis
  • Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
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 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 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.

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:

Other Pollutants

In addition to TSS and phosphorus, infiltration practices can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that infiltration practices are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD). A compilation of the pollutant removal capabilities from a review of literature are summarized below.

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

Pollutant Category Constituent Treatment Capabilities

(Low = < 30%; Medium = 30-65%;

High = 65 -100%)
Metals1 Cr, Cu, Zn High2
Ni, Pb
Nutrients Total Nitrogen, TKN Medium/High
Bacteria Fecal Coliform, E. coli High
Organics High

1 Results are for total metals only
2 Treatment capabilities are based mainly on information from sources that referenced only metals as a category and did not provide individual efficiency for specific metals


References and suggested reading


Related pages

This page was last edited on 26 July 2022, at 14:22.