Calculating credits for infiltration basin

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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 infiltration practices can achieve stormwater credits. Infiltration practices include infiltration basins, infiltration trenches, and underground infiltration systems.

Overview

Schematic showing Infiltration Basin Detailed Cross Section

An Infiltration basin is a large earthen structure designed to capture, store, and infiltrate stormwater water runoff. Infiltration basins rely on naturally permeable soils to fully infiltrate the designed water quality volume. Infiltration basins are typically off-line practices utilizing an emergency spillway or outlet structure to capture the volume of stormwater runoff for which the basin is designed. Volumes that exceed the rate or volume of the infiltration basin are allowed to bypass the BMP.

Pollutant Removal Mechanisms

Infiltration basins reduce stormwater volume and pollutant loads through infiltration of the stormwater runoff into the native soil. Infiltration basins 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 basins.

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. 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 followed by Best Management Practices that reduce the pollutant concentration and/or volume of stormwater runoff. Because Infiltration basins 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. Specific methods for calculating credits are discussed later in this article. Infiltration basins 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.

Infiltration basins generate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP).

Assumptions and Approach

In developing the credit calculations, it is assumed the infiltration basin 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 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 soil or engineered media and below the overflow elevation. 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.

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

Volume credits are calculated based on the capacity of the BMP to permanently remove stormwater runoff from the existing stormwater collection system via infiltration into the underlying soil. These credits are assumed to be instantaneous values entirely based on the capacity of the infiltration basin for any storm event. Instantaneous volume reduction, or event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

Volume reduction credits are dependent on the time required for drawdown, the infiltration rate of the underlying soil, and the area at the bottom of the BMP. The volume credit, V, is given by

\( V = A_B\ DDT\ (I_R/12) \)

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 water (hours).

The required drawdown time for the Construction Stormwater General Permit is 48 hours.

The volume reduction credit (V) can be converted to annual volume reduction percentage (VA%) if the annual volume reduction quantity is 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 the infiltration basin, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness.

Total suspended solid (TSS) calculations

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

\( M_{TSS}_i = 0.0000624\ V\ EMC_{TSS} \)

where

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

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

Related Articles within the Minnesota Stormwater Manual

Information about Infiltration Basins

Overview of Stormwater Credits

Credit Calculation Methods

Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on volume reduction and BMP parameters presented in this credit article
2. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
3. MIDS Calculator approach
4. Quantifying volume and pollution reductions based on values reported in literature
5. Quantifying volume and pollution reductions based on field measurements

This section provides specific information on generating and calculating credits from infiltration basins for volume, TSS, and phosphorus. Infiltration basins are also effective at reducing concentrations of other pollutants including nitrogen and metals. This article does not provide information on calculating credits for pollutants other than TSS and phosphorus, but references are provided that may be useful for calculating credits for other pollutants; see Other Pollutants, and References, for more information.

Alternative techniques for calculating credits associated with volume and pollutant reductions may be proposed to the Minnesota Pollution Control Agency or other permitting agency for their consideration and approval.

Water Quality Credit Calculations

Quality credits applied to infiltration basins can be calculated per rain event or based on total annual rainfall. Though there is little available data to demonstrate load reductions in infiltration basins, when properly designed, constructed, and maintained, the entire volume of stormwater entering the basin, and the pollutant loads carried by that runoff, should be removed entirely. This does not include any stormwater in excess of the capacity of the BMP that ultimately bypasses the system.

Total Suspended Solids

TSS reduction credit corresponds directly with the volume reduction capacity of the infiltration basin. Because infiltration basins are designed entirely offline, 100% TSS removal is assumed for infiltrated stormwater.

The annual TSS credit (MTSS) for infiltration basins is given by

\( M_TSS=2.72*V_Annual*〖EMC〗_TSS \)

where: M_TSS =Annual or event TSS removal (lb/yr or lb/event).

V_annual = Annual volume reduction credit calculated above (acre-ft).

EMC_TSS = Event Mean Concentration, concentration of TSS in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMC_TSS should be dependent on the MTSS effluent (mg/L) from the next upstream tributary BMP.

Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.

The storm event based TSS credit (MTSS) for infiltration basins is given by \( M_TSS=2.72*V/43,560*〖EMC〗_TSS \)

where: M_TSS =Annual or event TSS removal (lb/yr or lb/event).

V = Event volume reduction credit calculated above (cf).

EMC_TSS = Event Mean Concentration of TSS in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTSS should be dependent on the MTSS effluent (mg/L) from the next upstream tributary BMP.

Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

Total Phosphorus

Similar to TSS, TP reduction credits correspond with volume reduction through infiltration of water captured by the infiltration basin. 100% removal of TP in captured stormwater is also assumed.

The annual TP credit (MTP) for infiltration basins is given by

\( M_TP=V_annual*〖EMC〗_TP*2.72 \)

where:

M_TP =Annual or event TP removal (lb/yr or lb/event).

V_annual = Annual volume reduction credit calculated above (acre-ft).

EMC_TP = Event Mean Concentration of TP in runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.

Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.

The storm event based TP credit (MTP-I) for infiltration basins is given by

\( M_TP=2.72*V/43,560*EMC_TP \)

where: M_TP =Annual or event TP removal (lb/yr or lb/event).

V = Event volume reduction credit calculated above (cf).

EMC_TP = Event Mean Concentration of TP in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMC_TP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.

Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

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 basins. 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.

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

Model Selection

2.3.3 MIDS Calculator=

Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.

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 here

Designers may use the pollutant reduction values reported here 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 stormwater pond or wetland device with site characteristics and climate similar to the device being considered for credits.
• When using data from an individual study, review the article to determine that the design principles of the studied stormwater pond or wetland are close to the design recommendations for Minnesota, as described in this WIKI, and/or by a local permitting agency.
• Preference should be given 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.

• 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
• BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
  • Appendix B provides pollutant removal performance curves
  • Provides values for TP, TSS, and Zn.
  • Pollutant removal broken down according to land use.
  • Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement.

Credits Based on Field Monitoring

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 constructed basins 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 in Table 3-1.

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

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

• 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. "California Stormwater BMP Handbook-New Development and Redevelopment." California Stormwater Quality Association, Menlo Park, CA (2003)
• Caltrans. 2004. BMP Retrofit Pilot Program Final Report, Report No. CTSW-RT-01-050. Division of Environmental Analysis. California Dept. of Transportation, Sacramento, CA.
• CDM Smith. 2012. Omaha Regional Stormwater Design Manual. Chapter 8 Stormwater Best Management Practices. Kansas City, MO.
• Caraco, Deborah, and Richard A. Claytor. Stormwater BMP Design: Supplement for Cold Climates. US Environmental Protection Agency, 1997.
• Denr, N. 2007. Stormwater Best Management Practices Manual. North Carolina Department of Environment and Natural Resources, Raleigh, North Carolina.
• Dorman, M. E., H. Hartigan, F. Johnson, and B. Maestri. Retention, detention, and overland flow for pollutant removal from highway stormwater runoff: interim guidelines for management measures. Final report, September 1985-June 1987. No. PB-89-133292/XAB. Versar, Inc., Springfield, VA (USA), 1988.
• Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical *Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
• Consultants, Geosyntec, and Wright Water Engineers. "Urban stormwater BMP performance monitoring." (2002).
• Gulliver, J. S., A. J. Erickson, and PTe Weiss. "Stormwater treatment: Assessment and maintenance." University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. http://stormwaterbook. safl. umn. edu (2010).
• Muthukrishnan, Swarna. "Treatment Of Heavy Metals In Stormwater Runoff Using Wet Pond And Wetland Mesocosms." In Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy, vol. 11, no. 1, p. 9. 2010.
• Hathaway, J. M., W. F. Hunt, and S. Jadlocki. "Indicator bacteria removal in storm-water best management practices in Charlotte, North Carolina." Journal of Environmental Engineering 135, no. 12 (2009): 1275-1285.
• Jaffe, et. al. 2010. The Illinois Green Infrastructure Study. Prepared by the University of Illinois at Chicago, Chicago Metropolitan Agency for Planning, Center for Neighborhood Technology, Illinois-Indiana Sea Grant College Program.
• Jurries, Dennis. "Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal." Quality, S. o. OD o. E.(Ed.). Kidd, R., y N. Colletta.(1980).
• Kurz, Raymond C. "Removal of microbial indicators from stormwater using sand filtration, wet detention, and alum treatment: best management practices." PhD diss., University of South Florida, 1998.
• Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
• New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B. Concord, NH.
• Transportation Officials, Oregon State University. Dept. of Civil, Environmental Engineering, University of Florida. Dept. of Environmental Engineering Sciences, GeoSyntec Consultants, and Low Impact Development Center, Inc. Evaluation of Best Management Practices for Highway Runoff Control. No. 565. Transportation Research Board, 2006.
• Schueler, T.R., Kumble, P.A., and Heraty, M.A. 1992. A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source Pollution in the Coastal Zone, Metropolitan Washington Council of Governments, Washington, D.C
• Schueler, Thomas R. Controlling urban runoff: A practical manual for planning and designing urban BMPs. Washington, DC: Metropolitan Washington Council of Governments, 1987.
• State of California, Department of Transportation. 2013. Caltrans Stormwater Monitoring Guidance Manual. Sacramento, CA.
• TetraTech. 2008. BMP Performance Analysis. Prepared for US EPA Region 1, Boston, MA.
• Water Environment Federation. 2014. Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices. A White Paper by the National Stormwater Testing and Evaluation of Products and Practices (STEPP) Workgroup Steering Committee.
• WEF, ASCE/EWRI. 2012. Design of Urban Stormwater Controls, WEF Manual of Practice No. 23, ASCE/EWRI Manuals and Reports on Engineering Practice No. 87. Prepared by the Design of Urban Stormwater Controls Task Forces of the Water Environment Federation and the American Society of Civil Engineers/Environmental & Water Resources Institute.
• Weiss, Peter T., John S. Gulliver, and Andrew J. Erickson. "The Cost and Effectiveness of Stormwater Management Practices Final Report." (2005).
• Wossink, G. A. A., and Bill Hunt. The economics of structural stormwater BMPs in North Carolina. Water Resources Research Institute of the University of North Carolina, 2003.