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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 Section 3, 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.

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 Section 3, Other Pollutants, and Section 4, Resources, 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.

Assumptions and Approach

For this section, several key assumptions were necessary in developing the credit calculations. These assumptions include that the infiltration basin:

• Is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual.

• Follow the REQUIRED, HIGHLY RECOMMENDED, and RECOMMENDED design criteria.

If any of these assumptions are not valid, the credit will be reduced.

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

• The credit for volume reduction represents the volume of runoff that can be retained and ultimately infiltrated between the basin bottom and the overflow elevation. Credit is also given for infiltration and evapotranspiration occurring within the drawdown time of the basin.

• Native soils of the site should be evaluated to achieve representative infiltration rates either through field soil measurements or by using a county soil survey or the NRCS Web Soil Survey. Design infiltration rates should use the infiltration rate of the least permeable soil within the first 5 feet below the bottom elevation of the proposed infiltration practice. Infiltration rates can be related to either tested values or the typical design infiltration rates for A, B, C, and D soil groups. It is REQUIRED that the measured infiltration rates shall be divided by a minimum safety factor of 2, though a higher factor of safety between 2 and 10 is recommended.

• Water Quality credits for infiltration basins are calculated assuming all stormwater entering the basin is infiltrated and all pollutants associated with this volume are removed.

Volume Reduction and Water Quality Credits

Volume Credit Calculations

Volume credits for infiltration basins are calculated based on the volume capture ability to permanently remove stormwater runoff from the existing stormwater collection system. 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 volume capacity of the basin between the overflow elevation, and the basin bottom, infiltration through the bottom of the basin, and evapotranspiration. Credit is only obtainable for evapotranspiration and infiltration values if there is available space for water within the ponding area to be infiltrated or evapotranspirated within the drawdown time. If the ponding volume is greater than the infiltration and evapotranspiration capacities of the native soils and plantings, then the volume credit for the infiltration pond is limited to the ponding area volume.

Volume credits (V) for infiltration basins are given by

If \( V_p<V_(inf⁡_b)+V_ET, V=V_P+V_(inf⁡_b)+V_ET \)

If \( V_p>V_(inf⁡_b)+V_ET, V=V_P \)

where:

V = Total event based volume credit for BMP (cf).

V_P = Volume reduction credit for ponding area storage capacity (cf).

V_{inf_b} = Volume reduction of infiltration through bottom of basin (cf).

V_ET = Volume reduction through evapotranspiration (cf).

Volume capacity of the ponding area (V_P) is given by

\( V_P=(A_O+A_B)/2*D_O \)

where:

A_O = Overflow surface area at elevation of outlet or overflow, or ponding surface area (sf).

A_B = Surface area at the bottom of the infiltration basin (sf).

D_O = Overflow depth, or depth from top of the filter bed to the top of the ponding area (ft).

Volume credits for infiltration through the bottom of the basin (Vinf_b) are given by

\( V_(inf⁡_b⁡)=I_R/12*A_B*DDT_calc \)

where:

I_R = Design infiltration rate of underlying soil group (in/hr).

DDT_calc = Required drawdown time (hrs). A conservative approach would be to assume a maximum of 24 hour drawdown time for credit calculations so as not to overestimate volume reduction capacity.

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

Volume credit for evapotranspiration (VET) is given by the lesser of

\( ET_mea=0.2/12*A_B*0.5*DDT_calc \)

or

\( ET_pot=D_B*A_B*(FC-WP) \)

where: ET_mea = The amount of water lost to ET as measured using available data (cf).

ET_pot = The amount of water stored between field capacity and the wilting point in the media (cf).

D_B= Depth below infiltration basin to seasonal high groundwater or bedrock (ft).

FC-WP = Water holding capacity of soils, equal to field capacity minus the wilting point of soils (cf/cf).

The volume reduction credit (V) can be converted to annual volume reduction percentage (V_A%) 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. The annual volume reduction (V_annual) credit is then given by

\( V_annual=V_(A%)*V_AR \)

where: V_annual = Annual volume reduction credit (acre-ft).

V_A% = Annual volume reduction percentage. Value calculated using MIDS calculator or other appropriate modeling techniques using the volume reduction (V) calculated above.

V_AR=Annual runoff volume (acre-ft).= \((DA*P_j*P*R_V)/12\)

DA = Total drainage area to BMP (acre).

P_j = Annual rainfall correction factor. Fraction of annual rainfall volume that produces runoff.

P = Annual rainfall precipitation (in).

R_V = Site runoff coefficient. Weighted average of area of land cover and associated runoff coefficient values.

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, 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.

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