BMPs for stormwater infiltration

Best Management Practices that infiltrate stormwater runoff into underlying soil include, but are not limited, to

• infiltration basins,
• infiltration trenches,
• underground infiltration,
• bioinfiltration,
• permeable pavements, and
• tree trenches and tree boxes.

These are discussed briefly below. Additional information about these BMPs can be found in the following tables.

• Stormwater infiltration BMPs - overview
• Stormwater infiltration BMPs – selection considerations
• Stormwater infiltration BMPs - treatment properties

Infiltration basin

Photo of an infiltration basin. Source: Clark County, Washington, with permission.

Applications and treatment capabilities for infiltration basins
Applications		Treatment capabilities3, 4, 5
Residential	Yes	TSS	High6
Commercial	Yes	TN	Medium/high
Ultra-urban	Limited1	TP	Medium/high
Industrial	Yes2	Chloride	Low
Highway/road	Limited	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
1 Due to a size restriction; 2 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fueling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 3Underground infiltration systems may have different (likely lower) pollutant removal capabilities than what is provided in this table. These systems may have a wider application range. 4 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 5 Low = < 30%; Medium = 30-65%; High = 65 -100%); 6 Assumes adequate pre-treatment Sources: Schueler, 1987, 1992; USEPA 1993a, 1993b; Maniquiz et al., 2010; NPRPD, 2007; California Stormwater Manual, 2009; Pennsylvania Stormwater Manual, 2006

An infiltration basin is a natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water. Drawdown of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. The required drawdown time is 48 hours or less. Water that is stored but not infiltrated must leave the BMP, typically through an outlet, within the required drawdown time. In the case of a constructed basin, the impoundment is created by excavation or embankment. Infiltration basins are commonly used for drainage areas of 5 to 50 acres with land slopes that are less than 20 percent. Typical depths range from 2 to 6 feet, including bounce in the basin. The sizing is to control stormwater volumes at the regional or development scale as opposed to bioretention basins (rain gardens) that are designed at the site scale. Typical dimensions range from 1,000 square feet up to an acre. Infiltration basins are commonly constructed with plant species that can tolerate and thrive in this unique growing environment.

For more information, see the following pages in this Manual.

• Overview for Infiltration basin
• Design criteria for Infiltration basin
• Construction specifications for Infiltration basin
• Operation and maintenance of Infiltration basin
• Cost-benefit considerations for Infiltration basin
• Calculating credits for infiltration basin
• External resources for Infiltration basin
• References for Infiltration basin
• Requirements, recommendations and information for using infiltration basin/underground infiltration BMPs in the MIDS calculator

Infiltration trench

Photo of a Infiltration trench in Lino Lakes

Applications and treatment capabilities for infiltration trenches
Applications		Treatment capabilities3, 4
Residential	Yes	TSS5	High5
Commercial	Yes	TN	Medium/high
Ultra-urban	Limited1	TP	Medium/high
Industrial	Yes2	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
1 Due to a size restriction; 2 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 3 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 4 Low = < 30%; Medium = 30-65%; High = 65 -100%); 5 Assumes adequate pre-treatment Sources: Schueler, 1987, 1992; USEPA 1993a, 1993b; Maniquiz et al., 2010; NPRPD, 2007; California Stormwater Manual, 2009; Pennsylvania Stormwater Manual, 2006

An infiltration trench is a shallow excavated trench, typically 3 to 6 feet deep, that is backfilled with a coarse stone aggregate allowing for the temporary storage of runoff in the void space of the material. Drawdown of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Infiltration trenches may be modified to become stormwater tree trenches and boxes where applicable with the addition of growing medium. All water captured by the BMP must be removed within 48 hours through infiltration and/or a drain. Trenches are commonly used for drainage areas less than 5 acres in size.

Caution: To avoid an infiltration trench being classified as a Class V injection well, it is strongly recommended that the length of the trench be at least 2 times greater than the depth of the trench.

For more information, see the following pages in this Manual.

• Overview for Infiltration trench
• Design criteria for Infiltration trench
• Construction specifications for Infiltration trench
• Operation and maintenance of Infiltration trench
• Cost-benefit considerations for Infiltration trench
• Calculating credits for infiltration trench
• External resources for Infiltration trench
• References for Infiltration trench
• Requirements, recommendations and information for using infiltration basin/underground infiltration BMPs in the MIDS calculator

Bioinfiltration basin

Bioinfiltration basin (Source: CDM Smith)

Applications and treatment capabilities for bioinfiltration basins
Applications		Treatment capabilities2, 3
Residential	Yes	TSS	High4
Commercial	Yes	TN	Low/Medium5
Ultra-urban	Limited7	TP	Medium/high6
Industrial	Yes1	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 This assumes no underdrain; 6 Certain soil mixes can leach P; 7 Due to a size restriction Sources: EPA Factsheet, 1999; Davis et al., 2001, 2003, 2006; Hsieh and Davis, 2005; Hong et al., 2006; Hunt et al., 2006; NPRPD, 2007; Li and Davis, 2009; Diblasi et al., 2009; Passeport et al., 2009; Brown et at., 2011a, b; Komlos et al., 2012; Denich et al., 2013; Li and Davis, 2013; California Stormwater BMP

Bioinfiltration basins, often called rain gardens, use soil (typically engineered media or mixed soil) and native vegetation to capture runoff and remove pollutants. Both the media and underlying soil typically have high infiltration rates that allow captured water to infiltrate within a required drawdown time, usually 48 hours. For more information, see the following pages in this Manual.

• Bioretention terminology (including types of bioretention)
• Overview for bioretention
• Design criteria for bioretention
• Construction specifications for bioretention
• Operation and maintenance of bioretention
• Cost-benefit considerations for bioretention
• Calculating credits for bioretention
• Soil amendments to enhance phosphorus sorption
• Summary of permit requirements for bioretention
• Supporting material for bioretention
• External resources for bioretention
• References for bioretention
• Requirements, recommendations and information for using bioretention with no underdrain BMPs in the MIDS calculator

Permeable pavement

Permeable pavement (Source: CDM Smith)

Applications and treatment capabilities for permeable pavement
Applications		Treatment capabilities2, 3
Residential	Yes	TSS	High4
Commercial	Yes	TN	Medium/High
Ultra-urban	Yes	Nitrate	Low/Medium
Industrial	Yes1	TP	Medium/High
Retrofit	Yes	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	5
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 Insufficient information Source: Schueler, 1987; Pratt et al, 1999; Adams, 2003; Brattebo and Booth, 2003; Adams, 2003; Bean et al, 2007; SEMCOG, 2008; International Stormwater Database, 2012

Permeable pavements are paving surfaces that allow stormwater runoff to filter through surface voids into an underlying stone reservoir for infiltration and/or storage. They are suitable for driveways, trails, parking lots, and roadways with lighter traffic. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). All permeable pavements have a similar design layering system, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains for filtration and geotextile over non-compacted soil subgrade. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. The drainage area leading to permeable pavements should not exceed twice the surface area of the final pavement surface.

For more information, see the following pages in this Manual.

• Overview for permeable pavement
• Types of permeable pavement
• Design criteria for permeable pavement
• Construction specifications for permeable pavement
• Assessing the performance of permeable pavement
• Operation and maintenance of permeable pavement
• Calculating credits for permeable pavement
• Additional considerations for permeable pavement
• Links for permeable pavement
• References for permeable pavement
• Requirements, recommendations and information for using permeable pavement BMPs in the MIDS calculator

Tree box/Tree trench

Tree box (Source: CDM Smith)

Applications and treatment capabilities for tree box/tree trench
Applications		Treatment capabilities2, 3
Residential	Yes	TSS	High4
Commercial	Yes	TN	Low/Medium
Ultra-urban	Yes	TP	Medium/High5
Industrial	Yes1	Chloride	Low
Highway/road	No	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 Certain soil mixes can leach P. Source: see [1]

Tree trenches are a system of trees that are connected by an underground infiltration structure. The system consists of a stormwater tree trench or box lined with geotextile fabric with structural stone, gravel or soil boxes in which the trees are placed. Tree systems consist of an engineered soil or rock layer designed to treat stormwater runoff via filtration through plant and soil/rock media, and through evapotranspiration from trees. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Tree species are carefully selected to survive both inundation and drought conditions in urban environments where they will be potentially affected by chloride and other traffic concerns. Tree trenches and boxes drainage areas should be less than five acres depending on the size of each trench. Irrigation, whether manual or automated, is strongly encouraged during the tree’s establishment period.

For more information, see the following pages in this Manual.

Trees - general

• Overview for trees
• Types of tree BMPs
• Plant lists for trees
• Street sweeping for trees
• References for trees
• Supporting material for trees

Tree boxes/tree trenches

• Design guidelines for tree quality and planting - tree trenches and tree boxes
• Design guidelines for soil characteristics - tree trenches and tree boxes
• Construction guidelines for tree trenches and tree boxes
• Protection of existing trees on construction sites
• Operation and maintenance of tree trenches and tree boxes
• Assessing the performance of tree trenches and tree boxes
• Calculating credits for tree trenches and tree boxes
• Case studies for tree trenches and tree boxes
• Soil amendments to enhance phosphorus sorption
• Fact sheet for tree trenches and tree boxes
• Requirements, recommendations and information for using trees as a BMP in the MIDS calculator

Underground infiltration and dry wells

Underground infiltration systems and dry wells have been installed below parking lots and other impervious surfaces on sites where insufficient space exists for a surface infiltration system. They are designed to temporarily store stormwater runoff before slowly infiltrating the water into the subsurface (Connecticut, 2004). There is limited information on the effectiveness of these systems in removing pollutants. Limited data suggests they may be less effective at removing mobile pollutants than surface-based infiltration systems.

One concern is that underground infiltration may meet the U.S. Environmental Protection Agency (EPA) definition of a Class V injection well. Class V injection wells are defined as any bored, drilled, or driven shaft, or any dug hole that is deeper than its widest surface dimension. Class V injection wells can also be an improved sinkhole, or a subsurface fluid distribution system (from U.S. EPA, June 2003). The U.S. EPA administers Class V injection well permits in Minnesota. Minimum requirements for installing, permitting, and operating a Class V well is defined by the USEPA.

A second concern pertains to the overall pollutant removal effectiveness of those underground infiltration systems that do not meet the definition of a Class V injection well. The document released by the Transport Research Synthesis titled “Issues of Concern Related to Underground Infiltration Systems for Stormwater Management and Treatment” provides a good overview of the concerns related to underground infiltration systems (MNDOT, 2009). Issues identified in this report include:

• There is potential that an underground infiltration system meets the criteria of a Class V injection well.
• There is insufficient knowledge of the fate of pollutants in the subgrade below the buried infiltration systems.
• Roadways and parking lots with high volumes of traffic have higher concentrations of certain pollutants, including heavy metals and PAHs.
• Underground systems do not allow for the pollutant removal that is accomplished through biological activity and vegetation uptake.
• The minimum separation requirement of 3 feet between the bottom of the infiltration system and the seasonally high groundwater elevation may be insufficient for adequate pollutant removal. Additional study is recommended.
• Maintenance of underground systems is critical for effective pollutant removal. However, access for maintenance is challenging. There are concerns that the difficult access is preventing owners from properly maintaining these systems.

This Manual currently does not provide guidance for design, construction, maintenance, and assessment of underground infiltration systems.

Enhanced turf

Infiltration can be enhanced on soils that have been improved or amended. This manual contains limited information on enhanced turf and does not provide guidance for design, construction, maintenance, and assessment of enhanced turf. Information on use of compost in soil and credits associated with improved turf can be found on the Turf page. A discussion of alleviating compaction from construction activities can be found here.

Warning: Enhanced turf cannot be used to meet requirements of the Construction Stormwater General permit because infiltrated water does not represent an instantaneous volume.

Unit processes for different infiltration BMPs

The following table provides a summary of unit processes for the different infiltration BMPs.

Unit processes of stormwater treatment techniques (Adapted from WEF, 2008)
Link to this table

Control	Infiltration basin	Infiltration trench	Bioinfiltration	Permeable pavement	Tree box/tree trench	Enhanced turf
Peak flow attenuation	X	X	X	X
Runoff volume reduction	X	X	X	X	X
Infiltration	X	X	X	X	X	X
Dispersion
Evapotranspiration			X		X
Runoff collection and usage			X1	X1
Sedimentation	X	X	X
Flotation	X		X
Laminar separation
Swirl concentration
Sorption	X	X	X	X
Precipitation	X	X	X	X
Coagulation	X	X	X	X
Filtration			X
Plant metabolism			X		X	X
Nitrification/denitrification	X	X	X
Organic compound degradation	X	X	X	X
Pathogen die off	X	X		X
Temperature reduction	X	X	X	X
Disinfection	X	X	X	X

¹ If underdrain is present

Related pages

• Overview of stormwater infiltration
• Pre-treatment considerations for stormwater infiltration
• BMPs for stormwater infiltration
• Pollutant fate and transport in stormwater infiltration systems
• Surface water and groundwater quality impacts from stormwater infiltration
• Stormwater infiltration and groundwater mounding
• Stormwater infiltration and setback (separation) distances
• Karst
• Shallow soils and shallow depth to bedrock
• Shallow groundwater
• Soils with low infiltration capacity
• Potential stormwater hotspots
• Stormwater and wellhead protection
• Stormwater infiltrations and contaminated soils and groundwater
• Decision tools for stormwater infiltration
• Stormwater infiltration research needs
• References for stormwater infiltration