Green Infrastructure benefits of infiltration practices

Photo illustrating permeable interlocking concrete pavement.

Bioinfiltration (rain garden) in a residential development. Photo courtesy of Katherine Sullivan.

Infiltration is the practice of draining water into soils, typically through engineered systems such as bioinfiltration (rain gardens), infiltration basins, dry swales with check dams, and permeable pavement. The practice of infiltration is beneficial for soils, maintaining natural hydrology, and has a significant water quality impact for downstream lakes, rivers, and ponds. Depending on design, stormwater infiltration practices can be a key component of GI to promote the health and well-being of animals, vegetation, and the people that rely upon these waters when designing sites.

Some of the more common infiltration practices include

• infiltration basins and trenches,
• rain gardens,
• vegetated swales,
• underground infiltration systems,
• permeable pavement, and
• tree trenches.

For further reading on different types of infiltration, see Stormwater infiltration Best Management Practices and BMPs for stormwater infiltration.

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here fore more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Benefit	Effectiveness	Notes
Water quality	●	Most pollutants are retained in the engineered media, soil, or vadose zone. If transported to groundwater, concentrations of most pollutants are below water quality standards. Chloride is an exception.
Water quantity/supply	◕	Can provide effective flood control for small- and medium-intensity storms.
Energy savings	◔
Climate resiliency	◔	Flood control. Impacts on carbon sequestration are uncertain.
Air quality	◑
Habitat improvement	◕	Use of perennial vegetation and certain media mixes promote invertebrate communities, pollinators, birds, and potentially small mammals.
Community livability	◕	When vegetation is incorporated, aesthetically pleasing and can be incorporated into a wide range of land use settings.
Health benefits	◔
Economic savings	◔	Generally provide cost savings vs. conventional practices over the life of the practice.
Macroscale benefits	◑	Macroscale effects depend on the size of the practice. Some infiltration practices, typically underground or tree trench systems, can be very large and have macroscale benefits.
Level of benefit: ◯ - none; ◔ - small; ◑ - moderate; ◕ - large; ● - very high

Green Infrastructure benefits of infiltration practices

• Water quality: Stormwater infiltration practices are excellent water quality treatment practices. Engineered media, soils, and the underlying vadose zone provide effective retention of most pollutants, as indicated in the accompanying table. Chloride and nitrate are exceptions, though concentrations of these are generally below water quality standards except for chloride during deicing season. Infiltration should be avoided in areas where contaminants in soil or groundwater may be mobilized by infiltrating water. For more information, see Surface water and groundwater quality impacts from stormwater infiltration

Minimum bioretention soil media depths recommended to target specific stormwater pollutants. From Hunt et al. (2012) and Hathaway et al., (2011). NOTE: The Construction Stormwater permit requires a 3 foot separation from the bottom of an infiltration practice and bedrock or seasonally saturated soils.
Link to this table

Pollutant	Depth of Treatment with upturned elbow or elevated underdrain	Depth of Treatment without underdrain or with underdrain at bottom	Minimum depth
Total suspended solids (TSS)	Top 2 to 3 inches of bioretention soil media	Top 2 to 3 inches of bioretention soil media	Not applicable for TSS because minimum depth needed for plant survival and growth is greater than minimum depth needed for TSS reduction
Metals	Top 8 inches of bioretention soil media	Top 8 inches of bioretention soil media	Not applicable for metals because minimum depth needed for plant survival and growth is greater than minimum depth needed for metals reduction
Hydrocarbons	3 to 4 inch Mulch layer, top 1 inch of bioretention soil media	3 to 4 inches Mulch layer, top 1 inch of bioretention soil media	Not applicable for hydrocarbons because minimum depth needed for plant survival and growth is greater than minimum depth needed for hydrocarbons reduction
Nitrogen	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing pollutant load to the receiving stream, and also improves nitrogen removal because the longer retention time allows denitrification to occur underanoxic conditions.	From top to bottom of bioretention soil media	Retention time is important, so deeper media is preferred (3 foot minimum)
Particulate phosphorus	Top 2 to 3 inches of bioretention soil media.	Top 2 to 3 inches of bioretention soil media.	Not applicable for particulate phosphorus because minimum depth needed for plant survival and growth is greater than minimum depth needed for particulate phosphorus reduction
Dissolved phosphorus	From top of media to top of submerged zone. Saturated conditions cause P to not be effectively stored in submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet, but 3 feet recommended as a conservative value; if IWS is included, keep top of submerged zone at least 1.5 to 2 feet from surface of media
Pathogens	From top of soil to top of submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet; if IWS is included, keep top of submerged zone at least 2 feet from surface of media
Temperature	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing volume of warm runoff discharged to the receiving stream, and also improves thermal pollution abatement because the longer retention time allows runoff to cool more before discharge.	From top to bottom of bioretention soil media	Minimum 3 feet, with 4 feet preferred

• Water quantity and hydrology:Infiltration practices reduce the volume of stormwater runoff and retard peak flow from rainfall events, thus reducing flood potential in areas downstream of the practice. They are most effective for small- and medium-intensity rain and runoff events unless sized to meet larger events. Infiltration promotes groundwater recharge, potentially increasing baseflow and/or recharge of deeper aquifers.
• Energy savings: Larger infiltration practices that incorporate trees and provide shade reduce air conditioning costs. Since infiltration reduces stormwater runoff, they help prevent road deterioration and reduce maintenance costs (Using Trees and Vegetation to Reduce Heat Islands;US EPA).
• Air quality: Benefits of infiltration practices are largely indirect, such as sequestration of carbon and other greenhouse gasses. Carbon sequestration is generally insignificant unless vegetation is incorporated into the practice and the soil or engineered media promotes biologic activity.
• Climate resiliency: Properly installed infiltration practices reduce the impact of flooding during rainfall events, particularly small- and medium-sized events. Vegetated infiltration systems promote photosynthesis and carbon sequestration. Incorporation of larger plants such as trees or vegetation that provides shade reduces effects of heat islands (Using Trees and Vegetation to Reduce Heat Islands;US EPA).
• Habitat improvement: Infiltration results in decreased runoff and erosion, which increases soil stability. This promotes vegetation growth that further stabilizes a site and creates habitat for birds, pollinator insects, and potentially small mammals. Soil or media may be engineered to promote invertebrate avtivity. Reduced runoff associated with increased infiltration reduces adverse effects of elevated temperatures that harm coldwater organisms.
• Community livability: Infiltration that results in groundwater recharge and improved baseflow provides increased recreational opportunities helps ensure safe and healthy access to water sources. Incorporating aesthetically pleasing landscaping when planning infiltration systems may help improve mental health of the site users (What are the physical and mental benefits of gardening? - Michigan State University Extension). Large infiltration practices that incorporate trees into the design provide shade that can reduce air temperatures (Reducing Urban Heat Islands: Compendium of Strategies: Trees and Vegetation; EPA).
• Health benefits: Infiltration provides cleaner waterways through the reduction of nutrients, pathogens, metals, TSS, and phosphorus and provides healthier environments to the humans, wildlife, and vegetation that use these waters.
• Economic benefits and savings: In addition to water quality and flood control benefits, properly designed infiltration can prevent downstream cleanup costs. Well maintained infiltration systems combined with vegetation may increase property aesthetics and property value.

Design considerations

Maximizing specific green infrastructure (GI) benefits of constructed areas requires design considerations prior to installation. While site limitations cannot always be overcome, the following recommendations for a designer are given to maximize the GI benefit.

Note: Under the Minnesota Construction Stormwater Permit GI, particularly infiltration, must be considered first when selecting stormwater treatment methods. However, if Class D soils are present on the site infiltration practices cannot be used. Class A soils are the most desirable for infiltration but infiltration systems can also be successful with B or C soils. --- Maybe add a chart indicating soil penetrability of different HSG groups —

• Water quality
  • Construction of the infiltration system should be done in a location where the majority of water will flow through
  • Design infiltration basins to maximize retention time and prevent short-circuiting
  • Infiltration systems with ecological diversity can help promote water quality treatment and maximize the amount of pollutants removed while providing healthy habitats for pollinators, and other key wildlife
  • Ensure an adequate treatment train is constructed to meet the desired water quality (Pre-treatment considerations for stormwater infiltration - Minnesota Stormwater Manual (state.mn.us))
  • Diversifying the vegetation will remove a wider range of pollutants and maximize the water treatment
• Water quantity and hydrology:
  • Distribute constructed infiltration systems throughout the site to increase the delivered benefits
• Climate resiliency:
  • Vegetation establishment is essential for carbon sequestration
  • Infiltration systems that treat water quality will reduce the offsite pollutant overloading that can be damaging to ecosystems
• Habitat improvement:
  • Providing a littoral shelf for the growth of macrophytes in an infiltration system promotes healthier wildlife populations
  • Planting plants for pollinators in infiltration systems is an effective area to establish healthier pollinator colonies — link to pollinator page
• Community livability:
  • Include recreational infrastructure and interpretative signs
  • Construct the infiltration system in a way that ensures safety and perceived safety of the area. A few examples would be to use shallower infiltration systems to avoid child accidents, attracting pollinators that are appropriate for the nearby community, or planting shrubs, fencing, or vegetation that prevents people from entering the system
  • Conduct surveys prior to and after development to identify community desires and construct features that enhance education, recreation, and other benefits of infiltration
  • Develop conveyance systems in such a way to minimize changes in temperature that can be detrimental to wildlife such a temperature sensitive fish
• Health benefits:
  • Infiltration that incorporates landscaping principles reduce heat stress associate with heat islands (Reducing Urban Heat Islands: Compendium of Strategies: Trees and Vegetation (epa.gov))
  • Infiltration that incorporates landscaping principles increases the mental health of the communities that use the area (What are the physical and mental benefits of gardening? - MSU Extension)
  • Infiltration systems naturally control mosquito habitats by going dry within a few days compared to stormwater ponds
• Economic benefits and savings:
  • Properly designed infiltration can prevent downstream cleanup costs
  • Infiltration systems that incorporate desired landscape vegetation may increase property aesthetics and value

Recommended reading

The Minnesota Stormwater Manual offers a chart to help designers with a cost-benefit analysis for infiltration linked here. The Pollution Control Agency allows for infiltration to be used as a credit source when meeting pollutant budgets for Total Suspended Solids (TSS) and Total Phosphorus (TP). The methodology for counting credits can be found here. Additional Information:

• Infiltration practices do not get water quality volume treatment credit when constructed in Class D soils.
• Class D soils are prohibited for use when building infiltration systems
• EPA recommended guidelines for designing areas prone to floods can be found in the following link A Guide to Assessing Green Infrastructure Costs and Benefits for Flood Reduction (noaa.gov)

References

Support material Outside MSM Links to include/reference: Y - good picture graphic for infiltration and groundwater recharge and simple explanation of groundwater - https://www.usgs.gov/special-topics/water-science-school/science/infiltration-and-water-cycle

Y - Reference for part of above built table - neat pictures - https://www.ashbyma.gov/plan/subdivision%20docs/breitmaier/160923%20plans-docs/Stormwater%20Report/BMP%20-%20Infiltration%20Basin.pdf

Y - Cost benefit table - https://stormwater.pca.state.mn.us/index.php/Cost-benefit_considerations_for_infiltration

M - good guidelines for developing green infrastructure though - https://coast.noaa.gov/data/docs/digitalcoast/gi-cost-benefit.pdf

Decent picture - Infiltration and the Water Cycle | U.S. Geological Survey (usgs.gov)

Related pages

Additional References from the Minnesota Stormwater Manual

• Infiltration
• Overview for infiltration
• Design criteria for infiltration
• Construction specifications for infiltration
• Operation and maintenance of infiltration practices
• Assessing the performance of infiltration
• Calculating credits for infiltration
• Case studies for infiltration
• External resources for infiltration
• References for infiltration
• Underground infiltration BMPs in the MIDS Calculator
• Understanding and interpreting soils and soil boring reports for infiltration BMPs
• Determining soil infiltration rates