Decision tools for stormwater infiltration

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This page provides guidance to help determine if infiltration Best Management Practices (BMPs) can be utilized at a site. Infiltration BMPs include bioinfiltration, infiltration basin, infiltration trench, underground infiltration, permeable pavement, and tree trench or tree box with no underdrain. Note that some infiltration occurs in BMPs with underdrains, particularly if the underdrain is raised above the bottom of the BMP. If no infiltration is desired at a site, a liner should be used.

Infiltration is often utilized at a site to meet a regulatory requirement or a performance goal for stormwater volume. The following discussion does not specifically focus on utilizing infiltration to meet a requirement or performance goal, although there is a section that provides information for determining the amount of infiltration feasible at a site.

Warning: The Construction Stormwater General Permit states “Where a project’s ultimate development replaces vegetation and/or other pervious surfaces with one (1) or more acres of cumulative impervious surface, the Permittee(s) must design the project so that the water quality volume of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site”

The following table can be used to determine if infiltration is prohibited or not recommended. Discussion of conditions where infiltration may be acceptable when site constraints exist is provided below.

Summary of situations where infiltration of stormwater runoff is prohibited or not recommended.
Link to this table

Condition	Prohibited	Not recommended	Not recommended unless conditions are met1
Local rule or ordinance prohibits infiltration	X
BMP is < 100 feet from drinking well in sensitive aquifer3	X
BMP is < 50 feet from drinking well in a non-sensitive aquifer3	X
BMP receives discharges from vehicle fueling and maintenance areas	X
There is less than 3 of separation to bedrock or saturated soil	if CGP applies2	X
BMP receives discharges from certain industrial facilities4	if CGP applies
BMP is in areas where high levels of contaminants in soil or groundwater will be mobilized by the infiltrating stormwater	if CGP applies		X
BMP is located in D soils	if CGP applies		X
BMP is 1,000 feet up-gradient or 100 feet down-gradient of active karst	if CGP applies	X
BMP is located within an Emergency Response Area in a DWSMA exhibiting high or very high vulnerability	if CGP applies	X
BMP is located within an Emergency Response Area (ERA) in a DWSMA exhibiting moderate vulnerability, or outside an ERA in a DWSMA exhibiting high or very high vulnerability	if CGP applies, unless a higher engineering review indicates infiltration is acceptable	X
BMP is located in soils with infiltration rate > 8.3 in/hr	if CGP applies		X
BMP is located within 1 year travel time of drinking supply well			X
BMP located within 10 feet of building or structure			X
BMP located within 35 feet of a septic system			X
BMP located less than 200 feet from crest of slope that exceeds 20%		X
BMP receives discharges from a confirmed stormwater hotspot			X
A groundwater mound formed beneath the BMP during infiltration extends into the BMP		X

¹see discussion for conditions under which infiltration is acceptable.
²CGP = Construction Stormwater General Permit
³See Minnesota Rules Chapter 4725
⁴To determine if infiltration is allowed for a specific industrial sector or subsector, go to the list of sectors, select the sector of interest, and review section 8 for that sector (Use of Infiltration Devices and/or Industrial Stormwater Ponds for Stormwater Treatment and Disposal)

Determine if infiltration is prohibited

• Determine if there are local ordinances or rules that prevent infiltration of stormwater runoff.
• Determine if there are other prohibitions on infiltration. Infiltration is prohibited if any of the following conditions apply.
  • The infiltration practice (BMP) is less than 100 feet from a drinking water supply well in a sensitive aquifer or less than 50 feet from a drinking water well in a non-sensitive aquifer. (link).
  • A Construction Stormwater General (CGP) permit is required and any of the following conditions applies.
    • The infiltration practice receives discharges from vehicle fueling and maintenance.
    • There is less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock. (link)
    • The infiltration practice receives discharges from industrial facilities which are not authorized to infiltrate industrial stormwater under an NPDES/SDS Industrial Stormwater Permit issued by the MPCA. To determine if infiltration is allowed for a specific industrial sector or subsector, go to the list of sectors, select the sector of interest, and review section 8 for that sector (Use of Infiltration Devices and/or Industrial Stormwater Ponds for Stormwater Treatment and Disposal)
    • The infiltration practice receives discharges from areas with high levels of contaminants in soil or groundwater and contaminants will be mobilized by the infiltrating stormwater. (link)
    • The infiltration practice is located in areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit. (link)
    • The infiltration practice is within 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features, unless allowed by a local unit of government with a current MS4 permit. (link)
    • The infiltration practice is within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13., unless allowed by a local unit of government with a current MS4 permit. (link)
    • The infiltration practice is located in areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour or as allowed by a local unit of government with a current MS4 permit.

Determine if infiltration is not prohibited but is not recommended

If infiltration is not prohibited but any of the following conditions apply, infiltration is not recommended.

• A Construction Stormwater General (CGP) permit is not required but one or more of the following conditions applies.
  • There is less than three (3) feet of separation distance from the bottom of the infiltration practice to the elevation of the seasonally saturated soils or the top of bedrock.
  • The infiltration practice is located in areas of predominately Hydrologic Soil Group D (clay) soils.
  • The infiltration practice is located in areas where soil infiltration rates are more than 8.3 inches per hour.
  • The infiltration practice is within 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features.
  • The infiltration practice receives discharges from areas with high levels of contaminants in soil or groundwater and contaminants will be mobilized by the infiltrating stormwater.
• The infiltration practice is located in the one (1) year travel time from a drinking water supply well. The area within the 1-year time of travel is also called the Emergency Response Area for a water supply well. To view shapefiles, a map, or metadata for Emergency Response Areas, link here and select Emergency Response Areas from the Geospatial Data Files table.
• The infiltration practice is within 10 feet of a building or structure unless a mounding analysis confirms that the building or structure will not be impacted by the infiltration practice.
• The infiltration practice is located within 35 feet of a septic drainfield, unless a mounding analysis confirms that the drainfield will not be impacted by the infiltration practice.
• The infiltration practice is less than 200 feet from the toe of a slope greater than or equal to 20 percent.
• The infiltration practice receives discharges from a confirmed stormwater hotspot (see guidance) and pretreatment practices cannot be used to achieve acceptable pollutant concentrations in the runoff. There is no specific pollutant concentration that defines a stormwater hotspot and the pollutant characteristics and removal capability of the BMP must be considered in making this determination. A suggested approach is to apply the expected pollutant removal for the BMP to the expected concentration of the pollutant and compare the remaining concentration to the water quality standard (see water quality standards for surface water and groundwater). For example, the drinking water standard for zinc is 2 milligrams per liter (mg/L). The concentration of zinc in runoff from an area is 6 mg/L. Expected pollutant removal rates are 65 percent for underground infiltration and 80 percent for bioinfiltration. The resulting concentrations in effluent beneath the infiltration systems are 2.1 mg/L for underground infiltration and 1.2 mg/L for bioinfiltration. Using the drinking standard as the criteria for determining the feasibility of infiltration, bioinfiltration would be acceptable and underground infiltration would not be acceptable.
• A groundwater mound forming beneath the infiltration practice will extend into the practice. The expected mound elevation can be calculated using guidance in this manual.

Information needed

Site review is necessary to determine if stormwater runoff can be infiltrated at a site. A site review includes both a review of information and field sampling. The following information may be of use in conducting a site review

• Aerial photos and topographic maps: used to determine slopes and distance to slopes, presence of surface water features, presence of sinkholes
• County soil surveys and other soil information as available: used to determine likely soil infiltration rates, depth to saturated soil
• County Geologic Atlas: used to determine depth to bedrock, presence of karst, information for conducting mounding assessment
• Local groundwater levels: used to determine depth to water, information for conducting mounding assessment
• DWSMA and Wellhead Protection maps: used to determine location of DWSMA, travel times to drinking water supply wells
• Soil borings: used to confirm depth to saturated soil and bedrock, presence of contaminated soil or groundwater, presence of karst features; information collected can be used to verify soil types and infiltration rates can be determined knowing the soil type
• Site survey: used to confirm presence of specific features such as buildings, structures, utilities, slopes, bedrock outcrops, septic systems, drinking water wells
• MPCA listing of potentially contaminated sites: used to identify confirmed or likely sources of contaminated soil or groundwater
• Phase 1 and 2 Environmental Site Assessments TMDLs and local water quality standards: used to determine appropriate stormwater treatment practices and location of those practices
• Wetland delineations, MNRAM assessments, and wetland classifications: used to determine presence of surface water features, depth to saturated soil
• Proposed conditions, conceptual/preliminary site design
• Local zoning and land use requirements/ordinances, including stormwater rate control requirements: used to determine prohibitions or restrictions
• Communication with local landowners, LGU, or others knowledgeable about the site
• Site inspection

Determining the amount of infiltration possible at a site

The amount of infiltration that can be achieved at a site depends on the soil properties and BMP size. The size of the BMP may be impacted by separation distances that can be achieved. Required or recommended separation distances are shown in the table below. For example, if bedrock is within 5 feet of the land surface, the BMP depth can only be 2 feet to achieve the 3 foot separation from bedrock unless the BMP is raised to achieve greater separation.

Required and recommended minimum vertical and horizontal separation distances. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is above-ground, the distance is measured from the edge of the BMP to the structure. If the structure is underground, the vertical separation distance represents the distance from the point of infiltration through the bottom of the system to the structure, while the horizontal separation (often called setback) distance is the shortest distance from the edge of the system to the structure.
Link to this table

Structure		Distance (feet)	Requirement or recommendation	Note(s)
Vertical	Saturated soil	3	Requirement1
	Bedrock	3	Requirement1
Horizontal	Public supply well	100 for sensitive wells; 50 for others3	Requirement
	Building/structure/property line2	10	Recommended
	Surface water	none unless local requirements exist		If nearby stream is impaired for chloride, see [1]
	Septic system	35	Recommended
	Contaminated soil/groundwater			No specific distance. Infiltration must not mobilize contaminants.
	Slope	200	Recommended	from toe of slope >= 20%
	Karst	1000 up-gradient 100 down-gradient	Requirement1	active karst

¹ Required under the Construction Stormwater General Permit
² Minimum with slopes directed away from the building
³If treating an average of 10,000 gallons per day; otherwise separation distance is 300 feet

Schematic showing dimensions for an infiltration basin. The average cross-sectional area is 6067 square feet (surface area is 6534 feet and the bottom area is 5600 feet) and the depth is 1 foot. Water is ponded in this BMP, resulting in a treatment volume of 6067 cubic feet.

Schematic showing the dimensions of a tree trench system. The average cross-sectional area is 4415 square feet (surface area of the system is 5600 square feet and the bottom surface area is 3230 square feet), the media depth is 4 feet, and the (porosity minus field capacity) is 0.25. The resulting treatment volume is therefore 4415 cubic feet.

The basic equation used to size a BMP is

• for BMPs with ponded water \( V = A * D \)

• for BMPs with water stored in soil or media (no ponded water) \( V = A * D * (n – FC) \)

Where

V = volume infiltrated by the BMP in 48 hours or less, in ft³;

A = average cross-sectional area of the BMP, in ft²;

D = depth of water ponded or stored in the BMP soil or media, in feet;

n = the porosity of the soil or media where runoff is retained, in ft³/ft³; and

FC = the soil or media field capacity, in ft³/ft³.

BMPs with ponded water include bioinfiltration, infiltration trench, and infiltration basin. BMPs with water stored in the soil or media include tree trench, tree box, permeable pavement, and swale with a bioretention base. Specific equations for sizing can be found within articles discussing credits for individual BMPs.

Recommended field verification

Field verification should be made for separation distances, depth to water, depth to bedrock, and presence of karst. Recommended numbers of borings and pits are presented in this Manual.

Evaluation

Field sampling and evaluation is recommended for determining soil properties (e.g. infiltration rate, presence of gleying and mottling). Recommended numbers of borings and pits are presented in this Manual.

Other guidance

Guidance for determining the potential for infiltrating stormwater has been developed for locations outside Minnesota.

Caution: Caution must be exercised in applying guidance from other locations to Minnesota since regulations may differ.

Two examples that may be useful are indicated below.

• Wisconsin: Site Evaluation for Stormwater Infiltration
• Strategies/Guidance for Managing Stormwater: Infiltration vs. Surface Water Discharge. Clark, S., Baker, K., Lalor, M., Mikula, J., and Burkhardt, C. (2005). Impacts of Global Climate Change: pp. 1-12. doi: 10.1061/40792(173)228.

In addition to the above two references, the Minimal Impact Design Standards (MIDS) design sequence flowchart can be used to make decisions about the potential for infiltrating stormwater at a site. Although the flowchart focuses on MIDS performance goals, it can be adapted to help determine if infiltration prohibitions exist at a site. Potential constraints to infiltration, with links to the MIDS flowchart and other guidance in this manual, are shown below.

• Are there constraints due to lack of available right-of-way, off-site drainage and/or rate control requirements?
• Are there zoning and land use requirements that constrain infiltration?
• Is the site located in a DWSMA, wellhead protection area or within 200 feet of a drinking water well? Also see guidance on wellhead protection.
• Are there existing or proposed structures or infrastructure that constrain infiltration?
• Is karst present on the site? Also see guidance on sites with karst.
• Is shallow groundwater or shallow bedrock present on site? Also see guidance for sites with shallow groundwater or shallow bedrock.
• Is there presence of contaminated soils and/or groundwater, or hotspot runoff? Also see guidance for sites with contaminated soils or groundwater or potential stormwater hotspots.
• Are there very low infiltrating soils (<0.2 inches per hour) on site? Also see guidance for sites with soils having low infiltration capacity.
• Are there very high infiltrating soils (more than 8 inches per hour)?
• Are there adverse surface water hydrologic impacts from infiltration practices (e.g. impacting perched wetland)?
• Also see guidance on setback distances

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