Decision tools for stormwater infiltration

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This site was recently updated and is undergoing development. Anticipated completion date is October, 2015.

This page provides guidance to help determine if and to what extent infiltration is feasible at a site. The discussion focuses on the physical ability to infiltrate stormwater runoff rather than determining the ability to meet a specific volume requirement of performance goal.

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”

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.

• 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 and
  • the practice cannot be raised to achieve three (3) feet of separation, or
  • the practice cannot be relocated to achieve three (3) feet of separation.
• The infiltration practice is located in areas of predominately Hydrologic Soil Group D (clay) soils and
  • soils cannot be amended to achieve an infiltration rate of 0.2 inches per hour or more, or
  • the practice cannot be relocated to an area with an infiltration rate of 0.2 inches per hour or greater.
• The infiltration practice is located in areas where soil infiltration rates are more than 8.3 inches per hour and
  • soils cannot be amended to slow the infiltration rate below 8.3 inches per hour or
  • the practice cannot be relocated to an area with an infiltration rate of less 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 and the practice cannot be relocated to achieve greater separation distances.
• 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 mound forming beneath the infiltration practice will extend into the practice. The expected mound elevation can be calculated using guidance in this manual.
• Presence of endangered species habitat.
• Presence of flood plains and flood fringes.

Information needed

Site review is necessary to determine the feasibility of infiltrating stormwater runoff 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, surface water features, 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
• FEMA and local floodplain maps: used to determine presence of surface water features, depth to saturated soil
• 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 estimate soil infiltration rate,
• Site survey: used to confirm presence of specific features such as buildings, structures, utilities, slopes, bedrock outcrops, septic systems, drinking water wells; used to identify appropriate locations for infiltration practice(s)
• 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 feasible 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

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 \( V = A * D * (n – FC) \)

Where V = volume infiltrated by the BMP in 48 hours or less, in ft3; A = area of the BMP, in ft2; D = depth of water ponded or stored in the BMP soil or media; N = the porosity of the media where runoff is retained, in ft3/ft3; and FC = the soil 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

Borings Distances

Evaluation

Mounding analysis Determining infiltration rates

Other guidance

Guidance for determining the feasibility of infiltrating stormwater has been developed for locations outside Minnesota. Examples can be found through a simple Google search on stormwater infiltration guidance.

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.