Stormwater infiltration and setback (separation) distances

Schematic showing some horizontal and vertical separation distances from an infiltration BMP. A separation distance may be required, such as with a drinking water well, or recommended, as with an underground tank. (Source: CDM Smith) Not to scale.

Separation distance is defined as the distance from the closest point of a best management practice (BMP) to the particular feature being considered. Example recommended and REQUIRED separation distances for infiltration BMPs are shown in the figure to the right and are discussed below. Note that all REQUIRED statements in this section refer to requirements contained in the NPDES/SDS (CSW) General Permit (CGP).

Warning: The following are required minimum setback distances:

• 50 feet from a water supply well or 100 feet from a sensitive water supply well
• 3 feet from the seasonally saturated soils (water table) or bedrock
• 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features

Vertical separation distances

Vertical separation distance is the vertical distance from the bottom of a BMP to the top of the feature of concern. The shortest vertical distance is used in measuring the separation distance.

Seasonally saturated soils

Schematic illustrating separation distance from bottom of infiltration BMP to water table or top of bedrock. This diagram includes a modified subsoil zone in which the subsoil has been ripped to alleviate compaction.

Under the CGP, a three (3) foot vertical separation is REQUIRED between the bottom of any infiltration BMP and the top of seasonally saturated soils. Seasonally saturated soil is defined as the highest seasonal elevation in the soil that is evidenced by the presence of redoximorphic features or other parameters as assessed by a soils specialist. The temporary groundwater mound that develops below an operational infiltration BMP is not required to be a minimum of 3 feet below the bottom of the infiltration BMP. However, it is HIGHLY RECOMMENDED that this mound not penetrate into the infiltration BMP which could inhibit the pollutant removal within the media contained in the BMP. The base of the BMP may be raised to achieve the 3 foot separation. For more information on infiltration in areas with seasonally saturated soils, link here.

Bedrock

Under the CGP a minimum 3 foot vertical separation is REQUIRED between the base of any constructed infiltration BMP and the top of the bedrock. The base of the BMP may be raised to achieve the 3 foot separation distance. For guidance on infiltration in areas with shallow bedrock, link here.

Karst bedrock

Karst topography is a geological formation shaped by the dissolution of a layer, or layers, of soluble bedrock. Groundwater in karst regions is particularly vulnerable to contamination. There is also concern about the formation of sinkholes resulting from focused infiltration. The CGP prohibits infiltration within areas that are 1,000 feet up gradient or 100 feet down gradient of active karst terrain. In addition, the 3 foot minimum separation to bedrock is applicable under the CGP. Additional detail on identifying and testing for karst is detailed here.

Horizontal separation (setback) distances

Horizontal separation distance is the horizontal distance from the closest point of a BMP to the feature of concern. The shortest horizontal distance is used in determining the separation. This distance does not include a mound that extends beyond the boundaries of the BMP.

Municipal water supply wells

The Minnesota Department of Health (MDH), in a memo dated November 15, 2013, determined that a stormwater infiltration system was "a source of pollution or contamination that may drain into the soil". Consequently, a minimum 100 foot horizontal separation is REQUIRED (by MDH) between any infiltration BMP and a sensitive public water supply well, and a 50 foot minimum horizontal separation is REQUIRED between any infiltration BMP treating an average of 10,000 gallons per day (11.25 acre-feet per year) or less and all other public water supply wells. For systems treating more than an average of 10,000 gallons per day the separation distance from a public water supply well is 300 feet. See MDH isolation distances for pollutants or contaminants that may drain into the soil, as required by Minnesota Rules Chapter 4725. This is to ensure there is adequate removal of pollutants before the infiltrating water reaches the drinking water well.

NOTE: You can use a simple tool such as the Minimal Impact Standards (MIDS) calculator to determine the annual volume treated by an infiltration practice. For example, an infiltration basin located in St. Paul, MN, that treats the MIDS goal of 1.1 inches must treat runoff from 5.25 acres or more of impervious surface to exceed the 10,000 gallons per day criteria.

The CGP prohibits infiltration in areas within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13., if the system will be located:

• in an Emergency Response Area (ERA) within a DWSMA classified as having high or very high vulnerability as defined by the Minnesota Department of Health; or
• in an ERA within a DWSMA classified as moderate vulnerability unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater; or
• outside of an ERA within a DWSMA classified as having high or very high vulnerability, unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.

Maps of DWSMA areas are available through the MN Department of Health.

Other water supply wells (e.g. private, transient)

Adequate separation must be maintained between all water supply wells and infiltration systems. Unlike municipal public supply wells, modeling and other technical analysis is rarely done to determine appropriate separation distances for these wells. We recommend one of the following three approaches to determining a separation distance between an infiltration system and a water supply well.

Screen shot of a segment of the surficial pollution sensitivity map from the Chisago County Geologic Atlas.

• Follow Minnesota Department of Health guidelines, which specify a minimum 100 foot horizontal separation between any infiltration BMP and a sensitive water supply well, and a 50 foot minimum horizontal separation between any infiltration BMP and all other water supply wells. Sensitive water supply wells may be identified using existing reliable groundwater maps (e.g. MDH vulnerability maps, Minnesota Department of Natural Resources (MDNR) County Atlases), or by applying MDNR groundwater sensitivity criteria (see page 8). The adjacent image shows a screen shot of the sensitivity map from the Chisago County Geologic Atlas.
• Applying the guidance found at this page, determine an appropriate separation distance based on a mounding analysis. We recommend adding an additional safety factor if runoff delivered to the infiltration practice is located in a stormwater hotspot or has the potential to contain mobile organic chemicals or pathogens. An example mounding calculation using a United States Geological Survey calculator is show here. Guidance on mounding beneath infiltration systems is located here.
• Calculate a travel time. This can be done if there are sufficient data to apply Darcy's equation V= KI/n, where V is the groundwater velocity, K is the horizontal hydraulic conductivity , I is the horizontal hydraulic gradient, and n is the effective porosity. Values for hydraulic conductivity can be found here. Porosities for soil can be found here. Porosities for consolidated geologic materials can be found at the following references - [1], [2], [3]. As an example, assume a coarse sand aquifer with a saturated hydraulic conductivity of 5 * 10^-5 meters per second, a hydraulic gradient of 0.01 m/m, and a porosity of 0.40. The one year travel time is (0.00005 * 0.01) / 0.4 = 39.4 meters per year or 130 feet per year. The setback distance for an infiltration system should therefore be a minimum of 130 feet, though it may be beneficial to build in a safety factor to account for potential mounding effects and uncertainty in the conductivity, hydraulic gradient, or porosity. Hydraulic gradients can be determined if regional groundwater elevations are known, such as reported in County Geologic Atlases. This reference provides basic information regarding calculating travel times in aquifers.

Building or other structural foundations

A horizontal separation of 10 feet is HIGHLY RECOMMENDED between any infiltration BMP and a structural foundation. Designers are encouraged to model the groundwater flow in the area to ensure the infiltration BMP will not impact any surrounding structural foundations.

Surface waters

Infiltration BMPs are not recommended if the infiltrating water contains a soluble pollutant that is likely to be transported via groundwater to a surface water impaired for that pollutant (e.g. chloride). Modeling may be useful for predicting the likelihood that infiltrated water will be discharged to a nearby surface water. There may also be local zoning or shoreline restrictions. Prior to the designing of any infiltration BMP that may impact surface waters, the user should consult the local city or county zoning office and watershed management organization, or the Soil and Water Conservation District.

Septic system tank/leach field

A 35 foot horizontal separation is recommended between any infiltration BMP and a septic tank or leach field.

Potential contamination sources

The CGP PROHIBITS infiltration when the infiltration system will be constructed in areas where high levels of contaminants in the soil or groundwater will be mobilized by the infiltrating water. At all other sites not regulated by the CGP, infiltration is not generally recommended in areas of contaminated soils for the same reason that the infiltrating water may mobilize the contamination. Stormwater infiltration may be feasible, however, on sites where the infiltration BMP can be isolated from the contaminants.

It is HIGHLY RECOMMENDED that stormwater infiltration BMPs not be installed in areas with contaminated groundwater down-gradient of the BMP, unless barriers or other devices that maintain a separation between the water infiltrating from the BMP and the contaminants are present. More information regarding stormwater management in contaminated areas can be found here.

Site topography and slopes

Unless slope stability analyses demonstrate otherwise, it is HIGHLY RECOMMENDED that infiltration practices be located a minimum horizontal distance of 200 feet up-gradient from the toe of a slope that is greater than 20 percent, and that the slopes in contributing drainage areas be limited to 15 percent. Guidance for managing infiltration and runoff to minimize the likelihood of slope erosion and slumping has been developed by Toronto and Region Conservation and The University of Wisconsin Sea Grant Institute.

Methods for determining separation distances

Horizontal separation distances can be determined using simple measurement devices such as a GPS, survey, or even something as basic as a tape measure or measuring wheel. Distances should be measured from the edge of the BMP to the edge of the area of interest.

Vertical separation distances require subsurface investigations. Information on determining the vertical distance to bedrock or groundwater can be found here.

Summary of separation distances

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 [4]
	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

Additional references

• Maine guidance on separation distance from infiltration practices
• Issues of Concern Related to Underground Infi ltration Systems for Stormwater Management & Treatment - see page 10

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