Minnesota Pollution Control Agency Logo

Minnesota Stormwater Manual

Difference between revisions of "Soils with low infiltration capacity"

m
 
(72 intermediate revisions by 3 users not shown)
Line 1: Line 1:
−
[[File:Minnesota dominant soil orders.jpg|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Statewide map of soil suborders. While soil infiltration rates vary widely within a suborder, infiltration rates tend to be lower on mollisols, histisols and vertisols compared to entisols, inceptisols, spodosols, and alfisols. Source: [http://www.extension.umn.edu/distribution/cropsystems/dc2331.html]</font size>]]
+
[[File:Technical information page image.png|right|100px|alt=image]]
 +
[[File:Pdf image.png|100px|thumb|left|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Soils_with_low_infiltration_capacity_-_Minnesota_Stormwater_Manual.pdf Download pdf]</font size>]]
 +
[[File:Minnesota dominant soil suborders.jpg|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Statewide map of soil suborders. While soil infiltration rates vary widely within a soil order or suborder, infiltration rates tend to be lower on mollisols, histisols and vertisols compared to entisols, inceptisols, spodosols, and alfisols. Source: [https://extension.umn.edu/soil-management-and-health/soil-orders-and-suborders-minnesota University of Minnesota]</font size>]]
  
−
[[File:Minnesota soil survey status.png|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Statewide map illustrating the availability of digital soil surveys. Cook, Koochiching, portions of Pine, and portions of Crow Wing counties are the only remaining areas in Minnesota lacking digital soil surveys. Source: [http://www.bwsr.state.mn.us/maps/Website/Technical%20Services/Soils%20Hydrology%20Vegetaion/Soils_Survey_Status.pdf]</font size>]]
+
{{alert|The [http://stormwater.pca.state.mn.us/index.php/Construction_stormwater_permit Construction General Permit] (CGP) prohibits infiltration when an infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils.|alert-danger}}
  
−
Soils with low infiltration capacity (defined as soils with infiltration rates less than 0.2 inches per hour) are found throughout the state. Details of where to find soils that can and cannot be used for infiltration systems should begin with available county soil surveys, most of which are available digitally. However, these surveys are not accurate enough to determine site specific characteristics suitable for infiltration systems, so a detailed site analysis is recommended. Stormwater management limitations in areas with tight soils generally preclude large-scale infiltration and ground water recharge (infiltration that passes into the ground water system). These soils will typically be categorized under Hydrologic Soil Group (HSG) D and have other characteristics as shown in Table 13.3. The infiltration rates noted in Table 13.3 are conservative estimates of long-term, sustainable infiltration rates that have been documented in Minnesota. They are based on in-situ measurement within existing infiltration practices in Minnesota, rather than national numbers or rates based on laboratory columns.
+
Soils with low infiltration capacity (tight soils) are defined as soils with steady-state infiltration rates equal to or less than 0.06 inches per hour. [https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm County soil surveys] are useful for initial screening to identify soils that may have low infiltration rates. Most county soil surveys are available digitally from the NRCS (National Resources Conservation Service). These surveys are not accurate enough to determine site-specific characteristics suitable for infiltration systems but are useful for initial screening. If there is a potential for slowly infiltrating soils to exist on a site, a detailed site analysis should be performed for all proposed infiltration BMPs ([[References_for_stormwater_infiltration#S|Susilo, 2009)]]. Additional information on soils can be found [http://stormwater.pca.state.mn.us/index.php/Soils_and_geology here].
  
−
==Overview on soils with low infiltration capacity==
+
Stormwater management limitations in areas with tight soils generally preclude large-scale infiltration and ground water recharge (infiltration that passes into the ground water system). These soils will typically be categorized under [[Glossary#H|Hydrologic Soil Group]] (HSG) D. The table below provides a general summary of infiltration rates for different soils. These are conservative estimates of long-term, sustainable infiltration rates that have been documented in Minnesota. They are based on in-situ measurement within existing infiltration practices in Minnesota, rather than national numbers or rates based on laboratory columns.
  
−
==Stormwater concerns on soils with low infiltration capacity==
+
The [http://stormwater.pca.state.mn.us/index.php/Construction_stormwater_permit Construction General Permit] (CGP) prohibits infiltration when an infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils.
−
Sites with poorly infiltrating soils (defined as soils with infiltration rates less than 0.2 inches per hour) limit the number of practices that can used for stormwater management on a site or specific area of a site. Certain watershed organizations in Minnesota do not allow the use (or strongly discourage the use) of infiltration practices where soil infiltration capacity is low. This does not mean, however, that these tight soils don’t have any infiltration and recharge capabilities. So it may be possible for sites to meet recharge objectives as long as appropriate design modifications have been incorporated.
 
  
−
==General stormwater management guidelines for sites with low infiltration capacity soils==
+
{{:Design infiltration rates}}
−
*Local soil surveys should be used for preliminary determination of infiltration capacity of site soils; however, on-site soil testing is recommended to accurately characterize site soils if local surveys characterize site soils as either HSG C or D.
+
 
−
*Recharge criteria, if applicable, can still be met using infiltration practices or modified filter designs (Figures 13.6 and 13.7), as long as they are appropriately designed.
+
==Why are soils with a low infiltration capacity a concern?==
−
*Soil compost amendments may be required to increase pervious area storage and filtration rates for sites with HSG C and D soils that are expected to receive either rooftop or surface IC disconnection in accordance with certain stormwater credits (see Chapter 11 for credits discussion, specifically Impervious Cover Disconnection and Rooftop Disconnection).  
+
Sites with low infiltration capacity soils may limit the type, location, number and/or sizing of infiltration BMPs that can be used for stormwater management. Low infiltration rates result in extended surface ponding of water, which may damage vegetation, lead to mosquito breeding, damage soil structure, and reduce pollutant treatment by the BMP. Certain watershed organizations in Minnesota do not allow the use, or strongly discourage the use, of infiltration BMPs where soil infiltration capacity is low. This does not mean, however, that these soils do not have any infiltration and recharge capabilities. It may be possible for sites to partially or fully meet infiltration objectives as long as appropriate design modifications have been incorporated, such as [http://stormwater.pca.state.mn.us/index.php/Turf#Turf_establishment_or_incorporation_in_soil_as_an_amendment amending the soil with compost] or sand, or incorporating an [[Glossary#U|underdrain]] into the practice.
−
*Where volume reduction is a primary objective for a site (e.g., potentially a receiving water-based goal due to channel erosion, nuisance flooding, or inadequate infrastructure capacity), emphasis should be placed on practices that promote runoff reuse and evapotranspiration such as cisterns, rain barrels, greenroofs, rain gardens, evaporative systems, and bioretention.
+
 
−
Table 1 provides an overview of low infiltration capacity soil related design considerations for several structural practice groups.
+
The following table provides an overview of design considerations for several groups of structural practices.
 +
 
 +
[[File:Modified sand filter design.png|thumb|300px|alt=schematic of a modified sand filter design|<font size=3>Modified media (sand) filter design</font size>]]
 +
 
 +
[[File:Bioretention with infiltration gallery.png|thumb|300px|alt=schematic of a bioretention system with infiltration gallery|<font size=3>Bioretention system with infiltration gallery</font size>.]]
  
 
{{:BMP use in soil with low infiltration capacity}}
 
{{:BMP use in soil with low infiltration capacity}}
  
−
==Investigation for Low Infiltration Capacity Soils==
+
==How to investigate soils with low infiltration capacity==
−
Soil testing is recommended for all proposed stormwater facilities that plan to have a recharge or infiltration component to their design. Testing can be less rigorous than that for karst areas or sites with shallow bedrock and soils. The purpose of the testing is to identify and confirm the soil characteristics and determine their suitability, if any, for infiltration practices. The guidelines outlined below should not be interpreted as all-inclusive. The design of any subsurface investigation should reflect the size and complexity of the proposed project.
+
Soil tests to determine infiltration capacity of soil should be performed at all proposed stormwater facilities that plan to have a recharge or infiltration component to their design. The purpose of the testing is to identify and confirm the soil characteristics and determine suitability, if any, for infiltration BMPs. Guidelines for investigating all potential physical constraints to infiltration on a site are presented [http://stormwater.pca.state.mn.us/index.php/Procedures_for_investigating_sites_with_potential_constraints_on_stormwater_infiltration in a table at this link]. These guidelines should not be interpreted as all-inclusive. The size and complexity of the project will drive the extent of any subsurface investigation.
  
−
===Location of Borings===
+
===Subsurface material investigation===
−
Borings should be located in order to provide representative area coverage of the of the proposed BMP facilities. The location of borings should be:
+
Soil testing is recommended for all proposed stormwater facilities that plan to have a recharge or infiltration component to their design. Testing can be less rigorous than that for karst areas or sites with shallow bedrock and groundwater. The investigation is designed to identify and confirm the soil characteristics and determine their suitability, if any, for infiltration practices.
−
*In each geologic unit present, as mapped by the Minnesota (MGS) and U.S. Geological Surveys (USGS) and local county records;
 
−
*Near the edges and center of the proposed practice and spaced at equal distances from one another; and
 
−
*Near any areas identified as anomalies from any existing geophysical studies.
 
  
−
===Number of Borings===
+
===Location of soil borings===
−
The number of recommended borings are:
+
Borings should be located in order to provide representative area coverage of the proposed BMP facilities. The location of borings should be:
−
*Infiltration trenches, bioretention, and filters - a minimum of two per practice.
+
*within each distinct major soil type present, as mapped in [https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm soil surveys];
−
*Ponds/wetlands - a minimum of three per practice, or three per acre, whichever is greater.
+
*near the edges and center of the proposed practice and spaced at equal distances from one another; and
−
*Additional borings - to define lateral extent of limiting horizons, or site specific conditions, where applicable.
+
*near any areas identified as anomalies from any existing geophysical studies.
  
−
===Depth of Borings===
+
===Number of soil borings===
−
Borings should be extended to a minimum depth of five feet below the lowest proposed grade within the practice unless auger/backhoe refusal is encountered.
+
The number of recommended borings is described below.
 +
*Infiltration trenches, bioretention, and filters - a minimum of 2 per practice. Note that more borings are recommended for infiltration BMPs greater than 5000 square feet in area. See [http://stormwater.pca.state.mn.us/index.php/Recommended_number_of_soil_boring,_pits,_and_permeameter_tests_for_bioretention_design here] for recommendations on number of borings for infiltration BMPs as a function of BMP size.
 +
*Ponds/wetlands - a minimum of 3 per practice, or 3 per acre, whichever is greater.
 +
*Additional borings – as needed to define lateral extent of limiting horizons, or site specific conditions, where applicable.
  
−
===Identification of Material===
+
===Depth of soil borings===
 +
Borings should be extended to a minimum depth of 5 feet below the lowest proposed grade within the practice unless auger/backhoe refusal is encountered.
 +
 
 +
===Identification of material===
 
All material penetrated by the boring should be identified, as follows:
 
All material penetrated by the boring should be identified, as follows:
−
*Description, logging, and sampling for the entire depth of the boring.
+
*Provide descriptions, logging, and sampling for the entire depth of the boring.
−
*Any stains, odors, or other indications of environmental degradation.
+
*Note any stains, odors, or other indications of environmental degradation.
−
*A minimum laboratory analysis of two soil samples, representative of the material penetrated including potential limiting horizons, with the results compared to the field descriptions.
+
*Perform a laboratory analysis of a minimum of 2 soil samples, representative of the material penetrated including potential limiting horizons, with the results compared to the field descriptions.
−
*Identified characteristics should include, as a minimum: color; mineral composition; grain size, shape, and sorting; and saturation.  
+
*Identify soil characteristic including, at a minimum: color; mineral composition; grain size, shape, and sorting; and saturation.
−
*Any indications of water saturation should be carefully logged, to include both perched and ground water table levels, and descriptions of soils that are mottled or gleyed should be provided.  
+
*Log any indications of water saturation to include both perched and ground water table levels, and descriptions of soils that are mottled or gleyed (sticky clay soils typically found in waterlogged soils).
−
*Water levels in all borings should be taken at the time of completion and again 24 hours after completion. The boring should remain fully open to total depth of these measurements.
+
*Measure water levels in all borings at the time of completion and again 24 hours after completion. The boring should remain fully open to total depth of these measurements.
 +
 
 +
===References for conducting geotechnical investigations===
 +
{{alert|A section providing information on soil borings is being developed for the Manual and should be available in early 2016|alert-info}}
 +
 
 +
The following references provide useful information for conducting geotechnical investigations. Note that some of these documents were written for investigations at contaminated sites.
 +
*[https://www.fhwa.dot.gov/engineering/geotech/pubs/reviewguide/checklist.pdf CHECKLIST AND GUIDELINES FOR REVIEW OF GEOTECHNICAL REPORTS AND PRELIMINARY PLANS AND SPECIFICATIONS]
 +
*[https://www.publications.usace.army.mil/portals/76/publications/engineermanuals/em_1110-1-1804.pdf Geotechnical Investigations]
 +
*[https://dtsc.ca.gov/wp-content/uploads/sites/31/2018/09/Guidelines_for_Planning_and_Implementing_GW_Characterization_of-Contam_Sites.pdf GUIDELINES FOR PLANNING AND IMPLEMENTING GROUNDWATER CHARACTERIZATION OF CONTAMINATED SITES]
 +
*[https://seawi.org/images/downloads/2017_Spring_Conference_Presentations/walters_how_to_read_a_geotech_rpt.pdf How to Read and Understand a Geotechnical Report]
 +
 
 +
===Infiltration rate testing===
 +
Soil permeability should be determined in the field using the [https://stormwater.pca.state.mn.us/index.php?title=Determining_soil_infiltration_rates#Soil_property_determination_for_infiltration_SCM_design procedures described here].
 +
<!--
 +
*Install casing (solid 6-inch diameter) to 36 inches below proposed BMP bottom.
 +
*Remove any smeared soiled surfaces and provide a natural soil interface into which water may percolate. Remove all loose material from the casing. Upon the tester’s discretion, a 2 inch layer of coarse sand or fine gravel may be placed to protect the bottom from scouring. Fill casing with clean water to a depth of 36 inches and allow to pre-soak for up to 24 hours.
 +
*Refill casing with another 36 inches of clean water and monitor water level (measured drop from the top of the casing) for 1 hour. Repeat this procedure (filling the casing each time) 3 additional times, for a total of 4 observations. Upon the tester’s discretion, the final field rate may either be the average of the 4 observations, or the value of the last observation. The final rate should be reported in inches per hour.
 +
*May be done through a boring or open excavation that is protected from access by the public.
 +
*The location of the test should correspond to the BMP location.
 +
Upon completion of the testing, the casings should be immediately pulled, and the test pit should be back-filled.-->
 +
 
 +
==What are general stormwater management guidelines for sites with low infiltration capacity soils?==
 +
*Local soil surveys should be used for preliminary determination of infiltration capacity of the soils on site; if the soil survey suggests soils with very low infiltration capacities, then alternative BMPs such as filters, wet sedimentation basins, etc. should be considered.  If the survey suggests the potential for infiltration, the  on-site soil testing should be done to accurately characterize site soils.  The testing should be conducted in the most restrictive layer of soil that is found within 5-feet below the bottom of the proposed BMP.
 +
*[http://stormwater.pca.state.mn.us/index.php/Turf#Turf_establishment_or_incorporation_in_soil_as_an_amendment Soil compost amendments] should be considered for lawns and other pervious surfaces to increase pervious area storage and/or decrease pervious surface runoff. Designers should also consider disconnection of impervious surfaces, by draining rooftops and other impervious surface runoff to compost amended pervious surfaces before collection and discharge into a structural BMP.
 +
*Where volume reduction is a primary objective for a site (e.g., required by permit, potentially a receiving water-based goal due to channel erosion, nuisance flooding, or inadequate infrastructure capacity), emphasis should be placed on practices that promote [http://stormwater.pca.state.mn.us/index.php/Stormwater_re-use_and_rainwater_harvesting runoff reuse] and evapotranspiration such as cisterns, rain barrels, [[Green roofs|green roofs]], and [[Bioretention|biofiltration]] in order to maximize volume reduction.
 +
*A [http://stormwater.pca.state.mn.us/index.php/Stormwater_infiltration_and_groundwater_mounding mounding analysis] should be conducted to ensure that any groundwater mound that develops under a BMP will not extend into the BMP. This mounding analysis is especially important for soils with low permeability since such soils cannot efficiently dissipate groundwater through the soil column.
  
−
===Infiltration Rate Testing===
+
==Related pages==
−
Soil permeabilities should be determined in the field using the following procedure (MDE, 2000), or an accepted alternative method.
+
*[[Overview of stormwater infiltration]]
−
*Install casing (solid 6-inch diameter) to 36” below proposed practice bottom.
+
*[[Pre-treatment considerations for stormwater infiltration]]
−
*Remove any smeared soiled surfaces and provide a natural soil interface into which water may percolate. Remove all loose material from the casing. Upon the tester’s discretion, a two-inch layer of coarse sand or fine gravel may be placed to protect the bottom from scouring. Fill casing with clean water to a depth of 36” and allow to pre-soak for up to twenty-four hours.
+
*[[BMPs for stormwater infiltration]]
−
*Refill casing with another 36” of clean water and monitor water level (measured drop from the top of the casing) for one hour. Repeat this procedure (filling the casing each time) three additional times, for a total of four observations. Upon the tester’s discretion, the final field rate may either be the average of the four observations, or the value of the last observation. The final rate should be reported in inches per hour.
+
*[[Pollutant fate and transport in stormwater infiltration systems]]
−
*May be done through a boring or open excavation.
+
*[[Surface water and groundwater quality impacts from stormwater infiltration]]
−
*The location of the test should correspond to the practice location.
+
*[[Stormwater infiltration and groundwater mounding]]
−
*Upon completion of the testing, the casings should be immediately pulled, and the test pit should be back-filled.
+
*[[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 infiltration and contaminated soils and groundwater]]
 +
*[[Decision tools for stormwater infiltration]]
 +
*[[References for stormwater infiltration]]
  
−
[[category:Stormwater articles 2]]
+
<noinclude>
 +
[[Category:Level 2 - Technical and specific topic information/infiltration]]
 +
[[Category:Level 2 - Technical and specific topic information/soils and media]]
 +
</noinclude>

Latest revision as of 13:24, 15 February 2023

image
pdf image
Download pdf
statewide map illustrating karst areas
Statewide map of soil suborders. While soil infiltration rates vary widely within a soil order or suborder, infiltration rates tend to be lower on mollisols, histisols and vertisols compared to entisols, inceptisols, spodosols, and alfisols. Source: University of Minnesota
Warning: The Construction General Permit (CGP) prohibits infiltration when an infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils.

Soils with low infiltration capacity (tight soils) are defined as soils with steady-state infiltration rates equal to or less than 0.06 inches per hour. County soil surveys are useful for initial screening to identify soils that may have low infiltration rates. Most county soil surveys are available digitally from the NRCS (National Resources Conservation Service). These surveys are not accurate enough to determine site-specific characteristics suitable for infiltration systems but are useful for initial screening. If there is a potential for slowly infiltrating soils to exist on a site, a detailed site analysis should be performed for all proposed infiltration BMPs (Susilo, 2009). Additional information on soils can be found here.

Stormwater management limitations in areas with tight soils generally preclude large-scale infiltration and ground water recharge (infiltration that passes into the ground water system). These soils will typically be categorized under Hydrologic Soil Group (HSG) D. The table below provides a general summary of infiltration rates for different soils. These are conservative estimates of long-term, sustainable infiltration rates that have been documented in Minnesota. They are based on in-situ measurement within existing infiltration practices in Minnesota, rather than national numbers or rates based on laboratory columns.

The Construction General Permit (CGP) prohibits infiltration when an infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils.


Caution: The table for design infiltration rates has been modified. Field testing is recommended for gravelly soils (HSG A; GW and GP soils; gravel and sandy gravel soils). If field-measured soil infiltration rates exceed 8.3 inches per hour, the Construction Stormwater permit requires the soils be amended. Guidance on amending these soils can be found here.

Design infiltration rates, in inches per hour, for A, B, C, and D soil groups. Corresponding USDA soil classification and Unified soil Classifications are included. Note that A and B soils have two infiltration rates that are a function of soil texture.*
The values shown in this table are for uncompacted soils. This table can be used as a guide to determine if a soil is compacted. For information on alleviating compacted soils, link here. If a soil is compacted, reduce the soil infiltration rate by one level (e.g. for a compacted B(SM) use the infiltration rate for a B(MH) soil).
Link to this table

Hydrologic soil group Infiltration rate (inches/hour) Infiltration rate (centimeters/hour) Soil textures Corresponding Unified Soil ClassificationSuperscript text
A
 Although a value of 1.63 inches per hour (4.14 centimeters per hour) may be used, it is Highly recommended that you conduct field infiltration tests or amend soils.b See Guidance for amending soils with rapid or high infiltration rates and Determining soil infiltration rates.

gravel
sandy gravel

GW - Well-graded gravels, fine to coarse gravel
GP - Poorly graded gravel
1.63a 4.14

silty gravels
gravelly sands
sand

GM - Silty gravel
SW - Well-graded sand, fine to coarse sand
0.8 2.03

sand
loamy sand
sandy loam

SP - Poorly graded sand

B
 0.45 1.14 silty sands SM - Silty sand
0.3 0.76 loam, silt loam MH - Elastic silt
C
 0.2 0.51 Sandy clay loam, silts ML - Silt
D
 0.06 0.15

clay loam
silty clay loam
sandy clay
silty clay
clay

GC - Clayey gravel
SC - Clayey sand
CL - Lean clay
OL - Organic silt
CH - Fat clay

OH - Organic clay, organic silt

1For Unified Soil Classification, we show the basic text for each soil type. For more detailed descriptions, see the following links: The Unified Soil Classification System, CALIFORNIA DEPARTMENT OF TRANSPORTATION (CALTRANS) UNIFIED SOIL CLASSIFICATION SYSTEM

  • NOTE that this table has been updated from Version 2.X of the Minnesota Stormwater Manual. The higher infiltration rate for B soils was decreased from 0.6 inches per hour to 0.45 inches per hour and a value of 0.06 is used for D soils (instead of < 0.2 in/hr).

Source: Thirty guidance manuals and many other stormwater references were reviewed to compile recommended infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration rates: (1) Rawls, Brakensiek and Saxton (1982); (2) Rawls, Gimenez and Grossman (1998); (3) Bouwer and Rice (1984); and (4) Urban Hydrology for Small Watersheds (NRCS). SWWD, 2005, provides field documented data that supports the proposed infiltration rates. (view reference list)
aThis rate is consistent with the infiltration rate provided for the lower end of the Hydrologic Soil Group A soils in the Stormwater post-construction technical standards, Wisconsin Department of Natural Resources Conservation Practice Standards.
bThe infiltration rates in this table are recommended values for sizing stormwater practices based on information collected from soil borings or pits. A group of technical experts developed the table for the original Minnesota Stormwater Manual in 2005. Additional technical review resulted in an update to the table in 2011. Over the past 5 to 7 years, several government agencies revised or developed guidance for designing infiltration practices. Several states now require or strongly recommend field infiltration tests. Examples include North Carolina, New York, Georgia, and the City of Philadelphia. The states of Washington and Maine strongly recommend field testing for infiltration rates, but both states allow grain size analyses in the determination of infiltration rates. The Minnesota Stormwater Manual strongly recommends field testing for infiltration rate, but allows information from soil borings or pits to be used in determining infiltration rate. A literature review suggests the values in the design infiltration rate table are not appropriate for soils with very high infiltration rates. This includes gravels, sandy gravels, and uniformly graded sands. Infiltration rates for these geologic materials are higher than indicated in the table.
References: Clapp, R. B., and George M. Hornberger. 1978. Empirical equations for some soil hydraulic properties. Water Resources Research. 14:4:601–604; Moynihan, K., and Vasconcelos, J. 2014. SWMM Modeling of a Rural Watershed in the Lower Coastal Plains of the United States. Journal of Water Management Modeling. C372; Rawls, W.J., D. Gimenez, and R. Grossman. 1998. Use of soil texture, bulk density and slope of the water retention curve to predict saturated hydraulic conductivity Transactions of the ASAE. VOL. 41(4): 983-988; Saxton, K.E., and W. J. Rawls. 2005. Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions. Soil Science Society of America Journal. 70:5:1569-1578.



Why are soils with a low infiltration capacity a concern?

Sites with low infiltration capacity soils may limit the type, location, number and/or sizing of infiltration BMPs that can be used for stormwater management. Low infiltration rates result in extended surface ponding of water, which may damage vegetation, lead to mosquito breeding, damage soil structure, and reduce pollutant treatment by the BMP. Certain watershed organizations in Minnesota do not allow the use, or strongly discourage the use, of infiltration BMPs where soil infiltration capacity is low. This does not mean, however, that these soils do not have any infiltration and recharge capabilities. It may be possible for sites to partially or fully meet infiltration objectives as long as appropriate design modifications have been incorporated, such as amending the soil with compost or sand, or incorporating an underdrain into the practice.

The following table provides an overview of design considerations for several groups of structural practices.

schematic of a modified sand filter design
Modified media (sand) filter design
schematic of a bioretention system with infiltration gallery
Bioretention system with infiltration gallery.

This table shows structural BMP use in soil with low infiltration capacity
Link to this table

BMP Low infiltration capacity soil considerations
Bioretention, dry swale, permeable pavement, tree trench/box Should be constructed with an underdrain. Recharge criteria, if applicable, can be met by modifying the design to include an infiltration gallery below the underdrain, so long as it is appropriately sized.
Media filter Recommended practice in tight soils. Some design variants can be modified to incorporate an infiltration gallery that can help meet recharge criteria, if properly sized.
Infiltration trench or basin
  • Not recommended as a practice
  • Soils analysis should be conducted to confirm limiting aspects of soil profile
Stormwater ponds Acceptable practice with tight soils. Soils should help maintain permanent pool.
Constructed wetlands
  • Acceptable practice with tight soils. Soils should help maintain permanent pool if practice is not tied into groundwater table.
  • Compost amendments may be necessary to establish suitable planting beds


How to investigate soils with low infiltration capacity

Soil tests to determine infiltration capacity of soil should be performed at all proposed stormwater facilities that plan to have a recharge or infiltration component to their design. The purpose of the testing is to identify and confirm the soil characteristics and determine suitability, if any, for infiltration BMPs. Guidelines for investigating all potential physical constraints to infiltration on a site are presented in a table at this link. These guidelines should not be interpreted as all-inclusive. The size and complexity of the project will drive the extent of any subsurface investigation.

Subsurface material investigation

Soil testing is recommended for all proposed stormwater facilities that plan to have a recharge or infiltration component to their design. Testing can be less rigorous than that for karst areas or sites with shallow bedrock and groundwater. The investigation is designed to identify and confirm the soil characteristics and determine their suitability, if any, for infiltration practices.

Location of soil borings

Borings should be located in order to provide representative area coverage of the proposed BMP facilities. The location of borings should be:

  • within each distinct major soil type present, as mapped in soil surveys;
  • near the edges and center of the proposed practice and spaced at equal distances from one another; and
  • near any areas identified as anomalies from any existing geophysical studies.

Number of soil borings

The number of recommended borings is described below.

  • Infiltration trenches, bioretention, and filters - a minimum of 2 per practice. Note that more borings are recommended for infiltration BMPs greater than 5000 square feet in area. See here for recommendations on number of borings for infiltration BMPs as a function of BMP size.
  • Ponds/wetlands - a minimum of 3 per practice, or 3 per acre, whichever is greater.
  • Additional borings – as needed to define lateral extent of limiting horizons, or site specific conditions, where applicable.

Depth of soil borings

Borings should be extended to a minimum depth of 5 feet below the lowest proposed grade within the practice unless auger/backhoe refusal is encountered.

Identification of material

All material penetrated by the boring should be identified, as follows:

  • Provide descriptions, logging, and sampling for the entire depth of the boring.
  • Note any stains, odors, or other indications of environmental degradation.
  • Perform a laboratory analysis of a minimum of 2 soil samples, representative of the material penetrated including potential limiting horizons, with the results compared to the field descriptions.
  • Identify soil characteristic including, at a minimum: color; mineral composition; grain size, shape, and sorting; and saturation.
  • Log any indications of water saturation to include both perched and ground water table levels, and descriptions of soils that are mottled or gleyed (sticky clay soils typically found in waterlogged soils).
  • Measure water levels in all borings at the time of completion and again 24 hours after completion. The boring should remain fully open to total depth of these measurements.

References for conducting geotechnical investigations

Information: A section providing information on soil borings is being developed for the Manual and should be available in early 2016

The following references provide useful information for conducting geotechnical investigations. Note that some of these documents were written for investigations at contaminated sites.

Infiltration rate testing

Soil permeability should be determined in the field using the procedures described here.

What are general stormwater management guidelines for sites with low infiltration capacity soils?

  • Local soil surveys should be used for preliminary determination of infiltration capacity of the soils on site; if the soil survey suggests soils with very low infiltration capacities, then alternative BMPs such as filters, wet sedimentation basins, etc. should be considered. If the survey suggests the potential for infiltration, the on-site soil testing should be done to accurately characterize site soils. The testing should be conducted in the most restrictive layer of soil that is found within 5-feet below the bottom of the proposed BMP.
  • Soil compost amendments should be considered for lawns and other pervious surfaces to increase pervious area storage and/or decrease pervious surface runoff. Designers should also consider disconnection of impervious surfaces, by draining rooftops and other impervious surface runoff to compost amended pervious surfaces before collection and discharge into a structural BMP.
  • Where volume reduction is a primary objective for a site (e.g., required by permit, potentially a receiving water-based goal due to channel erosion, nuisance flooding, or inadequate infrastructure capacity), emphasis should be placed on practices that promote runoff reuse and evapotranspiration such as cisterns, rain barrels, green roofs, and biofiltration in order to maximize volume reduction.
  • A mounding analysis should be conducted to ensure that any groundwater mound that develops under a BMP will not extend into the BMP. This mounding analysis is especially important for soils with low permeability since such soils cannot efficiently dissipate groundwater through the soil column.

Related pages

Retrieved from "https://stormwater.pca.state.mn.us/index.php?title=Soils_with_low_infiltration_capacity&oldid=67073"

This page was last edited on 15 February 2023, at 13:24.