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.
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|
|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.||
|GW - Well-graded gravels, fine to coarse gravel
GP - Poorly graded gravel
GM - Silty gravel
SP - Poorly graded sand
|0.45||1.14||silty sands||SM - Silty sand|
|0.3||0.76||loam, silt loam||MH - Elastic silt|
|0.2||0.51||Sandy clay loam, silts||ML - Silt|
GC - Clayey gravel
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
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.
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.
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||
|Stormwater ponds||Acceptable practice with tight soils. Soils should help maintain permanent pool.|
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.
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.
Borings should be located in order to provide representative area coverage of the proposed BMP facilities. The location of borings should be:
The number of recommended borings is described below.
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.
All material penetrated by the boring should be identified, as follows:
The following references provide useful information for conducting geotechnical investigations. Note that some of these documents were written for investigations at contaminated sites.
Soil permeability should be determined in the field using the procedures described here.