Shallow soils and shallow depth to bedrock

Bedrock Outcroppings Areas in Northern Minnesota (Source: MN DNR, with permission. Map is from the Minnesota DNR Lands and Minerals Division and depth to bedrock grid data is from the Minnesota Geological Survey)

Sites with shallow bedrock are defined as having bedrock within 6 feet or less of the ground surface. Shallow bedrock is found in many portions of the state, but is a particular problem in the northeastern region. When installing an infiltration Best Management Practice (BMP), there must be at least 3 feet of separation between the base of the BMP and the bedrock per the Minnesota Construction General Permit (CGP). Bedrock at the 6 foot depth is a trigger to perform a geotechnical investigation to determine the location of the bedrock in the area in and around the proposed BMP to ensure the 3 foot separation can be achieved.

Why is shallow depth to bedrock a concern?

Shallow bedrock limits the depth of BMPs, reduces the potential for subsurface infiltration, and reduces the depth over which treatment can occur. These sites present challenges to stormwater management; however these challenges can be managed. General guidelines for investigation and management are presented below. Special caution for steep slopes and fractured bedrock is urged.

How to investigate for shallow bedrock

It is important to understand the general depth to bedrock over the entire site, but more specifically it is important to know the depth to bedrock in and around the area of the proposed BMP. Geotechnical investigations are recommended for all proposed stormwater facilities located in regions with shallow bedrock. The purpose of the investigation is to identify subsurface conditions which can pose an environmental concern or a construction hazard to a proposed stormwater management practice. The guidelines for how to investigate for shallow bedrock are summarized below. 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

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.

The investigation is designed to determine the nature and thickness of subsurface materials, including depth to bedrock and to the water table. Subsurface data for depth to groundwater may be acquired by soil boring or backhoe investigation. These field data should be supplemented by geophysical investigation techniques deemed appropriate by a qualified professional, which will show the location of the geologic and groundwater formations under the surface. The data listed below should be acquired under the direct supervision of a qualified geologist, geotechnical engineer, or soil scientist who is experienced in conducting such studies. Pertinent site information should include the following:

• Known groundwater depth or bedrock characteristics (type, geologic contacts, faults, geologic structure, rock surface configuration)
• Soil characteristics (type, thickness, mapped unit)
• Bedrock outcrop areas

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;
• next to bedrock outcrop areas and/or in areas with known shallow groundwater if present;
• 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.
• Estimate soil engineering characteristics, including “N” or estimated unconfined compressive strength, when conducting a standard penetration test (SPT).

Evaluation of findings

At least 1 figure showing the subsurface soil profile cross section through the proposed practice should be provided, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed practice, using the actual or projected boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.

What are general stormwater management guidelines for areas with shallow bedrock?

The following investigations and guidelines are HIGHLY RECOMMENDED for infiltration and other BMPs proposed to be located in areas with shallow depth to bedrock.

• Conduct thorough geotechnical investigations in areas with suspected or documented shallow bedrock. Perform site geotechnical analysis similar to karst.
• Consider a non-infiltration BMP or moving the BMP to a location on site with sufficient depth to bedrock if the required 3-foot separation cannot be achieved. It may be possible to move the infiltration BMP to another location in order to achieve this separation.
• Consider shallow ponding depths up to 12 inches for filters, swales, and bioretention.
• Conclude that infiltration of stormwater runoff from stormwater hotspots is not feasible due to potential for connections with bedrock fracture zones.
• Consider stormwater wetlands which have shallower ponding depths than stormwater ponds. The disadvantage is that the shallow depths result in basins with large footprints which may not be feasible on small sites.

The following table provides an overview of shallow bedrock and soil related design considerations for different structural practice groups. Guidelines for investigating all potential physical constraints to infiltration on a site are presented in the table at the bottom of this page.

Recommendations for structural BMP use in settings with shallow soils and shallow depth to bedrock.
Link to this table

BMP	Shallow soil and shallow depth to bedrock considerations
Bioretention	Should be constructed with an underdrain or liner if minimum separation distance of three (3) feet is not present between practice bottom and bedrock.1
Media filter	Recommended practice in areas of shallow bedrock and soil Can be located in bedrock, but will be expensive due to blasting
Vegetative filter	Recommended practice in areas of shallow bedrock and soil. Dry swales with engineered soil media will need an underdrain if minimum separation distance of three (3) feet is not present between bottom of practice and bedrock
Infiltration trench or basin	Will be limited due to minimum separation requirement. Surface area to depth ratios of practices may need to be larger. Arch pipe and other perforated storage "vault" practices can help increase treatment volumes within limited spaces. If used, should have supporting geotechnical investigations and calculations Use with PSHs should be carefully considered. Pre-treatment should be extensive to limit risk of groundwater contamination if groundwater is close to the land surface. Local review authority should be consulted for approval
Stormwater ponds	Will have depth limitation to consider, making surface areas larger for a given storage volume. Shallower depths may be undesirable from an aesthetic standpoint, particularly if wide fluctuations in water level are expected. Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present
Constructed wetlands	Applied more easily than ponds, but will also require larger surface area to drainage area ratios. Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present

¹A liner is required under the Construction Stormwater General Permit.

Procedures for investigating sites with potential constraints on stormwater infiltration.
Link to this table

Investigation	Shallow groundwater	Shallow bedrock	Soils with low infiltration capacity	Karst
Preliminary site investigation	NA	NA	NA	The level of detail required will depend on the likelihood that karst is present and any local regulations. The preliminary site investigation should include, but not be limited to (Pennsylvania BMP, 2009): A review of aerial photographs, geological literature, sinkhole maps, previous soil borings, existing well data, and municipal wellhead or aquifer protection plans. A site reconnaissance, including a thorough field examination for features such as limestone pinnacles, sinkholes, closed depressions, fracture traces, faults, springs, and seeps. The site should be observed under varying weather conditions, especially during heavy rains and in different seasons to identify and map any natural drainageways.
Subsurface material investigation	The investigation is designed to determine the depth to seasonally saturated soils. Subsurface data for depth to seasonally saturated soil may be acquired by soil boring or studying existing wells on the site, if present. These field data should be supplemented by geophysical investigation techniques deemed appropriate by a qualified professional, which will show the location of the saturated soil formations under the surface. The data listed below should be acquired under the direct supervision of a qualified geologist, geotechnical engineer, or soil scientist who is experienced in conducting such studies. Pertinent site information should include the following: Known groundwater (water depth) depth Soil characteristics (type, thickness, mapped unit) Bedrock outcrop areas	The investigation is designed to determine the nature and thickness of subsurface materials, including depth to bedrock. Subsurface data for depth to bedrock may be acquired by soil boring or backhoe investigation. These field data should be supplemented by geophysical investigation techniques deemed appropriate by a qualified professional, which will show the location of the bedrock formations under the surface. The data listed below should be acquired under the direct supervision of a qualified geologist, geotechnical engineer, or soil scientist who is experienced in conducting such studies. Pertinent site information should include the following: Known bedrock characteristics (type, geologic contacts, faults, geologic structure, rock surface configuration) Soil characteristics (type, thickness, mapped unit) Bedrock outcrop areas	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.	The investigation should determine the nature and thickness of subsurface materials, including depth to bedrock and the water table. Subsurface data may be acquired by backhoe excavation and/or soil boring. These field data should be supplemented by geophysical investigation techniques deemed appropriate by a qualified professional, which will show the location of karst formations under the surface. This is an iterative process that might need to be repeated until the desired detailed knowledge of the site is obtained and fully understood. The data listed below should be acquired under the direct supervision of a qualified and experienced karst scientist. Pertinent site information to collect includes the following: Bedrock characteristics (ex. type, geologic contacts, faults, geologic structure, rock surface configuration) Depth to the water table and depth to bedrock Type and percent of coarse fragements Soil characteristics (ex. color, type, thickness, mapped unit, geologic source/history) Photo-geologic fracture trace map Bedrock outcrop areas Sinkholes and/or other closed depressions Perennial and/or intermittent streams, and their flow behavior (ex. a stream in a karst area that loses volume could be a good indication of sinkhole infiltration)
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 by the Minnesota (MGS) and U.S. Geological Surveys (USGS) and local county records. Next to bedrock outcrop areas and/or in areas with known shallow groundwater if present. Near the edges and center of the proposed practice and spaced at equal distances from one another. Near any areas identified as anomalies from any existing geophysical studies.	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 by the Minnesota (MGS) and U.S. Geological Surveys (USGS) and local county records. Next to bedrock outcrop areas and/or in areas with known shallow groundwater if present. Near the edges and center of the proposed practice and spaced at equal distances from one another. Near any areas identified as anomalies from any existing geophysical studies.	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 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. Near any areas identified as anomalies from any existing geophysical studies.	The local variability typical of karst areas could mean that a very different subsurface could exist close by, perhaps as little as 6 inches away. To accommodate this variability, the number and type of borings must be carefully assessed. If the goal is to locate a boring down the center of a sinkhole, the previous geophysical tests or excavation results can show the likely single location to achieve that goal. If the goal is to “characterize” the entire site, then an evaluation needs to occur to determine the number and depth needed to adequately represent the site. Again, the analyst must acknowledge the extreme variability and recognize that details can easily be missed. Some general guidance for locating borings include: Getting at least 1 boring in each distinct major soil type present, as mapped by the MGS and USGS and local county records. Placing an adequate number as determined by a site investigation near on-site geologic or geomorphic indications of the presence of sinkholes or related karst features. Locating along photo-geologic fracture traces. Locating adjacent to bedrock outcrop areas. Locating a sufficient number to adequately represent the area under any proposed stormwater facility. Documenting any areas identified as anomalies from any existing geophysical or other subsurface studies.
Number of soil borings	The number of recommended borings is described below. Infiltration trenches, bioretention, and filters - a minimum of 2 per practice. 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.	The number of recommended borings is described below. Infiltration trenches, bioretention, and filters - a minimum of 2 per practice. 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.	The number of recommended borings is described below. Infiltration trenches, bioretention, and filters - a minimum of 2 per practice. 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.	The number and depth of borings will depend entirely upon the results of the subsurface evaluation obtained from the observational, geophysical, and excavation studies, as well as other borings. There are no prescriptive guidelines to determine the number and depth of borings. These will have to be determined by the qualified staff conducting the BMP management evaluation and will be based upon the data needs of the installation. The borings must extend well below the bottom elevation of the designed BMP, however, to make sure that there are no karst features that will be encountered or impacted as a result of the installation.
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.	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.	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.	The number and depth of borings will depend entirely upon the results of the subsurface evaluation obtained from the observational, geophysical, and excavation studies, as well as other borings. There are no prescriptive guidelines to determine the number and depth of borings. These will have to be determined by the qualified staff conducting the BMP management evaluation and will be based upon the data needs of the installation. The borings must extend well below the bottom elevation of the designed BMP, however, to make sure that there are no karst features that will be encountered or impacted as a result of the installation. At least 1 subsurface cross section should be provided for the BMP installation, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed installation, using the actual geophysical and boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.
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. Estimate soil engineering characteristics, including “N” or estimated unconfined compressive strength, when conducting a standard penetration test (SPT).	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. Estimate soil engineering characteristics, including “N” or estimated unconfined compressive strength, when conducting a standard penetration test (SPT).	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.	All material identified by the excavation and geophysical studies and 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 laboratory analysis on a of 2 soil samples, representative of the material penetrated including potential limiting horizons, with the results compared to the field descriptions. Identify soil characteristics including, as a minimum: color; mineral composition; grain size, shape, sorting and degree of 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 should be provided. Be aware that ground water levels in karst can change dramatically in short periods of time and will not necessarily leave mottled or gleyed evidence. Record water levels in all borings over a time-period reflective of anticipated water level fluctuation. That is, water levels in karst geology can vary dramatically and rapidly. The boring should remain fully open to a total depth reflective of these variations and over a time that will accurately show the variation. Be advised that to get a complete picture, this could be a long-term period. Measurements could of course be collected during a period of operation of a BMP, which could be adjusted based on the findings of the data collection. Report an estimation of soil engineering characteristics including “N” or estimated unconfined compressive strength, when conducting a SPT
Evaluation of findings	At least 1 figure showing the subsurface soil profile cross section through the proposed practice should be provided, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed practice, using the actual or projected boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.	At least 1 figure showing the subsurface soil profile cross section through the proposed practice should be provided, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed practice, using the actual or projected boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.	NA	At least 1 figure showing the subsurface soil profile cross section through the proposed practice should be provided, showing confining layers, depth to bedrock, and water table (if encountered). It should extend through a central portion of the proposed practice, using the actual or projected boring data. A sketch map or formal construction plan indicating the location and dimension of the proposed practice and line of cross section should be included for reference, or as a base map for presentation of subsurface data.
Infiltration rate testing	NA	NA	Soil permeability should be determined in the field using the following procedure (MDE, 2000), or an accepted alternative method. 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.	NA
Geophysical and dye techniques	NA	NA	NA	Stormwater managers in need of subsurface geophysical surveys are encouraged to obtain the services of a qualified geophysicist experienced in karst geology. Some of the geophysical techniques available for use in karst terrain include: seismic refraction, ground-penetrating radar, and electric resistivity. The surest way to determine the flow path of water in karst geology is to inject dye into the karst feature (sinkhole or fracture) and watch to see where it emerges, usually from a spring. The emergence of a known dye from a spring grants certainty to a suspicion that ground water moves in a particular pattern. Dye tracing can vary substantially in cost depending upon the local karst complexity, but it can be a reasonably priced alternative, especially when the certainty is needed.

Related pages

• Overview
• Pre-treatment considerations for stormwater infiltration
• BMPs
• Pollutant fate and transport in stormwater infiltration systems
• Surface water and groundwater quality impacts from stormwater infiltration
• Groundwater mounding
• Setback distances
• Karst
• Shallow groundwater
• Soils with low infiltration capacity
• Potential stormwater hotspots
• Wellhead protection
• Contaminated soils and groundwater
• Decision tools
• Research needs
• References for stormwater infiltration