Karst geology makes up approximately 20 percent of the land surface in the United States. It is also found in other parts of the world such as China, Europe, the Caribbean, Australia, and Madagascar (USGS). Karst regions in Minnesota are predominantly found in the southeastern portion of the state. Use of infiltration BMPs in karst regions can be complicated and necessitates additional geotechnical testing, pre-treatment of stormwater runoff, and ponding of runoff. Caution must be used in interpreting the geographic depiction of karst lands as subsurface conditions can change rapidly over very short distances (Karst Working Group, 2009). Generalized maps of active karst will be less accurate than a county-scale map, as demonstrated by the two figures to the right. The following county-level maps have been developed.
In Minnesota there are three classifications of karst lands.
A site with Active Karst has the greatest potential for development of a sinkhole below a BMP and the recommendations contained in this section should be considered for all proposed BMPs in areas with Active Karst. For Transitional Karst sites, it is HIGHLY RECOMMENDED that the nature of the overlying soils be evaluated with respect to the potential for catastrophic failure given the increase in hydrostatic pressure created by a BMP.
Karst is a landscape formed by the dissolution of a layer or layers of soluble bedrock. The bedrock is usually carbonate rock such as limestone or dolomite but the dissolution has also been documented in weathering resistant rock, such as quartz. The dissolution of the rocks occurs due to the reaction of the rock with acidic water. Rainfall is already slightly acidic due to the absorption of carbon dioxide (CO2), and becomes more so as it passes through the subsurface and picks up even more CO2. Cracks and fissures form as the runoff passes through the subsurface and reacts with the rocks. These cracks and fissures grow, creating larger passages, caves, and may even form sinkholes as more and more acidic water infiltrates into the subsurface (American Rivers).
Subterranean drainage through karst geology limits the presence of surface water in places, explaining the absence of rivers and lakes. Many karst regions display distinctive surface features such as a sinkhole or natural pit (often termed cenotes or dolines), fissures, or caves (USGS, 2012). However, distinctive karst surface features may be completely absent where the soluble rock is below a deep layer of glacial debris (termed mantled), or is below one or more layers of non-soluble rock strata. Some karst regions include thousands of caves, although evidence of caves large enough for human exploration is not a required characteristic of karst.
Presence of sinkholes is an absolute indication of active karst. In these cases, an easement or reserve area should be identified on the development plans for the project so that all future landowners know of the presence of active karst on their property.
The following sources provide additional information on karst in Minnesota.
Infiltration BMPs in karst settings have the potential of creating sinkholes as a result of the additional weight of water in a structural BMP (termed hydraulic head) and/or water infiltrated from the BMP that can dissolve the carbonate rock (e.g., limestone). These conditions can lead to the erosion of bedrock underneath or adjacent to a BMP. In addition, the pollutants being carried by the stormwater runoff can pass rapidly through the subsurface into the groundwater, creating a greater risk of groundwater contamination than is found in other soil types.
Where karst conditions exist, there are no prescriptive rules or universally accepted management approaches. When underlying karst is known or suspected to be present at the site, stormwater runoff should not be concentrated and discharged into or near known sinkholes. Instead, the following strategies should be employed.
The Minnesota Department of Natural Resources has a relatively new product, Minnesota Regions Prone to Surface Karst Feature Development: GW-01, that shows karst areas in MN. This product can be used to outline such areas in a GIS environment. The GIS coverage is a superposition of Bedrock Geology and Depth to Bedrock maps prepared by the Minnesota Geological Survey (MGS). This dataset is managed by the Ecological and Water Resources Division, County Geologic Atlas Program. For additional information, visit Springs, Springsheds, and Karst on the Minnesota Department of Natural Resources website.
Developers, communities, public works agents, and other stormwater managers should conduct site investigations prior to designing and implementing stormwater BPMs in both active and transitional karst areas. A site with active karst has the greatest potential for development of a sinkhole below a BMP and the recommendations contained in this section should be considered for all proposed BMPs. For transitional karst sites, it is HIGHLY RECOMMENDED that the nature of the overlaying soils be evaluated with respect to the potential for catastrophic failure given the increase in hydrostatic pressure created by a BMP. The level of investigation required will depend on the likelihood of karst being present and the regulatory requirements in the area.
The purpose of the investigation is to identify subsurface voids, cavities, fractures, or other discontinuities which could pose an environmental concern or a construction hazard to an existing or proposed stormwater management facility. Of special concern are the construction hazards posed by karst geology, the formation of sinkholes, and the possibility of a preferential pathway that would provide a direct route for polluted runoff to enter the regional groundwater system. Because of the complexity inherent to active and transitional karst areas, there is no single set of investigatory guidelines that works for every location. Typically, however, the first step is a preliminary investigation that involves analyzing geological and topographic county maps, and aerial photography to determine if active karst is known to be present. Included in the preliminary investigation should be a site visit to perform a visual observation for karst features such as sinkholes. Results of the investigation should be reported to the appropriate agency, including the Department of Natural Resources (DNR), Minnesota Geological Survey (MGS), and local agencies (such as the city, township or county). These known and discovered karst features should be surveyed for specific location and permanently recorded on the property deed.
If it is determined that active karst is present, a detailed site investigation, including a subsurface materials investigation should be conducted. The design of any geotechnical investigation should reflect the size and complexity of the proposed project, as well as the local knowledge of the threat posed by the karstic geology. The geotechnical investigation should first assess the subsurface heterogeneity (variability). With this information in-hand, borings or observation wells can then be accurately installed to obtain vertical data surrounding or within karst features or within areas of instability that have the potential for development of karst. The vertical data should be used to determine the nature and thickness of the subsurface materials and needs to include information involving depth to the bedrock and depth to the groundwater table. The investigation will be an iterative process and should be expanded until the desired detailed knowledge of the site is obtained and fully understood (Karst Working Group, 2009). 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.
Additional information regarding site investigations in karst areas can be found in Appendix B of the Tennessee Permanent Stormwater Management and Design Guidance Manual (Tennessee Department of Environment and Conservation, 2014). The Tennessee manual provides a flowchart which will guide designers through the investigative process and will help designers determine if any special analysis is required.
The level of detail required for a site investigation will depend on the likelihood that karst is present and on any local regulations. The preliminary site investigation should include, but not be limited to
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.
In conducting subsurface investigations, all applicable State regulations must be met. For more information, see Minnesota Department of Health's Wells and Borings website.
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:
Exploratory borings must comply with Minnesota statutes, 4727.0100
The number and depth of borings depends 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 to ensure 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.
Under Minnesota law all soil borings must be sealed by a licensed well contractor or a licensed well sealing contractor. A property owner may not seal any well or boring. See Minnesota Statutes, Chapter 103I For more information see these Minnesota Department of Health websites.
All material identified by the excavation and geophysical studies and penetrated by the boring should be identified, as follows.
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.
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.
For good basic information on use of dye techniques in karst settings, see the United States EPA document, Application of Dye-tracing techniques for Determining Solute-transport Characteristics of Ground Water in Karst Terranes (1988).
In karst settings there are special considerations and potentially additional constraints needed prior to implementing most structural BMPs. A growing emphasis is being placed on the implementation of strategies that preserve the pre-development hydrology and maintain critical vegetated areas. This is based on the idea that, in a pre-development setting, the runoff was spread across the landscape rather than directed to a certain area, which often results when there is a high concentration of pervious surfaces. When stormwater is concentrated in one area, it can lead to a more rapid dissolution of the underlying rock.
The uncertainty related to the actual presence of karst, the presence of unstable materials that have the potential for development of karst, and the water movement through karst terrain dictates the level of additional field information to be collected before proceeding with BMP design and construction in Active Karst and Transitional Karst classifications. The following guidelines are based on adaptations from a handful of communities (e.g., Carroll County, MD (2004a and b); St. Johns River Water Management District, FL (2010); Jefferson County, WV (Laughland 2003); Tennessee Permanent Stormwater Management and Design Guidance Manual – Appendix B (2014); and other documents (Chesapeake Stormwater Network, Karst Working Group, 2009; West Virginia Stormwater Management & Design Guidance Manual).
The following guidelines do not contain substantial prescriptive information because of the variability inherent to karst geology in Minnesota.
Stormwater BMP selection in karst settings. Sources Karst Working Group, 2009; Minnesota Stormwater Wiki; Tennessee Permanent Stormwater Management and Design Guidance.
Link to this table
|BMP||Suitability in karst regions||Karst considerations|
|Impervious area disconnect||Preferred||
|Bioretention with underdrain (biofiltration)||Preferred||
|Dry swale or grassed channel||Preferred||
|Soil compost amendment||Adequate|
|Small scale infiltration/micro-bioinfiltration||Adequate||
|Infiltration trench or basin||Adequate||
Warning: If the CSW permit applies, liners are required in areas of active karst
|Dry extended detention (ED) ponds and wet ponds||Adequate||
Warning: If the CSW permit applies, liners are required in areas of active karst
|Wet swale||Discouraged||Not feasible|
|Large scale infiltration||Discouraged||
There are several approaches to sinkhole remediation if such an approach is desirable. Sinkhole sealing involves investigation, stabilization, filling, and final grading. In the investigation phase, the areal extent and depth of the sinkhole(s) should be determined. The investigation may consist of excavation to bedrock, soil borings, and/or geophysical studies. Sealing small-sized sinkholes is normally achieved by digging out the sinkhole to bedrock, plugging the hole with concrete, installing several impermeable soil layers interspersed with plastic or geotextile, and crowning with an impermeable layer and topsoil. For moderate sinkholes, an engineered subsurface structure is usually required.
It is often not feasible to seal large sinkholes so other remediation options must be pursued. These could include construction of a low-head berm around the sinkhole, clean-out of the sinkhole to make sure all potentially contaminating materials are removed, landscaping and conversion of land use in the sinkhole to open space or recreation, provided it can be done in a manner that provides adequate safety. In any of these cases, pre-treatment of any stormwater entering the sinkhole is imperative. Final grading of sinkholes in open space settings should include the placement of low permeability topsoil or clay and a vegetative cover, with a positive grade maintained away from the sinkhole location to avoid ponding or infiltration, if feasible.
Additional information on sinkhole remediation can be found at:
A water quality monitoring system installed, operated, and maintained by the owner/operator is RECOMMENDED, particularly where drinking water supplies are derived from ground water or associated with known sources of contamination. The location of monitoring wells or BMP performance monitoring will depend upon the nature of the BMP and surrounding karst characteristics. This could mean the installation of a monitoring system designed to reflect variable water behavior typical of karst water flow in addition to the monitoring of the performance of the BMP. Monitoring of groundwater and stormwater runoff behavior requires a thorough understanding of the local geology as the hydrology of karst terrains is vastly different from that of non-karst terrains (EPA, 1989).
Below is a list of resources that provide additional information on runoff and/or groundwater monitoring in karst areas.
In case of BMP failure in karst areas,