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{{alert|This site is undergoing updates. Anticipated completion date is June, 2015.|alert-under-construction}}
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{{alert|The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”. Active karst is a terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface.|alert-danger}}
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[[File:Technical information page image.png|right|100px|alt=image]]
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[[File:Pdf image.png|100px|thumb|left|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Karst_-_Minnesota_Stormwater_Manual.pdf Download pdf]</font size>]]
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[[file:Check it out.png|200px|thumb|alt=check it out image|<font size=3>[https://gisdata.mn.gov/dataset/geos-surface-karst-feature-devel Minnesota Regions Prone to Surface Karst Feature Development].</font size>]]
  
 
[[file:Minnesota karst lands.png|300px|thumb|alt=map of MN karst lands|<font size=3>Minnesota karst lands. Karst may also occur in sandstone formations in the eastern part of the state (e.g. Pine County). (Source: E. Calvin Alexander, University of Minnesota, with permission)</font size>]]
 
[[file:Minnesota karst lands.png|300px|thumb|alt=map of MN karst lands|<font size=3>Minnesota karst lands. Karst may also occur in sandstone formations in the eastern part of the state (e.g. Pine County). (Source: E. Calvin Alexander, University of Minnesota, with permission)</font size>]]
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[[File:Fillmore County sinkhole probability map.png|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Map illustrating sinkhole probability in Fillmore County, Southeast Minnesota. The highest sinkhole densities are on flat hilltops between or adjacent to river valleys in the western half of the county (shown in red and orange). In these locations, limestone and dolomite is the first bedrock encountered and is typically within 50 feet of the land surface. Source: [http://www.dnr.state.mn.us/waters/programs/gw_section/mapping/platesum/fillcga.html Minnesota Department of Natural Resources], October 2005.</font size>]]
 
[[File:Fillmore County sinkhole probability map.png|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Map illustrating sinkhole probability in Fillmore County, Southeast Minnesota. The highest sinkhole densities are on flat hilltops between or adjacent to river valleys in the western half of the county (shown in red and orange). In these locations, limestone and dolomite is the first bedrock encountered and is typically within 50 feet of the land surface. Source: [http://www.dnr.state.mn.us/waters/programs/gw_section/mapping/platesum/fillcga.html Minnesota Department of Natural Resources], October 2005.</font size>]]
  
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 ([http://pubs.usgs.gov/sir/2008/5023/pdf/06hollings.pdf 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 requirements for 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 ([http://stormwater.pca.state.mn.us/index.php/References_for_stormwater_infiltration Karst Working Group], 2009). In general, 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.
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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 ([http://pubs.usgs.gov/sir/2008/5023/pdf/06hollings.pdf 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 ([https://chesapeakestormwater.net/2011/07/technical-bulletin-no-1-stormwater-design-guidelines-for-karst-terrain/ 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.
*[http://conservancy.umn.edu/bitstream/handle/11299/58436/sinkholes%5B1%5D.pdf?sequence=4&isAllowed=y Olmstead]
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*[http://conservancy.umn.edu/bitstream/handle/11299/58436/sinkholes%5B1%5D.pdf?sequence=4&isAllowed=y Olmsted]
 
*[http://conservancy.umn.edu/bitstream/handle/11299/58557/plate5%5B1%5D.pdf?sequence=5&isAllowed=y Wabasha]
 
*[http://conservancy.umn.edu/bitstream/handle/11299/58557/plate5%5B1%5D.pdf?sequence=5&isAllowed=y Wabasha]
 
*[http://reflections.mndigital.org/cdm/ref/collection/mgs/id/623 Winona]
 
*[http://reflections.mndigital.org/cdm/ref/collection/mgs/id/623 Winona]
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*[http://files.dnr.state.mn.us/waters/groundwater_section/mapping/cga/c12_goodhue/pdf_files/plate10.pdf Goodhue]
 
*[http://files.dnr.state.mn.us/waters/groundwater_section/mapping/cga/c12_goodhue/pdf_files/plate10.pdf Goodhue]
 
*[http://files.dnr.state.mn.us/waters/groundwater_section/mapping/cga/c08_fillmore/pdf_files/plate08.pdf Fillmore]
 
*[http://files.dnr.state.mn.us/waters/groundwater_section/mapping/cga/c08_fillmore/pdf_files/plate08.pdf Fillmore]
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{{alert|The following definitions were based on a map developed by Alexander and others (2006) which defined active karst. These definitions need updating based on recent work in east-central Minnesota.  [http://files.dnr.state.mn.us/waters/groundwater_section/mapping/gw/gw01_report.pdf Barry et al.] (2016) state: "In Minnesota surface karst features primarily occur where 50 feet or less of unconsolidated sediment overlies Paleozoic carbonate bedrock, the St. Peter Sandstone, or the Hinckley  Sandstone." We are working with the Minnesota Department of Natural Resources to update these definitions.|alert-info}}
  
 
In Minnesota there are three classifications of karst lands.
 
In Minnesota there are three classifications of karst lands.
*Active Karst. Active karst is defined as areas underlain by carbonate bedrock with less than 50 feet of sediment cover.
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*Active Karst. Active karst is a terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface
 
*Transition Karst. Transition karst is defined as areas underlain by carbonate bedrock with 50 to 100 feet of sediment cover.
 
*Transition Karst. Transition karst is defined as areas underlain by carbonate bedrock with 50 to 100 feet of sediment cover.
 
*Covered Karst. Covered karst is defined as areas underlain by carbonate bedrock but with more than 100 feet of sediment cover.
 
*Covered Karst. Covered karst is defined as areas underlain by carbonate bedrock but with more than 100 feet of sediment cover.
  
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 should be evaluated with respect to the potential for catastrophic failure given the increase in hydrostatic pressure created by a BMP.  
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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.
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*Recommended websites pertaining to Minnesota karst
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**[http://www.mgwa.org/meetings/2007_spring/2007_spring_alexander.pdf Fractured sandstone karst aquifers, the St. Peter, Jordan and Hinckley Formations: Examples from Askov, Woodbury, Rochester and elsewhere] (Alexander, 2007)
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**[http://www.mgwa.org/meetings/2012_fall/karst/barr-alexander.pdf Examples of Hypogenic Karst Collapse Structures, Twin Cities Metropolitan Area, Minnesota] (Barr and Alexander, 2012)
  
 
==What is karst?==
 
==What is karst?==
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 (CO<sub>2</sub>), and becomes more so as it passes through the subsurface and picks up even more CO<sub>2</sub>. As the runoff passes through the subsurface and reacts with the rocks, cracks and fissures forms. These cracks and fissures grow, creating larger passages, caves, and may even form sinkholes as more and more acidic water infiltrates into the subsurface ([http://www.eahcp.org/files/uploads/09-03-10Attachment4aUsing_Green_Infrastructure.pdf American Rivers]).
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[[file:Illustration of karst features.jpg|thumb|300px|alt=illustration of karst features|<font size=3>The features of a karst system. Environmental Science Institute, The University of Texas at Austin. [http://www.esi.utexas.edu/outreach/caves/ ''Caves as a Window into the Edwards Aquifer''.]</font size>]]
  
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 ([http://stormwater.pca.state.mn.us/index.php/References_for_stormwater_infiltration 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.  
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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 (CO<sub>2</sub>), and becomes more so as it passes through the subsurface and picks up even more CO<sub>2</sub>. 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 ([https://aquiferalliance.org/Library/LibraryFiles/Resources/Using_Green_Infrastructure_in_Karst_Regions_American_Riv.pdf American Rivers]).
  
Knowledge of the 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.
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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 ([https://www.usgs.gov/news/earthword-karst 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.
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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.
 
The following sources provide additional information on karst in Minnesota.
*[http://www.pca.state.mn.us/index.php/water/water-types-and-programs/groundwater/groundwater-basics/karst-in-minnesota.html Karst in Minnesota] - Minnesota Pollution Control Agency
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*[https://www.pca.state.mn.us/water/karst-minnesota Karst in Minnesota] - Minnesota Pollution Control Agency
 
*[http://www.mngs.umn.edu/indx.html Karst features of Minnesota] - Minnesota Geological Survey
 
*[http://www.mngs.umn.edu/indx.html Karst features of Minnesota] - Minnesota Geological Survey
*[https://84a69b9b8cf67b1fcf87220d0dabdda34414436b-www.googledrive.com/host/0B0PLtJjhTxnkZDAzOGQxY2EtOTIzOS00ZjlkLWJhYmMtYWYzY2QwYmQ2ZjFi/Documents/LEFHE%20Studies%20Real%20Time%20%28C.%20Avon%29/Journal%20of%20Cave%20and%20Karst%20Studies/v64n1-Goa.pdf THE DEVELOPMENT OF A KARST FEATURE DATABASE FOR SOUTHEASTERN MINNESOTA] - Alexander and Tipping
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*[https://pdfs.semanticscholar.org/11a4/ef843ccdb7bf215840306df07ccac0a8b80a.pdf THE DEVELOPMENT OF A KARST FEATURE DATABASE FOR SOUTHEASTERN MINNESOTA] - Alexander and Tipping
  
 
==Why is karst geology a concern?==
 
==Why is karst geology a concern?==
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[[File:Dancing Waters sinkhole collapse 2.jpg|thumb|300px|alt=pond water disappears in karst setting|<p><font size=3>Woodbury sinkhole collapse looking toward the south.</font size>]]
 
[[File:Dancing Waters sinkhole collapse 2.jpg|thumb|300px|alt=pond water disappears in karst setting|<p><font size=3>Woodbury sinkhole collapse looking toward the south.</font size>]]
  
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 because the subsurface conditions and subsurface drainage pattern variability intrinsic to karst geology.
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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.
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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.
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*Runoff should be dispersed
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*Runoff should be pretreated and then infiltrated only if subsurface investigations and geotechnical analysis confirm that there are no unstable zones below the BMP
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{{alert|The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”. Active karst is defined as areas underlain by carbonate bedrock with less than 50 feet of sediment cover.|alert-danger}}
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*Additional borings and deeper borings may be warranted to target evaluation of transitional karst zone
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*Once ponds are constructed, include contingency plans for cases where karst features open up and impact a pond, including conducting geotechnical borings to appropriate depths trying to identify unstable zones, then target those zones for grouting
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*Convey runoff to a collection and transmission system away from the area via vegetated drainageways
  
In general 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.
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==Where can I get more information on karst in Minnesota?==
*Runoff should be dispersed.
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The Minnesota Department of Natural Resources has a relatively new product, [https://gisdata.mn.gov/dataset/geos-surface-karst-feature-devel 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 [https://www.dnr.state.mn.us/waters/groundwater_section/mapping/springs.html Springs, Springsheds, and Karst] on the Minnesota Department of Natural Resources website.
*Runoff should be pretreated and then infiltrated only if subsurface investigations and geotechnical analysis confirm that there are no unstable zones below the BMP.
 
*Additional borings and deeper borings may be warranted to target evaluation of transitional karst zone.
 
*Once ponds are constructed, include contingency plans for cases where karst features open up and impact a pond, including conducting geotechnical borings to appropriate depths trying to identify unstable zones, then target those zones for grouting.
 
*Convey runoff to a collection and transmission system away from the area via vegetated drainageways.
 
  
 
==How to investigate for karst on a site==
 
==How to investigate for karst on a site==
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.  
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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.
 
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 ([http://stormwater.pca.state.mn.us/index.php/References_for_stormwater_infiltration Karst Working Group], 2009). Guidelines for investigating all potential physical constraints to infiltration on a site are presented in the table at the bottom of this page. These guidelines should not be interpreted as all-inclusive. The size and complexity of the project will drive the extent of any subsurface investigation.
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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 ([https://chesapeakestormwater.net/2011/07/technical-bulletin-no-1-stormwater-design-guidelines-for-karst-terrain/ Karst Working Group], 2009). 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.
  
Additional information regarding site investigations in karst areas can be found in Appendix B of the [http://tnpermanentstormwater.org/manual/27%20Appendix%20B%20Stormwater%20Design%20Guidelines%20for%20Karst%20Terrain.pdf Tennessee Permanent Stormwater Management and Design Guidance Manual] (Tennessee Department of Environment and Conservation, 2014). This manual provides a chart which will guide designers through the investigative process and will help designers determine if any special analysis is required.
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Additional information regarding site investigations in karst areas can be found in Appendix B of the [http://tnpermanentstormwater.org/manual/27%20Appendix%20B%20Stormwater%20Design%20Guidelines%20for%20Karst%20Terrain.pdf 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.
  
 
===Preliminary site investigation===
 
===Preliminary site investigation===
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 ([http://stormwater.pca.state.mn.us/index.php/References_for_stormwater_infiltration Pennsylvania BMP], 2009):
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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
*A review of aerial photographs, geological literature, sinkhole maps, previous soil borings, existing well data, and municipal wellhead or aquifer protection plans.
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*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.
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*a site reconnaissance, including a thorough field examination for features such as limestone pinnacles, sinkholes, closed depressions, fracture traces, faults, springs, and seeps; and
*The site should be observed under varying weather conditions, especially during heavy rains and in different seasons to identify and map any natural drainageways.
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*site observations made under varying weather conditions, especially during heavy rains and in different seasons to identify and map any natural drainageways.
  
 
===Subsurface material investigation===
 
===Subsurface material investigation===
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:
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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.
 
*Bedrock characteristics (ex. type, geologic contacts, faults, geologic structure, rock surface configuration)
 
*Bedrock characteristics (ex. type, geologic contacts, faults, geologic structure, rock surface configuration)
 
*Depth to the water table and depth to bedrock
 
*Depth to the water table and depth to bedrock
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*Sinkholes and/or other closed depressions
 
*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)
 
*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)
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In conducting subsurface investigations, all applicable State regulations must be met. For more information, see Minnesota Department of Health's [https://www.health.state.mn.us/wells Wells and Borings website].
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{{alert|In conducting subsurface investigations, all applicable State regulations must be met.|alert-danger}}
  
 
===Location of soil borings===
 
===Location of soil borings===
 
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:
 
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.
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*getting at least 1 boring in each distinct major soil type present, as mapped in [http://www.nrcs.usda.gov/wps/portal/nrcs/surveylist/soils/survey/state/?stateId=MN soil surveys];
*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.
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*placing an adequate number near on-site geologic or geomorphic indications of the presence of sinkholes or related karst features;
*Locating along photo-geologic fracture traces.
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*locating along [https://pubs.usgs.gov/of/1996/0479/report.pdf geologic fracture traces];
*Locating adjacent to bedrock outcrop areas.
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*locating adjacent to bedrock outcrop areas;
*Locating a sufficient number to adequately represent the area under any proposed stormwater facility.
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*locating a sufficient number to adequately represent the area under any proposed stormwater facility; and
*Documenting any areas identified as anomalies from any existing geophysical or other subsurface studies.
+
*documenting any areas identified as anomalies from any existing geophysical or other subsurface studies.
 +
 
 +
Exploratory borings must comply with [https://www.revisor.mn.gov/rules/?id=4727.0100 Minnesota statutes, 4727.0100]
 +
 
 +
{{alert|Exploratory borings must comply with State regulations|alert-danger}}
 +
 
 +
===Number and depth of soil borings===
 +
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.
 +
 
 +
{{alert|Note that this manual contains a section on [http://stormwater.pca.state.mn.us/index.php/Design_criteria_for_bioretention#Determine_site_infiltration_rates_.28for_facilities_with_infiltration_and.2For_recharge.29 recommended number of borings]. The number of borings needed at sites that have or potentially have active karst is likely to be greater than the number indicated in the above referenced section of the manual.|alert-info}}
 +
 
 +
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.
  
===Number of soil borings===
+
====Abandoning soil borings====
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.
+
{{alert|Under Minnesota law all soil borings must be sealed by a licensed well contractor or a licensed well sealing contractor.|alert-danger}}
  
===Depth of soil borings===
+
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 [https://www.revisor.mn.gov/statutes/?id=103I Minnesota Statutes, Chapter 103I] For more information see these Minnesota Department of Health websites.
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.
+
*[https://www.health.state.mn.us/communities/environment/water/wells/disclosures/wellsatpropertytransfer.html What You Should Know About Wells at Property Transfer]
 +
*[https://www.health.state.mn.us/communities/environment/water/wells/lwc/index.html Well and Boring Contractor Directory]
  
 
===Identification of material===
 
===Identification of material===
All material identified by the excavation and geophysical studies and penetrated by the boring should be identified, as follows:
+
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.
 
*Provide descriptions, logging, and sampling for the entire depth of the boring.
 
*Note any stains, odors, or other indications of environmental degradation.
 
*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.
+
*Perform laboratory analysis on a minimum of 2 soil samples, representative of the material penetrated, including potential limiting soil 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.
+
*Identify soil characteristics including 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.
+
*Log any indications of water saturation to include both perched and groundwater table levels; include descriptions of soils that are mottled or gleyed. 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.
+
*Record water levels in all borings over a time-period reflective of anticipated water level fluctuation, noting that water levels in karst geology can vary dramatically and rapidly. Borings 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.
+
*Estimate soil engineering characteristics, including “N” or estimated [http://www.uta.edu/ce/geotech/lab/Main/Soil%20Lab/09_UCS/UCS.pdf unconfined compressive strength], when conducting a [https://www.sciencedirect.com/topics/engineering/standard-penetration-test standard penetration test] (SPT).
  
 
===Evaluation of findings===
 
===Evaluation of findings===
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===Geophysical and dye techniques===
 
===Geophysical and dye techniques===
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.
+
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 [https://www.dnr.state.mn.us/waters/groundwater_section/geophysics/index.html 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.
 +
 
 +
{{alert|Dye tracing requires expertise and should only be done by experienced karst hydrogeologists. Contact the [http://www.mngs.umn.edu/about.html Minnesota Geological Survey] for more information.|alert-warning}}
 +
 
 +
For good basic information on use of dye techniques in karst settings, see the United States EPA document, [http://karstwaters.org/wp-content/uploads/2015/04/dye-tracing.pdf Application of Dye-tracing techniques for Determining Solute-transport Characteristics of Ground Water in Karst Terranes] (1988).
  
 
==General stormwater management guidelines for karst areas==
 
==General stormwater management guidelines for karst areas==
{{alert|The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features unless allowed by a local unit of government with a current MS4 permit”.|alert-danger}}
+
{{alert|The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”.|alert-danger}}
  
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 ([http://www.eahcp.org/files/uploads/09-03-10Attachment4aUsing_Green_Infrastructure.pdf American Rivers]).
+
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); [http://tnpermanentstormwater.org/manual.aspTennessee Permanent Stormwater Management and Design Guidance Manual – Appendix B] (2014); and other documents ([http://chesapeakestormwater.net/training-library/design-adaptations/stormwater-in-karst-topography/ Chesapeake Stormwater Network] [Karst Working Group, 2009]; [http://www.dep.wv.gov/WWE/Programs/stormwater/MS4/Documents/Appendix_C_Stormwater_Mgmt_Design_Karst_WV-SW-11-2012.pdf West Virginia Stormwater Management & Design Guidance Manual]; [http://www.lccwc.com/pdf/Sem2_2011/Adams-KarstGeology_09_20_2011.pdf Infiltration and Karst: Design Considerations for Success], Adams, M.).
+
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); [https://tnpermanentstormwater.org/manual/27%20Appendix%20B%20Stormwater%20Design%20Guidelines%20for%20Karst%20Terrain.pdf Tennessee Permanent Stormwater Management and Design Guidance Manual – Appendix B] (2014); and other documents ([http://chesapeakestormwater.net/training-library/design-adaptations/stormwater-in-karst-topography/ Chesapeake Stormwater Network], [[References for stormwater infiltration#K|Karst Working Group, 2009]]; [http://www.dep.wv.gov/WWE/Programs/stormwater/MS4/Documents/Appendix_C_Stormwater_Mgmt_Design_Karst_WV-SW-11-2012.pdf 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.
 
The following guidelines do not contain substantial prescriptive information because of the variability inherent to karst geology in Minnesota.
 
*Conduct a thorough geotechnical investigation in areas with suspected or documented active karst. Karst geology can change rapidly over very short distances so additional soil borings may be required in comparison to geotechnical investigations for shallow groundwater or bedrock.
 
*Conduct a thorough geotechnical investigation in areas with suspected or documented active karst. Karst geology can change rapidly over very short distances so additional soil borings may be required in comparison to geotechnical investigations for shallow groundwater or bedrock.
*Investigate non-infiltration BMPs on sites subject to the requirements of the CGP, where the infiltration BMP is “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features unless allowed by a local unit of government with a current MS4 permit”.  For purpose of these guidelines, active karst features are defined as identified karst or identified unstable subsurface materials that have the potential for development of karst.
+
*Investigate non-infiltration BMPs on sites where infiltration is not allowed under requirements of the [http://stormwater.pca.state.mn.us/index.php/Construction_stormwater_permit CGP] (i.e. where the BMP would be “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”).
 
*Preserve the maximum length of natural swales as possible at the site to increase the infiltration and accommodate flows from extreme storms.
 
*Preserve the maximum length of natural swales as possible at the site to increase the infiltration and accommodate flows from extreme storms.
*Minimize the area of impervious surfaces at the site. This will reduce the volume and velocity of the stormwater runoff. Consult with a geotechnical engineer prior to the design and construction of a BMP. The effect of stormwater BMPs in a karst area is complex and hard to predict. Consulting with a professional can reduce the risks of geological hazards and groundwater contamination.
+
*Minimize the area of impervious surfaces at the site. This will reduce the volume and velocity of the stormwater runoff. Consult with a geotechnical engineer prior to the design and construction of a BMP.
*Capture the runoff in a series of small runoff reduction practices where sheet flow is present. This technique will help keep the stormwater runoff from becoming channelized and will disperse the flow over a broad area. Practices such as swales, bioretention with underdrains, media filters, and vegetated filters should be considered first at a site. However, not all sites lend themselves to this type of management approach and could require use of the active karst region for proper management. Under these conditions, adequate precautions should be taken to assure that runoff water is adequately pretreated.
+
*Capture the runoff in a series of small runoff reduction practices where sheet flow is present. This technique will help keep the stormwater runoff from becoming channelized and will disperse the flow over a broad area. Practices such as [[Filtration|swales]], [[Bioretention|bioretention with underdrains]], [[Filtration|media filters]], and [[Vegetated filter strips|vegetated filters]] should be considered first at a site. However, not all sites lend themselves to this type of management approach. Adequate precautions should be taken to assure that runoff water is adequately [[Pre-treatment|pretreated]].
*Design BMPs to be off-line such that volumes of runoff greater than the capacity of the BMP are bypassed around the BMP. This approach will limit the volume through the BMP to a quantity that is manageable in the karst.  
+
*Design BMPs to be <span title="a flow system where only stormwater runoff treated by a BMP enters the BMP, with remaining water bypassing the BMP"> '''off-line'''</span> such that volumes of runoff greater than the capacity of the BMP are bypassed around the BMP. This approach will limit the volume through the BMP to a quantity that is manageable in the karst.  
 
*Install multiple small BMPs instead of a centralized BMP. Centralized BMPs are defined as any practice that treats runoff from a contributing drainage area greater than 20,000 square feet, and/or has a surface ponding depth greater than 3 feet. Centralized practices have the greatest potential for karst- related failure, and will require costly geotechnical investigations and a more complex design.
 
*Install multiple small BMPs instead of a centralized BMP. Centralized BMPs are defined as any practice that treats runoff from a contributing drainage area greater than 20,000 square feet, and/or has a surface ponding depth greater than 3 feet. Centralized practices have the greatest potential for karst- related failure, and will require costly geotechnical investigations and a more complex design.
 
*Direct discharge from stormwater BMPs to surface waters and not to the nearest sinkhole. Because karst areas can be quite large in Minnesota, discharges should be routed to a baseflow stream via a pipe or lined ditch or channel to redirect the flow away from the karst, provided the stream does not disappear into a karst feature.
 
*Direct discharge from stormwater BMPs to surface waters and not to the nearest sinkhole. Because karst areas can be quite large in Minnesota, discharges should be routed to a baseflow stream via a pipe or lined ditch or channel to redirect the flow away from the karst, provided the stream does not disappear into a karst feature.
*Incorporate additional precautions where infiltration practices are used. Impermeable liners and underdrains should be used in infiltration practices in active karst areas, effectively converting the infiltration BMP to a filtration BMP. These measures will be used to prevent the infiltrating water from discharging into the soils surrounding the karst feature. In addition, large scale practices (drainage area is greater than 20,000 square feet) are strongly discouraged.
+
*Design  [[Stormwater ponds|ponds]] and [[Stormwater wetlands|wetlands]] with a properly engineered [http://stormwater.pca.state.mn.us/index.php/Liners_for_stormwater_management synthetic liner]. It is HIGHLY RECOMMENDED that a professional geotechnical engineer investigate and recommend the depth of unconsolidated material between the bottom of the pond and the surface of the bedrock.  A minimum of 3 feet of unconsolidated soil material is the minimum separation; however an expert may recommend 10 feet or greater. Pond and wetland depths should be fairly uniform and limited to no more than 10 feet in depth.
*Design  ponds and wetlands with a properly engineered synthetic liner. It is HIGHLY RECOMMENDED that a professional geotechnical engineer investigate and recommend the depth of unconsolidated material between the bottom of the pond and the surface of the bedrock.  A minimum of 3 feet of unconsolidated soil material is the minimum separation , however an expert may recommend 10 feet or greater. Pond and wetland depths should be fairly uniform and limited to no more than 10 feet in depth.
+
 
 +
{{alert|Section 18.10 of the Construction Stormwater permit requires that "Permittees must design basins using an impermeable liner if located within active karst terrain".|alert-danger}}
 +
{{alert|At sites within areas of active karst and that do not require a Construction Stormwater permit, a geotechnical investigation following guidance on this page is highly recommended.|alert-warning}}
 +
 
 
*Minimize site disturbance during BMP construction. Seek the recommendations of a geotechnical engineer for management of heavy equipment, temporary storage of materials, changes to the soil profile - including cuts, fills, excavation and drainage alteration - on sites that have been found to contain a karst feature.
 
*Minimize site disturbance during BMP construction. Seek the recommendations of a geotechnical engineer for management of heavy equipment, temporary storage of materials, changes to the soil profile - including cuts, fills, excavation and drainage alteration - on sites that have been found to contain a karst feature.
 
*Report sinkholes  as soon as possible after the first observation of sinkhole development. The sinkhole(s) should then be repaired or the stormwater management facility abandoned, adapted, managed and/or observed for future changes, whichever of these is most appropriate.
 
*Report sinkholes  as soon as possible after the first observation of sinkhole development. The sinkhole(s) should then be repaired or the stormwater management facility abandoned, adapted, managed and/or observed for future changes, whichever of these is most appropriate.
 
*Develop a contingency plan for how to manage the stormwater should a BMP fail as a result of the development of a karst feature.   
 
*Develop a contingency plan for how to manage the stormwater should a BMP fail as a result of the development of a karst feature.   
 
*If a karst feature is encountered report to the appropriate state agency, such as the Department of Natural Resources (DNR), Minnesota Geological Survey (MGS), and local agencies (such as the city, township or county). These known occurrences should be surveyed for specific location and permanently recorded on the property deed. For transition karst areas, local discretion and the likelihood of karstic features should be used to determine the amount of geotechnical investigation. An easement or reserve area should be identified on the development plats for the project so that all future landowners know of the presence of active karst on their property.
 
*If a karst feature is encountered report to the appropriate state agency, such as the Department of Natural Resources (DNR), Minnesota Geological Survey (MGS), and local agencies (such as the city, township or county). These known occurrences should be surveyed for specific location and permanently recorded on the property deed. For transition karst areas, local discretion and the likelihood of karstic features should be used to determine the amount of geotechnical investigation. An easement or reserve area should be identified on the development plats for the project so that all future landowners know of the presence of active karst on their property.
 +
*Incorporate additional precautions where infiltration practices are used. For example, infiltration of stormwater from [http://stormwater.pca.state.mn.us/index.php/Potential_stormwater_hotspots stormwater hoptspots] is discouraged unless pollutant concentrations can be significantly reduced through pretreatment practices.
  
 
{{:Structural BMP use in karst settings}}
 
{{:Structural BMP use in karst settings}}
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Additional information on sinkhole remediation can be found at:
 
Additional information on sinkhole remediation can be found at:
 
*[http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=1109&context=sinkhole_2013 Problems Associated with the Use of Compaction Grout for Sinkhole Remediation in West-Central Florida] (Zisman, 2013). This paper presents information regarding the improper application of compaction grouting in sinkholes.
 
*[http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=1109&context=sinkhole_2013 Problems Associated with the Use of Compaction Grout for Sinkhole Remediation in West-Central Florida] (Zisman, 2013). This paper presents information regarding the improper application of compaction grouting in sinkholes.
*[http://semanticommunity.info/@api/deki/files/838/=BobDenton09172008.pdf The Characterization and Remediation of Sinkholes] (Denton, N.D. – PowerPoint Presentation). The PowerPoint presentation presents an overview of karst geology and some common remediation techniques.  
+
*[https://potomacpartnership.org/wp-content/uploads/2015/11/RobDenton_KarstTopography.pdf The Characterization and Remediation of Sinkholes] (Denton, N.D. – PowerPoint Presentation). The PowerPoint presentation presents an overview of karst geology and some common remediation techniques.  
 
*[http://cedb.asce.org/cgi/WWWdisplay.cgi?256932 Development Mechanism and Remediation of Multiple Spontaneous Sinkholes: A Case Study] (Jammal et al., 2010). This journal article provides information on how to remediate when multiple sinkholes are present.
 
*[http://cedb.asce.org/cgi/WWWdisplay.cgi?256932 Development Mechanism and Remediation of Multiple Spontaneous Sinkholes: A Case Study] (Jammal et al., 2010). This journal article provides information on how to remediate when multiple sinkholes are present.
 
*[http://www.hydroworld.com/articles/hr/print/volume-32/issue-7/cover-story/sinkholes-and-seepage-embankment-repair-at-hat-creek-1.html Sinkholes and Seepage: Embankment Repair at Hat Creek 1] (Bowers et al., 2013). This article discusses geotechnical investigation and engineering solution regarding a sinkhole that was discovered near a Pacific Gas and Electric Company hydroelectric project.
 
*[http://www.hydroworld.com/articles/hr/print/volume-32/issue-7/cover-story/sinkholes-and-seepage-embankment-repair-at-hat-creek-1.html Sinkholes and Seepage: Embankment Repair at Hat Creek 1] (Bowers et al., 2013). This article discusses geotechnical investigation and engineering solution regarding a sinkhole that was discovered near a Pacific Gas and Electric Company hydroelectric project.
Line 145: Line 189:
 
Below is a list of resources that provide additional information on runoff and/or groundwater monitoring in karst areas.
 
Below is a list of resources that provide additional information on runoff and/or groundwater monitoring in karst areas.
 
*[http://www.epa.gov/OUST/cat/gwkarst.pdf Ground-Water Monitoring in Karst Terranes: Recommended Protocols & Implicit Assumptions] (EPA, 1989). This report provides information on the monitoring procedures and common monitoring pitfalls in karst areas. It describes where to monitor for pollutants, where to monitor for background water quality, when to monitor the groundwater, and how to do all this reliably and economically.
 
*[http://www.epa.gov/OUST/cat/gwkarst.pdf Ground-Water Monitoring in Karst Terranes: Recommended Protocols & Implicit Assumptions] (EPA, 1989). This report provides information on the monitoring procedures and common monitoring pitfalls in karst areas. It describes where to monitor for pollutants, where to monitor for background water quality, when to monitor the groundwater, and how to do all this reliably and economically.
*''Highway Stormwater Runoff in Karst Areas – Preliminary Results of Baseline Monitoring and Design of a Treatment System for a Sinkhole in Knoxville Texas'' ([http://stormwater.pca.state.mn.us/index.php/References_for_stormwater_infiltration Stephenson et al.], 1999). The authors of this report discuss the use of quantitative dye tracing and hydrograph analyses to monitor stormwater runoff and resulting groundwater flow from runoff from a highway system.
+
*''Highway Stormwater Runoff in Karst Areas – Preliminary Results of Baseline Monitoring and Design of a Treatment System for a Sinkhole in Knoxville Texas'' ([https://www.sciencedirect.com/science/article/abs/pii/S0013795298000544 Stephenson et al.], 1999). The authors of this report discuss the use of quantitative dye tracing and hydrograph analyses to monitor stormwater runoff and resulting groundwater flow from runoff from a highway system.
 
*[http://digitalcommons.wku.edu/cgi/viewcontent.cgi?article=2414&context=theses Evaluating the Effectiveness of Regulatory Stormwater Monitoring Protocols on Groundwater Quality in Urbanized Karst Regions] (Nedvidek, 2014). This report looks at monitoring techniques and frequencies. It also discusses how to make these monitoring programs cost effective, while still providing a sufficient amount of information.
 
*[http://digitalcommons.wku.edu/cgi/viewcontent.cgi?article=2414&context=theses Evaluating the Effectiveness of Regulatory Stormwater Monitoring Protocols on Groundwater Quality in Urbanized Karst Regions] (Nedvidek, 2014). This report looks at monitoring techniques and frequencies. It also discusses how to make these monitoring programs cost effective, while still providing a sufficient amount of information.
  
{{:Procedures for investigating sites with potential constraints on stormwater infiltration}}
+
===BMP failure===
 +
In case of BMP failure in karst areas,
 +
*if waters of the state are or may be impacted, contact the state duty officer (651-649-5451 or 800-422-0798; or 651-297-5353 TDD Line/800-627-3529 TDD Watts Line);
 +
*contact appropriate local authorities; and/or
 +
*repair the practice [https://stormwater.pca.state.mn.us/index.php?title=Karst#How_to_remediate_after_a_sinkhole_appears following guidance above].
  
 
<noinclude>
 
<noinclude>
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</noinclude>
 
</noinclude>
  
<!--
+
==Related pages==
THE TEXT BELOW IS HIDDEN AND IS FROM THE ORIGINAL MANUAL
+
*[[Overview of stormwater infiltration]]
[[File:statewide karst areas.jpg|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Statewide distribution of karst features. Karst primarily occurs in limestone and dolomite formations in Southeastern Minnesota, but may occur in sandstone formations in the eastern part of the state (e.g. Pine County). Note that a statewide map such as this will be less accurate than local maps depicting karst features. Source: [http://www.mngs.umn.edu/ Minnesota Geological Survey], October 2005.</font size>]]
+
*[[Pre-treatment considerations for stormwater infiltration]]
 +
*[[BMPs for stormwater infiltration]]
 +
*[[Pollutant fate and transport in stormwater infiltration systems]]
 +
*[[Surface water and groundwater quality impacts from stormwater infiltration]]
 +
*[[Stormwater infiltration and groundwater mounding]]
 +
*[[Stormwater infiltration and setback (separation) distances]]
 +
*[[Karst]]
 +
*[[Shallow soils and shallow depth to bedrock]]
 +
*[[Shallow groundwater]]
 +
*[[Soils with low infiltration capacity]]
 +
*[[Potential stormwater hotspots]]
 +
*[[Stormwater and wellhead protection]]
 +
*[[Stormwater infiltrations and contaminated soils and groundwater]]
 +
*[[Decision tools for stormwater infiltration]]
 +
*[[Stormwater infiltration research needs]]
 +
*[[References for stormwater infiltration]]
  
Karst regions are predominantly found in the southeastern portion of the state and have important implications with respect to geotechnical testing, infiltration, pre-treatment and ponding of runoff. Caution must be used in interpreting the geographic depiction of karst lands. Figures showing generalized maps of active karst will be less accurate than a county-scale map of actual karstic features.
+
<noinclude>
 
+
[[Category:Infiltration]]
Karst regions are predominantly found in the southeastern portion of the state and have important implications with respect to geotechnical testing, infiltration, pre-treatment and ponding of runoff. Caution must be used in interpreting the geographic depiction of karst lands as illustrated in the two figures. The figure showing a generalized map of active karst will be less accurate than a county-scale map showing karstic features.
+
</noinclude>
 
 
==Overview of karst geology==
 
Karst topography is a geological formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks, such as quartzite, given the right conditions.
 
 
 
Due to subterranean drainage, there may be very limited surface water, even in the absence of any rivers and lakes. Many karst regions display distinctive surface features, with cenotes, sinkholes or dolines being the most common. However, distinctive karst surface features may be completely absent where the soluble rock is mantled, such as by glacial debris, or confined by one or more superimposed 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.
 
 
 
The following sources provide additional information on karst in Minnesota.
 
*[http://www.pca.state.mn.us/index.php/water/water-types-and-programs/groundwater/groundwater-basics/karst-in-minnesota.html Karst in Minnesota] - Minnesota Pollution Control Agency
 
*[http://www.mngs.umn.edu/indx.html Karst features of Minnesota] - Minnesota Geological Survey
 
*[https://84a69b9b8cf67b1fcf87220d0dabdda34414436b-www.googledrive.com/host/0B0PLtJjhTxnkZDAzOGQxY2EtOTIzOS00ZjlkLWJhYmMtYWYzY2QwYmQ2ZjFi/Documents/LEFHE%20Studies%20Real%20Time%20%28C.%20Avon%29/Journal%20of%20Cave%20and%20Karst%20Studies/v64n1-Goa.pdf THE DEVELOPMENT OF A KARST FEATURE DATABASE FOR SOUTHEASTERN MINNESOTA] - Alexander and Tipping
 
 
 
==Stormwater concerns in karst settings==
 
In karst settings where karstic conditions are known to exist, additional constraints and considerations need to be evaluated prior to implementing most structural BMPs. Of particular concern in karst settings is the formation of sinkholes as a result of hydraulic head build up and/or dissolution of carbonate rock (e.g., limestone) or erosion of bedrock (see sidebar) present underneath or adjacent to BMPs. Where karst conditions exist, there are no prescriptive rules of thumb or universally accepted management approaches because of the variability intrinsic to karst terrain. An adaptation of a familiar old saying is very appropriate: the only thing predictable about the behavior of water in a karst system is its unpredictability.
 
 
 
[[File:Fillmore County sinkhole probability map.png|thumb|300px|alt=statewide map illustrating karst areas|<font size=3>Map illustrating sinkhole probability in Fillmore County, Southeast Minnesota. The highest sinkhole densities are on flat hilltops between or adjacent to river valleys in the western half of the county (shown in red and orange). In these locations, limestone and dolomite is the first bedrock encountered and is typically within 50 feet of the land surface. Source: [http://www.dnr.state.mn.us/waters/programs/gw_section/mapping/platesum/fillcga.html Minnesota Department of Natural Resources], October 2005.</font size>]]
 
[[File:Dancing Waters sinkhole collapse 1.jpg|thumb|300px|alt=pond water disappears in karst setting|<font size=3>A collapsed karst feature where a stormwater pond previously existed. The site was in Woodbury, Minnesota.</font size>]]
 
[[File:Dancing Waters sinkhole collapse 2.jpg|thumb|300px|alt=pond water disappears in karst setting|<p><font size=3>Woodbury sinkhole collapse looking toward the south.</font size>]]
 
 
 
In general when underlying karst is known or even suspected to be present at the site, stormwater runoff should not be concentrated and discharged into known sinkholes, but should rather be dispersed, or soaked into the ground after adequate pre-cleaning, or conveyed to a collection and transmission system away from the area via vegetated drainageways. In other cases, it may be impossible to remove water from an area with sinkholes or away from karst geology, so common sense clean-up of the water and discharge into the karstic area is a reasonable management approach, especially if some filtering soil is available between the land surface and the karst formation.
 
 
 
Some communities around the country have developed karst area design specifications and soil investigation procedures for siting and designing stormwater BMPs. The following sections represent adaptations from a handful of these communities (e.g., Carroll County, MD (1996a and b); St. Johns River Water Management District, FL (2001); and Jefferson County, WV (Laughland 2003). See [[References]].) and should be viewed only as a potential starting point. That is, the complete Minnesota experience is not represented by these resources, but they do represent products that have been put together to assist local stormwater managers deal with karst problems. Additional input was obtained from Professor Calvin Alexander (University of Minnesota) and Jeff Green (Minnesota DNR).
 
 
 
==General stormwater management guidelines for karst areas==
 
The following general guidelines are based on advice offered by many different sources. Again, the uncertainty characteristic of karst terrain and water movement should be the primary dictate when considering how much additional information to collect in these areas before proceeding with BMP installation. The following guidelines do not contain substantial prescriptive information because of the variability inherent to karst geology in Minnesota.
 
*Developers, communities, public works agents and others managing stormwater should conduct thorough geotechnical investigations prior to proceeding with projects or building in active karst areas. The level of geotechnical investigation will depend on the likelihood of active karst being present and the regulatory requirements within the area. They should identify the karst features encountered and report to the appropriate state agency, such as the Department of Natural Resources (DNR) and Minnesota Geological Survey (MGS), and local agencies (such as the city, township or county) any existing sinkholes on a piece of land intended for development. These known occurrences should be surveyed for specific location and permanently recorded on the property deed. For transition karst areas, local discretion and the likelihood of karstic features should be used to determine the amount of geotechnical investigation.
 
*Knowledge of the presence of sinkholes is an absolute indication of active karst. In these cases, an easement or reserve area should be identified on the development plats for the project so that all future landowners know of the presence of active karst on their property.
 
*In many cases, identified sinkholes can and should be remediated and stormwater directed away. In other cases, remediation is not possible and the normal regional hydrologic patterns must be maintained. In this case, however, precautions should be taken to pre-treat any water that drains into a known sinkhole area. If at all possible, runoff should be routed away from active karst features because of the possibility of subsurface flow into the karst formation.
 
*BMPs should be designed off-line to better manage volumes and flow rates from individual facilities.
 
*Discharges from stormwater management facilities or directly from impervious surfaces should not be routed directly to the nearest sinkhole. Because active karst areas can be quite large in Minnesota, discharges may be routed to a baseflow stream via a pipe or lined ditch or channel to remove flow from the area, provided the stream does not disappear into an active karst feature.
 
*Sinkholes developing within stormwater management facilities should be reported as soon as possible after the first observation of occurrence. They should then be repaired, abandoned, adapted, managed and/or observed for future changes, whichever of these are appropriate for proper management.
 
*Sinkhole formation is less likely when water is allowed to soak diffusely into the soil and when stormwater is managed for smaller, more diffuse quantities that limit the volume and rates of flow handled by each BMP. Practices such as swales, bioretention, and vegetated filters should be considered first at a site. However, not all sites lend themselves to this type of management approach and could require use of the active karst region for proper management. Under these conditions, adequate precautions should be taken to assure that all potential contaminants are removed from the infiltrating stormwater.
 
*Where ponds and wetlands are deemed necessary, they should be designed and constructed with a properly engineered synthetic liner. A minimum of three feet (ten feet is preferred) of unconsolidated soil material should exist between the bottom of the pond or wetland and the surface of the bedrock layer. Pond and wetland depths should be fairly uniform and limited to no more than ten feet in depth.
 
Table 1 provides an overview of karst related design considerations for different structural BMP groups.
 
 
 
{{:Structural BMP use in karst settings}}
 
 
 
==Investigation for karst areas==
 
Karst investigations are recommended for all stormwater facilities that are located in an active karst area with known karstic features (sinkholes, solution cavities, direct hydraulic connection between surface water and ground water). The purpose of a karst 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 is preventing the possibility that an unimpeded route will be provided to move polluted runoff into the regional ground water system. The guidelines outlined below should not be interpreted as all-inclusive. The design of any geotechnical investigation should reflect the size and complexity of the proposed project, as well as local knowledge of the threat posed by the karstic geology.
 
 
 
Because of the complexity inherent to active karst areas, there is no single set of investigatory guidelines that works for every location. Typically, however, the sequence involves some visual observation for the presence of sinkhole features (the single easiest evidence that active karst is present), followed by an assessment of the subsurface heterogeneity (variability) of the site through geophysical investigation and/or excavation. With this information in-hand, borings or observation wells can then be accurately installed to obtain vertical data surrounding or within a karst feature. The following sections describe general guidance that may or may not be used depending upon the local situation and information deemed as needed.
 
 
 
===Subsurface material===
 
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);
 
*soil characteristics (ex. type, thickness, mapped unit, geologic source/history);
 
*photo-geologic fracture trace map;
 
*bedrock outcrop areas;
 
*sinkholes and/or other closed depressions; and
 
*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)
 
 
 
===Geophysical and Dye Techniques===
 
There are many different techniques available to view the nature of the subsurface in karst areas. These techniques can be used to detect the presence of karst features or to collect additional data on the character of a known feature. 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.
 
 
 
====Location of Borings====
 
Once the character of the cover material is known and understood, borings can be used to obtain the details of the subsurface karst features at specific locations. It must be noted, however, that the local variability typical of karst areas could mean that a very different subsurface could exist a very small distance away, perhaps as little as 6 inches. 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 one boring in each geologic unit present, as mapped by the Minnesota (MGS) and U.S. Geological Surveys (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; and
 
*documenting any areas identified as anomalies from any existing geophysical or other subsurface studies
 
 
 
====Number and Depth of Borings====
 
The number and depth of borings will depend entirely upon the results of the subsurface evaluation obtained from the observational, geophysical, and excavation studies, and 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 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.
 
 
 
====Identification of Material====
 
All material identified by the excavation and geophysical studies and penetrated by the boring should be identified, as follows:
 
*description, logging, and sampling for the entire depth of the boring;
 
*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;
 
*identified characteristics should include, as a minimum: color; mineral composition; grain size, shape, sorting and degree of 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. 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;
 
*water levels in all borings should be recorded 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; and
 
*when conducting a standard penetration test (SPT), estimation of soil engineering characteristics, including “N” or estimated unconfined compressive strength, should be reported.
 
 
 
====Evaluation====
 
At least one 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.
 
 
 
===Sinkhole Remediation===
 
There are several approaches to sinkhole remediation if it is found that 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.
 
 
 
==Monitoring of BMPs in Karst Regions==
 
A water quality monitoring system installed, operated and maintained by the owner/operator may be desirable or even required under some circumstances, particularly where drinking water supplies are derived from ground water or in association with known sources of contamination. The location of monitoring wells or BMP performance monitoring will again depend upon the nature of the BMP and surrounding karst characteristics. As with all nonpoint source related monitoring, the capture of runoff events is the key goal. In karst areas, this could mean the installation of a monitoring system designed to reflect variable water behavior typical of karst water flow. Attempting to monitor this behavior without a thorough understanding of the local geology will be difficult and could lead to a wasted effort.
 
-->
 

Latest revision as of 21:34, 23 June 2021

Warning: The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”. Active karst is a terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface.
image
map of MN karst lands
Minnesota karst lands. Karst may also occur in sandstone formations in the eastern part of the state (e.g. Pine County). (Source: E. Calvin Alexander, University of Minnesota, with permission)
statewide map illustrating karst areas
Map illustrating sinkhole probability in Fillmore County, Southeast Minnesota. The highest sinkhole densities are on flat hilltops between or adjacent to river valleys in the western half of the county (shown in red and orange). In these locations, limestone and dolomite is the first bedrock encountered and is typically within 50 feet of the land surface. Source: Minnesota Department of Natural Resources, October 2005.

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.

Information: The following definitions were based on a map developed by Alexander and others (2006) which defined active karst. These definitions need updating based on recent work in east-central Minnesota. Barry et al. (2016) state: "In Minnesota surface karst features primarily occur where 50 feet or less of unconsolidated sediment overlies Paleozoic carbonate bedrock, the St. Peter Sandstone, or the Hinckley Sandstone." We are working with the Minnesota Department of Natural Resources to update these definitions.

In Minnesota there are three classifications of karst lands.

  • Active Karst. Active karst is a terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface
  • Transition Karst. Transition karst is defined as areas underlain by carbonate bedrock with 50 to 100 feet of sediment cover.
  • Covered Karst. Covered karst is defined as areas underlain by carbonate bedrock but with more than 100 feet of sediment cover.

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.

What is karst?

illustration of karst features
The features of a karst system. Environmental Science Institute, The University of Texas at Austin. Caves as a Window into the Edwards Aquifer.

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.

Why is karst geology a concern?

pond water disappears in karst setting
A collapsed karst feature where a stormwater pond previously existed. The site was in Woodbury, Minnesota.
pond water disappears in karst setting

Woodbury sinkhole collapse looking toward the south.

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.

  • Runoff should be dispersed
  • Runoff should be pretreated and then infiltrated only if subsurface investigations and geotechnical analysis confirm that there are no unstable zones below the BMP
Warning: The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”. Active karst is defined as areas underlain by carbonate bedrock with less than 50 feet of sediment cover.
  • Additional borings and deeper borings may be warranted to target evaluation of transitional karst zone
  • Once ponds are constructed, include contingency plans for cases where karst features open up and impact a pond, including conducting geotechnical borings to appropriate depths trying to identify unstable zones, then target those zones for grouting
  • Convey runoff to a collection and transmission system away from the area via vegetated drainageways

Where can I get more information on karst in Minnesota?

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.

How to investigate for karst on a site

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.

Preliminary site investigation

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

  • 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; and
  • site observations made under varying weather conditions, especially during heavy rains and in different seasons to identify and map any natural drainageways.

Subsurface material investigation

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.

  • 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)

In conducting subsurface investigations, all applicable State regulations must be met. For more information, see Minnesota Department of Health's Wells and Borings website.

Warning: In conducting subsurface investigations, all applicable State regulations must be met.

Location of soil borings

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 in soil surveys;
  • placing an adequate number near on-site geologic or geomorphic indications of the presence of sinkholes or related karst features;
  • locating along geologic fracture traces;
  • locating adjacent to bedrock outcrop areas;
  • locating a sufficient number to adequately represent the area under any proposed stormwater facility; and
  • documenting any areas identified as anomalies from any existing geophysical or other subsurface studies.

Exploratory borings must comply with Minnesota statutes, 4727.0100

Warning: Exploratory borings must comply with State regulations

Number and depth of soil borings

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.

Information: Note that this manual contains a section on recommended number of borings. The number of borings needed at sites that have or potentially have active karst is likely to be greater than the number indicated in the above referenced section of the manual.

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.

Abandoning soil borings

Warning: Under Minnesota law all soil borings must be sealed by a licensed well contractor or a licensed well sealing contractor.

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.

Identification of material

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 minimum of 2 soil samples, representative of the material penetrated, including potential limiting soil horizons, with the results compared to the field descriptions.
  • Identify soil characteristics including color, mineral composition, grain size, shape, sorting, and degree of saturation.
  • Log any indications of water saturation to include both perched and groundwater table levels; include descriptions of soils that are mottled or gleyed. 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, noting that water levels in karst geology can vary dramatically and rapidly. Borings 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.
  • 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.

Geophysical and dye techniques

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.

Caution: Dye tracing requires expertise and should only be done by experienced karst hydrogeologists. Contact the Minnesota Geological Survey for more information.

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).

General stormwater management guidelines for karst areas

Warning: The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”.

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.

  • Conduct a thorough geotechnical investigation in areas with suspected or documented active karst. Karst geology can change rapidly over very short distances so additional soil borings may be required in comparison to geotechnical investigations for shallow groundwater or bedrock.
  • Investigate non-infiltration BMPs on sites where infiltration is not allowed under requirements of the CGP (i.e. where the BMP would be “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”).
  • Preserve the maximum length of natural swales as possible at the site to increase the infiltration and accommodate flows from extreme storms.
  • Minimize the area of impervious surfaces at the site. This will reduce the volume and velocity of the stormwater runoff. Consult with a geotechnical engineer prior to the design and construction of a BMP.
  • Capture the runoff in a series of small runoff reduction practices where sheet flow is present. This technique will help keep the stormwater runoff from becoming channelized and will disperse the flow over a broad area. Practices such as swales, bioretention with underdrains, media filters, and vegetated filters should be considered first at a site. However, not all sites lend themselves to this type of management approach. Adequate precautions should be taken to assure that runoff water is adequately pretreated.
  • Design BMPs to be off-line such that volumes of runoff greater than the capacity of the BMP are bypassed around the BMP. This approach will limit the volume through the BMP to a quantity that is manageable in the karst.
  • Install multiple small BMPs instead of a centralized BMP. Centralized BMPs are defined as any practice that treats runoff from a contributing drainage area greater than 20,000 square feet, and/or has a surface ponding depth greater than 3 feet. Centralized practices have the greatest potential for karst- related failure, and will require costly geotechnical investigations and a more complex design.
  • Direct discharge from stormwater BMPs to surface waters and not to the nearest sinkhole. Because karst areas can be quite large in Minnesota, discharges should be routed to a baseflow stream via a pipe or lined ditch or channel to redirect the flow away from the karst, provided the stream does not disappear into a karst feature.
  • Design ponds and wetlands with a properly engineered synthetic liner. It is HIGHLY RECOMMENDED that a professional geotechnical engineer investigate and recommend the depth of unconsolidated material between the bottom of the pond and the surface of the bedrock. A minimum of 3 feet of unconsolidated soil material is the minimum separation; however an expert may recommend 10 feet or greater. Pond and wetland depths should be fairly uniform and limited to no more than 10 feet in depth.
Warning: Section 18.10 of the Construction Stormwater permit requires that "Permittees must design basins using an impermeable liner if located within active karst terrain".
Caution: At sites within areas of active karst and that do not require a Construction Stormwater permit, a geotechnical investigation following guidance on this page is highly recommended.
  • Minimize site disturbance during BMP construction. Seek the recommendations of a geotechnical engineer for management of heavy equipment, temporary storage of materials, changes to the soil profile - including cuts, fills, excavation and drainage alteration - on sites that have been found to contain a karst feature.
  • Report sinkholes as soon as possible after the first observation of sinkhole development. The sinkhole(s) should then be repaired or the stormwater management facility abandoned, adapted, managed and/or observed for future changes, whichever of these is most appropriate.
  • Develop a contingency plan for how to manage the stormwater should a BMP fail as a result of the development of a karst feature.
  • If a karst feature is encountered report to the appropriate state agency, such as the Department of Natural Resources (DNR), Minnesota Geological Survey (MGS), and local agencies (such as the city, township or county). These known occurrences should be surveyed for specific location and permanently recorded on the property deed. For transition karst areas, local discretion and the likelihood of karstic features should be used to determine the amount of geotechnical investigation. An easement or reserve area should be identified on the development plats for the project so that all future landowners know of the presence of active karst on their property.
  • Incorporate additional precautions where infiltration practices are used. For example, infiltration of stormwater from stormwater hoptspots is discouraged unless pollutant concentrations can be significantly reduced through pretreatment practices.

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 Construction stormwater permit restriction1
Impervious area disconnect Preferred
  • Strongly recommended for most residential lots less than 6,000 square feet.
  • Discharge point from the disconnect should extend at least 15 feet from any building foundation.
No
Bioretention with underdrain (biofiltration) Preferred
  • Requires 3 foot minimum separation distance between practice bottom and bedrock. If this is not possible, an underdrain should be used to convey the water away.
  • It’s recommended that the drainage area to an individual bioretention BMP be kept less than 20,000 square feet.
  • Larger designs that rely on exfiltration of treated runoff into the soils below are not recommended.
No
Rain tank/cistern Preferred
  • Above ground tanks are preferred to below ground.
  • Overflow of tank should extend at least 15 feet from building foundation.
No
Rooftop disconnect Preferred
  • 15 feet foundation set back.
  • Runoff should be spread diffusely across landscape.
No
Green roofs Preferred
  • Runoff should be spread diffusely across landscape.
No
Dry swale or grassed channel Preferred
  • Line with underdrains.
  • Incorporate compost amendments into the bottom of the channel to improve runoff reduction for vegetative uptake, and transpiration.
  • Do not incorporate check dams unless the swale or channel incorporates an underdrain. Otherwise small areas of infiltration would develop in the zones upgradient of the check dam where the water is captured without adequate outlet.
Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst
Yes if designed for infiltration
Media filter Preferred
  • Recommended practice in areas of shallow bedrock and soil.
No
Vegetative filter Preferred
  • Recommended practice in areas of shallow bedrock and soil.
No
Soil compost amendment Adequate No
Small scale infiltration/micro-bioinfiltration Adequate
  • Create multiple small sized infiltration BMPs to infiltrate impervious areas of 250 to 2,500 square feet.
  • Not recommended for sites identified as a stormwater hotspot.
Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst
Yes
Permeable pavement Adequate
Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst
Yes if designed for infiltration (no underdrain)
Infiltration trench or basin Adequate
  • Surface area to depth ratios of practices may need to be larger than typical basin designs to minimize depth of the BMP.
  • Confirm suitability with supporting geotechnical investigations and calculations.
  • Not recommended for sites identified as a stormwater hotspot.
  • Incorporate pre-treatment to limit risk of groundwater contamination in the event of future failure of the BMP associated with development of a karst feature. Local review authority should be consulted for approval.
Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst
Yes
Constructed wetlands Adequate
  • Requires larger surface area to drainage area ratios to limit the depth of the wetland.
  • Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present.
  • Consider using liner and liner cells.
Warning: If the CSW permit applies, liners are required in areas of active karst
No
Dry extended detention (ED) ponds and wet ponds Adequate
  • Requires larger surface area to drainage area ratios to limit the depth of the basin or pond. If analysis shows that the soils can support the weight of a pond then a liner should be used to prevent infiltration
Warning: If the CSW permit applies, liners are required in areas of active karst
No
Wet swale Discouraged Not feasible No
Large scale infiltration Discouraged
  • Use small scale infiltration practices instead.
Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst
Yes

1Section 16.20 of the CSW Permit prohibits prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst features.


How to remediate after a sinkhole appears

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:

Monitoring of BMPs in karst regions

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.

BMP failure

In case of BMP failure in karst areas,

  • if waters of the state are or may be impacted, contact the state duty officer (651-649-5451 or 800-422-0798; or 651-297-5353 TDD Line/800-627-3529 TDD Watts Line);
  • contact appropriate local authorities; and/or
  • repair the practice following guidance above.


Related pages

This page was last edited on 23 June 2021, at 21:34.

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