Minnesota's southeastern limestone country is a beautiful area of farms, rolling hills, hollows, caves, and dramatic bluffs and valleys (figure 1).
These same characteristics make southeastern Minnesota water resources most challenging to protect. Petroleum and other chemicals released from underground storage can quickly move into ground water supplies. Manure released from agricultural spills can cause fish kills many miles from the release point. Chemicals used on the landscape can reappear at unexpected times and in unexpected locations.
Figure 1. Minnesota's Karst country
Southeastern Minnesota is so challenging to protect because limestone is slowly dissolved by infiltrating rainwater, sometimes forming hidden, rapid pathways from pollution release points to drinking water wells or surface water. These pathways can be widened, interconnected fractures or caves in the subsurface. Sometimes the process of dissolving limestone forms distinctive landforms on the ground surface, and in other places there is no distinctive landform at all. Together, the processes that dissolve limestone bedrock and the landforms that result are called karst (figure 2). This Web page discusses the process leading to the formation of Minnesota's karst, karst landforms, environmental problems that occur in karst landscapes, and what MPCA is doing about environmental problems related to karst.
Figure 2. Idealized karst landscape, showing many karst landforms
and the subsurface
Source: Graphics provided courtesy of the Southeast Minnesota Water Resources Board as part of a grant funded by the Minnesota Environment and Natural Resources Trust Fund.
How Minnesota's karst landscapes form
Karst is an efficiently drained landscape that forms on soluble rock. Karst is characterized by caves, sinkholes, a lack of surface drainage and other climatically controlled features, and is mainly, but not exclusively, formed on limestone. (For more information, see the Sandstone Karst in Pine County, Minnesota Web page on the Geological Society of America's Web site.)
Karst features arise when rain falls and infiltrates the soil, where the availability of carbon dioxide causes the formation of weak carbonic acid (figure 3). If the slightly acidic soil water never reaches soluble bedrock, nothing happens. But if is able to move into contact with soluble carbonate rocks, dissolution occurs, with calcium, magnesium and bicarbonate ions as by products.
Figure 3. Dissolution of carbonate rock
Source: Image from British Columbia Ministry of Forests
Dissolution of the rock is focused where water flow and surface area are greatest, and this is usually along areas of pre-existing fractures, and partings or bedding planes. These features easily conduct the water, and are gradually widened by dissolution, sometimes widening greatly until they become caves or a collapse feature occurs at the surface.
The water, now ground water, moves within pore space in the rock, and will flow under pressure or in response to gravity until it emerges either as surface water (springs or seeps) or as drinking water (well water).
Karst aquifers are called triple-porosity aquifers (see Worthington, 1999) because water moves within three distinct, but connected, frameworks: matrix, fractures, and conduits. In the matrix, ground water moves relatively slowly, in fractures velocities can be higher, and in conduits the water can move exceedingly fast.
For the same reason, karst aquifers are very difficult to protect from activities at the ground surface. For while pollutants are quickly transported to drinking water wells or surface water, conventional hydrogeologic tools such as monitoring wells are of limited usefulness. The best strategy is pollution prevention.
Karst landforms in Minnesota
Karst landforms are concentrated in southeastern Minnesota. In figure 4 (courtesy of E. Calvin Alexander, University of Minnesota), most karst landforms are found in the red zone ("active karst"), and some are found in the yellow zone ("transitional karst"). Relatively few karst landforms are found in the green zone ("covered karst").
Figure 4. Minnesota Karst Lands
Source: E. Calvin Alexander, University of Minnesota
Minnesota's most common karst features are discussed and illustrated below (definitions supplied by Calvin Alexander, University of Minnesota, and Jeff Green, Minnesota Department of Natural Resources).
The MPCA, with assistance from several partners, recently produced an educational poster that specifically highlights karst features in Minnesota. The poster can be downloaded below. A limited number of copies may still be available. Please contact the following to enquire: Nancy Miller, MPCA Rochester Office, 800-657-3864 or 507-206-2631, or Sandeep Burman, MPCA St. Paul Office, 800-657-3864 or 651-757-2569.
Subsurface drainage (lack of surface water)
When rainwater infiltrates quickly, as in a karst landscape, water is not collected into streams, and cannot cut valleys. Instead the most distinctive feature may be the total lack of drainage features (figure 5).
Figure 5. Subsurface drainage (lack of surface water)
Blind valley
A valley that terminates abruptly at a point where its stream sinks, or once sank, underground. Blind valleys (figure 6) are completely enclosed valleys that water can not flow out of on the surface. Disappearing streams often sink in blind valleys.
Figure 6. Aerial Photograph Showing Location of Blind Valley
Cave
A feature generally formed by solution of limestone containing a natural underground room or series of rooms and passages large enough to be entered by people (figure 7).
Figure 7. Cave
Photograph courtesy of Allen Lewerer
Disappearing stream
Surface streams that run into holes in the ground and partially or completely cease flowing on the surface (figure 8).
Figure 8. Disappearing stream
Photograph courtesy of Jeff Green, MDNR
Sinkhole
A closed depression caused by a collapse of soil or overlying formation above fractured or cavernous bedrock (figure 9).
Figure 9. Air photo of sinkholes on a sinkhole plain near
Fountain, Minnesota
Spring
Any natural discharge of water from rock or overlying soil onto the surface of the land or into a body of surface water (figure 10). A disappearing stream may re-emerge at a spring.
Figure 10. Cold Spring, near Mazeppa, Minnesota
Environmental problems that occur in Minnesota's karst country, and MPCA programs that address them
In karst country, bedrock dissolution and rapid ground water flow result in a hydrogeologic setting sensitive to land use. Sometimes chemical changes occur that render water unsafe to drink. Or in some settings, chemical changes cause environmental damage to surface waters. The most spectacular environmental problems occur when bedrock dissolves so completely that it collapses.
Pollution of drinking water wells
The high yields prized by supply-well drillers are due to the solution enhanced porosity and resulting high transmission zones that may also cause carbonate aquifers to be easily polluted. Rapid water flow usually means that little filtration has occurred, and water originating at a pollution source may rapidly move, perhaps undetected, to a drinking water well.
Pollution of surface water
In karst landscapes, the distinction between ground water and surface water is commonly blurry, and sometimes very tenuous. Ground water may emerge as a spring, flow a short distance above ground, only to vanish in a disappearing stream, and perhaps re-emerge farther downstream again as surface water.
The intimate connection between ground water and surface water gives rise to large number of cold water streams in southeastern Minnesota where trout and other important species thrive. Pollution traveling rapidly along a ground water path may emerge at a lake or stream, thus posing a threat to the animals and plants living there. In the same way, pollution that has reached surface water can easily become ground water pollution, thus posing a pollution risk to people whose drinking water is ground water.
Problems related to the collapse of dissolving bedrock
Collapse of carbonate bedrock beneath liquid storage basins has been reported in many states, including North Carolina, Missouri, Iowa, and Minnesota.
Municipal sewage lagoons have collapsed in three southeastern Minnesota communities (Altura, Bellechester, and Lewiston) since 1976. All three lagoons were built in similar hydrogeologic settings: shallow carbonate bedrock beneath a thin layer of sand or sandstone (Alexander and Book, 1984; Dalgleish and Alexander, 1984). Geologists theorize that the lagoons' high leakage rates saturated the sandy material beneath with carbonate-poor water, which readily dissolved the underlying carbonate bedrock, or washed soil into preexisting solution cavities. In either case, the undermined lagoons collapsed, sending millions of gallons of sewage to the aquifer (Alexander and Book, 1984; Dalgleish and Alexander, 1984).
MPCA programs addressing environmental problems in Minnesota's karst country
The best strategy to prevent problems related to pollution in the subsurface is pollution prevention: Preventing waste and pollution
The Hazardous Waste program and the Feedlot program adopt the pollution prevention approach. See the following documents for more information:
Where a pollution problem has already occurred, several remedial responses are available to repair damage. Remedial responses usually focus on breaking the link within the ground water path connecting the pollution source to the drinking water well. The MPCA remediation programs (LUST, VPIC, VIC, Superfund, and Landfills) adopt this approach. The remediation program approach to cleanups in karst areas is available on the following fact sheet:
Links to karst-related web sites
Government
Academic
Online publications
Organizations focused on karst
Karst in the news
Karst aerial or space imagery
Companies concerned with karst
Journals related to karst
References cited
- Alexander, E. Calvin Jr., and Book, Paul R. 1984. Altura Minnesota lagoon collapses. In Proceedings of the First Multidisciplinary Conference on Sinkholes, Orlando, Florida, October 15-17, 1984, pp. 311-318.
- Dalgleish, Janet, and Alexander, E. Calvin, Jr. 1984. Sinkhole distribution in Winona County, Minnesota. In Proceedings of the First Multidisciplinary Conference on Sinkholes, Orlando, Florida, October 15-17, 1984, pp. 79-85.
- Living with Karst—A fragile foundation. AGI Environmental Awareness Series, 4. American Geological Institute, 2001. 65 pp.
- Worthington, Stephen R. H. 1999. A comprehensive strategy for understanding flow in carbonate aquifers. In Palmer, Arthur N., Palmer, Margaret V., and Sasosky, Ira D. Karst Modeling--Proceeding of the symposium held February 24-27, 1999, Charlottesburg, Virginia. Special publication 5 of the Karst Waters Institute, Charles Town, West Virginia. Pages 30-37.
More information
For more information about karst in Minnesota, contact Steve Thompson, 651-757-2778.
