Southeastern Minnesota is characterized by an unusual type of geography called karst. It features rolling hills, hollows, caves, sinkholes, and dramatic bluffs and valleys.
These features, formed primarily of limestone, make the topography "porous" and makes the area's water resources more challenging to protect. Contaminants can quickly find routes from the surface into groundwater. Petroleum and other chemicals leaking from underground storage tanks can quickly move into groundwater. Spilled manure can cause fish kills many miles from the release point. Chemicals used on the landscape can reappear at unexpected times and in unexpected locations.
Some features of karst geography:
- Subsurface drainage (lack of streams and other surface water) — When rainwater filters quickly into the soil, as in a karst landscape, water is not collected into streams, and cannot cut valleys.
- Blind valleys — A valley that terminates abruptly at a point where its stream sinks, or once sank, underground. Blind valleys are completely enclosed valleys that water can not flow out of on the surface.
- Disappearing streams — Surface streams that run into holes in the ground and partially or completely cease flowing on the surface
- Sinkholes — Closed depressions caused by a collapse of soil or overlying formation above fractured or cavernous bedrock
- Springs — Any natural discharge of water from rock or soil onto the surface of the land or into a body of water. Disappearing streams may re-emerge at springs.
In the figure at right, most Minnesota 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”).
Protecting water in karst region
In karst landscapes, the distinction between groundwater and surface water is blurry. Groundwater 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. This connection between groundwater and streams makes southeastern Minnesota home to many cold water streams, where trout and other important species thrive. In addition, pollution affecting groundwater can quickly threaten a lake or stream, and the animals and plants living there. And contaminated surface water can easily become groundwater pollution, and pose a health risk to those using it for drinking.
Another concern is the potential for carbonate bedrock beneath liquid storage basins to collapse, which 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 were built in similar hydrogeologic settings: shallow carbonate bedrock beneath a thin layer of sand or sandstone. 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. The collapsed lagoons sent millions of gallons of sewage into the aquifer.
Pollution prevention is the best strategy for protecting water quality in the karst region. Learn more: