What is karst?
“Karst” is a landscape created when water dissolves rocks. In Wisconsin, dolomite and some limestone are typical soluble rocks. The rocks are dissolved mostly along fractures and create caves and other conduits that act as underground streams. Water moves readily through these openings, carrying sediment (and pollutants) directly into our groundwater.
Karst landscapes may have deep bedrock fractures, caves, disappearing streams, springs, or sinkholes. These features can be isolated or occur in clusters, and may be open, covered, buried, or partially filled with soil, field stones, vegetation, water or other miscellaneous debris.
How do sinkholes form?
Sinkholes are holes or depressions that form when water washes sediment down into cracks and voids in karst bedrock. Sinkholes form from the bottom up as the sediment immediately above the bedrock is the first to be washed into the voids. The land above a sinkhole often appears normal until a critical amount below has been washed away. When the soil surface can no longer support the weight, it collapses.
Not all sinkholes are the result of karst. Manmade sinkholes occur when a water main break washes sediment out of the area, creating a large cavity.
Where is karst found?
Karst occurs worldwide. Some of the largest sinkholes are found in Florida and Guatemala.
In Wisconsin, karst is most likely to occur in a V-shaped swath that extends southeast from St. Croix County along the Mississippi River, across the bottom two tiers of counties, and northeast along Lake Michigan up to Marinette County. Much of it is masked by thick layers of glacial deposits, particularly in the south-central and south-eastern parts of the state.
How big do sinkholes get?
Depending on the type of underlying bedrock, sinkholes can range in size from tiny depressions in the surface to gaping building-eaters that are hundreds of feet wide. Sinkholes in Wisconsin tend to be smaller than 10 feet across. The depth of sinkholes can be highly variable, although most are about as deep as they are wide.
Do I need to worry about my house falling into a sinkhole? The short answer is that it’s highly unlikely. Although other parts of the world have house-eating sinkholes, Wisconsin’s sinkholes are relatively small. The difference lies in our geology. In Wisconsin, the karst bedrock forms in dolomite, which is much less easily dissolved than the limestone that forms the karst bedrock in Florida. As a result, we have fewer and smaller voids and cavities in Wisconsin’s karst. The sinkholes are proportionally smaller as well.
Karst and groundwater contamination
The cracks and crevasses in karst act as direct conduits for pollutants to enter groundwater, wells, springs, and streams. If you’ve got a sinkhole, you’ve got karst.
Protect your groundwater and wells by being careful about what you spread in these areas.
What to do if you’ve got a sinkhole
If a sinkhole appears on your land, first determine whether it’s a safety hazard. If it is, mark the location, restrict access, and, if necessary, call 911. Don’t allow anyone to crawl into the hole or be lowered into it—newly collapsed sinkholes may still be unstable. Also, please do not toss trash or anything you wouldn’t want in your drinking water into a sinkhole.
Your next step depends on the size of the hole:
- Small sinkholes (less than 20 feet across)—Randomly filling a sinkhole with soil and rocks will not permanently fill it. Instead, the best strategy may be to fill it using cement or using a procedure known as reverse grading. With this technique, fill the hole first with large rocks, then use progressively smaller rocks to fill the hole, and finish with 8–12 inches of soil. Placing large material at the bottom provides support and helps prevent another collapse; smaller material helps stop water from moving soil downward. This method may not work unless the large material is placed directly on bedrock.
- Large sinkholes (more than 20 feet across)—It may not be economical to fill large sinkholes. Instead, fence the area off to permanently limit access and ensure that no homes or other buildings are ever built on it. To prevent unfiltered surface runoff from entering the groundwater system, build up a low earthen berm around the hole.
A final note: Don’t be surprised if more sinkholes form nearby or if the same sinkhole returns. No matter how thorough the efforts are to fill a sinkhole, the conditions that created it—water and karst bedrock—are still present.