Learning More about Karst Landscape in the Midwest
By kevin.erb | 1:22 PM, 09/15/2014
The word “karst” describes a landscape where, over time, water has dissolved part of the rock (typically limestones), creating wider fractures, sinkholes, and over long periods of time, caves and caverns. Water moves very quickly through these features, carrying sediment, nutrients, pathogens and other contaminants into the groundwater.
In certain areas of the Midwest (including Wisconsin, Illinois, Iowa and Minnesota), these karst features occur in areas of intensive agricultural production. Elevated levels of nitrate and bacteria are often found in the groundwater in karst areas—some counties in Wisconsin report >50% of rural wells in problem areas are impacted (Kewaunee, Calumet). Both cash grain and livestock farms in karst areas need to take extra precautions to reduce the chances of contributing to groundwater contamination.
Sinkholes are areas where the water has dissolved the bedrock (or widened existing cracks), allowing soil to fall in. These start out as very small features (the size of a chipmunk burrow), and over a long period of time, can grow to be an acre or more in size. Smaller ones may appear each year, only to be covered up by tillage. They provide a direct route for surface water to enter groundwater. In the past, some farms routed drainage tile directly to sinkholes, but this practice is now illegal.
Swallets occur when a creek or stream is “swallowed” or enters a sinkhole or other feature. In some cases, the water re-appears a short distance away in a spring. In other areas, the water does not have an exit point.
Fracture Traces are often visible in late summer in alfalfa fields. These area cracks in the rock that have filled in with soil. When the weather turns dry, plants directly above these cracks have access to deeper soil, and so grow much taller than plants just a few feet away. This is a sign that nutrients and other contaminants may be able to enter the groundwater, as the soil is too shallow for adequate filtering.
The concern is not just limited to areas of very shallow soil. Cracks in the soil (in particular, the heavier clays), root channels and earthworm burrows can allow surface water to bypass normal soil filtering and reach deeply buried karst features. Surface runoff carrying manure had been documented moving 17 feet through cracked soil.
For conservationist and resource managers, the first step is learning more about karst geology and how to find and recognize these features in the landscape. Then susceptible areas can be identified and management practices implemented to reduce the risk.
You can find out more by taking the upcoming Conservation in Karst Landscapes/Highly Permeable Soils course on October 2, 2014 in Chilton, WI and signing up for the October 9, 2014 Karst Areas and Stormwater Management Workshop in Kimberly, WI.
author Kevin Erb is a University of Wisconsin-Extension Specialist