Karst Bedrock & Well Contamination

Why You Should Read This Web Page

Good quality drinking water is one of the building blocks for good health. Understanding your drinking water quality can help protect the health of you and your family. You should read this web page if you consume water from a private water well that is finished in any shallow aquifer.  Shallow aquifers are more susceptible to contamination due to their close proximity to the ground surface and the influences caused by local land use activities.

Why this Topic is Important

Even though your water may look clear and taste good, you will not know if it's safe to consume unless you take the necessary steps to confirm it's safe. For those connected to a public water supply, the water supply operator performs routine testing and informs the water users if the water is not safe to consume. For private wells, the responsibility of water testing falls solely onto the well owner and anyone else who uses the well water. This means you - as a well user - need to take the steps to ensure that your water supply is tested for the contaminants that may be present in your local area and inform yourself by understanding the analytical results provided by the testing laboratory.

Anyone who consumes unsafe water may experience health issues. Your risks increase if your water is over the EPA maximum contaminant level (MCL) for any contaminant and when you consume contaminated water for longer periods of time. Anytime you consume water that contains unsafe levels of any contaminant, you increase your risk for negative health effects.

The Potential Problem

Some wells finished in shallow aquifers can contain high levels of bacteria, nitrates, pesticides and other chemicals and organisms. Any of these contaminants can adversely impact health if consumed. Even older wells finished in currently safe aquifers can be affected when steel well casing used in the well's construction corrodes and the casing develops small leaks that allows poor quality shallow groundwater to enter the well.

Many farms and homes in Iowa obtain their water from bedrock wells. Deep bedrock wells are generally safe water supplies because they tap deeper aquifers that have natural geologic barriers in place that helps keep contaminants from entering the aquifer. But in some areas of our state, shallow bedrock aquifers exist. Some of this bedrock is defined as "Karst".

The term "Karst" refers to terrain characterized by the presence of easily dissolved bedrock (limestone and dolomite) near the ground surface. Because carbonate rocks can be dissolved by groundwater, karst areas are often characterized by sinkholes, springs, and losing streams - a stream where some or all of the surface water is diverted into the groundwater system through bedrock surface features that connect the stream bed to openings in the bedrock below the stream bed. Karst bedrock is characterized as bedrock that is close to the land's surface and contains a vast network of underground drainage systems that have direct connections to the land's surface.

In areas of Karst, much of the rainfall that would normally flow to rivers and streams instead flows into the shallow bedrock and becomes part of the groundwater some water wells may utilize. Some of the water that originates at the surface also flows undetected into the ground. This water can contain contaminants that are found on the land's surface and those not bound or utilized by the areas soils and land cover. Once in the ground, this water that was once on the surface becomes part of the groundwater supply.

The shallow groundwater located in or near Karst areas can be highly vulnerable to contamination because contaminants can travel quickly from surface water to the local shallow aquifers through features like losing streams, sinkholes, bedrock fractures, or cave systems. This bypasses the natural tendency for water to be naturally filtered by the soils. A well that obtains part or all of its water from a shallow aquifer can have higher levels of contaminants when compared to deeper wells in the same area.

When you live in a Karst area and use a water supply well for your drinking water needs, it's important to understand how protected your water source is based on your specific location, the land use practices in your area, your well's construction, and most importantly of all - frequent sampling and analysis of your well water. With this additional information you will be able to understand the quality of your drinking water and any health risks that can be attributed to the shallow groundwater.

Back to top

The Areas Affected

Shallow aquifers are less protected than deeper aquifers, and Karst bedrock aquifers are some of the most susceptible to contamination. Although Karst features can be found in a number of locations across Iowa, they are most abundant in the NE corner our state.

This map shows the areas of northeast Iowa where Karst bedrock is likely to occur. Shallow wells located in areas of Karst bedrock can have poor water quality. The darker shaded areas are those where sinkholes and bedrock exposures are most likely and water quality concerns are the greatest.

Whether or not your well may be affected will depend on a number variables, like:

  • The location of the well
  • The well's finished depth
  • The actual aquifer(s) the well taps
  • The well's primary casing depth
  • The presence of casing grout around the well casing
  • The age of the well
  • The historic and current land use activities nearby the well.

In general, if you use a shallow aquifer for your water supply, you may be affected. If your well is located in one of the shaded areas of the NE Iowa Karst Map and obtains any of its water from the shallow aquifer, your well has the potential to be influenced by Karst groundwater. Regardless of location, shallow well users should test their well water to make sure it's safe to consume.

Back to top

Challenges in Karst Areas

Because of the potential for surface water influence and the types of land use activities that may take place in Karst regions, localized shallow groundwater may contain infectious bacteria, viruses, agricultural or industrial chemicals, and other hazardous agents. In addition, the groundwater flow path through Karst bedrock can be unpredictable. This means that some shallow wells in Karst areas may be unaffected by contaminants while other wells show signs of contamination.

Constructing and maintaining a well in Karst areas requires greater care to ensure your drinking water comes from a deep, safe aquifer using construction standards that help protect your drinking water quality and the aquifer. Most modern water supply wells include protective construction features designed to exclude the shallow groundwater associated with Karst terrain. These features include setting the well casing to a greater depth to exclude the upper groundwater; full depth grouting/sealing of the well casing to help reduce/eliminate the chance that shallow groundwater will move downward along the well casing and into the well; and the use of groundwater from only known deeper, protected, and safe aquifers. If you live in a Karst area and are considering a new well, make sure that you hire only DNR Certified Well Drillers and ask them to document what construction features they will use to help protect your well and drinking water quality.

How Surface Water becomes Groundwater in Karst Areas

Normally, rain events create surface water that runs off into rivers and streams and is absorbed by soils and till. But in Karst areas, the run off can rapidly flow into the bedrock - either directly through shallow openings, surface sinkholes, vertical and horizontal fractures, or solution channels that are exposed near or at the ground's surface, or indirectly through the pore openings and areas of thin soil overlying the limestone bedrock.

In most areas, we depend on the soils to act as a filter in which biological and chemical interactions take place that help improve groundwater quality. But in Karst areas, there is little or no soil for the water to flow through before reaching the shallow aquifer. This means there is little or no natural filtering or treatment that takes place before the water mixes with the shallow groundwater. In addition, there is also no natural water filtering when water flows directly into the bedrock via sinkholes, losing streams, or a system of voids or fractures that reach deep into the ground.

In Karst systems, soil infiltration, surface water run-off and streams can drain directly into the shallow bedrock and become part of the shallow groundwater and aquifer. This means that the contaminants found on the land surface and in surface water may also be present in wells that draw water from the shallow aquifer. When surface water moves into a shallow aquifer, the potential for poor well water quality in the aquifer increases.

Back to top

How to Find Out if Your Water is Safe

When you live in a Karst area and use a water supply well for your drinking water needs, it's important to understand how protected your water source is based on your specific location, the land use practices in your area, your well's construction, and most importantly of all - frequent sampling and analysis of your well water. 

Please see the DNR Well Testing webpage for more information on testing your well.

Back to top

Additional Resources

Back to top