Karst Bedrock & Well Contamination

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.

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.

The Challenges with Karst

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 Iowa 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.

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. 

Older water supply wells - especially those constructed before 1982 - may or may not have adequate protections in place to ensure that the well obtains its water from deeper, protected aquifers. If you have an older well, or if you don't know the depth and construction details of your well, you should have your well water sampled at least yearly so that you know if you should consume the water. You should also consider hiring an DNR Certified Well Contractor to perform a thorough well inspection to determine if your well includes construction features that will help ensure the well only accesses water from known safe aquifers and that your well provides safe drinking water.

The only way to determine if your well water is safe to consume is to take a sample of the water and send it to a drinking water laboratory. Your local county environmental health office can help you arrange for testing or you can obtain a water sample test kit, sample the water yourself and send it to a certified drinking water laboratory for analysis.

Private wells do not fall under mandated state or national water quality standards. Because of this, you should use the EPA contaminant standards for public water supplies as the recommended standards for your private water supply. The presence of any contaminant above the EPA's maximum contaminant level (MCL) means that you should not consume the water without proper treatment, water system repairs that eliminates the source of contamination, or replacing the well with one constructed to provide safe water.

For more information on specific water testing you may want to consider, please contact the State Hygienic Laboratory at the University of Iowa, 800-421-4692, or any other Iowa certified drinking water laboratory, your local Iowa DNR Certified Well Contractor or the Iowa DNR.

Laboratory Analysis

All drinking water samples must be analyzed by a State Certified Drinking Water Laboratory. Laboratory analysis of drinking water requires controlled conditions, expensive technical equipment and highly trained professional staff.

There are currently over 40 certified labs who can provide water analysis for private well owners. There is a statewide grant program called the Grants to Counties Water Well Program that can provide you with basic water testing for free. For more information, please contact your local county environmental health specialist.

The Risk You Take When You Consume Unsafe Water

You increase your risk for illness and disease if you drink contaminated water. Each type of contaminant has its own health impacts depending on the level of contamination, how much of the contaminated water you consume and your individual sensitivity to the contaminant(s).

Here is some basic information on the most commonly detected contaminants.

• Bacteria, viruses and protozoa are microorganism groups that contain pathogens that can cause waterborne diseases. When these microbes are present in your water supply, symptoms such as diarrhea, cramps, nausea, headaches and others are possible. Microbes can pose a much greater health risk for infants, young children, senior citizens and those with severely compromised immune systems.
• Nitrate in drinking water above 10 mg/L should never be given to infants less than six months old as it can cause a potentially fatal disease called blue baby syndrome" There are also indications that long term exposure to nitrate levels may lead to other health issues, but more research is needed in this area.
• Studies have shown that chronic or repeated ingestion of water contaminated with arsenic over a person’s lifetime is associated with increased risk of cancer of the skin, bladder, lung, kidney, nasal passages, liver or prostate, and noncancerous health effects like diabetes and cardiovascular, immunological and neurological disorders.
• Other contaminants. EPA has a website available that contains a list of common contaminants and the potential health risks associated with each. You can view the website at the following web links:
  • http://www.epa.gov/ccl/types-drinking-water-contaminants
  • http://www.epa.gov/dwstandardsregulations/drinking-water-contaminant-human-health-effects-information.

For additional information on water borne diseases, please see the Centers for Disease Control website, Iowa Department of Public Health website, or contact your local county environmental health office.

Contaminants

The primary indicator tests normally performed include coliform bacteria, nitrate, and nitrite. Testing for these contaminants is easy, inexpensive and can be used to provide basic information on drinking water safety. There are also other water tests you should consider. Contaminants like arsenic, fluoride, nitrate, nitrite, and lead are naturally occurring in some aquifers and may require specialized water treatment to reduce or eliminate the exposure risk.

You should also consider testing shallow wells periodically for pesticides.

• Pesticides are mostly modern chemicals that are used to control weeds and insects, and improve crop and turf production. Pesticides are used on farms and in communities. You likely have a number of pesticides that you use routinely in and around your home.
• Atrazine is one of the most commonly found pesticides discovered during well water analysis. Learn more about Atrazine in drinking water and health concerns that may be attributed to this chemical by visiting the CDC Atrazine Information web page.
• Volatile Organic Compounds (VOCs) such as gasoline, vinyl or plastics, adhesives, dry-cleaning fluids, refrigerants, paints and solvents.
• Animal waste like manure and compost.
• Or a number of emerging contaminants that may be linked to our modern lifestyle and activities.

The following table lists water-related diseases and contaminants that may be found in unsafe drinking water and provides links with additional information. Source - Centers for Disease Control and Prevention

Managing Risks

Knowledge of your water quality will empower you and help you make informed decisions about your consumable water and long term health. First and foremost, ensure that all the water you consume is safe. Laboratory analysis of your well water is the only way to determine this. Your local county environmental health specialist or certified drinking water laboratory can help you decide which tests make sense for your well depth and location.

How often you test your supply should be based on several things. They include what your previous water testing found, who is using the water, changes in the water quality you can see, taste or smell, and the type of potential contaminants applied to land near your well. 

At a minimum, all private well users should test their water supply at least once each year for a minimum of coliform bacteria and nitrates, and have at least one arsenic test performed on the well water.

You should test your well more often if:

• You suspect or know your well obtains part of all of its water from a shallow aquifer or Karst bedrock.
• Previous testing found contamination and the level of contamination was near maximum recommended MCLs or above the MCLs.
• You have young children, senior citizens or immune compromised individuals using the water.
• Your water quality suddenly changes. Things like unusual tastes, odors, color or sediment.
• You have any well repair or pump and water system services performed.
• Neighboring wells of similar depth and construction have tested positive for contamination.
• One or more of your family members become ill and the symptoms do not seem to improve within a time period that seems normal based on your previous experiences.

If your water analysis finds the water is unsafe for any contaminant, do not drink the water. You should obtain all of your drinking and consumable water from a known safe water source, like a well recently proven safe by water testing and analysis, a water source with a water treatment device connected to the supply that is designed to remove the contaminant(s), a public water supply, or you should purchase bottled water. This reduces or eliminates your immediate risk and allows time for you to investigate the long term options for managing your water system.

If you live in a Karst area and your well obtains its water from shallow groundwater sources that have been proven to be unsafe to drink and renovating or replacing your well is not an option, you should use a known safe alternative source for your drinking water needs or consider the use of a reverse osmosis water treatment system at each point of use where you want to obtain your drinking water. Properly designed, installed, and maintained, reverse osmosis systems can provide you with a safe source of drinking water under Karst conditions.

Consumable Water

The term "consumable water" includes water that you drink and water used for mixing beverages, making ice cubes, preparing food, brushing teeth and rinsing mouth, and used for child's play. Depending on the type and level of contamination, consumable water also includes water used to wash dishes, wash hands, and shower or bathe.

Short term options if your water supply is contaminated:

• Use the well only for purposes it does not pose any health risk and use bottled water for all consumable purposes
• Install and maintain approved water treatment devices at each tap you obtain consumable water
• Regardless of which method used, contact an Iowa DNR Certified Well Contractor for consultation about your water supply.

Long term options if your water supply is contaminated:

• First, contact an Iowa DNR Certified Well Contractor for consultation. They can help you make informed decisions like:
  • Can you modify your existing well to keep-out the poor quality water?
  • Do you need to replace your well with a new well that includes additional construction features designed to improve your water quality?
  • Is it possible to install and maintain approved water treatment devices at all taps where you obtain consumable water?
  • Is it best for you to connect to a nearby public water supply if one is available?
  • And as a last resort, can you use the well only for purposes where it does not pose any health risk and use bottled water for all consumable purposes?

Additional Resources

• Water Treatment Options - Informational booklet by the State Hygienic Laboratory at the University of Iowa that provides a guide to well water contaminants and effective water treatment devices.
• Well Consumer Information Booklet - This booklet is designed to help private well owners understand private well construction and ownership.
• Iowa's Groundwater Basics - A geological guide to the occurrence, use and vulnerability of Iowa's Groundwater.  The publication is full of good information, interesting diagrams and wonderful pictures. Published by the Iowa Geological Survey.

DNR Fact Sheets

• Karst in NE Iowa
• How to Sample Your Well Water
• Understanding Your Water Testing (Lab) Report