Groundwater Contamination in Karst
Why Protecting Groundwater in Karst Is Important
Any source of water for a public supply, whether rivers, lakes, granular aquifers, or karst aquifers, can become polluted. The contaminant may not be treatable with the equipment at the local water plant, or in some cases, by any water treatment plant. Because about half of the surface area underlain by shallow, unconfined aquifers in Kentucky is karstic, a significant percentage of the groundwater in Kentucky moves through karst aquifers. Most karst springs previously used for a public water supply have been abandoned because of groundwater contamination. Despite that, water from karst aquifers remains vital to the commonwealth, because karst springs support the base flow of the streams to which they discharge. This means that most public systems in karst areas are still using water from a karst aquifer when they withdraw from a stream or reservoir.
Modern water treatment plants can remove many of the contaminants from a water source. The two most common contaminants, suspended sediment and bacteria, are routinely and cheaply eliminated. But it is much more efficient and less expensive to filter and sanitize if the plant begins with clear, clean water. For example, disposing of the suspended sediment that has been removed from "raw" water is a significant expense for some municipal water supplies, and a largely sediment-free source is highly desirable. As another example, the number of fecal bacteria in water collected from springs draining urban and agricultural areas can reach counts of hundreds of thousands of bacteria per 100 milliliters of water, making the water more expensive to make safe for people to drink. Some contaminants, such as nitrate-nitrogen (fertilizer), cannot be cheaply removed. Pesticides, some types of microorganisms, hydrocarbons, and some metals are also technically difficult to remove. The discovery of a contaminant that is new to a source, which then reoccurs or persists, can lead to drastic redesign and construction costs at a water treatment plant. It is technically much easier, and conserves tax dollars, to keep the water clean than be compelled to treat for every possible contaminant.
A significant number of Kentucky households are not on any public water system. Many homes will, for the foreseeable future, continue to use wells in karst aquifers or springs. An estimated 11 percent of karst springs in Kentucky are used for domestic water supplies: this affects over 10,000 homes. These people have a critical interest in protecting the quality of the water they are drinking. Most homes have no water treatment system, and those that homes that have water treatment only remove sediment and sanitize the water, but cannot remove organic compounds or metals.
Finally, if there were not a single person consuming water from karst aquifers, it would still be worthwhile to protect the groundwater. Of major economic importance are the livestock that are watered from springs, and aquiculture is also a growing use of karst groundwater. A unique and potentially valuable ecosystem ekes out a precarious and easily destroyed existence in karst aquifers. The potential economic value of the knowledge yet to be obtained from the animals that live in karst aquifers has only recently been realized.
Why Karst Aquifers Are Vulnerable to Pollution
Water recharge to karst aquifers occurs directly, either through swallow holes and sinkholes, or indirectly through the pores in the soil overlying the limestone bedrock. Although the soil overlying a karst aquifer provides some filtration of contaminants from in-flowing water, almost none takes place between swallow holes and springs. The water also has little opportunity to be filtered or for the contaminants to become bound to the bedrock as the water flows rapidly through the karst conduits.
Reasons for Karst Groundwater Vulnerability
- Recharge to karst aquifers bypasses the filtering capability of soil through macropores and swallow holes
- Groundwater flows through conduits so that there is little opportunity for filtration or sorption of contaminants onto aquifer material
- The movement of pollutants cannot be directly observed as in a surface-flowing stream
- Flow paths may take routes that are not apparent from the topography or slope of the land
- Flow velocities in karst aquifers are fast compared to velocities in granular aquifers, allowing little time to warn downstream users following a reported spill
- Flow is in converging conduits; therefore pollutants are not diluted through dispersal
How to Protect Karst Groundwater
The best way to protect a karst aquifer from pollution is for humans to limit industry and other activity over the groundwater basin. This is easy when the basin is within the control of a single user. It is very difficult when the groundwater basin is large and many landowners are involved. Ask your neighbors if they know of the impact land use has on groundwater in karst aquifers. Let them know that you think clean water is important and that disposal of waste in a sinkhole may ruin a neighbor's well or spring, their own water supply, or a city's water supply. No matter where you are in Kentucky, you are upstream from the next person to use the water.
The most important practice to protect groundwater in urban areas is to be certain domestic sewage is disposed of in a properly operating septic system or into a sanitary sewer. Older homes may have sewage discharge pipes routed directly into sinkholes, resulting in health and environmental hazards. Another important practice is to retain parking-lot and street runoff until solids settle and grease and oil can be filtered. The cautious application of pesticides and fertilizer in suburban areas is important to limit the runoff of contaminants into storm sewers, which may flow to sinkholes.
If you live in a rural area, countywide garbage pickup and programs to refurbish domestic sewage disposal systems may be available. If you farm, get involved in U.S. Department of Agriculture cost-share programs to promote soil and groundwater protection, or take action on your own. Manure, fertilizer, pesticides, and eroded soil are the most common pollutants from agriculture. Livestock should be excluded from sinking streams and karst windows. Extra precautions should be taken to prevent manure holding lagoons from leaking. Use chemicals judiciously and in conjunction with conservation tillage and cover crops.
Best management practices (BMPs) are now available for protecting sinkholes from polluted runoff. BMPs have also been devised for construction sites, timber harvesting, and urban areas. Grass buffer strips around sinkholes, if well maintained, can effectively trap soil, bacteria, and agricultural chemicals in runoff from farm fields. Detailed information on BMP's can be obtained from your local University of Kentucky agriculture extension agent or Natural Resources Conservation Service office. The UK College of Agriculture has examples of BMPs.