Karst: A Window To Groundwater
Many times we consider groundwater the "hidden resource" because it is not readily seen - like a tree, bird, or river. It is often difficult for people to understand groundwater and why human action affects its quantity and quality so readily. What some people don't realize is that there is a way to see, study, and experience groundwater "up close and personal" - this geologic phenomenon is called karst. A karst landscape is mostly limestone and dolomite bedrock which will dissolve in slightly acidic rainwater. Over the years, the bedrock dissolves and creates conduits, or underground "pipes". Some components of a karst terrain are features many people have seen before such as caves, sinkholes, springs, and losing streams. Together these features form a sort of window to groundwater allowing us to see a "hidden resource".
A sinkhole is a basin that funnels water quickly into the earth. Sinkholes can range in size from a few inches to many feet.
Another component of the karst terrain is springs. When groundwater flows out at the surface, the point of discharge is called a spring. Springs can be hot; they also can be small, just a tiny trickle of water, or very large, sometimes feeding an entire river. Many people are familiar with hot (thermal) springs, but springs can be cooler as well, depending on the depth at which they originate. Some spring environments can support unique plant and animal life.
One of the most familiar components of the karst terrain is the cave. A cave can be defined, from a human perspective, as an opening in the ground extending beyond the light zone and being large enough for a human to enter. Caves range in size from one small "room" extending only a few feet into the ground to immense systems of caverns, tunnels, and underground rivers or lakes that can extend for miles.
Losing streams are another prominent karst feature. In most waterheads, streams gain flow downstream, but streams in many karstic watersheds lose flow. This makes it tricky for such matters as siting sewage treatment plants, which usually depend on flow for "dilution" or "mixing".
Any number of human activities can have adverse effects on both groundwater quality and quantity within these karst systems. Karst features can also be adversely impacted by human activity.
Construction and development adversely impact groundwater and karst features. Land clearing, over-grazing, construction, and development - anything that causes erosion and increased surface water flow - can enlarge a sinkhole or cause a sinkhole to collapse. Cave-ins like this can cause serious property damage and even loss of life.
Debris (such as household trash, furniture, paint cans, auto parts, and even entire automobiles) are commonly dumped into sinkholes. Sinkholes clogged with debris no longer funnel water effectively, causing increased flooding. Anything dumped into a sinkhole, or spilled nearby, can also leach into, and contaminate, the groundwater.
Another potential contaminant in karst areas is salt. Whether caused by saltwater intrusion, runoff, or road salt in paved areas, salt affects drinking water supplies and is toxic to most aquatic cavelife.
Septic tanks are a major problem in karst areas. A leaking septic tank contaminates the groundwater and thereby contaminates the drinking water for any nearby wells. High water use or flooding can overload a poorly maintained septic tank system, releasing waste into the groundwater. Animal waste is also a major contaminant in karst areas. Large hog, chicken, and cattle farms must dispose of waste and dead animals properly (not thrown into sinkholes).
Human actions also have impacts on groundwater quantity. Excessive groundwater usage can dry up wells, cause springs to cease flowing, or can increase subsidence.
Maybe because groundwater is not readily visible to us, we take it for granted. We need to be aware of our actions and their impact on groundwater quality and quantity. Groundwater protection is more complicated in karst areas where unique geologic "windows" to groundwater make this important resource vulnerable to contamination and depletion.
Most of these problems we think of in terms of how they affect humans. Too many times, however, we forget that groundwater pollution affects other species as well. Karst systems support unique plant and animal species. Some species that live in karst systems are impacted by the same groundwater quality and quantity problems that affect humans.
Editor' Note: The information for this article came from an excellent chapter written by H.H. Hobbs III in Biodiversity of the Southeast U.S. Further information was obtained from Terri Jacobson of the U.S. Fish and Wildlife Service in Asheville, North Carolina.
Reprinted with permission of The Aquifer.