"Presently they came to a place where a little stream of water, trickling over a ledge and carrying limestone sediment with it had, in the slow-dragging ages, formed a laced and ruffled Niagara in gleaming and imperishable stone. Tom squeezed behind it in order to illuminate it for Becky's gratification" -The Adventures of Tom Sawyer, Mark Twain.
Mark Twain's cave scene is familiar to many who have visited caves as tourists, biologists or cave enthusiasts (a.k.a spelunkers) offering a glimpse at wonders such as those that greeted Tom and Becky. Mammoth Cave National Park in Kentucky, for example, plays host to hundreds of thousands of visitors a year who take tours ranging from sedate cave walks on smooth trails to "wild cave" tours in which they get a taste for the excitement and rigors of cave exploring. Rangers and naturalists acting as tour guides introduce visitors to the processes of cave formation. They often use the term "karst" to describe the unique geology that shapes the landscape around Mammoth Cave. When these visitors leave the park, they most likely look back at what beauty they have seen. Perhaps, they even think of the eerieness or the unbelievable job Mother Nature did. However, if they think that caves and karst have very little relevance to their everyday lives, they are wrong..
A geologic map of the globe reveals that about twenty percent of the land mass (forty percent of the continental US to the east of Tulsa, Oklahoma) is KARST. There are karst regions scattered on the map from West Virginia to China to the former Yugoslavia. Yet, people in the US probably know more about volcanoes and earthquakes than they do about karst. What is karst and what is its relevance to environmental science?
Karst is the result of a unique balance between two opposing geological forces: the weathering forces such as rain and ice that break down rock and the innate resistance of the rock itself to change. In areas of very resistant rocks, such as granite, weathering is an incredibly slow process (from a human perspective); in areas where softer rocks, such as gypsum, predominate the process is extremely fast. In areas where limestones and other similar rocks are common, karst landscapes can form. Limestone is fairly resistant to mechanical erosion, but can be dissolved in weak acid. In many temperate and tropical areas where limestone is exposed, environmental water may combine with CO2 in the atmosphere or in the litter layer above the rock to produce a weak solution of carbonic acid (H2CO3) which slowly dissolves the limestone. The weakness of the acid and the resistance of limestone to mechanical weathering results in the slow dissolution of rock. As a result, fissures may gradually open in the limestone and may spread throughout the subsurface. These fissures may grow larger as more and more surface water is funneled into the fissures. The fissures may eventually open into wide-mouth openings called sinkholes which direct more and more water into the subsurface as they grow. Sinkholes may also form when cavern roofs collapse.
Through this positive feedback process, the water slowly carves out larger and larger conduits over time through dissolution and by scouring, which occurs when undissolved particles are swept through the underground passageways. Karst caves are the results of years of this gradual dissolution. The unique and beautiful limestone formations in these caves develop when the water releases CO2 into the cave's atmosphere, resulting in the precipitation of CaCO3. The mere presence of limestone may not lead to karst geology. Limestone is very common in the North American southwest, but precipitation is too low to dissolve significant amounts of rock.
What impact does karst geology have on the ecosphere?
One of the obvious results of karst formations is the unique cave habitats which may be created. Caves have been explored and used by humans for centuries, but we are only recently appreciating how rich the cave fauna is. Caves are widely used as hibernacula (hibernation sites) for bats in temperate areas and may be important refugia for endangered species. A survey of the Blue River Basin Caves in 1993 revealed that these caves hosted 127,000 bats, more than one third of the known population. Surveys of caves in the US are continually finding new invertebrate species, especially arthropods.
The effects of karst geology reach beyond the caves into the surface features of karst landscapes. One of the striking features of many karst landscapes is the absence of surface water. Streams rapidly sink into the subsurface through the limestone fissures, sinkholes etc. This process produces unique communities which have evolved over long periods of time. The rapid run-off and sinking of water in karst lands has also had a dramatic effect on the ecology of one of the more recent inhabitants of the karst - humans.
When Europeans settled karst areas, they cleared and settled the land in the same way they had in non-karst areas. Their development of these unique landscapes has led to a number of unexpected and serious problems. In non-karst areas, water will usually filter slowly through a porous media like clay or silt before it reaches the aquifer. This slow percolation will often strip the water of nutrients and toxins and many pathogenic bacteria and viruses die before entering the groundwater. In karst areas however, the surface and subsurface are closely linked and materials that run-off the surface may appear in the groundwater within minutes of sinking beneath the surface.
Agricultural run-off poses a severe threat to groundwater in many karst systems. Studies in Mammoth Cave National Park, which is surrounded by agricultural areas, have recorded high levels of manure, fecal Coliforms (indicators for contamination by mammals), pesticides and herbicides in the cave system after rainfall events. Since there are hundreds of farms in the Mammoth Cave watershed, it is very difficult to control run-off into the subsurface conduits and cave system. The sinkholes located on farms have also been used as trash dumps because they could not be used for cultivation and because some farmers believed plugging these openings would prevent them from expanding. Toxic materials dumped in the sinkholes leach quickly into the connected subsurface flows.
Agricultural and ecological researchers have developed Best Management Practices (BMPs) which are designed to reduce run-off in karst areas. These include holding ponds which restrict manure flow away from barns and buffer zones of grass or trees around streams or sinkholes. BMPs are very effective when properly designed, but they are also very expensive to install. Many family farms do not have the funds to contract BMPs and rely on aid from federal programs to help them install them. Federal aid is gradually being reduced, so many farmers have an uncertain future in karst areas.
Some municipalities in karst areas require businesses to build retention ponds that will retain run-off water from storms of certain magnitudes, such as "100-year storm" events. Many have also passed legislation prohibiting construction near known sinkholes. Some landowners oppose such initiatives because of the cost of building and maintaining retention structures. They also object to building regulations which they fear may make their land unattractive to commercial developers and hence, less valuable.
To protect human life and property, the engineering problems posed by karst need to be addressed and often are on new construction sites; new technologies, such as ground-penetrating radar, make it easier to assess the structural stability of new building sites and there are now regulations in many karst areas calling for more extensive engineering analyses before new roads or buildings are located. Little can be done, however, to support structures already in place on weak karst rock. These assessment tests are also very expensive, driving up the cost of public and private projects. One of the least known-and potentially most costly-problems is that most private insurance policies exclude "earth-movement" damages from coverage, although most homeowners are not aware of this. Individuals on karst restrict catastrophic and unrecoverable losses as a result of the geologic processes occurring daily beneath their feet.
Household waste treatment (or lack of it) is also problematic in the karst. In the towns, the underground caves were often used as public sewers, with storm and wastewater injected directly into the underlying caverns. In a newspaper article around the turn of the century, a city in Kentucky positioned over a cave system boasted of possessing "a natural sewer system more than a million years old." Septic systems in rural areas, which rely on slow filtration for the treatment of household waste, typically do not work in the karst. The waste from the household is directed virtually untreated in the groundwater. The USEPA recently introduced regulations that greatly limit the use of "injection wells" into karst systems.
Household waste problems are not as well studied in the karst as agricultural ones and perhaps will be a more serious and longer-term problem. There are constructed wetland systems and special sub-surface filter systems which can be installed for single home or small community use. These may become more widely used in the coming years, but many rural households still use the more affordable septic systems. County governments are reluctant to pass restrictions on septic tank usage because of economic factors, so household contamination of groundwater remains a serious and unresolved issue.
As cities were built in the karst, the areas covered by roads, parking lots and other impervious surfaces increased. This increased the volume of run-off from these areas, and many sinkholes and other conduits were plugged to provide a solid surface for pavement. The sinkholes that were left unplugged received the increased volume of run-off much more quickly then they had when the surface was forested, with the predictable result that the sinkholes backed up during heavy rains, and flooded the fields, homes and businesses. During low rainfall events, the sinkholes may not back up but they will introduce toxic surface run-off such as oils and coolant from parking lots into the groundwater.
Communities built on karst geology are not built on solid bedrock, but rather on a subsurface network of conduits which are constantly being altered by the action of water in the subsurface. While technology exists to map out the caves and passageways beneath the solid-looking surface, there may be only a few feet of actual rock supporting roads and homes. This point was brought home to Mammoth Cave National Park just recently, when the rock supporting one of the main park roads collapsed in to a cavern underneath. The National Park Service is studying how to deal with the problem, but all of the solutions, from detours to bridging the gap, are very expensive. It is not uncommon to see sinkholes swallowing Florida homes on news reports. In many cases, the home owner built on land which was on or near a sinkhole plugged with dirt. While plugged, these sinkholes are often still connected with subsurface networks, and during periods of high flow underground the water may excavate the fill, moving slowly upward with no sign on the surface that the karst was preparing to "eat" the home. The dynamic nature of the karst also makes it difficult for engineers to locate reliable , long-term wells. During droughts, karst wells often go completely dry, and even where there is sufficient water, new channels may be cut that divert the water away from an existing well.
Radon orradon gas is produced by the natural degradation of uranium to lead and is a particularly common problem in karst areas. The same network of cracks which direct water down, transport radon gas up into basements and crawl spaces. Radon has been implicated as a causative agent for lung cancer. Of all of the problems posed to humans by karst geology, this problem is uncharacteristically easy to evaluate and often can be corrected. Radon test kits are readily available and inexpensive. If levels above 4.0 picocuries are detected, EPA recommends a variety of preventive measures, including sealing crawl spaces with plastic, improving crawl space and basement ventilation etc.
As we have seen, karst poses a number of environmental challenges to human populations. As the population growth increases in these karst areas in the US, these potential problems must be addressed to ensure the quality of life for citizens living over the unique karst geology.
A review of acid-base chemistry is found on pages 400-404. The hydrologic cycle/groundwater review is on pages 263-269. The information presented on stormwater management, pages 286-290, apply to karst systems.
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