karst
INTRODUCTION TO KARST ENVIRONMENTAL PROBLEMS
Sinkhole Collapse
While some sinkholes form slowly by solution of the underlying carbonate rock, other sinkholes develop as a result of the collapse of surface or near-surface material
There are twobasic types of sinkhole-forming collapses: 1) bedrock collapses, and 2) regolith collapses. Bedrock collapses are rare and generally occur due to enlargement of cave passages in limestone. The enlargement causes the roofs above the passages to weaken and eventuall collapse to create sinkholes. Regolith collapses are much more common than bedrock collapses and generally result from regolith falling into openings in the underlying limestone.
In areas where the water table is usually above the regolith-bedrock contact, collapses often occur when the water table drops below the regolith-bedrock contact, either during droughts or during high-volume pumping ).
Physically, the collapses in this case are caused by loss of buoyant support for the regolith arches that span openings in the limestone. Collapses are also caused by spalling of saturated regolith down the opening, enlarging the arch, and eventually causing collapse at the land surface. When the water table fluctuates above and below the regolith-bedrock contact, collapse may result from repeated wetting (swelling) and drying (shrinking) of material supporting regolith arches.
Regolith collapses also may occur in situations where the water table is usually below the regolith-bedrock contact ).
Construction and land use changes that concentrate surface runoff in drains and impoundments may locally increase the downward movement of water resulting in the piping of saturated regolith into openings in the limestone. Loading of the surface by structures, fill, or ponded water, or vibrating the surface by blasting may also occasionally cause the collapse of regolith arches (Crawford and Whallon, 1985). Plate 2 shows the areas of Kentucky most vulnerable to sinkhole flooding and sinkhole collapse problems (Crawford and Webster, 1986b)
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Shallow aquifers in karst areas are probably the most vulnerable in the world to groundwater contamination. The aquifers receive distributed recharge from percolation through the soil and concentrated recharge from surface runoff that flows directly into the aquifer at stream sinks (swallets) and sinkhole drains ).
Many subsurface streams are simply surface streams which after sinking, flow through caves to resurgences at springs where they become surface streams again. Because of the rapid velocities of these underground streams, contaminants may travel several miles through the aquifer in only a few hours.
Contaminants associated with agricultural activities, such as nitrates, bacteria from livestock waste, and pesticides, are potential problems in karst terrain. Also, contaminants associated with urban storm water runoff, such as lead, chromium, oil and grease, and bacteria from pet-animal wastes may be a threat to people using karst water supplies and to cave aquatic life. Dye traces have shown that septic tank effluent can travel through the thin soils which are characteristic of most karst areas into the aquifer and then to a spring in only a few hours (Crawford, 1979). Also, percolating soil water can push effluent from septic tanks down into the aquifer resulting in high fecal coliform counts and high fecal coliform/fecal streptococcus ratios at some springs following heavy rains.
Contamination problems are aggravated in karst areas by the practice of disposing of solid and liquid wastes in sinkholes where they may be washed directly into the aquifer (Figure 8). The recent development and widespread use of hazardous materials has increased the threat from this practice. Leaks, spills or deliberate dumping of toxic or explosive chemicals are a particularly serious hazard in karst areas. Not only are these materials a threat to water supplies and cave aquatic life, but upon vaporizing they may become highly concentrated in the cave atmosphere and rise through fractures in the limestone and enter inhabited structures on the surface. Chemicals that leak from underground tanks may be carried into the caves below by percolating soil water following heavy rains. Most caves become completely water-filled at some point downstream. This results in natural traps for floating chemicals such as gasoline, which accumulate against the ceiling since they cannot continue with the cave stream past these sumps. Fumes may then fill the cave and rise into buildings on the surface. Occasionally homes in urban areas must be evacuated because fumes reach explosive levels in basements (Crawford, 1984a).
The degree of contamination of shallow karst aquifers depends on whether they receive primarily diffuse or concentrated recharge and on proximity and types of sources of contamination. Karst springs and water wells if supplied entirely by distributed recharge through thick regolith may be free of contaminants and therefore good sources of potable water. However, many karst springs and water wells receive concentrated recharge from a nearby area where sources of contamination are present (Crawford and Whallon, 1985).
