The Karst Pages

INTRODUCTION TO KARST ENVIRONMENTAL PROBLEMS Groundwater Contamination

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 (Figure 8). 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).