Monday, February 27, 2012
Protecting Drinking Water in Karst Terrain
Most of us are familiar with the caves and the more striking karst features that occur in karst terrain though visits to the National Park System and caverns. My own introduction to karst features was Howes’ Caverns in New York where the stalactites, stalagmites and flowstone created from the calcite dissolved from overlying rock was a wondrous site for a child. My husband, a native son of Virginia, went to Luray Caverns as a child. In Virginia karst terrain covers much of the Valley and Ridge Province in the western third of the state. Smaller karst areas occur in the Cumberland Plateau, Piedmont and Coastal Plain Provinces, too. Cave systems are just some of the features that occur in karst terrain. Karst terrain may including sinkholes, fractured bedrock, sinking streams, and sinkhole ponds that are all direct routes of groundwater recharge that provides little if any containment or removal of contaminants of surface waters that recharge karst aquifers. Ground water flows rapidly through karst aquifers, through the enlarged solution channels, discharging from springs and supplying base flow to surface streams and rivers.
Karst terrain occurs in areas where the underlying rocks are carbonate rock such as limestone (CaCO3), dolomite (CaMg(CO3)2) and gypsum (CaSO4.2H2O). These rocks are soluble in dilute acids and typically have only a thin soil overlay with areas of rock outcroppings. Rain water becomes slightly acidic when passing through decaying organic debris in the surface soils. The decaying organic material is a ready source of carbon dioxide, CO2. The CO2 and H2O chemically react to form a weak acid called carbonic acid. The slightly acidic water percolates down through the soil into fractures in the carbonate bedrock. These types of rocks are the very type of rocks used in the filters to neutralize acidic or corrosive well water because they easily react with the slightly acidic rain water. The carbonic acid in the moving ground water slowly dissolves the bedrock forming passageways and caves. This geological process results in unusual surface and subsurface features ranging from sinkholes, disappearing streams and springs to complex cave systems and caverns that are the characteristic Karst features.
These Karst features are very important to understand because approximately 20% of the land in the United States is classified as karst topography, but these areas produce 40% of the groundwater used for drinking water in the United States. World wide approximately 10% of the earth's surface is classified as karst; with an estimate 25% of the world's population living in karst areas. The hollow nature of karst terrain results in a very high pollution potential. The thin soils over fractured limestone allow precipitation to enter the subsurface with minimal natural filtration. Streams and surface runoff enter sinkholes, fissures and caves, carrying surface contaminants and without the natural filtration provided by soil and sediment cover the contaminants quickly flow to depth. Groundwater can travel quite rapidly through these underground networks. In tests by the U.S. Geological Survey and the U.S Environmental Protection Agency, groundwater was documented to travel thousands of feet, even miles, per day transmitting tracer dyes and potentially contaminants to wells and springs throughout the vicinity. In karst terrain groundwater can flow like an underground river.
Karst aquifers are among the most highly vulnerable to contamination, particularly where the overlying soil is thin. This vulnerability results from: sinkholes, widened flow paths, and rapid velocities of ground water and contaminants. Contaminants can be transmitted quickly from entry in a sinkhole to wells and springs in the vicinity. A sinkhole is generally a funnel-shaped or steep-sided depression that is caused by the underlying carbonate rocks dissolving away and the subsidence of the land surface into a subterranean passage, cavity, or cave. Sinkholes proved a direct path for surface contaminants to enter the groundwater. Sinkhole creation, sinkhole flooding, and groundwater contamination are the major hazards associated with karst terrain, and unlike other natural hazards they are chronic in nature. Rapid infiltration of surface water allows bacteria to reach groundwater depth while still alive. Also, rapid infiltration does not consume a lot of the oxygen in the water and nitrate does not denitrify making karst aquifer highly susceptible to bacterial and nitrate contamination (major contaminants in human and animal waste).
Sinkholes are easily formed in karst terrain. Alterations to surface runoff during development can cause sinkholes. Groundwater pumping can quickly lower the water level and result in a subsidence or sinkhole formation. Failing septic systems are a significant source of groundwater contamination in karst terrain. Also, there are many cases of septic tanks simply sinking into the underlying cave system in karst areas. Rather than devastating natural disasters, karst terrain unwisely developed has resulted in long-term economic burdens on individual property owners and communities for sewage and water treatment in sparsely developed areas. The residents of karst areas need to be aware of how day-to-day activities affect the groundwater and fragile ecosystems in their karst regions. In addition, surface water can directly influence groundwater carrying with it all the surface bacteria and contaminants that source groundwater is not typically treated for.