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NAME: Karst activity
(with contributions from A.H. Cooper, A.R. Farrant and A.N. Palmer. Revised January 2007)
BRIEF DESCRIPTION: Karst is a type of landscape found on carbonate rocks (limestone, dolomite, marble) or evaporites (gypsum, anhydrite, rock salt) and is typified by a suite of landforms including a wide range of closed surface depressions, a well-developed underground drainage system, and a paucity of surface streams. Karst in carbonate rocks is formed by their dissolution by acidic water. Most dissolution occurs when rainwater picks up carbon dioxide from the air, and decaying organic matter in the soil, becoming more acidic and then percolates through cracks dissolving the rock. When the bedrock becomes saturated with water, dissolution continues as the water moves sideways along bedding planes (horizontal cracks between rock layers) and joints (or fractures) in the rock itself. These conduits enlarge over time, and move the water, via a combination of gravity and hydraulic pressure, further enlarging the conduits through a combination of dissolution and abrasion of the surrounding rock.
The varied interactions among chemical, physical and biological processes have a broad range of geological effects including dissolution, precipitation, sedimentation and ground subsidence. Diagnostic features such as sinkholes (dolines), sinking streams, caves and large springs are the result of the dissolutional action of circulating groundwater, which may exit to entrenched effluent streams. Initially, most of this underground water moves by laminar flow within narrow fissures, which gradually become enlarged above, at or below the water table to form subsurface caves. Once a certain conduit size threshold is exceeded, typically around 10-20mm, flow becomes turbulent. Caves contain a variety of dissolution features, sediments and speleothems (deposits with various forms and mineralogy, chiefly calcite), all of which may preserve a record of the geological and climatic history of the area.
Carbonate karst can be either a sink or a source of CO2, for the karst process is part of the global carbon cycle in which carbon is exchanged between the atmosphere, surface and underground water and carbonate minerals. Dissolution of carbonates, which is enhanced by the presence of acids in water, ties up carbon derived from the rock and from dissolved CO2 as aqueous HCO3-. Deposition of dissolved carbonate minerals is accompanied - and usually triggered - by release of some of the carbon as CO2. In many karst locations, CO2 emission is associated with the deposition of calcareous sinter (tufa, travertine) at the outlet of cold or warm springs.
Though most abundant in humid regions, karst can also be found in arid terrains, either as relict karst formed during former wetter periods, or where H2S in groundwater rising from reducing zones at depth oxidizes to produce sulphuric acid. Such processes can produce large caves, such as the Carlsbad Caverns of New Mexico. Similar processes also operate in humid regions but tend to be masked by the CO2 reaction. Sulphates and rock salt are rarely exposed in humid climates. They are susceptible to rapid dissolution during periodic rains where they are at the surface in drier terrains.
SIGNIFICANCE: Karst is important for society because of its global distribution (20 million km2, about 12% of the Earth's land surface), and for the resources it contains. As much as a quarter of the world's population is supplied by water derived from karstic aquifers. Karst regions are host to many mineral deposits (including significant lead, zinc and placer tin deposits, and form important hydrocarbon reservoirs. Karst regions are also important scenic attractions and can contribute significantly to the local economy, for example the Tower Karst around Guilin and Mammoth Cave National Park, Kentucky. The karst system is sensitive to many environmental factors and can form a significant geohazard. Karst hazards include sinkhole flooding, sudden cover collapse, leakage around dams, collapse of lagoons resulting in waste spills, irregular rockhead and radon infiltration into homes. The presence and growth of caves may cause short-term problems, including bedrock collapse, disparities in well yields, poor groundwater quality because of lack of filtering action, instability of overlying soils, and difficulty in designing effective monitoring systems around waste facilities. Instability of karst surfaces leads annually to millions of dollars of damage to roads, buildings and other structures in North America alone [see surface subsidence]. Radon levels in karst groundwater tend to be high in some regions, and underground dissolution conduits can distribute radon unevenly throughout a particular area.
Because the great variety of subsurface voids and deposits are protected from surface weathering and disturbance, karst preserves a record of environmental change more faithfully than most other geological settings. Temperature, rainfall, nature of soil and vegetation cover, glaciation, fluvial erosion and deposition, and patterns of groundwater flow can usually be read from cave patterns and deposits. This record can be resolved on an annual scale in the case of certain fast-growing speleothems [see coral chemistry and growth patterns].
HUMAN OR NATURAL CAUSE: Karst processes are naturally occurring. However, they can be influenced by human activities such as land-use modification (e.g. deforestation), alteration of surface drainage, waste disposal, and opening or blocking of cave entrances, all of which can substantially affect sedimentation, speleothem deposition and groundwater quality over the short term. Overgrazing in Europe several centuries ago caused severe erosion of soils in many karst areas, leaving only bare, fissured rock surfaces. Although most sinkhole collapse is triggered by high discharge of underground streams, lowering of water tables by over pumping in areas underlain by thick soils or weak rocks can induce ground failure and collapse into subsurface voids. Sinkhole collapse can also be triggered by concentrating surface water runoff especially around urban and industrial development or along roads. Warmer winters and longer active periods of precipitation are causing increased surface water run-off, which feeds groundwater and accelerates karst development.
ENVIRONMENT WHERE APPLICABLE: Karst is most common in carbonate terrains in humid regions of all kinds (temperate, tropical, alpine, polar), but processes of deep-seated underground dissolution can also occur in arid regions. Evaporite karst in humid regions is characterized by much higher rates of mainly subsurface development. Evaporite karst is present at surface mainly in relatively arid climates.
TYPES OF MONITORING SITES: Caves provide unique, productive and extensive field sites, because they allow direct observation and mapping of underground features and their relation to the surface and to groundwater flow. Furthermore, their origin, morphology and distribution patterns are the dominant factors in controlling the nature of the overlying land surface (e.g. distribution of sinkholes) and the directions of groundwater movement. However, caves are difficult places to access and monitoring may prove problematical. Wells, borings and quarries are less useful because they provide only discontinuous points of information. Hydrographic networks that track the dynamics and chemical composition of water flowing out from and into karstic terrain can also be useful.
SPATIAL SCALE: Patch / regional. The scale of karst features ranges from microscopic (e.g. zonation in chemical precipitates) to entire drainage basins (with caves that drain hundreds of square kilometers) and broad karst plateaus.
METHOD OF MEASUREMENT: A holistic approach is required for karst studies, one that addresses the entire suite of interacting features and processes: geology, chemistry, engineering, soil science, biology, meteorology and, especially, hydrology must all be involved. An essential starting point to any monitoring is the hydrogeological and karst inventory. This can be stored in either a database or GIS (Geographic Information System) format. It should record springs, stream sinks, dolines, cave systems, dye-tracing and other related information. This provides a framework to which detailed work can then be related. Much of this information can be gathered from maps, air photographs, journals and reports.
Hydrological and geochemical measurements of springs, sinking streams, drip waters into caves, and cave streams provide records of short-term changes in water quality and chemical processes. The most important variables include pH, temperature, Ca, Mg, Na, Cl, HCO3, and SO4. Gypsum denudation could be measured by the amount of Ca SO4 2H2O (m3 from 1km2 per year) carried away by underground runoff. Salt dissolution can be estimated in a similar way. In some evaporite karst areas the amount of surface subsidence occuring can give a good indication of the rate of subsurface dissolution.
Pumping tests on wells are useful in clarifying the nature of the porosity and permeability of karst aquifers, as is simple monitoring of natural changes in water levels in cave streams. Natural and artificial tracers and dyes can be useful for demonstrating patterns of underground flow and delineating drainage divides, which may vary with time. Studies of the mineralogy and geochemistry of cave precipitates (using X-ray diffraction, luminescence, isotope ratios and trace elements) can reveal past changes in temperature, humidity, infiltration rates and groundwater chemistry. These changes can be placed in a temporal context by dating deposits using U-Th, palaeomagnetic and Electron Spin Resonance methods.
In urban areas it is important to locate sinkholes and buried cavities and to monitor their potential for collapse, using a combination of geophysical surveys, exploratory drilling and repeated leveling. Factors that threaten the integrity of cave and karst systems include major land or hydrological changes, ground water pollution, and direct human impact.
FREQUENCY OF MEASUREMENT: Surface features and soils in karst terrains are notoriously unstable and can change rapidly, commonly at catastrophic rates. In humid climates, most surface collapses occur during or soon after floods, when soil and debris is either eroded from beneath incipient sinkholes, or washed down from above. Groundwater chemistry and contamination change so rapidly during floods that continuous measurements are needed in order to interpret the karst system. Hydrochemistry and runoff values of water flowing in/out of the karstic terrain should be measured once per month or at least once per season.
LIMITATIONS OF DATA AND MONITORING: Surface studies of karst are hampered by the fact that surface features are controlled by underground water movement, without knowledge of which it is impossible to interpret the surface features properly. Changes in karst are often so sudden and unpredictable that it is difficult to design a valid monitoring strategy. Caves are also difficult environments to work in and require specialist expertise and equipment.
APPLICATIONS TO PAST AND FUTURE: Karst responds with great sensitivity to environmental changes, and karst features (especially speleothem) contain many clues to past climatic and hydrological events and changes at a variety of time scales. It is uncertain whether future conditions can be interpreted from karst features, because many changes tend to be abrupt and discontinuous. Karst deposits and landforms may persist for extraordinarily long times in relict caves and paleokarst.
POSSIBLE THRESHOLDS: The slow, gradual movement of soil tends to fill depressions in the karst bedrock surface, keeping pace with the dissolutional growth of sinkholes. However, where this material can be transported away from the site by cave streams, a metastable arch of rock and soil can be produced over an underground void, resulting in sudden collapse. The threshold between gradual and catastrophic subsidence is not generally predictable from the surface. There is, however, an important threshold in carbonate rocks between dissolution and precipitation, which is governed by the degree of saturation of karst water with respect to minerals, especially calcite. The threshold can be crossed for a number of different reasons, with CO2 level enhanced by decay processes and reduced by aeration. Calcite and CO2 solubility both decrease with temperature, but high temperatures generate greater CO2 production, which in turn offsets the diminution of CO2 solubility. Dissolution conduits form along paths of greatest groundwater discharge, with their rate of enlargement at first determined by discharge rates and saturation concentration. Once the water is able to pass through the conduit without exceeding the threshold for calcite solubility (about 70% saturation), the enlargement rate becomes almost independent of discharge and is determined by dissolution kinetics.
Beck, B.F. 1989. Engineering and environmental implications of sinkholes and karst. Rotterdam: Balkema.
Drew, D. & Hotzl, H. 1999. Karst hydrogeology and human activities. Balkema.
Ford, D.C. & P.W.Williams 1989. Karst geomorphology and hydrology. London: Unwin Hyman.
Klimchouk, A., Lowe, D., Cooper, A. and Sauro, U. (Eds). 1997. Gypsum karst of the world. International Journal of Speleology. Vol. 5 (3-4) for 1996, 307pp
Jennings, J.N. 1985. Karst geomorphology Oxford: Basil Blackwell.
Alexander B. Klimchouk, Derek C. Ford, Arthur N. Palmer, and Wolfgang Dreybrodt (eds) 2000. Speleogenesis: Evolution of Karst Aquifers. National Speleological Society, Huntsville, Alabama.
Kuniansky, E. L. (ed) 2001. U.S. Geological Survey Karst Interest Group Proceedings. St. Petersburg, Florida. February 13-16, 2001. U.S. Geological Survey, Water-Resources Investigations Report 01-4011.
White, W.B. 1988. Geomorphology and hydrology of karst terrains. Oxford: Oxford University Press.
OTHER SOURCES OF INFORMATION: Water/hydrological agencies, geological surveys, IAH, IGA, IGCP 379 (Karst processes and the carbon cycle), Karst Waters Institute, INQUA, International Speleological Union. IAH-Karst Commission, IGCP projects: 299 (1990-1994): Geology, Climate, Hydrology and Karst Formation, 379 (1995-1999): Karst Processes and the Carbon Cycle, 448 (2000-2004): World Correlation of Karst Geology and its Relevant Ecosystems.
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Some karst landscapes are sensitive to external change, partly because of extreme permeability and thin soils as a result of intensive karstification. This sensitivity, coupled with population pressures and malpractice has resulted in soil erosion, deterioration of water supplies, and groundwater pollution in many areas. Flooding of caves in highly populated areas can disperse contaminants over wide areas. For example, in the mid-1980s, flood-induced ponding of water under high pressure in caves beneath the city of Bowling Green, Kentucky dispersed hydrocarbons (from industrial wastes) throughout many fissures, bringing their concentration to nearly explosive levels in overlying basements and nearby wells. Under steep hydraulic gradients, fissures may enlarge sufficiently to cause significant leakage through the ground during a human lifetime, as around some of the Tennessee Valley Authority dams in the mid-twentieth century. The most vexing problem in karst today is the lack of rational regulations concerning groundwater monitoring, a situation complicated by a common misunderstanding of the great differences in flow behaviour between karst and non-karst (porous-media) aquifers.
Karst is very vulnerable to groundwater pollution, due to ease of water flow: natural filtration is nearly non-existent. The use of cave conduits as natural sewer lines, and sinkholes as garbage dumps in small towns and rural areas puts the local drinking water supplies at risk. Urban expansion in karst areas often means the building of houses on land which cannot physically support them and problems with septic tanks, underground pipeline breaks and landfills.
OVERALL ASSESSMENT: Karst landscapes are particularly dynamic and subject to rapid change. They preserve a valuable record of environmental change, and should be monitored closely for their effect on human settlements and built structures.
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