II. ADVANCE IN KARST RESEARCH
Peter A. Bull
Environmental reconstruction from clastic sediments is a well tried and tested procedure. Grain size analysis (morphometry), mineralogy and macro- textural analysis are the mainstays of sedimentology. Recent innovations have enabled microscopic analyses to be undertaken by the use of scanning electron microscopes. Such SEM analysis of clastic sediments, particularly quartz, enables detailed paleo-history reconstructions of both environment and energy levels of sediment transportation and deposition. The work pioneered by David Krinsley (Krinsley and Doornkamp, 1973) has lead to the proliferation of publications utilizing the technique (Bull et al, 1986 for review). Successful reconstructions have been undertaken in all natural regimes although it is in the province of karst regions that the technique becomes a really powerful tool.
Clastic sediment deposition on the "surface" inevitably leads to post depositional weathering and erosion. Very little of the continental record is preserved. However, clastic sediments that are washed into caves take with them an imprint of surface conditions at the time of their transportation and deposition. Once in the cave, and particularly in southern China, tectonic uplift leaves cave levels abandoned and dry. The sediments remain relatively unaltered and normally completely undisturbed for hundreds, thousands or even millions of years. The karst terrain development in active tectonic areas acts in the case of clastic sediments as a well preserved three dimensional river valley and flood plain.
This is of particular importance in southern China since there is currently a debate as to the extent of the major glaciations of the Pleistocene in China as a whole. One school of thought suggests that much of southern China was glaciated whilst others would argue that the area escaped the full effects of glacial action. The main aim of this study therefore was to examine by scanning electron microscopy, surface and underground clastic deposits to identify particularly whether the deposits had been modified in glacial or periglacial environments.
Sediments were collected from both "river terraces", major sediment deposition sites and from the nearby Guilin Devonian sandstones (Yuan et al, 1985). The surprising results were that the materials had not been transported by river, nor had they undergone any aeolian or other subaqueous modification. Furthermore, post-depositional modification by pedogenic activity was also extremely limited. The sediments collected around Guilin were, in all intents and purposes, identical to the Devonian Sandstones surrounding the Guilin area at topographically higher sites. It was the conclusion of this study that the valley deposits must have derived from soliflucted or mudflow deposits from the mountains. A series of negative occurrences implied that the material was most probably transported in cool conditions, although most certainly not in a glacier.
This study was extended when sediments from a local karst tower were analyzed (Bull et al, 1989). Clastic sediments were taken from four distinct cave levels in Chuan Shan Tower. The sediments from the lower three cave levels suggested that the materials derived from nearby sources having been colluviated, soliflucted or transported in a mudflow, then to be windblown into the caves. The upper two layers of the cave, however, contain materials derived from aeolian, fluvial and direct mudflow deposits predating the development of the sixty metre tower karst feature. The fluvial aeolian mix of materials in the upper layers of the cave conforms with the Williams model of tower karst development (Williams, 1987).
Allied studies were undertaken on the microtexture analysis of surface sediments around the region of Lushan in Jiangxi province near Nanchang in central China. The Quaternary glaciations of Lushan, studied in detail by Lee (1947), and presented in a detailed monograph identifying the moraines and glacial deposits from at least two glaciations, the Taku and Lushan. Detailed analysis (Bull et al, in prep, a) shows that these sediments on and around Lushan bear characteristics of cold climate modification with little or no fluvial and aeolian transportation.
The distinctive diagenetic history of the Lushan Sandstones can be identified in almost all of the samples studied from the moraines on the lower slopes of the mountain. Superimposed, however upon these inherited textures are breakage and grinding features suggestive of modification in a harsh, mechanically dominant environment of modification. It would appear therefore that some, if not all, of the samples derive from glacial-type activity. It appears that this analysis supports the pioneer work of Lee (1947).
Surface samples were also collected in the region around Guilin (Bull et al, in prep, b) where debate existed as to the convoluted nature of the sedimentary structures within deposits. Field studies showed that the deposits suffered greatly from warping and deformation caused by external forces and in many instances the matrix-supported sediments showed signs of shearing and faulting. It is possible that these macrotextures could support the contention that the area around Guilin had been glaciated. Micro-morphological studies by SEM, however, show that none of the samples had any signs of having been modified in a glacier, that only a few samples had suffered very low energy fluvial modification, that the sediments had not been transported by aeolian agencies and, more importantly, that the deposits appeared very similar to the Devonian sandstones outcropping on topographic highs in the region around Guilin. It is contented that these deposits represent at least two major mudflow events, albeit during a much cooler climatic phase than at present. Details are provided in Bull et al. (in prep, b).
During 1990, it is intended that cave sediments be collected from a number of other towers in the region around Guilin to establish correlation between the distinct levels of caves that can be seen in the field. Particular attention will be focused upon the upper levels of caves in order to establish the identity of these upper most level, gravity-fed, clastic deposits which in the field exhibit matrix-supported textures.
Comparative work needs to be carried out on the present-day fluvial terraces particularly with reference to the Guilin mudflow study outlined above. Furthermore, a more extensive sampling programme will be undertaken of the surface deposits around Guilin to try and substantiate the suggestions provided in the mudflow study presented herein.
The author wishes to hear from any other scientist who is studying sediments in southern China with a view to collaborating over further research.
RATIONALE FOR KARST INVESTIGATIONS
ANDRES DIAZ ARENAS
"Karst terrain" denotes a landscape underlain by rocks that have been dissolved by either surface water or ground water; dissolution has created characteristic features such as scarcity of soils, enclosed depressions, sinkholes, caves, and absence of surface streams. Drainage occurs through subterranean fractures and solution openings. "Karst" is used to describe the many processes that developed these features and also to designate landscapes where these processes occur. The word has its origin in a region in Yugoslavia, the Krs, Kras, or Karst plateau, initially studied by Jovan Cvijic in the past century.
Karst terrains are sites for development of mining, energy, and water resources; they are highly fragile environments that commonly respond more rapidly and dramatically to environmental stress than do other types of terrains. Most of these responses are deleterious and lead to environmental degradation. Population increases, urban and suburban expansion, and increased demands on limited resources are compounding this environmental stress. Human-induced or natural phenomena can occur unexpectedly and have serious consequences. For example, a sinkhole collapse that occurred in August 1986 in a village near Sao Paulo, Brazil, destroyed so many houses and utilities that about 2,000 people had to be evacuated. The collapse was caused by dewatering of an abandoned limestone quarry that was being reactivated to satisfy increased demand for additional construction material. Under temperate climatic conditions, similar catastrophic phenomena have been reported in central Europe. Heavy rains accompanying a hurricane can induce collapse of natural sinkholes after infiltration of intense rainfall increases head and ground-water velocity and washes out cave sediments, as has happened in the western part of Cuba and elsewhere.
The soluble rocks are most commonly limestone and dolomite which consist mainly of the minerals, calcite, and dolomite. To a lesser degree, kal-st features, called "pseudokarst," are found in other soluble rocks such as marl, gypsum, salt, and even in some igneous rocks. Approximately 75 percent of the total land area of the earth is directly underlain by sedimentary rocks, and the carbonate rocks of limestone and dolomite constitute about 15 percent of sedimentary deposits. The major known karst areas in the world are found in the Mediterranean Basin (mainly in Algeria, Cyprus, France, Greece, Italy, Morocco, Spain, Tunisia, and Yugoslavia); North and Central America and the Caribbean Basin (as in Bahamas, Barbados, Belize, Cuba, the Dominican Republic, Guatemala, Haiti, Jamaica, Mexico, Puerto Rico, and the United States of America); Southeastern Asia (especially Cambodia, Laos, and Viet Nam), China, and Oceania (Australia, Indonesia, Java, and Papua-New Guinea). Karst terminology has been enriched by incorporation of many local terms for naming specific features in different latitudes; among the classical examples are the following: cenote from Central America, cockpit from Jamaica, mogote from Cuba, polje from Yugoslavia, furnia from Spain, lapies from France, havara from Cyprus, and ouadi from North Africa arabian countries.
It is most appropriate that an article on karst appear in this journal because of the long-standing support that the United Nations Educational, Scientific, and Cultural Organization (UNESCO), and other agencies of the United Nations, have given to the study of karst regions and publication of the results. These publications include, among others, the Proceedings of the Symposium on the Hydrology of Fractured Rocks, held in Dubrovnik in 1965; "Hydrogeology of Karstic Terrains," prepared by the International Association of Hydrogeologists in 1975; "Case Histories of Hydrogeology of Karstic Terranes," in 1984; "Guide to the Hydrology of Carbonate Rocks" in 1984; and "Karst Water Resources," the proceedings of a symposium held in Ankara in July 1985. In fact, the opportunity for collaboration of the two authors of this article was provided by a symposium held under the auspices of UNESCO in Cuba in December 1982. At the present time, a major publication on karat is being prepared by the Karst Commission of the International Association of Hydrologist titled, "Hydrology of Selected Karst Regions of the World," which will include many articles resulting from investigations supported by the United Nations agencies. A second workshop on the Hydrology of the Caribbean Karst was held in Havana in December 1988, cosponsored by UNESCO and other agencies of the United Nations.
The differentiating feature between carbonate rocks and most other types of rocks is their relative high solubility, which facilitates the development of both surface and subsurface characteristics of karat. The outstanding characteristics are (1) the ubiguitous presence of enlarged fissures and voids that permit a rapid infiltration and flow of water, and (2) the large variation in permeability that causes a complex pattern of ground-water flow. These properties are so distinctive that the hydrology of carbonate rocks forms a special facet of the science of hydrogeology. Investigations of karst areas require an emphasis on the hydrodynamic properties of the aquifer and on the chemistry of both the water and the rock. The principles, concepts, and techniques that were developed largely by studies of aquifers with uniform distribution of permeability must be modified or adjusted when applied to ground-water flow in the fractures and solution channels of carbonate rocks. Much of the early scientific understanding of worldwide karat -phenomena was gained by studies in the classic karat areas in the Mediterranean region, particularly in the Dinaric region of Yugoslavia. At the present time, karst areas of the Caribbean, China, many of the Mediterranean Basin countries, and North America continue to be areas of significant research by scientists and engineers.
FEATURES AND PROCESSES
Karst features, caused by dissolution of carbonate rocks may be divided into two groups, (1) surficial features extending not more than a few meters below land surface, and (2) karst features that extend to greater depths and that affect the permeability of the rocks and the flow of water; for example, dolines, solution shafts, and cavities. Most lateral or horizontal caves that are now dry formed when the water table was above their level. There is general consensus that most horizontal dissolution occurs near, and at shallow depths below, the water table, where ground-water circulation tends to be greater than in deeper zones.
The infiltration of water and its circulation underground is the essence of the karstification process, which is dominated by the chemical reactions governing the dissolution of carbonate rocks. Therefore, the application of chemical thermodynamics during the past two decades and, more recently, the laboratory and field studies on rates of calcite dissolution have provided additional insight to these important processes. International cooperative studies have been carried out in the framework of the International Programme on the Genesis and Evolution of Karst in several research stations in different climates of Cuba, Poland, and Germany. Rates of dissolution are affected by climate, particularly temperature and rainfall, and the amount of vegetation; consequently, karat terrains are sensitive to climatic variations and change. Karst processes are at the maximum in areas characterized by (1) strong, compact limestones that have numerous joints; (2) rainfall sufficiently high to provide large quantities of water; (3) temperature warm enough to permit growth of extensive vegetation that provides the carbon-dioxide gas required for dissolution of carbonate rocks; and (4) elevation differences between the inlets and outlets of the system that provide an energy gradient causing large amounts of water to flow.
Another requisite for the development of spectacular karat, such as that in southeastern Asia (China and Viet Nam) and Yugoslavia, is tectonic activity that has elevated and formed joints, faults, and folds in the limestones far enough above sea level to permit development of high relief, that is, steep mountains and deep canyons. However, many areas of karat have low elevation above sea level, such as the Indiana-Kentucky area, and the peninsula of Florida of the United States, the Habana-Matanzas Southern Coastal Plain of Cuba and the Yucatan Peninsula of Mexico. In addition, coastal areas underlain by carbonate rocks can exhibit karstic features such as those present in the Burren of Ireland, the Balearic Islands of Spain, the Greek Islands in the Ionian Sea, and the Mediterranean coast of Turkey.
Two of the fundamental controls that generate the distinctive characteristics of karat terrains are the mineralogic and textural nature of the carbonate rocks. The texture and mineralogic composition of the carbonate rocks are, in turn, controlled by such factors as the particular marine environment in which the calcareous sediments were originally deposited; the effects of early diagenetic changes as the sediments were uplifted from the ocean to the freshwater regime; continued processses of lithification in the freshwater; and the tectonic activity that elevated the carbonate rocks above sea level and generated fractures and joints through which water can flow.
The initial permeability of most carbonate rocks is quits low and depends on the degree of fissure connections. The important permeability of carbonate rocks results from dissolution that enlarges fracture systems. Dissolution cannot take place without openings through which infiltrating water begins this process. Therefore, most water in carbonate rocks occurs in fairly well-defined solution channels that gradually enlarge by continued dissolution; this enlargement continues to concentrate the ground-water flow. The rocks develop an extremely uneven distribution of permeability, with the unaffected portions being essentially non-water-bearing, whereas, the solution channels have extremely high water-yielding capability.
This uneven distribution of permeability is manifested in various ways and is responsible for many of the hydrogeologic features associated with karst terrains. These include (1) absence of streams or well-developed surface drainage systems attributal to the rapid infiltration of water into the subsurface; (2) sparse or thin soil cover; and (3) high permeability of near-surface carbonate rocks that permits rapid drainage of ground water; this drainage promotes the continuous lowering of water levels, and in some high areas, permits the rocks to drain entirely. This heterogeneous distribution of permeability further contributes to the extremely rugged and commonly high-relief surface associated with karst. Areas where such uneven limestone surfaces are covered with soil and unconsolidated deposits can be extremely unstable. The surface if stable over the blocks of limestone, but unstable over solution channels which, under certain conditions, may eventually collapse to form sinkholes.
The position of the water table provides important information for deducing the functioning of karst aquifers. In karstified limestones near the surface, a water table commonly exists; however, if fracturing and extensive dissolution have not occurred, the rocks may be so impermeable that the water table is discontinuous or absent. Where diffuse-flow aquifers exist, the location and position of the water table can be used to identify the general direction of ground-water flow, the hydraulic gradient, and the areas of recharge and discharge. In addition, seasonal fluctuation of the water table provides information about the permeability distribution, and the position of the water table can also indicate which solution cavities contain water.
Topographic configuration and differential permeability combine to control the position of the water table. For example, in areas of high topography and high permeability the water table is far beneath the surface, whereas in areas of low topography and low permeability the water table is shallow. Shallow water tables also underlie in small, low-lying islands where the location of ground water is influenced by sea-level fluctuations.
Seasonal changes in elevation of the water table in carbonate rocks can be of considerable magnitude; seasonal fluctuations of water levels of 20 to 50 meters are not unusual and can be as great as 80 to 100 meters. Such extreme fluctuation is readily observable in poljes. Poljes, first defined and quite numerous in Yugoslavia, are a tectonic or solutional valley commonly without a through-flowing surface stream; the water enters or exits the valley through a sinkhole. Commonly, depending upon the season, the same sinkhole serves as either the recharge or discharge point. In the dry season the water lies far enough below the valley floor to make the valley suitable for growing crops. During the summer, boat docks that extend from the valley sides can appear as mysterious constructions in the hayfields. Their purpose becomes obvious only when the rising water table has created a lake that may be several meters deep. Holes through which water drains in karst areas either from poljes or streams are known as "swallow holes" or "ponors." When these holes alternate seasonally between serving as swallow holes or as discharge points for springs, they are known as estavelles. Some estavelles exist at the bottom of the sea, as in the Adriatic along the Yugoslavian coast or in Bay of Pigs, Cuba, and sea water can drain into the aquifer, often with velocity great enough to form large whirlpools.
In coastal areas where karst is exposed to influence of the sea, as in the Yucatan, West Indies, and Mediterranean region, sea level is a controlling factor on the discharge of ground water. The position of coastal and submarine springs depends on the relation of fluid potential, also called "head," of the fresh water to that of the saltwater. These springs discharge either freshwater or brackish water depending on their position in relation to the freshwater-saltwater interface, and can have extremely high flows, such as Caleton spring in Cuba which discharges 14 cubic meters per second. Mineral alteration can occur and porosity and permeability can increase in this mixing zone. Fluctuations of sea level, such as those during Pleistocene time, have shifted the location of the zones of flow of fresh- water. During low stands of the sea, flow and dissolution were active in zones at elevations slightly lower than the present sea level; therefore, in many coastal areas, large solution channels extend to depths below sea level. For example, in Cuba, dozens of caves exist at depths as great as 70 meters below sea level. In these water-filled caves, scuba-diving explorations have recognized not only the present mixing zone but also paleomixing zones that are associated with dolomitization of limestone.
In many coastal areas where the water table is only slightly above sea level, this dissolution causes development of shallow zones of high permeability. Consequently, the thinness of freshwater zones and presence of salty water only a few meters below land surface contribute to the problems of withdrawing freshwater in coastal areas. Effects of dissolution in this mixing zone can be seen at higher elevations where the coastal rocks have been uplifted by tectonic activity, such as on the Greek Islands in the Ionian Sea, the Balearic Islands in the Mediterranean, and some islands of the Caribbean.
Water and Power
One of the currently active projects that is being supported by United Nations agencies in a major karst area is at Hacetepe University, Ankara, Turkey, where one of the authors has been involved for several years. The characteristics of the karst water resources of Turkey provide an example of the carbonate-rock formations that exist in Southern Europe, the Alps, the Dinaric Alps, Northern Africa, and the Middle East. Turkey is characterized by the wide extent of the karstified areas, existence of some of the largest karst springs and karst aquifers of the world, some of the largest and most complex karst-water regimes in poljes, and numerous coastal karst features throughout the southern and western coastline along the Mediterranean Sea.
Carbonate rocks crop out in about 30 percent of Turkey and are buried at shallow depths in about 40 percent. Most of the carbonate rocks of Turkey are intensely karstified. The basic reasons for the karstification are (1) strong orogenic movements that have lifted the carbonate rocks high above sea level; (2) a great difference in water- levels that created high energy gradients for both the surface water and the ground water; (3) intense fracturing of rocks by the orogeny that provided the initial openings for water flow and dissolution; and (4) the formation of high mountain ranges that act as a barrier to th6 movement of air masses and promote heavy rainfall and great quantities of water for rapid infiltration into and dissolution of, carbonate rocks.
In many parts of the world, karst terrains contain abundant amounts of water and, consequently, are extremely important in the national economy of developing countries. In some areas high mountain ranges influence the existence of these water resources. These influences include (1) abundant precipitation, (2) great differences in elevations of water-levels between the mountains and the sea within relatively short distances which provide large hydroelectrical potential, (3) melting of accumulated snow in the mountain that sustains low flows of streams even during dry summer seasons, and (4) high mountains with rapid stream runoff that prevents extensive water loss from evaporation.
Other Karst Resources
Even though water is the most important resource for the economy and society of karst regions, and many of the world's major aquifers are in karstified limestone, karst produces other resources. For example, tourism associated with caves and picturesque karstic valleys, as in Cuba, can be a major contribution to local economies. Tourist accommodations in caves in many parts of the world include electric lights, walkways, and boat trips, some of which are in flat- bottom boats containing small musical bands or pianos for concerts and special theatrical performances. Because of stable temperature and hydrological conditions within the caves, both in temperate and tropical climates, caves have other economic uses, such as cultivation of mushrooms, aging of wines and cheeses, and storage of petroleum products.
Some caves, such as the subterranean laboratory near Moulis, France, have been instrumented for scientific investigations of earth tides and the physiology of cave-dwelling animals. Karst caves were used by ancient civilizations for religous and other ceremonials; the associated murals and artifacts provide impressive representations of their mythology, culture, and social organization.
Extensive hydrogeologic investigations have been undertaken in karat regions of Hungary to dewater mines beneath the water table that produce bauxite (aluminum ore), low-grade coal, and manganese ore. In the Silei3ia-Cracow area of Poland, secondary concentrations of galena (lead ore), calamine (zinc ore), limonite (low-grade iron ore), manganese oxide, and fire clays fill karst depressions and have been mined. Molding sands for metal castings also have been mined extensively from these depressions. In many areas, calcite from the dripstone deposits in caves and bat guano used for fertilizer are common products obtained from caves.
Limestone is used extensively as a building stone and provides a valuable source of lime, an essential ingredient for cement and agricultural soil improvement. Dolomite provides an agricultural source of magnesium. More than one-half of the world's petroleum is in carbonate reservoirs. Karstified limestones also are important host rocks for many ore deposits, such as lead and zinc in the mid- continent area of the United States, important manganese deposits in Cuba, and bauxite deposits in Jamaica. Water from carbonate rocks has traditionally been used for domestic, agricultural (the main consumption in developing countries), and municipal purposes, and also is being used extensively to generate hydroelectric Power in Yugoslavia, France, Turkey, and in the United States by the Tennessee Valley Authority. Hydroelectric plants in small fluvial basins have become feasible for developing countries in the Caribbean Region and elsewhere. In addition, reservoirs built at the outlet of subterranean rivers ate successful damsites, partly because loss by evaporation is minimal in underground channels.
Karst terrains present numerous types of hazards and engineering-problems in development of water and other resources. These include land subsidence, underground cave-ins, water and mud break-ins during excavations, landslides and rock falls, salt-water intrusion, and dry holes drilled for water. Another problem, caused largely by uneven distribution of permeability, is leakage of reservoirs after construction of a dam. During attempts to fill the surface reservoir, water is lost through dry caves and solution openings that had previously been above the water table. At some sites, the presence of these solution channels was unknown before construction of the dam; the reservoir remains unused where the cost of injecting concrete grout is not economically feasible. However, with proper geologic engineering, it may be possible to use the storage capacity of the underground caves and caverns as part of the reservoir and thereby decrease water loss by evaporation.
Traditional engineering concerns about the suitability of karst areas for construction of dams for flood control and generation of hydroelectric power were well founded because of the numerous early failures. However, with the expanded need for energy and the engineering experience in using geologic principles, karst areas are being successfully used for water storage, excavation of underground chambers, and construction of high dams.
One of the more dramatic and terrifying phenomena of karst terrains is the sudden collapse of land surfaces that form holes large enough to destroy houses and other buildings. These sinkholes, ranging in size from less than a meter in diameter to several tens of meters can occur instantly and continue to increase for several days or weeks. These collapse features are not caused by the rapid dissolution or collapse of the limestone, but rather by infilling of sand and clay overlying a solution cavity in the limestone. Most sinkholes result from failure of the roof of cavities in unconsolidated deposits that overlie solution cavities in the carbonate rocks. Although many sinkholes develop under natural conditions, most sinkholes in inhabited areas are induced by human activities.
Sinkholes can be induced by lowering or raising the water level and by various construction activities. The sudden occurrence of sinkholes results from the downward migration of soil and other unconsolidated material into solution openings near the top of the limestone. Triggering mechanisms can result from the lowering of water levels by pumping or loss of buoyant support, water-level fluctuations, and increase in water velocity, and induced recharge. The loss of buoyant support resulting from decline of water levels is equivalent to addition of weight on the unconsolidated roof. Large water-level fluctuations contribute to sinkhole formation because of the repeated loss and recovery of the buoyant support and the repeated saturation and drying of the unconsolidated deposits that tend to decrease the cohesive strength of the surface material. Also, a rapid lowering of the water table from within the unconsolidated material into the limestone results in downward drainage of water that can cause subsurface erosion, which, in turn, can produce a cavity in the overburden and contribute to its enlargement. Several examples in Cuba demonstrate that a rapid rise of water level caused by the sudden and continuous infiltration of water during intense rains associated with hurricanes, was a triggering mechanism for sinkhole formation.
Pumpage can steepen the hydraulic gradient, and thereby increase the flow velocity of ground water and, hence, the erosion and transport of the sediments into the limestone cavities. This subsurface erosion can eventually remove the roof material and cause an opening to form at the land surface. A decline of the water table below the top of the limestone can increase recharge of water and, thereby, provide another mechanism for subterranean erosion and ultimately formation of sinkholes.
Construction activities that modify the landscape can induce sinkholes. The erection of buildings or storage tanks can add additional weight to the supporting surficial material. Any modification of the hydrologic regime in sinkhole-prone areas can provide the triggering mechanism for sinkhole development. Inadvertent construction activities, such as the rupture of a sewer line or, water main also can induce development of sinkholes.
In sinkhole-prone areas, even minor construction activities can have severe consequences. For example, during a drought in July 1988, in the Guangxi province of southern China, two small charges of dynamite were used to develop an intermittent spring. After the second explosion, a geyser formed with a water column 2 meters high and 40 sinkholes developed in a nearby rice field. The damaged region continued to expand, and by October, had covered an area nearly 2 kilometers square containing 157 sinkholes.
Floods, Droughts, and Contamination
One of the primary problems in many karst areas is the seasonal alteration of floods and drought. For example, areas in southern China where the rainy season is well defined and annual precipitation is 2,000 millimeters, can often suffer from drought because of rapid infiltration of precipitation through subterranean drains. Even during the wet season, rice fields will become dry without rain every 10 days. During the dry season, the drought can become so severe that the people obtain their drinking water from caves in which the water table may have declined as much as 100 meters below land surface. During the wet seasons, the poljes and karst depressions may be flooded because the underground channels are not large enough to discharge the excess water from repeated rainstorms.
In the cockpit karst of China, called the "peak cluster depression," the bottoms of some closed depressions are so deep that no vegetation grows because of frequent flooding. Therefore, an inverted "timberline" develops in which the forest grows only above a certain elevation controlled by the flood level. In addition, growth of vegetation at low elevations is prevented because of the short duration of sunshine in some of the cylindrical karst depressions that are several hundred meters deep. Also in contrast to the usual situation, the lower elevations are cooler because of the lack of sunshine, and temperatures around the karst depression increases with altitude, rather than decreasing, as in normal mountain ranges. Consequently, the densest forests are usually on the middle of the slopes around the karst depressions. However, the soil mantle on these slopes is thin and the water table is commonly quite deep. The trees necessarily develop a deep root system that penetrates tens of meters of hard rock to obtain water. These observations in China demonstrate the difficulty of restoring forests that have been destroyed in karst areas. The destruction of forests increases the problems of drought and floods.
During rainy periods, water from wells and springs in many karst regions has high turbidity; is contaminated by elevated concentrations of nitrogen, chloride and sulfate; has increased oxygen demand; and has a marked increase in the concentration of coliform bacteria. The character and degree of such contamination depends largely on landuse within the recharge area, the hydraulic head, and the nature of conduit-filling sediments. Contamination is generally slight in uninhabited or uncultivated areas and greater in populated or highly fertilized areas.
In many areas, water from carbonate rocks is not safe for human consumption without treatment. For example, after World War II, water treatment eliminated water-borne diseases that had previously been endemic in communities pin the Dinaric region in Yugoslavia. Water in regions underlain by carbonate rocks is much more susceptible to contamination because of the characteristics of karstic terrains, such as: (1) the lack of cover of soil or permeable sediments precludes the normal filtering of water as it moves from land surface to the water table, (2) the rapid infiltration of water through solution channels that does not provide enough time for degradation of organic material and elimination of bacteria, and (3) the high permeability of the rocks that permits contaminants to move with relatively high velocity from the recharge area to the point of water collection. These factors preclude the completion of the biological processes that would provide natural self-purification. In addition, the great number of solution openings from the surface to the aquifer permits direct access of potential contaminants to the aquifer.
A CHALLENGE FOR THE FUTURE
It is clear that engineers, geologists, and other scientists in many parts of the world are gradually accumulating a body of scientific knowledge and engineering experience to understand the many phenomena associated with karst terrains. It is also equally clear that people living in karst terrains frequently tend to destroy the quantity and quality of land and water resources by not applying knowledge of the laws governing its complex hydrogeologic behavior. This is especially critical now when the magnitude of the influence of human activities on the biosphere is largely known. The increased demands on karst terrains during the present century because of population growth, water and land use, mining, energy production, industrialization, and agriculture often combine to degrade the fragile karat environment.
It is a responsibility of scientists to educate members of society, such as managers, planners, and politicians, of the consequences of activities undertaken in karst terrains. These consequences may be direct and immediate or may be indirect and gradual over a long period of time. As in. other natural sciences, this challenge for the scientific community to provide usable information demands more than ever, effective communication, meaningful cooperation, and active collaboration among scientific researchers and technicians of all nations. Professional organizations, such as the International Association of Hydrogeologists and the International Hydrological Programme of UNESCO offer the needed framework.
THE MODERN INTENSITY OF KARST DEVELOPMENT IN SOUTH CHINA
The modern intensity of karst development is concerned seriously by geologist. It is not only adopted to study the rate of modern karst development, but also to estimate its trend.
For estimating the intensity of karst development, the Cobel formula of erosional rate has been used. But in this paper, the method of chemical runoff module of karst water is introduced. The chemical runoff module of karst water is the amount that solute removed together with ground water runoff in special time and area within a basin. It is the product of runoff of groundwater and its mineralization degree. The formula is:
Mc=Q´ C/F or Mc=M´ C
Where Mc is ground water chemical runoff modules
The quantity of matter removed by water at a square kilometer in south China karst areas has been calculated (Tab.1). The data show that erosional rate in east Yunnan is the biggest one, where the quantity removed by water at a square kilometer is 38.47 T. Next to east Yunnan, it is 35.65 T in Guizhou and is 32.48 T. in Guangxi. Guangxi is a well karstified area in China, where there are many caverns, big karst springs, and subterrenean rivers, but the chemical runoff module of karst water is small in comparison with Guizhou and east Yunnan.
Tab.1 The erosional rate in South China karst areas
where C.R.M is chemical runoff modules of karst water,
S.A is statistic area,
C.V.A is corrosion unit area,
T.C.A is total corrosion amount in the whole year.
The explanation to the anomaly is that strong karstification of Guangxi taken place in geological history, but today they can not keep strong erosional rate like before. This argument would be also supported by the data in Puding, Guizhou, where the karst conduits of trellis subterrenean rivers develop very well. For example, within Houzhai subterrenean river basin the density of karst conduits is up to 720 m/km2, but chemical runoff module of karst wateris only 1.55 g/s/km2, which is small when comparison it with other subterrenean rivers. There are many subterrenean rivers such as Lontan with conduits development not fully yet, but their chemical runoff modules are up to 2.2 g/s/km2, i.e. bigger than trellis subterrenean rivers'.
The figures in Tab.1 reveal an important fact that in the area already well karstified, the modern karstification is not necessary still intensive. There are three prerequisites for modern erosional rate to be high: 1) Carbonate rock must have high solubility; 2) karst water runoff intensity is strong; 3) karst water has high erosional capacity. Based on the data of erosional quantity and above conditions, the inference can be drawn: the intensity of modern karst development, generally speaking, is stronger in Yunnan-Guizhou plateau than the karst plains of Guangxi.
ON THE FRAGILITY OF KARST ENVIRONMENT
Karst environment is a specific natural environment structurally. It consists of both surface and subsurface spatial systems that are closely interrelated.
Matter and energy in all three phases (solid, liquid, gas) organic and inorganic, life and non-life of the calcium-rich lithosphere, hydrosphere, atmosphere and human-biosphere interchange vigorously through the three-dimensional boundary surface of the karst system and become dynamical-equilibrated.
Therefore, the karst environmental system with the activities of the matter and energy is a large open system of complicated structure. In karst regions, under the adjustment and control of a set of parameters, the karst environment experiences evolution, equilibrium and variation.
Guizhou province, the biggest contiguous karst area of China with its well- known plateau-gorge landscape is a typical fragile karst environment. The author takes Guizhou karst as an example to discuss the fragility of karst environment and points out:
1. High sensibility of the ecosystem variation
The transform ways of ecosystem energy is fragile in the karst ecological environment. With high sensibility of entropy increase, for instance, once a forest is destroyed, with the destruction of desirable circulation of forest vegetation- soil cover and the balance of matter and energy in ecological chain, the desirable circulation will be discontinued, and the selectivity of environment on plant species will be increased and the process of rock desertification will be speeded up which can be expressed as follows.
Karst forest ---- karst tree bush ---- karst splinter ...
Vine bush ---- karst rock-desertification.
Under the accelerate destruction by human being, the rate of rock desertification in Guizhou reaches 0.0048ha/yr.
2. Low karst environment capacity
In bare karst areas, the capacity of crop offerings to population in per unit area is low because of low Productivity and poor quality of cultivable land and multi-factors influencing efficiency of phisiological radiant.
3. Low threshold of elasticity to endure catastrophes
In Guizhou, the frequency of drought and flood in karst areas is about 30-40% higher than nonkarst areas. With the poor bearing capacity and rapid diffusion of pollutant and low natural purification, surface and subsurface Pollution has been expanded in all three-dimensional space. Moreover, there appear lackness of buffer space for polluted water and of adequate time for filtration, absorption, oxidizing and ion exchanging. In addition, collapses occur frequently and in large scale.
Jiang Zhong Cheng
Tumen region is in the northwestern part of Yiyuan county, Shandong province. It is on the southern flank of Mt. Lushan with a total area of about 120 sq. km The carbonate rock totals about 50 per cent of the region, surrounded by impermeable gneiss of Archaeozoic era. So this region is a complete hydrogeological unit with clear boundaries. The limestone of middle Ordovician series underlying central part of the region can get the recharge of a lot of allogenic water. The modern climate of this region is between semihumid and semiarid, with mean annual rainfall of about 800 mm, and mean annual temperature of about 13ºC .
A major karst feature of this area is many significant caves. The number of cave totals over 80, with mean density 1.4 cave/sq. km. This is the area with highest cave concentration in Shandong province or even in north China. The Xiayan cave is the longest one. Its length having been surveyed is about 800m, but its real length is claimed by local people to be about 10 km. Besides there are 17 big and media caves distributed in a multi-leveled feature. Within the caves, there are various solutional forms and deposits. In some caves, the chemical deposit is very abundant. wondful , and valuable for tourism . So far 5 caves have been exploited for tourism. It is worthwhile to be mentioned that, in early 1980s. homo erectus Yiyuanensis and other animal fossils were unearthed from cave deposits in this area . It is identified to be contemporary with the homo erectus Pekinenis of middle Pleistocene .
T'he karst feature complex of the region could be distinguished into two sets. The first set is in accordance with environment conditions, including karst hill, dry valley, karst spring shallow karren and rock shelter or single conduit cave. and corresponding sediment is brown-yellow silty clay containing limestone scree , but speleothem is rare . Another set is composed of such features as sulational cap rock , karst depression , large conduit cave system , terra rossa and abundant speleothem . Their distribution is less than the first set, altitude is generally higher and the relative heights are over 100m. This kind of karst feature combination represents a humid environment, the K-Ar age of igneous rock vein Penetrating limestone in the bottom of the karst depression is 45 million years and the thermoluminecene age of overlain terra rossa is 1.2 million years, so the depression was formed between 1.2 to 45 million years ago , i.e. Neogene period. The age of speleothem is younger and they formed in the latter period of cave development. The U isotope ages of the flowstone in cave are from 0.1 to 0.25 million year, i.e. middle Pleistocene epoch. Therefore, the high level karst feature complex is a good indicator to reconstructed paleoenvironment history.
Lelic karst belongs to the so-called Ophiolite Belt of the Yugoslav Inner Dinarides laying on its NE edge. It occupies approximately 248 square kilometers of intensively karstified limestones visible on the surface, though by including parts covered with the low-permeable non-karstified rocks, its widespread is much bigger. It could be easily reached from Belgrade which is around 120 km far to the North East (Figure 1).
Lelic karst owns its name to the famous Serbian and Yugoslav karstologist Javan Cvijic who described it in several papers, the monograph published in 1920 being the most important. Among many other karst areas studied throughout Europe, Lilic karst certainly has served as a prototype of merokarst, the term introduced by this exceptional scientist.
Groundwater discharging from Lelic karst, mainly along its northern boundary, is now widely used for the water supply which is also planned to be more intensified in the following years.
The oldest discovered rocks in the wider area of Lelic karst are of Devonian and Carboniferous age. They are presented mainly by clayey shales thus acting as a impermeable base of karst aquifers which are developed in Middle and Late Triassic limestones. On several places, along the NW edge of Lelic karst, Paleozoic shales are separted from the karstified limestones by dolomites, marly and sandy limestones belonging to Late Permian and Early Triassic.
The depth of Middle and Late Triassic limestones exposed to the surface exceed 600 meters, while their thickness under the Ophiolites and Neogene sediments is still unknown (a borehole in Vrujci Spa discovered more than 300m of karstified limestones without reaching their base). Outcrops of Middle Triassic porphyrites are scattered throughout Lelic karst, mostly in its central and eastern parts where two long dikes caused emergence of large karstic springs - Paklje and Gradac.
The Ophiolites of Jurassic age are presented by the Diabase-Chert Formation and ultramafics lying over karstified limestones along the southern edge of Lelic karst. These low-permeable rocks act as a hanging or complete side-barrier for karst aquifers, which depend on their thickness and the presence of deep vertical and reverse faults. Flysh and flysh-like sediments of Late Cretaceous along the eastern boundary of Lelic karst have the same role.
Neogene low-permeable sediments occupy large area North and North-West from Lelic karst covering older rocks, with thickness varying between 60 and 400 meters. Considered as a one whole, Neogene sediments represent impermeable cover of the artesian karst aquifer. However, when Tortonian limestone or more sandy sediments are present in Neogene formation, the communication with surface could be present.
Groundwater flow in aquifers of Lilic karst is mainly oriented from the South-West towards North and North-East edge of large limestone body. This is directed by faults of the predominant system having SW-NE strike. Practically all surface streams coming to Lelic karst from SW, i.e. Ophiolites, are loosing water by sinking during the summer period. A rough calculation of water-balance shows that only part of this karst aquifer recharge, as well as of infiltrated precipitation, emerge on surface at several large permanent karst springs along the northern boundary of Lelic karst. The rest (roughly one fourth) flows under the Neogene sediments towards NE and is only occasionally tapped by water supply wells.
Almost 90 millions of cubic meters of groundwater is discharged from Lelic karst during the annual cycle, only small part being used for public water supply of the city of Valjevo and other settlements.
A usual problem of lack of water during the dry period is planned to be solved by the construction of dam and impoundment Stubo-Rovni on Jablanica intermittent stream flowing through Lelic karst. This construction would result in significant "smoothing" of seasonal discharge peaks, retention for the period and increasing of springs' discharge.
Intensive researches of possible loosing of water from
the future impoundment and corresponding directions have been recently
conducted. Numerous dye test shows that, though boundaries of particular
karst aquifers within Lelic karst could be defined, a significant
interchange of groundwater between them is present.
1: Paleozoic shales (impervious base), 2: Karstified Triassic limestones(main karst aquifers), 3: Porphyrites (complete side-barrier), 4: Ultramafics (side-barrier or low-permeable cover), 5: The Diabase-Chert Formation(same as ultramafics), 6: Cretaceous flysh and flysh-like sediments (low permeable cover), 7: Cretaceous limestones (karst aquifer), 8: Neogene sediments (low-permeable cover), 9: Karstic spring, 10 and 11: Directions of karst groundwater flow.
Kazuhisa YOSHiMURA, Youji INOKURA, Hisashi NAKAMURA, Akihiro SUGIMURA and Takehiko HAIKAWA
Akiyoshi-dai Plateau, most of which is a Quasi-national Park in Yamaguchi Prefecture, western Japan, is one of the biggest Karst plateaus in Japan. The yearlies mean temperature is 13.9ºC at nine a.m. During 1964-1988, the annual mean precipitation was 1,974 mm. On the basis of the values observed during two periods, from 1965 to 1968 and from 1983 to 1986, the average run-off from Akiyoshi-do Cave (a special natural treasure), located at the southern foot of the plateau, was estimated to be 955 mm.
The groundwater issuing from the cave has been measured for temperature, pH, and Na+, K+, Mg++, Ca2+, Cl-, HCO3- , NO3-,SO42-, and HPO42- contents. The equilibrated partial pressure of CO2 and saturation index for calcite were computed. The contents of major components (Ca2+ and HCO3-), produced by the dissolution of the Akiyoshi limestone in the presence of dissolved CO2, showed seasonal fluctuations, which followed changes in CO2 partial pressure in the soil. The contents of the major components Peaked 2 month after the Maximum CO2 Partial Pressure in the soil. The seasonal trend of calcite saturation was characterized by a winter maximum and a summer minimum. Although, according to the hydrography, the contents of dissolved components decreased during the early part of a storm hydrography because of recharge via sink holes, the contents of Ca2+ and HCO3- increased during the middle of the storm peak, followed by an increase in the contents of the other chemical components. Piston-flow type infiltration through soil near the conduits of the cave groundwater system strongly affects the quality of the groundwater.
At a given discharge, the calcium content could be predicted by using regression equations of calcium content vs. discharge. This made it possible to evaluate the mean solutional denudation rate in the cave basin: 51 mm/ka. A yearly average of 2,420 tons of limestone was dissolved in 3.5×107 m3 of groundwater issuing from the cave, whose limestone catchment basin area is 16.5 km2 : about half of the Akiyoshi-dai Plateau limestone area.
In the Akiyoshi-dai Plateau, the frequency distribution of the PH values of the Precipitation in the Akiyoshi-dai Plateau, ranging from 3.7 to 6.6 (from 1987 to 1989), is representative for all-Japan. The chemistry of rainwater was characterized by fairly high contents of nitrate and sulfate. Although acid rain, in this area has not yet produced any notable changes in the landscape and vegetation (because the limestone buffers the effect of the acid), it has dissolved 30% more calcium carbonate than non-polluted rain water. The estimation was carried out by the procedure shown in Table 1. The limestone dissolved by acid rain was estimated to be about 1% of the total amount dissolved.
Table 1. Estimation of the amounts of limestone dissolved in
Annual mean precipitation: 1,974 mm (1964--1988; Inokura et al., 1989)
Annual mean temperature: 13.9C (1964--1983; Maeda and Haikawa, 1985)
H' added into the Akishi-do drainage basin by precipitation:
9.4×105 mol (Feb.9--Dec.8, 1989; total precipitation:1,827 mm)
Dissolution reaction of limestone: CaCO3 + H+ ó Ca2+ + HCO3-
Limestone dissolved by non-polluted precipitation equilibrated with
the air of PCO2 = 10-3.5 atm:7.2×105 mol at 14ºC
Amounts of limestone dissolved by acidic wet deposits:
2.2×105mol = 23 tons/year
Averaged amount of limestone dissolved in the groundwater: