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Community news

Speleology in Kazakhstan

Shakalov on 04 Jul, 2018
Hello everyone!   I pleased to invite you to the official site of Central Asian Karstic-Speleological commission ("Kaspeko")   There, we regularly publish reports about our expeditions, articles and reports on speleotopics, lecture course for instructors, photos etc. ...

New publications on hypogene speleogenesis

Klimchouk on 26 Mar, 2012
Dear Colleagues, This is to draw your attention to several recent publications added to KarstBase, relevant to hypogenic karst/speleogenesis: Corrosion of limestone tablets in sulfidic ground-water: measurements and speleogenetic implications Galdenzi,

The deepest terrestrial animal

Klimchouk on 23 Feb, 2012
A recent publication of Spanish researchers describes the biology of Krubera Cave, including the deepest terrestrial animal ever found: Jordana, Rafael; Baquero, Enrique; Reboleira, Sofía and Sendra, Alberto. ...

Caves - landscapes without light

akop on 05 Feb, 2012
Exhibition dedicated to caves is taking place in the Vienna Natural History Museum   The exhibition at the Natural History Museum presents the surprising variety of caves and cave formations such as stalactites and various crystals. ...

Did you know?

That uniform flow is flow with constant velocity at all points and at all times [16].?

Checkout all 2699 terms in the KarstBase Glossary of Karst and Cave Terms


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Featured articles from Cave & Karst Science Journals
Chemistry and Karst, White, William B.
See all featured articles
Featured articles from other Geoscience Journals
Karst environment, Culver D.C.
Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs, Bontognali, Tomaso R.R.; D’Angeli Ilenia M.; Tisato, Nicola; Vasconcelos, Crisogono; Bernasconi, Stefano M.; Gonzales, Esteban R. G.; De Waele, Jo
Calculating flux to predict future cave radon concentrations, Rowberry, Matt; Marti, Xavi; Frontera, Carlos; Van De Wiel, Marco; Briestensky, Milos
Microbial mediation of complex subterranean mineral structures, Tirato, Nicola; Torriano, Stefano F.F;, Monteux, Sylvain; Sauro, Francesco; De Waele, Jo; Lavagna, Maria Luisa; D’Angeli, Ilenia Maria; Chailloux, Daniel; Renda, Michel; Eglinton, Timothy I.; Bontognali, Tomaso Renzo Rezio
Evidence of a plate-wide tectonic pressure pulse provided by extensometric monitoring in the Balkan Mountains (Bulgaria), Briestensky, Milos; Rowberry, Matt; Stemberk, Josef; Stefanov, Petar; Vozar, Jozef; Sebela, Stanka; Petro, Lubomir; Bella, Pavel; Gaal, Ludovit; Ormukov, Cholponbek;
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Your search for thrust (Keyword) returned 75 results for the whole karstbase:
Showing 16 to 30 of 75
Induced seismicity at Wujiangdu Reservoir, China: A case induced in the Karst area, 1996,
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Hu Y. L. , Liu Z. Y. , Yang Q. Y. , Chen X. C. , Hu P. , Ma W. T. , Lei J. ,
To date 19 cases of reservoir-induced seismicity have been acknowledged in China and 15 of them are associated with karst. The Wujiangdu case is a typical one induced in the karst area. The dam with a height of 165 m is the highest built in a karst area in China. Seismic activity has been successively induced in five reservoir segments seven months after the impoundment in 1979. A temporary seismic network consisting of 8 stations was set up in,ne of the segments some 40 km upstream from the dam. The results indicate that epicenters were distributed along the immediate banks, composed of karstified carbonate, and focal depths were only several hundred meters. Most of the focal mechanisms were of thrust and normal faulting. It is suggested that karst may be an important factor in inducing seismicity. It can provide an hydraulic connection to change the saturation and pressure and also weak planes for dislocation to induce seismicity

Geology, geochemistry, and origin of the continental karst-hosted supergene manganese deposits in the western Rhodope massif, Macedonia, northern Greece, 1997,
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Nimfopoulos M. K. , Pattrick R. A. D. , Michailidis K. M. , Polya D. A. , Esson J. ,
Economic Mn-oxide ore deposits of commercial grade occur in the Rhodope massif near Kato Nevrokopi in the Drama region, Northern Greece. The Mn-oxide mineralization has developed by weathering of continental hypogene rhodochrosite-sulphide veins. The vein mineralization is confined by tectonic shear zones between marble and metapelites, extending laterally into the marble as tabular, pod or lenticular oreshoots (up to 50 m x 20 m x 5-10 m). Supergene oxidation of the hypogene mineralization led to the formation of in-situ residual Mn-oxide ore deposits, and secondary infills of Mn-oxide ore in embryonic and well developed karst cavities. Whole rock geochemical profiles across mineralized zones confirm the role of thrusts and faults as solution passageways and stress the importance of these structures in the development of hydrothermal and supergene mineralization at Kato Nevrokopi. Three zones an recognized in the insitu supergene veins: (A) a stable zone of oxidation, where immobile elements form (or substitute in) stable oxide mineral phases, and mobile elements are leached; (B) a transitional (active) zone in which element behavior is strongly influenced by seasonal fluctuations of the groundwater table and variations in pH-Eh conditions; and (C) a zone of permanent flooding, where variations in pH-Eh conditions are minimal. Zone (B) is considered as the source zone for the karst cavity mineralization. During weathering, meteoric waters, which were CO2-rich (P-CO2 similar to 10(-3.8) to 10(-1.4)) and oxygenated (fO(2) -10(-17) for malachite), percolated downward within the veins, causing breakdown and dissolution of sulfides and marble, and oxidation of rhodochrosite to Mn-oxides. Karat cavity formation was favored by the high permeability along thrust zones. Dissolved Mn2 was transported into karst cavities in reduced meteoric waters at the beginning of weathering (pH similar to 4-5), and as Mn(HCO3)(2) in slightly alkaline groundwaters during advanced weathering (pH similar to 6-8). Mn4? precipitation took place by fO(2) increase in ground waters, or pH increase by continuous hydrolysis and carbonate dissolution. In the well developed karst setting, some mobility of elements occurred during and after karst ore formation in the order Na>K>Mg>Sr>Mn>As>Zn>Ba>Al>Fe>Cu>Cd>Pb. (C) 1998 Canadian Institute of Mining, Metallurgy and Petrolem. Published by Elsevier Science Ltd. All rights reserved

Hydrogeological exploration of the Rjecina river spring in the Dinaric Karst., 1997,
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Biondic B. , Dukaric F. , Kuhta M. , Biondic R.
The Rjecina spring is one of the major springs in the Dinaric Karst. It appears at the contact between permeable carbonate and impermeable clastic rocks, with a discharge of up to 120 m3/s but it dries up during the dry seasons. The spring occurs close to the town of Rijeka, 325 m above sea level and offers an outstanding opportunity to cover gravitationally the public water demand of a town of about 200 000 inhabitants, and turistic needs of the whole region. This hydrogeological research project is a part of efforts to solve the problems of water deficiency during the dry summer seasons up to a maximum of three months. It was necessary to enter the parts of karst aquifer that are active in time of any outflow from the Rjecina spring by complex geological, hydrogeological and geophysical exploration accompanied with deep exploratory boreholes. During earlier explorations, it was determined that there are no active inflows in the immediate hinterland of the spring and that it is necessary to discover the inflows from other karst structures, that behave as retentions of karst springs in the zones of permanent discharge. The presence of multiple overthrusted structures in the zone around the spring site suggests the existence of deep zones of water retention, which may be reached by an access gallery from the Rjecina canyon. This work represents a substantial change in the exploration methodology for Dinaric Karst aquifers, because it directs the researchers toward deep, unknown retention spaces, which contain large reserves of high-quality groundwater outside urban areas.

Controls on the evolution of the Namurian paralic basin, Bohemian Massif, Czech Republic, 1997,
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Kumpera O. ,
The Namurian A paralic molasse deposits of the Upper Silesian Coal Basin form erosion remnants of an extensive foreland basin located in the eastern part of the Bohemian Massif. This basin represents the latest stage of development of the Moravian-Silesian Paleozoic Basin (Devonian-Westphalian). The paralic molasse stage of the foreland basin evolved from foreland basins with flysch and with marine molasse. The deposition of the thick paralic molasse (Ostrava Formation) started in the Namurian A. In comparison with other coal-bearing foreland basins situated along the Variscan margin in Europe, this is characterized not only by earlier deposition, but also by a different tectonic setting. It is located in the Moravian-Silesian branch of the Variscan orocline striking NNE-SSW, i.e. perpendicularly to the strikes of more western European foreland basins. In the Visean and Namurian, the foreland basin developed rapidly under the influence of the western thrustfold belt in the collision zone. The deposition was influenced by contrasting subsidence activities of the youngest and most external trough -- Variscan foredeep -- and the platform. The Upper Silesian Basin shows therefore a distinct W-E lithological and structural polarity and zonation

Origin and development of cave system in the Rosandra Valley (Classical Karst - Italy), 1998,
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Cucchi F. , Potleca M. , Zini L.

The system Savi - Fessura del Vento caves occurs in the Rosandra Valley Tertiary limestones in an area where intense tectonic influences have caused a series of overthrusts mixed with faults. In Rosandra Valley's area and in its caves there are some clues reporting recent tectonics which have conditioned both, epigean (slope irregularities, falls and variations of directions along the course of Rosandra creek) and hypogean morphologies (displaced solution forms, breakdowns and other morphologies).


Geological Characteristics of Desert and Upper Desert Caves (NE Blue Diamond Hill, Nevada, USA), 1998,
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Š, Ebela Stanka, Hess John W.

Desert Cave (72 m) and Upper Desert Cave (62 m) are the longest known caves in Blue Diamond Hill and are situated in one of its canyons. Caves are developed in limestone of latest early Permian age (Kaibab formation). In lowest part of Desert Cave, which is 12,6 m deep, cave passage reaches lithological contact between Kaibab limestone and sedimentary breccia (lower part of Kaibab formation). The contact breccia-limestone is one of initial structures in phreatic period of cave development but not before Mesozoic thrusting tectonics. Surface distance between entrances to Desert and Upper Desert Cave is 81 m, the caves are not connected. Desert Cave is situated 2 km W from Bird Spring thrust. Prevailing fissure directions in the cave are NW-SE and NE-SW. Passage direction is almost parallel with the NE-SW fissures direction. There is obvious connection in occurrence of breakdown in the cave with strongly expressed fissures in NW-SE direction.


Karstification and tectonic evolution of the Jabal Madar (Adam Foothills, Arabian platform) during the Upper Cretaceous, 2000,
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Montenat C. , Soudet H. J. , Barrier P. , Chereau A. ,
A palaeokarst system of Turonian age, located on the Arabian platform, at the front of the ophiolitic nappes of Oman (Jabal Madar, Adam foothills), is described and placed in its geodynamic context. The development of the karst network in a vadose context was favoured by an episode of fracturing (N-S to NW-SE fractures) that affected the Cenomanian platform carbonates of the Natih Formation. The karstic filling comprises two main types of speleothems: - laminated bioclastic calcarenites with graded bedding essentially deposited by gravity currents in a vadose regime; - crystallisation of large masses of white calcite in a saturated regime. The calcite was deposited during several episodes, and often constitutes most of the filling. The episode of uplift and emergence, accompanied by fracturing which favoured the development of the Madar karstic system, was probably induced by the swelling of the Arabian platform, in response to the initiation of the ophiolitic nappe obduction. Karstic filling probably occurred during the rise of marine level, what is suggested by mixing of vadose and marine influences (production of bioclastic calcarenites and later dolomitisation of these ones; crystallisation of white calcite of various origins as evidenced by cathodoluminescence data and carbonate isotopes). At the beginning of the Senonian, the Jabal Madar area was again submerged and incorporated in a relatively deep foreland basin where pelagic marls and turbidites were deposited (Muti Formation). The Jabal Madar (and its karstic system) and the whole of the Adam foothills were affected by folding towards the end of the Cretaceous, during the final phase of thrusting of the Omani nappes. The folding was strongly reactivated by post-obduction compressional movement which occurred during Miocene times

The influence of tectonic structures on karst flow patterns in karstified limestones and aquitards in the Jura Mountains, Switzerland, 2000,
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Herold T. , Jordan P. , Zwahlen F. ,
The development of karst systems is often assumed to be related to tectonic structures, i. e. joints and faults. However, detailed studies report many of these structures to be indifferent ui even obstacles to karst development. The aim of our study is to present a systematic which helps to explain or even predict whether a specific fault or joint, or a class of such structures are permeable (and therefore likely to be widened to karat conduits) or impermeable. Therefore three extended multi-tracer experiments followed by three months of monitoring were performed at some 95 springs and streams in the Eastern Jura fold-and-thrust belt. In addition, detailed mapping of tectonic and hydrogeological structures, including sinkholes and some 600 springs, has been carried out. The study area is characterised by two large anticlines, which have been affected by pre-fold normal faulting and synorogenic folding and thrusting as well as oblique reactivation of pre-existing faults. Hydrogeologically, two karst aquifers can be distinguished, the lower Mid Jurassic Hauptrogenstein (Dogger Limestone) and the upper Late Jurassic Malm Limestone. Both karst aquifers are confined and separated From each other hy impermeable layers. This study has shown that karst development and groundwater circulation is strongly controlled by tectonic structures resulting in specific meso- to macro-scale anisotropies. Fast long distance transport along fold axes in crest and limb at cas of anticlines is found to be related to extension joints resulting from synorogenic folds. Concentrated lateral drainage of water now from anticline limbs is exclusively related to pre-orogenic normal faults, which have been transtensively reactivated during folding. The same structures are also responsible for the significant groundwater exchange between the lower (inner) and upper (outer) aquifer. This water now, through otherwise impermeable layers, which is reported at several places and in both directions, is suspected to take place in porous calcite fault gouges or fault breccias. Transpressively reactivated normal faults and synorogenic reverse faults, on the other band. are found to have no influence on karst development and groundwater circulation. It is proposed that the systematic found in the Weissenstein area, i.e. that karst conduit development is mainly controlled by extensive or transtensive (reactivated) joints and faults, may also be applied to other tectonically influenced karat regions. Transpressive structures have no significant influence on karst system development and may even act as obstacles

The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wa, 2000,
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Chetty D, Frimmel He,
The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association? stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17-33 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370-405 degrees C) with early sulphide remobilisation in Tsumeb at 275 degrees C, whereas they are lower at Berg Aukas (up to 255 degrees C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct

Speleogenesis of sistema Cheve, Oaxaca, Mexico, 2000,
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Hose L. D.
The Cheve hydrologic complex in southern Mxico is the deepest known karst conduit system in the world, with a proven relief of 2540 m. The explored upper portion of the system, Sistema Cheve, extends to a depth of -1386 m. The water re-emerges from a cave in the Santo Domingo canyon, about 18 km north of the main sink. The entire cave system developed in various carbonates along (and under) the footwall of Laramide-age thrust fault. Speleogenesis of Sistema Cheve began in the Neogene or Pleistocene with exposure of a carbonate block isolated by faults. Aggressive allogenic waters entered the ground near the contact and formed mostly vertical passages until they arrived at the local base level, which was ~100 m higher than its present level. The stream removed ceilings and wall in the extensively fractured carbonates, resulting in local passage enlargement. Phreatic tubes formed in the middle and lower portions of the hydrologic system. Most of the conduits drained as downcutting continued in the Santo Domingo canyon, leaving mostly air-filled cave passages with a trunk stream and phreatic loops.

Karst hydrogeology of Kusluk-Dilmetas karst springs, Van-Eastern Turkey, 2001,
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Ozler H. M. ,
Permian marbles and recrystallised limestone nappes outcrop in the Artos Mountain range and comprise an aquifer with a small storage reservoir. Carbonate units are underlain by the impervious Yuksekova ophiolites. Between the marble and ophiolites, there is a transition zone by the northward thrusting, which varies between 500-1,000 m thickness. Fissures and fractures systems are well-developed in this transition zone because of the effects of tectonic movement, and extensive karstification has resulted in a high infiltration although its storage capacity is low. Because of the impermeable ophiolites at the base, groundwater discharges as springs flowing from the plane of the thrust faults. Numerous karst springs (48 springs) issue from this fissured and fractured zone, which are characterised by small discharge rates, a long residence time, and well-regulated spring flows. In addition, a selective enlargement is observed from west to east, which is greatly effected by strike-slip faults. All these springs are mostly fed by snowmelt during 6 months of the year

Late Archaean foreland basin deposits, Belingwe greenstone belt, Zimbabwe, 2001,
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Hofmann A. , Dirks P. H. G. M. , Jelsma H. A. ,
The c. 2.65 Ga old sedimentary Cheshire Formation of the Belingwe greenstone belt (BDB), central Zimbabwe, has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt. The Cheshire Formation rests sharply on a mafic volcanic unit (Zeederbergs Formation) and comprises a basal, eastward-sloping carbonate ramp sequence built of shallowing-upward, metre-scale sedimentary cycles. The cycles strongly resemble Proterozoic and Phanerozoic carbonate cycles and might have formed by small-scale eustatic sea level changes. The top of the carbonate ramp is represented by a karst surface. The carbonates are overlain by and grade laterally to the east into deeper water (sub-wave base) siliciclastic facies. Conglomerate, shale and minor sandstone were deposited by high- to low-density turbidity currents and were derived from the erosion of Zeederbergs-like volcanic rocks from the east. Shortly after deposition, the Cheshire Formation and underlying volcanics were affected by a northwest-directed thrusting event. Thrusting gave rise to the deformation of semi-consolidated sediments and resulted in the juxtaposition of a thrust slice of Zeederbergs basalts onto Cheshire sediments. The stratigraphy, asymmetric facies and sediment thickness distribution, palaeogeographic constraints and evidence for an early horizontal tectonic event suggest that the Cheshire Formation formed in a foreland-type sedimentary basin. (C) 2001 Elsevier Science B.V. All rights reserved

The speleological objects in the area of the Rjecina nappe structure., 2001,
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Kuhta M.
According to the new geotectonic concept of the Dinarides (Herak 1986, 1991), the investigated carbonate region of the Rjecina nappe structure is locate along the margin of the mega-structural unit of the Adriatic carbonate platform, the Adriaticum. Folds and numerous reverse faults and thrusts of the Dinaric strike manifest the main structural and tectonical features. These structures are incised by younger diagonal faults. The whole structure is inclined towards the north-west, with a succession of younger deposits towards that direction and the structure terminates with flysch beds. The flysch forms a rim around the structure in contact with the Obruc nappe, which represents the boundary with the Dinaricum mega-structural unit. In a geomorphologic sense the studied area is a part of a karst plateau situate at an altitude of 580 m above sea level. The typical karst morphology on the surface extends to the underground were it is manifested by the existence of caves and pits formed by karstification. During the most recent speleological investigations 10 up to now unknown caves and pits were studied. For six objects located in the vicinity the data used was presented from previous studies (Bozicevic, 1969, Biondic and Dukaric, 1986). Together with four objects which were not investigated as jet in the studied region of the Rjecina nappe structure there are 20 registered speleological objects. The main data concerning these objects are given in the table and the morphological features are presented on topographic maps.

Karst characteristics of thrust contact limestone-dolomite near Predjama, 2001,
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Č, Ar Jož, E, Š, Ebela Stanka

With detailed lithological and tectonic-structural mapping at the scale 1:5000 we studied the area near Bukovje NE from Predjama. In the studied area and further towards the NE we can follow a strong thrust contact between Upper Cretaceous limestones of Snežnik thrust sheet over which Upper Triassic Norian-Rhaetian dolomite of Hrušica thrust sheet is overthrust. The thrust is cut by systems of dextral faults with expressed vertical component. Beneath the studied area underground passages of Predjama cave can be found; they are developed in limestones as in dolomites. Dolines in Upper Cretaceous limestone are uniformly arranged and are in structural-genetic view broken (D), near-fault (E) or fault (F). Along the thrust edge we have contact dolines (G) in dolomite. The rest of the dolines on dolomite are connected to differently broken rocks in fault zones and are called reproduced fault-broken dolines / HF(D)do /. For described karst features along thrust contact we suggest the name contact karst on dolomite along thrust.


Coastal karst springs in the Mediterranean basin : study of the mechanisms of saline pollution at the Almyros spring (Crete), observations and modelling, 2002,
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Arfib B, De Marsily G, Ganoulis J,
Variations in salinity and flow rate in the aerial, naturally salty spring of Almyros of Heraklion on Crete were monitored during two hydrological cycles. We describe the functioning of the coastal karstic system of the Almyros and show the influence of the duality of the flow in the karst (conduits and fractured matrix) on the quality of the water resource in the coastal area. A mechanism of saltwater intrusion into this highly heterogeneous system is proposed and validated with a hydraulic mathematical model, which describes the observations remarkably well. Introduction. - Fresh groundwater is a precious resource in many coastal regions, for drinking water supply, either to complement surface water resources, or when such resources are polluted or unavailable in the dry season. But coastal groundwater is fragile, and its exploitation must be made with care to prevent saltwater intrusion as a result of withdrawal, for any aquifer type, porous, fractured or karstic. In karstic zones, the problem is very complex because of the heterogeneous nature of the karst, which makes it difficult to use the concept of representative elementary volume developed for porous or densely fractured systems. The karstic conduits focus the major part of the flow in preferential paths, where the water velocity is high. In coastal systems, these conduits have also an effect on the distribution of the saline intrusion. As was shown e.g. by Moore et al. [1992] and Howard and Mullings [1996], both freshwater and salt-water flow along the fractures and conduits to reach the mixing zone, or the zone where these fluids are superposed in a dynamic equilibrium because of their differences in density ; but the dynamics of such a saltwater intrusion are generally unknown and not represented in models. Such coastal karstic systems are intensely studied at this moment in the Mediterranean region [Gilli, 1999], both as above sea-level or underwater springs, for potential use in areas where this resource would be of great value for economic development. This article discusses the freshwater-saltwater exchange mechanisms in the karstic aquifer of the Almyros of Heraklion aquifer (Crete) and explains the salinity variations observed in the spring. First, the general hydrogeology of the study site is described, then the functioning of the spring : a main conduit drains the freshwater over several kilometres and passes at depth through a zone where seawater is naturally present. The matrix-conduit exchanges are the result of pressure differences between the two media. These processes are represented in a mathematical model that confirms their relevance. General hydrogeology of the studied site. - The karstic coastal system of the Almyros of Heraklion (Crete) covers 300 km2 in the Ida massif whose borders are a main detachment fault, and the Sea of Crete in the north, the Psiloritis massif (highest summit at 2,456 m) in the south and west, and the collapsed basin of Heraklion filled in by mainly neo-geneous marl sediments in the east. The watershed basin consists of the two lower units of characteristic overthrust formations of Crete (fig. 1) : the Cretaceous Plattenkalk and the Cretaceous Tripolitza limestones. The two limestone formations are locally separated by interbedded flysch or phyllade units that form an impervious layer [Bonneau et al., 1977 ; Fassoulas, 1999] and may lead to different flow behaviour within the two karstic formations. Neo-tectonic activity has dissected these formations with large faults and fractures. The present-day climate in Crete is of Mediterranean mountain type, with heavy rain storms and snow on the summits in winter. Rainfall is unevenly distributed over the year, with 80 % of the annual total between October and March and a year-to-year average of 1,370 mm. The flow rate of the spring is high during the whole hydrologic cycle, with a minimum in summer on the order of 3 m3.s-1 and peak flow in winter reaching up to 40 m3.s -1. The water is brackish during low flow, up to a chloride content of 6 g.l-1, i.e. 23 % of seawater, but it is fresh during floods, when the flow rate exceeds 15 m3.s-1. During the 1999-2000 and 2000-2001 hydrologic cycles, the water was fresh during 14 and 31 days, respectively. The water temperature is high and varies very little during the year (see table I). In the areas of Keri and Tilissos (fig. 1), immediately south of the spring, the city of Heraklion extracts water from the karstic system through a series of 15 wells with depth reaching 50 to 100 m below sea level. Initially, when the wells were drilled, the water was fresh, but nowadays the salinity rises progressively, but unequally from well to well (fig. 2). The relatively constant temperatures and salinities of the wells, during the hydrological cycle, contrast with the large salinity variations at the spring (fig. 2 and table I). They show that the karstic system is complex and comprises different compartments, where each aquifer unit reacts to its individual pressures (pumping, rainfall) according to its own hydrodynamic characteristics [Arfib et al., 2000]. The Almyros spring seems disconnected from the surrounding aquifer and behaves differently from that which feeds the wells (upper Tripolitza limestone). It is recharged by fresh water from the mountains, which descends to depths where it probably acquires its salinity. The spring would thus be the largest resource of the area, if it was possible to prevent its pollution by seawater. A general functioning sketch is proposed (fig. 3), which includes the different geological units of interest. Identification of the functioning of the Almyros spring through monitoring of physical and chemical parameters. - The functioning of the aquifer system of the Almyros spring was analysed by monitoring, over two hydrological cycles, the level of the spring, the discharge, the electric conductivity and the temperature recorded at a 30 min time interval. In the centre of the watershed basin, a meteorological station at an altitude of 800 m measures and records at a 30 min time interval the air temperature, rainfall, relative humidity, wind velocity and direction ; moreover, an automatic rain gauge is installed in the northern part of the basin at an altitude of 500 m. The winter floods follow the rhythm of the rainfall with strong flow-rate variations. In contrast, the summer and autumn are long periods of drought (fig. 7). The flow rate increases a few hours after each rainfall event ; the water salinity decreases in inverse proportion to the flow rate a few hours to a few days later. Observations showed that the water volume discharged at the Almyros spring between the beginning of the flow rate increase and the beginning of the salinity decrease is quite constant, around 770,000 m3 (fig. 4) for any value of the flow rate, of the salinity and also of the initial or final rainfall rates. To determine this constant volume was of the upmost importance when analyzing the functioning of the Almyros spring. The lag illustrates the differences between the pressure wave that moves almost instantaneously through the karst conduit and causes an immediate flow rate increase after rainfall and the movement of the water molecules (transfer of matter) that arrives with a time lag proportionate to the length of the travel distance. The variation of the salinity with the flow rate acts as a tracer and gives a direct indication of the distance between the outlet and the seawater entrance point into the conduit. In the case of the Almyros, the constant volume of expelled water indicates that sea-water intrusion occurs in a portion of the conduit situated several kilometres away from the spring (table II), probably inland, with no subsequent sideways exchange in the part of the gallery leading up to the spring. As the lag between the flow rate and the salinity recorded at the spring is constant, one can correct the salinity value by taking, at each time step, with a given flow rate, the salinity value measured after the expulsion of 770,000 m3 at the spring, which transforms the output of the system so as to put the pressure waves and the matter transfer in phase [Arfib, 2001]. After this correction, the saline flux at the spring, equal to the flow rate multiplied by the corrected salinity, indicates the amount of sea-water in the total flow. This flux varies in inverse proportion to the total flow rate in the high-flow period and the beginning of the low-flow period, thereby demonstrating that the salinity decrease in the spring is not simply a dilution effect (fig. 5). The relationship that exists between flow rate and corrected salinity provides the additional information needed to build the conceptual model of the functioning of the part of the Almyros of Heraklion aquifer that communicates with the spring. Freshwater from the Psiloritis mountains feeds the Almyros spring. It circulates through a main karst conduit that descends deep into the aquifer and crosses a zone naturally invaded by seawater several kilometers from the spring. The seawater enters the conduit and the resulting brackish water is then transported to the spring without any further change in salinity. The conduit-matrix and matrix-conduit exchanges are governed by the head differences in the two media. Mathematical modelling of seawater intrusion into a karst conduit Method. - The functioning pattern exposed above shows that such a system cannot be treated as an equivalent porous medium and highlights the influence of heterogeneous structures such as karst conduits on the quantity and quality of water resources. Our model is called SWIKAC (Salt Water Intrusion in Karst Conduits), written in Matlab(R). It is a 1 D mixing-cell type model with an explicit finite-difference calculation. This numerical method has already been used to simulate flow and transport in porous [e.g. Bajracharya and Barry, 1994 ; Van Ommen, 1985] and karst media [e.g. Bauer et al., 1999 ; Liedl and Sauter, 1998 ; Tezcan, 1998]. It reduces the aquifer to a single circular conduit surrounded by a matrix equivalent to a homogeneous porous medium where pressure and salinity conditions are in relation with sea-water. The conduit is fed by freshwater at its upstream end and seawater penetrates through its walls over the length L (fig. 6) at a rate given by an equation based on the Dupuit-Forchheimer solution and the method of images. The model calculates, in each mesh of the conduit and at each time step, the head in conditions of turbulent flow with the Darcy-Weisbach equation. The head loss coefficient {lambda} is calculated by Louis' formula for turbulent flow of non-parallel liquid streams [Jeannin, 2001 ; Jeannin and Marechal, 1995]. The fitting of the model is intended to simulate the chloride concentration at the spring for a given matrix permeability (K), depth (P) and conduit diameter (D) while varying its length (L) and its relative roughness (kr). The spring flow rates are the measured ones ; at present, the model is not meant to predict the flow rate of the spring but only to explain its salinity variations. Results and discussion. - The simulations of chloride concentrations were made in the period from September 1999 to May 2001. The depth of the horizontal conduit where matrix-conduit exchanges occur was tested down to 800 m below sea level. The diameter of the conduit varied between 10 and 20 m, which is larger than that observed by divers close to the spring but plausible for the seawater intrusion zone. The average hydraulic conductivity of the equivalent continuous matrix was estimated at 10-4 m/s. A higher value (10-3 m/s) was tested and found to be possible since the fractured limestone in the intrusion zone may locally be more permeable but a smaller value (10-5 m/s) produces an unrealistic length (L) of the saline intrusion zone (over 15 km). For each combination of hydraulic conductivity, diameter and depth there is one set of L (length) and kr (relative roughness) calibration parameters. All combinations for a depth of 400 m or more produce practically equivalent results, close to the measured values. When the depth of the conduit is less than 400 m, the simulated salinity is always too high. Figure 7 shows results for a depth of 500 m, a diameter of 15 m and a hydraulic conductivity of 10-4 m/s. The length of the saltwater intrusion zone is then 1,320 m, 4,350 m away from the spring and the relative roughness coefficient is 1.1. All the simulations (table II) need a very high relative roughness coefficient which may be interpreted as an equivalent coefficient that takes into account the heavy head losses by friction and the variations of the conduit dimensions which, locally, cause great head losses. The model simulates very well the general shape of the salinity curve and the succession of high water levels in the Almyros spring but two periods are poorly described due to the simplicity of the model. They are (1) the period following strong freshwater floods, where the model does not account for the expulsion of freshwater outside the conduit and the return of this freshwater which dilutes the tail of the flood and (2) the end of the low-water period when the measured flux of chlorides falls unexpectedly (fig. 5), which might be explained by density stratification phenomena of freshwater-saltwater in the conduit (as observed in the karst gallery of Port-Miou near Cassis, France [Potie and Ricour, 1974]), an aspect that the model does not take into account. Conclusions. - The good results produced by the model confirm the proposed functioning pattern of the spring. The regulation of the saline intrusion occurs over a limited area at depth, through the action of the pressure differences between the fractured limestone continuous matrix with its natural saline intrusion and a karst conduit carrying water that is first fresh then brackish up to the Almyros spring. The depth of the horizontal conduit is more than 400 m. An attempt at raising the water level at the spring, with a concrete dam, made in 1987, which was also modelled, indicates that the real depth is around 500 m but the poor quality of these data requires new tests to be made before any firm conclusions on the exact depth of the conduit can be drawn. The Almyros spring is a particularly favorable for observing the exchanges in the conduit network for which it is the direct outlet but it is not representative of the surrounding area. To sustainably manage the water in this region, it is essential to change the present working of the wells in order to limit the irreversible saline intrusion into the terrain of the upper aquifers. It seems possible to exploit the spring directly if the level of its outlet is raised. This would reduce the salinity in the spring to almost zero in all seasons by increasing the head in the conduit. In its present state of calibration, the model calculates a height on the order of 15 m for obtaining freshwater at the spring throughout the year, but real tests with the existing dam are needed to quantify any flow-rate losses or functional changes when there is continual overpressure in the system. The cause of the development of this karstic conduit at such a great depth could be the lowering of the sea level during the Messinian [Clauzon et al., 1996], or recent tectonic movements

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