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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. ...

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. ...

Speleology in Kazakhstan

Shakalov on 11 Jul, 2012
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 facies is the lithologic appearance of a rock [16].?

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Chemistry and Karst, White, William B.
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Geochemical and mineralogical fingerprints to distinguish the exploited ferruginous mineralisations of Grotta della Monaca (Calabria, Italy), Dimuccio, L.A.; Rodrigues, N.; Larocca, F.; Pratas, J.; Amado, A.M.; Batista de Carvalho, L.A.
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
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Your search for numerical-model (Keyword) returned 23 results for the whole karstbase:
Showing 1 to 15 of 23
The role of tributary mixing in chemical variations at a karst spring, Milandre, Switzerland, , Perrin J. , Jeannin P. Y. , Cornaton F. ,
SummarySolute concentration variations during flood events were investigated in a karst aquifer of the Swiss Jura. Observations were made at the spring, and at the three main subterraneous tributaries feeding the spring. A simple transient flow and transport numerical model was able to reproduce chemographs and hydrographs observed at the spring, as a result of a mixing of the concentration and discharge of the respective tributaries. Sensitivity analysis carried out with the model showed that it is possible to produce chemical variations at the spring even if all tributaries have constant (but different for each of them) solute concentrations. This process is called tributary mixing. The good match between observed and modelled curves indicate that, in the phreatic zone, tributary mixing is probably an important process that shapes spring chemographs. Chemical reactions and other mixing components (e.g. from low permeability volumes) have a limited influence.Dissolution-related (calcium, bicarbonate, specific conductance) and pollution-related parameters (nitrate, chloride, potassium) displayed slightly different behaviours: during moderate flood events, the former showed limited variations compared to the latter. During large flood events, both presented chemographs with significant changes. No significant event water participates in moderate flood events and tributary mixing will be the major process shaping chemographs. Variations are greater for parameters with higher spatial variability (e.g. pollution-related). Whereas for large flood events, the contribution of event water becomes significant and influences the chemographs of all the parameters. As a result, spring water vulnerability to an accidental pollution is low during moderate flood events and under base flow conditions. It strongly increases during large flood events, because event water contributes to the spring discharge

Hydrogeology of the Umm Er Radhuma aquifer, Saudi Arabia, with reference to fossil gradients, 1982, Bakiewicz W, Milne Dm, Noori M,
Much of North Africa and the Arabian peninsula, lying in the Saharan climate zone, are underlain by huge tabular sandstone and carbonate aquifers, ranging in age from Cambrian to Tertiary. These are often saturated with water of reasonable quality and form very valuable resources in an area often desperately short of water. The Palaeocene Umm Er Radhuma carbonate aquifer is one such formation which has been the subject of intensive recent investigation. The formation contains groundwater of a reasonable quality, has adequate transmission and storage characteristics and hence considerable potential for future development. The origin of the water in such aquifers is the subject of continuing controversy. It is not disputed that the water is moving under the influence of regional groundwater gradients but origins of these gradients are the subject of considerable argument. On the one hand, there are those who hold that the presently observed gradients are fossil remnants of conditions created by a much wetter climatic regime prevalent some thousands of years ago. Against this are those who maintain that the gradients, at least in part, reflect a present day system with groundwater discharge in approximate dynamic equilibrium with recharge. This paper examines the hydrogeology of a typical Middle Eastern formation of the disputed kind, the Umm Er Radhuma aquifer in Saudi Arabia, and, with the aid of analytical and numerical models, attempts to resolve the problem of the origin of the observed groundwater gradients and to discover the extent to which the past must influence present day plans for future development

LATE-STAGE DOLOMITIZATION OF THE LOWER ORDOVICIAN ELLENBURGER GROUP, WEST TEXAS, 1991, Kupecz J. A. , Land L. S. ,
Petrography of the Lower Ordovician Ellenburger Group, both in deeply-buried subsurface cores and in outcrops which have never been deeply buried, documents five generations of dolomite, three generations of microquartz chert, and one generation of megaquartz. Regional periods of karstification serve to subdivide the dolomite into 'early-stage', which predates pre-Middle Ordovician karstification, and 'late-stage', which postdates pre-Middle Ordovician karstification and predates pre-Permian karstification. Approximately 10% of the dolomite in the Ellenburger Group is 'late-stage'. The earliest generation of late-stage dolomite, Dolomite-L1, is interpreted as a precursor to regional Dolomite-L2. L1 has been replaced by L2 and has similar trace element, O, C, and Sr isotopic signatures, and similar cathodoluminescence and backscattered electron images. It is possible to differentiate L1 from L2 only where cross-cutting relationships with chert are observed. Replacement Dolomite-L2 is associated with the grainstone, subarkose, and mixed carbonate-siliciclastic facies, and with karst breccias. The distribution of L2 is related to porosity and permeability which focused the flow of reactive fluids within the Ellenburger. Fluid inclusion data from megaquartz, interpreted to be cogenetic with Dolomite-L2, yield a mean temperature of homogenization of 85 6-degrees-C. On the basis of temperature/delta-O-18-water plots, temperatures of dolomitization ranged from approximately 60 to 110-degrees-C. Given estimates of maximum burial of the Ellenburger Group, these temperatures cannot be due to burial alone and are interpreted to be the result of migration of hot fluids into the area. A contour map of delta-O-18 from replacement Dolomite-L2 suggests a regional trend consistent with derivation of fluids from the Ouachita Orogenic Belt. The timing and direction of fluid migration associated with the Ouachita Orogeny are consistent with the timing and distribution of late-stage dolomite. Post-dating Dolomite-L2 are two generations of dolomite cement (C1 and C2) that are most abundant in karst breccias and are also associated with fractures, subarkoses and grainstones. Sr-87/Sr-86 data from L2, C1, and C2 suggest rock-buffering relative to Sr within Dolomite-L2 (and a retention of a Lower Ordovician seawater signature), while cements C1 and C2 became increasingly radiogenic. It is hypothesized that reactive fluids were Pennsylvanian pore fluids derived from basinal siliciclastics. The precipitating fluid evolved relative to Sr-87/Sr-86 from an initial Pennsylvanian seawater signature to radiogenic values; this evolution is due to increasing temperature and a concomitant evolution in pore-water geochemistry in the dominantly siliciclastic Pennsylvanian section. A possible source of Mg for late-stage dolomite is interpreted to be from the dissolution of early-stage dolomite by reactive basinal fluids

Principles of early development of karst conduits under natural and man-made conditions revealed by mathematical analysis of numerical models, 1996, Dreybrodt W,
Numerical models of the enlargement of primary fissures in limestone by calcite aggressive water show a complex behavior. If the lengths of the fractures are large and hydraulic heads are low, as is the case in nature, dissolution rates at the exit of the channel determine its development by causing a slow increase of water flow, which after a long gestation time by positive feedback accelerates dramatically within a short time span. Mathematical analysis of simplified approximations yields an analytical expression for the breakthrough time, when this happens, in excellent agreement with the results of a numerical model. This expression quantifies the geometrical, hydraulic, and chemical parameters determining such karat processes. If the lengths of the enlarging channels are small, but hydraulic heads are high, as is the case for artificial hydraulic structures such as darns, it is the widening at the entrance of the flow path which determines the enlargement of the conduit. Within the lifetime of the dam this can cause serious water losses, This can also be explained by mathematical analysis of simplified approximations which yield an analytical threshold condition from which the safety of a dam can be judged. Thus in both cases the dynamic processes of karstification are revealed to gain a deeper understanding of the early development of karst systems. As a further important result, one finds that minimum conditions, below which karstification cannot develop, do not exist

Mixed transport reaction control of gypsum dissolution kinetics in aqueous solutions and initiation of gypsum karst, 1997, Raines M. A. , Dewers T. A. ,
Experiments with gypsum in aqueous solutions at 25 degrees C, low ionic strengths, and a range of saturation states indicate a mixed surface reaction and diffusional transport control of gypsum dissolution kinetics. Dissolution rates were determined in a mixed flow/rotating disc reactor operating under steady-state conditions, in which polished gypsum discs were rotated at constant speed and reactant solutions were continuously fed into the reactor. Rates increase with velocity of spin under laminar conditions (low rates of spin), but increase asymptotically to a constant rate as turbulent conditions develop with increasing spin velocity, experiencing a small jump in magnitude across the laminar-turbulent transition. A Linear dependence of rates on the square root of spin velocity in the laminar regime is consistent with rates being limited by transport through a hydrodynamic boundary layer. The increase in rate with onset of turbulence accompanies a near discontinuous drop in hydrodynamic boundary layer thickness across the transition. A relative independence of rates on spinning velocity in the turbulent regime plus a nonlinear dependence of rates on saturation state are factors consistent with surface reaction control. Together these behaviors implicate a 'mixed' transport and reaction control of gypsum dissolution kinetics. A rate law which combines both kinetic mechanisms and can reproduce experimental results under laminar flow conditions is proposed as follows: R = k(t) {1 - Omega(b)() zeta [1 - (1 2(1 - Omega(b)())(1/2)]} where k(t) is the rate coefficient for transport control, and Omega(b)() is the mean ionic saturation state of the bulk fluid. The dimensionless parameter zeta(=Dm(eq)()/2 delta k() where m(eq)() = mean ionic molal equilibrium concentration, D is the diffusion coefficient through the hydrodynamic boundary layer, delta equals the boundary layer thickness and k() is the rate constant for surface reaction control) indicates which process, transport or surface reaction, dominates, and is sensitive to the hydrodynamic conditions in the reactor. For the range of conditions used in our experiments, zeta varies from about 1.4 to 4.5. Rates of gypsum dissolution were also determined in situ in a cavern system in the Permian Blaine Formation, southwestern Oklahoma. Although the flow conditions in the caverns were not determinable, there is good agreement between lab- and field-determined rates in that field rate magnitudes lie within a range of rates determined experimentally under zero to low spin velocities A numerical model coupling fluid flow and gypsum reaction in an idealized circular conduit is used to estimate the distance which undersaturated solutions will travel into small incipient conduits before saturation is achieved. Simulations of conduit wall dissolution showed-member behaviors of conduit formation and surface denudation that depend on flow boundary conditions (constant discharge or constant hydraulic gradient and initial conduit radius. Surface-control of dissolution rates. which becomes more influential with higher fluid flow velocity, has the effect that rate decrease more slowly as saturation is approached than otherwise would occur if rates were controlled by transport alone. This has the effect that reactive solutions can penetrate much farther into gypsum-bearing karst conduits than heretofore thought possible, influencing timing and mechanism of karst development as well as stability of engineered structures above karst terrain

Numerical simulation as a tool for checking the interpretation of karst spring hydrographs, 1997, Eisenlohr L, Kiraly L, Bouzelboudjen M, Rossier Y,
A schematic representation of karst aquifers may be that of a high hydraulic conductivity channel network with kilometre-wide intervals, surrounded by a low hydraulic conductivity fractured limestone volume and connected to a local discharge area, the karst spring, The behaviour of the karst spring (hydrographs, chemical or isotopic composition, etc.) represents the global response of the karst aquifer to input events. The available data an karst aquifer hydraulic parameters are limited, Global response is therefore more easily obtained and is commonly used to make inferences on the recharge and groundwater How processes, as well as on the hydraulic parameter fields. Direct verification of these interpretations is, obviously, very difficult. We have used an indirect method of verification, consisting of introducing well-defined theoretical karst structures into a finite element model and then analysing the simulated global response according to presently accepted interpretation schemes. As we know what we put into the numerical model, the validity of any interpretation may be checked. The first results indicate that some of the generally accepted interpretations are not necessarily true. In particular: (i) separation of simulated recession hydrographs into several components shows that different exponential components do not necessarily correspond to aquifer volumes with different hydraulic conductivities: (ii) non-exponential parts of recession hydrographs do not always give information about the infiltration process: and (iii) the recession coefficient of the baseflow (i.e. the last, nearly exponential part of the recession hydrograph) depends on the global configuration of the whole karst aquifer, not just on the hydraulic properties of the low hydraulic conductivity volumes. (C) 1997 Elsevier Science B.V

Physical response of a karst drainage basin to flood pulses: Example of the Devil's Icebox cave system (Missouri, USA), 1998, Halihan T. , Wicks C. M. , Engeln J. F. ,
In karst aquifers, water moves from the recharge area (sinkhole plains and swallets) to the discharge area (springs), traveling kilometers through the groundwater system in a period of hems to days. Transit rimes through karst aquifers are a function of the conduit geometry and connectedness, intensity and duration of the recharge event, and antecedent soil moisture. Often many of these factors are unknown or difficult to quantify. Therefore, predicting the response of a karst basin to recharge is difficult. Numerous researchers have attempted to understand the response of a karst basin, but a good understanding of whether the response is dependent on local features or regional effects is currently lacking. From April 1994 to May 1995, flood pulse hydrographs from a karst aquifer with well-developed and well-documented conduits (Devil's Icebox cave system) were obtained from a gaging station near the spring of the karst basin. Data were also collected from within the conduit system in an attempt to determine whether flow was locally controlled by constrictions in the conduits. Based on an application of Bernoulli's equation, analyses of the changes in kinetic head and potential head over time indicated local control during storm events. The observed sediment patterns and water level variations also support localized flow control during storm events. A numerical model of the constrictions was rested and reproduced the responses observed at the spring during initial periods of storm events. The model illustrated that the constricted flow was very sensitive to recharge. It also illustrated the transition from local control due to constriction to regional controls due to the aquifer matrix. (C) 1998 Elsevier Science B.V

Modeling of storm responses in conduit flow aquifers with reservoirs, 1998, Halihan Todd, Wicks Carol M. ,
In aquifers containing large voids, such as karst aquifers with caves or basaltic aquifers with lava tubes, hydrographs at wells or springs are used to analyze the structure and response of the hydrogeological system. Numerical modeling of hydrograph response is commonly based on either inverse techniques or postulated flow geometries. However, the range of mechanisms for generating hydrograph responses have not been fully investigated.Physical modeling of these complex non-Darcian systems permits better understanding of the storm responses that conduit systems may generate. Using a numerical model of conduit flow systems which incorporates turbulent flow, some of the mechanisms that can alter storm pulses were investigated by treating them as combinations of pipes that connect reservoirs.The results indicate that the response of a conduit-flow aquifer can range from what has been called 'diffuse' or 'steady' to 'conduit' or 'flashy', without employing a diffusive component. A full range of behavior can be the result of changes from phreatic to epiphreatic conditions in a conduit, changes in conduit geometry, or multiple springs draining the same system. The results provide a quantitative tool to assess spring and well hydrographs, and illustrate mechanisms that can generate observed responses, which have previously been qualitatively interpreted

Infiltration measured by the drip of stalactites, 2000, Sanz E. , Lopez J. J. ,
The hydrodynamic processes and mechanisms involved in rain infiltration and recharge in local areas of karst terrain can be identified and quantified by using measurements of the seepage of cave stalactites, Detailed measurements of the seepage of stalactites in seven caves located in an area close to the land surface, or the subcutaneous area of the karst, show a diversity of complex factors involved in infiltration: type of precipitation (rain or snow), air temperature, soil type and thickness, etc., which give rise to larger or smaller variations of flow in the espeleothem hydrographs, In some cases, no explanation can be found for the response of stalactites to rainfall, while in others there is a relationship between outer atmospheric parameters and the recharge represented by the stalactite drip. Romperopas Cave (Spain) has both a rapid and a basic flow, with hydrograph recessions similar to those observed in other caves. Water seepage in this cave varies greatly both in space and in time. The infiltration in Altamira Cave (Spain) was calculated and a multiple regression was found between infiltration, rain and outside air temperature. In other cases, the balance of the water on the soil is responsible for the seepage, Thus, a precipitation runoff numerical model that simulated the stalactite hydrographs could be applied to the Baradla and Beke Caves (Hungary), The complex properties of the ground, which are required for other flow numerical models for the unsaturated zone, were not taken into consideration

HJWFTAC: software for Hantush-Jacob analysis of variable-rate, multiple-extraction well pumping tests, 2002, Fleming Sw, Ruskauff Gj, Adams A,
Analytical well test solutions are a powerful approach to aquifer characterization and the parameterization of comprehensive numerical models. In addition, wellfield drawdown tests, which consist of coordinated pumping and data collection at a suite of monitoring and operating production wells, are of growing significance due to increasing pressures upon groundwater resources and the consequent management and planning requirement for superior hydrogeologic characterization of existing production wellfields. However. few pumping test analysis codes accommodate the multiple extraction wells involved, particularly for more sophisticated analytic aquifer test solutions. We present and demonstrate here a FORTRAN code for analysis of drawdown at a monitor well due to simultaneous variable-rate pumping at multiple independent production wells, which we developed in response to a need to refine an existing numerical, coupled groundwater/surface water resource management model, Spatial and temporal superposition are used to accommodate the typical operational properties of wellfield pumping tests, The software invokes the well-accepted Hantush-Jacob method for semiconfined or 'leaky' aquifers in a forward simulation procedure and effectively assumes homogeneity in applicable aquifer parameters (transmissivity, coefficient of storage, and leakance). Intended for both professionals and students, the code is widely applicable and straightforward to use as written. However, it can be modified with relative ease to use alternative well test solutions and/or formal inverse modeling techniques, or to accommodate spatial hydrogeologic variability. An application to a pumping test conducted in a karst limestone aquifer at the Cross Bar Ranch wellfield in Tampa Bay, Florida, demonstrates the utility of the software. (C) 2002 Elsevier Science Ltd. All rights reserved

Is the water still hot? Sustainability and the thermal springs at Bath, England, 2002, Atkinson Tc, Davison Rm,
The hot springs at Bath are the largest natural thermal source in Britain. Sustainable use of the waters for a spa requires maintenance of their temperature and flow rate. Together with smaller springs at Hotwells, Bristol, they form the outflow from a regional thermal aquifer that occurs where the Carboniferous Limestone is buried at depths > 2.7 km in the Bristol-Bath structural basin. The aquifer is recharged via limestone outcrops forming the south and west portions of the basin rim. Current knowledge of the basin's structure is reviewed, and important uncertainties identified concerning the hydrogeological role of thrust faults which may cut the limestone at depth. A simple numerical model is used to determine the possible influence of thrusts upon groundwater flow within the thermal aquifer. Comparison of the modelled flow patterns with geochemical data and structure contours eliminates the hypothesis that thrusts completely disrupt the continuity of the aquifer. The most successful model is used to simulate the possible impact of dewatering by large quarries at the limestone outcrops north and south of Bath. Substantial reductions in modelled flow at Bath result from proposed dewatering in the eastern Mendips, although the steady-state approach adopted has severe limitations in that it does not take account of the incremental staging of actual dewatering, nor allow for partial restitution of groundwater levels. The geological uncertainties highlighted by the modelling could be addressed by future research into the effect of thrusts on the continuity of the Carboniferous Limestone. More refined modelling to predict the timing of possible impacts of quarry dewatering will require measurements of the storativity of the thermal aquifer

Karst aquifer evolution in a changing water table environment - art. no. 1090, 2002, Kaufmann G. ,
[1] A vertical cross section through a karst aquifer is modeled by means of the finite element method to study the evolution of fractures and flow in the aquifer. The karst aquifer receives a constant recharge along the top boundary by precipitation and drains toward a resurgence assumed to be the base level in a valley. Flow is allowed both in the permeable rock matrix and the fracture network, and the fractures are enlarged with time by chemical dissolution. Hence during the early evolution of the karst aquifer the conductivity increases over several orders of magnitude, and the initially high water table drops to a steady state base level niveau. As a consequence, fractures above the final water table change from phreatic to vadose flow conditions. A systematic parameter study is carried out to investigate the aquifer evolution over a wide range of parameters, such as recharge rate, initial fracture width and density, and initial calcium concentration. The numerical models cover a wide range of drainage patterns, from phreatic water table caves to deep bathyphreatic caves to vadose river caves. The models suggest that a single theoretical approach is capable of explaining most common cave passage patterns

Hydrogeological overview of the Bure plateau, Ajoie, Switzerland, 2003, Kovacs A. , Jeannin P. Y. ,
This study presents a hydrogeological synthesis of the most recent data from the Bure plateau in Ajoie, canton Jura, NW Switzerland. Included is a complete reappraisal of aquifer geometry and aquifer boundaries, the delineation of catchment areas based on tracing experiments, and the evaluation of the hydraulic role of different hydrostratigraphic units. Furthermore, it presents GIS-based calculations on the mean piezometric surface, the thickness of the unsaturated zone and on the thickness of the minimum and mean saturated zones. The spatial extension of the shallow karst zone is also evaluated. A coherent conceptual model and the two-dimensional steady-state combined discrete channel and continuum type numerical model of the aquifer has been constructed. The research site is 83 km(2) in area and is underlain by slightly folded layers of Mesozoic limestones and marls. The Bure plateau is dissected by normal faults, which form a succession of elongated horst and graben structures. The main aquifer consists of Maim limestones, with thicknesses varying between zero (eastern border) and 320 m (south-eastern regions). The aquifer is bounded from below by the Oxfordian Marls. The underlying sediments of Middle Jurassic age are considered to be hydraulically independent. The surface topography of the Oxfordian Marls reveals the periclinal termination of a wide anticline over the plateau and a syncline in the southern parts. The aquifer contains three marly intercalations. Tracing experiments prove that marl layers do not act as regional aquicludes. These experiments also allow for the division of the aquifer surface into several water catchments. Based on tracing tests and piezometric data a NW-SE oriented groundwater divide seems to extend in the regions of Porrentruy-Bure-Croix. Calculations of the average (matrix flow) and minimum (conduit flow) water tables indicate an extended shallow karst zone in the region of Boncourt-Buix-St-Dizier. The thickness of the saturated zone increases towards the extremities of the research site, being thickest in the South. The thickness of the unsaturated zone shows a large variation, reaching its maximum in the central areas. Numerical model calculations roughly reproduce the observed hydraulic heads and mean spring discharges, they confirm current ideas about hydraulic parameters and suggest the existence of extended karst subsystems throughout the model domain

Modeling of karst aquifer genesis: Influence of exchange flow, 2003, Bauer S, Liedl R, Sauter M,
[1] This paper presents a numerical model study simulating the early karstification of a single conduit embedded in a fissured system. A hybrid continuum-discrete pipe flow model (CAVE) is used for the modeling. The effects of coupling of the two flow systems on type and duration of early karstification are studied for different boundary conditions. Assuming fixed head boundaries at both ends of the conduit, coupling of the two flow systems via exchange flow between the conduit and the fissured system leads to an enhanced evolution of the conduit. This effect is valid over a wide range of initial conduit diameters, and karstification is accelerated by a factor of about 100 as compared to the case of no exchange flow. Parameter studies reveal the influence of the exchange coefficient and of the hydraulic conductivity of the fissured system on the development time for the conduit. In a second scenario the upstream fixed head boundary is switched to a fixed flow boundary at a specified flow rate during the evolution, limiting the amount of water draining toward the evolving conduit. Depending on the flow rate specified, conduit evolution may be slowed down or greatly impaired if exchange flow is considered

Numerical models for mixing corrosion in natural and artificial karst environments, 2003, Kaufmann G. ,
[1] The enlargement of initially small fractures in a karst aquifer by chemical dissolution is studied. Flow in the aquifer is driven by head differences between sinks and resurgences, and flow depends on the permeability of small fissures and fractures in the aquifer. Enlargement of fractures is controlled by the chemical composition of the recharge, as water undersaturated with respect to calcite is able to dissolve material from the fracture walls. As fractures are enlarged with time, permeability within the aquifer increases significantly, and flow becomes very heterogeneous. Two different processes are considered: enlargement due to normal corrosion, where water is undersaturated with respect to calcite, and enlargement due to mixing corrosion, where two solutions saturated with respect to calcite but with different carbon dioxide concentrations mix and the resulting solution becomes undersaturated again. The importance of mixing corrosion is discussed for two boundary conditions: A natural karst aquifer is modeled with fixed recharge boundary conditions representing sinking streams, and an artificial karst aquifer is simulated with fixed head boundary conditions representing a reservoir. In both cases, mixing corrosion is important, especially if recharge is characterized by an almost saturated chemistry. Mixing corrosion significantly changes the evolving passage pattern, as dissolution due to mixing of solutions is possible deep in the aquifer. Mixing corrosion also reduces breakthrough times of the aquifer and can result in dramatic leakage underneath dam sites, even if the impounded water is almost saturated with respect to calcite

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