Karst aquifers have complex and original characteristics which make them very different from other aquifers: high heterogeneity created and organised by groundwater flow; large voids, high flow velocities up to several hundreds of m/h, high flow rate springs up to some tens of m3/s. Different conceptual models, known from the literature, attempt to take into account all these particularities. The study methods used in classical hydrogeology—bore hole, pumping test and distributed models—are generally invalid and unsuccessful in karst aquifers, because the results cannot be extended to the whole aquifer nor to some parts, as is done in non-karst aquifers. Presently, karst hydrogeologists use a specific investigation methodology (described here), which is comparable to that used in surface hydrology. Important points remain unsolved. Some of them are related to fundamental aspects such as the void structure – only a conduit network, or a conduit network plus a porous matrix –, the functioning – threshold effects and non-linearities –, the modeling of the functioning – double or triple porosity, or viscous flow in conduits – and of karst genesis. Some other points deal with practical aspects, such as the assessment of aquifer storage capacity or vulnerability, or the prediction of the location of highly productive zones.

Karst groundwater: a challenge for new resources

https://pubag.nal.usda.gov/download/7446/PDF

A review and evaluation of catchment transit time modeling

624 km 2 ) that represent diverse geologic and geomorphic conditions in the western Cascade Mountains of Oregon Our primary objective was to determine the dominant physical controls on catchment-scale water residence time and specifically test the hypothesis that residence time is related to the size of the basin Residence times were estimated by simple convolution models that described the transfer of precipitation isotopic composition to the stream network We found that base flow mean residence times for exponential distributions ranged from 08 to 33 years Mean residence time showed no correlation to basin area (r 2 < 001) but instead was correlated (r 2 = 091) to catchment terrain indices representing the flow path distance and flow path gradient to the stream network These results illustrate that landscape organization (ie, topography) rather than basin area controls catchment-scale transport Results from this study may provide a framework for describing scale-invariant transport across climatic and geologic conditions, whereby the internal form and structure of the basin defines the first-order control on base flow residence time

/pdf/the-role-of-topography-on-catchment-scale-water-residence-2m0hfx1v8a.pdf

The role of topography on catchment-scale water residence time

Karst regions represent 7–12% of the Earth's continental area, and about one quarter of the global population is completely or partially dependent on drinking water from karst aquifers. Climate simulations project a strong increase in temperature and a decrease of precipitation in many karst regions in the world over the next decades. Despite this potentially bleak future, few studies specifically quantify the impact of climate change on karst water resources. This review provides an introduction to karst, its evolution, and its particular hydrological processes. We explore different conceptual models of karst systems and how they can be translated into numerical models of varying complexity and therefore varying data requirements and depths of process representation. We discuss limitations of current karst models and show that at the present state, we face a challenge in terms of data availability and information content of the available data. We conclude by providing new research directions to develop and evaluate better prediction models to address the most challenging problems of karst water resources management, including opportunities for data collection and for karst model applications at so far unprecedented scales.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RG000443

Karst water resources in a changing world: Review of hydrological modeling approaches

Noble gases within the crust originate from three main sources: the atmosphere, introduced into the crust dissolved in groundwater; the mantle, in regions of magmatic activity; and those produced in the crust by the result of radioactive decay processes. The continental crust contains approximately 40% of the terrestrial radioelements (Rudnick and Fountain 1995) that produce noble gases and, after the mantle and the atmosphere, forms the third major terrestrial noble gas reservoir (neglecting the core). In addition to these sources, contributions from interplanetary dust particles (IDP), cosmic ray interaction with the crustal surface and anthropogenic noble gases can in some cases be a significant source of noble gases in crustal materials. The use of noble gases to understand the role of fluids in different geological settings relies on their low natural abundance and chemical inertness. The low abundance of noble gases in crustal systems and their distinct isotopic character means that contributions from these different sources can often be resolved and quantified. With this, information is gained about the source of associated fluids, the environment from which they originated the physical manner in which they have been transported to the sampling site and the different phases that may have interacted within the crustal fluid system. This is only possible, however, with a detailed understanding of the processes that control the concentration and isotopic composition of the noble gases in different crustal environments.

The first part of this chapter deals with the three different mechanisms of noble gas production within the crust—radiogenic, nucleogenic, fissiogenic (Fig. 1⇓). We show how production ratios are affected not only by the source region radioelement concentration, but in the case of nucleogenic reactions, also by the spatial distribution and concentration of the target elements. For completeness we consider cosmogenic noble gas production rates and Interplanetary …

Production, Release and Transport of Noble Gases in the Continental Crust

On the theory of tracer experiments in fissured rocks with a porous matrix

Several published tracer tests are reexamined using the single fracture dispersion model (SFDM), i.e., a solution to the transport equation in which fractured rock is represented by a single fracture and the tracer is allowed to diffuse into the matrix. The model has three fitting parameters, i.e., the mean time of flow, dispersivity, and a diffusion parameter which combines such physical parameters as matrix porosity, coefficient of matrix diffusion, and adjusted fracture aperture. The model is easily calibrated yielding as good, or better, fits as did the models applied in original works, which were based on an assumption of negligible matrix diffusion and were characterized by larger numbers of fitting parameters. The SFDM better describes the short-term transport in investigated systems and/or their parameters than models applied so far. Validation of the model was obtained by showing that the values of its physical parameters either agree with those known from other methods or are within expected ranges. Another validation was achieved for tests performed in the same pair of wells with two tracers characterized by distinctly different coefficients of molecular diffusion. In spite of distinct differences in the shapes of the experimental curves, the SFDM yielded the same values of the fracture aperture, matrix porosity, and intrinsic dispersivity. The ratio of the diffusion coefficients in the matrix was close to that known from the diffusion coefficients in free water.

Tracer experiments in fractured rocks: Matrix diffusion and the validity of models

Transport equations for a single fissure in a porous matrix are coupled with a chemical model allowing for both an instantaneous equilibrium governed by a linear adsorption isotherm and a nonequilibrium kinetic reaction of the first order. The fitting procedure of the obtained solution is improved by additional fitting of the mass recovery curves; i.e., the fitting consists of a trial-and-error procedure applied in turn to the concentration tracer curve and the mass recovery curve until a given set of parameters gives the best fit of both theoretical curves to the experimental data. The model is tested against known multitracer experimental data from a fissured chalk formation. It is shown how some physical parameters can be obtained by a combined interpretation of the tracer and pumping data. However, in the case of adsorbable tracers some of the parameters must be known from a nonsorbable tracer experiment. In spite of its approximate nature the model works surprisingly well for nonsorbable tracers and reasonably well for sorbable solutes.

Mathematical modeling of tracer behavior in short‐term experiments in fissured rocks

Identifying the flow systems in a karstic-fissured-porous aquifer, the Schneealpe, Austria, by modelling of environmental 18O and 3H isotopes

Principles of the lumped-parameter approach to the interpretation of environmental tracer data are given. The following models are considered: the piston flow model (PFM), exponential flow model (EM), linear model (LM), combined piston flow and exponential flow model (EPM), combined linear flow and piston flow model (LPM), and dispersion model (DM). The applicability of these models for the interpretation of different tracer data is discussed for a steady state flow approximation. Case studies are given to exemplify the applicability of the lumped-parameter approach. Description of a user-friendly computer program is given. (author). 68 refs, 25 figs, 4 tabs.

Andrzej Zuber

Papers

On the theory of tracer experiments in fissured rocks with a porous matrix

Tracer experiments in fractured rocks: Matrix diffusion and the validity of models

Mathematical modeling of tracer behavior in short‐term experiments in fissured rocks

Identifying the flow systems in a karstic-fissured-porous aquifer, the Schneealpe, Austria, by modelling of environmental 18O and 3H isotopes

Lumped parameter models for the interpretation of environmental tracer data