Factors affecting the variation of permeability with depth in carbonate aquifers
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Factors affecting the variation of permeability with depth in carbonate aquifers Stephen R. H. Worthington 1 Received: 29 May 2020 / Accepted: 16 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The permeability of carbonate aquifers varies widely, but the major factors that influence changes in permeability with depth are not well established. Trends in permeability and solute concentration data were analysed for four carbonate aquifers where data were available over a wide range of depths. The Deep Geologic Repository (Ontario, Canada), Edwards Aquifer (Texas, USA), and Chalk aquifer (England, UK) all had permeability data from wells, supplemented by numerical groundwater flow models. There were no permeability data from wells for the fourth site, the Arabika Massif in the Caucasus Mountains (Abkhazia, Georgia). However, the permeability could be calculated from the water-table gradient. It was found that high permeabilities are associated with low solute concentrations, but there is a weak correlation between permeability and depth. The highest permeabilities are found in the freshwater zone, where total dissolved solids (TDS) concentrations are 2,000 m below the surface in the Caucasus Mountains. This study highlights the importance of dissolution, the link between permeability and both solute concentrations and flow rate, and how aquitards can play a pivotal role in how permeability varies as a function of depth. Keywords Carbonate rocks . Hydraulic properties . Karst . Permeability . Weathering front
Introduction Weathering enhances permeability in all bedrock lithologies, but it is particularly important in carbonate aquifers, where dissolution is the main weathering process (Worthington et al. 2016). However, the factors that influence the changes in permeability with depth in carbonates have not been well studied, except in petroleum reservoirs. The study of how permeability varies with depth in the crust has largely focussed on igneous and metamorphic rocks. This is understandable because these lithologies are predominant in the Published in the special issue “Five decades of advances in karst hydrogeology” Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10040-020-02247-2) contains supplementary material, which is available to authorized users. * Stephen R. H. Worthington [email protected] 1
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middle and lower crust (Ingebritsen and Gleeson 2017). Data from thermal and metamorphic heat flow models show that there is a consistent decrease in permeability in at least the uppermost 10–15 km in the crust (Manning and Ingebritsen 1999). Below that depth there is a transition to stresses being accommodated by ductile rather than brittle responses, and there are both fewer data and more uncertainty. Ranjram et al. (2015) compiled permeability measurements from crystalline rocks to a depth of 2,500 m, largely from research projects for
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