Preprints
https://doi.org/10.5194/hess-2020-473
https://doi.org/10.5194/hess-2020-473

  16 Oct 2020

16 Oct 2020

Review status: a revised version of this preprint is currently under review for the journal HESS.

Early hypogene Carbonic Acid Speleogenesis in unconfined limestone aquifers by upwelling deep-seated waters with high CO2 concentration: A first modelling approach

Franci Gabrovšek1 and Wolfgang Dreybrodt1,2 Franci Gabrovšek and Wolfgang Dreybrodt
  • 1Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia
  • 2Faculty of Physics and Electrical Engineering, University of Bremen, Germany

Abstract. Hypogene caves originate from upwelling deep-seated waters loaded with CO2 that mix with meteoric waters in a limestone aquifer. Here we present first results on digital modelling of Carbonic Acid Speleogenesis (CAS). We study an unconfined aquifer where meteoric water seeps through the vadose zone and becomes saturated with respect to calcite when it arrives at the water table. From below deep-seated water with high pCO2 and saturated with respect to calcite invades the limestone formation by forced flow. Two flow domains arise that host exclusively water from the meteoric or the deep-seated source. They are separated by a water divide. There by dispersion of flow, a fringe of mixing arises and widening of the fractures is caused by mixing corrosion (MC). The evolution of the cave system is determined by its early state. At sites with high rates of fracture widening regions of higher hydraulic conductivity are created. They attract flow and support one by one mixing with maximal dissolution rates. Therefore, the early evolution is determined by karstification originating close to the input of the upwelling water and at the output at a seepage face. In between these regions, a wide fringe of moderate dissolution is present. In the later stage of evolution, this region is divided by constrictions that originate from statistical variations of fracture aperture widths that favour high dissolution rates and focus flow into this region. This MC-fringe-instability is an intrinsic property of cave evolution and is present in all scenarios studied. We have investigated the influence of defined regions with higher fracture aperture widths. These determine the cave patterns and suppress MC-fringe-instabilities. We have discussed the influence of the ratio of upwelling water flux rates to the rates of meteoric water. This ratio specifies the position of the mixing fringe and consequently that of the cave system. In a further step, we have explored the influence of time dependent meteoric recharge. Furthermore, we have modelled scenarios where waters are undersaturated with respect to calcite. These findings give important insight into mechanisms of hypogene speleogenesis. They also have implications to the understanding of sulfuric acid speleogenesis (SAS).

Franci Gabrovšek and Wolfgang Dreybrodt

 
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Franci Gabrovšek and Wolfgang Dreybrodt

Franci Gabrovšek and Wolfgang Dreybrodt

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Short summary
The evolution of karst aquifers is often governed by solutions gaining their aggressivity in depth. Although the principles of Hypogene Speleogenesis are known, modeling studies based on reactive flow in fracture networks are missing. We present a model, where dissolution at depth is triggered by the mixing of waters of different origin and chemistry. We show how the initial position of the mixing zone and flow instabilities therein, determine the position and shape of the final conduits.