Climate and early karstification: What can be learned by models?

Abstract

First an overview is given on the present state of modelling of karst aquifers and karst conduits. Emphasis is placed to early karstification in rock massives with low fissure density as suggested for states 1 and 2 in Ford’s four-state-model. In this case early karstification proceeds under the condition of a constant hydraulic head. The evolution of a single isolated karst conduit, as well as evolution of karst conduits in two-dimensional networks of fractures are discussed. From these models the parameters determining early karstification can be identified. These are the initial aperture widths of the fractures, their lengths, the hydraulic head, and the viscosity of water, as well as the parameters of the non-linear dissolution kinetics of limestone, and the equilibrium concentration of calcium with respect to calcite. Early karstification under constant head conditions is characterized by a feedback-mechanism which couples flow rates through the conduits to the dissolutional widening of the fracture. After an initially slow increase in flow and in aperture width of the fracture a dramatical increase of flow rates and fracture widening occurs at breakthrough. The breakthrough time, when this event happens can be quantified from the parameters defined above. This time can be considered as a measure of intensity of karstification. Large scale climatic parameters, especially temperature exert influence to breakthrough time. Under otherwise identical geological conditions breakthrough times in tropic and moderate climates are about 5 times shorter than in arctic/alpine climate. Micro-climatic conditions, however, are of similar importance. If the vegetation on a karst plateau exhibits regions with different CO2 partial pressure in the soil, waters from these differing regions may mix at fracture-confluences in the karst massive. Mixing corrosion causes renewed solutional power at these confluences. Therefore breakthrough times can be reduced significantly. We present details of this mechanism and its influence to breakthrough times. Already moderate differences in in differently vegetated areas are sufficient to reduce breakthrough times by a factor of four. Although macro- and micro-climatic variables exert a significant influence to karstification it is not possible to draw conclusions on climatic conditions from the structure of a karst aquifer in retrospective.