Processes in Karst Systems

Carbon dioxide is an ubiquitious gas present in the earth’s atmosphere, in the soil air, and in dissolved form in rainwater as well as in groundwater, in rivers, lakes and in the ocean. In dissolved form it constitutes H₂ CO₃ , carbonic acid, which is the motor of erosion and weathering of rocks. Figure 1.1 shows the CO₂ pressure as its negative decadic logarithm for various environments. For CO₂ dissolved in water this is the pressure which would be in equilibrium with the aqueous CO₂^(aq) dissolved.

Weathering of limestone rocks, at the surface and underground, is due to the reaction:

Mass exchange between a liquid phase and a solid, such as in dissolution and precipitation of calcite, requires some kind of transport mechanism, which in the case of dissolution removes the ionic species released from the solid surface into the bulk of the solution, and vice versa in the case of precipitation transports the species from the bulk to the surface, where they are built into the crystal lattice.

In the dissolution of limestone, transport of the ions from the surface of the solid into the bulk of the solution by diffusion is only one step. When the chemical processes at the surface are so fast that ions removed by diffusion into the bulk are immediately replaced by those released from the solid, i.e. the surface concentration of Ca²⁺ is at saturation, then diffusion determines the dissolution rate entirely. If, however, this is not so, then chemical rate laws, i.e. chemical kinetics, have to be considered when dealing with dissolution rates. In CaCO₃ dissolution there are two chemical processes which are sufficiently slow to play an important role in determining dissolution rates, i.e. the amount of CaCO₃ removed from the solid per unit area and time.

Flow of groundwater in karst areas covers a wide field of hydrodynamic conditions. There is diffuse flow through partings in the rocks, such as joints or beddings, which initially, i.e. prior to widening by dissolution, might have apertures as small as 2 × 10⁻³ cm (Davis 1968). To describe the hydrodynamic properties of these pathways of flow, one may visualize these partings either as two parallel planes with a fixed distance, or as a two-dimensional porous medium. In any case flow is laminar and flow velocities are small, in the order of 10⁻³ cm s⁻¹. Thus, diffuse flow is characterized by a long retention time of karst water in the rock. The springs fed by this kind of water, which has sufficient time to equilibrate with respect temperature and chemical composition, usually show only small variations in temperature and are close to calcite saturation. Due to the long retention time of water and the large storage volume of the aquifer, they react slowly to flood pulses.

As already stated dissolution of calcite is a complex process comprising three different, simultaneously acting mechanisms:

Karst evolution in its dimensions of space and time depends on the dissolution kinetics of limestone. The time needed to excavate conduits by solutional removal of limestone depends on the rates of dissolution. The length of these flow conduits is determined by the distance calcite-aggressive water can travel until it becomes saturated. This penetration distance is related to the velocity of flow and also to the kinetics of dissolution.

Karst systems, in general, can be observed only in their mature state of karstification. From their morphology at the surface and underground (caves) one has to infer the history of evolution of those systems. From the relation of tectonic structures in the rock, such as fracture systems, to the orientation of cave passages one agrees nowadays that secondary permeability for groundwater flow is established along those fracture planes, which represent the least resistance to water flow, and are therefore most effectively enlarged by dissolution of the confining limestone rock. Thus, karst systems evolve due to increasing secondary porosity and the corresponding secondary permeability by the dissolving action of calcite-aggressive water, penetrating into the rock along joints, bedding plane partings, faults or intersections of those structures.

The evolution of secondary permeability in soluble rocks is determined by two fundamental factors. The first is the field of hydraulic gradients within the primary voids and partings of the rock which establishes flow of aggressive water from points of input to points of output. The second factor is the kinetics of the dissolution reactions. These determine to which extent the earliest initial flow paths are enlarged by dissolution. If saturation, with respect to calcite, is attained very quickly, dissolution can be active only at very short distances from the input, and only insignificant changes of the first flow channels result. On the other hand, at slow approach to equilibrium the earliest flow channels are modified by dissolution along their entire length. This changes the distribution of the pressure field and accordingly changes the flow fields. It is the purpose of this chapter to describe the evolution of karst according to these mutually interrelated factors.

CO₂ flux into or out of water influences the saturation state with respect to calcite. The result is either dissolution or precipitation of calcite. Outgassing of CO₂ can lead to precipitation of calcite in surface streams, which are fed by karst springs, as observed by Jacobson and Usdowski (1975) and Michaelis et al. (1984). Other studies show that the states of the saturation index Ω up to ten can be maintained without precipitation (Suarez 1983, Herman and Lorak 1987). In this chapter we will discuss these processes with respect to precipitation kinetics of calcite and its dependence on the flow properties of the precipitating solution.

In this book we have presented only one view of karst systems by discussing the physics and chemistry of dissolution of limestone and its consequences for the development of karst. One important outcome of this procedure is the recognition of important external and internal parameters, which determine karst processes. Internal parameters are, for example, the dimensions of karst channels or fissures, their connectivity, and hydraulic gradients which determine the type of flow, i.e. laminar or turbulent. As has been shown in Chapter 7, the type of flow is of utmost influence to the dissolution rates of limestone and thus to the further development of the karst system.