THE ROLE OF DISSOLUTION KINETICS IN THE DEVELOPMENT OF KARST AQUIFERS IN LIMESTONE - A MODEL SIMULATION OF KARST EVOLUTION

To model the development of karst aquifers from primary fissures in limestone rock, a numerical model of solutional widening of such fractures by calcite agressive water is suggested. The geological setting determines relevant geometrical parameters, i.e. length of the fracture, its initial width and the hydraulic gradient driving water from the input to the output. To simulate the solutional widening as it proceeds in time, the solution rates must be known as a function of concentration c of dissolved calcite. They are given as a first-order kinetic rate law. The model is applied to estimate evolution times of real karst aquifers in Germany, yielding results in agreement with geological observations. Depending on the geometrical parameters, especially length and hydraulic gradient, breakthrough times range from 104 to several 106 yr. The model is also applied to small-scale karst features, such as the evolution of solution dolines. In these cases fracture lengths are in the order of several tens of meters, and hydraulic gradients are high. This is also the case in land use projects where the groundwater table is lowered or artificial canals or dams are constructed. Karstification times of several tens to several hundred years are derived for these cases. -from Author