EARLY DEVELOPMENT OF KARST AQUIFERS ON PERCOLATION NETWORKS OF FRACTURES IN LIMESTONE

We have modeled flow and dissolution processes within percolation netw orks representing stochastic primary fracture systems in limestone. At the beginning of karstification, flow is evenly distributed on all fr actures available. As the system develops by dissolutional widening of the fractures, preferred flow pathways evolve, which attract more and more flow, until at breakthrough the total flow rate increases dramat ically. These breakthrough times have been investigated with regard to their dependence on the initial fracture aperture width, the dimensio n of the evolving karst aquifer, hydraulic gradients, connectivity of the percolation network, and the chemical parameters, such as higher-o rder rate constants and exponents, and equilibrium concentration. They exhibit a behavior similar to that of breakthrough times for one-dime nsional conduits [Dreybrodt, 1996]. Our results show that the structur e of the evolving karst aquifer is determined by the initial geologica l setting. However, the final conduits are selected from competing pat hways with potential breakthrough times close to each other, whereby t he details of the distribution of undersaturation with respect to calc ite play an important role.