Role of mixing corrosion in calcite-aggressive H2O-CO2-CaCO3 solutions in the early evolution of karst aquifers in limestone

Two cave-forming mechanisms in limestone are discussed currently. First, wh en two H2O-CO2-CaCO3 solutions, saturated with respect to calcite but with different chemical compositions mix, renewed aggressiveness to limestone di ssolution occurs. This process called mixing corrosion [Bogli, 1964, 1980], in combination with linear dissolution kinetics, has been suggested as cav e forming. Second, it has been shown that solely the action of nonlinear di ssolution kinetics can generate extended karst conduits. This paper combine s both mechanisms. By digital modeling of the evolution of the aperture wid ths of a confluence of two fractures into a third one it is shown that the first mechanism does not create large cave conduits. The combination of mix ing corrosion and nonlinear kinetics, however, considerably intensifies kar stification, compared to that of nonlinear kinetics solely. The times to te rminate early evolution of karst are significantly reduced when the CO2 con centrations of the inflowing solutions differ by no more than 30%. We discu ss the underlying mechanisms by inspection of the time dependence of the ev olution of aperture widths, flow rates through them, and of the renewed und ersaturation of the mixed solution at the confluence of two fractures. Fina lly, the evolution of a karst aquifer on a two-dimensional percolation netw ork is modeled when mixing corrosion is present, and compared to that on an identical net with identical nonlinear dissolution kinetics, but mixing co rrosion excluded. Large differences in the morphology of the net of cave co nduits are found and also a reduction of the time of their evolution. From these findings we conclude that climatic changes, which influence the p(CO2 ) in the soil, can divert the evolving cave patterns.