Evolution of Karst Aquifers in Natural and Man Made Environments: A modeling approach

Abstract

The evolution of karst aquifers under various hydrological and chemical boundary conditions is studied.The influence of flow from a prominent fracture into a 2D network of fissures is compared to the evolution of a fracture isolated from this net. The modeling domain is a limestone block dissected by fissures. A prominent fracture extends along its center. Under constant head conditions between the left and the right hand side of the domain it looses flow into the network and aggressive solution from the input enhances dissolution. Breakthrough times are reduced. An isolated one-dimensional fracture with an additional single point of outflow from it is also investigated.The evolution of a karst aquifer below dam sites is studied. The modeling domain is a 2D, 1 m wide vertical section of soluble rock (gypsum and limestone), perpendicular to the dam. The chemical composition of the inflowing water is equal at all input points. A large zone of increasing permeability is created below the structure. The dependence of breakthrough times on different parameters is investigated.The effect of chemical boundary conditions on the karstification is studied. The model domain is a limestone block with two input points. The hydrological boundaries are equal for all simulated scenarios. The chemical composition of the inflowing water at both inputs is varied. Mixing corrosion is the reason for zones of increased permeability deep inside the aquifer. Two types of evolution are observed: Breakthrough type (BT) for low values of the input Ca concentration, and mixing corrosion (MC) type for values close to the equilibrium concentration. The BT - is characterized by enlarged pathways connecting an inflow point with the outflow boundary. For high Ca concentrations, MC is dominating. There is no considerably widened connection between the inflow points and the out flow boundary, but an enlarged channel along the mixing zone is observed. Also the timescale is considerably longer.