MODELLING THE DEVELOPMENT OF CARBONATE AQUIFERS – APPROACHES AND PRACTICAL APPLICATIONS

Mature karst aquifers are characterized by an extreme heterogeneity comprising a highly permeable, low storage conduit system and a less permeable, high storage fissured system. This heterogeneity results from the enlargement of fractures with small initial apertures to conduits. This paper provides insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems, the dissolution kinetics, the associated temporal decrease in flow resistance and the impact on these processes associated with long-term variations in climatic parameters. For the simulation of karst aquifer development it is of major importance to account for a fundamental feature of karst water hydraulics, i.e. the interaction conduit network and fractured rock matrix under variable boundary conditions. Only if this coupling of flow mechanisms is considered, can an appropriate representation of other relevant processes be achieved, e.g. carbonate dissolution, transport of dissolved solids and limited groundwater recharge. Here, the model CAVE (Carbonate Aquifer Void Evolution) is presented, which allows a simulation of the genesis of karst aquifers during geologic time periods. CAVE integrates important features relevant for different scenarios of karst evolution: (i) hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix; (ii) laminar as well as turbulent flow conditions; (iii) time-dependent and non-uniform recharge; (iv) widening of the conduits taking into account appropriate physico-chemical relationships governing calcite dissolution kinetics; (v) various boundary conditions may be applied, e.g. spatially and temporally variable recharge distribution, exchange of water between conduits and fissured system. This is achieved by pre-defining an initial network of karst conduits (“proto-conduits”) which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions. Decrease of hydraulic resistance is associated with the increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time. Practical applications and case studies are also presented.