The initial phase of cave formation: Aquifer-scale three-dimensional models with strong exchange flow

Limestone, dolomite, gypsum, anhydrite, and salt are rocks, which can be dissolved, either by water alone, or in the case of limestone and dolomite more efficiently by water enriched with carbon dioxide. With time, the secondary porosity in these rocks increases substantially, and flow through these soluble rocks becomes very effective and fast. The time period for a substantial increase in secondary porosity ranges from years (salt) to 10,000-100,000 years (limestone, dolomite). The temporal evolution of such a soluble rock can be described by numerical models, coupling flow, transport, and dissolution. On the fracture-scale, numerous models describe fingering and wormhole formation in soluble rocks. On the aquifer-scale, numerical models predict preferential flow patterns and a strong exchange-flow component. We present results from three-dimensional numerical models describing the evolution of a karst aquifer in its early stage. Our numerical approach, which describes the large aquifer-scale, is capable to simulate the strongly heterogeneous development of secondary permeability in the aquifer. Based on irregularities in the model (e.g. geometry, chemistry, lithology, hydrology), preferential pathways evolve under a catchment-control situation, limiting the inflow into the system through the amount of water available from a catchment. We compare qualitatively our large aquifer-scale results to small fracture-scale results and show that the fingering described by the latter models is driven by similar mechanisms as the preferential growth in the former models.