Karst hydrology: recent developments and open questions

Karst aquifers are those that contain dissolution-generated conduits that permit the rapid transport of ground water, often in turbulent flow. The conduit system receives localized inputs from sinking surface streams and as storm runoff through sinkholes. The conduit system interconnects with the ground water stored in fractures and in the granular permeability of the bedrock. As a conceptual framework, the basic components of karstic aquifers seem to be generally accepted. Progress in the decade of the 1990s has focused mainly on quantifying the conceptual model. The equilibrium chemistry of the limestone and dolomite dissolution has been reliably established, and there are formal models for the kinetics of dissolution. Kinetic models have been used to calculate both fracture enlargement to protoconduits (0.01-m aperture) and the enlargement of protoconduits to the size of typical cave passages. Modeling of ground water flow in karstic aquifers has been less successful. Progress has been made in the use of water budgets, tracer studies, hydrograph analysis and chemograph analysis for the characterization of karstic aquifers. Topics on which progress is needed include (a) the construction of models that describe the complete aquifer including the interactions of all components, (b) models for clastic sediment transport within the aquifer, and (c) working out processes and mechanisms for contaminant transport in karst aquifers. An optimistic assessment at the end of the millennium is that a complete model for karstic aquifers is visible on the horizon.