AQUIFER-SCALE PROPERTIES FOR HYDRAULIC CHARACTERIZATION OF CARBONATE AQUIFERS

A major problem with understanding flow in carbonates is the lack of test methods to assess the effects of solutional enlargement of fractures, resulting in karstification. One useful approach is to compare hydraulic properties in a proven karst aquifer to those in an aquifer one wishes to understand. We use the Mammoth Cave area as the type example since there is abundant hydraulic data available and there is consensus that the aquifer is karstified. Six specific testable properties which we use to differentiate karst aquifers from other fractured rock or porous media aquifers are tributary flow to springs, turbulent flow in conduits, troughs in the potentiometric surface, downgradient decreases in hydraulic gradient and increases in hydraulic conductivity, and aquifer-wide scaling effects in hydraulic conductivity.

There are many carbonate aquifers where the role of karstification and the presence of conduits have not been well addressed. Examples tested for the six above properties include the Edwards Aquifer in Texas, the Floridan aquifer in the Ocala area, the Yucatan aquifer in Mexico, and the dolostone aquifer in the Niagara Falls area. In each case the data show that aquifer behavior is much more similar to the Mammoth Cave karst aquifer than to a porous medium. This accords with lab experiments and numerical modeling which indicate that these aquifers should indeed behave as karstic rather than as porous media aquifers.

Simulation of the above aquifers using porous medium models such as MODFLOW is not ideal. However, by using a small cell size and continuous high K lines of cells terminating at springs it is possible to provide a first approximation of these aquifers as double porosity media.