Development and Distribution of Permeability in Carbonate Aquifers

Abstract

Unlike aquifers in nonsoluble rocks in which the permeability tends to be both inherent and fairly even in local distribution, aquifers in carbonate rocks tend to have their permeability developed through circulation of water and solution of the rock and to have an uneven distribution of permeability. The extent to which near‐surface carbonate rocks have permeability developed in them depends on the degree to which water that is high in dissolved carbon dioxide has moved through joints and other openings to places of discharge.The following well‐established generalizations indicate that the geometry of carbonate aquifers is commonly different from that of other aquifers. (1) The circulation of water and solution activity tend to be greatest in the upper part of the zone of saturation and tend to lessen with increased depth. (2) The moderately large openings in the path of bulk flow of groundwater tend to enlarge by solution action; contrastingly, the small openings not in the path of bulk flow enlarge only slightly. The texture of permeability in a few carbonate aquifers is fine and even but in most carbonate aquifers is more commonly coarse and uneven. Where the texture of permeability is fine and fairly even, as in the Biscayne aquifer of southeastern Florida, the aquifer may be riddled with a close network of solution channels or interstitial openings. Where the texture of permeability is coarse and uneven, ramifying solution channels are bounded by a large volume of relatively dense impermeable carbonate rocks. The channel network system is especially prominent in the upper part of the zone of saturation. In many cases the gross permeability seemingly decreases exponentially with increasing depth. The dynamic water table, becoming flatter and moving lower as permeability increases, is bounded above by a cavernous unsaturated zone that is a relic of the former permeable zone of saturation. With passing geologic time, the solution channel network loses its aquifer characteristics when (1) the water table moves downward to the base of the aquifer, (2) the cavernous rocks are destroyed by erosion, or (3) the aquifer becomes buried under later deposits and lies below the groundwater circulation system. The history of some carbonate aquifers includes early development under water table conditions, burial and preservation under later deposits, resurrection, and reactivation in the modern groundwater circulation system. Observable or discernible recognition can be made of (1) aquifers having fine textured permeability, (2) aquifers having coarse textured permeability, and (3) aquifers that have been resurrected and are now reactivated; each type of aquifer has distinctive characteristics that help in hydrologic evaluations of carbonate rock systems.