The Karst Pages

HYDROLOGY OF CARBONATE AQUIFERS- Classification of Carbonate Aquifers

(This Section is taken from Reeder, 1988)

White (1969, 1977) has classified carbonate aquifers according to groundwater system and hydrologic setting. Groundwater moves in carbonate aquifers by diffuse flow, retarded flow or free flow. Diffuse flow occurs in less soluble rocks such as shaly limestones or crystalline dolomites. In diffuse flow carbonate aquifers, integrated conduits are rare, caves are small irregular features which are often only solutionally modified joints (White, 1969) (Figure 3) . Retarded flow aquifers are characterized by slow flow under artesian environments or in thin beds sandwiched between impervious rocks (Gunn, 1985). Retarded flow carbonate aquifers possess network cave patterns because of a lack of concentrated recharge, which is inhibited by overlying rocks of low permeability. Hence, solutioning takes place along any available joint and generates a dense cave network (White, 1969) (Figure 4) .  Free flowing carbonate aquifers have well- developed subsurface drainage systems similar in pattern to surface drainage systems. Inputs to the subsurface may be from sinking surface streams, as well as flow from sinkholes and general infiltration. In this type aquifer, groundwater flow paths have been enlarged by solutioning into a well-integrated conduit system (Figure 5) . Flow velocities in free flowing carbonate aquifers can reach tenths of feet/second and are often in the turbulent regime (White, 1969). Turbulent flow occurs through voids which range in size from 0.01 inches in solutionally enlarged fissures and bedding planes, to major phreatic conduits over 30 feet in size. These voids are within a mass of limestone which has very low primary permeability (Gunn, 1985). Secondary permeability is well-developed in free flow carbonate aquifers due to the solutioning process that forms these voids.