GEOLOGIC CONTROLS ON WATER EXCHANGE BETWEEN SURFICIAL LAKES AND FLORIDAN AQUIFER SYSTEM OF NORTHEAST FLORIDA

Fluid exchange between surficial waters and groundwater, as well as the processes that control this exchange, are of critical concern to understanding the mechanism by which ground water quality can be impacted by surficial water quality. Digital high-resolution seismic equipment was used to collect geophysical data from > 40 lakes and rivers in Florida. In each case study, geologically controlled solution and/or mechanical processes determined the geomorphology of lakes. Processes that control lake development are twofold: 1) karstification or dissolution of the underlying limestone, and 2) collapse, subsidence, or slumping of overburden to form sinkholes. Initial lake formation is directly related to the karst topography of the underlying host limestone. Lake size and shape are factors determined by the thickness of overburden and size of the col-lapse or subsidence, and/or the clustering of lake-forming depressions.

The lakes form through sequential stages that create a predictable progression of the following four geomorphic types: (1) active subsidence or collapse phase (young) - collapse structures typically associated with sinkholes: (2) transitional phase (middle age) - the sinkhole becomes plugged as the voids within the collapse are filled with sediment, periodic reactivation may occur; (3) baselevel phase (mature) – permanently plugged by the continual erosion of material into the basin; and (4) polje (drowned prairie) - broad flat-bottom basins located within the epiphreatic zone that are inundated at high stages of the water table and have one or more phases of sinkhole development and types of karst and karren features.

Water-well gamma logs correlated with seismic profiles of subsurface features were used to define the geologic structure and to locate possible breaches in the confining layer of these lakes. Using acoustic signatures, Florida lakes can be classified by size, sediment fill, subsurface features, and geomorphology. Knowledge of these parameters and identifying the location and magnitude of subsurface conduits is important in understanding the interaction between surficial waters and the underlying aquifer.