Subaerial exposure and weathering is one of the most important factors affecting the hydraulic properties of Surficial aquifer sediments (Perkins, 1977). Exposure to the atmosphere and chemical weathering of the exposed rock by dissolution resulted in formation of discontinuous bands of dense caliche-type crusts, paleosols, freshwater limestone, and laminated crusts (Perkins, 1977; Beach, 1982). Identifying subaerial exposure surfaces is an accepted means of identifying the boundaries between different stratigraphic subdivisions of the Quaternary units. In practice, these subaerial exposure surfaces are difficult to identify because their thicknesses may be appreciably less than 3 ft (Perkins, 1977). Detailed continuous geologic logs are, therefore, the only reliable means to identify these surfaces.

Perkins (1977) differentiated the depositional framework of south Florida into five distinct marine units punctuated by episodes of subaerial exposure. The estimated age of the Q5 unit was assigned based upon radiometric dating of the Key Largo Limestone and Miami Limestone (Osmond and others, 1965; Broecker and Thurber, 1965). Dates for the remaining four Quaternary sediments were estimated from amino acid racemization results from the Fort Thompson Formation (Mitterer, 1974, 1975; Giddings, 1999). Drilling at ENR site MP3 fully penetrated this complete Quaternary sequence. The chronostratigraphic details of these units are shown in table 1.

Another potentially important type of weathering surface was described by Krupa (1999). Krupa (1999) established the presence of radioactive crusts in the sediments that he referred to as Secondary Depositional Crusts (SDC). These SDCs are less distinctive than the Q unit indicators and they usually are not visible. They typically are associated with increases in the natural gamma measurements at a depth just below a subaerial exposure surface. Evidence indicates the SDCs are a function of the paleo ground-water interface location (Krupa, 1999). Krupa proposed that the crusts are formed by residual leachate deposited at the ground-water interface as a result of downward percolation of rainfall through the vadose zone. The SDCs were found to exhibit levels of elements such as Ca, Mg, Al, Sr, Fe, U, Th, and K above the natural background level of the area. Krupa (1999) suggested those elements could be important in the hydraulic properties of the rock because of the effects of secondary cementation within the pore network.