Bulletin No- 64


Hydrogeology of the Middle Confining Unit of the Floridan Aquifer System

 The upper and lower Floridan aquiler systems are separaled by a middle confining unit (Miller, 1956) (Plate 3). Locally this zone of confinement may contain thin zones of moderate to high permeabitity; however, as a whole, the unit acts as a single confining unit within the main body o! the permeable carbonates of the Floridan aquifer system. The Middle Eocene sediments that make up the middle confining unit are (MilLer, 1986) similar in composition to both the upper and lower Floridan aquifer systems. The middle confining unit is considered a leaky confining unit because of the lack of strong contrast in permeability between these three zones (Miller, 1986).

 The middle confinng unit in the study area consists of dense dolostone with interbedded limestones located immediately below the A8marker bed oP the Avon Park Formation (Plate 3). The middle confining unit in the study area is defined as a zone of slightly lower permeability separating two zones of higher permeability. This determination is based upon estimated porosities of less than 20 percent, and lilhologic character determined from geophysical logs and sample descriptions (Plate 3).

 The middle confining unit is recognized on geophysical logs by a slight increase in gamma ray activity and (when the carbonates are not fractured) a decrease in interval transil time on the sonic logs as the limestones of the upper Floridan aquifer system abruptly grade into low porosity (less than 20 percent), dense. microcrystalline dolostones that make up the middle confining unit (Plate 3)The top of the middle confining unit in the study area is between .600 la -1,100 feet NGVD and depth generally increases to the southeast (Figure 20 and Plate 3). The thickness of the middle confining unil ranges from 110 to 250 feet and decreases loward the south (Figure


21). Based on lithologic criteria, it appears that the middle confining unit is absent at the South Beaches injection well, demonstrating the signilicance of local geologic variation (Plate 4).

 Quantitative tield data and aquifer tests that describe the water transmtting characteristics of the middle confining unit were analyzed for the Merrilt Island injection well (Appendix Al). Horizontal hydraulic conductivity is estimated at 2-7 X 10.4 cm/s, vertical hydraulic conductivity is 1.5 X 10-B cms and transmissivity is 609 gpd/ft (Geraghty and Miller, 1984). Geophysical evi. dence coupled with borehole video observations indicate that the middle confining unit contains fractures at several of the well sites (Plate 3). Locally, vertical fractures may hydraulically con. nect the upper and lower Floridan aquifer system: however, the available data is insufficient to accurately make this determination.

 Hydrogeology of the Lower Floridan
 Aquifer System

 The lower Floridan aquifer system consists of all beds 1hat lie below the middle confining unit (Plate 3) and above the sub-Floridan confining unit (Miller, 1986). If the middle confining unit is absent (Le., Soulh Beaches injection well), the upper boundary of the lower Floridan can be defined geochemically, The geochemicaL boundary (Meyer, 1989) is where the total dissolved solids in the ground water is equal to or greater than 10,000 mg/L (Plate 3),

 The rocks of the lower Floridan aquifer system are comprised of a thick, complex sequence 01 limestones and dolostones with highly variable carbonate mairices. The higher porosity, less dense limeslones of the lower Floridan aquifer system are geophysically identified where a slight decrease in gamma-ray activity and an increase in sonic interval transit time occurs (Plale 3}.

 Geophysical and lithologic evaluations of the injection wells indicates that the lop of the lower