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Groundwater Characterization and Assessment of Contaminants in Marine Areas of Biscayne National Park


>Geologic Setting
References Cited
Tables and Figures
The Miami Limestone composes the Biscayne Aquifer in large part, making it one of the most permeable aquifers in the U.S. (Fish and Stewart, 1991). In contrast, the modern sediments are generally less permeable (Enos and Sawatsky, 1981). The relative difference in permeability between sediment and rock, and the juxtaposition of sediment above the limestone, have led to the hypothesis that modern sediments may act as a partial aquiclude or seal over more permeable limestone on the seaward shelf. The surficial Pleistocene limestone of the mainland and the Florida Keys has been shown to be approximately 125,000 years old (Multer and others, 2002). Younger Pleistocene reef deposits, approximately 80,000 years old, have been identified along the shelf edge farther south in the Keys (Lidz and others, 1991; Toscano and Lundberg, 1998). Similar relations may exist in the Pacific Reef area, and the limestone below modern reef sediments may be as young as 80,000 years but has not been dated by appropriate techniques to verify that age. The Pleistocene limestone in the region has been exposed to weathering and karstification during periods of lowered sea level (Multer and others, 2002), evidenced particularly well in the Cutler Ridge area where karst surfaces and small sinks and caves occur south of the Deering Estate Preserve. Within Biscayne Bay, many of the seagrass patches grow in sediment-filled solution holes, similar to those documented by Ziemann (1972) in Florida Bay. Although the influence of karst on water flow has been recognized for many years (Parker and others, 1955; Shinn and Corcoran, 1987), only recently are attempts being made to integrate detailed knowledge of limestone dissolution with hydrogeology (Cunningham and others, 2003). About 15 miles south of BNP, a large, sediment-filled sinkhole occurs on the shelf behind a reef known as The Elbow (Shinn and others, 1996). No evidence of groundwater flow was observed from the sinkhole during study of this particular karst feature.

The Upper and Lower Floridan (Boulder Zone) Aquifers, respectively, are roughly 1000 to 1800 ft and 2500 to 3000 ft below the surface of BNP. Historically, the aquifers are believed to be flowing slowly toward the shelf edge where they empty into the Florida Straits. Locally, these aquifers deepen eastward, and there is concern about leakage from the deep aquifer that is used for sewage disposal (McNeill, 2000). Regional changes in hydraulic head play the major role in flow of the Upper Floridan that has its recharge area in northwestern Florida. Lower Floridan flow is driven by an additional component of geothermal warming that causes warmer, lessdense water to rise beneath the Florida Platform and flow outward both on the Atlantic and Gulf of Mexico sides of the peninsula (Kohout, 1965). In Dade and Monroe Counties, artesian flow from wells drilled into these aquifers was encountered during exploratory well drilling. Natural springs and seeps from these aquifers are known to occur in north and central Florida as far south as 27° N, but not south of that latitude. Mud Hole Submarine Spring, believed to emanate from the Lower Floridan Aquifer, occurs in the Gulf of Mexico off Ft. Myers at 26° 15' 50" N (Fanning and others, 1981). No natural springs flowing from the Floridan Aquifer are known in Dade or Monroe Counties (Rosenau and others, 1998).

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