Conclusions 7 5 Suggestions for Future Studies 7 Acknowledgments References 7 Appendix A 29. Schematic geologic cross section of middle Key Largo and bar graph of major nutrient concentrations 69 30. Schematic geologic cross section of upper Key Largo and bar graph of major nutrient concentrations 7 0 IV Florida reef tract. Grainstone is approximately an order of magnitude less permeable than the coralline Key Largo Limestone facies. The Q3 surface, a major subsurface unconformity thought to form an effective confining zone elsewhere in south Florida, was not detected in wells drilled more than 1 mile from shore. This unconformity, however, was detected in all wells drilled on or near the Keys. What was found to be a more effective and widespread confining layer is the Holocene sediment deposited on the Pleistocene limestone during the past 6,000 to 7,000 years. These relatively impermeable sediments are extensive, forming a belt up to 5 miles wide beginning about 0.5 mile offshore. Holocene sediments generally consist of low-permeability lime mud just above the Pleistocene surface, overlain by more permeable carbonate sands and reefs. Leakage of ground water by tidal pumping is not likely to occur through lime-mud-dominated areas such as Hawk Channel but is likely to occur through isolated porous and permeable Holocene reefs situated on Pleistocene limestone highs, and in places where Holocene sediment does not cover the limestone bedrock. Leakage is therefore limited to 1) a shallow-water 0.5-mile-wide nearshore belt of exposed Key Largo Limestone, 2) Holocene patch reefs, which grow on mud-free topographic rock highs, and 3) along the seaward side of the outermost reef in 35 to 65 ft (10-20 m) of water, where Holocene reef and sediment accumulations are thin or absent. This study did not address direct measurements of lateral groundwater movement or a hydrologic mechanism for transporting hypersaline ground water away from the Florida Keys. More recent work, however (Halley et al., 1994), shows that sea level in Florida Bay is higher than on the Atlantic side of the Keys more than 50% of the time. Higher sea level on the bay side of the Keys provides a potential for groundwater flow toward the Atlantic most of the time. Use of tracers (dyes or harmless bacteriological tracers) injected into the center of tightly spaced clusters of monitoring wells is a simple way to ascertain the net direction and rate of groundwater movement. Knowing the direction and rate of groundwater movement is needed for prediction and modeling efforts in the future. growths of algae. One well-documented cause of increased algal growth was the disappearance of the herbivorous sea urchin Diadema. Although Diadema suffered near extinction throughout the Caribbean in 1983 (Lessios, 1984), coral mortality and algal proliferation appeared most pronounced on Florida's reefs (Dustan, 1985). While Diadema were dying in unprecedented numbers, reefs, especially in the Florida Keys, were also experiencing accelerating human exploitation. Along wit...

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2. Chart showing lithostratigraphic units in the study area, generalized lithology, and correlation to hydrogeologic units .

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