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Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Introduction:
Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
Introduction
Depth to Pleistocene Bedrock Surface
Reef & Sediment Thickness
Benthic Ecosystems & Environments
Sedimentary Grains in 1989
Summary Illustration Index Map
Evolution Overview
Tile-by-Tile Analysis
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,
11
Summary
Acknowledg-
ments
References
Disclaimer
Related
Publications

Tile 1

Hydrology: Multi-depth water-monitoring wells have been installed on the Florida Bay and Atlantic Ocean sides of Key Largo due north of Molasses Reef (Figs. 22B, 23). Rates and direction of groundwater flow in the area have been measured using dye tracers (Reich, 1996; Reich et al., 2002). Fluorescent dyes injected into central wells of routinely monitored well clusters are detected in extremely small quantities (detection limit is <1 part per trillion, or ppt). Two mechanisms drive groundwater flow: oceanic tides caused by gravitational pull of the moon and meteorologic tides caused by storms.

 (A) Index map shows location of water-monitoring well study. (B) Diagram shows sites of groundwater-flow experiments off Key Largo and layout and position of well clusters in Florida Bay and the Atlantic Ocean. Figure 23. (A) Index map shows location of water-monitoring well study. (B) Diagram shows sites of groundwater-flow experiments off Key Largo and layout and position of well clusters in Florida Bay and the Atlantic Ocean (modified from Reich et al., 2002). Well clusters consist of nested, capped monitoring wells. Wells labeled 'A' are deep (13.6 m). Wells labeled 'B' are shallow (6 m). Onshore wells (black dots labeled 'I-IV') between offshore clusters were drilled toward the end of the groundwater-flow project for another study on onshore tidal fluctuations. Several months after the end of the groundwater-flow project, dye was detected in wells I, II, and IV. [larger version]

Because Florida Bay lacks regular tides and the Atlantic, directly across the island, oscillates with an ~1-m tidal range on a semi-diurnal cycle (Fig. 24), groundwater shifts back and forth beneath the keys between the bay and ocean (Fig. 25A, 25B). However, Florida Bay maintains, on average, a higher water level (head gradient) than that in the Atlantic by ~12 to 15 cm (Fig. 24). A bay-to-ocean head gradient exists most (about two-thirds) of the time (Reich et al., 2002; Fig. 25A). During non-storm conditions, the gradient forces net groundwater flow from the bay toward the ocean. Flow rates at these times have been measured at 2.5 m/day on the bay side and 0.6 m/day on the ocean side. The combination of tidal pumping and typical southward bay-to-ocean slope is a key forcing factor for a generally persistent transport of nutrient-rich groundwater toward the offshore reef system. A similar Gulf-to-Atlantic slope in sea level (~30-cm difference) allows a semi-steady non-tidal flow through passes in the lower and middle Keys (Smith, 1994, 1998).

Graph shows five-month plot of water-level fluctuations in Florida Bay and the Atlantic Ocean
Figure 24. Graph shows five-month plot of water-level fluctuations in Florida Bay and the Atlantic Ocean (from Reich et al., 2002). Bay tides are meteorologically driven and fluctuate due to changes in wind forcing. Ocean tides are astronomically driven and semi-diurnal. Average bay-water level is ~12 to 15 cm higher than that of the ocean. This difference creates a drive for seaward cross-key groundwater flow. Water-level data are referenced to the North American Vertical Datum of 1988 (NAVD88). [larger version]

Conceptual models show bay-to-ocean groundwater flow during a falling and rising ocean tide. Figure 25. Conceptual models show bay-to-ocean groundwater flow during a falling and rising ocean tide (from Reich et al., 2002). (A) Net flow direction is seaward in the upper Keys approximately two-thirds of the time. (B) Graph shows pressure head in underwater monitoring wells on both sides of Key Largo during a rising and falling ocean tide. Corresponding conceptual model shows ocean-to-bay groundwater flow that occurs approximately one-third of the time. Pressure head at both well clusters was measured with an underwater manometer (Reich, 1996). Tidal fluctuation in Florida Bay was 3 cm over the 10-hour observation period. Arrows above wells indicate direction water would flow if well caps were removed. Models are not to scale. [larger version]

During periods of sustained high winds, usually associated with strong cold fronts, tropical storms, or hurricanes, groundwater flow in the upper Keys is reversed (Reich et al., 2002). As a result, bayside water level is depressed relative to a slightly elevated ocean-side level (Fig. 25B). During passage of a cold front with sustained 30-mph easterly winds in March 1997, groundwater-flow rates into the bay were 3 m/day. Ocean and bay water levels during this cold front most likely resembled water-level fluctuations that occurred when Hurricane Georges passed to the south of the study site in September 1998 (Fig. 26). During Hurricane Georges, easterly winds of 45 to 50 mph depressed the bay level 1 m and raised the ocean level ~0.5 m, creating, in essence, an approximately 2-m head-gradient flow into Florida Bay.

Plot shows surface-water level at well clusters in Florida Bay and the Atlantic Ocean during passage of Hurricane Georges Figure 26. Plot shows surface-water level at well clusters in Florida Bay and the Atlantic Ocean during passage of Hurricane Georges, September 24-26, 1998 (from Reich et al., 2002). Bay-water level dropped ~1 m and the ocean level rose ~0.5 m. The storm surge occurred during a low tide, minimizing flooding of the low-lying keys (highest natural elevation in the keys is ~5.5 m; Lidz and Shinn, 1991). Water-level data are referenced to the North American Vertical Datum of 1988 (NAVD88). [larger version]

This discussion on hydrology applies only to the site-specific area off Key Largo where the water-monitoring wells were installed and measurements of surface- and groundwater-flow rates and directions were made. Among many other hydrology-oriented studies conducted elsewhere in south Florida are:

  • geology and groundwaters (Matson and Sanford, 1913);
  • hydrogeology and potable and freshwater lenses on the keys (Parker and Cooke, 1944; Parker et al., 1955; Klein, 1970; Hanson, 1980; Mackenzie, 1990; Halley et al., 1997);
  • diagenetic alteration of skeletal carbonates (Land, 1967; Halley and Harris, 1979);
  • weathering features such as laminated soilstone crusts (calcretes of Multer and Hoffmeister, 1968) and karst, from widespread minor karstification (Dodd and Siemers, 1971) to localized deep sinkholes and broad underground caverns (Shinn et al., 1996; Kindinger et al., 1999, 2000);
  • loss of high porosity with depth (Halley and Schmoker, 1983);
  • tidal pumping (Carballo et al., 1987; Shinn et al., 1997);
  • effects of dredge-and-fill canals on freshwater resources (Beaudoin, 1990);
  • Gulf-to-Atlantic flow through tidal channels in the keys (Smith, 1994, 1998).

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

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