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Hydrogeology of a Dynamic System in the Florida Keys: A Tracer Experiment

Part 1

satellite image of Florida Bay
Satellite image of South Florida. Image shows Florida Everglades at the top of the picture; Florida Bay with its anastimozing mudbanks and mud islands in the center; the arcuate chain of islands that make up the Florida Keys; and the reef tract that runs parallel to the Keys. Our study site is located in the upper right hand side of the photograph.(Click on photo above for larger version.)

Abstract

Fluorescein and rhodamine dyes and an inert gas solution, sulfur hexafluoride (SF6), were injected into the shallow subsurface to determine flow velocities and direction within the groundwater-flow system in the upper Florida Keys. In the Florida Keys, ground water is entirely saline or hypersaline with isolated freshwater/brackish-water lenses on the larger islands. Ground water flows through a highly permeable and porous Pleistocene limestone, known as the Key Largo Limestone. The Key Largo Limestone is a coralline boundstone containing numerous primary and secondary porosity features, making it a very transmissive unit. Estimates of hydraulic conductivity for the Key Largo Limestone Formation range from 1,400 m/d (Vacher et al., 1992) to 8,800 m/d (Fish and Stewart, 1990).

Two 60.6-m-diameter underwater well clusters were installed in the nearshore waters of the Florida Keys in approximately 0.6 to 2.4 m of water depth. One well cluster is located in Florida Bay on the northwest side of Key Largo and the other is located directly across the island to the southeast in the Atlantic Ocean. There are nine holes to a depth of 13.6 m; eight are in a 30.3-m-radius around a central site. Well clusters were designed with a deep and shallow well in a single borehole. The deep piezometers (13.6 m) are completed with 1.5-m-long well screens set in a quartz sand pack and capped with Portland cement. Shallow piezometers (6.1 m) have a 1.5-m-long screen and are suspended within the open hole. The wells are sealed with a cap of cement at the seafloor surface, leaving approximately 25 to 35 cm of the wellhead exposed for subsequent sampling. Caps are placed on each piezometer head to prevent sea flora and fauna from inhabiting the wells.

Ground water from the piezometers was analyzed for fluorescence using a filter fluorometer. Groundwater-flow velocities were calculated from breakthrough curves. Flow velocities were as high as 2 m/d in an easterly and southeasterly direction (i.e., toward the reef tract). However, it was noted that velocity on the bay side was 3 to 5 times greater than on the ocean side of Key Largo. The drive for horizontal groundwater flow in this region is thought to be the difference between sea level in Florida Bay and the Atlantic Ocean. On average, Florida Bay is 15 to 20 cm higher than the Atlantic Ocean. In the upper Florida Bay region, astronomical tides are absent, but on the Atlantic side of Key Largo, tides are semi-diurnal and have a range of approximately 1 m.<

Dye injected in the deep piezometer appeared first in the shallow peripheral piezometers, indicating an upward transport of ground water. This observation is the result of a phenomenon known as tidal pumping. Tidal pumping is a measure of pressure head in underwater piezometers relative to sea level. Piezometers in both Florida Bay and the Atlantic Ocean directly reflect the pressure changes due to ebbing or flooding tides. Also, deeper piezometers show a greater dampening of the tidal signal than do the shallow piezometers. Pressure heads can exceed ±20 cm, with shallow piezometers having a slightly greater pressure than the deeper piezometer in the same borehole, and negative head pressures are generally greater than positive head pressures. The effects of the pressure gradients result in a vertical component of groundwater flow. 

Major Findings

  • Dye results indicate groundwater flow to the southeast toward the reef tract. BSWC generally has higher groundwater flow rates than OSWC. During the first dye injection, flow rates were as high as 2.5 m/d at the BSWC (bayside well cluster) site and 0.61 m/d at the OSWC (oceanside well cluster) site.
  • Lateral movement of ground water is thought to be caused by the head differences between Florida Bay and the Atlantic Ocean. On average, the water level in Florida Bay is higher than in the Atlantic Ocean and 66% of the time the bay level is higher than ocean level. This suggests that two-thirds of the time groundwater flows toward the Atlantic Ocean from Florida Bay.
  • It is suspected that groundwater flows into Florida Bay, opposite its normal flow direction, during storm events or during periods of persistent easterly winds. This is a direct result of the lowering of the bay level and raising of the ocean sea level. This was observed after the second injection of dye. Flow velocities for the second injection were up to 3.0 m/d (BSWC) and 1.9 m/d (OSWC) and the flow direction was toward Florida Bay. Results from an injection of SF6 suggested a similar flow velocity and direction.
  • Vertical movement of groundwater is caused by tidal fluctuations. This phenomenon is called tidal pumping. Tidal pumping creates vertical pressure gradients causing groundwater to flow upward and seep into the overlying surface water. It is a near-shore effect that decreases as you traverse offshore and also decreases with increasing depth. This phenomenon is responsible for the upward movement and diffusion of Rhodamine injected at 45 ft (13.6 m) and recovered in the shallow piezometers.


Next: Concerns and Local Ecosystem Impacts

Related information:

SOFIA Project: Determination of Groundwater-Flow Direction and Rate Beneath Florida Bay, the Florida Keys and Reef Tract

Related Links:

South Florida Information Access

Center for Coastal Geology

U.S. Geological Survey



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