Construction of canals and levees in south Florida has altered the natural hydrologic conditions of the Everglades. The canals and levees were constructed to convey water, prevent flooding, and store water in conservation areas for future use. The U.S. Army Corps of Engineers is evaluating various structural and operational changes to the canal system in an attempt to restore natural sheetflow conditions to Everglades National Park. Questions related to these potential changes are: (1) Is ground water flowing to Biscayne Bay a significant component of the water budget in south Florida? (2) Would the quantity of ground water flowing to Biscayne Bay be greatly affected by changes in the operation of gates and control structures in canals? and (3) How much change in ground-water discharge to Biscayne Bay has occurred due to the present modifications to the hydrologic system?

The U.S. Geological Survey is conducting a study to: (1) define the geohydrologic characteristics of the surficial aquifer system adjacent to and beneath Biscayne Bay, (2) estimate ground-water discharge to Biscayne Bay using geochemical and numerical modeling techniques, and (3) assess the potential effects of some proposed water-management operations in canals and water-conservation areas on ground-water discharge to Biscayne Bay. Preliminary results from the geochemical analysis only are presented here.

During May 1-17, 1997, personnel of the U.S. Geological Survey drilled test holes and installed 29 water-quality/water-level monitoring wells in Biscayne Bay. The wells are arranged in three transects, with two or three sites per transect, beginning 164 to 246 ft from shore opposite Coconut Grove to the north, Cutler in central Biscayne Bay, and Mowry Canal to the south, and extending 3,000 to 6,000 ft into Biscayne Bay. Each site consists of one shallow well, 6.5 to 10 ft into the limestone and above a major unconformity, and one deep well, 23 to 39 ft into the limestone and below the major unconformity.

Water samples were collected from all wells for analysis of major inorganic constituents, nutrients, and oxygen-18 and deuterium stable isotopes. The presence of freshwater in some of the offshore wells was reflected by the specific conductance, which ranged from 16,300 to 29,000 microsiemens per centimeter in certain water samples. Oxygen-18 and deuterium stable isotopes are influenced by processes affecting the water (rather than the solutes) and can be used to identify the source of water. A graphical analysis was made of the relation between oxygen-18 and deuterium in water samples collected from wells open to varying depth intervals under Biscayne Bay (0 to 10 ft, 10 to 20 ft, and greater than 20 feet) and in canal water, bay water, and open ocean water samples. Water samples that were collected from the different sources plotted in clusters that can be taken as representative of the sources. Some of the clusters overlap, which might indicate mixing or movement of water from one source to the other. Oxygen-18 values ranged from -2.1 to -0.97 per mil in canal water samples and from +2.51 to +2.56 per mil in ocean water samples. Deuterium values ranged from -9.3 to -3.3 per mil in canal water samples and from +6.9 to +12.4 per mil in ocean water samples. The graphical analysis showed a cluster of water samples collected from four sites in Biscayne Bay, three along the Mowry Canal transect and one along the Cutler transect at the site farthest from shore. This cluster of water samples consists of oxygen-18 values ranging from -0.13 to +0.34 per mil and deuterium values ranging from -7.3 to -5.0 per mil. For purposes of analysis, this cluster is considered to be representative of bay water, which actually is a mix of ocean, canal, and ground waters.

Water samples collected from Biscayne Bay along the Coconut Grove transect plotted within the range of values that is representative of ground water (oxygen-18 values from -1.5 to +2.19 per mil and deuterium values from -6.1 to +15.1 per mil), with oxygen-18 values of +1.23 and +1.30 per mil and deuterium values of +8.9 and +7.9 per mil. This result suggests that a significant quantity of ground water is discharging to Biscayne Bay along the Coconut Grove transect. Two water samples collected from Biscayne Bay along the Cutler transect plotted mainly within the range of values that is representative of canal water, with oxygen-18 values of -1.23 and -1.55 per mil and deuterium values of -8.5 and -7.4 per mil. However, the previously mentioned water sample (collected from the site most distant offshore along the Cutler transect) departed to the left of the graph, plotting on the cluster that is considered to be representative of bay water (oxygen-18 values from -0.13 to +0.34 per mil and deuterium values from -7.3 to -5.0 per mil) with an oxygen-18 value of -0.13 per mil and a deuterium value of -5.0 per mil. This shift to the left of the graph can be interpreted as a result of evaporation, which is characteristic of bay and ocean water.

In summary, it appears from the geochemical analysis that ground water is discharging offshore along the Coconut Grove transect (north transect). Water of canal origin represents a significant part of the ground water discharging along the Cutler transect (central transect). An insignificant amount of ground water is discharging along the Mowry Canal transect (south transect).

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