The U.S. Army Corps of Engineers (COE) is planning to construct gated spillways and culverts to allow for the restoration of natural sheetflow conditions to Everglades National Park (ENP). These proposed changes may further affect the hydrologic conditions of ENP and other parts of the ecosystem, thus leading to the following questions:
(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?
(3) How much change in ground-water discharges to Biscayne Bay has occurred due to modifications to the hydrologic system?
Quantification of ground water flowing to Biscayne Bay is needed as input to two interagency projects: the South Florida Ecosystem Restoration Program and the Biscayne Bay Feasibility Study. The principal objective of the Biscayne Bay Feasibility Study is to investigate ongoing construction/dredging projects and propose solutions to alleviate adverse factors that affect the bay and to aid in the development of guidelines for future management of the natural resources of Biscayne Bay. The Biscayne Bay Feasibility Study includes the implementation of a surface-water circulation model which will be developed by the Waterway Experimental Station of the COE. Quantification of ground-water discharges to Biscayne Bay is needed as input to the bay water circulation model.
Ferguson, G. E.; Love, S. K.; and others
Deszcz-Pan, Maria; Stoddard, Carl E.
Langevin, Christian D.
Shoemaker, W. Barclay; Guo, Weixing
Skougstad, M. W.; Fishman, M. J.
Reese, R. S.; Reich, C. D.
The field investigation was initiated by installing ground-water monitoring wells at each of the three transects. In an effort to fully characterize the transition zone between fresh and saline ground water, monitoring wells were installed both inland and offshore. Inland monitoring wells were installed by the Florida Geological Survey, and the offshore monitoring wells were installed by the USGS. The offshore wells were installed from a floating barge using the methods presented in Shinn and others (1994).
During the installation of selected monitoring wells, lithologic cores were collected and analyzed to provide a better understanding of the stratigraphy and hydrogeologic characteristics at the monitoring well locations. Permeameter analyses were performed on several rock samples extracted from the cores, but the analyses were inconclusive. For selected inland monitoring wells, geophysical logging was performed by the South Florida Water Management District prior to setting the steel surface casing.
Water samples were collected with a centrifugal pump from selected monitoring wells for each month from March 1998 to February 1999. Measurements of depth to water were recorded prior to sampling, and if the well had been leveled, a water-table elevation was calculated. During the first 3 months, water samples were analyzed by the USGS for chloride concentration, [Cl-], using the titration method (Brown and others, 1974). Measurements of specific conductance (SC) also were performed on the ground-water samples. After 3 months of directly measuring chloride concentrations, it was determined that chloride concentrations could be adequately estimated from measurements of specific conductance, which are easier to perform. Chloride concentrations for all subsequent ground-water samples were estimated from specific conductance using the following equation: [ Cl- ] = 1 . 10-6 . SC 2 + 0.3224 . SC - 177.7,
where [ Cl- ] is in milligrams per liter and SC is in microsiemens per centimeter. This polynomial equation was created by a fit to 120 measurements of specific conductance and chloride concentrations and represents the data with a correlation coefficient (R2) of 0.9967. (See the Introduction to OFR WRI 00-4251 for the correct format for the equation.)
The numerical model used in this study requires concentrations of total dissolved solids (TDS) rather than chloride concentrations. Chloride concentrations were linearly converted to TDS by assuming that seawater has a chloride concentration of 19,800 mg/L (milligrams per liter) and a TDS value of 35,000 mg/L (Parker, and others, 1955). Fish (1988) estimates that water-rock interactions in the surficial aquifer of southeastern Florida can affect the TDS value by 350 to 550 mg/L; therefore, the TDS values in this study, which were estimated from chloride concentrations, may contain a 1 to 2 percent error relative to the observed range of TDS values. This suggests that a linear relation between chloride and TDS is reasonable, even for ground-water samples.
During the initial part of the field investigation, much time was spent trying to obtain reliable results from seepage meters. A seepage meter is a cylindrical tube that is pressed into the bottom sediments of a surface-water body; seepage rates are determined by measuring liquid volumes in a bag attached to the tube. After many unsuccessful attempts, it was determined that seepage meters could not be used within the tidal environment of Biscayne Bay because flow rates measured at seepage meters were not in agreement with tidal phase, or were not proportional to vertical head differences at nested offshore monitoring wells. There is evidence that seepage meters may not work in tidal environments or under certain conditions because they may be artificially pumped from tides, waves, and fluctuations in barometric pressure (C. Reich, U.S. Geological Survey, oral commun., 2000). This artificial pumping can result in seepage measurements that are not representative of the actual seepage rate.
To better delineate the position of the saltwater interface, time-domain electromagnetic (TDEM) soundings were made at the Mowry Canal transect. The TDEM method has been successfully used in southern Florida to locate the saltwater interface (Sonenshein, 1997; Fitterman and others, 1999; and Hittle, 1999) and lithologic boundaries (Shoemaker, 1998). The TEMIX software (Interpex Limited, 1996) was used to invert the geophysical data. The approach described by Fitterman and others (1999) was used to interpret the inverted TDEM data and determine approximate depths of the saltwater interface.
U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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