Home Introduction Hydrogeology Simulation of GW Discharge Conclusions References Appendix 1 Appendix 2 Appendix 3 Appendix 4 Plates PDF Version

Introduction

Biscayne Bay is a coastal barrier-island lagoon that relies on substantial quantities of freshwater to sustain its estuarine ecosystem. During the past century, field observations suggest that the mechanism for the delivery of freshwater to Biscayne Bay has changed from a system largely controlled by widespread and continuous submarine discharge and overland sheetflow to one controlled by episodic releases of surface water at the mouths of canals. Current ecosystem restoration efforts in southern Florida are examining alternative water-management scenarios that could further change the quantity and timing of freshwater delivery to the bay. There is concern that these proposed modifications could adversely affect bay salinities.

To evaluate the effects of the modifications on Biscayne Bay, the U.S. Army Corps of Engineers (USACE) is constructing a surface-water hydrodynamic circulation model. To achieve a reasonable calibration, this model requires the accurate specification of freshwater discharges to the bay. The two most important mechanisms for freshwater discharge to Biscayne Bay are thought to be canal discharges and submarine ground-water discharge from the Biscayne aquifer. Canal discharges are routinely measured and recorded, but few studies have attempted to quantify the rates and patterns of submarine ground-water discharge. Depending on the method, estimates of submarine ground-water discharge can range over several orders of magnitude.

As part of the Place-Based Studies program, the U.S. Geological Survey (USGS), in cooperation with the USACE, initiated a project in 1996 to quantify the rates and patterns of submarine ground-water discharge to Biscayne Bay. This was accomplished through field investigation and ground-water flow simulation at three sites along the coastline of Biscayne Bay and development of a numerical ground-water flow model that covers most of Miami-Dade County and parts of Broward and Monroe Counties (fig. 1). Study results have been incorporated into the hydrodynamic circulation model under development by the USACE.

Figure 1. Southern Florida showing location of study area, domain of regional-scale model, and location of field transects. [larger image]

Purpose and Scope

The purposes of this report are to: (1) document the development of a regional-scale, three-dimensional numerical model that simulates variable-density ground-water discharge to Biscayne Bay, and (2) present an estimate of submarine ground-water discharge to Biscayne Bay. To properly simulate ground-water flow, processes affecting ground-water flow were characterized and represented mathematically. Two local-scale models were developed in cross section to simulate the complex ground-water discharge patterns near the coast of Biscayne Bay. Ground-water data collected for this study from March 1997 to February 1998 are presented and used with an assumption of steady-state conditions to calibrate the cross-sectional models. Results from the cross-sectional models were used to aid development of the regional-scale model that simulates transient ground-water discharge in three dimensions. The regional-scale model was calibrated with field data from 1989 through 1998 to ensure it is a reasonable representation of the physical system.

Methods of Field Investigation

A field investigation was conducted to collect data that would help quantify ground-water discharge to Biscayne Bay. The design of the field investigation was based on the general and widely accepted concept that fresh ground water flowing toward a coastal boundary will flow up and over a saltwater wedge. To better characterize this flow pattern within the study area, three transects, each located on a ground-water flow line toward Biscayne Bay, were selected for further study. The locations of these transects, referred to as Coconut Grove, Deering Estate, and Mowry Canal, are shown in figure 1.

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). Coordinates, screened intervals, and other specifications for these and other monitoring wells installed for this study are given in table 1.

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 (app. I). 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. These data are included in appendix II. 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.

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.

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