Groundwater Conductivity Beneath Florida Bay: Does the Biscayne Aquifer Discharge into Florida Bay?
Topical Area: Water Quality
Christopher D. Reich, Eugene A. Shinn, and Robert B. Hally, USGS, St. Petersburg, FL
The primary objective of this project is to document temporal and spatial variations in shallow groundwater conductivity (salinity) beneath Florida Bay. This project is a first approach to answering the question, "Does the Biscayne Aquifer discharge fresh ground water into Florida Bay?" The hypothesis proposed here is that if groundwater conductivity from monitoring wells in northern Florida Bay is saline or hypersaline, then the probability that the Biscayne Aquifer is discharging into Florida Bay is extremely low.
Florida Bay has exhibited extreme swings in surface salinity over the past several decades (McIvor et al., 1994) and years (Halley et al., 1995). Construction of canals and levees to redirect Everglades water to the east is primarily blamed for increasing salinity of Florida Bay. Prior to canal dredging, surface-water levels in the Everglades were about 6 ft (1.8 m) higher (Parker et al., 1955; Kohout and Kolipinski, 1967). The decrease in water level had a great impact on the recharge and potentiometric surface of the Biscayne Aquifer, the primary potable water source for south Florida residents. Discharge of aquifer water was also greatly impacted. Prior to 1900, discharge into Biscayne Bay occurred through well-known solution holes in the Miami oolite. Flow was sufficient to produce numerous offshore freshwater springs. The offshore springs, up to three-quarters of a mile (1.2 km) seaward of the Cutler area, were exploited by mariners (Kohout and Kolipinski, 1967). The greater head potential of the Biscayne Aquifer also acted as a salinity barrier. However, dredging of canals from the early 1900s until recently reduced the potentiometric surface of the Biscayne Aquifer, in turn reducing the occurrence of offshore springs and ultimately allowing salt-water encroachment.
Subsurface discharge of fresh water into Florida Bay is thought to have occurred but has not been documented. Prior to canal dredging, undisturbed sheet flow from the Everglades and possibly the addition of groundwater discharge caused Florida Bay to function as an estuary. In recent years, salt water has encroached approximately 6 miles (9.6 km) or greater landward from the northern coast of Florida Bay (Price, pers. com.; Fitterman, 1996). At present, the primary source of fresh water entering Florida Bay is from surface-water runoff. Several approaches have been undertaken or proposed by scientists to locate and quantify areas of groundwater discharge into the bay. Approaches include measuring radon, nutrients, conductivity (this project), stable isotopes, other chemical constituents, and employing age-dating techniques using Freon and CFC. Helicopter electromagnetic techniques have been applied with great success to map the extent of salt-water intrusion along the southern margin of the Biscayne Aquifer (Fitterman, 1996).
Methods and Timing
Data for this project were obtained by monitoring 28 of 52 wells installed throughout Florida Bay from July 1992 through July 1997. Data from four sampling rounds (July 1994, November 1994, September 1997, and November 1997) were collected and the results displayed as contour plots (generated in Surfer). Groundwater sampling was conducted primarily for nutrient analysis. In addition, surface-water and groundwater conductivity/temperature were recorded in the field at each well site. A hand refractometer was used to measure salinity during the first nutrient sampling round in July 1994. The salinity values (in parts per thousand, ppt) were converted to conductivity (in milliSiemens per centimeter, mS/cm) using the relation, 35 ppt equals 55.0 mS/cm. Subsequent sampling trips employed a conductivity/temperature probe to evaluate both surface water and ground water.
Summary of Results
Conductivity data from shallow monitoring wells were entered into Surfer and contour plots generated. Wells range in screened-interval depths from 4 ft (1.2 m) to 65 ft (19.7 m) below seafloor. The contour plots are based on data only from wells screened between 15 ft (4.5 m) and 65 ft (19.7 m). Shallow wells less than 15 ft (4.5 m) were only slightly less conductive than the deeper well water.
Hypersaline ground water is thought to have originated from past hypersaline events in Florida Bay. Surface salinities were extremely high during low precipitation periods, ranging from 59 ppt (1989-1990) to 70 ppt (1956) (McIvor et al., 1994). As warm hypersaline surface water becomes dense, it sinks downward into underlying permeable limestones and unconsolidated quartz sand layers. Once in the subsurface, groundwater flows laterally, driven by a head gradient or possibly by tidal pumping. The rate of groundwater movement is slow as indicated by relatively constant groundwater conductivity over the three-year period from 1994 to 1997.
This project will conclude by the end of 1998. The wells and surface water will be sampled during early summer 1998 to observe possible effects from the El Niño-driven precipitation events of 1997-1998.
Future work is warranted to ascertain whether groundwater discharge from the Biscayne Aquifer is occurring in Florida Bay. This project is a first-hand approach at examining the possibilities of groundwater discharge into Florida Bay as well as groundwater flow beneath the bay. The use of groundwater conductivity (salinity) is a simple, good first approximation, but advanced techniques are needed. Future work to develop a better understanding of groundwater discharge along the northern boundaries of Florida Bay may include stable isotopes (e.g., oxygen), natural tracers (e.g., radon), radioactive isotopes (e.g., radium), CFCs, and other chemical tracers. Many of the aforementioned tracers are currently being investigated in Florida Bay and the Everglades or are being proposed for future funding.
Fitterman, D.V., 1996, Geophysical mapping of the freshwater/saltwater interface in Everglades National Park, Florida, U.S. Geological Survey Fact Sheet; FS-96-173.
Halley, R.B., D. Smith, and M. Hansen, 1995, Surface salinity of Florida Bay, U.S. Geological Survey Open-File Report; 95-634.
Kohout, F.A., and M.C. Kolipinski, 1964, Biological zonation related to groundwater discharge along the shore of Biscayne Bay, Miami, Florida, ESTUARIES, p. 488-499.
McIvor, C.C., J.A. Ley, and R.D. Bjork, 1994, Changes in freshwater inflow from the Everglades to Florida Bay including effects on biota and biotic processes: a review, in Everglades: The ecosystem and its restoration, S.M. Davis and J.C. Ogden (Eds.), St. Lucie Press, Delray Beach, L., chap. 6, p.117-142.