The urban and agricultural corridor of southern Florida lies between the Everglades and water-conservation areas to the west and the Atlantic Ocean to the east. This area includes eastern Dade, Broward, and Palm Beach Counties and is subject to widely conflicting stresses on the environment. The Shark River Slough is located immediately west of this corridor. A highly controlled water-management system has evolved during this century largely to provide drained land for a rapidly expanding population. Reclamation of Everglades wetland areas during the last 75 years provided the opportunity for westward expansion of agricultural, mining, and urban activities. Surface water is impounded in water-conservation areas that lie west of the protective levee system partly to sustain an Everglades ecosystem, partly to keep overland sheetflow from moving eastward and flooding urban and agricultural areas, and partly for water supply. In coastal areas of the urban-agricultural corridor, parallel environmental conflicts exist. Coastal residential and urban areas must be drained for flood control; the underlying aquifer system simultaneously serves as the principal source for water supply and heads must be maintained to prevent saltwater intrusion. Changes in predevelopment ground-water flow patterns and the associated reduction in ground-water discharge to coastal bays have altered salinity and affected the local ecology.

Saltwater intrusion in the surficial aquifer system is a direct consequence of water-management practices, concurrent agricultural and urban development, and natural drought conditions. The objectives of this synthesis are to: (1) provide a temporal and spatial overview of coastal saltwater intrusion in southeastern Florida; (2) identify the principal factors that control the extent of saltwater intrusion; (3) evaluate long-term trends in ground-water withdrawal rates, ground-water-level change, rainfall, and increases in chloride concentration; and (4) illustrate causal relations between the position of the saltwater interface, water-management practices, and the expansion of agricultural and urban areas. Hydrologic maps and interpretive analyses, land-use and population density maps, and geologic information are being used in combination to illustrate the effect of anthropogenic change during the 20th century on the coastal ground-water hydrology of southeastern Florida.

Urban and agricultural growth and land-use change has greatly impacted the ecological health and stability of the Everglades and Biscayne Bay area. A review of 100 years of land use and population changes is being conducted, mapping key historical events such as the development of the Florida East Coast Railroad, the 1920's land boom and bust, Cuban immigration, and post-war development and redevelopment. The population growth has been explosive; in 1900, southeastern Florida included a few small towns that has grown to a modern-day megapolis with almost 4 million inhabitants. Canal construction, designed to drain lands west of the Atlantic Coastal Ridge, has helped to provide the impetus for the westward expansion of agricultural and urban development. Urban areas are encroaching and replacing agricultural areas in Miami-Dade and Palm Beach Counties.

Average ground-water levels have risen in coastal areas, but have declined in western developed areas developed between the 1940's and 1990's. Some declines in water levels can be directly attributed to municipal ground-water withdrawals; however, water-level declines over wider areas are a direct result of canal drainage. Canal discharge to the ocean has declined, but coastal canal stage has been increasing, largely as a hedge to impede saltwater intrusion into the major canals and the aquifer. Conversely, canals near the western margin of the urban areas do not exhibit increasing or declining flows. A potential factor in this process is that municipal ground-water withdrawals from the surficial aquifer system in the Miami-Dade, Broward, and Palm Beach tri-County area increased from 85 million gallons per day in 1940 to 756 million gallons per day in 1995. Possible factors that contribute to a decrease in canal discharge to the ocean include: Flow in major canals recharge the surficial aquifer system along the urban reach, and surface-water flow is being rerouted to secondary canals to elevate coastal ground-water levels.

The landward movement of the saltwater interface has been an issue of local to regional concern since the 1940's. Miami-Dade, Broward and southeastern Palm Beach Counties are the areas most affected by saltwater intrusion. In decreasing importance, canal overdrainage, overpumpage from wells located near the coast, and upconing of relict water are the primary sources of saltwater in the surficial aquifer system.

The marine ecosystem of Biscayne Bay has been profoundly affected over the past 150 years. Significant changes in land and water uses and within the bay, in addition to natural phenomena, have contributed to the deteriorating conditions of its marine ecosystem. Water quality has greatly deteriorated with increased nutrient loads, heavy metals, and other pollutants. Dredging and channelization within the bay decreased the water quality by removing natural seagrass beds and increasing turbidity. Sediment cores from Biscayne Bay record distinct changes in the marine ecosystem over the past 150 years. Shallow-water marine organisms are very sensitive to environmental changes such as salinity, temperature, nutrient input, and dissolved oxygen, among others.

The Biscayne Bay ecosystem was different in the early to mid-1850's than it is today. The salinity of the bay was much lower than the normal marine salinities found today. The southern extreme of Biscayne Bay was nearly of freshwater salinity. The central bay also was lower in salinity, possibly with lowered oxygen or higher organic concentrations in the surficial sediments, which may have had a profound effect on the limited seagrass abundance or density present during that time. The early 1900's recorded the first profound salinity change, reaching close to normal marine salinity, reflected by a significant increase in the abundance of typical Atlantic Continental shelf fauna components. Seagrass indicators became increasingly more abundant from the early 1900's to present time in the central bay. A similar record of ecosystem change is recorded in the coastal region, Manatee Bay, with a significant change in salinity to mesohaline conditions occurring in the early 1900's. Coastal vegetation changes at that time are consistent with marine records, indicating increased salinity conditions by the initial presence of red mangrove in this region. Bay salinity remained relatively stable until the early 1940's when it increased to euhaline and polyhaline conditions, but was subject to broad salinity fluctuations. The occurrence and increasing abundance of epiphytal and macro-algal habitat dwelling organisms indicate a change in substrate conditions and increased seagrass abundance during this time. From the late 1980's to present, a slight salinity decrease in Manatee Bay has been found, and field observations in this region suggest deteriorating conditions in the health of the seagrasses.

Observations of salinity and seagrass changes in Biscayne Bay over the past 150 years have been consistent with records obtained for Florida Bay. The timing and magnitude of the change are correlative, indicating that the southern Florida marine ecosystem has been affected on a broad geographic scale and that the impact of Everglades restoration measures will be as great on Biscayne Bay as it will be on Florida Bay.

Next: Saltwater Intrusion in the Surficial Aquifer

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For more information contact:

Bob Renken
U.S. Geological Survey
9100 N.W. 36th Street Suite 107,
Miami, FL, 33178
Phone: (305) 717-5822
Fax: (305) 717-5801
EMAIL: rarenken@usgs.gov

Related information:

SOFIA Project: Synthesis on the impact of 20th Century water-management and land-use practices on the coastal hydrology of southeast Florida

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