Prepared as part of the
U.S. Geological Survey Greater Everglades Priority Ecosystems Science Program
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Purpose and Scope
Description of Study Area
Changes in Land Use and Population Trends
Water and Land Uses
Agricultural Water and Land Uses
Methods Used to Estimate Water Use
Findings on Agricultural Water Use
Use of Alternative Aquifers for Supply and Wastewater Management
Development of Water-Management System and Impact on the Hydrology of Southeastern Florida
Water-Table Fluctuations of the Surficial Aquifer System
Predevelopment Water-Table Conditions and Ground-Water Flow
Average 1940-44 Water-Table Conditions
Average 1970-74 Water-Table Conditions
Average 1990-94 Water-Table Conditions
Ground-Water Level Changes Over Time
Assessment of Saltwater Intrusion
Changes in the Marine Ecosystem of Biscayne Bay
Southeastern Florida is an area that has been subject to widely conflicting anthropogenic stress to the Everglades and coastal ecosystems. This stress is a direct consequence of the 20th century economic competition for limited land and water resources needed to satisfy agricultural development and its expansion, its displacement by burgeoning urban development, and the accompanying growth of the limestone mining industry. The development of a highly controlled water-management system designed to reclaim land for urban and agricultural development has severely impacted the extent, character, and vitality of the historic Everglades and coastal ecosystems. An extensive conveyance system of canals, levees, impoundments, surface-water control structures, and numerous municipal well fields are used to sustain the present-day Everglades hydrologic system, prevent overland flow from moving eastward and flooding urban and agricultural areas, maintain water levels to prevent saltwater intrusion, and provide an adequate water supply. Extractive mining activities expanded considerably in the latter part of the 20th century, largely in response to urban construction needs.
Much of the present-day urban-agricultural corridor of southeastern Florida lies within an area that is no more than 15 feet above NGVD 1929 and formerly characterized by freshwater marsh, upland, and saline coastal wetland ecosystems. Miami-Dade, Broward, and Palm Beach Counties have experienced explosive population growth, increasing from less than 4,000 inhabitants in 1900 to more than 5 million in 2000. Ground-water use, the principal source of municipal supply, has increased from about 65 Mgal/d (million gallons per day) obtained from 3 well fields in 1930 to more than 770 Mgal/d obtained from 65 well fields in 1995. Water use for agricultural supply increased from 505 Mgal/d in 1953 to nearly 1,150 Mgal/d in 1988, but has since declined to 764 Mgal/d in 1995, partly as a result of displacement of the agricultural industry by urban growth. Present-day agricultural supplies are obtained largely from surface-water sources in Palm Beach County and ground-water sources in Miami-Dade County, whereas Broward County agricultural growers have been largely displaced.
The construction of a complex canal drainage system and large well fields has substantially altered the surface- and ground-water hydrologic systems. The drainage system constructed between 1910 and 1928 mostly failed to transport flood flows, however, and exacerbated periods of low rainfall and drought by overdraining the surficial aquifer system. Following completion of the 1930s Hoover Dike levee system that was designed to reduce Lake Okeechobee flood flows, the Central and Southern Florida Flood Control Project initiated the restructure of the existing conveyance system in 1948 through canal expansion, construction of protective levees and control structures, and greater management of ground-water levels in the surficial aquifer system.
Gated canal control structures discharge excess surface water during the wet season and remain closed during the dry season to induce recharge by canal seepage and well withdrawals. Management of surface water through canal systems has successfully maintained lower ground-water levels inland to curb urban and agricultural flooding, and has been used to increase ground-water levels near the coast to impede saltwater intrusion. Coastal discharge, however, appears to have declined, due in part to water being rerouted to secondary canals, and to induced recharge to the surficial aquifer system by large municipal withdrawals.
Southeastern Florida is underlain by Holocene- to Tertiary-age karstic limestone deposits that form (in descending order): a highly prolific surficial aquifer system, a poorly permeable intermediate confining system, and a permeable Floridan aquifer system. Prior to construction of a complex drainage network, a widespread uppermost veneer of fresh wetland peat and muck deposits served to store water, maintained a higher water table (prolonging the Everglades hydroperiod), and ultimately helped to limit movement of the coastal saltwater interface. The highly permeable Biscayne aquifer, which is part of the surficial aquifer system, yields 1,000 to more than 7,000 gallons per minute to wells. By 2000, the Floridan aquifer system was used for aquifer storage and recovery and reverse osmosis at some sites in southern Florida, but primarily was used for wastewater injection purposes; the efficacy of such systems has been increasingly the subject of public scrutiny.
Prior to construction of the drainage system, the ground-water table reflected Atlantic Coastal Ridge topography, and springs reportedly discharged as freshwater boils. Everglades surface waters discharged southward toward Florida Bay and in the transverse glade areas. The modern-day ground-water table reflects the effect of canal systems, levees, impoundments, and the drawdown effects of larger well fields.
The saltwater interface forms a broad zone of diffusion, and its position is largely a function of lateral movement of seawater from the Atlantic Ocean, seepage from tidal canals, and upconing of relict seawater. Emplacement of conveyance and drainage canals, subsequent compaction and oxidation of inland peat and muck soils (which served previously to maintain higher water levels within the surficial aquifer system, including the Biscayne aquifer), and increased withdrawals from municipal supply wells collectively altered the natural balance between freshwater and saltwater considerably. Saltwater intrusion has been a concern in southeastern Florida since the early 1930s; its effects were most pronounced in Miami-Dade and Broward Counties during the 1940s and 1950s, respectively. Canal drainage appears to have had the most widespread impact on saltwater intrusion, lowering water levels in the surficial aquifer system and contributing to landward movement of the interface.
Core sample paleontologic observations of salinity and the distribution of seagrass in Biscayne Bay and Florida Bay suggest that the coastal marine ecosystem system during the 20th century has been impacted considerably by anthropogenic activities. Land use and water-management practices have increased nutrient loads and other pollutants and increased bay turbidity. Prior to 1900, the Biscayne Bay ecosystem was characterized by much lower marine salinities, including the extreme southern part of the bay, which contained waters that were nearly fresh. Consistent with the progressive inland saltwater intrusion into the surficial aquifer system and the Biscayne aquifer, the increase in salinity interpreted for surface- and ground-water resources in the early 1900s through the 1970s is the result of increased urban development and construction of a canal drainage system. Post-1940 water-management practices to control water discharge greatly affected the Biscayne Bay ecosystem by increasing the frequency, and particularly the magnitude of salinity fluctuations in the 1940s. Clearly, the changes in land use and water-management practices over the long term have had a profound effect on the ground-water hydrology of southeastern Florida.
Renken, R.A., Dixon, Joann, Koehmstedt, John, Lietz, A.C., and others, 2005, Impact of Anthropogenic Development on Coastal Ground-Water Hydrology in Southeastern Florida, 1900-2000: Reston, Va., U.S. Geological Survey Circular 1275, 77 p.
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