Summary

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. The area, which includes eastern Miami-Dade, Broward, and Palm Beach Counties, has experienced explosive population growth (from less than 4,000 residents in 1900 to 5 million residents in 2000), and as such is subject to widely conflicting stresses on the environment. A highly controlled water-management system evolved during the 20th century largely to provide drained land for a rapidly expanding population. Reclamation of Everglades wetland areas 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: (1) sustain an Everglades ecosystem, (2) keep overland sheetflow from moving eastward and flooding urban and agricultural areas, and (3) use for water supply. Parallel environmental interests exist in coastal areas of the urban-agricultural corridor. Coastal residential and urban areas must be drained for flood control, whereas the underlying surficial aquifer system must simultaneously serve as the principal source for water supply, and ground-water 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. Active since the early 1920s, extractive mining has increased considerably, largely to satisfy urban construction needs. The limited availability of limestone that meets construction requirements and simul-taneous competition for land by both industries have placed both in direct conflict.

Surface- and ground-water systems were altered considerably by the construction of a complex system of canals and levees as well as by heavy municipal withdrawals. Between 1900 and 1948, uncontrolled canal drainage increased the rate of flow from the Everglades, reduced the extent of inundated land, and lowered ground-water levels. These canals failed, however, to transport the load imposed during flood events and worsened drought conditions through overdrainage of the surficial aquifer system. Drought and hurricane-related flooding provided the impetus for the 1949 establishment of the Central and Southern Florida Flood Control Project and District; canals were enlarged, conveyance structures and controls were installed, and protective levees were constructed. The southern Dade conveyance system was completed as the final phase of the project during the 1980s, and involved redirecting surface-water flow and controlling ground-water levels in southeastern Miami-Dade County.

Ground water represents the principal source of water for municipal supply in the tri-county area of Miami-Dade, Broward, and Palm Beach Counties and has increased from three well fields producing about 66 Mgal/d in 1930 to about 65 well fields producing 770 Mgal/d in 1995. West Palm Beach is the only large municipality that uses surface water for supply purposes. Miami-Dade County uses a centralized well-field infrastructure in which five large-capacity well fields withdraw the majority of the supply. A decentralized well-field infrastructure has been constructed in Broward and Palm Beach Counties, in which municipalities have relied on developing their own source of supply to meet local demand. On the basis of temporal analysis of well-field locations and production levels, there has been a historic shift from large well fields near the coast to more western facilities partly designed to mitigate saltwater intrusion.

Surface water is the primary source of water for cultivation of sugar, field, and row crops in much of Palm Beach County, particularly in the Everglades Agricultural Area; conversely Miami-Dade agricultural growers primarily rely on ground water withdrawn from shallow wells and conveyed using truck-mounted pump and spray irrigation systems. The agricultural industry of Broward County has been largely displaced by residential and urban development, despite once having been the Nation’s primary winter producer of tomato, pepper, and bean crops, between the 1920s and 1940s. Broward County producers mostly relied on surface-water supplies until about 1960, when they converted to the use of ground water. Whereas agricultural activities in Broward County had become a minor factor in the county economy, cultivated lands expanded considerably in Miami-Dade and Palm Beach Counties between 1953 and 1988. Damage caused by Hurricane Andrew, which resulted in an agricultural financial loss exceeding $1 billion, appears to have contributed to the decline in cultivated lands in Miami-Dade County since 1992. Agricultural water use in the tri-county area increased from 505 Mgal/d in 1953 to almost 1,150 Mgal/d in 1988, declining to 764 Mgal/d in 1995.

The surficial aquifer system, the principal source of ground water in southeastern Florida, is a wedge-shaped, eastward thickening sequence of limestone, quartz sand, shell, and terrigeneous mudstone of Pliocene to Holocene age. The prolific Biscayne aquifer, a sole-source aquifer, is the most transmissive of three separate aquifers that comprise the surficial aquifer system. Transmissivity of limestone-rich areas is greater than 1,600,000 ft²/d but decreases to 54,000 ft²/d where the surficial aquifer system mostly consists of sand; yields of 1,000 to 7,000 gal/min are reported from some wells completed in the cavernous part of the surficial aquifer system.

Well fields have been constructed farther inland during the latter part of the 20th century because of coastal saltwater intrusion. Competition between agriculture, the Everglades ecosystem, and mining interests ultimately limits construction of new well fields along the western margin of the urban corridor. The underlying Floridan aquifer system is viewed as an alternative source of water for municipal supply either by treatment of saline water through reverse-osmosis processes, or by storage of freshwater by use of aquifer storage and recovery (ASR). A large network of ASR wells is being evaluated for Everglades restoration purposes if regional hydrologic and local geotechnical, hydraulic, and water-quality issues can be resolved. The lower part of the Floridan aquifer system is used for disposal of liquid waste by deep well injection for municipal industrial wastewater and reverse-osmosis concentrate. More than 20 Class I injection wells were operating in the tri-county area by 2000, injecting treated wastewater into the Boulder Zone at depths of 2,000 to 3,000 ft below NGVD 1929. Deep well injection is under greater scrutiny in recent years largely because of a growing need to use the aquifer for purposes other than for waste disposal.

Few data are available to accurately document the predevelopment conditions within the surficial aquifer system; the water table probably subtly reflected the Atlantic Coastal Ridge topography. Peat and muck deposits, an important predevelopment component of Everglades surface- and ground-water hydrology, functioned as a storage reservoir to a water column that extended upward from the underlying aquifer and maintained a higher water table that prolonged the hydroperiod and restricted movement of a coastal saltwater interface. Surface-water stage within the adjoining Everglades was sufficient to allow water to discharge through traverse glades areas, and shoreline and submarine springs discharged freshwater. Uncontrolled canal drainage and a lengthy drought in 1945-46 caused water levels to reach their lowest recorded levels, exacerbating municipal well-field saltwater intrusion problems. The modern-day water table largely reflects the hydrologic influence of numerous engineering features, including primary and secondary canal systems, gated control structures, levees, impoundments, pump systems, and the drawdown effects of the larger well fields. Ground-water movement is largely coastward and water levels are highest near the water-conservation areas, except locally in southeastern Palm Beach County and northeastern Broward County, where surface water is pumped from the Hillsboro Canal into secondary canals to artificially maintain water levels. Regional water-level comparison maps of the difference in “average conditions” show that improved drainage systems built during the 1950s lowered inland ground-water levels and increased land areas for urban and agricultural development.

Gated coastal canal structures are used to retard landward movement of saline water during the dry season through maintenance of stage higher than local water levels, inducing seepage into the aquifer. Management of canal stage has helped to increase ground-water levels in some coastal areas. Long-term canal coastal discharge appears to have declined, but coastal canal stage has been maintained gradually at higher levels, presumably to impede saltwater intrusion. Diminished coastal discharge is attributed to the rerouting of surface water to secondary canals, and induced recharge to the aquifer caused by increased municipal withdrawals.

Calcium bicarbonate water is dominant in shallow parts of the surficial aquifer system, whereas sodium bicarbonate and sodium chloride water are dominant in more deeply buried parts of the aquifer system or along the coast. Chloride concentrations generally are less than 100 mg/L at depths shallower than 50 ft, except in coastal areas and southeast of Lake Okeechobee. Chloride concentrations are less than 100 mg/L at the 150-ft depth in eastern Palm Beach County, eastern Broward County, and much of central and northwestern Dade County.

A broad zone of diffusion characterizes the saltwater interface in southeastern Florida in which the position of the interface is a consequence of three principal mechanisms: westward lateral movement of seawater within the surficial aquifer system, seepage from tidal canals, and upconing of relict seawater. Prior to 1945, uncontrolled drainage contributed considerably to lowering the water table of the surficial aquifer system along the Miami Canal. Water levels were lowered further by heavy municipal withdrawals, inducing tidal seepage into the aquifer system. Canal drainage contributed greatly to intrusion of the saltwater interface in Broward County, lowering ground-water levels with the subsequent landward movement of saltwater in the surficial aquifer system from the Atlantic Ocean. Well-field withdrawals and tidal seepage are an important, but less decisive, source of saltwater intrusion.

Predevelopment freshwater spring discharge in Biscayne Bay diminished considerably following the emplacement of canal drainage networks and the loss and compaction of inland peat deposits that formerly maintained higher water levels in the ecosystem, and stored excess surface water that helped to recharge the underlying aquifer. Changes in land use and water-management practices have greatly impacted the marine ecosystem of Biscayne Bay, resulting in increased nutrient loads and other pollutants, and increased turbidity. Prior to construction of the major canals, the salinity of the southernmost part of Biscayne Bay was much lower than normal marine salinity, especially near the coastline from Manatee Bay to possibly as far north as the Coral Gables Canal. The increase in salinity interpreted for both Biscayne and Florida Bays in the early 1900s through the 1970s is likely related to the increased development of the canal system and modifications in surface-water drainage. This is consistent with the progressive inland saltwater intrusion. 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. By altering the natural variability in fresh-water discharge to Biscayne Bay, the natural cycles of the nearshore marine organisms were disrupted, resulting in biotic fluctuations similar to the frequency and magnitude of the salinity changes.