However, a network of control structures also allows water to be diverted through the canal system to points where it may be needed to help maintain ground-water levels, such as near municipal well fields. Rapid interchange of water from the canals to the Biscayne aquifer is possible in most places because of the high permeability of the aquifer. Control structures near the coast on the major canals are particularly important in helping to prevent encroachment of saltwater into the canals, and subsequently into the aquifer, during periods of less than normal precipitation.
The major features of the flow system in the Biscayne aquifer are shown by a generalized water-table map fig. The configuration of the water table is a subdued replica of the land surface; that is, the water table is at a higher altitude under hills and at a lower altitude under valleys.
The water table fluctuates rapidly in response to variations in recharge precipitation , natural discharge, and pumpage from wells. Natural discharge is by seepage into streams, canals, or the ocean; by evaporation; and by transpiration by plants.
The contours in figure 31 , and the arrows superimposed on them, show that the general movement of water in the Biscayne aquifer is seaward. Water levels are generally highest near the water-conservation areas and lowest near the coast.
Contours are not drawn in the conservation areas because they represent impoundments, and, accordingly, there is no slope in the water table there. The effects of natural surface drainage and uncontrolled canals on the water table are shown by the irregular patterns of the contours, particularly where they point upstream in a sharp "V" shape, showing that the aquifer is discharging to the canals. Near the coast, the contours point downstream, showing that the aquifer is being recharged from the canals.
The water level of an unconfined aquifer typically is markedly affected by surface drainage. Some of the local variations in the water table are due to other causes.
The local high area in eastern Palm Beach County fig. The closed depressions in eastern Broward and Dade Counties reflect large-scale pumpage from major well fields supplying Miami and Fort Lauderdale compare figs.
Withdrawal of large volumes of ground water has locally reversed the natural flow direction note westward-pointing arrows adjacent to depressions , thereby increasing the possibility of saltwater encroachment. The wide spacing of contours in Dade County and southeastern Broward County indicates a slight gradient slope in the water table, as compared to a steep gradient to the north where the contours are closely spaced.
The wide spacing of contours reflects areas where the Biscayne aquifer consists mostly of highly permeable limestone; permeability is less in the steep-gradient areas where the aquifer is sandier. Major fluctuations in the water table of the Biscayne aquifer result from variations in recharge and natural or artificial discharge, or both. Fluctuations may range from 2 to 8 feet per year, depending primarily on variations in precipitation and pumpage.
Pumpage is generally greater during periods of less than normal precipitation, as farmers and homeowners apply irrigation water to maintain crop production and lawn growth. Extremely low water-table conditions, such as those shown in figure 33 , result from prolonged periods of less than normal precipitation. Total precipitation for the 2 years preceding the date of the water levels shown in figure 33 barely exceeded the long-term average precipitation for a single year.
As a result, water levels declined slightly below sea level throughout a large area in southern Dade County, primarily due to transpiration by plants coupled with domestic pumpage. Water levels also were below sea level in a smaller area at Miami Springs, due to pumpage from the municipal well field. Under these conditions, saltwater migrated inland for considerable distances. Most of the drainage canals also were uncontrolled at the time 's represented by figure 33 , thus the lowering of the water table; saltwater encroachment was accelerated by continuous drainage to canals.
Extensive flooding also occurs during periods of greater than average precipitation, such as that preceding the high-stage water levels of October , shown in figure Water overflowed the banks of many of the canals, and a large part of the inland area was inundated.
West of Biscayne Bay, water levels were almost 11 feet higher than those shown in figure In Hialeah, water levels that had declined to about 0. The numerous types of control structures in southern Florida were constructed largely to avoid the problems associated with such extreme water-level fluctuations as those indicated by these two figures. The highly permeable rocks of the Biscayne aquifer are covered in most places only by a veneer of porous soil. Accordingly, water levels in the aquifer rise rapidly in response to rainfall.
The rise in the water level in well G, located in Miami, following two periods of intense rainfall in April , is shown in figure Eleven inches of rainfall during a 4-hour period in the early morning of April 16 produced a 4. Six inches of rainfall during the late morning and early afternoon of April 17 was responsible for an additional rise of 1. The hydraulic connection between the Biscayne aquifer and the canals that cross it is direct. Water passes freely from the canals into the aquifer and vice versa.
A decline in the water level of a canal lowers the adjacent water table of the aquifer almost immediately. Similarly, a rise in the water level in a canal is rapidly followed by a rise in the water table of the aquifer adjacent to the canal. These canal-aquifer water-level relations are shown schematically in figure The arrows show the direction that water moves when the water level of the canal is lower fig.
The degree of connection decreases as fine sediment settles out of the canal water and lines the canal bottom. Accordingly, the degree of connection may change from time to time because of either accumulation of these sediments or their removal during runoff from intense storms.
The hydraulic connection between the canals and the aquifer results in both benefits and problems. Perhaps the most obvious benefit is the ability of the canals to rapidly remove excess surface and ground water, thereby preventing flooding in low-lying interior areas.
A more subtle benefit is the ability to move water from inland parts of the aquifer to coastal areas through the canals, allowing ground-water levels near the coast to remain high enough to retard saltwater encroachment during periods of less than normal precipitation. Problems also can result from the direct hydraulic connection. For example, aquifer contamination by any pollutants in the canal water can be both rapid and widespread.
In addition, the canals provide channels by which saltwater can encroach into the aquifer for considerable distances inland during periods of low water. The latter problem has been greatly alleviated by the construction of large-scale canal control structures near the coastal ends of the major canals fig. These structures prevent the movement of saltwater up the canals when water levels in the canals are low. The delicate natural balance between freshwater and saltwater in the Biscayne aquifer is tipped when canals and well fields are superimposed on it.
Where a highly permeable aquifer, such as the Biscayne, is hydraulically connected to the ocean, inland movement of saltwater is offset by a slightly higher column of freshwater. Because freshwater is lighter than saltwater, a foot column of freshwater is necessary to balance a foot column of saltwater. This means that, for each foot of freshwater above sea level, there is approximately a foot column of freshwater below sea level. Accordingly, lowering of freshwater levels by drainage canals or by intensive pumping creates an imbalance that causes the inland movement of saltwater.
How saltwater can encroach coastal areas as a result of development is shown diagrammatically in figure In the natural, balanced condition shown in figure 38A , saltwater is present only near the shoreline and is balanced by a thick inland column of freshwater.
Construction of a drainage canal, however fig. In addition, the canal becomes a tidal channel that conveys saltwater inland and, thence, laterally into the Biscayne aquifer. Where municipal well fields withdraw large quantities of ground water, the water level in the aquifer is lowered still farther, and saltwater can enter the well field fig.
Some coastal well fields have been abandoned for this reason. Control structures fig. Thus, further saltwater encroachment is prevented and, in some instances, has even been reversed.
The saltwater body in the aquifer is approximately wedge-shaped, as shown in figure 39 , being thickest near the coast and tapering inland. Therefore, the maximum inland extent of saltwater is located near the base of the aquifer.
The cross section shown in figure 39 represents conditions near Biscayne Bay, where the aquifer is highly permeable and free interchange of freshwater and saltwater is possible. Farther northward, especially in Palm Beach County, the Biscayne aquifer is sandy and less permeable, and saltwater encroachment does not extend as far inland. The exact position of the saltwater front, defined by a chloride concentration of 1, milligrams per liter, varies in response to the height of freshwater in the aquifer, which in turn varies directly with precipitation.
Movement of the saltwater front is inland and upward in response to low ground-water levels and seaward and downward in response to high ground-water levels. The arrows in figure 39 show that freshwater at the bottom of the aquifer flows upward and then discharges seaward along the saltwater front.
The sequence of maps in figure 40 shows the inland movement of saltwater in the Biscayne aquifer in response to development. The colored area on all the maps shows the inland extent of saltwater at the base of the aquifer. Under natural conditions, as shown by the map, saltwater was limited to a narrow band along the coastline and to short tidal reaches of natural water courses. The benthic communities, in turn, were able to flourish because of the clear waters. Landward, freshwater sheet flow, natural tributaries, and shallow depressions that cut through the coastal ridge known as transverse glades , fed water from the Everglades to the margins of the bay.
Freshwater even entered the bay through springs in the limestone. Salinity channels and resulting freshwater upwelling affected the bay from the coastal cliffs to likely far offshore. The seaward margin of the bay was a series of sandy barrier islands to the north, channel-dissected shallow marine sand and mud banks along the central portion, and islands of an emergent coral limestone ridge to the south.
The entire south Florida coastal ocean ecosystem, including Biscayne Bay, has undergone major environmental change due to a century of extensive regional population growth that accelerated coastal and watershed development, pollution, and habitat loss and degradation. Miami began to grow at the beginning of the twentieth century, and Biscayne Bay became the site of one of its most important population centers.
By , four canals dissected the Everglades from Lake Okeechobee to the Atlantic Ocean, including the channelization of the Miami River. Alterations continued throughout this period, culminating with significant changes that resulted from the Central and Southern Florida Project for Flood Control and Other Purposes, which began in This project dramatically lowered freshwater levels in the Biscayne Aquifer by approximately four feet by cutting drainage canals to drain surface and groundwater to prevent intermittent coastal flooding and expand agricultural production.
As a result of a century of modifications to hydrology, Biscayne Bay has changed from a subtropical estuary fed by coastal rivers, tidal creeks, and groundwater seepage, including submarine springs, to a pulsed system that alternates between marine conditions and extreme low salinity conditions near canal discharge sites.
Freshwater now enters the bay as an intense point source rather than as distributed input over time and space.
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