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South Dade County is at the south end of a regional water system managed by the Central and Southern Florida Flood Control District. The District maintains and operates a system of levees, canals, control structures, pumping stations, and water storage areas by which the fresh water resources of Palm Beach, Broward, and Dade counties are managed. The levees impound water in Lake Okeechobee and in three conservation areas. They prevent sheet flow from the Everglades and Lake Okeechobee through agricultural and urban areas during the rainy season, June through October. Pumping stations at the edges of the conservation areas provide flood protection by pumping excess rainfall from the Everglades agricultural area and other flood-prone areas into Lake Okeechobee and the three conservation areas. Thus, the system not only protects against flooding, but also conserves water. Water is transferred from the conservation areas into Everglades National Park, or to coastal cities as needed.

The water-management system of southeast Florida consists of levees, water-conservation areas, canals and control structures, and pumping stations. The system is operated to provide both surface drainage and recharge for the Biscayne aquifer. 1/ [larger image]

Canals drain the urbanized strip along the coast. They also transfer water from Lake Okeechobee and the conservation areas to the Everglades agricultural area and to the east coast to replenish ground-water reservoirs near municipal well fields and to prevent sea-water intrusion. Flow in the canals is regulated by coastal control structures which are normally open or partly open during the rainy season. The structures are generally closed during dry seasons to prevent inland movement of salt water.

Water levels in southeast Florida rise during the rainy season until they reach annual highs, usually in October. Water levels are highest in Lake Okeechobee and the water-conservation areas and slope toward the coast. Flow in the canals is regulated so that ground-water levels in the urban and bordering farmlands are maintained at levels low enough to prevent flooding.

Outflow from the canals varies from year to year depending upon rainfall. At the end of the rainy season, most coastal controls are closed to conserve water; thereafter, water levels normally decline to their annual lows, usually in May. On the average, 40 or more inches of the 60 inches of yearly rainfall is lost by evapotranspiration; the remainder is lost by canal discharge to the ocean, by ground-water seepage along the coast, and by withdrawals for municipal supplies and irrigation that are not returned to the hydrologic system. Although withdrawals represent the smallest part, they are ever-increasing.

The extensive canal network in south Dade County, constructed after 1960, lowered peak water levels to reduce flooding. The effect of the network was a general downward displacement of water levels. Shown here are hydrographs of ground-water levels for 2 wells in the area. 3/ [larger image]

Sea-water intrusion has been and will continue to be a major threat to the water resources of southeast Florida, particularly those of south Dade County. The greatest inland penetration occurred before 1946, when canal flows were not regulated by control structures. Intrusion then was the result of overdrainage by canals rather than pumping ^2/. Water management practices of the Central and Southern Florida Flood Control Project, beginning in 1952, have stabilized the sea-water intrusion. However, during the drought of 1970-71, ground-water levels declined abnormally and in parts of south Dade County salt water temporarily advanced more than half a mile inland from its normal position ^3/. Preliminary studies indicate that a drought of this magnitude may be expected only once in several centuries.

The chief water problems presently facing water managers are: (1) satisfying the water demands of an everincreasing population while still retaining sufficient water in the system to meet the needs of the aquatic environments; and (2) upgrading and maintaining the water quality in the water-storage areas, the canals, and the aquifer. Reducing discharge of surplus water to the ocean can moderate water shortages in south Dade County and the National Park.

All municipal water systems from Boca Raton southward are supplied from the productive Biscayne aquifer, which extends from the south tip of Florida northward into Palm Beach County. The aquifer is wedgeshaped, thick near the coast, and thin along the western and central parts of the Everglades. In coastal Miami it extends to a depth of about 120 feet, and in coastal south Dade County to about 80 feet. The aquifer is very productive in south and central Dade County. It contains fresh water, except along the coast where it is intruded by sea water.

Residential clusters suburban to Miami began to appear in the Snapper Creek Canal area in the mid 1950's. Before then only small scattered areas, primarily along the high parts of the Coastal Ridge, between Miami and Homestead, were occupied by housing. Most of the remaining area was agricultural. Flooding in most of south Dade County during rainy seasons tended to retard urban development. Flooding and heavy damage south of the Snapper Creek Canal associated with Hurricane Donna in September 1960, however, provided the impetus for extension of the drainage canal network into south Dade County. The network, completed in 1967, resulted in rapid urban development since the mid 1960's.

In some coastal areas annual minimum water levels were raised as a result of the canal network. Shown here are water levels for 2 wells near the coast.3/ [larger image]

The canals have lowered water levels in the interior of south Dade County sufficiently to prevent flooding; but the range in annual water-level fluctuations has increased. Along the coast, maximum levels were lowered 2 to 4 feet, but minimum levels are higher than in pre-canal times.

The rise in low-water levels near the coast (wells F-319, S-182) helps retard sea-water intrusion. Water levels are lowest in the south part of the area where they have declined below sea level in 8 of the 10 years between 1961 and 1970. ^4/

Water levels in south Dade County are low each dry season. When they decline below sea level for a long time, plants and animals in Everglades National Park suffer from lack of water. Such a condition existed in the spring of 1971 (see map). At that time, the water level in the south was lower than 1.5 feet below sea level so that hydraulic gradient there was northward and inland, from Florida Bay and Biscayne Bay toward the south part of the Park and Homestead. As the southernmost water conservation area was nearly dry, the only fresh-water replenishment was ground water moving eastward and southward from that conservation area. Fortunately, this low-water condition was temporary. After the rains in early June 1971, water levels began to rise and the normal seaward gradient was restored.

Low-water conditions exist each year. Consequently, the threat of sea-water intrusion will recur with equal or greater severity as water demands increase, unless additional water is supplied to south Dade County.

Near the end of the prolonged drought of 1970-71, water levels were below sea level throughout the south half of the area. Salt water moved inland and plants and animals in Everglades National Park were severely stressed.3/ [larger image]

Sea water has intruded the Biscayne aquifer along the coast from less than half a mile inland to about 8 miles inland (see map). Salty water in the canals and in the aquifer moves in response to seasonal rainfall. During the rainy season, increased runoff dilutes sea water in the tidal reaches of canals; at the same time, rising ground-water levels throughout the area push the salty ground water downward and seaward in the coastal part of the aquifer.

Sea-water intrusion is a problem in coastal south Dade County. It can be minimized through strict water-conservation measures and by increasing fresh-water replenishment from the water-storage system. [larger image]

During the dry season, runoff to the ocean decreases or stops, and sea water moves upstream toward the coastal control structures. As water levels decline, sea water moves inland and upward in the coastal parts of the aquifer. The dense salt water moves inland along the lower part of the aquifer and the fresh water moves seaward in the upper part of the aquifer (see image below) ^5/. Salt water has intruded a wide coastal zone in the south because water levels there during each dry season are lower than they are in the remainder of the area also the land is low, only 1 to 4 feet above sea level, and is subject to periodic tidal flooding.

Distance from Bay, in Feet

The pattern of salt-water intrusion in the Biscayne aquifer is shown schematically. [larger image]

Further changes in the salt-water front in the Biscayne aquifer can be expected as a result of the drainage. However, movement of sufficient water to the south during dry season in accordance with existing plans, will control further intrusion of sea water. Largest increases in user demands in the area will be from the Key West (Navy) and Homestead systems. However, supplying these increases will require that supplemental water for local recharge of the Biscayne aquifer be made available. ^6/

The Orr-Southwest well-field complex, the largest in south Dade County, furnishes water to much of the city of Miami as well as to Coral Gables, West Miami, South Miami, Key Biscayne, and adjacent unincorporated areas. It will eventually serve the northern third of south Dade County. The combined well field includes 10 wells of 70 mgd capacity in the Orr section, and 10 wells of 100 mgd in the Southwest section. The wells average 90 feet deep. Because of the high permeability of the Biscayne aquifer and the good hydraulic connection between the aquifer and the Snapper Creek Canal, a major part of the water pumped by the city of Miami well field is derived by infiltration from the canal (see map). New well fields probably will be needed to serve the growing population. These fields are best located far enough inland to negate the threat of sea-water intrusion. Pumping of well fields in selected intercanal areas would lower water levels reduce evapotranspiration. Water-level drawdowns due to pumping would finally be stabilized by inducing infiltration from distant canals. 7/

The largest municipal well field in south Dade County, the Orr- Southwest field of the city of Miami, is recharged primarily by seepage from the controlled reach of Snapper Creek Canal. The well field is protected from sea-water intrusion by the ability of the canal to maintain relatively high water levels.3/ [larger image]

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