Abstract Introduction Study Area Aquifer Framework and Definitions Transmissivity, Hydraulic Conductivity and Storage Coefficient Hydrogeology Ground-Water Flow System >Summary References Cited PDF Version

In 1981, the U.S. Geological Survey, in cooperation with the South Florida Water Management District, began an investigation of the surficial aquifer system in Broward County, as part of a regional study of the system in southeast Florida. The purpose of this report is to characterize the ground-water hydrology, including defining and delineating the surficial aquifer system and aquifers within it, to portray the permeability (hydraulic conductivity) framework of the surficial aquifer system by hydrogeologic sections, to map the generalized transmissivity distribution, and to describe the general pattern of ground-water flow. The methods used were: (1) hydrogeologic test drilling; (2) aquifer testing of wells open to selected zones or lithologies; and (3) analyses of previously available geologic logs, long-term water-level records, aquifer tests, and specific capacity tests of large supply wells.

The surficial aquifer system in Broward County is defined as all materials from land surface to the top of the intermediate confining unit which confines the Floridan aquifer system. Ground- water circulation in the surficial aquifer system is driven by or closely related to the water table. The base of the surficial aquifer system (also top of the intermediate confining unit) in most places is the permeability contrast between slightly clayey sands in the lower part of the Tamiami Formation and thick deposits of clay, silt, or sandy clay or silt, either in the upper part of the Hawthorn Formation or lowermost part of the Tamiami Formation. The surficial aquifer system, having materials ranging more than about seven orders of magnitude of hydraulic conductivity, contains two aquifers with an intervening semiconfining bed, which is mostly clayey sand.

The definition of the Biscayne aquifer is refined to include permeability and thickness criteria. The Biscayne aquifer is that part of the surficial aquifer system composed of Pamlico Sand, Miami Oolite, Anastasia Formation, Key Largo Limestone, Fort Thompson Formation, and contiguous subjacent highly permeable limestone or sandstone of the Tamiami Formation where at least 10 feet of the section is very highly permeable (horizontal hydraulic conductivity of about 1,000 ft/d or more). The delineation of the Biscayne aquifer differs from previous work in that less of northwest and north-central Broward County are included within the area of the aquifer, and the aquifer is substantially thicker near the coast. The upper part of the Biscayne aquifer is composed primarily of quartz sand in the east and dense limestone, peat, and lime mud in sand in the west; the lower part is a highly to very highly permeable zone of limestone and calcareous sandstone that have abundant solution cavities and some interbedded sand. Hydraulic conductivities of the highly permeable zone of the Biscayne aquifer may exceed 10,000 ft/d.

The gray limestone aquifer is defined as that part of the limestone beds in the lower and locally the middle part of the Tamiami formation that are highly permeable (hydraulic conductivity of about 100 ft/d or more) and at least 10 feet thick. Lateral changes of this limestone to less-permeable clayey, sandy limestone or carbonate sand are excluded from the aquifer, but depths between 20 and 100 feet below sea level in west Broward County, and it extend to between 102 and 170 feet below sea level. Calculated hydraulic conductivities in the gray limestone aquifer range from 590 to 930 ft/d for most of the aquifer, but the poorly consolidated upper part in northwest Broward County is less permeable.

Materials ranging from moderate to very low permeability occur within semiconfining beds separating or underlying the aquifers of the surficial aquifer system and as less permeable layers within the aquifers, especially the Biscayne aquifer. On the basis of two permeability tests and values reported in the literature, most of the relatively clean sands found in Broward County would be moderately permeable, having hydraulic conductivities of about 30 to 100 ft/d. Clayey or silty sands, which are common in the interval that separates the Biscayne aquifer from the gray limestone aquifer and as the basal unit of the surficial aquifer system throughout the area, are less permeable. Silt, clay, and mixtures of lime mud, shell, and sand form even less-permeable layers in the system, especially in the semiconfining unit that overlies the gray limestone aquifer in west Broward County, and have hydraulic conductivities that generally range from about 0.001 to 1.000 ft/d. Many types of limestone in the area also have relatively low permeability.

Analysis of test drilling, specific capacities, and pumping tests indicates that transmissivity of the surficial aquifer system is locally variable but has a definite areal pattern. Transmissivity in south-central, southeast, and part of northeast Broward County generally exceeds 300,000 ft²/d and may exceed 1,000,000 ft²/d in the southeastern coastal area of Hallandale and Hollywood. Transmissivity rapidly decreases to less than 75,000 ft²/d over a large area in northwest and north-central Broward County. In areas with high transmissivity, most of the transmissivity occurs in the cavernous zone of the Biscayne aquifer. Transmissivity of the gray limestone aquifer in west Broward County ranges from about 20,000 to 88,000 ft²/d. In northwest Broward County, the gray limestone aquifer may have higher transmissivity than overlying limestone beds of the Fort Thompson Formation and upper part of the Tamiami Formation.

Circulation in the ground-water flow system is likely to be different in some respects after development of the area because water levels, distance to discharge areas, and patterns of recharge have changed. Features of the water-management system that affect circulation include canals, drainage districts, irrigation or artificial recharge areas, water-conservation areas, pumping stations, control structures on canals, and well fields. Canals quickly remove much ground water during storms, greatly shortening ground-water flow paths compared to predevelopment conditions. However, it is frequently unclear whether canals act as fully penetrating boundaries, thereby dividing the system into many independent flow cells, or as partially penetrating boundaries of flow systems. Coastal water levels have been lowered, the threat of saltwater intrusion into coastal well fields a serious concern. The anomalous occurrence of freshwater to depths of several hundred feet below the coastal ridge, where water levels are generally less than 2 feet above sea level, suggests that the saltfront in not at an equilibrium position and that the surficial aquifer system. Under the present conditions of development, south and generally similar to the interpreted predevelopment pattern. Locally, differences may occur due to the drainage by the major canals (such as the Miami Canal), differences in water level between water-conservation areas, and underflow along the eastern edge of the water-conservation areas.

Prior to alterations of the hydrologic system by man, the likely development of a wet-season ground-water ridge under the topographic coastal ridge in east Broward County may have caused seasonal, deep, downward, and westward movement of ground water; flooding of the Everglades and of swamps in the Sandy Flatlands; and southward movement of ground water in west Broward County. The natural water-quality characteristics of Broward County are primarily related to the flushing of old seawater from the aquifer by circulation of fresh ground water prior to development and to solution of calcite. Circulation in the predevelopment ground-water flow system was controlled by water levels (such as the coastal ground-water ridge), distance to discharge areas, pattern of recharge, and by the permeability framework of the surficial aquifer system. On the basis of these controls during predevelopment conditions, water apparently entered the gray limestone aquifer in Broward County by lateral movement from upgradient areas in Hendry, Collier, and Palm Beach Counties and by downward leakage from overlying sediments and the Everglades. Ground-water movement was to the south into Dade County and to coastal discharge areas. The hydrologic interpretations are supported by the natural water-quality pattern areally and vertically in the surficial aquifer system.