Abstract Introduction Study Area Aquifer Framework and Definitions Transmissivity, Hydraulic Conductivity and Storage Coefficient Hydrogeology - Sediment Hydraulic Conductivity   -  W. Broward Co.   >  Central and E. Broward Co. - Aquifer Delineation - Transmissivity Distribution Ground-Water Flow System Summary References Cited PDF Version

A major transition on the geology, and consequently, the hydraulic conductivity framework of Broward County occurs in the vicinity of the boundary between west and central Broward County. The depositional environments of the sediments in east and central Broward County were sufficiently complex and variable in space and time that there is less similarity of lithology between the test holes there than in west Broward County where the continuity of lithology is greater. Logs given in reports by Parker and others (1955), Sherwood and other (1973), and drilling experiences (Joe Voegtle, U.S. Geological Survey, oral commun., 1983) indicate that even locally considerable variation in lithology may occur. The sediments of the surficial aquifer system in central Broward County tend to be fine grained and poorly sorted–mostly clayey, fine-grained sands and fine- grained limestone. From about the boundary between central and east Broward County, the aquifer system and the principal geologic formations thicken toward the coast. Accompanying this trend, the sediments gradually become generally cleaner and coarser. Locally, near the coast, some coralline limestones are interpreted to have been reefs (Causarás, 1985). Although there are prominent changes in characteristics of the geologic formations from west to east (a coast parallel depositional pattern), there is also an important trend from south to north of decreasing limestone content (especially cavernous limestone) and increasing sand and mud content.

In north-central Broward County, at the Twenty-Six Mile Bend site (G-2312, fig. 20), a very different hydrogeologic section than the generalized west Broward County section occurs. Underlying near-surface peat deposits and extending to a depth of 106 feet below land surface are muddy sands either interbedded with limestone or within cavities in limestone; thus, the interval has lower permeability than sites in south-central Broward County (G-2321, G-2311, G-2317, and G-2318; fig. 17, fig. 18, and fig. 20). Coarse shell sand, quartz sand, and shell-the thickest found at any of the test sites–occur from 106 to 140 feet (fig. 27). An aquifer test indicates and average hydraulic conductivity of 650 ft/d for this zone (G-2312J, table 4). Although the extent of this shell sand is not known, it may be in hydraulic continuity laterally with part of the gray limestone aquifer to the south or north and presently included in that aquifer (fig. 20). The coarse layer is underlain in sequence by about 15 feet of limestone if low permeability (a lateral equivalent of the gray limestone); by soft, sandy limestone or carbonate and quartz sand; by clayey, fine sand; and by green, sandy silt at the base of the aquifer system. The general lack of very highly permeable limestone, the abundance of muddy sands, and the presence (though less) of highly permeable shell or shell sand beds also is characteristic of the Cypress Creek Canal west site (G-2341, fig. 16) and Hillsboro Canal west site (PB-1428, fig. 15) on the east side of Conservation Area 2A. Hence, these lithologies and permeabilities appear to be characteristic of the northern half of the north-central Broward County area.

Along the western edge of south-central Broward County (G-2317, G-2311, and G-2319; fig. 20), the upper 35 to 40 feet of the Fort Thompson Formation contains low or moderately permeable sand with lime mud and

Figure 27. Coarse, shell sand and quartz sand of the Tamiami Formation in west Broward County. Occurs from 126 to 129 feet deep, site G-2312. [larger image]

shell, and dense limestone interbedded with some very highly permeable limestone beds that have thin horizons with cavities. The cavities may be partially filled with muddy sand. A lower zone of the Fort Thompson Formation, extending to depths of about 60 to 70 feet, is solution-riddled limestone that is relatively free from sand or lime mud (fig. 24a, b). Hydraulic conductivities of this zone may often exceed 10,000 ft/d (G-2311B, G-2317J, table 4). The Fort Thompson Formation is very highly permeable, especially in a cavernous zone at 71 feet (G-2321J, table 4). Subjacent beds of the Tamiami Formation are locally of high permeability and are included in the Biscayne aquifer.

Except for shell sand zones in north-central Broward County or local highly permeable limestone included in the Biscayne aquifer, most of the Tamiami Formation in central Broward County consists of unconsolidated sediments of low or moderate permeability. The gray, shelly limestone of the Tamiami Formation in west Broward County, which composes most of the gray limestone aquifer, generally thins and grades eastward in central Broward County into less-permeable, sandy, silty, or clayey limestone or clayey, carbonate and quartz sands (fig. 7b, fig. 17, and fig. 18). In most places, the other Tamiami Formation sediments are thick sequences of clayey or silty fine sand, sandy silt, or sandy lime mud (see G-2341, fig. 16; G-2321, fig. 17; and G-2311 and G-2317, fig. 20). A notable exception to the characteristic low permeability of Tamiami Formation sediments in this area occurs at the Hillsboro Canal west site (PB-1428, fig. 15). There, 20 feet of gray limestone suggests that the aquifer is continuous across the northern edge of central Broward County.

(a)	(b)
(c)
Figure 28. Near-surface sands in east Broward County. (a) fine, brownish quartz sand (6 to 9 feet deep, site G-2345); (b) clean, white, fine quartz sand (29 to 33 feet deep, site G-2327); and (c) fine to very fine quartz sand with 5 to 10 percent lime mud matrix (43 to 46 feet deep, site G-2327). [click on images above for larger version]

In east Broward County, the uppermost materials are mostly fine to medium quartz sand (fig. 15, fig. 16, fig. 17, fig. 18 and fig. 21) of the Pamlico sand and the Anastasia Formation. The sands are usually clean and moderately permeable, but locally the Anastasia sands may have some lime mud in the matrix (fig. 28) or thin limestone interbeds of low or moderate permeability. Two slug tests at the Miramar east site (G-2327, fig. 18 and fig. 21) in fine sand with minor lime mud indicated hydraulic conductivities of 44 and 27 ft/d (G-2327A and G2327B, respectively, table 5). At sites located 6 to 7 miles inland, this sand body is thick, reaching depths of 80 feet in the northeast and 50 feet in the southeast (fig. 21). The sand thins westward toward central Broward County and eastward toward the coast where it overlies or interfingers with limestone of the Anastasia Formation and Key Largo Limestone. Interfingering with discontinuous beds of the Miami Oolite may occur at shallow depths over much of east Broward County.

The highly to very highly permeable zone of the Biscayne aquifer in east Broward County underlies the sand body and the Miami Oolite. It is composed of part of the Anastasia Formation, the Key Largo Limestone, the Fort Thompson Formation, and part of the Tamiami Formation (fig. 16 and fig. 17). The municipal supply wells listed on table 2 (at the end of report) are open to this zone and were used as and aid in mapping highly permeable materials. To allow comparisons among four areas in east and south-central Broward County, histograms of the depth to the top of production intervals (fig. 29) and thickness of production intervals (fig. 30) were prepared. These show that few wells are less than 50 feet deep, that most are cased to depths 50 and 120 feet below land surface (depending on the area), and that the production intervals (open hole or screen lengths) are usually between 5 and 20 feet thick. The very highly permeable zone is usually thicker than indicated by the histograms of production interval thickness because large quantities of water can often be obtained from thin cavernous zones and because the zone commonly has interbeds of sand. In a given area, production wells are often finished at a variety of depths. In some instances, deeper highly permeable zones may occur but are not used, such as reported by Bearden (1974, p. 25) for the Hollywood area.

Figure 29. (above) Casing depth (depth to top of production interval) of production wells in four areas in east and south-central Broward County. [larger image]

Figure 30. (above) Thickness of production zone (open hole or screened interval) of production wells in four areas in east and south-central Broward County. [larger image]

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Figure 31. Some common lithologies in the Biscayne aquifer in east Broward County. (a) carbonate grainstone with abundant quartz, Anastasia Formation near the coast (70 to 74 feet deep, site G-2347); (b) carbonate grainstone showing linear structure and tubular porosity, Anastasia Formation (80 to 95 feet deep, site G-2322); (c) various calcite- cemented "concretionary" structures, Anastasia and Tamiami Formations near the coast (0 to 300 feet, site G-2347); and (d) calcareous sandstone (296 to 299 feet deep, site G-2347). [click on images above for larger version]

A wide variety of lithologies with differing degrees of cavity-related permeability development occurs in the very highly permeable zone, including coarse-grained limestone (fig. 31a), micritic limestone often containing large cavities, sandy micritic limestone, limestone composed of calcite-cemented mixtures of carbonate and quartz sand (fig. 31b), and calcareous sandstone or sandy limestones commonly occur as round or tubular concretionary structures (fig. 31c), usually lying loose in sand and of moderate permeability, or small cavities that probably are highly permeable (fig. 31d). These sandstones occur in both the Anastasia Formation and Tamiami Formation. Sand is commonly found interbedded with permeable rock layers or found in partially filled cavities. Several of these lithologies usually occur at any given site. However, highly permeable coralline limestone (fig. 32), assigned by Causarás (1985) to the Key Largo Limestone, was found only in wells near the coast (fig. 22).

Several miles inland from the coast, the base of the highly to very highly permeable zone of the Biscayne aquifer is about 150 to 200 feet below land surface (fig. 21). Bearden (1974, p. 25) reported that in the Hollywood area, highly permeable limestones of the Tamiami Formation extend to a depth of about 200 feet (about midway between wells G-2327 and G-2345, fig. 21). Also, at the Dixie well field (near site G-2345, fig. 17 and fig. 21), municipal production well open at 181 to 189 feet (see table 2 at the end of report) and geologic logs reporting large cavities (Parker and others, 1955) indicate highly permeable limestones exist there from about 190 to 200 feet. However, the geologic logs indicate that lithologies probably range from very high to low permeability in this zone. Laboratory tests on tree samples of limestone from 176 to 193 feet deep at the north Dixie well field site indicated hydraulic conductivities of only about 4.3 ft/d, but porosity averaged 39 percent (table 6). The limestone from about 140 to 193 feet in this well (G-2345) shows little or no development or small interconnected cavity networks; hence, as suggested by Parker's logs, cavities may be few but several inches to greater than 1 foot wide. As shown in figure 21, along the northernmost and southernmost parts of the section, Tamiami limestone or sandstone deeper than about 150 to 160 feet is less permeable than at Hollywood and the Dixie well field and is not included in the Biscayne aquifer. At the Cypress Creek Canal east site (G-2342, fig. 16 and fig. 21), a specific capacity test of greenish, lightly to moderately cemented quartz and carbonate sand and sandy, shelly limestone between depths of 170 and 190 feet gave a hydraulic conductivity of 75 ft/d, comparable to a medium sand (G-2342B, table 4).

In east Broward County inland from the coast, the Tamiami Formation sediments that underlie the very highly permeable zone of the Biscayne aquifer form a thick section of quartz and carbonate sand (fig. 33a) of moderate permeability. Locally, there may be an admixture of minor silt or minor beds of light-greenish or grayish, soft limestone that locally may be sandy (fig. 33b). At the north Dixie well-field site (G-2345, fig. 17), for example, these sediments extend from 193 to 310 feet below land surface. The average vertical hydraulic conductivity of this sand zone is less than the horizontal hydraulic conductivity, especially at the north Dixie well field where there are many thin layers of silt, siltstone, and greenish limestone of low permeability. The lowest part of this section, immediately overlying the base of the surficial aquifer system, is clayey or silty quartz and phorphorite sand similar to west Broward County.

(a)	(a)
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Figure 32. (left) Coralline, very highly permeable Key Largo Limestone. (a) 97 to 100 feet deep, site G-2347; and (b) 87 to 90 feet deep, site G-2347. [click on images above for larger version]	Figure 33. (right) Quartz and carbonate sand or soft, sandy limestone of the Tamiami formation in east Broward County. (a) 266 to 269 feet, site G-2345; and (b) 203 to 206 feet deep, site G-2342. [click on images above for larger version]

Nearer the coast at the Snyder Park site and the Pompano Beach well-field site (G-2347 and G-2344, fig. 16, fig. 17, and fig. 22), the quartz and carbonate sands of the Tamiami formation are generally coarser and better sorted.

Figure 34. Greenish, sandy calcarenite in east Broward County. Occurs from 419 to 423 feet, site G-2347. [larger image]

Differential cementation and solution processes acting on these sediments have produced more permeable zones (than farther inland) composed of calcareous sandstones or sandy limestones (fig. 31d). Although no aquifer test and little production data are available for this deep zone, the hydraulic conductivities probably range from 100 to more than 1,000 ft/d, depending on the development of solution cavities. Thus, the Biscayne aquifer extends to depths of about 330 feet in this area. A few hard sandstone layers apparently lack solution features and probably have hydraulic conductivities of about 10ft/d.

The uppermost part of the intermediate confining unit underlying the surficial aquifer system in east Broward County is predominantly silt rather than the clay found in west Broward County. However, near the coast, no clay or silt was found. Instead, various types of limestone of low permeability occur. At the Snyder Park site (G-2347, fig. 17 and fig. 22) and the Pompano Beach well- field site (G-2344, fig. 16 and fig. 22), at least 100 feet of soft, fine-grained, greenish calcarenite (fig. 34) with up to 25 percent very fine quartz sand was found. Laboratory permeability tests on four samples from the Snyder Park site indicated horizontal hydraulic conductivities ranging from 0.61 to 8.9 ft/d and averaging 3.6 ft/d (table 6). Although this limestone is much more permeable than clay, it is much less permeable than the overlying materials, which are predominantly highly permeable limestone or sandstone, and it is a lateral equivalent of the clay and silt to the west. Therefore, the top of this thick sequence is taken as the base of the surficial aquifer system along the coast.