Abstract Introduction Study Area Aq. Framework and Definitions Estimates of Transmissivity and Hydraulic Conductivity Hydrogeology - Hydraulic Conductivity   >  W. Dade Co.   -  E. Dade Co. - Delineation of the Surficial Aq. System - Transmissivity Distribution of the Surficial Aq. System Ground-Water Flow System Summary and Conclusions References Cited Plates PDF Version

A generalized western Dade County hydrogeologic section (fig. 6c) begins in the northwestern area with a few feet of peat, muck, and lime mud. At most drill sites, some or all of these sediments have been replaced with road or levee fill. Although no tests were performed in the organic deposits (peat and muck) for this investigation, Parker and others (1955, p. 109) indicate a relatively low permeability for these sediments. The lime mud layers, referred to as the Lake Flirt Marl (Holocene age), lie between the organic deposits and the rock floor of The Everglades or as thin layers intercalated with the organic deposits. These marl layers are unconsolidated to relatively indurated and are relatively impermeable, thereby retarding movement of water down ward to, or upward from, more permeable layers below. These marls are absent or very thin in west-central Dade County, but lime mud is present in the lower Everglades and coastal marshes of southwestern Dade County.

The Miami Oolite forms the bedrock that underlies The Everglades over all of western Dade County, except the northwesternmost corner where it does not occur (pl. 1, well G-3296; pl. 6, wells G-3301 and G-3295). It thickens eastward and southward in western Dade County, reaching a maximum thickness of about 16 ft (pls. 5 and 6, well G-3318). In northwestern Dade County, the Miami Oolite may be either well cemented and very hard throughout its thickness (pl. 2, well G-3302), or have alternating layers of harder and softer limestone. The hydraulic conductivity of the Miami Oolite in this area is low to moderate, depending upon the presence of soft layers that have minor development of secondary-solution porosity. To the south and east, hydraulic conductivity increases as secondary porosity becomes better developed (pls. 6-9 and fig. 10a). Pumping of several wells open only to the Miami Oolite indicates that large yields can be obtained from this formation in some areas. However, test drilling also indicates that the cavities in many areas are at least partly clogged with lime mud and sand, thereby reducing the average hydraulic conductivity to much less than the underlying limestone. In general, the Miami Oolite does not appear to have as well developed a network of open cavities as the Fort Thompson Formation.

The Fort Thompson Formation occurs throughout western Dade County. It underlies the Miami Oolite everywhere in the county, except in the northwesternmost corner where the Fort Thompson Formation is the uppermost rock unit. The Fort Thompson Formation is only a few feet thick in the latter area, but it increases in thickness southward and eastward (pls. 1-7). Marly limestone or hard, dense limestone layers with low hydraulic conductivity are predominant at or near the top of the Fort Thompson Formation in northwestern Dade County (pls. 1, 2, and 6-8). Only a few feet of highly permeable limestone of the Fort Thompson Formation are found at the two westernmost sites along the Tamiami Trail (pl. 2, wells G-330l and G-3302; table 5, wells G-3301D and G-3302D). Hence, the Biscayne aquifer, as defined by Fish (1988) and in this report, is not present in the western part of northwestern Dade County (fig. 6b); however, the thin permeable zone is contiguous with the Biscayne aquifer to the south and east. Eastward and southward from that area, the permeable part of the Fort Thompson Formation thickens (pls. 1, 2, 6, and 7). The marine limestones of the Fort Thompson Formation generally are riddled with secondary-solution cavities and are very highly permeable (fig. 10b). The cavities generally are 2 in. or less across but are so abundant that the limestone resembles a sponge, making collection of representative samples difficult. Interbedded with the marine limestones are much more dense, less-permeable, freshwater limestones. Tests conducted during this investigation (table 5) and other studies (table 3 and table 4) indicate the average hydraulic conductivity of the Fort Thompson Formation over most of the area is tens of thousands of feet per day, possibly exceeding an average of 40,000 ft/d. Therefore, in western Dade County, the Fort Thompson Formation is the most significant unit of the Biscayne aquifer in total volume and in hydraulic conductivity. No attempt was made to distinguish the freshwater limestones on the hydraulic conductivity cross sections.

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Figure 10. (a) Highly porous and permeable Miami Oolite in western Dade County (from 8-10 feet below land surface, site G-3311), and (b) porous, shelly, highly permeable limestone of the Fort Thompson Formation in western Dade County (from 28 feet below and surface, site G-3296). [click on images above for larger version]

In places, highly to very highly permeable limestones or calcareous sandstones of the Tamiami Formation (fig. 11a) immediately underlie the Fort Thompson Formation. These limestones and sandstones form the basal zone of the Biscayne aquifer where they occur, primarily along the eastern boundary of western Dade County (pls. 2 and 9). The hydraulic conductivity of these rocks ranges from 240 to 2,000 ft/d (table 5, wells G-3294E. G-3297E, and G-3304G), much less than limestones of the Fort Thompson. Where the hydraulic conductivity of the uppermost Tamiami Formation rock or rock and sand is less than 100 ft/d (for example, well G-3294 in pl. 9 and well G-3309 in pl. 7), those sediments are excluded from the Biscayne aquifer and are included in the underlying hydrogeologic unit.

A sequence of sand, silt, clay, shell, organic sediment, and mixtures of these components, as well as minor limestone and sandstone, occur below the Biscayne aquifer. These sediments are hydrogeologically grouped as a semiconfining unit that separates the overlying Biscayne aquifer from the underlying gray limestone aquifer (fig. 6b and fig. 6c). The unit is thinner in northwestern Dade County than in west-central and southwestern Dade County, ranging from about 15 ft thick (pl. 9, well G-3304) to more than 100 ft thick (pl. 8, well G-3314). Causaras (1987) assigns the sequence to the Tamiami Formation.

The hydraulic conductivity of the semiconfining unit generally is low to moderate because silty or clayey sand and relatively clean sand, sometimes partly cemented, are the most common lithologies. A low-rate, specific-capacity test of a poorly sorted mixture of fine sand and shell fragments indicates a hydraulic conductivity of 94 ft/d (table 5, well G-3303I), which approaches the upper limit of the hydraulic conductivity range. Clean sands in the area usually have a lower hydraulic conductivity. Locally, shelly beds or shell fragment layers that contain less fine sand than the tested zone in well G-3303I probably have hydraulic conductivities of several hundred feet per day. Examples of such highly permeable beds within this unit are from 70 to 87 ft deep at the Context Road West site (pls. 4 and 7, well G-3394) and from 64 to 70 ft deep and 90 to 114 ft deep at the Levee 31N site (pls. 3 and 9, well G-3311). Silty limestones or sand stones in the semiconfining unit may have low hydraulic conductivities. Locally, silt or clay may be abundant and hydraulic conductivity is very low. An example is the thick layer at about 110 ft below land surface at the Levee 67 extension and Tamiami central sites (pls. 7 and 8). The sediments of the semiconfining layer-commonly have hydraulic conductivities that are 2,000 to 100,000 times less than the average for the Biscayne aquifer and 10 to 1,000 times less than the average for the gray limestone aquifer.

Underlying the semiconfining unit in nearly all of western Dade County is the gray limestone aquifer. It is composed primarily of gray, shelly, occasionally sandy lime stone (fig. 11b) of the middle to lower part of the Tamiami Formation, but minor sandstone (pl. 6, 88-120 ft in well G-3322) or contiguous shelly beds (pls. 4 and 7, 110-126 ft in well G-3394) are also included in the aquifer. This aquifer occurs throughout western Broward County (Fish, 1988) and in part of southwestern Palm Beach County (W.L. Miller, U.S. Geological Survey, oral commun., 1984). In Dade County, the aquifer is closest to land surface and is thickest in the northwestern area, reaching a maximum thickness of 95 ft at the Levee 67A site (pls. 1 and 8, well G-3296). The aquifer generally becomes thinner to the east and south as the upper part of the limestone is replaced with sand, silt, and clay. In the southeastern part of western Dade County, the aquifer thins and becomes less than 10 ft thick; however, in northern Dade County, the aquifer continues from the west into the eastern part of the county (fig. 6b and pls. 1 and 2).

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Figure 11. (a) Calcareous sandstone with shells, porous and highly permeable, of the Tamiami Formation in southwestern Dade County (from 47-50 feet below land surface, site G-3323); and (b) gray, sandy, shelly, highly permeable limestone of the Tamiami Formation in western Dade County (from 117-120 feet below land surface, site G-3301). [click on images above for larger version]

For this investigation, three aquifer tests were conducted in the gray limestone aquifer (table 5). At the Forty-Mile Bend site (well G-3301E), the average hydraulic conductivity is 780 ft/d (table 5), which is slightly lower than values of about 900 ft/d indicated by tests in western and southwestern Broward County (Fish, 1988, table 4). At the next two sites (wells G-3302E and G-3303E), eastward along Tamiami Trail, measured hydraulic conductivities are 420 and 430 ft/d, respectively (table 5). This is consistent with drilling data, which indicate a general decrease in hydraulic conductivity and transmissivity from west to east in the aquifer in western Dade County. At the Levee 31N site (well G-3311H), along the eastern margin of western Dade County and about 8 mi south of Tamiami Trail, the gray limestone is more dense, with a hydraulic conductivity of 210 ft/d; at the Context Road West site (well G-3394B), which includes both a shell layer and gray limestone, the hydraulic conductivity is about 400 ft/d (table 5).

Underlying the gray limestone is a sandy unit usually with some silt, clay, or shell that forms the lowest part of the Tamiami Formation. These sediments may be unconsolidated or loosely cemented into calcareous sandstone. Where the gray limestone aquifer does not occur in the southeastern part of western Dade County, a clastic unit in the lower part of the Tamiami Formation merges with the semiconfining unit of the upper part of the Tamiami Formation. The sediments of this unit are predominantly of moderate or low hydraulic conductivity. However, a shelly layer forms a local zone of high hydraulic conductivity at a depth of about 140 ft in southwestern Dade County (pl. 6, wells G-3322 and G-3317) that may not be connected with the gray limestone aquifer. Also, thin and silty sandstone, siltstone, and claystone layers (usually less than a few inches thick) are common and are grouped together on the hydrogeologic sections as relatively thicker, low or very low hydraulic conductivity layers because of the scale of the sections. The base of this unit is also the base of the surficial aquifer system, which regionally is considered the top of the Hawthorn Formation. The top of the Hawthorn Formation usually is marked by a significant increase in clay and silt and a decrease in hydraulic conductivity. However, there are many sandy interbeds in the upper part of the Hawthorn Formation in western Dade County in contrast with western Broward County (Fish, 1988).