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Hydrogeology of Southern Florida: Hydrogeology at the Alligator Alley Test Well Site

Floridan Aquifer System

The Alligator Alley test well penetrated about 67 percent of the estimated thickness of the Floridan aquifer system in southern Florida. The top of the Floridan aquifer system in this test well is considered to coincide with the top of the Suwannee Limestone of Oligocene age at 770 ft (fig. 9), on the basis of hydraulic head and water chemistry data. Miller (1986), in describing the regional hydrogeologic framework of the Floridan, placed the top of the Floridan at about 950 ft at this test well on the basis of apparent porosity changes within the Suwannee Limestone. This discrepancy of placing the top of the Floridan aquifer system at two different depths at this test well site is due to the difference in the criteria defined for the regional framework and for the area studies; more refinements are needed for area studies than for a regional study. The test well was terminated at 2,811 ft in the Oldsmar Formation of early Eocene age. According to Miller (1986), the base of the Floridan aquifer system (or top of the lower confining unit) is at about 3,800 ft in depth in the Cedar Keys Formation of Paleocene age (fig. 3). The formations that compose the system in the test well are (from shallowest to deepest) the Suwannee Limestone of late Oligocene age and the Ocala Limestone, Avon Park Formation, and Oldsmar Formation of Eocene age.

Discrete water-bearing zones in the Floridan aquifer system are recognized in the test well by changes in permeability, pressure, water quality, and temperature. The zones are chiefly related to dissolution of the limestone, and they are generally located at or near unconformities. Permeability contrasts within the aquifer system suggest that locally, and perhaps regionally, some of the major water-bearing zones act as distinct aquifers (fig. 9, zones 3 and 13).

Although the 16-in casing penetrated most of the Suwannee Limestone, a 2-in monitor tube with perforations from 811 to 816 ft provided data from the part of the aquifer that was cased off. The test well flows at about 1,000 gal/mm from the interval between 895 and 2,811 ft and produces a blend of saline water (25,500 µS/cm) that compares to a 50-percent mixture of freshwater with seawater. Most of the water is produced from two major water-bearing zones. A reverse geothermal gradient is indicated, and the coolest water temperature (76.1 °F) is at the bottom of the well (fig. 9).

Fluid resistance and temperature logs show the cumulative effects of inflow from the water-bearing zones in the borehole (fig. 9). Superimposed on the fluid resistance logs is a scale showing the approximate conductance. The fluid resistance and temperature logs of July 16, 1983, were obtained after the well had flowed sufficiently for the water chemistry to stabilize. Both logs indicate several water-bearing zones, but, as previously mentioned, two are the most significant–a zone at about 1,030 ft which contributes a significant amount of warm (79.2 °F) brackish water and a zone at about 2,560 ft which contributes cooler (77.1 °F) saltwater whose specific conductance is comparable to that of modern seawater. According to the fluid resistance log, the cumulative conductance of all water-bearing zones was about 26,000 µS/cm, which is similar to the average specific conductance of 14 samples.

All temperature logs that were collected during the drilling and testing showed a reverse geothermal gradient that is related to the cooler saltwater in an underlying water-bearing zone called the Boulder Zone. The coldest temperature is at the bottom of the well, and the flow becomes progressively warmer uphole by contributions of warm water from shallower water-bearing zones. The cumulative effect of inflowing ground water uphole produces a blend that has a temperature of about 78.8 °F.

The temperature and fluid resistance logs (fig. 9) show that between 2,560 and 2,811 ft the temperature decreased from about 78.3 °F to 76.7 °F and that, concomitantly, there was an increase in resistivity. The temperature decrease is probably related to a very slight upward flow of cool saltwater from the lowermost water-bearing zone (the Boulder Zone) that probably occurs at about 2,900 ft (Meyer, 1974, 1984) or to heat loss to the cooler underlying zone.

Water-bearing zones of the Floridan aquifer system in the test well were identified primarily from flowmeter, fluid resistance, and fluid temperature logs. The percentage of the total flow (the discharge measured at land surface) was calculated at about 50-ft intervals from point velocities on the flowmeter log and from the hole diameter as derived from the caliper log. The flowmeter-caliper calculations were supplemented by calculations based on the contributions (blending) of inflowing water from the water-bearing zones. The fluid resistance log of July 16, 1983 (fig. 9) and miscellaneous specific conductance measurements of water samples obtained during the packer tests and by a thief sampler were used to identify and evaluate the quantity and quality of water from each zone. Acoustic televiewer photos and borehole television surveys were also used to identify the sources. The water-bearing zones in the Floridan aquifer system as indicated in the borehole are numerous, but 14 were identified and evaluated (table 2).

Table 2. Estimated distribution of flow and fluid conductance for the Floridan aquifer system at the Alligator Alley test well [Conductance in µS/cm (microsiemens per centimeter). Well located at site 10, fig. 2]
Zone	Depth (feet)	Percent of flow	Cumulative percent of flow1	Estimated average conductance	Estimated conductance load (percent of flow times µS/cm)	Remarks
Upper Floridan aquifer
1	770 - 840	0		6,200	0	Zone cased off. Sampled from perforated monitor tube at 811 to 816 feet.
2	920 - 940	1	100	4,600	46	Minor inflow from numerous cavities.
3	1,020 - 1,034	34	99	3,300	1,122	Major inflow from large cavities at 1,025 and 1,032 feet.
4	1,110 - 1,154	6	65	3,300	198	Major inflow from large cavities at 1,114, 1,120, 1,125, 1,127, and 1,132 feet.
5	1,180 - 1,192	1	59	2,500	25	Minor inflow from small cavities.
6	1,248 - 1,256	2	58	2,500	50	Minor inflow from small cavities at 1,248 and 1,256 feet.
7	1,280 - 1,310	1	56	3,300	33	Minor inflow from small cavities at 1,284, 1,286, 1,288, 1,304, and 1,308 feet.
8	1,350 - 1,370	1	55	3,300	33	Minor inflow from small cavities at 1,356, 1,360, 1,365, and 1,367 feet.
9	1,430 - 1,645	4	54	25,000	1,000	Major inflow from cavities at 1,642 feet; minor inflow from small cavities at 1,430, 1,468, 1,476, 1,506, 1,570, 1,578, 1,592, 1,600, 1,606, 1,610, and 1,625 feet.
Middle confining unit of the Floridan aquifer system
10	1,645 - 1,840	8	50	35,000	2,800	Major inflow from cavities at 1,715 feet; minor inflow from cavities at 1,678, 1,690, 1,739, 1,754, 1,764, 1,793, and 1,809 feet.
11	1,840 - 2,200	5	42	36,900	1,845	Major inflow from cavities at 1,896, 2,070, and 2,172 feet; minor inflow from cavities at 1,856, 1,874, 1,960, 2,028, and 2,126 feet.
Lower Floridan aquifer
12	2,200 - 2,457	5	37	42,600	2,130	Major inflow from cavities at 2,250 feet; minor inflow from cavities at 2,228, 2,258, 2,308, and 2,340 feet.
13	2,457 - 2,580	32	32	50,000	16,000	Major inflow from cavities at 2,490 to 2,491, 2,544 to 2,546, 2,550 to 2,552, and 2,560 to 2,562 feet.
14	2,580 - 2,811	<1		50,000		Very minor inflow from cavities at 2,616, 2,635, 2,653, 2,672, 2,703, and 2,715 feet.
Total					25,282
1 In reverse order with depth.

Zones 3 and 13 contributed 66 percent of the total borehole flow. Zone 3 was the principal contributor (34 percent) of brackish ground water (specific conductance of about 3,300 µS/cm) from the Upper Floridan aquifer, and zone 13 was the principal contributor (32 percent) of salty ground water (specific conductance of 50,000 µS/cm) from the Lower Floridan aquifer. The remaining 34 percent of the total flow was contributed by many less permeable zones within the remaining 12 zones. Zones 2 through 9, the interval from 920 to 1,645 ft, collectively contributed about 50 percent of the flow, with composite specific conductance at about 5,000 µS/cm. Zones 10 through 14, the interval from 1,645 to 2,811 ft, collectively contributed the other 50 percent of the flow, with composite specific conductance of 45,500 µS/cm. Zones that contributed little or no water to the well (that is, those that contributed 1 percent or less) probably constitute the confining units within the individual aquifer systems. Zones 1 through 9, which collectively contributed about 50 percent of the flow, are identified as the Upper Floridan aquifer. Zones 10 and 11, which contributed about 13 percent of the flow, are identified as the middle confining unit of the Floridan aquifer system. Zones 12 through 14, which contributed about 37 percent of the total flow, are identified as the Lower Floridan aquifer.

Pressure gradients for 11 water-level measurements (table 3) were calculated from estimated densities and depths. Measurements that have similar densities and pressure gradients (for example, measurements 4, 6, 8, and 10, table 3) are generally representative of a common pressure (flow) system, and measurements that have dissimilar densities and pressure gradients (for example, measurements 3 and 10) are generally from different pressure (flow) systems. The fact that static conditions were reached only for measurements 3, 10, and 11 raises some doubt about the calculations of total static pressure for the other measurements.

Table 3. Measurements of head and pressure in the Floridan aquifer system at the Alligator Alley test well [Pressure gradient in pounds per square inch per foot; pressure at depth in pounds per square inch. Well located at site 10, fig. 2]
Measurement No.	Measured head (feet above sea level)	Depth (feet)1	Estimated pressure gradient2	Estimated representative depth3 (feet below sea level)	Estimated pressure at depth
1	>52.0	895 - 934	0.43284	879.6	403.23
2	>51.1	895 - 2,457	.43518	879.6	405.02
3	7.0	2,463 - 2,811	.44426	2,447.6	1,090.48
4	>51.5	895 - 1,428	.43253	879.6	402.73
5	>50.5	1,433 - 1,618	.43323	1,417.6	636.02
6	>56.6	895 - 1,249	.43253	879.6	404.93
7	>52.4	1,254 - 2,811	.43435	1,238.6	560.75
8	>57.7	895 - 1,124	.43253	879.6	405.41
9	>54.1	1,129 - 2,811	.43388	1,113.6	506.64
10	58.8	1,030 - 1,154	.43253	1,014.6	464.28
11	55.7	811 - 816	.43314	795.6	368.73
1 Datum is land surface, which is 15.4 feet above sea level. 2 Estimated pressure gradient is on the basis of estimated fluid density and representative depth. 3 Estimated representative depth is top of measured depth minus 15.4 feet.

For comparison, the pressure versus depth data for each measurement is shown in figure 10, a pressure-depth diagram. The plotted data suggest two distinct relations, as indicated by lines of brackish water gradient (G_BW), represented by water at the depth of measurement 10, and saltwater-like gradient (G_SW), represented by water at the depth of measurement 3. The lines through the points represent the respective pressure gradients for each measurement. The points for measurements 1, 2, 4 through 9, and 11 in the upper (brackish) part of the Floridan aquifer system (water-bearing zones 1 through 9) generally fall near or on the line (G_BW) represented by water at the depth of measurement 10, thereby suggesting that they are part of the same flow system (although minor variations in respective pressures and pressure gradients suggest the presence of local confining units). Pressures at selected depths within the body of brackish ground water in the upper part of the aquifer system may be approximated by the following equation:

P = G_BW(D + 43.4)   (1)

where
P = pressure, in pounds per square inch;
G_BW = pressure gradient of brackish water (0.43253 pound per square inch (lb/in²) per foot of depth), represented by the water at depth of measurement 10 (1,030 to 1,154 ft at the Alligator Alley test well site);
D = depth below land surface, in feet; and
43.4 = head above land surface of the water at depth of measurement 10 (58.8 ft - 15.4 ft = 43.4 ft).

Measurement 3, which represents the deeper seawater-like zones below a depth of 2,463 ft, plots slightly above the downward extension of the line (G_BW) that represents the pressure-depth relation for the upper part of the system. Pressures in the deep, saltwater part of the Floridan aquifer system may be approximated by the following equation:

P = G_SW(D - 8.4)   (2)

where
G_SW = pressure gradient of saltwater (0.44426 lb/in² per foot of depth);
8.4 = head below land surface of the water at depth of measurement 3 (15.4 ft - 7 ft = 8.4 ft); and
D = depth below land surface, in feet.

The upward extension of the line G_SW representing the pressure-head relation for the saltwater part, intersects that for the brackish water part at 1,918.5 ft, the point of equal pressure. Two interpretations of the data are possible: (1) the saltwater and brackish water systems are unrelated and function independently because of intervening confining units; and (2) the two systems are interconnected and related by buoyancy, and the point of intersection (1,918.5 ft) is the approximate brackish water-saltwater contact, or interface.

The conductance or resistance of water that entered the borehole from all water-bearing zones (fig. 9, table 2) while the well was flowing suggests that the base of the brackish water part of the system is in zone 9, which ranges in depth from 1,430 to 1,645 ft, and that the top of the saltwater part is in zone 12, which ranges in depth from 2,200 to 2,457 ft. Between the upper brackish water zones and the lower saltwater zones are zones 10 and 11, which contain mixtures of both - much the same as the zone of diffusion in unconfined coastal aquifers such as the Biscayne aquifer (Cooper and others, 1964, fig. 8).

According to the fluid resistance log of July 16, 1983, while the well was flowing (fig. 9) there was no obvious indication that the saltwater-brackish water contact occurred at 1,918.5 ft, as projected by buoyancy relations in figure 10. The fluid resistance logs of November 13, 1980, and April 13, 1981 (not shown), obtained while the well was shut-in (not flowing), suggest that the pressure in the upper brackish water part is sufficient to displace the saltwater in the borehole to a depth of about 2,250 ft. The maximum head for the upper zone was 43.4 ft above land surface, or 58.8 ft above sea level, on April 21, 1981, when the average density of the 2,250-ft fluid column was estimated to be 1.002 grams per milliliter (g/mL) at ambient temperature. Theoretically, given sufficient time, brackish water from the high-pressure upper zone would have completely displaced the saltwater to about 2,250 ft with a water column of density 0.998 g/mL. The brackish water head required for the displacement would, however, be about 9.2 ft higher than the maximum measured on April 21, 1981. Therefore, the static head in the upper part of the Floridan aquifer system could be as high as 68 ft above sea level. The discrepancy between the heads (measured and displacement) could be caused by intraborehole flow (from high-pressure zones to low-pressure zones) during shut-in.

The static head for zone 1 (table 3, measurement 11) was 55.7 ft above sea level at the ambient density on April 24, 1981. Comparisons show that the head in zone 1 (measurement 11) was about 3.1 ft lower than that in zone 3 (measurement 10) at ambient density. At the same density, the difference in head would only be 2.1 ft. The slight differences in head and in density suggest that confining beds separate these zones (at least locally) or that the differences are due to significant permeability contrasts, which suggests that ground water moves faster and more freely through zone 3. The widespread occurrence of fractures and sinkholes in the limestones that make up the Floridan aquifer system rules out the possibility that water-bearing zones within the aquifer system are isolated from each other.

Comparison of the highest measured head (table 3, 58.8 ft above sea level) in the well with the 1974 potentiometric surface map by Healy (1975b) indicates that the head extrapolated from the map was about 9 ft lower than the measured head at the Alligator Alley test well. Potentiometric surface maps by Johnston and others (1980, 1981) were recently modified on the

Figure 10. Pressure-depth relation for water-level measurements in the Floridan aquifer system at the Alligator Alley test well (well G-2296 at site 10, fig. 2). [larger version]

basis of the head measured at the Alligator Alley test well. As more detailed information on the vertical distribution of head in the Floridan aquifer system is obtained from other test wells in southern Florida, the mapped configuration of the potentiometric surface can be expected to change, particularly in the area between the Alligator Alley test well and the potentiometric surface high in central Florida.

Flowmeter, fluid resistance, and fluid temperature logs indicated that zone 13 contributed a significant amount of saltwater to the well during natural flow. Prior to the packer tests it was assumed that the static head of saltwater in zone 13 was above land surface in order to account for the saltwater flow. That assumption proved to be incorrect. The pressure-depth diagram (fig. 10) suggests that at 1,030 ft the static pressure for the saltwater column (extension of line G_SW) is lower than the static pressure in zone 3. The pressure at 1,030 ft in terms of the saltwater gradient (G_SW) would be 453.86 lb/in², and that for the brackish water gradient (G_BW) would be 464.28 lb/in². The fluid pressure in zone 3, therefore, would be 10.42 lb/in² greater than the fluid pressure at 1,030 ft in the static column of saltwater above zone 13. The difference in static pressure is equivalent to about 23.5 ft of saltwater head or about 24.1 ft of brackish water head. Because the borehole provides physical connection between the upper and lower zones, the fluid pressure in zone 3 is sufficiently greater to displace the saltwater column below the point of intersection (about 1,030 ft).

If the brackish water head is reduced by 24.2 ft or more (a reduction that would occur when the well is permitted to flow naturally), the pressure of the brackish water column at the intersection (1,030 ft) is exceeded by that of the saltwater column, and saltwater will move up the borehole from zone 13 to displace and mix with brackish water from zone 3. The inflowing brackish water from zone 3 effectively dilutes and entrains the saltwater in the upper part of the saltwater column and transports the saltwater to the surface as a blend that is equivalent to a 50-percent concentration of saltwater. This phenomenon is, in some respects, comparable to the operation of an airlift pump, and its effects have led to misinterpretation of static head distribution in flowing wells.