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Summary and Conclusions

In southern Florida, the Floridan aquifer system is divided into three general hydrogeologic units: (1) the Upper Floridan aquifer, which contains brackish ground water, (2) the middle confining unit, which contains salty ground water, and (3) the Lower Floridan aquifer, which contains ground water whose chemical composition compares closely with that of seawater. The aquifer system is about 3,000 ft thick and is composed chiefly of carbonate rocks that range in age from early Miocene to Paleocene. Zones of high permeability are present in the Upper Floridan aquifer at the unconformable contacts of the Suwannee Limestone with the Ocala Limestone and the Ocala Limestone with the Avon Park Formation. Zones of high permeability in the Lower Floridan aquifer are present in three dolostones in the Oldsmar Formation; the lowermost, locally called the Boulder Zone, is perhaps one of the most permeable units in the world. The transmissivity of the Upper Floridan aquifer is estimated to range from 10,000 to 60,000 ft²/d, whereas that of the Lower Floridan aquifer (Boulder Zone unit) is as much as 2.5 x 10⁷ ft²/d. The porosity of both aquifers is estimated at 0.3. In southeastern Florida, the salinity of the ground water in the aquifer system generally increases with increasing depth, whereas water temperature decreases with increasing depth. Temperatures of salty ground water in the Lower Floridan aquifer (Boulder Zone) range from 50.0°F at Fort Lauderdale on the southeastern coast to about 110.0°F near Punta Gorda on the southwestern coast.

Seismic reflection profiles in the western Straits of Florida show submerged karst on the Miami Terrace at about a 1,000-ft depth and in deeper parts of the straits. Ground-water movement in the Upper Floridan aquifer is generally southward to the Gulf of Mexico and the Atlantic Ocean from recharge areas in central Florida. Hydraulic gradients in the Upper Floridan aquifer in southeastern Florida suggest that eastward-flowing, brackish ground water is actively discharging through unfilled sinkholes on the Miami Terrace as submarine springs. The middle confining unit is relatively less permeable than the Upper and Lower Floridan aquifers, and it separates the two flow systems. However, hydraulic connection between the aquifers is inferred from the presence of sinkholes and fractures and from local temperature and salinity anomalies in the Upper Floridan aquifer.

Samples of ground water from the Upper and Lower Floridan aquifers were analyzed for natural carbon and uranium isotopes to determine rates of ground-water movement. Analyses of principal anions and cations were also performed. Tritium was determined in selected samples to check for contamination by modern water, and oxygen isotopes were determined in selected samples to assess their use as climate indicators.

Vertical variations in carbon-14 activity were indicated earlier by samples of ground water from selected depths in the Alligator Alley test well (site 10), and areal variations in carbon-14 activity were reported by Hanshaw and others (1965). Carbon-14 activity in the ground water in the Upper Floridan aquifer decreases southward and downgradient from the principal recharge area in central Florida to site 5 near Lake Okeechobee, where the activity is hardly detectable (apparent age, 50,000 Libby yr), and increases slightly downgradient from the lake to the coastal area of Dade and Broward Counties. The apparent reversal in the carbon-14 activity gradient at site 5 near Lake Okeechobee is coincident with the area where the southward-flowing fresh ground water becomes brackish and is no longer potable. Southward and downgradient from the Lake Okeechobee area, the increase in salinity and in carbon-14 activity is probably related to upwelling of relatively young seawater (with relatively higher carbon-14 activity) from the Lower Floridan aquifer. Carbon-14 activity in ground water in the Lower Floridan aquifer is highest at Fort Lauderdale (which is closest to the source area in the Straits of Florida) and decreases radially inland. Carbon-14 activity in the ocean is estimated at 94 percent of modern carbon, the activity at Fort Lauderdale (well G-2333) was 62.9 percent, and the activity at well G-2296 (site 10) about 44.5 mi west of Fort Lauderdale was 37.1 percent. The apparent carbon-14 ages of the samples suggest that the past rate of inland flow was decreasing and may have been affected by the rise in sea level during the Holocene transgression. The average rate of movement between Fort Lauderdale (site 9) and well G-2296 (44.5 mi west of Fort Lauderdale) is estimated at 54.6 ft/yr, and the average rate between Fort Lauderdale and the subsea source (about 10.5 mi east of Fort Lauderdale) is estimated at 17.3 ft/yr.

The results of the uranium isotope analyses of ground water from the Upper Floridan aquifer show a relation that apparently parallels the carbon-14 data. The uranium concentration gradient generally decreases down the hydraulic gradient to about the latitude of Lake Okeechobee and then increases slightly along the southeastern coast, again implying upwelling of uranium-rich saltwater from the Lower Floridan aquifer and (or) lateral encroachment of seawater from the Straits of Florida. Concentrations of dissolved uranium averaged 0.75 µg/L in the outcrop area of west-central Florida, 0.080 µg/L in central Florida, 0.048 µg/L in southern Florida, and 0.568 µg/L in distal southeastern Florida. The alpha-activity ratios for the Upper Floridan aquifer generally showed an inverse relation to the dissolved uranium. Ratios generally increased downgradient from the outcrop area in west-central Florida but decreased in southern and distal southeastern Florida. Increasing ratios indicate enrichment of uranium-234 with respect to uranium-238, and decreasing ratios indicate depletion of uranium-238 with respect to uranium-234. Ratios averaged 0.98 in the outcrop area of west-central Florida, 1.83 in central Florida, 1.24 in southern Florida, and 1.11 in distal southeastern Florida.

In the Lower Floridan aquifer, the uranium concentration gradient parallels the carbon-14 gradient. The dissolved uranium concentration at Fort Lauderdale is almost identical to that for seawater, and the concentration decreases radially inland as did the carbon-14. The alpha-activity ratio decreased radially inland, suggesting enrichment of uranium-234 with respect to uranium-238 during transit. Therefore, the concentration gradients for both carbon-14 and uranium indicate inland flow of seawater from the Straits of Florida; the relative carbon-14 ages of the seawater in the Lower Floridan aquifer imply that velocity is greatest toward the center of the Floridan Plateau, where temperatures are highest.

Measurements of water levels in the Upper and Lower Floridan aquifers were used to estimate hydraulic gradients, which were then used to estimate rates of ground-water movement. The hydraulic rates were then compared with estimates of rates based on relative carbon-14 ages of the ground water. Rates and transit times were calculated for segments of the flow lines from Polk City to Key Largo (227 mi) and from Polk City to Fort Lauderdale (194 mi) based on estimated predevelopment hydraulic gradients and assumed aquifer characteristics. Although predevelopment gradients represent a relatively short transient event compared with long-term averages represented by the carbon-14 data, the estimated rates and transit times for predevelopment conditions are considered an indication of the long-term flow pattern. For the predevelopment gradient, a particle of water traveling 227 mi from Polk City to Key Largo would be in transit from 159,000 to 779,000 yr, and a particle of water traveling 152 mi from the southernmost sinkhole lakes in central Florida to Key Largo would be in transit from 155,000 to 768,000 yr. Both estimates of transit time exceed the maximum dating capability of carbon-14. Only the transit time between site 9 (Fort Lauderdale) and site 10 (well G-2296, 44.5 mi west of Fort Lauderdale) is within the dating capability of carbon-14. Therefore, the occurrence of measurable carbon-14 activity in the brackish ground water of the Upper Floridan aquifer in southern Florida strongly suggests that infiltrating freshwater in central Florida is not the only source.

For the Lower Floridan aquifer, water-level measurements and estimates of fluid density at site 9 (well G-2334 at Fort Lauderdale) and site 10 (well G-2296, 44.5 mi west of Fort Lauderdale) were used to calculate the difference in head and the apparent hydraulic gradient. The head at site 9 (in terms of equivalent density) is about 0.2 ft higher than the head at site 10, thereby indicating an inland hydraulic gradient of about 8.5x 10^-7. The transit time between wells is from 3,900 to 7,900 yr based on the estimated hydraulic gradient and assumed aquifer characteristics. Comparison of the hydraulically derived rates and transit times with those based on relative carbon-14 ages suggests that the estimated hydraulic gradient based on the water-level measurements may be too high. The particle velocities, transit times, and hydraulic gradients based on relative carbon-14 ages of the seawater in the Straits of Florida and ground water at sites 9 and 10 probably represent average values for the period represented by the differences in apparent age of the water samples.

Changing sea level, and perhaps changing climate, had significant effects on hydraulic gradients in the Floridan aquifer system. During periods of rising sea level and constant climate, freshwater in aquifer storage would gradually be displaced by seawater. Estimates of sea-level fluctuation during the past 150,000 yr suggest that sea level ranged from about 23 ft above present sea level to about 330 ft below present sea level. The last rise in sea level, called the Holocene transgression, began about 18,000 yr ago. Sea level rose about 330 ft during this period, and it is still rising, although at a slower rate. The rise of freshwater levels in the Florida peninsula during the past 9,000 yr was recorded by changes in vegetative types. Pollen studies of bottom sediments in lakes by Watts (1971) indicated a change from oak forest to pine forest as water tables in central Florida rose in response to rising sea level about 5,000 yr ago. A hypothetical relation between sea level and the water table in central Florida resulted from the pollen studies. The apparent carbon-14 ages of samples of saltwater from the Lower Floridan aquifer at sites 9 and 10 in southern Florida suggest that the inland transit of seawater from the Straits of Florida is also related to the rise in sea level during the Holocene transgression. Estimates of the rates based on assumed aquifer characteristics and relations between sea-level rise and the apparent carbon-14 ages of the samples ranged from about 172 ft/yr at the beginning of the Holocene transgression to about 5 ft/yr at present. The estimated differences in the head between sites 9 and 10 (44.5 mi apart) associated with these rates are 0.87 and 0.026 ft, respectively.

The present-day scenario of circulation proposed for the Floridan aquifer system of southern Florida is, therefore, (1) continuing rise in sea level and concurrent displacement of stored fresh ground water by inland-moving cold seawater chiefly through the Lower Floridan aquifer, (2) heating of the seawater in the Lower Floridan aquifer during inland transit, which results in lowered fluid density, (3) upwelling of seawater from the Lower Floridan aquifer through sinkholes and fractures that transect the middle confining unit, and (4) dilution of the seawater (again reducing the fluid density) and transport of the seawater back to the ocean by seaward-flowing freshwater in the Upper Floridan aquifer. This circulation theory is generally similar to that proposed earlier by Kohout (1965). The inland circulation of seawater is an argument against the theory that the saltwater in the Floridan aquifer system in southern Florida is connate or residual from former high stands of sea level.

Evidence of the upwelling phenomenon is indicated by salinity and temperature anomalies (also by anomalous carbon-14 activity) in the ground water of the Upper Floridan aquifer. In the Martin County-St. Lucie County area of southern Florida, wells ranging from 800 to 1,200 ft in depth produce brackish water that exceeds the background temperature of 80.0°F by 10.0°F. Associated with the local temperature anomalies are salinity anomalies. Beneath the temperature and salinity anomalies, at a depth of about 3,000 ft in the Lower Floridan aquifer, is saltwater having temperatures that compare with anomalous temperatures in the Upper Floridan aquifer. The temperature-salinity anomalies generally trend northwestward to southeastward and seem to originate from sinkholes or vertical solution pipes that are aligned with the major system of fractures or joints in the Tertiary limestone.

The Floridan aquifer system has been used as a receptacle for oil field brines since 1943. During 1943-83, 8.1 Ggal of brine were produced with 3.2 Ggal of oil. Of the 8.1 Ggal of brine, about 7.1 Ggal were injected into the Florida aquifer system. During 1959-83, 112 Ggal of nontoxic liquid waste were injected into the Floridan by municipal wastewater-treatment systems and industry. The average rate of injection increased from 268,000 gal/d in 1959 to 73.75 Mgal/d in 1983. In 1984, the estimated rate of injection was 112 Mgal/d. Injection of nontoxic liquid wastes is chiefly into the Boulder Zone of the Lower Floridan aquifer, although small amounts have been injected into the Upper Floridan aquifer.

Pilot studies indicate that the Upper Floridan aquifer can be used for temporal storage of freshwater. However, storage of freshwater in the Lower Floridan aquifer is not feasible.