Home Introduction Hydrogeology of the Floridan Aquifer System Collection and Analysis of Salinity Data Evaluation of Formation Salinity Distribution of Salinity in the Floridan Aquifer System Origin of Salinity >Summary and Conclusions References Cited Appendix: Inventory of Wells PDF Version

Summary and Conclusions

The Floridan aquifer system is considered to be a valuable supplemental source for public-water supply in southeastern Florida in spite of the brackish nature of its contained water in this area. The primary purpose of the study reported here was to describe the distribution of salinity in this thick and complex aquifer system and relate this distribution to the local hydrogeology, thereby allowing for some understanding of processes that control this distribution.

Geophysical logs and lithologic descriptions of wells penetrating the Floridan aquifer system were used to produce a detailed structure map of the top of the rocks of Eocene age. This top at which erosional relief was found to be present in the study area represents an important regional unconformity. The top of the rocks of Eocene age ranged in depth below sea level from 950 ft in northeastern Dade County to at least 1,300 ft in southern Dade County.

The Floridan aquifer system consists of the Upper Floridan aquifer, the middle confining unit, and the Lower Floridan aquifer, and the dominant lithology in the aquifer system is limestone. The unconformity present at the top of the rocks of Eocene age was found to approximate the top of the Upper Floridan aquifer, and the most transmissive zone in this aquifer, normally only 10 to 40 ft thick, occurs in association with this unconformity. Generally, porosity is high and permeability is low in the Upper Floridan aquifer; however, another important permeable zone of lower transmissivity than that of the upper zone occurs at about 500 ft below the upper zone. The Upper Floridan aquifer is generally 500 to 600 ft thick, and its transmissivity has been measured to be as high as 31,000 ft²/d. Except for the relatively thin permeable zones in the Upper Floridan aquifer, the porosity and permeability in the middle confining unit are not considerably less than in the Upper Floridan aquifer; however, hydraulic conductivity in the middle confining unit can be as low as 10^-4 ft/d. The top of the Lower Floridan aquifer is usually placed at or near the top of the highest thick dolomite because of the high permeability often present in this rock. This top is variable, ranging from at least 2,140 to 2,460 ft deep. Thick confining units can be present in the Lower Floridan aquifer.

Salinity was defined on the basis of chloride and dissolved-solids concentrations, and the relation between these two values was determined using data from the Floridan aquifer system. Relations were also determined between chloride concentration and specific conductance at low and high salinities. These relations were used to determine formation water resistivity in the Floridan aquifer system at two threshold salinity values, 10,000 and 35,000 mg/L of dissolved-solids concentration. The formation resistivity of the Floridan aquifer system containing water with these two salinity values was then computed.

Vertical variations in salinity in the Floridan aquifer system, defined on the basis of water-quality and geophysical log data, indicate that the Floridan aquifer system can be divided into three salinity zones. These zones, in order of increasing depth, are the brackish-water zone, the transition zone, and the saline-water zone. Salinity increases with depth rapidly in the transition zone. The transition zone was defined using a salinity of 10,000 mg/L of dissolved-solids concentration (about 5,240 mg/L of chloride concentration) at its top and 35,000 mg/L of dissolved-solids concentration (about 18,900 mg/L of chloride concentration) at its base. The concentration used at its base is a salinity value similar to that of seawater.

The base of the brackish-water zone and the top of the saline-water zone were approximately defined using geophysical logs. Using the extreme values computed for formation resistivity, the base of the brackish-water zone in one well ranged from 1,615 to 1,667 ft (a depth interval of only 50 ft). The thickness of the transition zone averaged 143 ft, and ranged from 60 to 124 ft in 10 of the 18 wells in which it was measured. The salinity of water samples obtained from completed intervals near the base of the brackish-water zone supported the position of the base as determined using geophysical logs.

The altitude of the base of the brackish-water zone in the study area ranges from about 1,150 ft below sea level in Key Largo of the Florida Keys to about 2,150 ft below sea level inland. The base of the brackish-water zone lies in the Upper Floridan aquifer along the coast but extends into the middle confining unit of the Floridan aquifer system inland. The brackish-water zone is as much as 1,200 ft thick inland.

Salinity determined by chloride concentration ranged from 850 to 5,640 mg/L in the upper interval of the brackish-water zone. Salinity increases substantially in this interval in some areas along or near the coast, particularly in one area in northeastern Broward County in which salinity becomes slightly saline (dissolved-solids concentration greater than 10,000 mg/L and chloride concentration greater than 5,240 mg/L). In this anomalous area, salinity decreases with depth in the brackish-water zone, whereas salinity typically increases downward in the zone. In the lower interval of the brackish-water zone, chloride concentrations ranged from 1,410 to 3,330 mg/L and averaged 2,280 mg/L. At one location, salinity increased by a factor of three from the top of the brackish-water zone to its lower part.

Salinity from water samples collected in saline-water zone did not vary greatly. However, salinity in the upper part of the zone can be lower than that of seawater (36,000 mg/L of dissolved-solids concentration) by about 5,000 mg/L of dissolved-solids concentration. Salinity determined by dissolved-solid concentration averaged about 37,000 mg/L in the Bolder zone of the Lower Floridan aquifer, which slightly higher than that of seawater.

The rapid, steady increase in salinity with depth in the transition zone indicates that the zone is primarily the result of diffusion across a salinity interface. The thickness of the brackish-water zone can be explained by assuming density equilibrium exists between the brackish-water and saline-water zones. Density equilibrium exists far inland where the base of the brackish-water zone extends well down into the lower permeability sediments of the middle confining unit.

Salinity in the brackish-water zone could be residual, being derived from original saline pore water of deposition or later invaded seawater. The fine-grained, low-permeability, but porous nature of the sediments in the brackish-water zone could make complete flushing by meteoric recharge difficult. The presence of the interface would seem to precluded convective upwelling of saline water from the Boulder zone into the Upper Floridan aquifer. Even at the highest temperature expected for Boulder zone water in southern Florida, this water is still more dense than that in the brackish-water zone.

In areas of anomalous higher salinity in the upper part of the brackish-water zone present along or near the coast (chloride concentrations greater than 3,000 mg/L), available data indicate that permeability or transmissivity is also higher in the Upper Floridan aquifer than is typical for the study area. The anomalous salinities in these areas could have resulted from the preferential encroachment of seawater into zones of higher permeability in the Upper Floridan aquifer during Pleistocene high stands of sea level. These anomalies then persisted because of incomplete flushing of this seawater by the present flow system. Flushing by freshwater would be minimized because of the low transmissivity of the aquifer upgradient of the areas and the long distance of transport from the recharge area. This explanation would make salinity much younger in these areas than in the rest of southeastern Florida.