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 - Vertical Variations in Salinity - Brackish-Water Zone   -  Upper Interval   -  Lower Interval > Saline-Water Zone Origin of Salinity Summary and Conclusions References Cited Appendix: Inventory of Wells PDF Version

Distribution of Salinity in the Floridan Aquifer System: Saline-Water Zone

The top of the saline-water zone was determined in wells in which geophysical resistivity logs were run. The depths of this top are given in table 7, and the altitude of the top is mapped in figure 18. A formation resistivity (R₀) of 1.7 ohm-meters was used to determine the depth of the top of the saline-water zone.

Water-quality data collected from the upper part of the saline-water zone often indicate a salinity that is less than the 35,000 mg/L of dissolved-solids concentration used in resistivity calculations. Dissolved-solids concentrations were 29,800 mg/L in well BCN-I4 from 1,963 to 2,013 ft deep, 30,000 mg/L in well CS-I2 from 2,300 to 2,340 ft deep, 32,600 mg/L in well MDS-M3 from 1,771 to 1,892 ft deep, 31,500 mg/L in well PBP-M1 from 2,000 to 2,050 ft deep, 27,600 mg/L in well PLT-I2 from 2,190 to 2,230 ft deep, and 19,900 mg/L in well PLT-ROI1 from 2,150 to 2,300 ft deep (tables 2 and 3). Additionally, chloride concentration was only 5,110 mg/L in well PLT-M1 from 2,130 to 2,230 ft deep. Although these values were lower than anticipated, except for the values from PLT-ROI1 and PLT-M1 they still indicate a salinity of at least three times higher than that at the top of the transition zone. These lower salinity values in the saline-water zone could have been because of invaded drilling fluid still present in the formation when the water samples were collected. Alternatively, the upper part of the saline-water zone might have once contained water with a salinity much lower than that of seawater, and this water was incompletely flushed when displacement by saline water occurred.

Figure 18. Altitude of the top of the saline-water zone in southeastern Florida. Method used to determine this altitude in each well is given in table 7. [larger image]

The thickness of the salinity transition measured in 18 wells ranged from 60 to 257 ft and averaged 143 ft (table 7). This thickness is 124 ft or less in 10 wells and 200 ft or greater in 5 wells.

Water with salinity similar to or higher than of seawater is present in the saline-water zone of the Floridian aquifer system. Water samples with a chloride concentration of 22,000 mg/L and a dissolved-solids concentration of 40,100 mg/L were collected from well S-1533 at a depth interval of 2,100 to 2,304 ft (table 2). Water samples from 10 wells completed in the Boulder zone (BCN-I1, FTL-I2, FTL-I5, MDS-I5, MDS-BZ1, PBP-I1, PLT-I1, PU-I1, PU-I2, and SUN-I1) had an average chloride concentration of 19,760 mg/L (tables 2 and 3). The chloride concentration of Boulder zone water from well CS-I2 (24,700 mg/L) was anomalously high and was not included the average. The average dissolved-solids concentration of Boulder zone water from eight wells was 37,000 mg/L. These averages indicate a salinity similar to seawater if chloride concentration is used for comparison, but slightly more saline than seawater if dissolved-solids concentration is used. This indicates there has been some mineralization of Boulder zone water due to mineral dissolution.