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publications > wri > 94-4010 > distribution of salinity > brackish-water zone
Hydrogeology and the Distribution and Origin of Salinity in the Floridan Aquifer System, Southeastern Florida
The approximate depth to the base of the brackish-water zone was determined in each well in the study area where data were available (table 7). When resistivity geophysical logs were available, the base of the brackish-water zone was determined using the average R0 value of 5 ohm-meters. The type of resistivity logs and number of wells in the study area used for each type are listed as follows: (1) combination tool with deep and medium induction devices along with a shallow-reading device, such as a spherically focused log (dual induction log)--10 wells; (2) induction-short normal combination tool--4 well; and (3) conventional electric logs--8 wells (four of these did not include a lateral device).
Water samples collected during well drilling by the reverse-air rotary method were also used to determine the base of the brackish-water zone. The base of the brackish-water zone in well PU-I1 was placed at about 1,750 ft based on where the chloride concentration of drilling fluid samples first began to increase more rapidly with depth (fig. 13). The highest rate of increase in chloride concentration was at about 1,820 ft. The short normal and deep induction resistivity curves read about 2 ohm-meters at 1,830 ft and deeper, indicating a salinity greater than 10,000 mg/L of dissolved-solids concentration. As shown in this example, the base of the brackish-water zone can easily be placed too deep in a well if only drilling fluid sample data are used. Reverse-air rotary water sampling data were solely used to determine the base of the brackish-water zone in wells MAR-I2 and PU-I2 (table 7).
|Table 7. Depths to salinity zone boundaries in the Floridan aquifer system as determined for this study
[Well locations shown in figure 1; local well identifier used for this report only; method used to determine salinity zone boundaries: 1, short normal (16 inches) and long normal (64 inches) resistivity curves (borehole geophysical log); 2, short normal (16 inches), long normal (64 inches), and long lateral (usually 18 feet 8 inches) resistivity curves; 3, deep induction with short normal resistivity curves; 4, dual induction (medium and deep) and shallow resistivity curves; and 5, water-quality data collected during drilling by reverse-air rotary method; --, top recorded in nearby well; E, estimated; NL, not logged]
|Depth to base of
|Depth to top of
|Method used to
|Thickness of salinity
|1 Average thickness is 143 feet.|
The low resistivity in the thin beds in the brackish-water zone could also be caused by a change in lithology, such as to limestone with a high clay content or chalk. Calculations for porosity were made in well CS-I2 from 1,860 to 1,873 ft, a zone in which resistivity was as low as 2 ohm-meters (fig. 14). Using a salinity of 5,000 mg/L of dissolved-solids concentration (a salinity not unusual for the brackish-water zone), a cementation factor of 1.8, and a formation temperature of 25°C, a very high porosity of 75 percent is required to give an R0 of 2.0 ohm-meters (eqs. 1, 2, 4, and 5). This required porosity would decrease only to 72 percent if the cementation factor were lowered to 1.6. The lithologic descriptions of drill cuttings in wells CS-I2, CS-M2, MDS-I12, and S-1533 were examined, and there is no evidence that the thin beds with low resistivity in these wells are of a substantially different lithology than is normal for the Avon Park Formation. However, a zone of higher permeability is apparent in well MDS-I12 from 1,500 to 1,550 ft based on geo-physical log analysis (Miami-Dade Water and Sewer Department, 1991, p. 67).
The conventional electrical log run in well SUN-I1 to determine salinity boundaries presented some problems. The log run on November 3, 1984, across the interval from 1,790 to 3,207 ft, had resistivity readings that were abnormally low. From the top of the logged interval downward, the short normal and long normal curves decrease to a minimum of 0 to 1 ohm-meter at 2,000 ft and remain in this range for most of the logged interval. This response pattern indicates that the top of the saline-water zone is at about 2,000 ft in the well based on knowledge of other wells in the study area, such as well MDS-I12 (fig. 12). However, resistivity in the saline-water zone should be at least 1 ohm-meter as shown by logs run on other wells in the study area and in table 6.
An earlier conventional electrical log survey run in well SUN-I1 on March 27, 1984, across the interval from 1,014 to 1,822 ft, indicates that the base of the brackish-water zone is at least 1,822 ft deep in this well. The depth of the base of the brackish-water zone was estimated to be at 1,857 ft in well SUN-I1 using the November 3, 1984 log run by assuming an average thickness for the salinity transition zone and subtracting this thickness from the depth of the top of the saline-water zone at 2,000 ft. This average thickness of the salinity transition zone (143 ft) was determined using all of the wells in which the thickness was determined in the study area (table 7).
The altitude of the base of the brackish-water zone in the study area is highest along the coast, particularly at Key Largo, as shown in figure 15. The mapped surface dips inland with contours that parallel the coast. The magnitude of the dip seems to be steepest along the coast. The surface apparently reaches a maximum depth from 2,100 to 2,200 ft below sea level in central to east-central Broward County and northwesternmost Dade County.
The thickness of the brackish-water zone can be approximated using the attitude of the top of Eocene rocks and the base of the brackish-water zone (figs. 6 and 15). This thickness ranges from 1,190 ft in well G-2296 in western Broward County to 24 ft in well MO-130 on Key Largo.
The altitude of the base of freshwater flow in the Floridan aquifer system was mapped by Sprinkle (1989, fig. 23). A chloride concentration in ground water of 10,000 mg/L was used to define the base. The depth of the base was determined using sample data or by calculation using predevelopment freshwater heads. The altitude of the base of freshwater flow determined by Sprinkle (1989, fig. 23) is similar to the altitude of the base of the brackish-water zone shown in figure 15.
For the purpose of describing salinity variations, the brackish-water zone was divided into an upper interval and a lower interval. The break used between the upper and lower intervals is 200 to 300 ft below the top of the rocks of Eocene age. More water-quality data were available for the upper interval of the brackish-water zone than for the lower interval. This reflects the importance of the flow zone at or near the top of the rocks of Eocene age previously described in this report. The upper and lower intervals of the brackish-water zone are described in the subsequent sections of this report.
U.S. Department of the Interior, U.S. Geological Survey
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