Distribution of Salinity in the Floridan Aquifer System

This section begins with a description of the vertical variations in salinity in the Floridan aquifer system in a well located in southeastern Dade County. The Floridan aquifer system is divided into three salinity zones which, in order of increasing depth, are the brackish-water zone, the transition zone, and the saline-water zone.

The base of the brackish-water zone is determined in all wells with adequate data in the study area. A problem with the determination of this boundary using only water-quality data collected during well drilling by the reverse-air rotary method is described using an example. Slightly saline water can occur above the base of the brackish-water zone in the Floridan aquifer system, and the vertical variations of salinity in the brackish-water zone in one such area are described using a geophysical well log and water-quality data. In this area, the variation is anomalous, with salinity decreasing substantially downward from the Upper Floridan aquifer to the middle confining unit. In some wells, the formation had been invaded with saline drilling fluid during drilling prior to logging, and the difficulties found in using the geophysical logs from these wells to determine salinity boundaries are discussed. In some cases, this invasion was too deep for a resistivity log to be of use. A map showing the altitude of the base of the brackish-water zone in the study area is discussed.

For the purpose of describing salinity variations, the brackish-water zone is divided into upper and lower intervals. A map showing lateral changes in chloride concentration of water from both the upper and lower intervals in the study area is discussed, and the salinity of the two intervals is contrasted. A map showing chloride concentration of water samples from the lower part of the intermediate confining unit is presented, and these values are compared to those from the upper interval of the brackish-water zone.

Lastly, a map showing the top of the saline-water zone is presented and discussed. Variations of salinity in the saline-water zone are described, and this salinity is compared to that of seawater.