WRIR 99-4061


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Reese, R.S., and Memberg, S.J., 2000, Hydrogeology and the distribution of salinity in the Floridan aquifer system, Palm Beach County, Florida: U.S. Geological Survey Water-Resources Investigations Report 99-4061, 52 p., 2 plates.

Abstract

The virtually untapped Floridan aquifer system is considered to be a supplemental source of water for public use in the highly populated coastal area of Palm Beach County. A recent study was conducted to delineate the distribution of salinity in relation to the local hydrogeology and assess the potential processes that might control (or have affected) the distribution of salinity in the Floridan aquifer system.

The Floridan aquifer system in the study area consists of the Upper Floridan aquifer, middle confining unit, and Lower Floridan aquifer and ranges in age from Paleocene to Oligocene. Included at its top is part of a lowermost Hawthorn Group unit referred to as the basal Hawthorn unit. The thickness of this basal unit is variable, ranging from about 30 to 355 feet; areas where this unit is thick were paleotopographic lows during deposition of the unit. The uppermost permeable zones in the Upper Floridan aquifer occur in close association with an unconformity at the base of the Hawthorn Group; however, the highest of these zones can be up in the basal unit. A dolomite unit of Eocene age generally marks the top of the Lower Floridan aquifer, but the top of this dolomite unit has a considerable altitude range: from about 1,200 to 2,300 feet below sea level. Additionally, where the dolomite unit is thick, its top is high and the middle confining unit of the Floridan aquifer system, as normally defined, probably is not present.

An upper zone of brackish water and a lower zone of water with salinity similar to that of seawater (saline-water zone) are present in the Floridan aquifer system. The brackish-water and saline-water zones are separated by a transition zone (typically 100 to 200 feet thick) in which salinity rapidly increases with depth. The transition zone was defined by using a salinity of 10,000 mg/L (milligrams per liter) 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 base of the brackish-water zone and the top of the saline-water zone were approximately determined mostly by means of resistivity geophysical logs. The base of the brackish-water zone in the study area ranges from about 1,600 feet below sea level near the coast to almost 2,200 feet below sea level in extreme southwestern Palm Beach County. In an area that is peripheral to Lake Okeechobee, the boundary unexpectedly rises to perhaps as shallow as 1,800 feet below sea level.

In an upper interval of the brackish-water zone within the Upper Floridan aquifer, chloride concentration of water ranges from 490 to 8,000 mg/L. Chloride concentration correlates with the altitude of the basal contact of the Hawthorn Group, with concentration increasing as the altitude of this contact decreases. Several areas of anomalous salinity where chloride concentration in this upper interval is greater than 3,000 mg/L occur near the coast. In most of these areas, salinity was found to decrease with depth from the upper interval to a lower interval within the brackish-water zone: a reversal of the normal salinity trend within the zone. These areas are also characterized by an anomalously low altitude of the base of the brackish-water zone, and a much greater thickness of the transition zone than normal. These anomalies could be the result of seawater preferentially invading zones of higher permeability in the Upper Floridan aquifer during Pleistocene high stands of sea level and incomplete flushing of this high salinity water by the present-day flow system.