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Chemical hydrogeology 81

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Hydrochemical evolution in the Floridan aquifer system
3.2

The Floridan aquifer system occurs in the south-east of the United Florida. Most discharge is to rivers and springs with only a small
States (Fig. 1) and is one of the most productive aquifers in the fraction (<5%) discharged directly into the sea. Where the system
world. The aquifer system is a vertically continuous sequence of is unconfined, recharge is rapid and groundwater circulation and
Tertiary carbonate rocks of generally high permeability. Limestones discharge rates are high, and secondary permeability is developed
and dolomites are the principal rock types, although in south- by mineral dissolution. Where confining units are thick, the carbon-
western and north-eastern Georgia and in South Carolina, the lime- ate chemistry of the groundwater evolves in a closed system
stones grade into lime-rich sands and clays. The Floridan aquifer is (Section 3.7) and the development of secondary permeability and
composed primarily of calcite and dolomite with minor gypsum, flushing of residual saline water within the aquifer system is slow
apatite, glauconite, quartz, clay minerals and trace amounts of (Sprinkle 1989).
metallic oxides and sulphides. In an extensive study of the hydrochemistry of the Floridan
The aquifer system generally consists of an Upper and Lower aquifer system, Sprinkle (1989) identified the following major
Floridan aquifer separated by a less permeable confining unit hydrochemical processes:
having highly variable hydraulic properties (Fig. 1). The Upper • dissolution of aquifer minerals towards equilibrium;
Floridan aquifer is present throughout but the Lower Floridan is • mixing of groundwater with seawater, recharge or leakage;
absent in most of northern Florida and Georgia. Recharge occurs • sulphate reduction;
primarily in outcrop areas of Alabama, Georgia and north-central • cation exchange between water and rock minerals.

Fig. 1 Map of the extent of the Upper
Floridan aquifer showing the relation
between the spatial distribution of
chloride concentrations and areas of
confined–unconfined groundwater
conditions. After Sprinkle (1989).