Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 28 Number 3: 119-126 - July 1966

A publication of the National Speleological Society

Chemical Equilibrium Between the Water and Minerals of a Carbonate Aquifer
William Back, Rodney N. Cherry and Bruce B. Hanshaw


The departure of ground water from chemical equilibrium with several carbonate minerals is being studied in the major artesian aquifer of central Florida. The aquifer is composed predominately of Tertiary age limestone with small amounts of dolomite and gypsum. Field measurements of pH, temperature, and concentration of bicarbonate are used in conjunction with standard laboratory analyses of water samples to calculate an ion-activity product for calcite, aragonite, and dolomite. The water in the major area of recharge is characterized by calcium bicarbonate type water, by low concentrations of dissolved solids, and by undersaturation with respect to calcite and dolomite. Water from a depth of several hudred feet is undersatruated within a large part of the recharge area. This indicates that solution of dolomite and limestone can occur at depths several hundred feet below the water table to form caverns and channels.

Concentration of total dissolved solids increases with length of flow path and duration of residence time in the aquifer. At a distance away from the recharge area, the water is supersaturated with respect to calcite and dolomite. Throughout most of the area studied, the water has not reached equilibrium with aragonite. Superaturation with respect to dolomite downgradient from the recharge area is due to an increase in magnesium content of the water probably derived from the solution of magnesium calcite. Radiocarbon ages indicate that within the area of little recharge, the ground-water velocities are about 8 meters per year. The combined study of mineral equilibrium and isotopic composition of the water provides an understanding of its geologic history from which the rate of solution in various parts of the hydrologic system may be determined.

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