11.5 The Black Creek Sandstone Aquifer, South Carolina, USA

Dental fluorosis was known to occur in Georgetown and Horry Counties, South Carolina, USA, and an investigation by Zack (1980) revealed the source of the high F concentrations in several groundwater supply wells. These counties border the Atlantic Ocean, and some seawater mixing affects some of the wells, although it is not clear what is recent seawater intrusion and what is ancient, trapped seawater. The Black Creek aquifer is a Late Cretaceous formation with thin continuous layers of calcite-cemented quartz sand interlayered with unconsolidated quartz sand and Na-rich clays. Carbonaceous material and lignite are commonly found in the formation. Fossil shark teeth, containing fluorapatite, are also common in the cemented sand. This formation lies in the belt of phosphorite deposits that occurs in coastal states from North Carolina to Florida; phosphate nodules have also been found. The mineralized material in shark teeth can be nearly pure fluorapatite (Enax et al., 2012) and francolite is also likely present in the phosphate nodules. Wells occur at depths from about 70 to 600 m with groundwater pH values approaching 9 inland and decreasing to about 8 on the coast with increasing NaCl content. Fluoride concentrations range from 0.5 to 5.5 mg/L and bicarbonate concentrations range from 350 to 1300 mg/L, making a strong positive correlation (Figure 18). Bicarbonate concentrations in Zack (1980) were computed from the WATEQ code (Plummer et al., 1976) before the revised data of Plummer and Busenberg (1982) on calcite solubility and CO₂-H₂O equilibria were published. Hence, the analytical data were revised with the phreeqc.dat database in PhreeqcI to obtain revised DIC concentrations and plotted as HCO₃ concentrations in Figure 18.

Figure 18 – Fluoride concentrations plotted against HCO₃ concentrations for the Black Creek aquifer, South Carolina. Data from Zack (1980).

Saturation indices for calcite and fluorite were recalculated and plotted from data in Zack (1980) in Figure 19a and b, respectively. An interesting trend for the fluorite saturation indices is the consistent undersaturation. This result would argue in favor of Zack’s hypothesis that shark’s teeth or a fluorapatite mineral is the source of aqueous fluoride.

Figure 19 – a) Calcite saturation indices for Black Creek aquifer groundwaters showing both undersaturation and oversaturation (data from Zack, 1980). b) Fluorite saturation indices showing consistent undersaturation with respect to fluorite.

Using the wateqf.dat database which contains thermodynamic data for fluorapatite and the few analyses that contain P determinations (Zack, 1980), Figure 20 shows that the SI values for fluorapatite are close to saturation. The SI values have been divided by 9, the total formula stoichiometry, as a normalization procedure (Nordstrom, 1999).

Figure 20 – Saturation indices (SI) for fluorapatite using data from Zack (1980). The SI values have been normalized to the total stoichiometry of the mineral formula.