High concentrations of fluoride in groundwater supplies in certain areas of Horry and Georgetown Counties, S.C., have been the cause of dental fluorosis (tooth mottling) among persons who have lived in these areas and have ingested the water as children. Geochemical evidence and laboratory experiments demonstrate that fluorapatite in the form of fossil shark teeth is the source of fluoride, and that the fluoride ions are liberated to the groundwater system through anion exchange, rather than by dissolution. Calcite-cemented quartz sand in the upper third of the Black Creek Formation of Late Cretaceous age contains the fossil shark teeth. As ground water progresses downdip, the calcite matrix dissolves and hydrolyzes, releasing bicarbonate, hydroxyl, and calcium ions. The calcium ions are immediately exchanged for sodium ions adsorbed on sodium-rich clays, and the bicarbonate ions accumulate. As the shark teeth are exposed, the hydroxyl ions in solution exchange with fluoride ions on fluorapatite surfaces. Experiments using fossil shark teeth show that sodium chloride in solution inhibits the rate of exchange of fluoride ions from tooth surfaces for hydroxyl ions in solution. The amount of fluoride removed from water and exchanged for hydroxyl ions in the presence of pure hydroxylapatite (hog teeth) was greater in saline water than in freshwater.
Cafio Tiburones on the north coast of Puerto Rico originally was a shallow, coastal lagoon, 9 square miles in area. It accumulated freshwater from adjacent rivers and springs and drained to the ocean through unobserved subterranean conduits. Forced drainage by pumping of Cafio Tiburones for agriculture was begun in 1949, and resulted in lowering the water level to below sea level. The reversed hydraulic gradient permitted seawater to enter the Cafio through the conduits forming seawater "springs". Seawater flooding caused widespread sodium chloride contamination in the canals and laterals and in the soils. Agricultural expectations of the area were not realized. The present investigation into the freshening potential of Cafio Tiburones (1979-1982) determined that groundwater discharge does not contribute to salty conditions. On-site experiments were performed to segregate freshwater from seawater using differences in hydraulic head. Dams were constructed on canals and laterals, forcing accumulated saltwater to the ocean. Ring levees were placed around the discharging seawater conduits until sea level was reached and flow to the system stopped. These efforts have succeeded in reducing the amount of seawater in Cafio Tiburones to one-third of its original concentration. Most of the water presently in Cafio Tiburones is suitable for irrigation.
Introduction 1 Background 1 Purpose and scope 1 Study area: location and hydrologic setting 1 Theory and operation of recovery wells 2 Hydraulic theory of vertical saltwater movement 3 Scavenger/production well systems 5 Description of demonstration well 5 Vertical movement of saltwater at well site 5 Determination of optimal freshwater discharge from production well 7 Field testing the scavenger/production well system 9 Application of design to other wells and aquifers 11 Summary and conclusions 13 Selected references 15
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