springs at the head of Crystal River varied from about 200 to 1,100 mg/l during the January 1964-June 1966 period, and mineral content of water from the main springs at the head of Chassahowitzka River varied from about 300 to 2,000 mg/l during the August 1964-June 1966 period. Sodium chloride, common table salt, was the principal dissolved mineral constituent in the more mineralized water. The reason for the variation in mineral content is complex and only partly understood; it is probably controlled by a fluctuation of the dynamic balance between the denser sea water and the level of the fresh water above sea level and the dynamics imparted by the diurnal tidal cycle. Because the density of sea water is greater than that of fresh water, a 41-foot column of fresh water is theoretically required to balance 40 feet of sea water, figure 12. Thus, in theory, in a coastal aquifer each foot of fresh water above sea level would indicate 40 feet of fresh water below sea level. In the coastal area, the balance between the two heads constantly changes: whenever fresh-water levels at the coast decline to or near sea level, sea water moves inland; if fresh-water levels are at or below sea level, salt water moves up coastal streams and into the aquifer.
The zone in the aquifer between fresh and salt water that exists because of this dynamic balance is known as the fresh-salt water interface. Because of changes in hydrostatic pressure in the aquifer, this interface can move both horizontally and vertically within an aquifer. The interface is normally not sharp but is a transitional zone of some thickness. In the Miami, Florida area where it has been extensively studied in great detail, Parker (1955, p. 620) reports that the zone is about 60 feet thick.
Figure 13 illustrates how movement of this interface could cause variation in quality of the water discharging from the springs. During periods of high fresh water levels in the aquifer (wet season) the fresh-salt-water interface is below the spring vent as shown in figure 13-A, and discharge from the spring is fresh. During low fresh water levels (dry season) the depth to the interface decreases--the interface zone may be located within the spring vent as shown in figure 13-B. Under this condition, discharge from the spring would be salty to some degree. The interface could remain relatively stationary for a year or more, could change seasonally as illustrated, or could change as a result of ground-water withdrawals in an area. Depending on the location of the fresh-salt-water interface, the quality of the spring flow could be changing seasonally or as physical changes are imposed on the aquifer system.