THE APPLICATION OF THE STABLES ISOTOPES OF OXYGEN AND HYDROGEN TO DISTINGUISH THE ORIGIN OF FRESHWATER DELIVERED TO THE COASTAL ESTUARIES OF SOUTH FLORIDA

The salinity of the ecologically sensitive coastal estuaries of South Florida involves mixing between freshwater, derived from direct runoff, precipitation, and groundwater, and normal marine seawater. We have been investigating the application of stable oxygen and hydrogen geochemical tracers in order to quantitatively distinguish the amounts of freshwater derived from each of the relevant sources. This method relies on the fact that each of the end members has distinctive salinities and stable isotopic compositions. Therefore a cross plot between salinity and either the oxygen or hydrogen isotopic composition can reveal the isotopic composition of the original freshwater water end-member. This method has been applied in Florida and Biscayne Bays and suggests that in Florida Bay ground water contributes only a minor amount of fresh-water to the Bay. In fact the salinity over the majority of the area is controlled by evaporation and direct input from precipitation rather than from runoff from the adjacent mainland. In contrast in Biscayne Bay salinities are more strongly influenced by runoff rather than precipitation and groundwater.