USING NATURAL GEOCHEMICAL TRACERS TO DISCERN THE DOMINANT SOURCES OF FRESHWATER INTO BISCAYNE BAY, SOUTHEAST FLORIDA

The water occupying an estuary is a balance of saltwater influx from the open ocean and freshwater inputs from precipitation, surface water runoff, and submarine groundwater discharge. The goal of this study is to use naturally occurring geochemical constituents as tracers to identify, differentiate and quantify the sources of freshwater, i.e. rainfall, canal flow, and groundwater discharge to an estuarine system. The field area for this study is Biscayne Bay, a sub-tropical estuary located on the carbonate platform of south Florida. In this study discrete samples of precipitation, canal water, terrestrial groundwater, marine groundwater, and bay surface water are collected monthly and analyzed for the stable isotopes of hydrogen and oxygen as well as for major cations and anions. Initial results indicate that it is possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. However, it is not possible to separate the groundwater signal from the precipitation signal. To separate submarine groundwater inputs from precipitation, Sr, Mg, and Ca concentrations present in freshwater sources were measured and averaged creating a tertiary mixing diagram. Ion concentrations were then measured in surface waters in Biscayne Bay. These surface data were plotted on the tertiary mixing diagram to calculate percentages of each freshwater member present. Results show significant spatial variation from site to site in the bay. While canal water seems to be the dominate source of freshwater throughout the bay, sites near the northwestern shore of the bay show a strong groundwater signal, and sites further east nearing the Atlantic Ocean appear to be dominated by precipitation.