Novel Geophysical and Geochemical Techniques used to Study Submarine Groundwater Discharge in Biscayne Bay, Florida

Swarenski, Peter; Dulaiova, Henrieta; Meunier, Jeff

Moore (1996) and colleagues (cf Burnett et al., 2003 and references therein) have broadcast coastal submarine groundwater discharge (SGD) into the global spotlight by describing the ubiquitous nature of SGD along varied coastlines, and highlighting its importance to coastal water and geochemical budgets. For example, the discharge of nutrient-enriched groundwater into coastal waters may cause nutrient imbalances that can lead to eutrophication (Bokuniewicz, 1980; Giblin and Gaines, 1990) or near-shore micro-organism blooms (Valiela and D'Elia, 1990). Similarly, SGD can also directly affect threatened coastal freshwater resources and impact fragile coastal ecosystems, such as coral reefs. Recently much effort has been devoted to developing and adapting new tracer techniques and methods for the identification and quantification of SGD. As the discharge of coastal groundwater most often occurs as diffuse seepage rather than through a single vent feature (Swarzenski et al., 2001 ), assessing SGD has remained difficult for both oceanographers and hydrologists alike. Burnett and colleagues have developed a systematic approach to investigate SGD by using a combination of both physical seepage measurements and a suite of naturally occurring isotopic tracers in the U/Th decay chain - 222Rn and 223, 224, 226, 228 Ra. Manheim et aL (2002) further extended SGD investigations by adapting geophysical resistivity techniques to examine fine-scale change in conductivity fields within coastal sediments. Such streaming resistivity profiling has been successfully applied to identify sites of SGD (Belaval et al., 2003), as well as the dynamic position of the fresh water/ saltwater interface. In this paper, we report on the use of streaming resistivity profiling, continuous water column 222Rn mapping and the deployment of electromagnetic seepage meters to identify and quantify submarine groundwater discharge at select sites in Biscayne Bay, FL. Such data support and validate variable-density modeling results, and provide insight into the mechanisms and scales of SGD in Biscayne Bay.