publications > fact sheet > FS 2004-3117
U.S. Department of the Interior
Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Biscayne Bay, Florida
Submarine groundwater discharge (SGD) is a problem of major proportions on a world-wide scale. The ubiquitous nature of SGD along varied coastlines and its importance to coastal water and geochemical budgets have recently been thrust into the global spotlight [(Moore, 1996, and colleagues (cf. Burnett et al., 2003 and references therein)]. 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; LaRoche et al., 1997). 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 223,224,226,228Ra. 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.
A ~75 km survey of Biscayne Bay (Figure 2) for surface water 222Rn activities and streaming resistivity profiling was conducted during June 7-9, 2004. Simultaneous GPS positions, depth soundings, salinity, and temperature were obtained using a Lowrance echo sounder and an In Situ profiler, respectively.
Results from the near-continuous 222Rn survey (Figure 3) show greatest activities (> 11 dpm L-1) at Cutler Ridge, a site where F. Kohout worked on freshwater/saltwater dynamics (Kohout, 1960). Such elevated 222Rn activities can easily be discerned from background Biscayne Bay surface-water Rn activities (~2-3 dpm L-1).
Streaming Resistivity Profiling
In addition, continuous surface salinity, pH, temperature, and depth soundings were recorded to support post-processing of the resistivity data. Interpretations of the streaming resistivity data confirm enhanced freshened subsurface water masses at sites of increased 222Rn activities (Figure 4).
Electromagnetic Seepage Meter Deployments
The USGS has been developing and utilizing electromagnetic (EM) seepage meters to study groundwater/surface exchange (Rosenberry and Morin, 2004) and submarine groundwater discharge into coastal waters (Swarzenski et al., 2004). Such EM seepage meters were deployed at a site by Cutler Ridge in Biscayne Bay during March 2004. Electromagnetic seepage-rate data collected at this site show distinct and continuous discharge of groundwater. The rate of exchange across the sediment/water interface ranged from 10 to 50 cm day-1, with an average of 23.2 cm day-1 (Figure 5). It appears that tidal forcing at least partially controls the pattern of submarine groundwater discharge. These data were collected during the south Florida dry season and therefore such seepage rates would most likely increase during periods of higher rainfall (July-November). The average seepage rate (23.2 cm day-1) observed in this study corresponds very closely to modeled fluxes of groundwater into the bay (Langevin, 2001, 2003).
Near-continuous excess 222Rn measurements in the surface waters of Biscayne Bay show some striking anomalies that suggest enhanced submarine groundwater discharge at discrete sites within the bay. Interestingly, at Cutler Ridge the well-known site of Kohout's work on freshwater/saltwater dynamics water-column 222Rn activities are highest and indicate the most active submarine groundwater discharge. This is also supported by the streaming resistivity profiling data, which indicate greater freshened water masses in this region. Such data confirm the utility of these two techniques in identifying sites of SGD and provide direct evidence in support of ongoing modeling efforts on freshwater/saltwater interface processes in Biscayne Bay. The electromagnetic seepage-meter data provide the first continuous record of exchange rates across the sediment/water interface at Cutler Ridge and similarly support recent modeling predictions.
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