U.S. Department of the Interior
U.S. Geological Survey
FS 2004-3117

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 ^{223,224,226,228}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.

Figure 1. Landsat TM image of south Florida showing Biscayne Bay and Miami. Water-column 222Rn and streaming resistivity survey track lines (A-A’ and B-B’) as well as the electromagnetic seepage meter site (•) at Cutler Ridge are identified. [larger image]

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.

Figure 2. Operational configuration of the continuous 222Rn and streaming resistivity equipment deployed on a 25-ft-long pontoon boat. [larger image]

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).

Figure 3. Continuous water-column 222Rn (t1/2 = 3.8 d) activities as a tracer for identifying sites of enhanced submarine groundwater discharge. Note elevated activities (~ 11 dpm L-1) at Cutler Ridge relative to mid-bay background activities (~ 2-3 dpm L-1). [larger image]

Figure 4. A comparison of two modeled streaming resistivity profiles (top) and simultaneous water-column 222Rn activities from transects A – A’ and B – B’ (see Figure 1) in Biscayne Bay. Darker hues in the streaming resistivity interpretation correspond to freshened subsurface water masses (i.e., at Cutler Ridge). [larger image]

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).

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).

Cutler Ridge, Biscayne Bay March 2004

Figure 5. Ten-minute time-averaged electromagnetic (EM) seepage-meter results from Cutler Ridge site (see Figure 1). The EM seepage meter was likely exposed to air during a low-tide event at ~8:00 pm. [larger image]

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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Belaval, M., Lane, J.W. Jr., Lesmes, D.P. and Kineke, G.C., 2003, Continuous-resistivity profiling for coastal groundwater investigations: three case studies. In, SAGEEP Proceedings, Texas, 14 p.

Bokuniewicz, H., 1980, Groundwater seepage into Great South Bay, New York. Estuarine, Coastal Marine Science, v. 10, p. 437-444.

Burnett, W.C., Kim, G. and Lane-Smith, D., 2001, A continuous monitor for assessment of ²²²Rn in the coastal ocean. J. Radioanal. Nuc. Chem., 249 (1), 167-172.

Burnett, W.C., Bokuniewicz, H., Huettel, M., Moore, W.S. and Taniguchi, M., 2003, Groundwater and pore water inputs to the coastal zone. Biogeochemistry, 66, 3-33.

Giblin, A.E. and Gaines, A.G., 1990, Nitrogen inputs to a marine embayment: The importance of groundwater. Biogeochemistry, 10, 309-328.

Kohout, F.A., 1960, Cyclic flow of saltwater in the Biscayne aquifer of southeastern Florida, Journal of Geophysical Research, 65, 2133-2141.

Langevin, C.D., 2001, Simulation of ground-water discharge to Biscayne Bay, southeastern Florida. USGS Water Resources Investigation Report 00-4251, pp. 137.

Langevin, C.D. 2003, Simulation of submarine ground water discharge to a marine estuary: Biscayne Bay, Florida. Ground Water 41, no. 6: 758-771.

LaRoche, J., Nuzzi, R., Waters, R., Wyman, K., Falkowski, P.G. and Wallace, D.W.R., 1997, Brown tide blooms in Long Island's coastal waters linked to inter-annual variability in groundwater flow. Global Change Biology, 3, 397-410.

Manheim, F.T., Krantz, D.E., Snyder, D.S. and Sturgis, B., 2002, Streamer resistivity surveys in Delmarva coastal bays. In, SAGEEP Proceedings, Las Vegas, NV, p. 18.

Moore, W.S., 1996, Large groundwater inputs to coastal waters revealed by ²²⁶Ra enrichments. Nature, 380, 612-614.

Parker, G.G. et al., 1955, Water resources of southeastern Florida, with special reference to the geology and ground water of the Miami area. USGS Water Supply Paper 1255, pp. 965.

Rosenberry, D.O. and Morin, R.H., 2004, Use of an electromagnetic seepage meter to investigate temporal variability in lake seepage. Groundwater, 42, 68-77.

Swarzenski, P.W., Reich, C.D., Spechler, R.M., Kindinger, J.L. and Moore, W.S., 2001, Using multiple geochemical tracers to characterize the hydrogeology of the submarine spring off Crescent Beach, Florida. Chemical Geology, 179, 187-202.

Swarzenski, P.W., Charette, M. and Langevin, C., 2004, An autonomous, electromagnetic seepage meter to study coastal groundwater/surface water exchange, U.S. Geological Survey, Open File Report 2004-1369.

Valiela, I. and D'Elia, C., 1990, Groundwater inputs to coastal waters. Special Volume, Biogeochemistry, 10, 328.

For more information, please contact:
Peter Swarzenski
600 4th Street South
St. Petersburg, FL 33701
phone: 727-803-8747 x3072
fax: 727-803-2030
pswarzen@usgs.gov

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