Investigating Submarine Groundwater Discharge Into Florida Bay Using in situ ²²²Rn and Streaming Resistivity Profiling Surveys

Project Background

Submarine groundwater discharge (SGD) is an important factor in the water budget of Florida Bay and can play an important role in the delivery and transformations of groundwater borne chemical constituents. It is likely that a large component of SGD in Florida Bay is not fresh groundwater, but recycled seawater.  However, SGD discharge rates are not clear; current estimates of total (fresh + saline groundwater) SGD into Florida Bay vary widely (1-40 cm/day), depending on which particular geochemical tracer has been used, and if physical seepage meters were deployed to evaluate SGD exchange rates (Corbett et al., 1999; Top et al., 2001).  SGD rates into Florida Bay are likely controlled in part by tidally-driven water level fluctuations across the Florida Keys – the effects of which are dampened towards the northern fringe of the bay.  In this region, the fresh water/saltwater interface reflects a quasi-equilibrium between surface and subsurface fresh water run off and saline bay water interactions. 

It has been shown repeatedly (for example, Burnett et al., 2001; Burnett and Dulaiova, 2003), that these in situ techniques are accurate and precise when compared to standard but laborious emanation techniques.  The high temporal resolution of the radon measurements provides the ability to examine regional scale Rn SGD signatures.  Previous results have shown that SGD inputs are often not in steady state but change on tidal, seasonal, and likely climatic time scales. 

Objectives and Scope:

The primary objective of this study is to map the ²²²Rn distribution in surface waters of northern FLorida Bay and to conduct concurrent streaming resistivity profiling surveys (SRP).  Such resistivity data will provide unprecedented information of the fresh water / salt water interface to depths of 10’s of meters.  The Rn-222 data will identify sites of enhanced fluid exchange across the sediment / water interface where submarine ground water discharge rates can be estimated.

The project team will map the temporal and spatial patterns of ²²²Rn during wet and dry conditions using multiple automated radon detectors (RAD7 – Durridge, Co., Inc.) as a proxy for submarine ground water discharge.  A small subset of existing wells in Florida Bay will also be sampled during our bi-yearly effort for salinity and ²²²Rn to: 1) ground-truth the resistivity measurements, and 2) for developing radon mass budgets and Rn SGD fluxes.  At a minimum, two wells (one deep and one shallow) will be sampled.  Fieldwork will be coordinated with existing in situ SFWMD data collection efforts, if feasible.  The data collection will focus on the northern fringe of Florida Bay as recommended by our SFWMD collaborator (D. Rudnick). 

To address the spatial coverage necessary, two individual boat surveys will be conducted; one in the dry season and one in the wet season.  The geochemical and geophysical equipment will be connected to data loggers with GPS, a CTD meter and depth information that will continuously log data while the boat is moving. 

Select References:

Burnett, W., Kim, G., Lane-Smith, D., 2001. Use of a continuous radon monitor for assessment of radon in coastal ocean waters. J Radioanal. Nucl. Chem. 249, 167–172.

Burnett, W.C., Dulaiova, H., 2003. Estimating the dynamics of groundwater input into the coastal zone via continuous radon-222 measurements. J. Environ. Radio. 69, 21-35.

Cable, J. E., W. C. Burnett, J. P. Chanton, and G. L. Weatherly. (1996). Estimating groundwater discharge into the northeastern Gulf of Mexico using radon-222. Earth and Planetary Science Letters 144 (1996) 591-604.

Corbett, D.R., Chanton, J., Burnett, W., Dillon, K., Rutkowski, C., Fourqurean, J., 1999. Patterns of groundwater discharge into Florida Bay. Limnol. Oceanogr. 44, 973-1185.

Corbett, D.R., Dillon, K., Burnett, W.C., Chanton, J.P., 2000. Estimating the groundwater contribution into Florida Bay via natural tracers, ²²²Rn and CH₄. Limnol. Oceanogr. 45, 1546-1557.

Swarzenski, PW, Orem, WG, McPherson, BF, Baskaran, M and Wan, Y. 2006. Biogeochemical transport in the Loxahatchee river estuary: The role of submarine groundwater discharge. Mar. Chem. 101, 248-265.

Top Z, Brand LE, Corbett RD, Burnett W, Chanton J (2001) Helium and radon as tracers of groundwater input into Florida Bay. J Coast Res 17:859-868

Click on the images below to open a printable version of the USGS Report in Adobe Reader.

OFR 2004-1226 (808 KB PDF)

Submarine ground water discharge and its role in coastal processes and ecosystems http://sofia.usgs.gov/publications/ofr/2004-1226/

FS 2004-3117 (1.16 MB PDF)

Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Biscayne Bay, Florida http://sofia.usgs.gov/publications/fs/2004-3117/

OFR 2004-1369 (1.03 MB PDF)

An autonomous, electromagnetic seepage meter to study coastal groundwater/ surface-water exchange http://sofia.usgs.gov/publications/ofr/2004-1369/