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

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. 

Study area map for submarine groundwater discharge project in Florida Bay.

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