Tracing the Mixing and Movement of Groundwater into Florida Bay with Four Naturally Occurring Radium Isotopes

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Swarzenski, Peter W.
Holmes, Charles W.
Shinn, Eugene A.
Moore, Willard S.
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Four naturally occurring isotopes of radium (²²³, ²²⁴, ²²⁶, ²²⁸Ra) have a range in half-life that extends from a few days to over 1,600 years. Unique geochemical attributes make these radium isotopes ideal to examine sediment/water interface exchange processes in coastal waters. Here we present initial radium isotopic data of Florida Bay, a heavily impacted coastal system in south Florida. Florida Bay is a shallow, brackish, semi-enclosed water body that receives most of its limited freshwater supply from the Everglades, principally by surficial water runoff through Trout Creek/Taylor River. Because the entire region is underlain by highly porous Key Largo limestone and due to other hydrologic constraints, there is the possibility that ground water exchange may be significant in Florida Bay. To evaluate the extent of such a subsurface contribution, radium isotopes are being determined in shallow wells, seepage meter sites, and a series of water column samples across the Everglades National Park-Florida Bay boundary. All four radium isotopes were at least an order of magnitude greater in the two shallow well samples than in the water column samples. For example, ²²⁶Ra ranged from about 0.50 dpm L⁻¹ at a salinity of 5 to over 13 dpm L⁻¹ in Well B (salinity = 47.2). Isotopic radium ratios reveal that the well waters (i.e., marine ground water) are geochemically distinct from surficial waters and are regenerated on a time-scale of several days (i.e., ²²⁴Ra/²²³Ra). Results indicate that this radium quartet can be used effectively in Florida Bay to examine the exchange of surficial water and ground water.
Sponsored and Organized by: U.S. Geological Survey, Georgia Department of Natural Resources, The University of Georgia, Georgia State University, Georgia Institute of Technology
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