The complex exchange of fluvial, subsurface, and marine material within an estuary directly affects global biogeochemical cycles. Environmental scientists have few tools to accurately quantify such processes directly and must therefore rely on various tracer techniques. Satellite imagery, for example, provides an invaluable means for tracking some freshened river plumes into the open ocean, often for many hundreds of kilometers. However, the mixing of fresh water into seawater cannot always be tracked remotely and generally cannot yield information on the movement and rate of water mixing. Fortunately, natural and artificially produced radioactive tracers can be used to determine recent chronologies of such processes as: recently deposited sediments, water mass mixing, and exchange processes across the sediment/water interface.

In order for a chemical constituent to be implemented successfully as an environmental tracer, its source and sink functions, as well as all processes that regulate them, must be known and quantifiable. For example, methane (CH₄) and radon-222 (²²²Rn) are two natural tracers that have been successfully utilized in coastal groundwater studies because their respective concentrations in ground water are usually much higher than in surrounding seawater. There are, however, some known caveats in using these two tracers successfully. CH₄ is a product of organic matter decomposition and therefore has strong microbial component that can complicate its environmental behavior. Radon-222, while being chemically inert, has an additional atmospheric source that can be difficult to constrain from what is being produced within the sediments or a water column. The naturally occurring isotopes of radium are an additional suite of tracers ideal for freshwater/saltwater interface processes.

Figure 1: The radium quartet. (Click on image for full-sized version.)

Their wide range in half-lives (t_1/2) corresponds well with the duration of many coastal processes. A suite of highly particle-reactive thorium isotopes decays to form the four radium isotopes. In fresh water, radium is also chemically bound onto particle surfaces, yet as these particles become exposed to higher salinity water during estuarine mixing, radium will under go a phase transformation and will eventually reside exclusively in the dissolved phase in the open ocean. Thorium, in contrast, will continue to remain bound to particles, regardless of salinity. Estuarine sediments thus provide a continuous source for radium isotopes to coastal waters and the production rate is defined directly by their individual isotopic decay constants (Figure 2).

Figure 2: Examples of sources and sinks for Ra isotopes (such as 226Ra) across a sediment-water interface. (Click on image for full-sized version.)

In the past, only the long-lived isotopes of Ra were routinely used as geochronometers because radiometric counting techniques were inadequate for many short-lived radionuclides, such as ^223,224Ra. The U.S. Geological Survey, in partnership with the University of South Carolina, now has acquired two delayed-coincidence alpha scintillation counters that can accurately quantify very low activities of ^223,224Ra. This capability, in addition to standard gamma spectroscopy, allows for the rapid and precise analyses of all four radium isotopes.

South Florida has undergone rapid environmental changes since the 1950s. In response, the hydrology of south Florida has also been significantly modified. Today, an overabundance in nutrients and saltwater enrichment often threaten to contaminate freshwater reservoirs of many south Florida municipalities. Florida Bay receives the majority of its fresh water from Taylor Sough and is thus also vulnerable to deteriorating water-quality issues in the Everglades. Heightened subsurface flow through porous strata in south Florida may further introduce anthropogenic contaminants into the bay. To address the issue of groundwater flow and groundwater/surficial water exchange in upper Florida Bay, a series of samples was collected in March, 1998, for radium isotopes. In this bay system, the radium quartet can clearly differentiate surficial from subsurface water masses and the ratio of ²²³Ra/²²⁴Ra can provide information on the apparent age of water masses. We are currently developing models with which we hope to better constrain the exchange of ground water in upper Florida Bay and elsewhere.

Colaborators:

• South Florida Water Management District
• Florida Department of Environmental Protection
• University of Miami
• Florida International University
• Florida State University

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For more information contact:

Peter W. Swarzenski
Charles W. Holmes
U.S. Geological Survey
600 Fourth St. South
St. Petersburg, FL 33716
Telephone: (727) 803-8747
Fax: (727) 803-2032
E-mail: pswarzen@usgs.gov