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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.)
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The combined source functions for radium in an estuary thus include a) riverine
particulates/dissolved, b) oceanic dissolved, c) estuarine sediments and
d) ground water. The relative significance of each of these sources is
usually a function of the site-specific hydrogeology and where the samples are
taken relative to the salinity gradient (extent of freshwater/saltwater mixing).
Ground water, defined either as recycled marine water or fresh water, may contribute Ra to coastal water
column anywhere along this salinity gradient as long as the hydraulic gradient, hydraulic head and sediment
hydraulic conductivities are favorable for groundwater discharge. Because ground water is also commonly enriched in radium isotopes
relative to surficial water (Ra source in the sediments), a time-dependent groundwater influence can easily be distinguished even in
a dynamic water column. In surface sediments that are flushed either continuously or sporadically with ground water, a localized disequilibrium between 228Th and 228Ra will develop, because Ra is released into bottom waters by water movement, whereas thorium will remain attached to sediments. This isotopic disequilibrium can be used to assess a ground-water flux rate or an apparent water-mass age.
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.
Case Study: Florida Bay
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:
Click here for a printable version of this fact sheet (note: document will
open in a new browser window)
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
Related information:
SOFIA Project: Tracing the Mixing of Groundwater into Coastal Waters Utilizing a New Radiometric Technique: Radium Isotope Systematics to Look at the Geologic Control of Aquifers