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U.S. Department of the Interior
U.S. Geological Survey
FS-65-99

Examining Freshwater-Saltwater Interface Processes with Four Radium Isotopes

Tracking the cycling of nutrient-laden ground water in upper Florida Bay with a suite of radium isotopes yields information on the rate and quantity of water exchange.

Introduction | Radium Isotopes as Tracers | Case Study: Florida Bay

Introduction

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.

Radium Isotopes as Tracers

illustration of the radium quartet
Figure 1: The radium quartet. (Click on image for full-sized version.)
There are four radium isotopes (Figure 1) in the uranium-238, thorium-232 and uranium-235 decay series:

223Ra (t1/2= 11.4 d),
224Ra(t1/2= 3.7 d),
228Ra (t1/2= 5.7 yr)
and
226Ra(t1/2= 1600 yr).

Their wide range in half-lives (t1/2) 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).

illustration showing examples of sources and sinks for Ra isotopes across a sediment-water interface
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.)

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:



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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

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



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Last updated: 04 September, 2013 @ 02:03 PM(TJE)