Stable activable tracers

For the following analyses, indicate whether radiochemical neutron activation analysis would be preferred to instrumental neutron activation analysis. If radiochemistry is indicated, briefly sketch the separation procedures to be used (a) the determination of ppm levels of Mo in flathead minnows, (b) the determination of the trace-element content of agricultural field-burning particulate matter, (c) the use of stable activable tracers to determine flow patterns in an ocean estuary, and (d) the determination of Dy in pine needles. 

Consider you want to trace the deposition of particulate matter using the stable activable tracer In. The dilution factor between the point of release and the point of sampling is 106. Assume the samples that are collected are activated in a thermal neutron flux of 3 x 1012 n/cm2-s for 10 min. Further assume a 1% efficiency for detecting the emitted photons. Determine the minimum amount of In that must be released to ensure the uncertainty in the measured sample concentrations is 5%. 

Stable activable tracers are powerful tools for the study of many environmental problems. Compared with fluorescent dyes or radiotracers, they offer many advantages that are reviewed herein. Our research group has emphasized the use of rare earth nuclides with short-lived activation products as stable activable tracers. Use of rare earth nuclides is cost-competitive with conventional tracers and allows rare earth "fingerprints to mark individual sources with subsequent simultaneous tracing of effluents from several sources. We present results from the application of these tracers to monitor pollutant dispersal from multiple industrial stacks, monitor fluid-bound pollutant dispersal in estuarine and fresh-water systems, monitor herbicide dispersal, and trace toxic organic chemicals in the marine environment. 

Stable Activable Tracers. To overcome many of these problems and to open new possibilities of tracer use for the environmental scientist, there has been renewed interest in developing stable activable tracer technologies that will be attractive alternatives to conventional tracers. This chapter reviews some of these developments with illustrative examples from the author s published and impublished work. 

A stable activable tracer (SAT) is a stable material that is injected into a system under study and whose concentration in the system is measured by a post sampling activation analysis. The advantages of such "artificiar tracers as compared with the naturally occurring trace elements in various systems (which act as natural tracers) are as follows artificial tracers have a controlled emission rate (either pulse or continuous injection) and control of the amoimt injected, both of which are valuable in model validation studies they can be injected in amounts suflBcient to ensure easy detection in the system under study and they lend themselves better to simultaneous tracing of several similar pollutant sources. 

Advantages of SATs. What advantages do stable activable tracers possess in general When compared in specific situations with fluorescent dyes, radiotracers, or conventional chemical tracers, what advantages do they have The advantages are listed below. 

The detection sensitivity of these tracers is very good. For example, Dy can be readily detected in amounts down to 10"i2 g. Xms allows pollutant tracing over very long distances. Stable activable tracers can be chosen such that their natiurally occurring concentrations are very small. 

Non Rare Earth SATs. Several candidates that meet these criteria have been found, and some of their applications are discussed by Kruger (5). Krugers discussion also contains an excellent accoimt of the early history of this field. In a series of experiments at the Pennsylvania State University, Jester and coworkers (6,7) have used Br" and I" to mimic the movement of soluble species in natural waters. In-EDTA complexes have been used by Behrens et al. (8) to monitor groundwater movement, while Dahl (9) has shown that In(N03)2 could be used to trace water stream patterns and pollutant dispersal in and around the harbor of a Norwegian town. Indium and scandium were used as stable activable tracers for monitoring in-plant movements of water in waste water treatment plants by Craft and Eicholz (10). The entire subject of industrial uses of activable tracers has been reviewed recently by Van Dalen and Wijkstra (II). 

Rare Earth SATs. One class of elements that are especially appealing as stable activable tracers is the class of rare earth elements. They admirably fulfill the criteria for stable activable tracers, especially those of low environmental concentration and ease of detection. These qualities were first recognized by Channell and Kruger (12), who used the lanthanides as estuarine tracers. 

The rare earth tracers, especially those with short-lived activation products, are cost-competitive with other t) es of tracers. Tables I and II show typical rare earth stable activable tracers and the total cost of using them in tracing water pollutants. These cost estimates are, of course, rough approximations and are intended to demonstrate that the costs associated with different tracer technologies are similar. Also note, as shown in Table I, the small amounts of tracer that can be easily detected (give 10 counts). These detection sensitivities are emphasized further when one notes that the natural concentrations of the rare earths in fresh water are (17) in the ppt-ppb range and the atmospheric concentrations (19) are around 10" ng/m . 

LOVELAND Stable Activable Tracers in Environmental Science 527 Tracers with Short-Lived (ti/2 < 1 day) Activation Products... 

In is not a member of the rare-earth family. It is a well known stable activable tracer and is therefore included here for comparison. 

There are certain experimental decisions that must be made either explicitly or implicitly in the use of stable activable tracers in environmental science. In this section I shall review some of these factors. 

LOVELAND Stable Activable Tracers in Environmental Science 529... 

Dispersal, Collection, and Analysis of the Tracer. The dispersal of the stable activable tracer is similar to that of a chemical tracer, radiotracer, etc. Tracer aerosols can be produced by, for example, p)n otechnic flares, acetone burners, or spraying water solutions. Dispersal in water should cause little problems other than concern that the initial tracer concentration near the point(s) of dispersal may be high. Collection and analysis of the tracer species proceed like the analysis of the trace element content of any matrix, except that the tracer should be present at a level high enou to ensure easy detection. 

To gain an appreciation for some of the unique ways in which stable activable tracers may contribute to environmental investigations, let us review some of the applications of these tracers in environmental science. 

One of the most appropriate uses of stable activable tracers is in measuring removal of pollutants from the atmosphere. The concentrations of species are extremely low even in tracer experiments (/ ng/L), and the good detection sensitivity of activation analysis is a necessity along with the controlled emission of the tracer. Much of this work has been carried out by the Battelle Pacific Northwest Laboratories and is summarized in annual reports of that laboratory from 1972-1976. 

Hydrospheric Applications. Most applications of stable activable tracers have been to trace pollutant movement in water systems. In one example of such an application, Loveland (17) used DTPA anionic complexes of the rare earth elements to trace the movement of fluid-bound pollutants in fresh waters. Figure 3 shows a comparison of the dispersal pattern of a Dy tracer injected into a municipal sewage outfall and the dispersal pattern of Zn, a naturally occurring trace element in... 

An unusual application of stable activable tracers in the marine environment was reported recently by Ghannam and Loveland (20). They used SATs to mimic the physical and chemical behavior of toxic, biologically significant organic molecules in the marine environment. To... 

The use of stable activable tracers is an important alternative for today s environmental science. With proper future development, this use can expand. Hopefully, such developments will lead to an increased understanding of the many complex problems facing the environmental scientist. 

INAA La, Ce, Sm, Dy stable activable tracers added to stack gas Shum et al. (1974)... 

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