Types of Application

The many environmental applications of naturally occurring nuclides can be separated into three categories. The first category is the well-known use of isotopes as tracers. An example is measurement of the ratio of radiogenic, stable 87Sr to stable 86 Sr to trace the source of groundwater and its path in an aquifer. Another is the use of cosmogenic 7Be (53.28 d) as a tracer for river sediment and its resuspension.

The second category includes the study of chemical and physical processes associated with events and changes that are occurring or have occurred in and on the earth and in its atmosphere. Differences in chemical or physical properties associated with differences in the masses of the isotopes of an element result in isotope fractionation—a change in the relative abundance of isotopes of a given element. (See Chapter 13, Section Since isotope fraction-ation can be related to the environmental conditions on the earth when a change or event occurred in the past, it can be used as a research tool in atmospheric chemistry, geochemistry, paleogeology, paleoclimatology, and other fields related to the paleoenvironment.

The third category encompasses studies to determine the age of materials, date events that occurred in the past (e.g., for geochronometry), or measure the rates at which changes (e.g., climatic changes) have occurred, using radioactivity as a nuclear clock or chronometer. The length of time measured by radio-metric dating may be from a few years to the age of the earth. Commonly, a study in the second category will also include the use of radionuclides to establish the time when the process or event being studied occurred.

Another method of determining the age of an ancient rock, sand, or objects buried in soil, for example, is based on a clock that measures the time interval since the undisturbed sample was last exposed to heat or sunlight. Time is proportional to the absorbed dose of ionizing radiation received by the sample from natural sources of ionizing radiation during the time interval. When the sample is heated or exposed to a laser, it emits light called thermoluminescence or optically stimulated luminescence, the intensity of which is proportional to the dose and, therefore, the exposure time (assuming the dose rate is constant). Luminescence occurs when electrons that were detached from atoms by the ionizing radiation and became trapped in imperfections in the crystal lattice of the material are given sufficient thermal energy to become free. The potential energy stored in the crystal is emitted as light. The concentration of trapped electrons also can be measured by electron spin resonance (ESR) techniques.

Data obtained by the various methods have been used in the formulation of models of naturally occurring processes (e.g., climatic changes) and in the testing of existing mathematical models and hypotheses. Such models are intended to enable prediction of future changes in the earth's environment.

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