Resuspension Processes

Because a large amount of radionuclides deposit on the ground as the result of global fallout, resuspension of radionuclides attached to surface soil particles provides another mechanism of deposition on other surfaces. Historically, the concern with resuspension has been with isotopes of plutonium, which have a very long half-life. But, the resuspensions of 90Sr and 137Cs have raised the major concern of current radioactivity in the ecological environment. Resuspension has generally been treated by means of an empirical resuspension factor (RF) defined as the ratio of a resuspended radionuclide concentration in air (Bqm~ ) to the total ground deposition density (i.e., activity per unit area, Bqm~ ) of a radionuclide. Many empirical models for studying resuspension processes have been proposed but do not capture details of the mechanisms because the processes, which depend on the surface conditions (i.e., wet or dry soil, arid or semiarid) and need to take into account the soil particle size and density, air density, surface wind and atmospheric turbulence, are complex. There has not been universal agreement that resuspension is an important pathway, but it is now generally accepted that there are a few situations in which this pathway could be the dominant one. For example, the surface observations of 137Cs/90Sr activity ratios have shown the indication of long-range transport of resus-pended surface dust carrying 90Sr and 137Cs during the dust storm. The approach is to compare the measured concentrations of 90Sr and 137Cs and their activity ratio from the surface dust collected at remote site with the activity ratio and concentrations measured at the local site that is proved to be a receptor to the remote site during the transport. A group of scientists in Japan has used this approach for studying the resuspension of 90Sr and 137Cs attached to surface dust in a dust storm event.

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