go! INTRODUCTION Radioactive particles deposited on natural or man-made surface can be resuspended by wind and mechanical activity. Wind resuspension may occur over a wide area as well as a local area. In contrast, mechanical activity resuspension is usually more localized and may present an immediate inhalation problem to the worker in a contaminated zone. In both wide-area and local resuspension of radioactive particles, particles are transported downwind and in sufficient concentration could become a potential radiological hazard to man. Sources for resuspended particles include radioactive fallout as well as releases from nuclear facilities. Radioactive fallout is a worldwide phenomenon in which stratospheric debris ts deposited on environmental surfaces. Subsequently, deposited falTtout particles can be resuspended and transported downwind. At present, the significance of fallout resuspension is unknown. Data are needed to define the relative inhalation hazard between fallout particle resuspension and the direct delivery of stratospheric debris. Currently, fallout resuspension is assumed to contribute little radiological hazard to man. Radioactive particle resuspension is probably more important at nuclear facilities near which the environment has been contaminated with radioactive particles. These particles can be resuspended by both wind stresses and mechanical disturbances and trans- ported downwind. However, resuspensfon mechanisms are poorly understood and, consequently, resuspension rates and potential airborne inhalation hazards cannot now be adequately. predicted. The need for such predictions are not new: resuspension has been known for many years to be occurring at nuclear sites. Wilson et al. (1961) and Anspaugh et_al. (1969) obtained some of the earlfest of such data at the Nevada test site. Ground radioactivity contours were determined as a function of time after a test detonation, The contours indicated that deposited radioactive particles must have been resuspended by prevailing winds and that ground surface concentrations decreased with increasing distance from the test center during the initial time period after the test (Anspaugh et_a1., 1969). Subsequent ground radioactivity contours showed a migration of radioactivity away from the test site, indicating that resuspension had occurred. More recently, resuspension has been of considerable interest at the Rocky Flats nuclear plant in Colorado where ground surfaces were contaminated with plutonium from leaking storage barrels containing plutonium-contaminated cutting oi] (Sehmel and Lloyd, 1976a; Krey et _al., 1973; Krey et al., 1976; Volchok et al., 1972; Johnson et al., 1976}. After the leakage was discovered, the barrels were removed and corrective actions were taken, but plutonium resuspension from residually contaminated soil surfaces is still occurring. Although environmental plutonium resuspension is receiving attention, resuspension physics is poorly understood. Resuspension is frequently characterized by a "resuspension factor". The resuspension factor is defined as the ratio of airborne pollutant concentration (amount/m?) at breathing height divided by the ground surface contamination level (amount/m¢). Thus, the resuspension factor has units of m-}. Reported resuspension factors vary many orders of magnitude with values from 10-'! up to 600 m-! (Stewart, 1967, Mishima, 1964; Sehmel and Lloyd, 1976b). Resuspension factor variations have not been adequately explained as a function of experimental conditions. Resuspension factors from about 10-9 to 10-5 m-! are often used in hazard evaluations. The resuspension factor is useful since a worker's inhalation hazard is most likely related to the local resuspension caused by his work activities within a contaminated zone; however, resuspension factors are only a very rough estimate of the potential airborne contaminant concentra- tion since resuspension factors cannot be accurately predicted. In addition to local resuspension, airborne contaminated particles can reach workers from upwind contaminated areas. be considered. Hence, both local and upwind resuspension should The resuspension factor is an index of only the potential inhalation con- centration and not the total resuspension release rate from a surfacecontaminated area. Resuspension release rates are needed for source terms in calculating total downwind diffusion and transport of resuspended particles. It is only recently that particle resuspension rates have been measured (Sehmel and Lloyd, 1976b). OBJECTIVES The objective of this research is to measure particle resuspension rates from environmental surfaces caused by wind and mechanical disturbances. Subsequently, generalized models to predict resuspension rates can be developed and incorporated into downwind transport models (Horst, 1976). The objective of this paper is to summarize reported resuspension rates and parameters that have been determined by Battelle-Northwest between 1971 and early 1976. These include plutonium resuspension measurements at Rocky Flats in July, 1973, resuspended fallout concentrations in the smoke of a forest fire, and measurements using controlled tracer particles. EXPERIMENTS The experiments for measuring particle resuspension reported here have been reported in fuller detail] in the following references: 182 183