284 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



function of the source characteristics. However, very little is knovv'n about predicting 

 weathering. For fallout previous hterature has indicated that this half-life is between 30 

 and 45 days. However, results of '^^Cs and transuranic air concentration measurements 

 at resuspension study sites at Hanford reported here indicate a weathering half-life of 

 tVom 5 months (Sehmel, 1977c) to 1 yr or greater. Knowledge of this ill -de fined half-life 

 is important in resuspension modeling efforts to describe airborne effects of surface 

 contamination. Changes in surface contamination availability with time must be known if 

 models are to predict airborne concentrations. This range of from 5 months to 1 yr or 

 greater for a weathering half-Ufe must be considered when evaluating resuspension 

 changes with time. 



Transuranic-element resuspension rates have not been directly measured at surface 

 contaminated sites other than inferred from nonvalidated models since published 

 characteristics of the contaminated surface sources are not adequate for direct 

 measurement of particle resuspension rates. Consequently resuspension rates were 

 measured with controlled tracer-particle simulants using a uniform surface contamination 

 source. On the basis of those measurements, the following conclusions were reached. 



Particle resuspension rates are a function of at least wind speed and mechanical 

 disturbances. Mechanical disturbances, such as vehicular traffic or a man walking, can 

 cause high local resuspension rates. In comparison, average wind resuspension rates from a 

 local area could be less important per unit area than local mechanical-disturbance 

 resuspension. However, wind-caused resuspension rates apply to the entire contaminated 

 area. In the comparison of relative resuspension from wind-caused and mechanical 

 disturbances, one would need to know the total surface contamination area for wind 

 resuspension vs. small localized surface contamination levels for mechanical-disturbance 

 resuspension rates. Botli mechanisms, however, do resuspend and transport potentially 

 hazardous respirable particles. 



Resuspension rates for respirable and nonrespirable particles are needed for inclusion 

 as source terms in atmospheric diffusion and transport equations; however, model 

 predictions are no better than the uncertainty in the source data. In the case of 

 resuspension rates, uncertainties are very large. Much research is yet needed to develop 

 resuspension models to predict particle resuspension rates for any situation. 



Wind-caused resuspension rates from a sparsely vegetated area have only been directly 

 measured with submicrometer tracer particles and estimated for tracer particles larger 

 than 1 idm (Healy and Fuquay, 1958; 1959). The potential etTects of different particle 

 diameters and chemical properties on resuspension rates are unknown. It miglit be 

 hypothesized that similar results would be expected for other submicrometer particles of 

 interest since submicrometer particles are probably attached to host soil particles when 

 particles are resuspended. If the particles of interest were much larger, it is unknown 

 whether the particles would be resuspended attached to host soil particles or resuspended 

 as discrete parficles. 



The change in airborne concentration of a pollutant as a function of time is often 

 attributed to a weathering half-life, the fixation of the pollutant particle into the 

 ground-surface soil. In contrast, weathering half-lives for respirable tracer particles are 

 now estimated here as being on the order of years. Predictions using weathering half-lives 

 of months vs. years could have a significant implication in environmental hazards 

 evaluations. At tlie present time, credit for decreased airborne radioactivity from 

 resuspension could be attributed to a weathering halt~-life of months. If a weathering 

 half-life of years were applicable for transuranic elements, the potential downwind 



