Vegetation, radioactive deposition

The direct contamination of vegetation by deposition of radioactive material from the atmosphere onto above-ground organs is important under three main circumstances. Firstly, where a release of radionuclides occurs to atmosphere during routine discharges from nuclear installations in most cases such releases are effectively continuous, although spike releases can take place, such as during blow-down when... 

Atkins, D.H.F., Chadwick, R.C. Chamberlain, A.C. (1967) Deposition of radioactive methyl iodide to vegetation. Health Physics, 13, 91. 

The interaction of atmospheric HTO with vegetation and soil has several time scales. After a short release, airborne HTO is deposited to land or sea by vapour diffusion with a vg of about 10 mm s-1 (Garland, 1980). From land, about 75% of the HTO returns to the atmosphere within days or weeks depending on climate. At sea, HTO mixes downwards and only a small fraction re-enters the atmosphere within the radioactive life of tritium. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

An important parameter used to quantify dry deposition processes is the velocity of deposition (Kg) (Chamberlain, 1953). Vg is defined as the downward flux of aerosol or gas to a vegetation or soil surface, normalised to the ambient atmospheric gas or aerosol concentration above that surface. In the case of radionuclide deposition processes flux and concentration are, respectively, measured in units of radioactivity per unit area and volume, hence... 

Aside from anoxic or suboxic basins, the other marine environment suitable for radioactive geochronometry is salt-marsh deposits. As sea level has risen over the past 100 years, salt marshes have kept up by vertical growth of a vegetated framework that supports sediment accumulation. In addition, since high salt marshes are inundated by seawater only —5% of the year, the surface becomes an accumulator of atmospherically derived species including °Pb. The radioactive decay of °Pb can then be used to determine the age of levels in the salt marsh and thereby the accumulation rate of the salt marsh and its components. Since the salt-marsh vertical growth depends on the rise in sea level, the °Pb chronometer becomes a proxy for the rate of rise... 

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