Radon, radioactive

CAS RN: 10043-92-2

Environmental Fate

TERRESTRIAL FATE: Radon occurs naturally from the uranium-238 decay series present in rocks and soil of the earth(1-3). Radon-222 is a gas that is released normally from the soil since before the Earth took its current shape(1). Radon-222 and radon-220 are the gaseous radioactive products of the decay of radium-226 and radium-228, respectively, which are present in all terrestrial materials(2). The largest share of normal background radiation is contributed by radon (54% radon, 27% other natural radiation)(3). It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in minute amounts to the atmosphere(4).
TERRESTRIAL FATE: High porosity increases the diffusion rate. The release rate from a material depends also on its moisture content: if the moisture content is very low, the radon release is decreased by the effect of re-adsorption of radon atoms on surfaces in the pores. If the moisture content increases slightly, the radon release increases up to a certain moisture content, above which the release of radon decreasing again owing to a decreasing diffusion rate in water filled pores.
TERRESTRIAL FATE: The mechanism of radon release from rock, soil, and other materials is not very well understood and is probably not always the same. The main physical phenomena are recoil and diffusion of the radon atom through imperfections of the crystalline structures of the radium bearing particle followed by a secondary diffusion, which depends on the porosity of the material.
AQUATIC FATE: Radon is soluble in water(1). The concentrations of radon in water may range over several orders of magnitude with the highest usually in well water, intermediate in groundwater and less in surface water(2). Radon dissolved in water may enter indoor air via de-emanation when the water is used(2).
AQUATIC FATE: Uranium, thorium, radium, radon, lead, and polonium radionuclide concentrations in ground waters from the Hanford Site indicate that uranium, thorium, and radium are highly sorbed. Relative to radon, these radionuclides are low by factors of 1X10(-3) to 1X10(-6). Uranium sorption is likely due to its reduction from the hexavalent state, where it is introduced via surface waters, to the tetravalent state found in the confined aquifers. The distribution of radionuclides is very similar in all of the confined aquifiers and significantly different from the distribution observed in the unconfined and surface waters.
ATMOSPHERIC FATE: Radon-222 exists everywhere in the atmosphere and varies widely at the global level with high concentrations noted particularly in areas where terrestrial gamma radiation is high(1). Although radon is a gas, its decay products are not, and they occur either as unattached ions or atoms, condensation nuclei (approx 0.002 um) or attached to particles(2).
ATMOSPHERIC FATE: The diffusion rate and thereby the exhalation rate is influenced by meteorological factors such as rainfall, snowfall, freezing, and variations in atmospheric pressure. An increase in these parameters will decrease the exhalation rate. Measured values of radon exhalation rate from soil vary between about 0.0002 and 0.07 Bq/sq m/sec.
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