UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

5.7.2 General Geochemistry
Radon is a colorless, odorless, essentially inert gas. All radon isotopes are radioactive. The
longest-lived isotope of radon is 222 Rn which has a half life (t ½) of 3.8 d. The main health risk is
from inhalation of radon gas and its daughter products which are usually adsorbed on dust in the
air. Detailed descriptions of the geologic controls, migration, and detection of radon have been
included in published proceedings such as Graves (1987), Gesell and Lowder (1980), and
elsewhere. Of the 45 Superfund National Priorities List (NPL) sites considered in
EPA/DOE/NRC (1993), radioactive contamination of air, soil, surface water, and/or groundwater
by 220 Rn and/or 222 Rn has been identified at 23 sites.
Twenty isotopes of radon are known (Weast and Astle, 1980). Environmental radon
contamination typically results from radioactive decay of isotopes in the uranium-thorium series.
These include the formation of:
C
C
C
222 226 238
Rn by alpha decay from Ra in the U decay series
220
Rn (t½=54 sec) by alpha decay from 224 Ra in the 232 Th decay series
219
Rn (t½=3.9 sec) by alpha decay from 223 Ra in the 238 U decay series.
The final, stable daughter products in these 3 decay series are 206 Pb, 208 Pb, and 207 Pb, respectively.
Some noble gases (i.e., krypton, xenon, and radon) have very limited chemical reactivity with
other elements. The chemical reactivity of radon is difficult to assess because of its short half life.
Geologic and hydrogeologic processes that might influence radon mobility are discussed in detail
by Tanner (1980). As an inert gas, radon is not immobilized by precipitation processes along
migration pathways. According to data cited by Tanner (1980), the ratio (i.e., solubility
distribution coefficient) of 222 Rn in a water phase to that in a gas phase ranges from 0.52 at 0 " C to
0.16 at 40 " C. This ratio has been used, for example, for the solubility of radon in water in
mathematical models designed to calculate radon diffusion coefficients in soils (e.g., Nielson et
al., 1984). The solubility of radon in organic liquids is greater than that in water.
5.7.3 Aqueous Speciation
The existence of radon aqueous species was not identified in any of the references reviewed for
this study. Given the inertness of radon and the short half life (t ½=3.8 d) for 222 Rn, aqueous
speciation and complexation of dissolved radon would not be expected to be important.
However, as noted above, radon is soluble in water. The hydrostatic pressure on ground water
below the water table is sufficient to keep dissolved radon in solution. Above the water table, the
radon present in vadose zone pore water will exsolve from solution, enter the vapor phase, and
migrate as part of the air through the open rock and soil pore spaces.
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