Factors controlling the emanation of radon and thoron in soils of the eastern U.S.A.

Emanation coefficients (proportion of generated Rn atoms that escape from a solid material) have been measured for 222Rn (t12 3.8 days) and 220Rn (thoron, t12, 55 s) on 68 disaggregated soil samples from 12 soil profiles representative of the eastern U.S.A. Average values for sites are 0.20 for 222Rn and 0.16 for 220Rn. Based on distribution of 226Ra among the exchangeable, organic, Fe-oxide, sand, silt and clay fractions of the soils, a multiple regression indicates that the organic-exchangeable fraction, occurring mainly as coatings on grains, has an emanation coefficient for 222Rn of 0.46, and the silt-clay fraction has an emanation coefficient of 0.22. The latter value is verified by experiments. The organic component makes the largest single contribution to Rn in soil gas. Mineral grains have twice the 222Rn emanation as 220Rn, implying that about half the Rn atoms are emanated directly to pore space, and the remainder are freed by track-etching and diffusion over a period of days. ion for 220Rn appears to be dominated by Fe-oxides, the main host for pedogenic 232Th. Surface A- and E-horizons have low emanation coefficients for {Su220}Rn, probably because of a lack of Fe-oxides in these horizons.