Answer to Question #10780 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Is the terrestrial gamma radiation from dry soils always higher than from soils with moisture in the pores? Does this hold true for all types of soils? In clay, for example, when all the soil pores are filled with water, the transmission of gamma radiation is attenuated, but there is also radon building up in the top layer of soil. Would the radon increase the gamma radiation from water-filled clay soils?
Generally it is true that when water fills the open spaces in soil, the gamma radiation dose rate is reduced somewhat because of added attenuation of the gamma rays by the water (see, for example, the previous question number 10406 and its answer on the Ask the Expert website). So far as I know, this holds true for all soil types. A possible exception is when radioactive water fills the void space in the soil. This can occur, for example, when radon progeny are washed from the air during a rainstorm.
In such cases some radon and the gamma-emitting radon progeny, particularly 214Pb and 214Bi, are carried to surfaces, including soil, with the falling rain. The rainwater may seep into the ground and temporarily increase the gamma exposure rate from the earth. Radon progeny on the order of 105 Bq per liter of rainwater have been measured at times. If we consider a lower concentration of 214Pb and 214Bi of 104 Bq per liter in rainwater, seeping uniformly into soil that has a 30 percent porosity to a depth of about 2 cm, the resulting added gamma radiation dose rate from the radon progeny at 1 m above a circular area of soil 50 m in diameter would be about 0.2 µSv per hour, which would be in addition to the typical external gamma dose rate in many parts of the world of between about 0.05 and 0.1 µSv per hour. Radon progeny and the parent radon have relatively short half-lives so that such increases in gamma radiation levels are transient, declining as the progeny concentrations decay.
George Chabot, PhD