The flux of radon and thoron from Australian soils

Abstract

The accumulator technique was used to measure radon and thoron flux density at a variety of locations throughout Australia. This is the first such systematic study of Australia and, in the case of thoron, one of few such studies of any large land mass. Seasonally adjusted arithmetic mean flux densities from Australian soils were estimated to be 22 mBq m−2 s−1 (1.05 atom cm−2 s−1) for radon and 1.7 Bq m−2 s−1 (0.0135 atom cm−2 s−1) for thoron. Consideration of statistical sampling error, and systematic error with the accumulator method, leads to an error estimate of about ±20% for these numbers; projection of total flux to the atmosphere requires consideration of additional sources of error. Only modest correlations with variables easily measured in the field were observed. The strongest correlation was a positive one between flux density and gamma dose rate 1 m above ground. Weaker correlations were seen with soil temperature (positive) and soil moisture (negative at higher moistures). Radon and thoron flux density were strongly correlated, but only a weak correlation (negative) existed between them and vegetation. The amount of radon isotope released to the pore space seems particularly important for controlling the wide variation in observed flux densities, but it remains difficult to predict flux densities based on simple field measurements or information in conventional soil and geological maps.

A map of radon flux at the Australian land surface

“…This instrument was used to collect all of the data listed in Table 1 with the exception of the mainland survey which used a different device, but following the same principle (Schery et al, 1989). In a comparison with eight others at a field site, the radon flux density measured with our instrument was within one standard deviation of the mean and higher by 31% (Hutter and Knutson, 1998).…”

Effect of Soil Moisture Content on Radon and Thoron Exhalation

“…In this paper, we turned our attention to the relation between the exhalation rates and soil moisture content. For example, Schery et al 10) and Koarashi et al 3) reported on the relationship between the radon exhalation rate and the soil moisture content, using the accumulation method for in situ measurement. Megumi and Mamuro 9) and Iimoto 4) reported on the relationship between the thoron exhalation rate and the soil moisture content by way of an indoor experiment and in situ measurement, respectively.…”

“…The results of these studies suggest that regional variation in exhalation rates is mainly controlled by soil characteristics. [5][6][7][8] Much research has been conducted concerning the relationship between the exhalation rates and the soil moisture content, 3,4,9,10) pressure difference in soil air, 11) and effect of rainfall, 12) etc. In this paper, we turned our attention to the relation between the exhalation rates and soil moisture content.…”

Estimation of inhalation dose due to exposure to radon, thoron and their progeny in the indoor environment of Shahjahanpur, Uttar Pradesh, India

“…The flow of radon and thoron impedes into the interior of cemented houses due to the coating of ground surface and walls with the layers of cement in the cemented house. As the soil is an important source of indoor radon 13 , the emanation of radon is also higher from the ground surface of the house.…”