Factors affecting exhalation of radon from a gravelly sandy loam

Measurements of radon exhalation from a gravely sandy loam have been made in a semi‐arid climate by using a combination of closed accumulation, flow‐through accumulation, and 222 Rn and 210 Pb soil profiles. The meteorological factors that most affected the instantaneous value of exhalation of 222 Rn were atmospheric pressure and rain. Effects due to other parameters such as wind or temperature were either comparatively small or undetectable. No evidence was seen for migration of radon from distant (≫10m) sources or for an effect on exhalation due to limited nearby seismic activity. Measurements for 220 Rn indicated its exhalation was also sensitive to pressure variation but to a lesser extent than for 222 Rn. While instantaneous exhalation of 222 Rn could easily vary by a factor of 2 or more, time‐averaged exhalation was found to be close to that expected for pure diffusion. There is thus some indication that the time‐averaged effect of cyclic environmental variables is small for this soil. Comparison with transport equations indicates that it is difficult to explain the observed variation in surface flux density solely in terms of the radon concentration gradient in the top few decimeters of soil. A contribution to transport from direct flow, perhaps through inhomogeneities such as cracks or channels, is one possible explanation.