Experimental and theoretical study of radon distribution in soil

Radon concentration as a function of the soil depth (0-2.6 m) was measured during the years 2002-2003 and 2003-2004 on the Aristotle University campus. Radium distribution in soil was found constant. On the contrary, as expected, radon concentration increased with soil depth. However, the radon concentration did not follow the well known monotonous increase, which levels off to a saturation value. In both radon distributions, radon concentration increased up to a soil depth of about 80 cm, seemed to remain constant at depths of 80-130 cm, and then increased again. The experimental distribution was reproduced by solving the general transport equation (diffusion and advection). The main finding of the numerical investigation is that the aforementioned, experimentally observed, profile of radon concentration can be explained theoretically by the existence of two soil layers with different diffusion-advection characteristics. Soil sample analysis verified the existence of two different soil layers. Different boundary conditions of the radon concentration at the soil surface were used for the solution of the diffusion-advection equation. It was found that the calculated radon concentration in the soil is, away from the soil surface, the same for the two boundary conditions used. However, from the (frequently used) boundary condition of zero radon concentration at the soil surface, the experimental profile of the radon concentration at the soil surface cannot be deduced. On the contrary, with more appropriate boundary conditions the radon concentration at the soil surface could be deduced from the experimental profile. The equivalent diffusion coefficient could be uncovered from the experimental profile, which can then be used to estimate the radon current, which is important, for example, for the estimation of radon entrance to dwellings.