Experimental and modeling methods to study radon transport processes in porous media

Abstract

Radon is a naturally occurring radioactive gas that is produced in the Earth s crust as a result of alpha-decay of radium and is free to migrate through soil, either by molecular diffusion or by advection, and be released to the atmosphere where his behavior and distribution are mainly governed by meteorological processes. Advec-tion, in particular, may transfer radon over a wide range of distances, depending on the porosity and on the velocity of the carrier fluid. Due to its unique properties, soil gas radon has been established as a powerful tracer used for a variety of purposes, such as exploring uranium ores, locating geothermal resources and hydrocarbon deposits, mapping geological faults, predicting seismic activity or volcanic eruptions and testing atmospheric transport models. Much attention has also been paid to the health radiological hazard due to increased radon concentrations in the living and working environment. In order to exploit radon pro?les for geophysical purposes and also to predict its entry indoors, it is necessary to study its transport through porous soils. The complexity generated by the presence of a great number of uncontrollable and varying parameters and processes affecting the generation of radon in the soil grains and its transport in the source medium (pore-water distribution, permeability, porosity, radium content, radon emanation coefficient, advection, ), has led to many theoretical and/or laboratory studies. To measure these quantities in situ, in fact, it is not only hard, it is almost impossible to keep them constant during an experiment. Moreover, soil, as it is found in situ, is inhomogeneous due to mixing of different layers by geological and/or human activity, and it is influenced by flora and fauna presence, etc. The complexity is even larger if one considers, for example, that radon diffusion is mainly governed by porosity but not strongly influenced by pore size or pore size distribution. For these reasons, laboratory measurements are preferred, allowing for experiments to be conducted under well-speci?ed and controlled conditions. This approach constitutes the main basis for the study presented in this thesis. Therefore, a laboratory facility was built consisting of a large cylindrical vessel, homogeneously filled with different materials, with inserted sleeves that allow measurements of radon concentra-tions in the sample gas at various depths under the sample column surface. The vessel can be closed with a stainless steel cover, the space under the cover above the sample simulating a crawl space, and, in addition, a nearly homogeneous air-flow pattern can be induced in the column by means of two inlets at the bottom of the vessel. The results of the laboratory measurements are compared with expected concentrations, according to a transport model developed by C.E. Andersen (Risø National Laboratory, Denmark) and suitably adapted for our purposes. The main goal of the present study is to better understand how some parameters could affect radon transport in porous media, through both in situ and laboratory measurements.