Soil Radon Concentration and Permeability at “Valle della Caffarella” Test Site (Roma, Italy). Evaluation of Gas Sampling Techniques and Radon Measurements Using Different Approaches

Soil gas radon concentration at “Valle della Caffarella” test site (Roma, Italy) was monitored in December 2009 and June 2010, using three different gas sampling and measurement techniques (alpha spectrometry, scintillation cells and ionization chambers). Gas permeability of soil was associated to radon measurements to evaluate its effects on soil radon results. During the winter campaign, the three methods provided comparable results at depths of 50 and 80 cm (stations in areas A and B), where the gas permeability is low. Stations in area C apparently do not respond to this simple model. Determinations carried out at depths of 20 and 30 cm are generally 60 % lower using the continuous radon monitor (alpha spectrometry device), because larger volumes of air are extracted using the built-in pump. The proximity to soil/air interface probably drains atmospheric air into the subsoil and radon-poor air is then delivered to the instrument. This effect is enhanced by larger gas permeability. Results from the summer campaign substantially mirror those of the winter fieldwork, even if soil radon concentrations are strongly reduced both at 30 and 80 cm depth because of enhanced radon release to the atmosphere when soil is drier, warmer and more permeable. The recourse to a mixed approach (Radon v.o.s. probe + RAD7 continuous monitor) shows that the volume of extracted air affects soil radon results less than the combined effect of the probe tip size and the poor sealing of the sampling hole.