A new geostatistical tool for the analysis of the geographical variability of the indoor radon activity

The population is continuously exposed to a background level of ionizing radiation due to the natural radioactivity and, in particular, with radon (222Rn). Radon gas has been classifi ed as the second leading cause of lung cancer after tobacco smoke [1]. In the confi ned environment, radon concentration can reach harmful level and vary accordingly to many factors. Since the primary source of radon in dwellings is the subsurface, the risk assessment and reduction cannot disregard the identifi cation of the local geology and the environmental predisposing factors. In this article, we propose a new methodology, based on the computation of the Gini coeffi cients at different spatial scales, to estimate the spatial correlation and the geographical variability of radon concentrations. This variability can be interpreted as a signature of the different subsurface geological conditions. The Gini coeffi cient computation is a statistical tool widely used to determine the degree of inhomogeneity of different kinds of distributions. We generated several simulated radon distributions, and the proposed tool has been validated by comparing the variograms based on the semi-variance computation with those ones based on the Gini coeffi cient. The Gini coeffi cient variogram is shown to be a good estimator of the inhomogeneity degree of radon concentration. Indeed, it allows to better constrain the critical distance below which the radon geological source can be considered as uniform at least for the investigated length scales of variability; it also better discriminates the fl uctuations due to the environmental predisposing factors from those ones due to the random spatially uncorrelated noise.