Temporal and small-scale spatial variability of 222Rn gas in a soil with a high gravel content

To quantify the small-scale spatial and long-term temporal variability of the 222Rn concentration in a typical soil with a high gravel content, we monitored this radionuclide every week for 1 year, at 0.5 m and 1.0 m depth at nine sampling positions in a 20×20-m field, and at the four corners of a 1×1-m plot within this field. The data show that the 222Rn soil gas concentrations exhibited a spatial variability which is characterised in the 20×20-m field by coefficients of variation from 20 to 30% at 0.5 m depth, and from 15 to 20% at 1.0 m depth. Within the 1×1-m plot, these values were at both depths only 5–10%. In the winter months, the 222Rn soil gas concentration was higher at 0.5 m depth compared to that at 1.0 m depth. However, in the summer months, the opposite behavior was observed. Time series analysis of the data showed that the 222Rn concentrations in the soil gas determined at a given position and depth is strongly correlated with the preceding observation at this point. In addition, strong cross-correlations are present between the 222Rn concentration time series observed at different positions and depths. The above results are used to calculate the probability for estimating, within a given deviation, the annual mean 222Rn soil gas concentration from a single measurement on an arbitrary day of a given month at a limited number of sampling positions only. Because the 222Rn concentration in the soil gas can vary considerably even within 1 month, 222Rn measurement obtained only once in a given month (especially in January and February) cannot be used to obtain a good estimate of the mean annual radon concentration, even if a large number of samples in the field are taken.