TEMPORAL AND SMALL-SCALE SPATIAL VARIABILITY OF RN-222 GAS IN A SOIL WITH A HIGH GRAVEL CONTENT

To quantify the small-scale spatial and long-term temporal variability of the Rn-222 concentration in a typical soil with a high gravel cont ent, we monitored this radionuclide every week for 1 year, at 0.5 m an d 1.0 m depth at nine sampling positions in a 20 x 20-m field, and at the four corners of a 1 x l-m plot within this field. The data show th at the Rn-222 soil gas concentrations exhibited a spatial variability which is characterised in the 20 x 20-m field by coefficients of varia tion from 20 to 30% at 0.5 m depth, and from 15 to 20% at 1.0 m depth. Within the 1 x l-m plot, these values were at both depths only 5-10%. In the winter months, the Rn-222 soil gas concentration was higher at 0.5 m depth compared to that at 1.0 m depth. However, in the summer m onths, the opposite behavior was observed. Time series analysis of the data showed that the Rn-222 concentrations in the soil gas determined at a given position and depth is strongly correlated with the precedi ng observation at this point. In addition, strong cross-correlations a re present between the Rn-222 concentration time series observed at di fferent positions and depths. The above results are used to calculate the probability for estimating, within a given deviation, the annual m ean Rn-222 soil gas concentration from a single measurement on an arbi trary day of a given month at a limited number of sampling positions o nly. Because the Rn-222 concentration in the soil gas can vary conside rably even within 1 month, Rn-222 measurement obtained only once in a given month (especially in January and February) can not be used to ob tain a good estimate of the mean annual radon concentration, even if a large number of samples in the field are taken. (C) 1998 Elsevier Sci ence B.V. All rights reserved.