Assessment of spatial distribution of fallout radionuclides through geostatistics concept

After introducing geostatistics concept and its utility in environmental science and especially in Fallout Radionuclide (FRN) spatialisation, a case study for cesium-137 (137Cs) redistribution at the field scale using geostatistics is presented. On a Canadian agricultural field, geostatistics coupled with a Geographic Information System (GIS) was used to test three different techniques of interpolation [Ordinary Kriging (OK), Inverse Distance Weighting power one (IDW1) and two (IDW2)] to create a 137Cs map and to establish a radioisotope budget. Following the optimization of variographic parameters, an experimental semivariogram was developed to determine the spatial dependence of 137Cs. It was adjusted to a spherical isotropic model with a range of 30 m and a very small nugget effect. This 137Cs semivariogram showed a good autocorrelation (R2 = 0.91) and was well structured (‘nugget-to-sill’ ratio of 4%). It also revealed that the sampling strategy was adequate to reveal the spatial correlation of 137Cs. The spatial redistribution of 137Cs was estimated by Ordinary Kriging and IDW to produce contour maps. A radioisotope budget was established for the 2.16 ha agricultural field under investigation. It was estimated that around 2 × 107 Bq of 137Cs were missing (around 30% of the total initial fallout) and were exported by physical processes (runoff and erosion processes) from the area under investigation. The cross-validation analysis showed that in the case of spatially structured data, OK is a better interpolation method than IDW1 or IDW2 for the assessment of potential radioactive contamination and/or pollution.