Characterization of subsurface geometry and radioactivity distribution in the trench containing Chernobyl clean-up wastes
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Several radiometric and geophysical methods were applied to characterize the subsurface geometry and radioactivity distribution (especially 137Cs) in a trench with low-level wastes at the Chernobyl site. While surface dose rate measurements and electromagnetic soil conductivity survey produced uninterpretable fields of signals, the ground penetrating radar appeared to be an efficient method for characterization of the subsurface geometry of the waste burial. It was established that the trench had the following dimensions: the length was ≈70 m; average width ≈6–8 m, and depth ≈2–2.5 m. Data on 137Cs distribution in the trench were obtained by means of a borehole gamma-logging technique. The total inventory of 137Cs was estimated at 600±200 GBq. Geostatistical analysis using the semivariogram function has shown regular spatial correlation patterns for the logarithm-transformed 137Cs activity of waste material. The correlation length along the trench was ≈17 m, while across the trench, and in the vertical direction, it was ≈4 m. The observed correlation patterns supposedly were caused by the method used to dispose the contaminated topsoil: the bulldozing in the direction perpendicular to a trench axis. Obtained results may be useful for selecting a characterization method and for optimization of sampling strategies for similar waste sites.