Long-term study (1987-2023) on the distribution of 137Cs in soil following the Chernobyl nuclear accident: a comparison of temporal migration measurements and compartment model predictions

Abstract

After the Chernobyl accident, a designated area of ~1000 m2 within the University farm of Aristotle University of Thessaloniki in Northern Greece was utilized as a test ground for radioecological measurements. The profile of 137Cs in the soil was monitored from 1987 to 2023, with soil samples collected in 5-cm-thick slices (layers) down to a depth of 30 cm. The mean total deposition of 137Cs in the area, backdated to the time of the Chernobyl accident, was determined to be 18.6 ± 1.8 kBq m-2 based on four follow-up profile measurements of 137Cs in the soil for the years 2022 and 2023. It is noteworthy that this value is similar the total deposition at the site, which was independently measured to be about 20 kBq m-2 during the first year after the Chernobyl accident. The fractional contribution of each soil layer (e.g., 0-5 cm, 5-10 cm, 10-15 cm, etc.) to the total deposition of 137Cs (0-30 cm) is presented and analyzed. A compartment model was utilized to forecast the temporal evolution of fractional contributions of the different soil layers to the total deposition of 137Cs (0-30 cm). In this model, each soil layer is represented as a separate compartment. The model assumes that the transfer rates between adjacent compartments are equal. The agreement between the measured fractional contributions and the model predictions suggests that the compartment model with equal transfer rates can capture the broad patterns of 137Cs migration within the soil layers over the long period of 1987-2023. However, the use of a second compartment model with increasing transfer rates between consecutive soil layers did not align with the observed outcomes. This indicates that diffusion may not be the primary migration mechanism over the 36-y period covered by our study.

Figure 1

¹³⁷ Cs mass activity (Bq/kg) in different soil layers measured in 1987 and 2023.

Figure 2

¹³⁷ Cs mass activity (Bq/kg) in different soil layers measured in 1987 and 2023. The results are backdated at the time of Chernobyl accident.

Figure 3

Schematic representation of the compartment model. R_5, R_10, R_15, R₂₀, R_25, R₃₀ are defined by Equation (1) and k are the transfer rates in y⁻¹ between the compartments.

Figure 4

Time dependence of the R₅ ratio. The dashed line is the best exponential function passing through the experimental values, taking into account that at the time of Chernobyl accident (t = 0) R₅ = 0.71.

Figure 5

Time dependence of the R₁₀ ratio. In dashed line are the results of Equation (10).

Figure 6

Time dependence of the R₁₅ ratio. In dashed line are the results of Equation (11).

Figure 7

Time dependence of the R₂₀ ratio. In dashed line are the results of Equation (12).

Figure 8

Time dependence of the R₂₅ ratio. In dashed line are the results of Equation (13).

Figure 9

Time dependence of the R₃₀ ratio. In dashed line are the results of Equation (14).

Figure 10

The schematic representation of the modified compartment model. k₁, k₂ are the transfer rates in y⁻¹ between the compartments.

Figure 11

Time dependence of the R₁₀ ratio. In dashed line are the results of Equation (10) with k = 0.024 y⁻¹. In straight lines are the results of Equation (18) with different k₂ values 4%, 25% and 46% greater than k₁ = k = 0.024 y⁻.