Long-term migration of 137Cs in Swedish grassland soil profiles following the Chornobyl accident

This study presents long-term findings (1987–2008) on the vertical migration of ¹³⁷Cs from the Chornobyl accident in undisturbed grassland soils in central and northern Sweden. We examined five mineral and three organic soils, with ¹³⁷Cs deposition in 1986 ranging from 16 to 190 kBq m². ¹³⁷Cs activities were measured in 1 cm slices at depths of 0–10 cm and in 2.5 cm slices at 10–50 cm. Distribution (kBq m²) was calculated for different soil horizons, and migration rates were determined based on observed depths. During the initial period after the fallout (1987–1992), ¹³⁷Cs was primarily located in the upper 0–2 cm layers of both mineral and organic soils, comprising 77 % to 94 % of the radionuclide. During the intermediate period (1994–2003), the average migration depth was 4.0 cm in mineral soils and 5.5 cm in the organic soils while during the later period (2004–2008) it was 4.4 and 7.0 cm, respectively. After about 20 years, approximately 80 % of the ¹³⁷Cs activity was found in the upper 0–6 cm at five out of eight sites, and 75–78 % within 0–9 cm at two sites regardless of soil type. The average radionuclide migration rate of ¹³⁷Cs in the period 1987 to 2008 across various sites was 0.31 cm yr−¹. In mineral soils, the average migration rate was 0.28 cm yr−¹ (range 0.13–0.47) and in organic soils it was 0.3 cm yr−¹ (range 0.17–0.76). There was no significant relationship between soil clay content in mineral soils and the average migration depth of the radionuclide during the study period (P = 0.423).