Global fallout 137Cs accumulation and vertical migration in selected soils from South Patagonia

The spatial distribution and vertical migration of global fallout 137Cs were studied in soils from South Patagonia at the austral region of South America in semi-natural and natural environments located between 50–54° S and 68–74° W. The 137Cs areal activity density varied from 222 to 858 Bq m−2, and was found to be significantly positively correlated (p<0.001) with the mean annual precipitation rate. The fraction of the total activity density observed in steppe grass varied from <0.03% to 0.12% (median <0.07%) and is considerably lower than the results obtained at the South Shetland Islands (median 8%) and in other temperate environments in south-central Chile (median 0.2%). The median of the convection velocity vs of 137Cs in the soil in such polar isotundra climate has been determined to be 0.056 cm y−1. This value is higher than vs determined under polar climate (−0.012 cm y−1) and is near to the upper limit of vs-values determined in temperate environments from Chile (0.019 cm y−1). The median value of the diffusion coefficient Ds (0.048 cm2 y−1) is similar to Ds observed in an Antarctic region (0.043 cm2 y−1) and lower than Ds in temperate regions of Chile (1.24 cm2 y−1). About 35 years after the highest depositions, 137Cs had penetrated to a depth of 6–14 cm in the Patagonian soils and can be expected to remain in the rooting zone of grass for many decades. Nevertheless, because of its low transfer to steppe grass observed at this region, the radioecological sensitivity of this ecosystem with respect to fallout radiocesium seems to be lower than in other polar regions.