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

The spatial distribution and vertical migration of global fallout (137)Cs were studied in soils from South Patagonia at the austral region of South America in semi-natural and natural environments located between 50-54 degrees S and 68-74 degrees W. The (137)Cs 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 v(s) of (137)Cs in the soil in such polar isotundra climate has been determined to be 0.056 cm y(-1). This value is higher than v(s) determined under polar climate (-0.012 cm y(-1)) and is near to the upper limit of v(s)-values determined in temperate environments from Chile (0.019 cm y(-1)). The median value of the diffusion coefficient D(s) (0.048 cm(2) y(-1)) is similar to D(s) observed in an Antarctic region (0.043 cm(2) y(-1)) and lower than D(s) in temperate regions of Chile (1.24 cm(2) y(-1)). About 35 years after the highest depositions, (137)Cs 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.

Global weapons' fallout 137Cs in soils and transfer to vegetation in south-central Chile.

The contamination and depth distribution of 137Cs in soil due to the fallout from atmospheric weapons' tests were measured at 29 sites in the 9th and 10th administrative regions in Chile located in the 40 degrees latitude in the southern hemisphere. The depth distribution in most of the sites follows no systematic pattern in the upper few centimetres, but below this depth an exponential decline could be deduced. The calculated relaxation depth appears to be a good indicator for estimating the long-term 137Cs distribution in these soil profiles. It ranges from 4.4 +/- 1.9 cm in Palehumults to 8.4 +/- 4.4 and 9.7 +/- 5.1 cm in Hapludands and Psamments, respectively. For these soil types the value for the relaxation depth tends to increase with decreasing clay content and increasing volume of coarse pores. 137Cs activity densities at the selected sites ranged from 450 to 5410 Bq m(-2) and correlate significantly (r = 0.791) with the mean annual rainfall rate of the sampling sites. 137Cs concentration ratios of prairie plants/soil were found to be in the range 0.008-2.3 and could be related to relaxation depths in undisturbed soils.

Fallout radiocesium in an Antarctic region: deposition history, activity densities and vertical transport in soils.

To improve the knowledge about the (137)Cs spatial distribution and vertical migration in soils of the Southern Hemisphere, the total areal activity density and the vertical transport parameters of this radionuclide were measured in an Antarctic region. For this purpose vegetation and incremental soil samples were collected at 21 representative sites located at 4 islands of the South Shetland Archipelago: King George, Robert, Greenwich and Snow (62-63 degrees S and 58-62 degrees W). The total (137)Cs activity density varied considerably from 118 to 662 Bq m(-2) (median 384 Bq m(-2), reference date 1995), with a high percentage of the total activity retained in the vegetation cover (5-98% in moss, 3-20% in lichen and 4-12% in grass). At most sites, the maximum activity density in soil was observed in the top layer from where it decreased continuously. To evaluate the transport parameters of (137)Cs from the activity-depth profiles, the classical convection-diffusion model was used based on the time-course of the annual deposition density of (137)Cs at the studied region. The values for the diffusion coefficient D(s) (median 0.043 cm(2) year(-1)) and the convection velocity v(s) (median -0.012 cm year(-1)) of radiocesium observed under a polar climate are small compared to the transport parameters determined in temperate zones. The data also indicate that at these sites the convectional transport of (137)Cs is almost negligible compared to the transport by diffusion. The high vulnerability of the Antarctic soils to (137)Cs deposition, as a consequence of its very slow transport due to the extreme climatic conditions at these latitudes, has been confirmed.

Long-term decrease of atmospheric test 137Cs in the soil-prairie plant-milk pathway in southern Chile.

The time dependency of nuclear test 137Cs in soil, prairie plants, and milk was observed on pastures of seven dairy farms in the 10th Region, Chile, from 1982 to 1997, without any appreciable deposition of radioactive fallout after 1983. Whereas the 137Cs concentration in the soil decreased at a rate close to that of the radionuclide's physical decay during the whole observation period, the rate of decrease of the 137Cs concentration in the prairie plants and in the milk, having been very rapid between 1982-1990, became slower between 1991-1997. The effective half-lives of the concentration in plants were found to be 5.6 y and 12 y during the first and second observation periods, respectively. Similar half-lives of 5.5 y and 13 y were found for the concentration decline in milk during each period. These data clearly demonstrate a reduction in the long-term decrease of the 137Cs plant uptake, and consequently in the decrease of the 137Cs concentration in milk, resulting from a decline of 137Cs availability for prairie plants in the Hapludand soils over the whole 15-y observation period.