Long-term Decrease of Atmospheric Test 137cs in the Soil-prairie Plant-milk Pathway in Southern Chile

The time dependency of nuclear test {sup 137}Cs 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 {sup 137}Cs 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 {sup 137}Cs 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 {sup 137}Cs plant uptake, and consequently in the decrease of the {sup 137}Cs concentration in milk, resulting from a decline of {sup 137}Cs availability for prairie plants in the Hapludand soils over the whole 15-y observation period.