Trends in caesium activity concentrations in milk from agricultural and semi-natural environments after nuclear fallout

The radiocaesium contamination of milk and milk products is directly related to that in grass or hay and therefore the time trend to the effective half-life in these fodders. In the early phase the half-life in grass predominantly depends on effects such as dilution due to plant growth, translocation and weathering effects. The average effective half-life during this period (growing season) lies between 5 and 18 days. In upland pastures values of up to 25 days are observed. Studies performed on a great number of sites in particular countries after the Chernobyl accident showed half-lives for 137Cs in grass from 7.9±1.5 d to 10.5±1.4 d for the period of May to July. An equivalent biological half-life for 131I was observed. Only one measurement of half-lives in winter was performed up to now indicating a substantially longer value (50 d). No reliable data on effective half-lives at other periods of the year (late summer, fall) are available and would require further research. The long-term decline is determined by soil properties. Soils with low fixation capacity and low pH show higher aggregated transfer factors into milk than others. In certain semi-natural alpine pastures these factors remain high for years which cannot be explained by extreme soil properties only. Other reasons such as water logging, little dilution due to low plant growth, cycling of radionuclides within living and dead plant biomass and runoff effects have to be considered as main causes. A classification system for the long-term trend in Cs-availability to milk is proposed, but further research on the differences and possible measures with respect to seasonal variations and climatic conditions is required. (author)