TRENDS IN CESIUM ACTIVITY CONCENTRATIONS IN MILK FROM AGRICULTURAL AND SEMINATURAL ENVIRONMENTS AFTER NUCLEAR FALLOUT

The radiocaesium contamination of milk and milk products is directly r elated to that in grass or hay and therefore the time trend to the eff ective 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 ha lf-life during this period (growing season) lies between 5 and 18 days . In upland pastures values of up to 25 days are observed. Studies per formed on a great number of sites in particular countries after the Ch ernobyl accident showed half-lives for Cs-137 in grass from 7.9 +/- 1. 5 d to 10.5 +/- 1.4 d for the period of May to July. An equivalent bio logical half-life for I-131 was observed. Only one measurement of half -lives in winter was performed up to now indicating a substantially lo nger value (50 d). No reliable data on effective half-lives at other p eriods of the year (late summer, fall) are available and would require further research. The long-term decline is determined by soil propert ies. Soils with low fixation capacity and low pH show higher aggregate d transfer factors into milk than others. In certain semi-natural alpi ne pastures these factors remain high for years which cannot be explai ned by extreme soil properties only. Other reasons such as water loggi ng, little dilution due to low plant growth, cycling of radionuclides within living and dead plant biomass and runoff effects have to be con sidered as main causes. A classification system for the long-term tren d in Cs-availability to milk is proposed, but further research on the differencies and possible measures with respect to seasonal variations and climatic conditions is required.