TRANSFER OF CS-137 TO COWS MILK - INVESTIGATIONS ON DAIRY FARMS IN SWEDEN

Since 1986, the year of the nuclear accident at Chernobyl, Cs-137 acti vity concentrations in cow's milk on dairy farms were studied in Swede n. Transfer coefficients, F-m, of Cs-137 from pasture and fodder to co w's milk were determined on farms in the counties of Uppsala, Gavlebor g and Vastmanland in central Sweden for one month on winter-fodder, an d for their st month in 1987 and in 1988 on pasture. The average F-m f or all investigations (of 10 farms on winter-fodder and 11 farms on pa sture in 1987 and 4 farms on pasture in 1988) south of Gavle was estim ated to be 0.0055 with a range of 0.0039 to 0.0080. The Cs-137 activit y concentration in milk decreased with time. In summer 1992 and 1993, Cs-137 in milk, on the farms still producing milk, was determined. On these farms, Cs-137 activity concentration in milk was found to be <2- 21 Bqkg(-1). The effective ecological half-life from 1987 was estimate d to be 1.4 +/- 0.5 (sd) years for milk fr om 10 farms with a range of 0.8-2.0 years. One farm where Cs-137 in milk decreased at a slower ra te, or not at all, used semi-natural and uncultivated pasture, forest meadows. On intensely managed farms, where potassium fertilizer was di stributed, ploughing was performed and, in 1986, forage was cut at a h igher stubble-height, the decrease of Cs-137 in milk was observed to b e faster.