BEHAVIOR OF CS-137 FROM CHERNOBYL FALLOUT IN A SCOTS PINE CANOPY IN SOUTHERN FINLAND

The behaviour of Cs-137 from the Chernobyl nuclear power plant acciden t in the Ukraine on 26 April 1986 was monitored during a 3-year period in the canopy of a young Scots pine (Pinus sylvestris L.) stand in so uthern Finland. Needle samples were collected from the same whorls on 13 sample trees marked at the beginning of the study. The Cs-137 activ ity of throughfall water was determined in summer 1986. The oldest nee dles at the time of the radioactive deposition (formed in 1983) had th e highest Cs-137 activity (20 000 Bq/kg) in June 1986, when the monito ring was started, 58 days after deposition. About 50% lower Cs-137 act ivity was observed in the needles formed in 1984 and 1985, and an even lower activity of 3050 Bq/kg in the needles formed in 1986 following the incident. During the summer of 1986, the Cs-137 activity of the wh ole canopy decreased from 106 000 to 42 500 kBq/ha, corresponding to a weathering half-life of 72 days. Washout by throughfall accounted for 79% of the decrease, while the rest was mainly due to the gradual she dding of the highly contaminated 1983 needles. The needles formed in t he summers of 1987 and 1988 had rather high Cs-137 activities of 2700 and 4800 Bq/kg, respectively. The reduction in the Cs-137 activity of the canopy was considerably less in 1987 and 1988, with weathering hal f-lives of 435 and 242 days, respectively. The total Cs-137 activity o f the canopy was still 13 500 kBq/ha on 20 October 1988, which was 896 days after deposition. An increase in the Cs-137 activity was observe d in the autumns 1987 and 1988, and to a lesser degree in the autumn o f 1986, a time of year when needle potassium concentrations normally t end to increase. It was concluded that the Chernobyl Cs-137 had become mixed with the chemically analogous K, and was recirculated within th e trees together with K.