THE UPTAKE BY VEGETATION OF CHERNOBYL AND AGED RADIOCESIUM IN UPLAND WEST CUMBRIA

The uptake into vegetation of radiocaesium originating from the Chernobyl accident and from previous sources was compared at two upland sites in west Cumbria during November/December 1989. Both sites were in an area where restrictions are in place on the movement and slaughter of sheep due to high radiocaesium activities, and were known to have received comparatively high levels of deposition from both the 1957 Windscale accident and weapons fallout. The proportion of Chernobyl derived radiocaesium in the total radiocaesium inventory at each site was estimated using a Cs-134:Cs-137 ratio of 0.53 in Chernobyl fallout. Aged radiocaesium, mostly present for over 20 years, accounted for 59 +/- 2.3 and 44 +/- 2.8% (mean +/- SE) of the Cs-137 deposit at the two sites. Initially, after the Chernobyl accident, the transfer of the recently deposited radiocaesium was reported to be greater than of aged deposits. However, four years after the accident, the extent of transfer of Chernobyl radiocasium from the top 4 cm soil to vegetation is now similar to that of the aged radiocasium. The similarity in behaviour of Chernobyl derived and aged radiocaseium suggests that future reductions in radiocaesium levels in vegetation, and therefore in sheep, will be slow. The total deposit, measured per square meter down to bedrock or 40 cm, of aged radiocaesium was less available for plant uptake than that from Chernobyl. This is because a greater proportion of the earlier deposit has migrated further down the soil profile. The movement of Chernobyl radiocaesium down the soil profile and its radioactive decay will be the two major factors which contribute to its decline in vegetation and hence sheep.