Forest (re)vegetation impact on radium-226 cycling in U-mining area

In revegetated contaminated area such as former U-mining sites, the long-term vegetation impact on the radioactivity dissemination is still few documented. Ca is required in large amounts by trees with possible effect on 226Ra uptake. In this study, we compared the Ca and 226Ra redistribution by 37-years Scots pine developped on an afforested U-mining heap and we quantified the recycling dynamics of both elements in terms of annual fluxes. Both elements mainly accumulated in ligneous organs (stem + branches) but results provided further evidence of a preferential retention of 226Ra in stemwood on exchange sites of xylem tissues. The net root uptake of 226Ra amounted to 24 kBq ha-1 y-1, i.e. less than 0.002 % of the NH4-exchangeable 226Ra pool in the soil (0-30 cm). The efficiency of 226Ra uptake by trees was shown to be 400 times less than that of Ca and could be ascribed to the high level of available Ca in the mining debris and the resultant competitive interaction for uptake. The rate of 226Ra redistribution through vegetation was low and largely ruled by soil properties involving a high Ca availability for root uptake which dominates over discrimination effects in favor of 226Ra retention in stemwood.