226Ra bioavailability to plants at the Urgeirica uranium mill tailings site

Large amounts of solid wastes (tailings) resulting from the exploitation and treatment of uranium ore at the Urgeirica mine (north of Portugal) have been accumulated in dams (tailing ponds). To reduce the dispersion of natural radionuclides into the environment, some darns were revegetated with eucalyptus (Eucalyptus globolus) and pines (Pinus pinea). Besides these plants, some shrubs (Cytisus spp.) are growing in some of the dams. The objective of this study is to determine the Ra-226 bioavailability from uranium mill tailings by quantifying the total and available fraction of radium in the tailings and to estimate its transfer to plants growing on the tailing piles. Plant and tailing samples were randomly collected and the activity concentration of Ra-226 in plants (aerial part and roots) and tailings was measured by gamma-spectrometry. The exchangeable fraction of radium in tailings was quantified using one single step extraction with 1 mol dm(-3) ammonium acetate (pH = 7) or 1 mol dm(-3) calcium chloride solutions. The results obtained for Ra-226 uptake by plants show that Ra-226 concentration ratios for eucalyptus and pines decrease at low Ra-226 concentrations in the tailings and appear relatively constant at higher radium concentrations. For shrubs, the concentration ratios increase at higher Ra-226 solid waste concentrations approaching a saturation value. Percentage values of 16.0 +/- 8.3 and 12.9 +/- 8.9, for the fraction of radium extracted from the tailings, using 1 mol dm(-3) ammonium acetate or calcium chloride solutions, respectively, were obtained. The Ra-226 concentration ratios determined on the basis of exchangeable radium are one order of magnitude higher than those based on total radium. It can be concluded that, at a 95% confidence Level, more consistent Ra-226 concentration ratios were obtained when calculated an the basis of available radium than when total radium was considered, for all the dams. (C) 2001 Elsevier Science Ltd. All rights reserved.