Evolution of pH, organic matter and 226radium/calcium partitioning in U-mining debris following revegetation with pine trees

Natural attenuation processes resulting from the afforestation of some U-waste rock piles have the potential to limit the linkage of radioelements and other trace pollutants, thereby minimizing exposure risks. We determined the evolution of pH and organic matter and compared the 226Ra and Ca extractability in pyrite-containing mining debris which was revegetated 35 years ago with Scots pine. Oxidation of sulphidic minerals remaining in the substrate appeared to dominate over acidification processes due to vegetation inputs and litter decomposition. The accumulation of organic matter in forest floor had a negligible effect on the 226Ra upward recycling compared to the migration losses observed mainly from decarbonatation of the surface mining debris. 226Ra was overall less soluble than Ca in the soil profile but NH4Ac-pH 5 had the capacity to extract a 226Ra fraction of 31.1–41.5%, i.e. at least twice as much as for Ca. In deeper layers, a majority of both Ca and 226Ra were extractable from the same non-specific adsorption pool, which mainly involved carbonate. In the upper acidified layer, the incorporation of organic matter had no effect on 226Ra extractability. A further specific adsorption pool for 226Ra was attributed to the formation of sparingly soluble Fe–Al oxyhydroxides. However, that specific 226Ra-bearing phase was readily dissolved in NH4Ac-pH 5, indicating a relatively reversibility of the precipitation reaction of 226Ra with amorphous oxide. Trees are effective at reducing hydrological release of many pollutants but in the mining debris studied, four decades of pine growth did not significantly promote 226Ra remediation in the soil.