Accumulations and sources of uranium, of its daughters and of metallic trace elements in wetlands located around old uranium mining sites

Uranium mining and uranium ore processing increase the environmental activity of U and Th decay products and trace elements, in particular in case of releases to the adjacent rivers. Contaminants accumulate then preferentially in sedimentation areas (such as ponds or lakes) or in wetlands (peat lands, marshes or riverbanks) located downstream to the mine. Wetlands - generally located at the head of watershed - are particularly sensitive to environmental changes and anthropogenic pressure. This poses a risk of release of contaminants from these accumulation areas. The objective of the present study is to propose an easily reproducible methodology - in particular for the orphan mining sites - to identify and characterize accumulation areas. This study also aims to improve our understanding of the mechanisms of accumulation and release, in these areas. This study was performed around the former mining site of Bertholene (France). Standing and mobile in situ gamma spectrometry is used to accurately locate the accumulation areas. Soils, sediments, vegetation, water and peat are also sampled upstream and downstream of the mine, in order to (a) characterize the activities and the disequilibria of the U-Th decay chains and the associated trace elements according to the scale of observation, (b) understand the mechanisms of accumulation and release and (c) identify the potential sources using geochemical proxies and isotopic analyses. The results obtained show that radionuclides are mainly accumulated in a flooding area located downstream the mine. Strong U-238 activities (≥ 20000 Bq.kg^-1) and strong Ra-226/U-238 and Th-230/Ra-226 activity ratios are recorded, involving preferential inputs of U-238 and Th-230 during flooding events. Trace element contamination is low, except for Mn, Ba and S. Such contaminations are potentially explained by the geochemical composition of the uranium ore and by the past and current processes of ore and water mine. Sequential extractions highlight the importance of the acid-soluble and the amorphous oxides in the trapping of contaminants. Thus, pH or Eh potential changes could induce major releases of uranium to the environment. The large differences in Pb isotope composition between natural soils and uranium ores, as well the activity ratios of the U-238 and decay chain products, make these indicators helpful to identify the soils and the sediments impacted by uranium mining. (author)