Thorium and Uranium distribution in a passive system for mine water treatment

The abandoned Jales mining area is a sulphide-rich deposit (Northeast of Portugal). Acid mine drainage resulted from oxidation of sulphides is treated in a passive system with wetlands. The present work studies the thorium and uranium behaviour in the water and in the fine fractions of wetland soils throughout the passive treatment system. The evaluation of the efficiency of the all system was done determining metals concentration variation in the creek water upstream and downstream of the treated effluent discharge. The results point to higher efficiency to retain Th after summer than after winter. The opposite was found for uranium, which increases significantly in the creek water after summer and, in a lower extent, after winter. Also, Th and U have a tendency to increase in the fine fractions of the wetlands soils after summer, which can be explained by the longer water-soil contact/lower water dynamics. Nevertheless uranium has a much higher tendency to be in solution as revealed by a high concentration in the porewater of wetland soils. A relation of these actinides behaviour with the Fe and Mn distribution is not clear.- Thanks are given to EDM, S.A. for providing access to the water treatment plant, FCT (Portuguese Science and Technology Foundation, UID/Multi/04349/2013, and COMPETE 2020 (Programa Operacional da Competitividade e Internacionalizacao), project ICT (UID/GEO/04683/2013) with reference POCI-01-0145-FEDER-007690 and project Nano-MINENV number 029259