Naturally occurring radioactive material and risk assessment of tailings of polymetallic and Ra/U mines from legacy sites

Old mine tailings from Northern and Central Portugal were studied in order to perform a radiological and chemical characterization. The evaluation of massic activity of natural radionuclides and concentrations in tailings of polymetallic and Ra/U mines was performed by gamma spectrometry and neutron activation analysis. Iron speciation was carried out by Mossbauer spectroscopy. In polymetallic tailings with physical ore processing (Cumieira and Verdes - exploited for Sn, Nb-Ta) higher contents of Th, Ra-228 and Ra-226 in the coarser materials occur, probably due to their presence in host rock and ore fragments. In finer tailings, washing may explain the lower Ra-226 and Pb-210 massic activity. In tailings with physical/chemical ore processing (Covas - exploited for W and Sn) high U contents and a tendency for higher Ra-226 and Pb-210 massic activity in the fine materials is observed, probably due to their incorporation in nanosized particles of iron oxides. A high variation of the Pb-210/Ra-226 Ra ratio occurs in polymetallic tailings; a deficit of Pb-210 can be observed particularly in deposits of settling tanks drained from dumps of chemically treated ore. In Ervideira-Mestras tailings (Ra/U exploitation) where no ore process in situ was performed, a near equilibrium between Pb-210 and Ra-226 occurs. Dose risk assessment was carried out by calculating external outdoor Annual Effective Dose Rate; the dose rates in air due to terrestrial gamma radiation are low for the polymetallic tailings (<47 nGy/h), and higher for tailings of Rail) (up to 4130 nGy/h), in the worst scenario. (C) 2019 Elsevier Ltd. All rights reserved.