Radiological characterisation and ecological risk assessment of an alum shale waste site in Kvarntorp, Sweden

Industrial waste sites containing naturally occurring radioactive material represent long-term and globally relevant environmental challenges, particularly where chemical and radiological stressors affect terrestrial and aquatic ecosystems. The study site is a legacy alum shale mining waste pile and is one of the most contaminated sites in Sweden. Ongoing exothermic reactions within the pile maintain elevated temperatures, creating a challenging and complex environment with evolving contaminant mobility and exposure pathways. As the pile cools, increased water infiltration is expected to increase leaching of heavy metals and radionuclides into the environment. Thus, a comprehensive radiological characterisation and ecological risk assessment was conducted across multiple compartments to evaluate current and future environmental impacts. A total of 68 samples of soil, shale ash, sediment, surface water, groundwater, and plants were analysed for radionuclides from the ²³⁸U and ²³² Th decay series. Ambient dose rates were measured in situ, and radiological risk to humans and non-human biota was assessed using the ERICA Tool. Activity concentrations and dose rates across the surface of the site showed pronounced spatial variability associated with the presence of alum shale residues. Among the analysed radionuclides, ²³⁰Th dominated in soil and shale ash, uranium isotopes in surface water, and ²¹⁰Po in vegetation. Soil-to-plant concentration ratios for uranium, thorium, radium, and polonium were generally low, averaging a few percent across 15 plant species, indicating limited bioavailability despite elevated source term concentrations. Similar ratios were observed for isotopes of the same element, suggesting that gamma spectrometry, when applicable, could provide a cost-effective alternative for large-scale environmental risk assessment. The ecological risk assessment indicated that radiological risk to biota at the waste pile and aquatic systems cannot be considered negligible. Although radiological risk to members of the public from external exposure was low, prolonged daily visits revealed potential doses approaching regulatory reference levels.