Radiological risk and impact on soil microbial diversity of radionuclides in agricultural topsoils downstream of a decommissioned hydrometallurgical uranium plant

Containing only low levels of U-bearing minerals, U ores often have to undergo hydrometallurgical processing for the separation of other minerals. Hydrometallurgical operations, even after being shut down, could pose radiological risk to the ecosystem and human health due to the radionuclide contamination of surrounding environmental media. This study investigated the contamination of radionuclides in the agricultural topsoils downstream of a decommissioned hydrometallurgical U plant in southern China, and assessed the corresponding radiological risk and evaluated its impact on soil microbial communities. The values of geoaccumulation index and potential ecological risk index indicate that all soil samples were significantly contaminated with U and ²²⁶Ra, with their concentrations being 4.4-28.7 times and 4.4-114.8 times higher than the corresponding regional background values, respectively. The mean outdoor annual effective dose (OAED) in the sampling plot next to the drainage ditch downstream of the decommissioned plant was up to 3.9 and 8.2 times higher than the Chinese annual effective dose limit and global average, respectively, which is indicative of unacceptable radiological risk for the local farm workers. Soil microbial composition was obviously impacted by the soil physicochemical properties and radionuclides. Specifically, Cladophialophora, which belongs to the fungal genus, exhibited significantly positive correlations with the contents of total Cd, total U, organic U, residual U, and total K, while Methanosarcina, which belongs to the archaeal genus, exhibited significantly positive correlations with the contents of ²²⁶Ra and residual U. Soil pH and total N content were significantly correlated with the abundance of several bacterial genera and the dominant archaeal genus (i.e., Candidatus Nitrocosmicus). These findings demonstrate the existence of potentially significant radiological risk associated with the radionuclides released from historical hydrometallurgical processing of U ores to the surrounding environment, and the need for proper site management and remediation.