The contamination of aquatic environments by uranium (U) is a global concern due to its chemical toxicity and radioactivity, both of which pose significant health risks. Although U occurs naturally in the earth’s crust and is commonly present at low concentrations in natural waters, elevated levels in the environment can result from both natural and human-induced sources. While previous research has focused primarily on U release from U mining and processing sites, less attention has been given to U behavior at non-U mining sites such as iron ore mines, where U may still be present as a trace element. Under certain conditions, U can become mobilized posing a contamination risk to recipient rivers and lakes downstream. This study investigated U sources and mobility at the LKAB iron ore mine in Svappavaara, Northern Sweden, where U concentrations exceeding the Swedish annual average guideline value of 0.17 µg/L have been detected in a river receiving excess process water from the mine site. 

The results show that the iron ore fed into the processing plant is not a significant contributor to U in the process water circulating in the mine’s water management system. This is due to the iron ore’s low U content and the occurrence of U in thorite, a mineral that remains stable under the alkaline pH conditions prevailing in the processing plant. However, the ore also contains gypsum and anhydrite which dissolve after being liberated during grinding releasing calcium (Ca) and sulphate (SO₄^2-). The presence of Ca promotes the formation of ternary calcium-uranyl-carbonate complexes, which increase U solubility and reduce its tendency to adsorb onto mineral surfaces. As a result, U mobility is enhanced within the water management system and toward the recipient river downstream. 

Mine water pumped from Leveäniemi open pit was identified as the primary source of U at the mine site. This U originates from both groundwater infiltration into the open pit and weathering of U-bearing minerals in the open pit walls. Minewall weathering stations revealed that among the different rock types present in the open pit, pegmatite had the highest U leaching rates per unit area. However, trachyandesite, the dominant rock type in both the hanging and footwall, exhibited lower U leaching rates but likely contributes more U overall due to its abundance. Groundwater entering the open pit through fractures and the drainage pipes was also identified as a key U source influencing U concentration in the mine water. However, tracing U back to specific rock sources is challenging due to evolving groundwater flow paths as mining progresses and limited access to rocks along these flow paths. As a result, elemental and isotopic tracers are needed to better understand U release from bedrock into circulating groundwater. To support this, further investigation into U release from pegmatite and trachyandesite rock under varying geochemical conditions was carried out. 

Acidic environments significantly enhanced U release, particularly from uraninite compared to more refractory minerals such as fergusonite, and thorite. Under neutral pH conditions, mineral dissolution was limited, and U was primarily mobilized through surface desorption and complexation with dissolved ligands. Carbonate was the most effective ligand in promoting U mobility under these conditions. Elemental correlations suggested that trace elements such as Pb, Th, Nb, and Y could serve as indicators of pegmatite-derived U, as they are commonly associated with the primary U-bearing minerals in pegmatite. In contrast, U leached from trachyandesite showed similar leaching behavior to elements typically found in silicate minerals such as Fe, Al, Si, Mg, and Mn. These elements are not effective tracers for trachyandesite-derived U because they are ubiquitous in silicate minerals, some of which may dissolve under the same geochemical conditions, making it difficult to attribute their presence to U-bearing phases. U isotope analyses added a valuable dimension to the study revealing differences in source behavior not captured by elemental concentrations alone. While δ²³⁸U overlapped between the rock types, ²³⁴U/²³⁸U activity ratios varied distinctly. The lack of overlap in ²³⁴U/²³⁸U activity ratios between pegmatite and trachyandesite leachates under neutral pH conditions both in NaHCO3 and groundwater solutions suggests that these ratios are more suitable than δ²³⁸U for distinguishing between U rock sources in groundwater at the mine site.

These findings highlight the importance of understanding both the sources and geochemical controls on U mobility in non-U mining environments. This knowledge is essential for implementing effective source-specific remediation strategies and improving water management practices to minimize U transport to downstream ecosystems.