How mining unleashes uranium into groundwater—and what controls it
Researchers have identified the specific geochemical conditions that trigger uranium release from mine rocks into drinking water supplies. The findings could help mining operators and regulators design better containment strategies to prevent contamination in active and abandoned mining regions worldwide.
Originaltitel: Geochemical controls on uranium mobilization from open-pit wall rocks under environmentally relevant conditions: elemental and mineralogical constraints
<p>Uranium (U) is a chemically toxic contaminant, and elevated concentrations in groundwater pose environmental and public health concerns in mining-impacted regions. U mobilization from mine-affected bedrock can contribute to groundwater contamination, yet the geochemical controls governing U release and transport under environmentally relevant conditions remain incompletely understood. This study investigates the roles of pH, complexing ligands, and mineralogy on U mobilization from pegmatite and trachyandesite rocks collected from Leveäniemi open pit, an active iron ore mine in Northern Sweden. Elevated U concentrations have been detected in groundwater entering the open pit through rock fractures. A systematic experimental approach combining batch leaching and dynamic flow through experiments was applied across a range of geochemical conditions, including varying acid concentrations, pH (acidic to alkaline), and ligand concentrations representative of groundwater (SO₄²⁻, NO₃⁻, Cl⁻, and HCO₃⁻) at environmentally relevant concentrations. The results demonstrate that U mineral dissolution and U mobilization are governed by distinct geochemical controls. Significant dissolution occurs under acidic and strongly alkaline conditions, with uraninite identified as the primary reactive U-bearing mineral, whereas other U-bearing minerals are comparatively resistant. Under neutral pH conditions representative of groundwater, U release is limited and controlled by surface-mediated processes rather than bulk mineral dissolution. However, the carbonate ligand promotes the formation of stable uranyl-carbonate and Ca-uranyl-carbonate complexes, enhancing U solubility and transport. These findings show that even limited mineral reactivity can sustain dissolved U concentrations over time and that groundwater composition plays a critical role in controlling U mobility. This is environmentally significant because such conditions are typical of groundwater systems, indicating that U can remain mobile and contribute to long-term contamination of water resources and downstream aquatic ecosystems. This study distinguishes the geochemical conditions that dissolve U-bearing minerals from those that sustain dissolved U transport under environmentally relevant groundwater conditions, providing new insight into mechanisms governing U transport in mine-impacted groundwater and supporting improved prediction, monitoring, and mitigation of U contamination of downstream recipients.</p>