Identifying key uranium sources in mine water: Open pit wall rock leaching and groundwater contributions in Leveäniemi open pit

Uranium (U) release from mining is an environmental concern due to U's chemical toxicity and radioactivity. In Sweden, U is classified as a river basin-specific pollutant (RBSP), emphasizing the need to minimize its release from mining activities. This study aims to trace the sources of U contributing to elevated concentrations in mine water from Leveäniemi open pit, Northern Sweden, to inform point-source prevention measures to mitigate U release. The study investigates U leaching rates from key rock types forming the open pit walls and evaluates groundwater entering the open pit through drainage pipes and fractures as a potential source of U. Minewall weathering stations showed higher U leaching rates from pegmatites compared to other rock types. The pegmatite station where uraninite was the predominant mineral had a leaching rate averaging 1800 μg/m 2 /wk, compared to 430 μg/m 2 /wk at the station where pyrochlore was the predominant U mineral and uraninite occurred as inclusions in pyrochlore. However, pegmatites cover a small area of the exposed surface in the open pit compared to trachyandesite, which leached at a lower average rate of 30 μg/m 2 /wk. Groundwater entering the open pit through fractures and drainage pipes was also identified as a significant source of U in the mine water, further influencing mine water U concentrations. Careful handling of pegmatite-containing waste rock is essential to prevent increased U leaching in both the open pit and waste rock dumps. This study highlights the importance of identifying rocks with high U release potential before exposure during mining. Additionally, understanding the distribution of these high U-release rock types along groundwater flow paths can also help to predict groundwater U concentrations and inform site-specific management strategies to mitigate U contamination in downstream recipients. • Study investigates U sources in mine water from Leveäniemi open pit. • Pegmatite leaches U up to 1800 μg/m 2 /week but covers a small area in the open pit. • Trachyandesite covers a larger area but leaches U at a rate of 30 μg/m 2 /week. • Groundwater inflow is a significant U source in Leveäniemi open pit.