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/m2/wk, compared to 430 μg/m2/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/m2/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.

Uranium (U) release from mining has been typically associated with former U mine sites, but trace U levels in iron or base metal ores can also lead to U mobilization into ground and surface water posing potential risks due to U's chemical toxicity and radioactivity. This study investigates U sources and mobility at an iron ore mine site in Northern Sweden, where U concentrations (median 1.8 μg/l) exceeding the Swedish annual guideline value of 0.17 μg/l have been detected in a river receiving excess process water from the mine site. Drill core samples were characterized to identify the minerals hosting U in the iron ore and sequential extraction tests were conducted on solid samples from the processing plant to assess U mobility potential. Results indicate that, given its low U content, iron ore is not a significant source of the elevated U levels detected in the process water. Thorite, the main U-bearing mineral remains stable under the neutral to alkaline pH conditions in the processing plant. U speciation calculations on process water monitoring data, performed in PHREEQC with the PRODATA thermodynamic database, revealed dominant calcium uranyl carbonate complexes, specifically Ca2UO2(CO3)3 and CaUO2(CO3)32−. Mine water from Leveäniemi and Gruvberget open pits, particularly Leveäniemi, was identified as the main source of U to the process water in the recirculation system. The U in mine water originates from groundwater infiltration into the open pits and leaching of U from the open pit wall rocks. Further investigation of these sources and U's geochemical behavior in mine water before it mixes with process water in the processing plant is crucial for understanding the processes driving elevated downstream U concentrations.

The diel oxygen technique relying on automated in-situ measurements of dissolved oxygen was used as an indicator of seasonal and diurnal variations of photosynthesis in the circumneutral Laver pit lake in northern Sweden. From July to September 2017, surface water temperature, electrical conductivity, pH, and dissolved oxygen were continuously measured at a time resolution of 30 min using sensors mounted on a floating buoy. The data could be monitored in real-time using a browser-based software and permitted calculation of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) in the lake. The dissolved O2 concentration showed a consistent pattern of diel variations that reached up to 0.5 mg L−1 during the warmer summer period (July–August). Towards the end of August these variations decreased in magnitude and remained at ∼0.1 mg L−1 throughout September. pH showed diel variations that mimicked those of dissolved O2, with maximum daily variations of 0.4–0.5 pH units during July and August. A seven-day moving average of GPP showed a peak during July to mid-August, and the maximum GPP value of 0.55 mg O2 L−1 day−1 is similar to those found in natural oligotrophic lakes. A phytoplankton sample showed a total biomass concentration of 24 μg L−1, with the species Chrysophyceae, Chlorophyta, and Bacillariophyta occurring in the water. Diel oxygen data indicated that respiration by autotrophs and respiration of autochthonous labile organic matter by heterotrophs dominated in the lake, as is often the case in lakes where planktonic primary production is the main supplier of labile organic carbon. A close coupling between R and GPP suggests that nearly all GPP was respired in the epilimnion. The study shows that the diel oxygen technique can be used for real-time monitoring of seasonal and diurnal variations of dissolved oxygen and pH in pit lakes. This would be a useful technique in pit lake remediation projects where fertilization is used to stimulate algal growth and metal sequestering by algae.

Fifteen microcosms were installed in the Åkerberg pit lake for 15 days in the summer season (July) 2021. To stimulate algal growth, the microcosms were fertilized with two P-rich wood ashes, and KNO3. Chlorophyll-a was used as an indicator of algal growth while filtered (<0.2 μm) and particulate suspended element concentrations (>0.2 μm) were used to estimate algal metal uptake. Water quality measurements and water sampling were conducted on three occasions (every five days) and at the start of the experiment to monitor algal growth. The chlorophyll-a concentration in the microcosms fertilized with wood ash increased from 0.3-0.8 μg/L at the start of the experiment to 53–77 μg/L after 15 days. Algal element uptake of filtered concentrations (<0.2 μm) was observed for many elements including, Ni (33–36%), Zn (22–65%) and Cd (22–54%). This suggests that wood ash could be used to stimulate algal growth in pit lakes by acting as a source for P and potentially also other nutrients. The highest chlorophyll-a concentrations were seen on day 10, indicating that a breakdown of chlorophyll-a impacted the measured concentrations, which otherwise could have been higher.

Predictive modelling for three climate scenarios, based on the three greenhouse gas emission scenarios RCP 2.6, RCP 4.5 and RCP 8.5, was conducted for the Rävlidmyran pit lake, located in Västerbotten, northern Sweden. The model output for pH, temperature, dissolved oxygen, Cl, Fe3+, and Zn during the 10-year period 2090–2099 was compared to the model output during the 10-year period 2006–2015, for which measured meteorological data was used as input. Changes in thermocline, chemocline and water outflow were also evaluated. The results indicate that the water outflow from the pit lake will increase, as well as the number of days when the temperature in the mixolimnion exceeds 12 °C. The largest changes are seen for the highest greenhouse gas emission scenario (RCP 8.5). A small increase in Zn outflow (4.4%) could be observed for the RCP 8.5 emission scenario compared to the RCP 2.6 scenario. The results also indicate that the stratification of the lake is relatively stable, and it is predicted to remain meromictic for all climate scenarios. However, a sensitivity analysis indicates that a reduction of groundwater inflow element concentrations by 25–50% may result in a weakened stratification of the lake. Minor dilution could be observed in the monimolimnion of the lake as the modelled Cl concentration decreased by ∼0.3 mg/L in the RCP 8.5 emission scenario compared to the 2006–2015 period. The Cl concentration was also lower in the RCP 8.5 scenario compared to the RCP 2.6 and RCP 4.5 scenario, both in the mixolimnion and the monimolimnion.

A microcosm nutrient limitation experiment was conducted in the Åkerberg pit lake, located in Västerbotten, northern Sweden, in the summer of 2018. The microcosms were fertilized with N, P and N and P in combination. Chlorophyll-a concentrations were used to estimate algal growth. Filtered and suspended metal concentrations of the microcosms were compared to see if an increase in algal growth would lead to higher metal uptake. The results show that the microcosms fertilized with N and P had the highest chlorophyll-a concentrations (3–3.4 μg/l). This corresponds to an increase of 9.5–11 times compared to the initial chlorophyll-a concentrations, suggesting that the lake is nutrient poor with regards to both N and P. An increase of the metal concentration in the suspended particulate samples (>0.2 μm) of the microcosms fertilized with both N and P could be observed particularly for the mining-related metals Cd, Co, Ni, and Zn. The uptake of these metals amounted to 2.5–20% (Cd), 2.6–14% (Co), 0.87–1.8% (Ni), and 19–64% (Zn) of their filtered concentrations (<0.2 μm).

As extraction techniques are improving an increased number of low grade deposits can be economically mined. However, this also means that the number of open pit mines will increase and as a result the number of pit lakes as well. This is of environmental concern as the pit lakes, depending on the geology and other factors, potentially can have a negative impact on surrounding ecosystems, e.g. through high metal concentrations, low pH, and by affecting the hydrologic system. Sweden has a long history of mining and is one the largest metal suppliers in Europe, and many of the mines are located in the northern part of the country. The project aim was to better understand and be able to predict pit lake water quality and some of the underlying processes affecting it, with a focus on cold climates, where temperatures are low and the lakes are ice covered for prolonged periods of time.  The following field measurements were conducted in the Laver and Åkerberg pit lakes: 1) minewall stations measuring metal leaching rates from pit walls, 2) oxygen and hydrogen isotopic composition of precipitation and pit lakes to better understand pit lake hydrology (including also the Udden and Rävlidmyran pit lakes), 3) a nutrient limitation and metal uptake experiment in the Åkerberg pit lake, and 4) a continuously measuring buoy installed in the Laver pit lake where short term fluctuations in water quality parameters could be observed. A bathymetric map of the Åkerberg pit lake was also generated during the project. Additionally, modelling of the Rävlidmyran pit lake was conducted based on three different climate scenarios. Measurements of water quality parameters showed that both the Laver and Åkerberg pit lakes have relatively good water quality, pH 6.1–7.4, specific conductivity 41–352 μS/cm and pH 7.6-7.7, specific conductivity 137–140 μS/cm, respectively, and low concentrations of metals. Algal growth was successfully stimulated in microcosms in the Åkerberg pit lake, through addition of the nutrients N and P in combination. Chlorophyll-a concentrations, which were used to estimate algal growth, were 9.5-11 times higher than at the beginning of the experiment in these microcosms. It was also seen that the metal concentration in the suspended particulate phase increased, suggesting that metals were taken up by algae. In the Laver pit lake measurements from the continuously measuring buoy showed diurnal variations for pH, dissolved oxygen, and temperature. The pH and dissolved oxygen was increasing during daytime, indicating that algal growth was increasing. A local meteoric water line, which can be used to separate sources of water from precipitation, if their isotopic compositions are different, was constructed for the study area (δ2H = (7.80 ± 0.09) δ18O + (4.35 ± 1.35) ‰). It was seen that the groundwater had a similar stable isotopic composition as precipitation, as its composition fell on the local meteoric water line. It was also seen that the studied pit lakes had undergone evaporation as they plotted on a local evaporation line (δ2H = (6.88 ± 0.47) δ18O + (−12.75 ± 5.60) ‰). Residence times were calculated for the pit lakes, ranging from 2.9-44.9 years, using the isotopic mass balance method. Modelling of the Rävlidmyran pit lake suggests that it is fairly stable and will remain meromictic during a modelled 100 year period. No major differences in redox or oxygen levels were observed. The temperature is predicted to increase in the mixolimnion along with a slight decrease in dissolved oxygen concentration. The modelling also indicates that the water outflow from the lake might increase, and as a consequence, a slight increase of metal outflow was observed as well.  Based on minewall stations, installed at the Laver and Åkerberg mine sites, leaching rates (µg/m2/week) of metals were estimated. These were used to approximate the total contribution of metals from the pit walls to the pit lakes over the course of a year. A difference in metal leaching could be observed for the two mine sites which could be attributed to the deposit type.

A local meteoric water line based on rainwater and snow core samples was developed for an area comprising parts of eastern Norrbotten and Västerbotten in northern Sweden. The oxygen and hydrogen isotopic composition of groundwater and water from four pit lakes was compared to the local meteoric water line by line-conditioned excess. The isotopic mass balance method was used to estimate the evaporation over inflow ratio and the residence time of the lakes. The results show that the local meteoric water line for the study area is: δ2H = (7.80 ± 0.09) δ18O + (4.35 ± 1.35) ‰, which is close to the global meteoric water line. The surface water of the four pit lakes all have negative line-conditioned excess values which indicate that they have been affected by evaporation. The groundwater plots on the local meteoric water line making it hard to utilize in a mixing model for a lake where precipitation and groundwater are treated as two different sources. Two scenarios were used to estimate the starting composition of the lakes for the isotopic mass balance method. One was based on the intersection point of the local meteoric water line and the local evaporation line and gave evaporation over inflow ratios ranging from 0.23 to 0.74 and residence times ranging from 7.2 to 44.9 years. The second scenario was based on the weighted average composition of precipitation and gave evaporation over inflow ratios ranging from 0.07 to 0.32 and residence times ranging from 2.8 to 14.9 years.