Geochemistry and environmental impact of neutral drainage from an uraniferous coal waste heap

Mining in the former Novátor coal and uranium mine ceased in 1957 and a part of the wastes underwent spontaneous ignition in 1959-1970. Since the 1970s, the waste heap has been exposed to continuous weathering. The objective of this study was to assess to what extent the burnt and unburnt parts of the heap contribute to contamination of the local drainage system over a period lasting >40 years after the end of the mining. Analyses of water collected by lysimeters located on the slope of the heap have shown that the burnt part of the pile is still a source of aqueous sulfate and volatile elements (F, Se, and Pb) that have accumulated in this part of the heap during the self-ignition processes. The unburned part of the heap is mainly the source of aqueous carbonate, Mo, and U. The pH values of water flowing through the heap are in the narrow interval of 6.9-8.6. Rare layers of unburnt coal-bearing sediments rich in sulfides, where the pH values are between 2.9 and 3.7 are rather exceptional. These layers are the main source of Zn, Cu, and Co as a result of weathering of pyrite, chalcopyrite and sphalerite. The chemical composition of water draining from the heap ranges between the values determined in both its burnt and unburnt parts. The water collected at the foot of the heap in periods of low drainage contains high levels of aqueous sulfate (up to 400 mg/L), carbonate (up to 208 mg/L), U (up to 585 μg/L), Se (up to 50 μg/L), and Mo (up to 28 μg/L). On the other hand, the concentrations of these elements are lower in periods of high drainage. The pH values in the water drainage are in the near-neutral range, and uranium is mostly present as U(VI) with dominant phosphate and carbonate complexes in the order UO2(HPO4)22− > UO2(CO3)22− > UO2(CO3)34− > UO2CO30. The concentrations of sulfate and trace elements rapidly decrease with increasing distance from the heap as a result of dilution due to influx of water from uncontaminated streams. Stream sediments sampled in the marshy area at the foot of the heap contain high U contents (up to 460 mg/kg), and other elements (Cu, Pb, Zn, Cd, and Se) due to their sorption on Fe- and Mn-hydroxides, organic matter, and co-precipitation with carbonate or sulfate. Similarly, the amount of uranium and other trace elements in sediments decreases with the distance from the heap because of the influx of clastic material from uncontaminated streams. Consequently, potentially harmful elements in water and sediments represent environmental risks only in the close vicinity of the heap.