Abstract
TR-31 and TR-33 are post-mining lakes located within the Łuk Mużakowa Geopark (western Poland). They were created after the end of lignite exploitation in the second half of the nineteenth century. Although they are located in neighbouring excavations and currently have neutral pH levels, they developed differently. Based on the diatom communities, elemental analyses and isotopic data, TR-33 experienced a natural neutralization process from an acidic state to nearly neutral due to the presence of lignite residuum at the bottom of the excavation. Pyrite is common in lignite deposits and gangue, and its oxidation causes acidity in aquatic ecosystems. The primordial state of the acidic water in TR-33 was confirmed by the development of diatoms belonging to the Eunotia genus, which can tolerate acidic environments. The excavation in TR-31 was filled by rain and groundwater, and the pH of the water was neutral or nearly neutral from the beginning of lake’s existence. This is indicated by the diatom assemblages during the initial phase of lake formation and the reconstruction of the water pH. The relatively low values of the C/N ratio in the deep layers of the core indicate the lack of lignite deposits at the bottom of TR-31.
Highlights
In many post-mining areas around the world, the inactive excavations that remained after the mines closed became lakes (Harrison et al 2003; Moser and Weisse 2011; Sienkiewicz and Gasiorowski 2016)
The chemical composition of pit lakes depends on the alkalinity of the local groundwater, the presence of carbonate in the surrounding areas, the amount of ore remaining in the excavation, the composition of the host rocks and the quality and quantity of runoff from the catchment (Castro and Moore 2000)
The neutralization of acidity is accelerated by anaerobic microbial processes, but their effectiveness depends on the concentration and availability of organic carbon
Summary
In many post-mining areas around the world, the inactive excavations that remained after the mines closed became lakes (Harrison et al 2003; Moser and Weisse 2011; Sienkiewicz and Gasiorowski 2016). The chemical composition of pit lakes depends on the alkalinity of the local groundwater, the presence of carbonate in the surrounding areas, the amount of ore remaining in the excavation, the composition of the host rocks and the quality and quantity of runoff from the catchment (Castro and Moore 2000). In addition to the decrease of sulphates, SRB remove metals from solution by the precipitation of sulphide (Castro and Moore 2000) Another important feature influencing the water chemistry in pit lakes is the length of time since the end of mining. Both abiotic and biotic factors have significant influences on the length of time needed for the natural neutralization of pit lakes
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