Natural evolution of artificial lakes formed in lignite excavations based on diatom, geochemical and isotopic data

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.