The legacy of hard coal mining in Germany: hydrochemistry and stable isotopes of mine waters
Antragstellende: Oliver Weisser und Jessica Pohl
Fachbereich, Studienrichtung: FB 14, Institut für Geologie und Paläontologie; Geowissenschaften
Projekttitel: The legacy of hard coal mining in Germany: hydrochemistry and stable isotopes of mine waters
Fördersumme: 4.695,00 Euro
This study focused on the hydrochemical and isotopic characterisation of processes reflected in mine waters, notably acid mine drainage resulting from pyrite oxidation. The study was located in the Ruhr area, the principal former hard coal mining district in western Germany until coal mining was finished in Germany at the end of 2018. The prime process responsible for the chemical composition of acid mine drainage waters is pyrite oxidation. Briefly, the iron- and sulphur-containing mineral pyrite (FeS2) is oxidised in contact with water and oxygen forming sulphuric acid. Its deprotonation produces dissolved sulfate on the one hand and hydrogen ions on the other. The latter decrease the pH value of the drainage water, which may lead to the mobilisation of heavy metals.
The aim of this study was to investigate whether multiple oxygen isotope analyses (δ18O, Δ17O) of the sulfate can identify the source of oxygen, coming either from the atmosphere or the water molecule. This research contributes to our understanding of chemical reactions following the flooding of the abandoned coal mines and how to reduce their legacy.
During this research project, mine waters from 19 former hard coal mines located in the Ruhr area were investigated.
The sampled mine waters exhibit an average oxygen content of 6.2 mg/L. An electrical conductivity of up to 970 µS/cm indicates a high dissolved load, likely resulting from water-rock interaction. Using the δ34S and δ18O analyses, it was shown that the sulfate ions dissolved in the mine waters clearly originate from pyrite oxidation. As mentioned before, sulfate from pyrite oxidation may have different sources of oxygen (the atmosphere or the water molecule). Previous studies revealed that half to two-thirds of the sulfate oxygen usually derives from the water molecule (Balci et al., 2007). Alternatively, or additionally, sulfate oxygen can be derived from atmospheric oxygen with a strongly positive δ18O value of +23.5 ‰. Our data indicates that sulfate in most mine waters likely received its oxygen from the water molecule. Three samples, however, show rather positive δ18O values. Is this indicative of a larger contribution from atmospheric oxygen?
In order to distinguish between both possible sources of sulfate oxygen, we determined the Δ17O value of the dissolved sulfate, a somewhat novel analytical approach in environmental chemistry. The mine waters exhibited small positive as well as small negative Δ17O values, suggesting that both forms of pyrite oxidation reactions exist depending on the different mining sites. Interestingly, the three samples mentioned earlier showing the high δ18O values display negative Δ17O values which suggests a high proportion of atmospheric oxygen in the sulfate ion.
To conclude: The combination of sulfur and triple oxygen isotopes revealed that the dissolved sulfate load in mine waters from the former Ruhr coal mining district in western Germany results largely if not exclusively from pyrite oxidation. Based on triple oxygen isotopes, the two different oxidation processes can be distinguished. This pilot study should be followed up by large-scale studies in order to investigate the extent to which oxidation reactions occur and whether there is a correlation between the type of reaction and the quantity of heavy metals mobilised.
The overall aim of the study could be successfully completed. Difficulties in sampling (timing, access to sampling sites) caused a slight delay in the project. Moreover, only 19 of the originally planned 28 mine waters could be sampled. The cooperation with Prof. Dr. Andreas Pack from the Georg August University of Göttingen, where the Δ17O measurements were carried out, was very positive. All analytical data required for the project was available by the beginning of May 2022 and results were presented at the European Geosciences Union (EGU) 22 at the end of the same month. The EGU 22 was a science fair in Vienna where scientists from 89 countries presented the latest insights in numerous different geoscience disciplines in around 12,000 presentations.
Balci, Nurgul; Shanks, Wayne; Mayer, Bernhard; Mandernack, Kevin (2007): Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite. In: Geochimica et Cosmochimica Acta 71. (2007) S. 3796-3811.