Hard coal mining ended in Germany in the Ruhr Area in December 2018. The cessation of mining activities and the associated changes in mine water management lead to a controlled mine water rebound. The geochemical composition of mine water is fundamentally influenced by sulfur cycling. Research results enable a deeper understanding of the sulfur and carbon cycles in mine water, and thus provide important information about ongoing biogeochemical processes in the now inaccessible underground mine workings. This in turn allows projecting expected biogeochemical changes into the future which is important for risk assessment.

A controlled mine water rebound is conducted in former hard coal mines of the Ruhr and Saar regions. To avoid risks for humans and the environment, the identification of old mine workings, which are prone to hydrogen sulfide (H₂S) formation is important. Thus, the sulfur content in coal is implemented in a geochemical model to assess future H₂S formation. Given the natural variability within a coal seam, mean sulfur contents for each mine water management province shall be used. For the southern Ruhr area, no significant correlation of precipitation and mine water geochemistry is discernible from the data available.

The lignite-mining region of Lusatia is characterized by i) low precipitation (compared to the German average), ii) a negative climatic water balance in most years, iii) predominantly sandy soils with a low water storage capacity covered by highly managed ecosystems (forest and agricultural monocultures), and iv) long-term, large-scale open cast lignite mining activities. The lignite mining activities have substantially affected the water resources, both in terms of water quantity and quality, in the Lusatian river basins. Recent extreme heatwaves, coupled with low precipitation, have exacerbated water management challenges in this already water-scarce region.

To refine the understanding of hydrological processes in watersheds strongly affected in lignite mining, we combine the analysis of extreme climate indicators with the isotopic signatures of precipitation, groundwater and surface water bodies (rivers, streams, post-mining lakes). In a first step, we quantified long-term changes in climate and drought indicators (1948-2022) as well as the isotopic composition (δ¹⁸O, δ²H, d-excess) of precipitation (1978-2022, station located in Berlin). In the second step, more than 1000 water samples, consisting of ≈400 groundwater, ≈260 channels and streams and ≈450 post-mining lake samples, are collected in 2024 and analyzed to assess the groundwater-surface water interactions and evaporation losses of the post-mining lakes. The survey covers an area of approximately 4000 km².

The analysis of the climate indicators show substantial increases in the temperature-related indices. The frequency of severe and extreme droughts has increased. The observed climatic changes coincide with a positive trend in the 45-year record of isotopic composition (δ¹⁸O, δ²H) of precipitation, which is statistically significant (p>0.05) on the annual basis as well as during spring (only δ¹⁸O) and summer. We show that the positive trend in the isotopic composition of precipitation is strongly correlated (r > 0.5) with air temperature during winter and moderately (0.3 < r < 0.5) during all seasons except autumn (September-November). A screening of the first samples suggest distinct difference between the isotopic signatures of the different water sources depending on the location as well as the mining influence.

Our study shows the potential of combining stable isotope tracers with hydroclimatic records to investigate the spatial and temporal variability of the dominant hydrological processes in watersheds affected by large-scale lignite mining. For the Lusatian mining region, the results suggest that multiple water-related challenges are overlapping and that climate change needs to be considered in regional water management.