Der Braunkohleausstieg führt zu Veränderungen in allen Bereichen der Wasserwirtschaft. Im Rheinischen Revier werden sich nach dem Ende des Bergbaus einige der größten deutschen Seen bilden. Der Grundwasserspiegel steigt und die Fließrichtung des Grundwassers ändert sich. Einige Oberflächengewässer führen mehr Wasser, andere weniger. Wasser wird ein wichtiger Faktor in der gesamten Region sein, aber es wird auch einen Wettbewerb um Wasser auf lokaler Ebene geben. Die Veränderungen in der Wasserwirtschaft sind wesentliche Randbedingungen im Strukturwandel im Bergbaugebiet.

Im Rheinischen Revier verursachten die Entwässerungsmaßnahmen rund um die Tagebaue ein Defizit von mehr als 20 Milliarden Kubikmetern Wasser. Beim Ausgleich dieses Defizits spielt die Nutzung von Wasser aus dem Rhein, das durch große Wasserleitungen gepumpt wird, eine wichtige Rolle. Die Befüllung der Seen wird etwa 40 Jahre dauern. Die Auswirkungen des Klimawandels werden in diese Planung einbezogen.

Eine der Kernaufgaben im Rahmen des Braunkohleausstiegs ist die Sicherstellung der Trinkwasserversorgung. Der Zufluss von Grundwasser aus den Abraumdeponien, das aufgrund von Pyritoxidationseffekten etwa 1.500 mg/l Sulfat enthält, wird die Schließung von mindestens vier Wasserwerken erzwingen. Darüber hinaus werden ca. 12 Wasserwerke von infiltriertem Rheinwasser betroffen sein, das verschiedene organische Spurenstoffe enthält. Ein flächendeckendes Wasserversorgungskonzept wird die Trinkwasserversorgung von mehr als 2,5 Millionen Einwohnern sicherstellen.

The largest contiguous former opencast lignite mining district in the EU is located in Lusatia (Germany). Hydrochemical contaminations such as acid mine drainage from opencast mines as well as increased susceptibility to soil instabilities pose major challenges for the reclamation of the Lusatian mining district in the coming decades. Therefore, time- and cost-efficient methods investigating former opencast lignite mines are vital for a sustainable remediation and reclamation.

In Lusatia, loose Cenozoic sediments (sands, silts, clays, and glacial tills) form several tens of metres of heterogeneous lithological successions in extensive opencast dumps. However, the effects of newly formed depositional structures, spatial heterogeneities and pore water mineralisation of the opencast dumps on groundwater flow and mass transport remain largely unknown in the post-mining areas of Lusatia.

In this study, we developed a workflow that makes use of spatially variable and heterogeneous input data ranging from airborne geophysics, geological maps and drilling logs. In a first step, we created 3D-representations of an exemplary post-mining area in Lusatia by using both classical geological interpretations as well as machine learning approaches. In a second step, the 3D representations are compared to each other and are used as input data for numerical groundwater modelling in order to analyse the effect on the simulated groundwater flow. This approach allows us to optimise the 3D-modelling workflow and to refine our understanding of the flow and mass transport processes between groundwater and pit lakes in the subsurface of post-mining areas.

As a result of the rearrangement of tertiary sediments and the groundwater lowering during open pit lignite mining, the formerly stable iron sulfide compounds oxidize to iron and sulfate ions and acidity. With the end of mining operations and the resulting rise in groundwater levels, these substances are transported into surface water, causing negative effects on the ecosystem and water management.
Sulfate reduction reverses the preceding oxidation process. By adding a carbon source, sulfate-reducing bacteria are activated in the aquifer. These metabolize the added carbon, producing sulfide which combines with the iron present in the groundwater to form iron sulfide and is fixed in the subsurface.
The process has already been tested as a pilot project in the Lusatian coalfield. For the transfer to the Central German mining district, adjustments had to be made to the plant design due to the cohesive soils and higher concentrations of iron and sulfate.
The plant configuration provides for a modular structure consisting of six self-sufficient sections. In each module, 86 individually controllable lances are provided for both lifting and infiltration of the water. Infiltration takes place in one lance at a time, while water is being lifted from other lances or these are paused so that water flows in. In this way, the problem of groundwater surface drawdown caused by upstream wells or the disadvantages of using external water are avoided. The lifted water is mixed with glycerine as a carbon source and then infiltrated again.

Empirical trophic models (Vollenweider-type) are one of the basic "tools of the trade" in management of natural and artificial lakes. They are not applicable to open-cast mining lakes (PML) because they underestimate their resilience to phosphorus (P) inputs. The high iron availability causes an efficient binding for phosphorus (P) in water and sediment of PML. The main objective was use hydrogeochemical modelling to assess the risks of eutrophication in the following way:

i) to develop conceptual (empirical) models (Vollenweider type) that can be used to more accurately estimate tolerable P loading to PMLs

ii) to represent the transition range from conditions with Fe excess to natural conditions in a model structure

iii) to identify specific indicators and tipping points for different P retention.

We analysed hydrological data, loads, and sediment properties to derive a closed P balance for 29 neutral mining lakes. The latter cover a range of water retention times and external P inputs. We distinguished three phases of PML development or maturation and the relevance of P-binding processes or binding forms. The Fe:P ratio in sediment was found to be the most important predictor of P retention. By integrating this ratio into conceptual models it is now possible to predict in-lake P-concentration from P inputs for PMLs and even for lakes with decreasing Fe import at the transition to “natural lake” conditions. We conclude that processes such as Fe~P adsorption and vivianite formation under anoxic conditions most likely ensure high P retention in the long term.

Lake Runstedt (near Merseburg, Germany; area 2.3 km², volume 53x10⁶ m³, max. depth 32.8 m) is an artificial lake resulting from lignite mining. The lower part of the former mine void was filled by industrial wastes consisting mainly of ashes but also containing waste from nitrogen fertilizer production rich in ammonium. Ammonium concentrations exceed 300 mgL^-1 in the pore water in the deposited wastes and constitute a threat for the regional groundwater resources. For protecting the groundwater, Lake Runstedt was created by filling the remaining space of the mine void above the waste with water from Saale River. The neighbouring pit lakes are managed in a way that groundwater flows into Lake Runstedt from all directions and that there is no outflow except evaporation. Hypolimnetic aerators provide the hypolimnion with oxygen needed for nitrification and reed was established in the littoral as habitat for denitrification. Since completion of the filling in 2003, the system has worked well as documented by monitoring. Usually, limnologists look at lakes as valuable ecosystems that have to be protected. In case of Lake Runstedt, the lake is used as a reactor. This unusual approach should not be a common preference but considered as exceptional option in applied limnology. The presentation will report on the creation of Lake Runstedt, the results of monitoring and research, and discuss the use of lakes as reactors protecting other compartments of the environment.