Processes Turning Saline Settling Basins into Freshwater Bodies (Selected Examples from the Upper Silesian Coal Basin)

There are numerous deep coal mines in the Upper Silesian Coal Basin. Ensuring their proper exploitation requires constant drainage of the rock mass and the transfer of mine waters to rivers. Several technical solutions are used to prevent the adverse effects of saltwater discharge on the river ecosystem. One such solution is adapting the post-mining reservoirs into mine water settling basins. This article characterises two such facilities—the “Gliniok” and “Hubertus I” reservoirs. The physicochemical properties of their waters were analysed both when they served as settling basins and after their decommissioning. During their exploitation, the waters of the settling basins showed high salinity (>10 g/L). It was revealed that these basins turned into freshwater reservoirs very quickly after decommissioning. A sudden decrease in the electrolytic conductivity and the concentration of main cations and anions in the water was observed. The mixing processes also changed. The reservoirs were transformed from meromictic to polymictic. The processes that led to them turning into freshwater basins differed in the studied settling basins. The transformation of the Gliniok settling basin into a freshwater reservoir was a unique process, draining brines into the rock mass through cracks and crevices. The formation of cracks and crevices was a consequence of high-energy mining tremors. It is the first known case of this type in the world.