## 4 Discussion  
The processes turning saline mine water in settling basins into freshwater after they are no longer used are poorly understood. It would seem that in the case of saltwater endorheic reservoirs, such processes cannot occur because there is no possibility of draining the brine. This is how permanent salty endorheic lakes function, maintaining a more or less constant level of salinity [24]. Small changes in salinity in these lakes result from intensive evaporation in summer, causing an increase in salinity, or the inflow of fresh meltwater or precipitation, causing a decrease in salinity [25]. This is also how endorheic  
Figure 21: Changes in sulphate concentration in the “Hubertus I” reservoir. Figure 20: Changes in chloride concentration in the “Hubertus I” reservoir. mine water settling basins function, which do not turn into freshwater basins after the end of use [10; 12]. However, the studied reservoirs transitioned into freshwater basins. The factors that led to this transition are different in the case of the Gliniok and Hubertus reservoirs. A spectacular example is the Gliniok reservoir, where brine escaped into the rock mass. There are known cases of the disappearance of small streams in the Upper Silesian Coal Basin, whose waters infiltrate the rock mass [23]. So far, such a phenomenon has not been observed in the case of a water reservoir. The removal of brine from the reservoirs caused several changes in the physicochemical properties of the water. The most characteristic ones include the disappearance of meromixis and a hindered mixing of waters. In terms of mixing in the reservoirs, we again observe the processes characteristic of the reservoirs of moderate latitudes [10]. The disappearance of meromixis was also observed in the Rontok Wielki settling basin after decommissioning [15]. However, in the case of the Rontok Wielki settling basin, the water outflow was gravitational. In the Gliniok reservoir, the heliothermal water heating also disappeared. However, such changes are not observed in anthropogenic reservoirs with permanently maintained salinity. In the case of the \"Hubertus I\" reservoir, an abrupt decrease in the electrolytic conductivity and the concentration of chlorides and sulphates was observed in the first year after decommissioning (Figures 17, 20 and 21). Similar changes were observed in the Rontok settling basin [15]. There was a significant decrease in sulphates, although with substantial fluctuations between individual years. This situation resulted from the fact that the source of sulphates was no longer mine water. Their source was seepage water leached from the mining waste used to build the railway embankment in the shore zone of the reservoir. Additionally, dry and wet depositions are the source of sulphates. High concentrations of sulphates in seepage from mining waste dumps have been found in numerous cases [26]. [PERSON] and [PERSON] [27] write about the significant influence of dry and wet depositions on the concentration of sulphates in water. An analogous situation was also found in the case of the Rontok reservoir, which harbours a mining waste dump in its shore zone [15]. Despite the fact that the sedimentation basins turn into freshwater, they still show a different hydrochemical type than that commonly found in the waters of post-mining reservoirs of the Upper Silesian Coal Basin. The waters of these reservoirs are most often of a hydrocarbonate-calcium (HCO\({}_{3}-\)Ca\({}^{2+}\)) hydrochemical type [15]. The decrease in the water level in the control reservoir Grunfeld also influenced the change in the hydrochemical type of water from hydrocarbonate-calcium (HCO\({}_{3}-\)Ca\({}^{2+}\)) to hydrocarbonate-calcium-sulphate-chloride (HCO\({}_{3}-\)Ca-SO\({}_{4}-\)Cl). This situation should be associated with the exposure to the previously submerged Carboniferous rocks containing pyrites. In aerobic conditions, their weathering is an efficient source of sulphates [28]. This had an impact on the hydrochemical type of water.