## 1 Introduction  
There are many deep coal mines in the Upper Silesian Coal Basin. Ensuring their proper exploitation requires constant rock mass drainage and mine water removal. According to the Hydrogeological Dictionary [1], mine waters include water flowing into the workings from the drained rock mass and technological waters, most often introduced with hydraulic backfilling. Much of the mine water in the Upper Silesian Coal Basin is highly saline brine. Most of it shows mineralisation > 70 g/dm\({}^{3}\) and ion concentration (Cl\({}^{-}\) + SO\({}_{4}\)\({}^{-}\)) > 42 g/dm\({}^{3}\)[2]. They may also contain significant concentrations of nitrates [3].  
The discharge of brines into surface watercourses causes several adverse effects in freshwater ecosystems. An example was the 2022 mass fish kill in the Oda River [4; 5]. Such waters also become unsuitable for irrigation in agriculture and as drinking water [6; 7]. Several technical solutions are used to prevent the adverse effects of saltwater discharge [8; 9]. They are intended to reduce the negative impact of brines on the environment. One is adapting post-mining reservoirs as mine water settling basins [10]. Post-mining reservoirs are created due to flooding workings after the exploitation of mineral deposits (Figure 1), which can then be used as mine water receivers. In these reservoirs, mine water is mainly purifiedmechanically from suspension. The brine is also slightly diluted by fresh groundwater and rainwater inflow into the reservoirs [10].  
Mine water settling basins, including their hydrology [10; 11; 12], hydrochemistry [12] and biology [13; 14], are a very interesting research theme. Some settling basins are mermotic, a unique mixing type rarely found in temperate latitudes [12]. In the Upper Silesian Coal Basin, mine restructuring resulted in the closure of numerous mines and the cessation of mine water pumping. Thus, mine water settling basins were put out of use. Currently, the limmic processes in the discontinued mine water settling basins are poorly understood, and the number of studies in this field is limited. In addition, they only apply to retention-and-dosing reservoirs, not post-mining reservoirs used as settling basins [11; 15].  
The processes leading to the changes in the physicochemical properties of water that occur in settling basins after decommissioning may depend on several factors. To identify them, this research was undertaken with the primary objectives of:  
* examining the changes in the morphometric parameters and type of water management of settling basins after their decommissioning,
* examining the changes in the mineralisation and chemical composition of the water in settling basins after their decommissioning,
* examining the changes in the physicochemical properties of water in the vertical column of the basins,
* describing the processes (factors) that turned the settling basins into freshwater tanks,
* indicating development directions for brine settling basins facing decommissioning.  
Figure 1: Stages of the formation of excavation reservoir, according to [PERSON] (2011) [10]. 1—situation before exploitation, 2—situation during exploitation above the water table, 3—situation during exploitation below the water table, 4—situation after the exploitation cessation.