The quality of surface waters in the territory of Poland, despite the implemented remedial measures, is mostly classified as bad. This article presents the results of research on the composition of water macroelements in 24 artificial reservoirs located in Opole province (southern Poland), searching for the reasons behind their pollution. Concentrations of the anions F−, Cl−, Br−, NO2−, NO3−, PO43− and SO42− and cations Na+, NH4+, K+, Mg2+ and Ca2+ were taken into account. An analysis of seasonal changes was carried out and, on the example of a dam reservoir (Turawa Big Reservoir), a multi-point analysis of the distribution of ion concentrations in water at the reservoir surface was also carried out. It was shown that retention and flow-through reservoirs, where the main source of pollution is the water feeding the reservoirs, are the most vulnerable to pollution. The second group consists of closed reservoirs located in industrial and agricultural areas, which are exposed to pollution from rainwater run-off and field leachate. In the case of reservoirs created in marl mining areas, a significant effect was also indicated from the chemical composition of the substrate on the formation of ionic equilibria in waters filling the excavations. The research shows that there are no hard and fast rules regarding the seasonal variability of ion concentrations during the growing season, indicating that eutrophic reservoirs show greater variability of ion concentration. Graphical Abstract