Pit Lakes Affected by a River Contaminated with Brines Originated from the Coal Mining Industry: Evolution of Water Chemistry in the Zakrzowek Horst Area (Krakow, Southern Poland)

Long-term coal mining activities in the Upper Silesia significantly affect the environment in southern Poland. Discharges of brines (with TDS reaching over 110 g/L) from mines are the main source of pollution of many rivers in Poland, including the Vistula River. The Zakrzowek horst is a small geological structure composed of the Upper Jurassic limestones. These limestones were exploited in several quarries. In the largest one (the "Zakrzowek" quarry), exploitation reached the depth of 36 m below the water table, i.e., about 32 m below the average water level in Vistula River which flows 700 m from the quarry. An important part of this inflow into quarries came from the contaminated Vistula River, with a chloride concentration over 2 g/L. The exploitation ceased in 1991, and dewatering ended in 1992. In the old quarry area, pit lakes appeared, which are unique because they present an example of a post-mining site affected by the riverine water contaminated with brines. Investigations of physicochemical parameters of water in the Zakrzowek area were carried out in the period of 1990-2020. Results showed that the largest pit lake was initially meromictic with a distinct stratification. After several years, holomictic conditions developed due to the surface layer freshening and convective mixing.