The thermal regime of the Czarnogłowy, a post-mining lake studied in 2008-2010, showed the reservoir to be meromictic. Throughout the year, the water column of the lake was divided into two layers (the mixolimnion extending down to 20 m depth and the monimolimnion) separated by a distinct, thin (about 2.0 m thick) chemocline. The chemical composition of the mixolimnion water was highly variable, the highest concentrations being shown by Ca, Na, Cl, SO₄, and HCO₃ ions. The same ions were also dominant in the monimolimnion, but their concentrations there were much higher and much less variable. The Czarnogłowy meromixis was found to be crenogenic. The lake received an inflow of groundwater from springs in the reservoir and the elevated mineralisation may have resulted from the inflow of groundwater seeping through the sedimentary limestone in the catchment. However, due to the limited data, these findings need further investigation.