Post-exploitation opencast pits constitute a final stage of mining activities and turning them into public utility facilities is taking place now by their water reclamation. They constitute basins without outflows with the depth of 15 to 69 m and areas ranging from 2.5 to 692 ha. The bottoms of these basins are situated in pyrite-containing Miocene formations. Products of Fe2S weathering comprise sulphuric acid and iron compounds – sulphate II and sulphate III. This study presents the basic parameters of the reservoirs constructed in the final opencast pits of the “Konin” and “Adamów” brown coal mine,s as well as the properties of the formations making up the bottoms of two water reservoirs under construction: Lubstów and Władysławów. It is evident from the performed investigations that these formations are characterised by very acid reaction and very high exchangeable acidity. The total sulphur content ranges from 446 to 962 mg·kg^-1 of the ground and that of sulphate sulphur – from 71 to 187 mg·kg^-1 of the ground. The chemism of these grounds will exert influence on the quality of water accumulating in the opencast pits during the initial period of their spontaneous flooding. Together with the increase of the capacity of these reservoirs, the contact of waters with boulder clays of the Warta River glaciation abounding in calcium compounds will increase and concentrations of calcium ions in the waters of the reservoirs will also grow.