Analysis of physical and chemical properties of water runoff samples collected in the vicinity of a spoil heap near the coal mine in Libiąż showed that they represented a classic example of acid mine drainage. For the first time in the Upper Silesian Coal Basin, it has been documented that rainwater in contact with a surface layer of waste containing pyrite (FeS2) and scarce carbonate phases, produces strongly acidified wastewater. This process is accompanied by a dramatic increase in the concentration of dissolved ions (eg. Cl-, SO , Fe, Al, Mn). Further geochemical changes occur in the ditch collecting water runoff at the base of the heap. At this stage, oxidation of pyrite catalyzed by bacteria, followed by hydrolysis of iron Fe3+ to form goethite (FeO(OH)) leads to conversion of the less acidic (pH 2.6-3.5), short-time retention runoff water into the aggressive stagnant runoff water (pH 2.1-2.8). Further disintegration of Carboniferous claystones occurs in the acidic stagnant water environment, leading to the decomposition of mineral phases containing heavy metals dispersed in the rock matrix and to mobilization of Zn, Ni, Cu, Cd, Co, and Cr.