This research deals with pollution impact on natural water resources in the industrial area of Belchatów, central Poland, where a large brown coal deposit is exploited and the coal is burned in an electric power plant. To trace the sources of groundwater pollutants the stable isotope analysis of oxygen and sulfur in sulfates was applied. The mass-spectrometric analysis was performed on SO42-samples from numerous wells and piezometres in the excavation area. By repetitive sampling performed in November 1994, May 1995 and December 1996 significant changes of SO42-concentration and sulfur and oxygen isotopic ratios in several sites were recorded. The interpretation of isotope ratios allowed us to recognize three groups of sulfates: (1) from the leaching of Permian salt dome, (2) produced by the leaching of soluble sulfates from an ash pool and (3) produced by oxidation of natural sulfides in water-bearing rocks.