The influence of municipal waste water from a 70-year old settling tank on groundwater’s quality has been studied. Authors have observed contamination of groundwaters in the nearest vicinity of the tank - a significant increase of Cl, SO4, N-NH4, N-NO2, N-NO3and K content. The extent of contamination within the whole monitored area has been analyzed. Composition of groundwater in years 1985-2002 has been shown on diagrams. A correlation between the groundwater contamination and type of area (forest, farmland, village) is reported. Local pollution sources independent of the tank have been detected.