ABSTRACT: The area of the karstic spring "Kráľ Mátyás" in Lopej village was investigated according to the supposed contamination by the leachate outflowing from the metallurgical waste depository in Vajsková. Isotope analyses of the water molecule oxygen and deuterium, 34S sulphur in the dissolved sulphates, d13CHCO3, tritium activity and radioactive carbon 14C were perfonned for the spring groundwater, stream water and groundwater from 2 boreholes between. Surpluses to the "Kráľ Mátyás" spring from the surface stream are apparent from the isotope data, but much more lower As and Sb contents in the spring' s water show that the surface water flowing to the "Kráľ Mátyás" had been separated from the Vajskovský potok stream above the position of the waste depository in Vajsková. "Kráľ Mátyás" spring water is also partly fed by the deep circuit groundwater of the carbonate structure - the same as were present in the VHV-1 and VHV-1a boreholes.