One of the more serious problems occuring in the region of Tarnobrzeg in recent years is the liquidation of the sulphur mines Machow and Piaseczno in a manner that would minimize the threat to the natural environment. The liquidation program indicates that in place of the mining excavations recreational water reservoirs are to be built. In the Machow excavation a 25 metre thick layer of cracovian clay is being built in order to isolate the water in the future reservoir from contaminated Tertiary waters. Since 1981, when mining activities at the Piaseczno excavations were ceased, a water reservoir has existed with the water level being maintained by directing excess water into the Vistula river. As a result of financial difficulties a isolating layer was not constructed at the Piaseczno excavation. The prognosed changes to the water quality at the Piaseczno are based on simulation modelling results on a hydrological conditions model of the Tarnobrzeg Sulphur Depression. The various solutions considered the variations in the water level height at the Piaseczno reservoir and the expected proress of liquidation at the Machow mine. The basis for the hydrogeochemical balance, enabling an estimation of the water quality in the reservoir was the discharge quantity of feeder streams as well as the concentration of indicator chloride and sulphate ions. At present as well as in the prognosis, the largest contaminant load is transported into the reservoir by feeder streams filtration flows from the direction of waste disposal sites. In the final stage the concentration of the chloride ion in the reservoir waters caused by this feeder streams filtration stream flow will amount to about 42% of the total concentration while for sulphates it is about 84%. Prognostic calculations indicate that the water quality in the Piaseczno reservoir will be significantly influenced by progress and initial liquidation work relations in both post-wxploitation excavations. Periodically, degradation of the water quality may occur however under the expected conditions it will improve in relation to the water quality currently observed.