Both chemical and biological stability of water was assessed from the three water supply networks using different water sources: surface, infiltration and the mixed (surface and groundwater). The oxidizing agent content, Langelier and Ryznar corrosivity indexes and water aggressiveness index were chosen as criteria for the water chemical stability assessment. Water biostability was determined on the basis of nutrient content: inorganic nitrogen and phosphate forms as well as biodegradable organic carbon (BDOC). It was established that in respect of chemical stability, the waters had some common characteristics such as dissolved oxygen and chlorine oxidizing agent content, conducive to the electrochemical corrosion process, as well as lack of an aggressive carbon dioxide with the concurrent lack of calcium-carbonate equilibrium supporting scale formation. The Ryznar index values were indicative of the corrosive potential of water; the surface water samples belonged to a clearly ‘corrosive’ range. The biological instability was a constant feature of the tested waters and it was the excessive inorganic nitrogen content that was the most conducive to the bacterial regrowth in the water. In either case, disinfectant content was determined to be inadequate with regard to prevention of biological recontamination of water, which was in contrast with its excess in relation to water corrosivity criterion.