This article presents the results of a preliminary assessment of the radioactivity of natural waters and the isotope analysis of drinking water. It describes the methods for the radiochemical preparation of water samples, which include concentration of uranium isotopes from water samples, extraction from impeding radionuclides and preparation of electrically countable samples. The results of violation of radioactive equilibrium between the isotopes 234U/238U and the several factors affecting this process in water samples have been obtained. It is clear from the obtained result that 234U isotope concentration in groundwater is higher than that in surface water.