The content of the radionuclide Sr-90 in natural waters taken from 11 reservoirs and rivers on the territory of the Sverdlovsk and Chelyabinsk regions was determined using the methodology developed at the Department of Radiochemistry and Applied Ecology of the Ural Federal University. Analysis of water samples from the Volchikhinsky reservoir, the Ivdel River, the Central City, Verkh-Isetsky and Nizhne-Isetsky ponds of Yekaterinburg for the content of Sr-90 was carried out for the first time. Shown that the waters of the Techa River are the most polluted. The specific activity of Sr-90 (in equilibrium with Y-90) in the samples taken in the area of the highway and the village of Muslyumovo was 56±3 and 51±1 Bq/l, respectively. In all other samples studied, Sr-90 activity did not exceed the drinking water intervention level of 4.9 Bq/l. The highest pollution (except for the waters of the Techa River) is typical for the waters of the Nizhne-Isetsky Pond; in November 2021, the activity of Sr-90 was 0.16 Bq/l. The concentrations of alkaline earth metals in samples of the studied natural waters were determined by mass spectrometry, and the dependence of the yield of strontium on the concentration of calcium and the total hardness of water was established. Shown that the yield of strontium remains constant up to a calcium concentration of ≤ 76 mg/l and a total water hardness of ≤ 5.2 mmol/l and amounts to 81±2%. To reduce labor costs, the developed method for determining Sr-90 can be used for monitoring without constant control of the release of Sr-90 into the concentrate, but using the established dependence of the output on the concentration of calcium or the hardness of natural waters.