Natural mineral and curative waters often characterize elevated level of mineral components. If the mineralization grows, the concentration natural radionuclides, which are one population's exposure sources on ionizing radiation, also grows. The main purposes work were: (1) determine the concentration isotope potassium 40K in samples of water, (2) calculate effective doses caused by intake of water with potassium 40K, (3)- explore correlation between potassium 40K concentration and total dissolved solids (TDS). The concentration radioisotope 40K in analysed samples determined by means of gamma spectrometry. Received concentrations have values from the interval 0.65 to 28.42 Bq/dm3. Measured values compared to values calculated from physico-chemical analyses composition of water, which amounted from 0.07 to 22.87 Bq/dm3. The calculated effective dose resulting from intake water with potassium 40K, concentration 2.26 Bq/dm3 equaled 294 nSv by treatment and caused by inhalation water with concentration 40K 1.45 Bq/dm3 reached value 9 nSv by treatment.