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This document specifies a method for the measurement of 210Po in all types of waters by alpha spectrometry. The method is applicable to test samples of supply/drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic, and industrial wastewater after proper sampling and handling, and test sample preparation. Filtration of the test sample may be required. The detection limit depends on the sample volume, the instrument used, the counting time, the background count rate, the detection efficiency and the chemical yield. The method described in this document, using currently available alpha spectrometry apparatus, has a detection limit of approximately 5 mBq l-1, which is lower than the WHO criteria for safe consumption of drinking water (100 mBq l-1). This value can be achieved with a counting time of 24 h for a sample volume of 500 ml. The method described in this document is also applicable in an emergency situation. The analysis of 210Po adsorbed to suspended matter in the sample is not covered by this method. If suspended material has to be removed or analysed, filtration using a 0,45 µm filter is recommended. The analysis of the insoluble fraction requires a mineralization step that is not covered by this document [13]. In this case, the measurement is made on the different phases obtained. The final activity is the sum of all the measured activity concentrations. It is the user's responsibility to ensure the validity of this test method for the water samples tested.