This International Standard specifes the measurement of 210Po activity concentration by alpha spectrometry in all kinds of natural waters.The detection limit of this method depends on the volume of the sample, the counting time, the background count rate and the detection effciency. In the case of drinking water, the analysis is usually carried out on the raw sample, without fltration or other pretreatment.If suspended material has to be removed or analysed, fltration at 0,45 µm is recommended. The analysis of the insoluble fraction requires a mineralization step that is not covered by this International Standard (see NF M 60-790-4[4]). In this case, the measurement is made on the different phases obtained. The fnal activity is the sum of all the measured activity concentrations.