This part of ISO 13165 specifies the determination of radium-226 (226Ra) activity concentration in all types of water by coprecipitation followed by gamma-spectrometry (see ISO 18589-3[7]). The method described is suitable for determination of soluble 226Ra activity concentrations greater than 0,02 Bq l"1 using a sample volume of up to 100 l of any water type. For water samples smaller than a volume of 1 l, direct gamma-spectrometry can be performed following ISO 10703with a higher detection limit. NOTE This test method can be adapted to determine other naturally occurring isotopes of radium, such as 223Ra, 224Ra, and 228Ra if the respective ingrowth periods are taken into account.