This part of ISO 15587 specifies a method for extracting trace elements from a water sample using aqua regia as a digestion agent. The method is applicable to all types of waters with a suspended solids mass concentration of less than 20 g/l and a mass concentration of total organic carbon (TOC) expressed as carbon of less than 5 g/l. The aqua regia digestion method is empirical and it might not release elements completely. However, for most environmental applications, the result is fit for purpose. Aqua regia digestion is suitable for the release of: Ag, Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Se, Sn, Sr, Tl, V, Zn. It is not suitable for the digestion of refractory compounds such as SiO2, TiO2 and Al2O3. The presence of chloride in the digestion solution may limit the application of analytical techniques. The method is generic and may be implemented using a wide variety of equipment provided — the digestion composition is unchanged, — the digestion temperature is known, and — the digestion duration is in accordance with this temperature.