This is because hydrogen atoms (1,2H) are rapidly exchanged between water molecules.

[3] In normal water, about 1 molecule in 3,200 is HDO (1H2HO) (one hydrogen in 6,400 is 2H).

By comparison, heavy water D2O or 2H2O[4] occurs at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,4002).

HDO may be separated from normal water by distillation or electrolysis, or by various chemical exchange processes, all of which exploit a kinetic isotope effect.

Partial enrichment also occurs in natural bodies of water under certain evaporation conditions.