Due to the different atomic weight, isotopes physically behave slightly differently. This leads, for example, to a change in the isotope ratios during phase changes (e.g. evaporation and condensation). For example, water molecules with light isotopes preferentially evaporate from a water body, while the "heavy" molecules accumulate in the remaining water body. These changes of the isotope ratios are called "isotope fractionation".
In general and for water in particular, isotope fractionation is temperature dependent. Therefore, the annual variation of air temperature in our latitudes results in an annual variation of isotope signatures in precipitation (Figure 1). Thus, precipitation in the cold winter months shows significantly more negative δ-values than in the warmer summer months.
Figure 1: Annual variation of oxygen-18 and deuterium values in precipitation in Schweitenkirchen, averaged from monthly values of the years 1999 - 2015.
The isotope ratios in precipitation also differ regionally due to temperature, altitude and origin of the precipitating air masses. If the isotopic ratios of oxygen and hydrogen of precipitation are plotted against each other in the so-called oxygen-18-deuterium diagram, the corresponding δ-values scatter in the range of a straight line, the mean global precipitation line (GMWL, Figure 2), despite the large variation of isotopic ratios of precipitation worldwide. The intercept of the GMWL of 10 (‰) is called the "deuterium excess" (d) (see formula in Figure 2). If the deuterium excess of a water sample deviates significantly from 10 ‰, i.e. if it is below or above the GMWL, this indicates phase changes in the water.
Figure 2: Oxygen-18 deuterium diagram. Also plotted is the GWML as well as value ranges of precipitation that fell under different conditions and the water reservoirs formed by this precipitation, as well as the typical change in isotopic signatures due to evaporation and condensation.
As a result of isotopic fractionation, water bodies such as groundwater and the tap water usually derived from it exhibit regionally varying δ-values.
The differences in isotopic signatures of waters resulting from the effects described above are used to evaluate the origin and modification of water.