Stable Isotopes in Hydrology: Fractionation and Environmental Signatures

Isotopes are variants of a particular element that differ in their number of neutrons. Stable isotopes do not undergo radioactive decay and are commonly used in hydrological and biogeochemical analyses. Carbon, for example, has three naturally occurring isotopes: two stable isotopes (¹²C and ¹³C) and one radioactive isotope (¹⁴C). Hydrogen has two stable isotopes, ¹H and ²H (commonly called deuterium and abbreviated by the symbol D). Two stable isotopes of oxygen used in geochemical studies are ¹⁶O and ¹⁸O. The lighter isotopes are much more common than the heavier isotopes; ¹²C, ¹H, and ¹⁶O have natural abundances of 98.89 percent, 99.98 percent, and 99.76 percent, respectively. The heavier isotopes (¹³C, ²H, and ¹⁸O) are much less abundant.

The isotopic composition of a sample (given by the ratio of the heavy isotope to the light isotope) is measured relative to a standard. For ¹⁸O:

is reported as per mil (%o). The standard, by definition, has δ = 0%o. A negative value indicates a ratio of heavy-to-light isotope that is less in the sample compared with the standard. The sample is said to be lighter or depleted relative to the standard. A positive value indicates a ratio of heavy-to-light isotope that is greater in the sample compared with the standard. The sample is said to be heavier or enriched. The δ²H is based on the ratio ²H/¹H, and the δ¹³C is based on ¹³C/¹²C.

The isotopic composition of a product can differ from the source material. The process by which this occurs is termed isotope fractionation. In general, lighter isotopes are favored in evaporation and photosynthesis, leaving the source material heavier. This isotopic fractionation, or natural variation in the isotopic composition of substances, provides mechanistic understanding of geochemical cycles. For example, three isotopic types of water are ¹H₂¹⁶0 (>99%) and the heavier water ¹H²H¹⁶O (commonly abbreviated HDO) and ¹H₂¹⁸0. Isotopic fractionation occurs naturally through evaporation and condensation and imparts a discernible signature to the hydrologic cycle (Figure 10.11).

Fig. 10.11. Generalized representation of isotopes in the hydrologic cycle. Shown are representative values for δ²h with evaporation from tropical oceans and subsequent precipitation over land. adapted from dawson (1993b)

Lighter isotopes evaporate more easily than heavier isotopes and occur preferentially in the vapor phase. Heavier isotopes preferentially condense and occur preferentially in the liquid phase. Thus, water vapor that evaporates from tropical oceans is depleted in ¹⁸O or ²H (low δ¹⁸O and δ²H). Heavier isotopes condense and precipitate preferentially to lighter isotopes, so that rainfall is enriched in ¹⁸O or ²H (higher δ¹⁸O and δ²H) compared with water vapor. The air mass itself becomes further depleted in ¹⁸O and ²H as it moves inland. Temperature affects the isotopic composition of precipitation. With increasing temperature, precipitation is enriched in heavier isotopes (¹⁸O and ²H). Thus, precipitation is depleted of heavy isotopes in polar regions (low δ¹⁸O and δ²H). The effect of temperature and source region on the isotopic signature of precipitation is seen in comparison of δ¹⁸O for coastal California and inland Colorado sites (Figure 10.12). The coastal site has higher δ¹⁸O. The inland site is more depleted in ¹⁸O (lower δ¹⁸O), especially in winter.

Fig. 10.12. Average monthly δ¹⁸O of precipitation for a low elevation site near the California coast (Ca45) and an inland site in the Colorado rocky Mountains (Co02). data from Vachon et al. (2010)

The isotopic composition of water can identify the source of atmospheric water (Henderson-Sellers et al. 2004; Noone et al. 2013). Isotopes can additionally discriminate among processes within a plant canopy, especially partitioning evapotranspiration into soil evaporation and transpiration (Yakir and Sternberg 2000; Yepez et al. 2003; Williams et al. 2004; Dawson and Simonin 2011; Jasechko et al. 2013). This is because evaporation enriches soil water in ¹⁸O and ²H, but transpiration does not produce similar isotopic fractionation.