2.6 Stable Isotopes

Stable isotopes are atoms of the same chemical element (i.e. the same number of protons and electrons) with a different number of neutrons that do not undergo radioactive decay, but they may be present as a result of radioactive decay. Ratios of different stable isotopes of the same element can provide useful indicators of water sources and chemical reactions.

The ratios of the less abundant to the most abundant stable isotope of an element (e.g., ¹³C/¹²C, ¹⁸O/¹⁶O, ¹⁵N/¹⁴N) vary slightly between different materials and vary spatially within the same material. Differences in abundance of the different isotopes are usually caused by small differences in reactivity because of mass differences; lighter isotopes typically being more reactive than heavier isotopes. Natural materials will therefore have different isotopic abundances due to different source materials and different geochemical processes to which they have been exposed. Consequently, changes in isotopic abundance of natural materials can provide information on sources of dissolved substances, and chemical reactions that might have taken place after dissolution. The stable isotope ratios of the water molecule (¹⁸O/¹⁶O and ²H/¹H) are particularly useful in hydrology, as they can provide information on condensation and evaporation processes. Manufactured chemicals can have different isotopic ratios due to differences in source materials or manufacturing processes. Fractionation is the name given to processes that affect the relative abundance of light and heavy stable isotopes (light being the isotopic form with fewer neutrons).

Some of the key stable isotopes that have application to hydrology are listed in Table 2, along with their relative global abundances. The ratio of heavy to light isotopes can be measured with high accuracy and precision, therefore differences between relative abundances at a local level can be used to infer migration of groundwater and geochemical processes occurring along the flow paths. Isotope ratios can be measured either on individual ions of molecules, for example, ¹⁵N/¹⁴N and ¹⁸O/¹⁶O of NO₃^–, ³⁴S/³²S or ¹⁸O/¹⁶O of SO₄^2–, ¹⁸O/¹⁶O or ²H/¹H of H₂O, or on groups of molecules, such as with ¹³C/¹²C of total dissolved inorganic carbon (TDIC). Usually this ratio is expressed relative to a standard using the delta (δ) notation as shown in Equation 4.

‰

(4)

where:

Thus, the δ-value of a sample which is identical to the standard would be 0 ‰, positive values indicate a greater proportion of the less abundant isotope than the standard, and negative values indicate a lower proportion of the less abundant isotope (Table 1).

Table 2 – Relative global abundances of some commonly used stable isotopes. From Rosman and Taylor (1998).

Two of the most widely used stable isotope ratios are those of hydrogen and oxygen in the water molecule. Although there are two common stable isotopes of hydrogen and three of oxygen, giving a total of nine isotopically different water molecules, only three are commonly measured in natural waters: ¹H¹H¹⁶O, ¹H²H¹⁶O and ¹H¹H¹⁸O, with the first being the most abundant form. Variations in ²H/¹H and ¹⁸O/¹⁶O ratios in natural waters arise mostly from isotopic fractionation during evaporation and condensation processes, whereby the heavy isotopes of water preferentially remain in the liquid phase during evaporation or pass into the liquid phase during condensation.

Rainfall everywhere in the world is depleted in the heavy isotopes of water relative to seawater through a process known as Rayleigh fractionation. (Seawater is used as the standard for δ ²H and δ ¹⁸O measurement, so it has an isotopic composition of δ ²H = δ ¹⁸O = 0 ‰.) Because lighter isotopes evaporate more readily than heavier isotopes, clouds will contain a greater proportion of light isotopes (and thus have negative values for δ ²H and δ ¹⁸O). The isotopic composition of rainfall will be heavier than that of clouds, but lighter than the seawater which was the original source of the water vapor forming the clouds. As air masses progressively move inland, rainfall becomes progressively lighter (Coplen et al., 2000). Because ²H and ¹⁸O are affected in similar ways, δ ²H and δ ¹⁸O values in precipitation are highly correlated. The global meteoric water line (GMWL) is an empirical relationship that expresses the average correlation between δ ²H and δ ¹⁸O values in precipitation, and is given by Equation 5 (Craig, 1961).

Often, however, the stable isotopic composition of local rainfall differs from the global mean, and so it is sometimes useful to define a local meteoric water line (LMWL; Figure 7).

In many cases, the mean δ ²H and δ ¹⁸O composition of groundwater will be similar to the mean amount-weighted δ ²H and δ ¹⁸O composition of precipitation within its recharge area. However, fractionation processes result in variations in isotopic composition of precipitation, and sometimes these variations can be used to identify conditions under which the groundwater recharge occurred (Figure 8). Other factors being equal, the isotopic composition will be more depleted in heavy isotopes during heavy rainstorms than during lighter storm events. So, if groundwater recharge primarily occurs during large rain events, it may cause the mean isotopic composition of groundwater to be more depleted than mean rainfall. Also, rainfall at higher elevations will be more depleted than rainfall at lower elevations, and rainfall during cold climatic periods will be more depleted than rainfall during warmer periods (Ingraham, 1998; Mazor, 2004). Subsequent to a rainfall event, the ¹⁸O and ²H composition of water may be further modified by evaporation from the soil or from surface water bodies, and these effects are most notable in more arid climates. Water samples affected by evaporation are typically displaced to the right of the meteoric water line, and often fall on a line of lower slope (Figure 8), while transpiration does not significantly fractionate the stable isotopes. The International Atomic Energy Agency (IAEA) maintains a network of rainfall collection stations around the world, so there is a good database on the δ ²H and δ ¹⁸O composition of precipitation for comparison with groundwater samples.

Other stable isotopes that are widely used in hydrogeology include:

• Nitrogen (¹⁵N/¹⁴N) and oxygen (¹⁸O/¹⁶O) isotope ratios on NO₃^–, which are used to determine sources of nitrate contamination in groundwater, and biogeochemical processes that affect nitrate concentrations.
• Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) for identifying water sources and geological provenance of groundwater.
• Sulphur (³⁴S/³²S) and oxygen (¹⁸O/¹⁶O) isotope ratios on SO₄^2‑ are used to trace natural and anthropogenic sources of sulfur, and biogeochemical processes that affect sulfur and sulfate.
• Carbon isotope ratios (¹³C/¹²C) are used for determining chemical reactions involving dissolved organic and inorganic carbon. δ¹³C ratios on dissolved inorganic carbon are particularly useful for determining chemical reactions that have affected ¹⁴C activity, and hence impacted the ability to use ¹⁴C as a groundwater age tracer. δ¹³C of organic materials can provide evidence for identifying sources of contaminants.