Key Features

Stable Isotopes

Isotopes are forms of a given chemical element that have different atomic masses. For a particular element, the isotopes have the same numbers of protons, and so have the same atomic number. However, each isotope has a different number of neutrons and therefore has a different atomic mass. Stable isotopes are those isotopes that do not undergo radioactive decay; so their nuclei are stable and their masses remain the same. However, they may themselves be the product of the decay of radioactive isotopes. In hydrological studies, the stable isotopes of interest generally relate to H, C, N, O, S, B, and Li. In terms of the water molecule itself, oxygen has three stable isotopes, 16O, 17O, and 18O; and hydrogen has two stable isotopes, 1H and 2H (deuterium). The relative abundances of these stable isotopes of hydrogen and oxygen are given in Table 1. The stable isotopes of 18O (oxygen-18) and 2H (deuterium) are used to provide information on hydrological processes, including groundwater-surface water interactions

Table 1: Relative abundances of the oxygen and hydrogen isotopes
Hydrogen Oxygen
Isotope Abundance Isotope Abundance
1H 0.99985 16O 0.99757
2H 17O 0.00038
18O 0.00205

Numerous papers or books have been published over the past 40 years that deal with applications of environmental isotopes in hydrological investigations. Some of the most comprehensive information on isotopes appears in the Handbook of Environmental Isotope Geochemistry series edited by Fritz and Fontes in the 1980s as well as reviews by Fontes and Edmunds (1989), Coplen (1993) and Gat (1996). Textbooks recently published by Mazor (1997), Clark and Fritz (1997) and Cook and Herczeg (2000) are excellent reference works on isotopes application in hydrology.

Water samples collected for isotopic analysis should be stored in bottles with tight closures, such as caps with conical plastic inserts. Bottles for archived samples should be glass. End-member samples (representing the key components of the hydrological system) should be analysed prior to undertaking a detailed study to determine if sufficient isotopic discrimination exists in the hydrologic system. Background samples can also be very important. Often these will be samples of shallow local groundwater, near to, but outside, the area of investigation.

Oxygen and hydrogen stable isotopic ratios are measured by isotope mass spectrometry. Hydrogen analysis is done on hydrogen gas obtained through high-temperature reduction of water on metal (Kendell and Coplen 1985). Oxygen analyses are done on carbon dioxide that has equilibrated with water at a constant temperature (Epstein and Mayeda 1953). Oxygen and hydrogen isotope compositions are commonly reported relative to an agreed sample of ocean water, referred to as the Standard Mean Ocean Water (SMOW), representing the largest and most equilibrated water body. Stable isotope ratios of deuterium/hydrogen (2H/1H) and 18O/16O of water are conventionally expressed as units of parts per thousand (per mil , ‰) deviation from SMOW.

Isotopic fractionation of water molecules due to evaporation of seawater and subsequent precipitation in rainfall was recognised by Craig (1961). Based on about 400 water samples from rivers, lakes and precipitation, a linear relationship between deuterium and oxygen-18 was established for average global meteoric waters. This relationship (δD=8δ18O+10) is known as the Global Meteoric Water Line (GMWL) and provides a useful benchmark against which regional or local waters can be compared and their isotopic composition interpreted. The slope of this curve represents Rayleigh fractionation due to repeated evaporation and precipitation, the intercept (termed the deuterium excess) is largely a function of the mean relative humidity of the atmosphere above the ocean water (Clark and Fritz,, 1997). Local meteoric water lines can be established from isotopic analysis of local precipitation events.

Comparison of the stable isotope data for surface water and groundwater samples relative to the global or local meteoric water lines can provide information on processes. For example, isotopically light water molecules evaporate more efficiently than isotopically heavy water molecules. Due to this variability in isotopic vapour pressures, evaporation produces residual water enriched in the heavier isotopes relative to the initial isotopic composition. Therefore water that has undergone evaporation lies to the right of the local meteoric water line due to this enrichment (Coplen, 1993). The trend line for evaporation from surface water tends to have a slope between 4 and 6, with a slope less than 4 indicating evapotranspiration of soil water in the unsaturated zone (Allison, 1988).

References