Oxygen and hydrogen isotopes

Introduction

The stable isotopes of oxygen (¹⁸O/¹⁶O) and hydrogen (²H/¹H) are widely used in groundwater studies and in other hydrological and hydrogeological studies. The advantages of this method were noticed and the usage grew exponentially after a particular publication by Craig in 1961. In short, the main finding of this publication was that the isotopes of oxygen and hydrogen are partitioned, or fractionated, in a rather predictable way by meteorological processes. These processes imprint an isotopic fingerprint on waters through thermodynamic reactions, physicochemical reactions and/or molecular diffusion, and this fingerprint is fundamental in investigating the origin of groundwater (Clark & Fritz 1997). The method is well-established and widely used in various studies both as a main tool (e.g. Harris et al. 1999, Gammons et al. 2006) and as part of wider geochemical investigations (e.g. Laaksoharju et al. 1999, Kietäväinen et al. 2013).

Description of the method

The usage of oxygen and hydrogen isotopes of water is based on the variations in their abundance ratios in different phases caused by fractionation. Fractionation occurs in chemical and physical reactions and is caused by small differences in the masses of different isotopes. This results in different relative proportions of different isotopes of the same element in various compounds or phases, because heavier isotopes have higher bond strengths than lighter isotopes, and the bond with heavy isotopes is broken at a slower rate. Thus, the heavier isotope enriches in the compound and phase in which it is bound most strongly. (Clark & Fritz 1997)

Fractionation can be divided in equilibrium and non-equilibrium (kinetic) processes. Equilibrium fractionation occurs when a system is in thermodynamic equilibrium and no net reaction takes place, so that the rates of forward and backward reactions are equal. Equilibrium fractionation, which is most importantly dependent on temperature, may cause the isotope ratios of the reactants and products to be different: the lower the temperature, the larger the fractionation. A typical equilibrium process is water vapor condensing in rain clouds. Kinetic fractionation, on the other hand, is associated with unidirectional, incomplete processes and reactions, such as evaporation and diffusion. In these reactions the fractionation is caused by the differences in the masses of the isotopes and their vibrational energies, making the light isotopes react faster and hence become enriched in the product. An important factor affecting the kinetic fractionation is humidity. (Clark & Fritz 1997, Hoefs 1997)

As air masses move across continents they lose water by rainout, and become progressively depleted in the heavy isotopes (²H and ¹⁸O) because these isotopes are preferentially fractionated and enriched in the liquid phase. When the isotope compositions of precipitation from all over the world are measured, they are plotted linearly relative to each other on δ¹⁸O versus δ²H plots (Craig 1961). The δ values express the isotope ratios as deviations in per mil (‰) from a standard (usually from VSMOW, Vienna Standard Mean Ocean Water) for each element, for example between ¹⁸O and ¹⁶O. A positive δ value means the sample is enriched in heavier isotope relative to the standard, and negative value means the sample is depleted relative to the standard. The linear line describing the correlation between the δ¹⁸O and δ²H compositions is called Global Meteoric Water Line (GMWL). Its slope is 8, which is produced in equilibrium condensation of rain, and the line intersects the y-axis in 10, although the composition of average ocean water (VSMOW) is δ¹⁸O = δ²H = 0. The reason why the line does not intersect the average composition of ocean water is because the global atmospheric water vapor forms with an average humidity of 85%, causing 10‰ kinetic enrichment in ²H of vapor evaporating from the ocean. (Clark & Fritz 1997)

As noticed before, isotope composition of precipitation plots on the global meteoric water line. Surface waters that have undergone evaporation, however, plot below the GMWL with smaller slope. The intersection of this kind evaporation line with GMWL shows the composition of the original water, and the slope of the evaporation line most importantly depends on the ambient humidity (Gat 1971, Clark & Fritz 1997, Hoefs 1997). The parameter used to describe the shift of the isotope composition from the GMWL is deuterium excess (d-excess). When the isotope composition of a water sample plots on the GMWL its d-excess is 10. Precipitation and groundwater samples that are of meteoric origin are typical examples of this kind water. For those samples that plot below the GMWL the d-excess is less than 10, and usually indicates evaporated surface water (e.g. Dansgaard 1964, Clark & Fritz 1997, Hoefs 1997). When the d-excess of the sample is more than 10 (and the isotope composition has been plotted above the GMWL), it is likely from very saline groundwater. Usually with increasing salinity and depth the sample is plotted more distinguishably above the GMWL (Kloppmann et al. 2002, Kietäväinen et al. 2013).

Appropriate applications

Information of the stable isotope compositions of oxygen and hydrogen in water samples can be used in a variety of problems, and the typical isotopic fingerprints that different processes and reactions leave on the water are advantageous in tracing the evolution and sources of the water. For example, the distinct temperature-dependent fractionation is useful in studying seasonal and annual changes or past climate conditions. The method is also suitable in tracking geographical and spatial changes in precipitation, and in determining the origin of the water. In addition, the usage of oxygen and hydrogen isotopes has applications in groundwater-surface water interaction studies, and with similar isotope compositions possible hydraulic connections can also be found. In some cases the stable isotopes of oxygen and hydrogen in water can be used as a tracer, but this requires a sufficient difference in the isotope compositions of the ambient and tracer water, and also sufficient mixing ratios. Although the current analyzing methods are quite sensitive, the method itself is not adequately sensitive to trace minor changes in the isotope composition, for example due to low mixing ratio. This is a reason why this method would be most advantageous when used together with other geochemical analyzes.

A case study on the use of H and O stable isotopes in recognition of preferable groundwater flow paths in bedrock fracture zones is presented in the Closedure  pages:

• Use of H and O stable isotopes in recognition of preferable groundwater flow paths in bedrock fracture zones

References

Clark, I. & Fritz, P. 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, Boca Raton, FL, 328 p.

Craig, H. 1961. Isotopic variations in meteoric water. Science 133, 1702-1703.

Dansgaard, W. 1964. Stable isotopes in precipitation. Tellus 16, 436-468.

Gammons, C.H., Poulson, S.R., Pellicori, D.A., Reed, P.J., Roesler, A.J. & Petrescu, E.M. 2006. The hydrogen and oxygen isotopic composition of precipitation, evaporated mine water, and river water in Montana, USA. Journal of Hydrology 328, 319-330.

Gat, J.R. 1971. Comments on the stable isotope method in regional groundwater investigations. Water Resources Research 7, 980-993.

Harris, C., Oom, B.M. & Diamond, R.E. 1999. A preliminary investigation of the oxygen and hydrogen isotope hydrology of the greater Cape Town area and an assessment of the potential for using stable isotopes as tracers. Water SA 25, 15-24.

Hoefs, J. 1997. Stable Isotope Geochemistry. 4th ed. Springer, Berlin, 201 p.

Kietäväinen, R., Ahonen, J., Kukkonen, I.T., Hendriksson, N., Nyyssönen, M. and Itävaara, M. 2013. Characterisation and isotopic evolution of saline waters of the Outokumpu Deep Drill Hole, Finland – Implications for water origin and deep terrestrial biosphere. Applied Geochemistry 32, 37-51.

Kloppmann, W., Girard, J.-P. and Négrel, P. 2002. Exotic stable isotope compositions of saline waters and brines from the crystalline basement. Chemical Geology 184, 49-70.

Laaksoharju, M., Tullborg, E.-L., Wikberg, P., Wallin, B. & Smellie, J. 1999. Hydrogeochemical conditions and evolution at the Äspö HRL, Sweden. Applied Geochemistry 14, 835-859.