Stable Isotopes of Water in Groundwater Studies

​

By F. Tillman

​

In this our fourth (and last) discussion of how hydrologists use isotopes in water resources investigations, we look at the use of stable isotopes of water in groundwater studies.  As you may recall, isotopes of an element have the same number of protons but a different number of neutrons in their nucleus.  A “stable” isotope is one that does not spontaneously decay to other isotopes, as opposed to radioactive (or unstable) isotopes whose nuclei disintegrate and emit radioactive alpha or beta particles and gamma rays.  The stable isotopes of water that are of interest to hydrologists are deuterium (denoted as D or 2H) and oxygen-18. Deuterium is a hydrogen atom that has one neutron and one proton, which is approximately twice the mass of the most abundant isotope of hydrogen, protium (1H).  The ratio of terrestrial deuteurium to protium is 1 to 6410.  All isotopes of oxygen have 8 electrons and 8 protons, but oxygen-18, with a mass of 18, has 2 more neutrons than the more abundant oxygen-16 (16O), which has a mass of 16.   The ratio of terrestrial oxygen-18 to oxygen-16 is 1:500.  Even though deuterium and oxygen-18 make up such a small portion of the hydrogen and oxygen in water, the differences in mass of these isotopes from their more abundant siblings cause them to react slightly differently – a behavior called fractionation or partitioning.  It is this fractioning behavior of the stable isotopes of water that makes them a useful environmental tracer in groundwater studies.

 

In 1961, a geochemistry professor at the Scripps Institution of Oceanography, Harmon Craig, published a landmark study in which he showed that deuterium and oxygen-18 partition by meteorological processes, and in a very predictable fashion.  The fundamental control on the isotopic composition of precipitation is temperature.  As temperature increases, precipitation becomes enriched in the heavier isotopes of deuterium and oxygen-18, in a linear relationship.  Similar effects are seen with increasing elevation and increased distance from the equator – both of which relate to lower temperature.  As rain moves in from coastal areas, it is initially more enriched in deuterium and oxygen-18 than later rain.  That is, as rain events continue, the rainfall becomes progressively lighter in stable isotopes – a phenomenon known as “rainout effect” (figure 1).  Water samples from groundwater systems can be analyzed for deuterium and oxygen-18 abundance, and compared with results from precipitation samples in the area, preferable at different altitudes, to understand where the groundwater recharge came from. 

 

Other hydrologic processes affect the relative stable-isotope composition of water, including evaporation and relative humidity.  By plotting the stable isotope composition of groundwater samples, information can be inferred on the source of groundwater recharge including the climate, latitude, and altitude of recharge, as well as information on whether the water first underwent evaporation (such as in a river or lake) before becoming groundwater (fig. 2).