Isotopic techniques enable scientists to understand the components of the water cycle, helping them better assess the quantity, quality and sustainability of water.
In the water cycle, groundwater is the least understood component. Scientists use naturally occurring isotopes as tracers to find out whether groundwater is being replenished, where it comes from, how it moves underground and if it is vulnerable to pollution and changing climatic conditions.
Water from different places has different isotopic signatures or unique ‘fingerprints’. Scientists use these ‘fingerprints’ to track the movement of water along its path through the entire water cycle: from evaporation, precipitation, infiltration, to runoff and evapotranspiration, then returning to the ocean or the atmosphere, and repeat.
But what are isotopes?
A chemical element, like hydrogen, is made entirely from one type of atom. The type of atom comes in different varieties. These varieties are isotopes, and they all share the same chemical characteristics and number of protons and electrons, but a different number of neutrons. The difference in the number of neutrons makes each isotope weigh differently, and this weight difference is key for hydrological studies.
Isotope hydrology uses both stable and unstable isotopes. Stable isotopes are non-radioactive, meaning they don’t emit radiation. Unstable isotopes (or radioisotopes) undergo radioactive decay and are therefore radioactive.