Water Isotope hydrology: an overview By Lucía Ortega and Laura Gil Isotope hydrology uses both stable and Y Q unstable isotopes. Stable isotopes are T U I non-radioactive, meaning they don’t emit A radiation. Unstable isotopes (or radioisotopes) T L undergo radioactive decay and are therefore N radioactive. I A T Here is a simple overview of how the science Y U of isotope hydrology works. Q Water Origin and transport of water in S the water cycle Y U Every water molecule (H O) is made of two S T 2 T L I hydrogen (H) atoms and one oxygen (O) A I N A B I atom, but these are not all the same: some atoms’ isotopes are lighter and some are heavier. Scientists use precise analytical equipment to measure these tiny weight sotopic techniques enable scientists to differences in water samples. Why? understand the components of the water Icycle, helping them better assess the quantity, As water evaporates from the sea, the quality and sustainability of water. molecules with lighter isotopes tend to preferentially rise, forming clouds with In the water cycle, groundwater is the specific isotopic signatures. These clouds least understood component. Scientists have a mix of water molecules that fall in use naturally occurring isotopes as tracers the form of rain. The water molecules with to find out whether groundwater is being heavier isotopes fall first. Then, as the replenished, where it comes from, how it clouds lose these heavy isotopes and moves underground and if it is vulnerable to move further inland, lighter isotopes fall pollution and changing climatic conditions. in a greater proportion. Water from different places has different As water falls to the earth, it fills lakes, rivers isotopic signatures or unique ‘fingerprints’. and aquifers. By measuring the ratio between Scientists use these ‘fingerprints’ to track the heavy and light isotopes in these water movement of water along its path through bodies, scientists can decipher the origin and the entire water cycle: from evaporation, movement of water. precipitation, infiltration, to runoff and evapotranspiration, then returning to the ocean or the atmosphere, and repeat. Groundwater age Isotopes are the most direct and powerful tools available to estimate the age, But what are isotopes? vulnerability and sustainability of water A chemical element, like hydrogen, is made resources. When the groundwater in an entirely from one type of atom. The type aquifer is ‘old’, this means that the water of atom comes in different varieties. These flow is slow and that the aquifer can take a varieties are isotopes, and they all share the long time to replenish. On the contrary, young same chemical characteristics and number of groundwater is easily and quickly renewed by protons and electrons, but a different number rainwater, but can also be easily affected by of neutrons. The difference in the number pollution and changing climatic conditions. of neutrons makes each isotope weigh Understanding water’s age gives scientists differently, and this weight difference is key and governments a good idea of how quickly for hydrological studies. aquifers are being replenished. 4 | IAEA Bulletin, April 2019 Water Molecules with lighter isotopes fall further inland Molecules with heavier isotopes fall sooner Scientists take samples to study the origin of water In hydrology, some naturally occurring Agriculture, industry and households each radioactive isotopes present in water, such produce different kinds of pollutants. as tritium (3H), carbon-14 (14C) and noble By studying the chemical and isotopic gas radioisotopes, are used to estimate composition of a pollutant, scientists can groundwater age. This age can be from a few determine its origins. months up to a million years. - For example, nitrate ion (NO3 ), which is Because these isotopes decay over time, made up of nitrogen and oxygen, is a common their abundance decreases as the years go pollutant. Nitrogen has two stable isotopes of by. Higher values mean ‘younger’ water, different weight. This difference in weight is while lower values mean ‘older’ water. For not the same in human waste and in fertilizers. example, groundwater with a detectable Fertilizers use nitrogen from the air, whereas amount of tritium may be up to around humans and animals go through a biological 60 years old, whereas groundwater with no process that changes nitrogen into different tritium must be older. While tritium is used forms. As a result, pollutants derived from for dating groundwater that has been recently various sources can be identified based on recharged, i.e. that is younger than about these isotopic weight differences. 60 years, carbon-14 is used for water up to 40 000 years of age and krypton-81 for Knowing the origins of pollutants is the water that can be up to a million years old first step in addressing problems with water (see page 21). quality. The data isotope hydrologists gather are useful to policymakers in their strategic planning and management of water resources. Water quality Pollutants in surface water and groundwater The IAEA supports scientists from around come from various sources — such as the world by promoting the use of isotope agriculture, industry, or human waste — or techniques, transferring scientific know-how may be present naturally due to geochemical to local water professionals. To learn more processes taking place in aquifers. about how we do this, read on. IAEA Bulletin, April 2019 | 5 .