Did you ever wonder how old the water is that you see flowing at Sabino Dam?
Probably not, if you’re not a hydrologist. But this is an important question in relation to the water resources available in our water-stressed environment.
The age of stream water is the length of time that passes from when rainfall delivers water to a catchment, say during a monsoon storm in August, to when the water exits the catchment as stream flow, say at Sabino Dam in October.
The time water spends in a catchment defines its quality, such as the amount of solutes it contains, as well as how much water is available to ecosystems. Until recently, hydrologists had a hard time determining the age of stream water.
At the Landscape Evolution Observatory (LEO) at Biosphere 2, hydrologists with the University of Arizona are testing a revolutionary method to measure the age of water in the landscape.
It involves state-of-the-art laser spectroscopy that can measure the stable isotope composition of rain, soil and stream water and uses the relative differences between the various sources to estimate the age of water.
Not all water molecules are created equally. Rainwater falling at different times often differs in its content of specific rare isotopes, providing a “time stamp” imprinted on water entering a catchment. The rare stable isotopes of the atoms that make up the water molecule are deuterium or 2H, oxygen-17 or 17O, and oxygen-18 or 18O. The number in front of the chemical element indicates the atomic mass.
These isotopes have extra neutrons in the atomic nucleus and are therefore heavier than the much more abundant hydrogen (1H) and oxygen (16O) atoms.
They can be detected using mass or laser spectrometry because of the differences in mass or sensitive features of infrared light absorption.
Because water molecules that contain such rare isotopes behave otherwise identically to normal water molecules, they form the ideal tracers that nature provides to follow water particles as they move through our landscapes.
Combined with over 1,800 automatic sensors and samplers embedded in the soil of the LEO landscapes, the new isotope laboratory will allow hydrologists to measure in real-time the ages of soil and stream water.
After intensive testing and calibration of the system, it will be implemented in the Catalina Mountains to measure water ages in real landscapes, providing a scientific breakthrough in experimental research and water-resources management.