Lutte contre la sécheresse au Zimbabwe : utiliser la science nucléaire pour mieux comprendre la dynamique des eaux souterraines et des cours d’eau

A water shortage from droughts is not the only problem; a lack of clean water is a growing issue.

River water is susceptible to contamination and is open to many types of pollutants. “If the river is contaminated, the contamination will automatically flow through to the groundwater. As these two water bodies are not isolated from each other, it is extremely important to understand how they interact with each other, not only in terms of water amounts but also in terms of water quality,” said Ioannis Matiatos, isotope hydrologist at the IAEA.

“By using stable isotopes of oxygen and hydrogen and naturally occurring radioactive isotopes such as tritium and radon-222 to trace water, we can better understand the complex dynamics of river and groundwater systems,” he said. This allows scientists to understand which water body is contaminated and how to replenish it.

Both river and groundwater are important in providing clean water for drinking, urban, and agricultural use within the Save Catchment — the focus area of the IAEA project. Located in the eastern part of Zimbabwe, the catchment receives limited rainfall during its dry season, leaving it prone to drought. With a growing population and an economy reliant on agriculture, the demand for water in the catchment area is growing.

To better understand the relationship between river water and groundwater, counterparts from the University of Zimbabwe received training in 2018 at Addis Ababa University in Ethiopia, and at the end of that year, Mhizha learned at the IAEA laboratories in Vienna, how to evaluate geological, hydro-chemical and hydrological data, and design a field sampling campaign for the study sites.

In June 2021 experts from the Helmholtz Centre in Leipzig, Germany, conducted a five-day virtual training course with local experts. They were instructed in the basic principles of isotope hydrology with special emphasis on the use of stable and radioactive tracers as tools to investigate surface water-groundwater interactions; groundwater dating techniques; and the vulnerability of aquifers to contamination.

This online training will be followed with an in-person expert visit to the country by the end of the year, to further train Zimbabwean experts on sampling and analytical techniques, interpretation of results, and in using specialised laboratory equipment. Local scientists will then be able to independently continue isotopic analysis over several seasons to generate data and understand interactions between the different components of the water cycle.

“The isotopic results found by local researchers will help identify the challenges Zimbabwe’s national groundwater management system needs to tackle and, in turn, enable more sustainable management of the country’s water supply,” said Michael Schubert, a mineralogist at the Centre in Leipzig and one of the trainers of the course. 

Using environmental isotopes to trace surface water and groundwater pathways

Bodies of water are naturally labelled with unique isotopic signatures. A laser spectroscopic analyser is used to determine the unique isotopic signature or fingerprint based on the relative proportion of the different isotopes present in a water sample. By using these signatures, water origin and movement can be tracked through the entire water cycle.

Naturally occurring radioactive isotopes, such as radon-222 (Rn-222), tritium (hydrogen-3), carbon-14 (C-14) are powerful tools that allow scientists to date very young to old groundwater systems and trace their contribution in river water discharge. This information informs water specialists on the nature, history and flow of sampled river and groundwater and helps them calibrate and improve numerical models that predict the response of rivers and aquifers to abstractions and climate change.