Addressing water security using 36Cl

Semi-arid regions, characterized by low and erratic rainfall, need significant efforts in managing their water resources. Groundwater, a vital resource in these areas, is often overexploited, leading to depletion and degradation. The lack of suitable data and methods to quantify regional hydrological processes often requires a comprehensive understanding of groundwater systems, including their recharge rates, flow patterns, and water quality and the adaptation to climate change.

To date, groundwater management is primarily based on hydrogeological modeling and key parameters such as recharge rate and groundwater dynamics.

The use of radioisotopes makes it possible to date groundwater resources and evaluate its recharge times. By using a combination of residence time indicators (3H, 14C, 36Cl) and stable water isotopes (2H and 18O), it is possible to provide a greater constraint on the residence time of water in groundwater aquifers.

Thanks to the advancement analytical techniques on the use of 36Cl, present in the environment following nuclear tests, is a promising method for estimating water transit times and recharge rates of aquifers on a basin scale and for distinguishing water and chloride cycles.

Studies have already been carried out in the Chari-Logone aquifer of the emblematic Lake Chad basin, located in the central Sahel, where the analysis of 36Cl in the central areas shows the presence of very old groundwater (<2 Ma), suggesting that the aquifers in the Sahel host a significant amount of renewable water, which could therefore be used as a strategic freshwater resource.

Continued investment in developing reliable and less time-consuming analytical techniques is crucial to manage groundwater resources sustainably in semi-arid regions.