Summary of field activities :
Groundwater wells were hand-augered on the motus Honuea, Tahuna Iti, Onetahi and Tahuna Rahi. Water samples were collected and aquifer tests (measurements of the soil's capacity to store and transmit water) were carried out. Electromagnetic surveys (a technique for measuring underground properties from the surface, like an X-ray of the ground) were also conducted on these motus to measure the depth and thickness of the freshwater lenses.
Preliminary results :
Analyses reveal strong chemical variations in groundwater, closely linked to land use and land cover. The motus Tahuna Iti and Tahuna Rahi, which support dense seabird populations, show high nitrate concentrations (nutrients derived from bird droppings) despite generally low-oxygen conditions. An asymmetry was also identified in the geometry of the freshwater lenses, which are significantly thicker on the ocean-facing side than on the lagoon side. These findings suggest that the recovery of bird populations will directly affect the composition of fresh water on the motus and, ultimately, the health of the surrounding coral reefs.
Perspectives :
The next step will be the installation of permanent multi-depth wells on several motus to monitor changes in groundwater chemistry over time. A return visit is planned for August 2026. A scientific publication is in progress on evaluating atoll restoration through groundwater nutrient budgets.