Saline intrusion poses a threat to the use of coastal aquifers for water supply. To anticipate this problem, the Mayotte Water Agency asked BRGM to carry out a study to improve knowledge and understanding of the phenomena involved in eight coastal areas where groundwater contributes to the drinking water supply.
30 October 2023
Water abstraction at a piezometer (Kawéni, 2021)

Water abstraction at a piezometer (Kawéni, 2021)

© BRGM - A. L’hotelier

The need

For several years, there has been increased pressure on water resources used for drinking water supply on the island of Mayotte. In this context of water stress, increasing pumping at a few boreholes remains an option to reduce the pressure. The Mayotte Water Agency has therefore commissioned BRGM to improve knowledge of aquifers developed for water supply in coastal areas.


By implementing an electrical resistivity tomography campaign on the ground and a water sampling campaign, reinterpreting airborne electromagnetic data acquired in 2010, and carrying out geochemical and isotopic analysis of water samples and geochemical logging near water infrastructure, the study conducted by BRGM provided a summary of saline intrusion evidence and the risk of this impacting boreholes where drinking water is pumped, in each area of interest.

The results showed that, despite the boreholes being close to the coastline, most aquifers managed for water supply were not currently affected by the saline wedge. There is therefore no immediate threat connected to the use of these boreholes in the eight areas investigated.

Using the results

The results were useful for improving general knowledge of managed aquifers in Mayotte. All the data produced during this study will be used to implement appropriate checks at the managed boreholes and at the piezometers monitored by BRGM, in order to monitor saline intrusion and take action to prevent sea water intruding further. 

The study also provided reassurance about the few boreholes where pumping could be increased in the future.


  • Mayotte Water Agency