Site housing the R’Garonne infiltration facilities
© CD31 - Aurélien Ferreira
Friday 29 May marked the end of the recharge of the Garonne alluvial aquifer as part of the R’Garonne scientific experiment, carried out in Cazères-sur-Garonne by BRGM in collaboration with Réseau31, the joint water and sanitation authority for Haute-Garonne.
This experimental scientific and technical project was launched in 2023 with the aim of compensating for low water levels in the Garonne river and preserving aquatic habitats. The initiative addresses a growing challenge, highlighted by the Cerema assessment, which forecasts a reduction of up to 60% in the river’s summer flow, longer and more severe low-water periods, and water temperatures exceeding 25°C with increasing frequency in summer. Nonetheless, the water of the Garonne is a vital resource for the whole of the Haute-Garonne region, particularly for drinking water supplies, agricultural activities and industrial uses.
Controlled groundwater recharge is regarded as one of the necessary measures, set out in the climate change adaptation plan and included in the action programmes of the Entente pour l’eau (water partnership), and, at a more local level, in the Garon’amont (upstream Garonne) regional development project led by the Haute-Garonne Departmental Council.
The aim: recharge the aquifer to compensate for the Garonne’s low-water flow
The principle is based on a marked contrast between the flow rates of surface water and groundwater. Thus, a drop of water travelling down the Garonne reaches the ocean in about ten days, whilst the same drop of water travels only a few kilometres a year as groundwater.
The project involves replenishing the Garonne’s alluvial aquifer using water from the St Martory Canal during the high-water period (November to May), when the Garonne’s flow is high.
The water is channelled by gravity via the Tuchan secondary canal and then the natural Aygossau stream to the infiltration system, where it replenishes the alluvial aquifer. The water will be returned to the Garonne a few months later, when it is most needed (during the summer and when the Garonne is at its lowest water level).
The aim, therefore, is to help replenish the alluvial aquifer whilst respecting the natural water cycle.
35 piezometers fitted with data loggers continuously monitor water level, temperature and conductivity.
© BRGM
The results of the trial, to be presented in spring 2027
The R’Garonne project is a large-scale demonstration project, designed to overcome the scientific obstacles associated with this type of experimentation, with a view to eventually developing operational systems. The innovative aspect of this demonstration project for controlled recharge of the alluvial aquifer lies in the high infiltration rate involved (up to 350 l/s, or around 1 million m3 per month).
A number of monitoring devices have been installed to track changes in water levels and monitor environmental quality in the area. This network comprises 35 piezometers fitted with data loggers that continuously monitor water level, temperature and conductivity. Initial observations confirm that the system is functioning correctly, and show a significant rise in the alluvial water table due to infiltration: a rise of 2 to 4 metres has been observed within a radius of 800 metres, and of at least 1 metre over several kilometres, at the periphery.
Based on these initial observations and by extrapolating the results, it is estimated that continuous infiltration over the period in question could amount to a volume of 7 to 8 million m3 for an infiltration area of 1 hectare. This water would subsequently be returned to the river, providing the Garonne with a net benefit of around 2 to 3 million m3 of cooled water (between 10 and 15°C), which is particularly valuable for the aquatic ecosystem during the summer and low-water periods, when demand is at its highest.
The next stage of the experiment involves collecting data from the various piezometers, followed by a phase of in-depth analysis. The results will then be officially presented in the spring of 2027. This presentation will include the BRGM report as well as the Réseau31 report.
Budget for the pilot scheme: €2.5 million for the experimental projects carried out by Réseau31 and BRGM, funded by the Adour-Garonne Water Agency (56%), the Occitanie Regional Council (16%), the Haute-Garonne Departmental Council and Réseau31 (16%) and BRGM (12%).
To find out more
Press contact
