The Garonne river is the third longest river in France and the most important one for the southwest part of the country. Its hydrology is influenced by the Mediterranean and oceanic climate and the snow melt. The surrounding Quaternary alluvial aquifer is considered as a large regional reservoir and an important source for the agricultural activities well developed in the Garonne valley. The hydraulic exchange between this aquifer and the river is depending on the river’s water level. The overexploitation of water resources and the effects of climate change lead to river discharge and aquifer level to decrease over the past several years. However, this affects the economy and threaten the related ecosystems.
The Techno pole Agen-Garonne (TAG) project, green and temperate peri-urban zone, is under construction within an area of about 240 ha close to the Agen City. The project and surrounding areas, about 20 km2, is taken as a study area (Fig 1). Runoff from the TAG is collected in retention basins and used as a potential source to recharge the shallow alluvial aquifer.
In the present work, the three-dimensional (3D) groundwater model was used to evaluate the effects of the groundwater artificial recharge on the aquifer level and the maintenance of ecological low flows of the Garonne River during dry periods. The calibrated model demonstrates a good agreement between observed and simulated groundwater levels (Fig. 2).
Groundwater level measurements close to the retention basin show that the water level increases about 1 m after the rainstorm event (Fig.3). The result of the simulation illustrates that the infiltrated water, takes about 3-4 months to reach and sustain the river during low-flow summer period (Fig. 4). The relatively low temperature of inflowing groundwater into the river can be considered as an important factor to control the ecosystem function.
For better sustainable water resource management in the study area, further modelling can be performed in the context of using surface water and groundwater as a single source taking into account the agricultural and ecological needs and water scarcity.