Résumé
A multi-layered aquifer made up of alluvial formations from the Quaternary, overlying levels of Pliocene sands and clays is located beneath the Roussillon plain (Perpignan, France). Covering an area of 800 km² and up to 300 m thick on land, this aquifer extends several dozen kilometres beneath the Mediterranean Sea. It contains a freshwater resource which, at a greater or lesser distance from the shoreline, is likely to be mixed with seawater. A hydrogeophysical observatory (Dem'Mer) has been set up along the coastline for monitoring the piezometric and physicochemical (conductivity, temperature) behavior of the groundwater flowing through the various permeable levels described within this multi-layer aquifer. The time series of groundwater levels obtained from this observatory for several years have been processed using spectral analysis to explore the seasonal to daily influence of meteomarine forcings (seawater level, tides, storms) on groundwater flows. Complementary, crosscorrelation and coherence analyses have been performed to identify how the different signals are organised over time. This work provides field evidence of the influence of storm waves forces at sea bottom, water level at the shoreline and tide on the piezometric fluctuations in the aquifer nearby the coast. The results obtained constitute constraints that can be incorporated into the modelling work currently underway on this aquifer.