The aim of this study is to identify the processes that lead to the ground surface deformation above the fractured crystalline aquifer of Ploemeur (French Brittany). We conducted a four-year continuous GPS data survey in order to extract the hydrogeological contribution due to the variations of the hydraulic head of the confined aquifer. The GPS configuration used in this study, with a differential setup, short baselines and adapted processing parameters, may have removed tide effects, ocean tide loading and tectonics trends. We checked in particular the case of ocean tide loading because of its important contribution in Brittany. Time series calculated with GAMIT software indicate a seasonal deformation on both vertical direction (up to 16 mm of displacement) and horizontal plane (from 3 to 12 mm of displacement) that provide a 3D motion. This sub-annual deformation is induced by variations of the hydraulic head of the aquifer and depends on its 3D hydromechanical properties. However, an offset between ground uplift and piezometric rise could not be explained by pressure variations into the deep aquifer and was linked to the variations of the hydrous state of the ground. This GPS study highlights that the hydrogeological and hydrological processes act significantly on ground motion and should be integrated in any Earth deformation survey.