A study of the organisation and functioning of a hydrogeological system in a fractured aquifer in crystalline rocks was undertaken in the Kerharo river basin in Brittany (Finistere, France). Observations and measurements have been made from the scale of the river basin scale to that of a 0.35 km 2 experimental perimeter with more than 50 water sampling points. The plurimethodological approach has revealed some aspects of the geometry of the aquifer (heterogeneity, anisotropy, connectivity, partitioning) and has provided information on the hydrodynamic situation and especially the hydrochemical functioning of the aquifer. It is controlled by redox and hydrodynamic conditions that cause vertical stratification via an oxido-reduction zone between a thin oxidizing zone containing nitrates and a nitrate-free reducing zone.

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