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Summary

We interpret regional electromagnetic (EM) data and chemical analyses of water, sampled in springs and seepages, from an area in St. Anne, near Brest (Finistére, France). The area is covered by a thin homogeneous arenaceous soil. We surveyed the region with both a controlled-source EM system (Geonics EM31) and a very-low-frequency (VLF) system. The controlled-source survey was sensitive mainly to the soil layer; the VLF system, to structures at depth. Inversion of the VLF data shows evidence of two different structural units below the soil. The first, which is resistive, is interpreted as a massive crystalline gneissic structure. In this zone, the water drainage is restricted to the topsoil. In the southwest, there is a heterogeneous conductive domain that we interpret as a fracture network able to drain water along the structural strike. Differences in water quality in the St. Anne area are associated with the different electrical structures at depth.

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