Insight into Groundwater Flow Within a Crystalline Aquifer. Case study of the Ursuya Mount, Northern Basque Country (France)

Stable isotopes of the water molecule and tritium in conjunction with geochemistry have been used to understand the groundwater flow pattern and origin within the gneissic aquifer of the Ursuya Mount. This aquifer constitutes one of the main water supplies of the Northern Basque Country and improved knowledge about the water recharge, origin, quality and residence time is of strategic importance for a sustainable development of the resource. 16 springs, 4 boreholes and total rainfall have been sampled monthly since summer 2009. Preliminary results indicate complex and contrasted groundwater flows within the aquifer. Shallow groundwater restricted to the upper weathered horizons is characterized by a strong influence of anthropogenic inputs. At depth, groundwater mainly circulates along the major structural discontinuities of the gneiss in semi-confined conditions. These levels, providing the most promising resource for the water supply, show increased water mineralization caused by a longer residence time of groundwater and hence indicate a slower dynamic of the system. (author)