Heterogeneous fractured aquifers are extensively developed in crystalline rocks, such as schist or granite. In these aquifers, porosity ranges from the fractures to the microporosity contained in pores and fissures of the matrix. A better characterisation of such aquifers and especially adapted to search for new water reservoir in Brittany, was undertaken through: (1) temporal chronicles, isotopic analyses and laboratory experiments and (2) by the interpretation of residence times through CFC analysis. Groundwater hydrochemistry is related to rock heterogeneity leading to a more important spatial variability than the temporal or seasonal variability in the catchments. In the aquifer, nitrogen and sulphur biogeochemical cycles are linked through denitrification coupled to sulphide oxidation. A denitrification coupled to an iron oxidation in crystalline rock has been demonstrated in laboratory experiments. Sulphur and oxygen isotopic composition of sulphate show a strong relationship between the spatial localisation in the watershed and the processes in any studied site. In the deep part of the aquifer solutes from old circulating fluids are conserved in the microporosity. These solutes may contribute to the chemical signature of circulating groundwater. This type of aquifer can be divided in 3 compartments: recharge areas, weathered part and deep part. In each compartment, the groundwater residence times are homogeneous at the scale of Brittany: (1) young water in the recharge areas, (2) old waters (> 50 yr old) in the deep fractured part and (3) a 18 ± 2 yr old water in the weathered part of aquifers. While the weathered part supports the water flux to rivers, these results indicate that water management should to be done at the same timescale.
Détermination du temps de résidence des eaux souterraines : application au transfert d'azote dans les aquifères fracturés hétérogènes