Heterogeneous fractured aquifers are extensively developed in crystalline rocks, such as schistor granite. In these aquifers, porosity ranges from the fractures to the microporosity containedin pores and fissures of the matrix. A better characterisation of such aquifers and especiallyadapted to search for new water reservoir in Brittany, was undertaken through: (1) temporalchronicles, isotopic analyses and laboratory experiments and (2) by the interpretation ofresidence times through CFC analysis.Groundwater hydrochemistry is related to rock heterogeneity leading to a more importantspatial variability than the temporal or seasonal variability in the catchments. In the aquifer,nitrogen and sulphur biogeochemical cycles are linked through denitrification coupled tosulphide oxidation. A denitrification coupled to an iron oxidation in crystalline rock has beendemonstrated in laboratory experiments. Sulphur and oxygen isotopic composition of sulphateshow a strong relationship between the spatial localisation in the watershed and the processesin any studied site. In the deep part of the aquifer solutes from old circulating fluids areconserved in the microporosity. These solutes may contribute to the chemical signature ofcirculating groundwater.This type of aquifer can be divided in 3 compartments: recharge areas, weathered part anddeep part. In each compartment, the groundwater residence times are homogeneous at thescale of Brittany: (1) young water in the recharge areas, (2) old waters (> 50 yr old) in thedeep fractured part and (3) a 18 ± 2 yr old water in the weathered part of aquifers. While theweathered part supports the water flux to rivers, these results indicate that water managementshould to be done at the same timescale.
