Age distribution of groundwater in fractured aquifers of the St. Lawrence Lowlands (Canada) determined by environmental tracers (3H/3He, 85Kr, SF6, CFC-12, 14C)

Information about the age distribution of groundwater provides unique and particularly relevant insights about groundwater flow and mixing conditions, as well as about the vulnerability and sustainability of the resource. Constraining flow paths in heterogeneous and fractured aquifers requires spatially distributed and depth-related observations. Where multiple wellbores are not available, depth-specific samples from individual long open-borehole wells may provide useful information. In this study, results from depth-specific sampling of environmental tracers (3H, 85Kr, SF6, CFC-12, noble gases, 13C, 14C) are reported from seven long open-borehole wells (depths ≤100 m below the surface) in the area of the St Lawrence Lowlands (Quebec, Canada). The multitracing approach allowed for the determination of the groundwater mixing conditions within the fractured-rock aquifers. The results indicate that fracture networks induce complex groundwater mixing patterns, without any evidence that the residence times significantly increase with the depth. Under unstressed conditions, the investigated aquifers contain 20–50% “modern” water (less than 60 years old) and 50–80% “old” water with residence times of ~4–11,000 years. The observed groundwater bi-modal age distribution is caused by fast flows through interconnected fractures and slow flows through poorly connected fractures or the primary porosity of the rock. Within the catchment of the wellfields, the pumped groundwater becomes essentially “modern” because of the preferential draining of productive fractures, while the water within the less transmissive volumes cannot be leaked out as fast. Thus, groundwater abstraction increases the aquifer’s intrinsic vulnerability to anthropogenic pollution.