Groundwater residence time : tell me who you are and I will tell which information you may provide

International audienceGroundwater residence-time or ages have been widely used in hydrogeology during the last decades. Followingtritium measurements, anthropogenic gases (CFC, SF6, 35Kr) have been developed. They provide informationat the aquifer scale on long residence times. They complement the more localized data obtained from sparseboreholes with hydraulic and geophysical methods.Anthropogenic tracer concentrations are most generally considered as “Groundwater ages” using a pistonflow model providing an order of magnitude for the residence time. More advanced information can however bederived from the combined analysis of the tracer concentrations. For example, the residence time distribution overthe last 50 years can be well approached by the concentration of two sufficient different anthropogenic tracersin the group (CFC, SF6, 35Kr), i.e. tracers whose anthropogenic chronicles are sufficiently different. And, withadditional constrains on geological and hydraulic properties, groundwater ages contribute to characterize theaquifer structures and the groundwater resources.Complex geological environments also include old groundwater bodies in extremely confined aquifer sections.In such cases, various tracers are related to highly different processes. CFCs can be taken as a markerof modern contamination to track exchanges between shallower and deeper aquifers, leakage processes, andmodification of circulations linked to recent anthropogenic changes. 14C or 36Cl can be used to evidence mucholder processes but have to be related to the history of the chemical element itself. Numerous field studies in factdemonstrate the broad-range extent of the residence time distribution spanning in some cases several orders ofmagnitude. Flow and transport models in heterogeneous structures confirm such wide residence times and help tocharacterize their distribution.Residence times also serve as a privileged interface to the fate of some contaminants in aquifers or to tracegeochemical processes. Among others, it is the case of (i) slow geochemical processes effective at large residencetimes such as silicate dissolution, (ii) biogeochemical reactivity activated by some modification of the flowstructure, (iii) interactions between microbial diversity and biogeochemical reactivity. While first-order kinetics iscommonly assumed, more advanced relations are developed between the nutrient and energy supply. O2 and NO3nutrient relate to recharge processes on the one hand and Fe and S relate to dissolution in reducing environmentson the other hand. Detailed studies of fractured aquifers in Brittany (France) are discusse