Groundwater residence time : tell me who you are and I will tell which information you may provide
Résumé
Groundwater residence-time or ages have been widely used in hydrogeology during the last decades. Following
tritium measurements, anthropogenic gases (CFC, SF6, 35Kr) have been developed. They provide information
at the aquifer scale on long residence times. They complement the more localized data obtained from sparse
boreholes with hydraulic and geophysical methods.
Anthropogenic tracer concentrations are most generally considered as “Groundwater ages” using a piston
flow model providing an order of magnitude for the residence time. More advanced information can however be
derived from the combined analysis of the tracer concentrations. For example, the residence time distribution over
the last 50 years can be well approached by the concentration of two sufficient different anthropogenic tracers
in the group (CFC, SF6, 35Kr), i.e. tracers whose anthropogenic chronicles are sufficiently different. And, with
additional constrains on geological and hydraulic properties, groundwater ages contribute to characterize the
aquifer 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 marker
of modern contamination to track exchanges between shallower and deeper aquifers, leakage processes, and
modification of circulations linked to recent anthropogenic changes. 14C or 36Cl can be used to evidence much
older processes but have to be related to the history of the chemical element itself. Numerous field studies in fact
demonstrate the broad-range extent of the residence time distribution spanning in some cases several orders of
magnitude. Flow and transport models in heterogeneous structures confirm such wide residence times and help to
characterize their distribution.
Residence times also serve as a privileged interface to the fate of some contaminants in aquifers or to trace
geochemical processes. Among others, it is the case of (i) slow geochemical processes effective at large residence
times such as silicate dissolution, (ii) biogeochemical reactivity activated by some modification of the flow
structure, (iii) interactions between microbial diversity and biogeochemical reactivity. While first-order kinetics is
commonly assumed, more advanced relations are developed between the nutrient and energy supply. O2 and NO3
nutrient relate to recharge processes on the one hand and Fe and S relate to dissolution in reducing environments
on the other hand. Detailed studies of fractured aquifers in Brittany (France) are discussed