Evaluating radium as a time tracer for infiltrated surface water in unsteady surface -groundwater interaction areas

In the critical zone, infiltrated surface water within porous media drives many biogeochemical processes that lead to a natural attenuation of contaminants. In the case of induced bank filtration (IBF), quantifying residence time is required to anticipate water quality issues. Due to combined hydroclimatic regimes and variable pumping sequences, groundwater flow is unsteady and classical residence time quantification approaches often fail to account for such dynamics. Radium is a geogenic element with four isotopes offering ideal half-life ranges for estimating groundwater residence time. Its potential to characterize water mixing processes or date seawater has been widely proven. These advances were supported by the development of affordable analysis devices such as delayed coincidence counters (RaDeCC). In freshwater, the radium adsorption ability on the porous matrix affects its mobility and reduces its availability in dissolved form. These direct consequences of adsorption i) restrict the representativity of water samples, ii) impact the ability to measure dissolved radium by RaDeCC and iii) drastically reduce the temporal range for using radium as a chronometer of surface water residence time in an aquifer. Without a paradigm change, radium as a temporal tracer in freshwater won’t reveal its full potential. However, the third point may be appropriate for tracing dynamics of unsteady flow such as those prevailing for infiltrated surface water in IBF site. A two-years study led in an IBF site equipped with eight wells and 10 piezometers allowed 200 samples to be analysed with a RaDeCC for 223, 224, 228 and 226 radium isotopes. Preliminary results suggest that dissolved radium is more controlled by flow than by a differential adsorption, as no clear correlation was found between dissolved radium and geochemistry (ORP, pH, EC). The spatial distribution pattern of short-lived isotopes at the site was related to the pumping regimes. Annual amplitudes of dissolved radium isotopes presented a periodicity for all the wells. These first results show a satisfactory tracer response to spatial and temporal hydrodynamic flow changes, either from natural or anthropogenic origin in critical zone.