Water residence time bycoupling anthropogenic gases(CFC, SF6) and isotopic ratios(234U/238U, 87Sr/86Sr) in theStrengbach catchment (VosgesMassif, Eastern France)

International audienceWeathering processes are active in surface watersbut groundwater also represents no negligiblechemical fluxes. As residence-time in groundwaterare high, silicate weathering might take place andcontrol Si, Ca and C fluxes. Weathering processescan be deduced from U isotopic ratios but the kineticsof these processes remain relatively poorlyconstrained.In order to better characterize these processes,residence-times deduced from anthropogenic gases(CFC and SF6) analysis and 234U/238U isotopic ratiosdetermination have been coupled. Water sampleswere collected in both springs and boreholes in thegranitic Strengbach catchment (Hydro-geochemicalObservatory OHGE, Vosges Massif, Eastern France).Two campaigns were carried out in May and August2015 during highly contrasted hydro-climatic periods.A very clear geochemical distinction is observedbetween groundwater from surface springs anddeeper groundwater from boreholes. Springs showmuch lower residence time (few years) and specificchemical composition. Deeper groundwater hasresidence time of several decades and differentgeochemical composition.The 234U/238U isotopic ratios confirm this verticalzonation in the boreholes, with much higher activityratios in depth in the borehole than in the surface andspring waters; such high ratios are indicative of longwater-rock interactions, which is consistent with thelong residence times deducted from the CFC and SF6data