Coupling groundwater age tracers (CFCs) and in-stream solute time series (DSi) to decipher the reactivity of crystalline watersheds

International audienceIf weathered-derived solutes time series can characterize theexported weathering flux [kg/ha/yr] of a given catchment, thesetime series alone are uninformative to characterize theweathering reactivity of a given landscape, ie its weatheringdynamic, leading to a given water quality in streamwater.Similarly, weathering rates derived from batch reactors aredifficult to upscale to estimate the weathering reactivity at thefield scale (White and Brantley, 2003). For these reasons, wepropose here to develop a model-based coupling betweenintermediate age tracers (ages representative of circa 10 to 100years) with long and in stream dissolved silica time series (15 yrslong) to characterize the weathering enrichment rate (inmMol/yr) water in different Brittany catchments.We developed a quasi-2D hillslope model that represents thepartitioning between stratified groundwater flows and fast flowsarising close to the surface seasonally, when the aquiferintersects the land surface. Coupled to a Lagrangian particletracking technique, the model is able to represent thestreamwater transit time distributions dynamics at the catchmentscale. We then collected CFCs measurements in wells, springsand streams to characterize the mean transit time variability ofthe different water storages. We calibrate the different watercompartments (storage capacity and flux coming from thedifferent storages) with discharge time series and CFCsmeasurements.We found that the seasonal dynamic of the transit timedistributions is correlated with the DSi observations. By adding afirst order kinetic law to represent the apparent weatheringactivity, we were able to generate numerical DSi concentrationstime series and to reproduce the seasonal patterns observed. Wefound that the weathering rates in shallow Brittany aquifers arecirca 0.003 mMol/yr, consistent with previous weatheringestimated carried in crystalline aquifer