Quantifying shallow and deep groundwater inputs to rivers withgroundwater dating in hydrological observatories.

International audienceRiver water derives in part from groundwater—water that has spent some time in the subsurface (e.g. soil,unsaturated zone, saturated zone). However, because groundwater residence times vary from months to millennia,determining the proportion of shallow and deep groundwater contribution can be challenging. Groundwater datingwith anthropogenic gases and natural geochemical tracers can decipher the origin of groundwater contribution torivers, particularly when repeat samplings are carried out in different hydrological conditions. Here, we presenttwo different applications of this approach from three hydrological observatories (H+ hydrogeological network;Aghrys and Armorique observatories) in western France, all these observatories belonging to the OZCAR nationalnetwork.We carried out a regional investigation of mean groundwater ages in hard rock aquifers in Brittany, using long-termchronicles from hydrological observatories and regional monitoring sites. We determined the mean residence-time(RT) and annual renewal rate (RR) of four compartments of these aquifers: the direct contribution of a veryyoung water component (i.e. RT less than 1-2 yr), the upper variably saturated zone (RR 27-33%), the weatheredlayer (RR 1.8-2.1%) and the fractured zone (RR 0.1%). From these values and a nitrate chronicle, we wereable to determine the respective contributions of each compartment to the largest river in Brittany, the Vilaine,which drains 30% of the region. We found that the deep fractured compartment with very slow renewal timescontributed to 25-45% of river water in winter and 30-60% in summer. The very young water which includesdirect precipitation and soil fluxes constituted 40-65% of the winter river water (Aquilina et al., 2012).To complement these estimates, we investigated the relationship between dissolved silica and groundwater agein the Armorique hydrological observatory in northern Brittany. We computed the silica concentration expectedalong the river from the groundwater age deduced from a deterministic model of the aquifer (Kolbe et al.,2016). The relationship between silica concentration determined with anthropogenic gases and observed silicaconcentration was strong (R2= 0.54-0.92), indicating that silica was a reliable geochemical chronometer, thoughit systematically underestimated anthropogenic gas age estimates. The difference could be accounted for by thevery young water contribution : approximately 20 - 40% of overall discharge.Both approaches indicated that very young water is particularly important during winter and that deep groundwatercontributes at least a third of the river discharge throughout the year. This last result has implications for rivernitrate dynamics and understanding the potential limits of catchment management interventions which only reducenitrate dynamics in shallow groundwater on decadal timescales.Aquilina L. et al., 2012 - Nitrate dynamics in agricultural catchments deduced from groundwater datingand long-term nitrate monitoring in surface- and groundwaters. Sci of the total Environment 435, 167-178.Kolbe et al., 2016 - Coupling 3D groundwater modeling with CFC-based age dating to classify local groundwatercirculation in an unconfined crystalline aquifer. J. of Hydrology DOI: 10.1016/j.jhydrol.2016.05.02