Tracking groundwater signature in the stream to characterize flow and denitrification in the aquifer

International audiencePredicting the evolution of nitrate pollution at the catchment scale implies a global evaluation of residence timesand denitrification rates in the aquifer. Data acquired in wells only result from processes occurring in their capturezone and are rarely representative of the whole aquifer [1]. Stream data, on the opposite, integrate the processesoccurring along all contributing groundwater flowpaths and thus could be more relevant for upscaling. But streamsignal is also controlled by soil, hyporheic zone and in-stream processes. Here we investigate the possibility ofextracting groundwater signature from such a complex stream signal.In-stream spatially-distributed measurements were performed during low flow over a 35 km2 crystallineagricultural catchment in Brittany, Western France. Groundwater inputs into the streams were localized andquantified using radon mass balance. In conjunction with radon, dissolved gases (O2, CO2, N2, N2O, CH4, Ar),wet chemistry and nitrate isotopes were measured at each site. Quantification of in-stream processes was furtherrealized by performing tracer injections in three selected reaches. Injections were coupled with continuous in-situmeasurement of dissolved gases using membrane inlet mass spectrometry (MIMS). Helium injections allowedto calculate gas exchange rates, while nutrient injections (acetate, nitrate) allowed to characterize in-streambiological activity (oxygenic respiration, denitrification).Radon mapping reveals a high localization degree of groundwater inputs into the streams. Groundwatersurfacewater exchanges are strongly concentrated after a major topographic slope that separates two geologicalunits, showing that geomorphology is a major control of hydrogeological circulations. These results are consistentwith a 3D-groundwater flow model of the catchment calibrated with age tracer data acquired in wells [1]. Close togroundwater input zones, stream chemistry is strongly impacted by groundwater signature. Helium and nutrientinjections evidence that gas exchanges between stream and atmosphere constitute the main eraser of groundwatersignature as water is flowing dowstream. Groundwater signature is lost specifically at highly turbulent sites, suchas small cascades, where measured gas exchange rates are one to two orders of magnitude higher than in calmreaches.[1] Kolbe, T., J. Marcais, Z. Thomas, B. W. Abbott, J. R. de Dreuzy, P. Rousseau-Gueutin, L. Aquilina,T. Labasque & G. Pinay, 2016. Coupling 3D groundwater modeling with CFC-based age dating to classifylocal groundwater circulation in an unconfined crystalline aquifer. Journal of Hydrology 543:31-46doi:10.1016/j.jhydrol.2016.05.020