Quantification of nitrate removal by a flooded alluvial zone in the Ill floodplain (Eastern France)

The nitrate reducing capacity of a flooded system in the Ill floodplain (Ea stern France) was investigated for a period of 2 years. The methodology use d consisted of a spatio-temporal monitoring of stream flow and nitrate conc entrations in the groundwater and surface water, calculation of input and o utput fluxes and modelling of groundwater fluxes and nitrate transfer throu gh the alluvial area. A comparison of chloride flux (used as hydrological t racer) and nitrate flux was done to determine a floodplain effect on the re tention of nitrate. We show that up to 95% of the nitrate load in the groun dwater is retained by the system, whereas the retention in the stream netwo rk is very low. Ammonium fluxes increased from inputs to outputs in the str eam and in the groundwater. The chloride input in the groundwater is higher than the output, whereas in the surface water the output is higher than th e input, the amount evacuated in streams corresponding to the losses from g roundwater. The nitrate removal rate calculated for the whole modelized sur face area (40 km(2)) represented 559 t N yr(-1) or 1397.7 kg N ha(-1) yr(-) 1. The ammonium fluxes exported by the system represented 102 kg N ha(-1) y r(-1). A part of nitrate is reduced and exported by the groundwater and str eam network in the form of ammonium. These results can be explained by the duration of floods which controls the equilibrium between the various forms of nitrogen. Thus, long watering periods favour nitrogen removal (denitrif ication and plant uptake) and limit nitrate production which compensates el imination during the dry period.