Hydrological flowpaths and nitrate removal rates within a riparian floodplain along a fourth-order stream in Brittany (France)

International audienceThree main reservoirs were identified that contribute to the shallow subsurface flow regime of a valley drained by a fourth-order stream in Brittany (western France). (i) An upland flow that supplied a wetland area, mainly during the high-water period. It has high N-NO 3 and average Cl concentrations. (ii) A deep confined aquifer characterized by low nitrate and low chloride concentrations that supplied the floodplain via flow upwelling. (iii) An unconfined aquifer under the riparian zone with high Cl and low N-NO 3 concentrations where biological processes removed groundwater nitrate. This aquifer collected the upland flow and supplied a relict channel that controlled drainage from the whole riparian zone. Patterns of N-NO 3 and Cl concentrations along riparian transects, together with calculated high nitrate removal, indicate that removal occurred mainly at the hillslope–riparian zone interface (i.e. first few metres of wetland), whereas dilution occurred in lower parts of the transects, especially during low-water periods and at the beginning of recharge periods. Stream flow was modelled as a mixture of water from the three reservoirs. An estimation of these contributions revealed that the deep aquifer contribution to stream flow averaged 37% throughout the study period, while the contribution of the unconfined reservoir below the riparian zone and hillslope flow was more variable (from ca 6 to 85%) relative to rainfall events and the level of the riparian water table. At the entire riparian zone scale, NO 3 removal (probably from denitrification) appeared most effective in winter, despite higher estimated upland NO 3 fluxes entering the riparian zone during this period