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

Three 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-NO3- 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-NO3- 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-NO3- 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, NO3- removal (probably from denitrification) appeared most effective in winter, despite higher estimated upland NO3- fluxes entering the riparian zone during this period. Copyright (C) 2003 John Wiley Sons, Ltd.