title: Catchment-scale quantification of hyporheic denitrification using an isotopic and solute flux approach.
abstract: A dual-isotope and solute flux mass-balance was used to elucidate the processes that lead to attenuation of nitrogen contamination in an agriculturally impacted river. The River Wensum drains a lowland catchment with an area of 570 km² in East Anglia, eastern England. Analysis of nitrate concentration, δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of samples from the River Wensum collected from upstream locations to the catchment outlet through all seasons and flow conditions showed a consistent pattern of increasing isotope values with decreasing nitrate concentrations downstream. δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of catchment surface water and groundwater samples revealed a dominant influence from microbially cycled and nitrified source-nitrogen, which results in high nitrate concentrations in Chalk groundwater and upstream in the River Wensum. Denitrification of Chalk groundwater-baseflow in the hyporheic zone results in the downstream trend observed in the river. Hyporheic denitrification is estimated to remove 931 kg/day of nitrate-nitrogen by the catchment outlet, representing 31% of the potential riverine nitrate load. The use of dual-isotope and solute flux modeling at the catchment scale is a novel application to quantify denitrification within the river valley, demonstrating the importance of hyporheic zone processes in attenuating the impacts of anthropogenic contamination of hydrologic systems.
authors.affiliation: School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK. skw66@cornell.edu
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Wexler
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Hiscock
authors.forename: Paul F
authors.initials: PF
authors.lastname: Dennis
mesh_terms: D000383:Agriculture; D002712:Chlorides / Q000032:analysis; D058440:Denitrification; D004739:England; D004784:Environmental Monitoring; D005618:Fresh Water / Q000032:analysis*; D007554:Isotopes / Q000032:analysis; D008956:Models, Chemical; D009566:Nitrates / Q000032:analysis* / Q000302:isolation & purification; D009584:Nitrogen / Q000032:analysis* / Q000302:isolation & purification; D014872:Water Movements; D014874:Water Pollutants, Chemical
chemical_list: D002712:Chlorides; D007554:Isotopes; D009566:Nitrates; D014874:Water Pollutants, Chemical; D009584:Nitrogen