Excess nutrient loads have been recognized to be the major cause of serious water quality problems recently encountered in the North Carolina estuaries and coastal waters. There has been a particular concern in coastal watersheds because agricultural and forested lands are located in close proximity to recreational and environmentally sensitive waters. The key to nutrient management at the watershed scale is the understanding and quantification of the fate of nutrients at the field scale and after they enter the aquatic environment. There is no accepted method to describe and predict fate of nutrients in canals and streams. The purpose of this research was to investigate the magnitude of the effects of in-stream processes in agricultural canals of the lower coastal plain and to propose a modeling approach for quantifying nitrogen transformations in such canals. This was accomplished in four steps. The first step was an extensive review of the literature on nitrogen retention in agricultural streams. Nitrogen removal rates in most agricultural canals and streams vary between 50 and 800 mg N/m²/d, with mass transfer coefficient varying between 0.01 and 0.10 m/d. The magnitude of nitrogen retention in streams and canals of agricultural watersheds has been reported to vary between less than 5% to more the 60% of the gross load. In the second step, the effects of biogeochemical processes on chemical and nutrient loads was evaluated in a 1125-m long agricultural canal reach of the lower coastal plain near the town of Plymouth, NC. Chemical and nutrient loads at both ends of the reach were measured by continuous measurement of flow and concentrations. Flow measurements were made using trapezoidal flumes in which flow velocity and depth was continuously measured and recorded with velocity meters. Nutrient concentrations were measured on water samples taken both manually and automatically at strategic times along the hydrographs so that linear interpolation between two consecutive samples could be made. Nutrient addition due to seepage along the reach was estimated. After corrections for lateral contribution, it was estimated that, over the 14-month measuring campaign, 3% of the total nitrogen load entering the upstream end was retained within the reach. This was mostly due to the combination of nitrate retention and release of organic nitrogen (ON) within the reach. Up to 10.2 % of the total phosphorus load measured at the upstream station was retained while 10% of the total suspended solids was also retained. There was a release of inorganic carbon equal to 18.7% more that the load measured at the upstream end. Measurements of algae and macrophyte biomass within the reach, and, measurements of nitrogen and carbon concentration profiles at the sediment-water interface revealed that most of nitrate retention was likely due to denitrification after diffusion from the water-column to the sediment. Release of organic nitrogen was attributed to flux of refractory organic nitrogen from the sediment into the water-column. Assimilation by algae and macrophytes may have accounted for as much as 20% of the total retention of inorganic nitrogen. Rates of nitrate removal and release of organic nitrogen were estimated using the model DUFLOW. Nitrate removal rates varied between 200 and 800 mg NO3-N/m²/d, while release rates of organic nitrogen varied between 100 and 400 mg ON/m²/d. A mass transfer coefficient of 0.3 m/d was obtained for nitrate at two distinct periods of the year. A simple approach was proposed for modeling nitrogen transformations in canals of the lower coastal plain. Transformations are simplified as the combination of downward diffusion of water-column nitrate into the sediment and an upward diffusion of organic nitrogen from the sediment.

Les eaux résiduaires urbaines du bassin versant du lac du Bourget sont largement collectées et traitées. Les gestionnaires du lac s'interrogent, dans le contexte de l'évolution climatique et de l'évolution des usages des sols, sur l'impact et les modalités de maîtrise des pertes en phosphore et en particulier de la contribution des sources diffuses d'origine agricole. La thèse vise à modéliser les régimes d'exportation du phosphore vers le lac, dans la perspective de tester des scénarios de maîtrise des flux. Elle s'inscrit dans un dispositif de recherche opérationnelle CIFRE entre le bureau d'étude SAFEGE et l'UMR CARRTEL, en partenariat avec le CISALB (Comité Intersyndical pour l'Assainissement du Lac du Bourget). Dans ce cadre, deux sous bassins versants expérimentaux emboîtés représentatifs de la géographie du bassin du Bourget, (l'un rural dominé par l'élevage laitier de 250 ha, l'autre périurbain et agricole de 6800 ha) ont été équipés et suivis à leur exutoire durant une période de 18 mois. Les suivis hydrochimiques ont permis une caractérisation qualitative et quantitative des régimes d'exportations à l'exutoire de chaque bassin versant (flux hydrologique, de matières en suspension, du phosphore total, particulaire, et dissous, et de certaines formes d'azote – nitrate, ammonium). Ces flux ont été modélisés à l'aide du modèle SWAT. Pour rendre compte de certains processus spécifiques de mobilisation et de transfert de la charge en phosphore, des développements ont été mis en œuvre (extension de l'aire de saturation des sols en relation avec la mise en charge des nappes, stockages transitoires, remobilisations au sein du réseau hydrographique) qui constituent une des principales avancées de la thèse. Les résultats obtenus apportent une amélioration des connaissances scientifiques sur les mécanismes de transfert du phosphore. Les développements en termes de modélisation constituent un outil adapté pour accompagner la mise en place d'un plan d'actions visant la réduction des flux de phosphore parvenant au lac du Bourget, ceci dans la perspective des évolutions climatiques. / Urban wastewater are now widely collected and treated. Many questions still remain about the impact of climatic or anthropic evolution on the mobilisation and the transfer of phosphorus to Lake Bourget. This work deals with hydrological and nutrient modelling to understand phosphorus mobilization and transfers processes and to test mitigation options. Thesis is supported by a CIFRE agreement between SAFEGE office and UMR CARRTEL, in partnership with the CISALB. In this context, 2 experimental watersheds nested and variable in land uses (one agricultural of 250 ha, the other suburban and agricultural of 6,800 ha) were instrumented and followed at their outlet over a period of 18 months (continuous measurement of flow and some key parameters of physico-chemical water quality). Hydro-chemical measurements led to a qualitative and quantitative characterization of the exports regime at the outlet of each watershed (water flow, suspended solids, some forms of phosphorus – total, particulate, dissolved - and nitrogen). The exports were then modeled. To account for some specifics processes of mobilization and transfer of phosphorus (variable source areas, "in-stream" processes), some developments of the SWAT model were implemented. IIn this context, this works provides an improved scientific understanding of the mechanisms of transfer of phosphorus. Modeling tool can support the development of mitigation options for best management practices included in the framework of the "Lake Bourget watershed contract".