QUANTIFYING NITROGEN CYCLING BETWEEN SURFACE WATER AND GROUNDWATER IN A LOW GRADIENT STREAM IN AN AGRICULTURAL SETTING

The aim of this study is to characterize stream-groundwater exchange at the fluvial plane scale in terms of mass flux of nitrogen in a low gradient agricultural setting where major non-point sources of nitrogen are nitrogen fertilizers and biodegradation. A numerical model will be combined with the concentrations of major N species to estimate mass fluxes of N into and out of the stream. Water samples have been collected from the stream and a network of wells installed to monitor major ions and concentrations of dominant nitrogen species. Stream water shows high NO3- concentration (10 mg N/L) during winter and spring and very low to non detectable NO3- concentration during summer and fall. Groundwater dissolved organic nitrogen (DON) concentrations are low (around 1 mg N/L) in the floodplain samples whereas it is much higher (0.9 to 2.9 mg N/L) in the meander samples. NO3- is below the detection limit for most of the groundwater samples and varies spatially from 0.2 to 1.5 mg N/L with highest concentrations in meander samples. NO3- concentrations in groundwater also vary temporally and are below the detection limit for most of the samples during winter and spring. The concentration of NH4+ also varies spatially and temporally within the study reach. NH4+ concentrations range from 0.03 to 2.37 mg N/L in the samples collected from meanders, and increase during the summer. Results of the present study indicate that the groundwater reservoir serves as a source or a sink for N depending on the amount of nitrate in the stream water. During winter and spring when the stream NO3- concentrations are high, groundwater acts as a sink due to denitrification as stream water flows beneath the meanders. And during summer and fall, when the stream NO3- is low, groundwater serves as a small net source due to discharge of DON and NH4+ to the stream. Discrete seasonal Cl- plumes due to road salting enter the hyporheic exchange zones from the stream and flow beneath the meanders, and can be used to calibrate the numerical model. This study provides a conceptual model for a numerical model to quantify mass fluxes of dominant nitrogen species and groundwater flow where three independent parameters, hydraulic head, stream discharge and Cl- will be used to calibrate the 3D groundwater model.