QUANTIFYING NITROGEN TRANSPORT FROM GROUNDWATER TO A STREAM IN A LARGE AGRICULTURAL WATERSHED IN NORTH CAROLINA

We used spatial integration of point measurements of groundwater seepage and nitrogen concentrations to quantify groundwater-based nitrogen (N) input to a large stream in a heavily-agricultural watershed.  We focused directly on flux across the stream bed by making all field measurements (hydraulic head gradient J, hydraulic conductivity K, and groundwater N concentrations) in the stream bed (J and K apply to the top 36 cm of bed, water samples for N measurement were drawn from a depth of 34 cm).  Measurements were made at 38 points (Dec 2005) or 46 points (Feb, April, and June 2006) over a 263 m reach of West Bear Creek in the North Carolina Coastal Plain.  Spatial integration of fluxes over the stream bed was based on interpolating among point measurements on a fine grid with an exponential variogram model in SURFER software.  

Groundwater input to the reach was 802, 866, 660, and 484 m³/day for December (2005), February, April, and June (2006), respectively, and was related to mean stream bed K.  Groundwater discharge rate was about twice as large in the center of the channel (averaging about 0.5 m/day) as near the banks.  Groundwater-based total dissolved nitrogen (TDN) input to the reach was 192, 233, 229 mol/day for December, February, and April, respectively; the majority was in the form of nitrate, with low amounts of ammonium (<3 mol/day) and a decrease in DON from around 29 mol/day in December and February to 4 mol/day in April.  Although groundwater discharge to the reach is greater at the center of the channel, groundwater-based N flux is larger near the banks where N concentrations in groundwater are generally higher.  However, the increase in groundwater-based TDN input from December to February is due to a 2-fold increase in the flux of TDN at the center of the channel.  From February to April, greater groundwater input near the banks and lower input near the center of the channel allowed groundwater-based TDN input to remain about the same despite a 24% decrease in groundwater input to the reach.