Periodic Changes in the Nitrate Concentration of Stream and in Seepage Water in a Low-Relief Coastal Stream

Previous studies have shown relationships between evapotranspiration in riparian vegetation and diurnal changes in stage/discharge in adjacent streams. Such diurnal changes have been linked to diurnal changes in NO3- concentration in Cobb Mill Creek through alteration of retention time in biologically active stream sediment. Using stream stage records to confirm the diurnal effects of evapotranspiration on Cobb Mill Creek, the present study asked 1) if there is a periodic (i.e., diurnal) variation in water column NO3- concentration similar to that observed for the stream stage? 2) do spatially distributed measurements of inseepage also show such a diurnal pattern? 3) does the concentration of NO3- in the inseepage vary diurnally? To address these questions, seepage meters were distributed along a reach of Cobb Mill Creek. A 10-day study in June 2010 confirmed a diurnal signal in both the stream stage and the NO3- concentration of the water column. However, the discharge and NO3- concentration of the inseepage from the meters lacked a similar diurnal signal. A second 3-day field campaign (September, 2010) employed similar methods to the first, but with a 2-hr sampling interval. The diurnal signal in stage was damped in September compared to June due to reduced precipitation during the summer which was presumed to have reduced plant activity and led to a significantly lower stream stage. Again, no diurnal signal was observed in the streambed discharge, inseepage NO3- concentrations or the stream water column NO3- concentrations. The results of this study confirmed the connection between stream stage, NO3- water column concentration and evapotranspiration. Unfortunately, it did not clarify if there is a connection between evapotranspiration and NO3- concentration of inseepage water even though theory predicts strongly that one should exist. Additional work will be necessary to demonstrate that relationship.