Diurnal Variations in Nitrate Concentrations in the Cobb Mill Creek, Virginia

The spreading of nitrate-based fertilizers on agricultural fields of the Eastern Shore of Virginia has raised concerns about their effect on water quality in streams and groundwater of the Delmarva Peninsula and in adjacent seaside lagoons. During baseflow conditions, groundwater containing high nitrate concentrations (ca 15 mg/L) discharges through the streambed sediments where denitrification occurs. Nitrate concentration in the stream is approximately 2.5 mg/L at baseflow, ranging from 0.5 to 12 mg/L across all hydrological conditions. Records of stream stage collected every 10 minutes show diurnal variation: excursion in water elevation may be as great as 6 cm from the low stage of 1.36 m amsl around noon to the high stage of 1.42 m amsl around midnight. A 30% change in specific discharge of groundwater to the creek has been observed over the course of a summer day, and the impact upon the flux of nitrate into the stream has been questioned. Accounting for the range in observed specific discharge, the modeled flux of nitrate from groundwater into the stream through the streambed could range between 30 and 200 mg/m² d for this stream of approximately 3600 m² streambed area. Seeking confirmation of the impact on streamwater composition and overall nitrate flux from the watershed, multiple three-day sampling events were undertaken during baseflow conditions to determine the magnitude of diel variability in NO₃^- concentration in streamwater. A robot sampler was deployed at the bank of Cobb Mill Creek and programmed to sample streamwater hourly. Filtered samples were analyzed for Cl^-, NO₃^-, and SO₄^2-. Thrice-daily grab samples were analyzed for dissolved organic carbon (DOC). Annual storm-flux of nitrate is approximately 2.8 times the annual baseflow flux. With diurnal variation in streamwater composition and discharge, another factor in the overall flux of nutrients from this low-gradient stream has been quantified.