DIURNAL PATTERNS IN STREAM STAGE, GROUNDWATER SEEPAGE RATE, AND NITRATE CONCENTRATIONS IN A SMALL AGRICULTURAL WATERSHED

In a small watershed in Virginia, groundwater has been contaminated by fertilizers (~15 mg/L as N). Groundwater discharge through organic-rich streambed sediments is reduced in nitrate content by the process of microbial denitrification. Stream nitrate averaged 2.5 mg/L, ranging from 0.5–12 mg/L for a multi-year record. Residence time of groundwater within the sediments has been demonstrated to control extent of denitrification. We anticipated that recorded diurnal variations in stream stage were driven by evapotranspiration and coincident changes in groundwater gradient and residence time would impact biogeochemical processes that limit nitrate occurrence in surface waters. Stage was measured every 10 minutes; spectral density analysis confirmed a strong 24-hour signal in stage. High stage values occurred between 10:00 and 13:00, coincident with the calculated maximum in potential evapotranspiration (12:00–13:00). Stage minima occurred between 22:00 and 2:00, a 12-hour offset from the timing of greatest predicted evapotranspirative flux. We conducted 72-hour chemical sampling campaigns. Nitrate concentration varied diurnally in the summer when evapotranspiration and stage variation were greatest. Nitrate concentration values in June ranged 2.9–7.5 mg/L. The timing of streamwater nitrate maxima depended on date and location. At locations approximately 250 m apart, maxima in June were observed at 22:00–00:00 upstream and 3:00–6:00 downstream; this offset is larger than the travel time separation. At the downstream location, maximum nitrate values were observed at 9–11 hours later in August than in June. Seepage meters deployed at the downstream location revealed diurnal variation in groundwater discharge rates and nitrate values. Greatest groundwater discharge rates occurred 10:00–14:00, concurrent with high stream stage. Highest nitrate concentration in seepage water occurred at 6:00–12:00, not concurrent with high stream nitrate values. The complex integration of groundwater seepage rates and composition along the entire length of the stream determines streamwater chemistry. The physical relationship among the location and the timing of evapotranspiration, changes in groundwater gradient and residence time, and the extent of biogeochemical processes is the focus of our continuing research.