NITROGEN CYCLING IN AN URBAN STREAM NEAR PHILADELPHIA

Diurnal cycles of nitrogen in streams can indicate metabolic processes. Such processes are an important aspect of nutrient attenuation, but in urban streams, the factors that lead to attenuation are not well understood. Nitrogen can be attenuated both in the stream column and in the hyporheic zone, but the stream column is more likely to be influenced by autotrophic processes and diurnal cycles under well-oxygenated conditions. It is important to understand attenuation processes to evaluate how restoration affects stream health. We studied diurnal cycles of nitrogen in Pennypack Creek, an impaired stream in suburban Philadelphia with high levels of nitrate (5 -10 mg/L) due to water treatment plant (WTP) discharge. We used a nitrate logger recording at hourly intervals for a 1.5 month period. The nitrate logger was paired with a multiport sonde that logged fluorescent dissolved organic matter (fDOM), dissolved oxygen, conductivity, and turbidity. The fDOM provided a signature of the WTP discharge. The nitrogen fluctuated 1-2 mg/L over the course of a day, and diurnal patterns did not resemble the WTP signature. Furthermore, these patterns exhibited a 12-hour shift in peak time from mid-day to around midnight over the study period, while the other parameters did not exhibit such a shift. High frequency nitrate monitoring shows that attenuation processes are complex and change over relatively short time spans. These shifts affect how time of sampling can change concentrations and create uncertainty in interpretation of metabolic processes.