NITRATE VARIABILITY AT MULTIPLE TEMPORAL SCALES IN AN URBAN STREAM DUE TO GROUNDWATER-SURFACE WATER INTERACTIONS

A wastewater treatment plant (WTP) on Pennypack Creek, an urbanized stream near Philadelphia, PA, elevates nitrate concentrations to a mean of 8.5 mg/L (as N). Concentrations decrease to a mean of 5.5 mg/L about 7.5 km downstream. The objective of this study was to identify the relative influences of tributary dilution, groundwater discharge, and biogeochemical processing in the hyporheic zone on the observed decrease. Variation in these influences through time was also examined. A reach of the stream along which decreases in nitrate had previously been observed was sampled at a fine spatial scale to determine the sources of the decrease. Samples were taken upstream, downstream, and in tributaries to quantify their dilution effect, and continuous temperature monitoring of the streambed and water column aided identification of sites of groundwater discharge and hyporheic flow. In addition, loggers were installed upstream and downstream of a hyporheic flow site to monitor daily fluctuations in nitrate, fDOM, and DO. Tributaries did not appear to cause significant dilution, where differences in nitrate concentration between upstream and downstream sites could not be resolved at the available sensitivity (0.5 mg/L). Similarly, differences in nitrate concentrations measured upstream and downstream of groundwater discharge zones were not significant at relatively low nitrate concentrations (5 mg/L), though previous data showed steady declines in nitrate and chloride along the 7.5 km stretch. Thus groundwater discharge may be diluting nitrate at other times of year. A decrease of 0.9 mg/L was observed along a 420 m reach identified as a hyporheic flow site. Because no decrease in chloride concentrations was observed, biogeochemical processing is likely occurring. Continuous measurements of nitrate taken at the downstream end of this reach exhibit sub-daily variation of about 1 mg/L; however, the resulting pattern was not clearly associated with the diurnal cycles of DO and fDOM. Analysis of nitrate nitrogen isotopic composition will elucidate the effects of biological processing on nitrate concentrations. These results indicate that the processes governing nitrate concentrations change with time, suggesting that an accurate assessment of nitrate dynamics demands sampling at a variety of temporal scales.