Investigating nitrate attenuation in an urban stream using stable isotope geochemistry and continuous monitoring

Abstract

Urbanization affects in-stream biogeochemical processes that control nutrient export. Attempts to restore urban streams will not be successful unless the biological and physical controls on water quality are thoroughly understood. The objective of this study was to identify the relative influences of tributary dilution, groundwater discharge, and biological processing on nitrate concentrations in an urban stream during high and low flow periods. A wastewater treatment plant (WTP) on Pennypack Creek, an urban stream near Philadelphia, PA, increases nitrate concentrations to a mean of 8.5 mg-l-1 (as N). Concentrations decrease to 5.5 mg-l-1 about 7.5 km downstream. Reaches along this distance were sampled for nitrate concentration and delta-15N at fine spatial intervals to determine the reasons for this decrease. To quantify the effects of dilution, samples were collected from tributaries, groundwater springs, and upstream and downstream of tributaries or groundwater discharge zones identified through terrain analysis and continuous temperature modeling. These methods were also used to identify and sample reaches along which hyporheic flow occurred, where nitrate biological processing is often concentrated. In addition, loggers were installed at closely spaced sites to monitor daily fluctuations in nitrate, dissolved oxygen, and related parameters, which provided further indications of biological processing. Longitudinal sampling revealed decreases in nitrate concentration of 2 and 6.5 mg-l-1 during high and low flow, respectively. During high flow, delta-15N varied from 9.5 to 10.5 per mille downstream of the WTP, while delta-15N varied from 10.14 to 11.06 per mille throughout this reach during low flow. Mixing analysis indicated that groundwater discharge and biological processing both control nitrate concentration during both flow periods. Larger declines in nitrate concentration were observed during low flow than during high flow, and delta-15N fell between biological and groundwater signatures, indicating that both processes were enhanced. Continuous nitrate concentrations displayed distinct diurnal cycles often out-of-phase with dissolved oxygen cycles, indicating autotrophic processing. However, shifts occurred in nitrate cycle timing at a weekly scale wherein daily maximum concentrations were observed as many as 6 hours closer to noon than previously. These shifts were comparable to shifts observed across seasons in other studies, and by the end of the summer, nitrate and dissolved oxygen cycles were in-phase. Furthermore, shifts in nitrate cycles could not be linked to shifts in daily fluctuations of WTP discharge. Longitudinal sampling and continuous monitoring suggest that biological processing is an important control on nitrate concentrations in urban systems, though documenting its signature may be complicated by groundwater discharge and anthropogenic inputs.