The impact of stream-groundwater exchange on seasonal nitrate loads in an urban stream

Urbanization negatively impacts water quality in streams by reducing stream-groundwater interactions, which can reduce a stream's capacity to naturally attenuate nitrate. Meadowbrook Creek, a first order urban stream in Syracuse, New York, has an inverse urbanization gradient, with heavily urbanized headwaters that are disconnected from the floodplain and downstream reaches that have intact riparian floodplains and connection to riparian aquifers. This system allows assessment of how stream-groundwater interactions in urban streams impact the net sources and sinks of nitrate at the reach scale. We used continuous (15-min) streamflow measurements and weekly grab samples at three gauging stations positioned longitudinally along the creek to develop continuous nitrate load estimates at the inlet and outlet of two contrasting reaches. Nitrate load estimates were determined using a USGS linear regression model, RLOADEST, and differences between loads at the inlet and outlet of contrasting reaches were used to quantify nitrate sink and source behaviour year-round. We observed a nitrate load of 1.4 × 10⁴ kg NO₃⁻ per water year, on average, at the outlet of the urbanized reach while the nitrate load at the outlet of the downstream, connected reach was 1.0 × 10⁴ kg NO₃⁻ per water year, on average. We found the more heavily urbanized, hydrologically-disconnected reach was a net source of nitrate regardless of season. In contrast, stream-groundwater exchange caused the hydrologically connected reach to be both a source and sink for nitrate, depending on time of year. Both reaches alter nitrate source and sink behaviour at various spatiotemporal scales. Groundwater connection in the downstream, connected reach reduces annual nitrate loads and provides more opportunities for sources and sinks of nitrate year-round than the hydrologically disconnected stream reach. Mechanisms include groundwater discharge into the stream with variable nitrate concentrations, surface-water groundwater interactions that foster denitrification, and stream load loss to surrounding near-stream aquifers. This study emphasizes how loads are important in understanding how stream-groundwater interactions impact reach scale nitrate export in urban streams.