Comparing Stream Nitrate Concentrations in Baseflow and Stormflow Conditions across Urban Watersheds

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Degraded water quality is common in urban streams due to increased impervious surface cover, which can input stormwater runoff directly into the stream. Nitrate, which enters streams from sewage inputs or runoff from fertilizer and atmospheric deposition, is a common contaminant in urban watersheds. Nitrate concentrations can vary throughout a stream network due to land cover and urban infrastructure influences including proximity to sewer lines and stream burial. Three urban watersheds (5-15 km2) in Summit County, Ohio with similar impervious surface cover, but spatially variable land cover metrics to understand urbanization on nitrate concentrations. Biweekly sampling of 26 sites between October 2017 and October 2018 was accompanied by 5-minute conductivity and water level data, as well as storm event sampling in August and September 2018. Baseflow concentrations showed that nitrate has higher concentrations and more variation between sampling locations in the summer (2-9 mg/L), with little variation in the winter (<2 mg/L). During storm events, nitrate concentrations changed in response to water level, with generally lower concentrations at high water level due to dilution. Nitrate concentrations varied at baseflow and stormflow between watersheds, possibly due to differences in land cover metrics. Understanding water quality response to discharge provides greater insight into the mechanisms affecting urban water quality, allowing managers to better predict impairment and target land use and stormwater actions that will improve water quality.