Downpour dynamics: outsized impacts of storm events on unprocessed atmospheric nitrate export in an urban watershed

Abstract

Water quality impacts of stream water nitrate (NO3-) on downstream ecosystems are largely determined by the load of NO3- from the watershed to surface waters. The largest NO3- loads often occur during storm events, but it is unclear how loads of different NO3- sources change during storm events relative to baseflow or how watershed attributes might affect source export. To assess the role of storm flow and baseflow in NO3- source export and how these roles are modulated by hydrologic effects of land-use practices, we measured nitrogen (δ15N) and oxygen (Δ17O) isotopes of NO3- and oxygen isotopes (δ18O) of water in rainfall and stream water samples from before, during, and after eight storm events across 14 months in two Chesapeake Bay watersheds of contrasting land use. Storms had a disproportionately large influence on the export of unprocessed atmospheric NO3- (NO3Atm-) and a disproportionately small influence on the export of terrestrial NO3- (NO3Terr-) relative to baseflow in the developed urban watershed. In contrast, baseflow and storm flow had similar influences on NO3Atm- and NO3Terr- export in the mixed agricultural–forested watershed. An equivalent relationship between NO3Atm- deposition on impervious surfaces and event NO3Atm- stream water export in the urban watershed suggests that impervious surfaces that hydrologically connect runoff to channels likely facilitate the export of NO3Atm- during rainfall events. Additionally, larger rainfall events were more effective at exporting NO3Atm- in the urban watershed, with increased rainfall depth resulting in a greater fraction of event NO3Atm- deposition exported. Considering both projected increases in precipitation amounts and intensity and urban/suburban sprawl in many regions of the world, best management practices that reduce the hydrologic connectivity of impervious surfaces will likely help to mitigate the impact of storm events on NO3Atm- export from developed watersheds.