Downpour Dynamics: Outsized impacts of storm events on unprocessed atmospheric nitrate export in an urban watershed

Abstract. Water-quality impacts of streamwater 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 stormflow and baseflow on NO₃¯ source export and how these roles are modulated by hydrologic effects of land-use practices, we measured nitrogen (δ¹⁵N) and triple oxygen (Δ¹⁷O) isotopes of NO₃¯ and oxygen isotopes (δ¹⁸O) of water in rainfall and streamwater samples from before, during, and after 8 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¯ (NO₃¯_Atm) and a disproportionately small influence on export of terrestrial NO₃¯ (NO₃¯_Terr) relative to baseflow in the developed urban watershed. In contrast, baseflow and stormflow had similar influences on NO₃¯_Atm and NO₃¯_Terr export in the mixed agricultural/forested watershed. An equivalent relationship between NO₃¯_Atm deposition on impervious surfaces and event NO₃¯_Atm streamwater export in the urban watershed suggests that impervious surfaces that hydrologically connect runoff to channels likely facilitate export of NO₃¯_Atm during rainfall events. Additionally, larger rainfall events were more effective in exporting NO₃¯_Atm in the urban watershed, with increased rainfall depth resulting in a greater fraction of event NO₃¯_Atm 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 hydrologic connectivity of impervious surfaces will likely help to mitigate the impact of storm events on NO₃¯_Atm export from developed watersheds.