Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Water-quality impacts of streamwater nitrate (NO<sub>3</sub>¯) on downstream ecosystems are largely determined by the load of NO<sub>3</sub>¯ from the watershed to surface waters. The largest NO<sub>3</sub>¯ loads often occur during storm events, but it is unclear how loads of different NO<sub>3</sub>¯ 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 NO<sub>3</sub>¯ source export and how these roles are modulated by hydrologic effects of land-use practices, we measured nitrogen (<em>δ</em><sup>15</sup>N) and triple oxygen (Δ<sup>17</sup>O) isotopes of NO<sub>3</sub>¯ and oxygen isotopes (<em>δ</em><sup>18</sup>O) 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 NO<sub>3</sub>¯ (NO<sub>3</sub>¯<sub>Atm</sub>) and a disproportionately small influence on export of terrestrial NO<sub>3</sub>¯ (NO<sub>3</sub>¯<sub>Terr</sub>) relative to baseflow in the developed urban watershed. In contrast, baseflow and stormflow had similar influences on NO<sub>3</sub>¯<sub>Atm</sub> and NO<sub>3</sub>¯<sub>Terr</sub> export in the mixed agricultural/forested watershed. An equivalent relationship between NO<sub>3</sub>¯<sub>Atm</sub> deposition on impervious surfaces and event NO<sub>3</sub>¯<sub>Atm</sub> streamwater export in the urban watershed suggests that impervious surfaces that hydrologically connect runoff to channels likely facilitate export of NO<sub>3</sub>¯<sub>Atm</sub> during rainfall events. Additionally, larger rainfall events were more effective in exporting NO<sub>3</sub>¯<sub>Atm</sub> in the urban watershed, with increased rainfall depth resulting in a greater fraction of event NO<sub>3</sub>¯<sub>Atm</sub> 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 NO<sub>3</sub>¯<sub>Atm</sub> export from developed watersheds.
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