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

Water quality impacts of stream water nitrate (NOinline-formula $M1inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg5c4cefaf8b78d41c1ce2f2ef151f712f bg-20-2485-2023-ie00001.svg9pt16ptbg-20-2485-2023-ie00001.png ) on downstream ecosystems are largely determined by the load of NOinline-formula $M2inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg53e1f98be2cdf70dbe180d95894fc6b5 bg-20-2485-2023-ie00002.svg9pt16ptbg-20-2485-2023-ie00002.png from the watershed to surface waters. The largest NOinline-formula $M3inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg0aaab3ee324d7a9ba8e4b96f67d8036e bg-20-2485-2023-ie00003.svg9pt16ptbg-20-2485-2023-ie00003.png loads often occur during storm events, but it is unclear how loads of different NOinline-formula $M4inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg0723f17b5be9fc41c36a5585631feb47 bg-20-2485-2023-ie00004.svg9pt16ptbg-20-2485-2023-ie00004.png 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 NOinline-formula $M5inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg8c72af1edd6d67ed562efcaf5163d22b bg-20-2485-2023-ie00005.svg9pt16ptbg-20-2485-2023-ie00005.png source export and how these roles are modulated by hydrologic effects of land-use practices, we measured nitrogen (inline-formulaδ¹⁵N) and oxygen (inline-formulaΔ¹⁷O) isotopes of NOinline-formula $M8inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg7de8959157e6c258409d4c11688ca166 bg-20-2485-2023-ie00006.svg9pt16ptbg-20-2485-2023-ie00006.png and oxygen isotopes (inline-formulaδ¹⁸O) 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 NOinline-formula $M10inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg78ed0f7e81615226176402cdd6a1afd5 bg-20-2485-2023-ie00007.svg9pt16ptbg-20-2485-2023-ie00007.png (NOinline-formula $M11inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-})$ 28pt16ptsvg-formulamathimg9514b4a9c185f97f94d1417c44425043 bg-20-2485-2023-ie00008.svg28pt16ptbg-20-2485-2023-ie00008.png and a disproportionately small influence on the export of terrestrial NOinline-formula $M12inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimgee54bb0fff66afdafaf51bed1fde360d bg-20-2485-2023-ie00009.svg9pt16ptbg-20-2485-2023-ie00009.png (NOinline-formula $M13inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Terr}}^{-})$ 27pt16ptsvg-formulamathimg341113ab75fbe4206273a3461be5102c bg-20-2485-2023-ie00010.svg27pt16ptbg-20-2485-2023-ie00010.png relative to baseflow in the developed urban watershed. In contrast, baseflow and storm flow had similar influences on NOinline-formula $M14inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimgc0317f6ec721e17a6789623e2e5db62c bg-20-2485-2023-ie00011.svg24pt16ptbg-20-2485-2023-ie00011.png and NOinline-formula $M15inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Terr}}^{-}$ 23pt16ptsvg-formulamathimgc6d582a56792fded2e68a608adaf0646 bg-20-2485-2023-ie00012.svg23pt16ptbg-20-2485-2023-ie00012.png export in the mixed agricultural–forested watershed. An equivalent relationship between NOinline-formula $M16inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimg1a3c003a46c3b6482fa8a8b7b1b65180 bg-20-2485-2023-ie00013.svg24pt16ptbg-20-2485-2023-ie00013.png deposition on impervious surfaces and event NOinline-formula $M17inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimg10b1537e5e03e8d6082fcb78275dc75e bg-20-2485-2023-ie00014.svg24pt16ptbg-20-2485-2023-ie00014.png stream water export in the urban watershed suggests that impervious surfaces that hydrologically connect runoff to channels likely facilitate the export of NOinline-formula $M18inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimg79cb312466c10021b4f7bb5bfac3932f bg-20-2485-2023-ie00015.svg24pt16ptbg-20-2485-2023-ie00015.png during rainfall events. Additionally, larger rainfall events were more effective at exporting NOinline-formula $M19inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimg35996e5a90939ae316cd043c0b28f172 bg-20-2485-2023-ie00016.svg24pt16ptbg-20-2485-2023-ie00016.png in the urban watershed, with increased rainfall depth resulting in a greater fraction of event NOinline-formula $M20inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimgec1e01414f483dc9b931b5497627696e bg-20-2485-2023-ie00017.svg24pt16ptbg-20-2485-2023-ie00017.png 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 NOinline-formula $M21inlinescrollmathml{}_{\mathrm{normal\; 3}\mathrm{normal\; Atm}}^{-}$ 24pt16ptsvg-formulamathimgd5242401365ac8a6b03918e82fd5c130 bg-20-2485-2023-ie00018.svg24pt16ptbg-20-2485-2023-ie00018.png export from developed watersheds.