Title

Stream Water Nitrogen Inputs Reflects Groundwater Across a Snowmelt-Dominated Montane-to-Urban Watershed

Document Type

Article

Journal/Book Title/Conference

Environmental Science and Technology

Volume

50

Issue

3

Publisher

American Chemical Society

Publication Date

1-8-2016

First Page

1137

Last Page

1146

DOI

10.1021/acs.est.5b04805

Abstract

Snowmelt dominates the hydrograph of many temperate montane streams, yet little work has characterized how streamwater sources and nitrogen (N) dynamics vary across wildland to urban land use gradients in these watersheds. Across a third-order catchment in Salt Lake City, Utah, we asked where and when groundwater vs shallow surface water inputs controlled stream discharge and N dynamics. Stream water isotopes (δ2H and δ18O) reflected a consistent snowmelt water source during baseflow. Near-chemostatic relationships between conservative ions and discharge implied that groundwater dominated discharge year-round across the montane and urban sites, challenging the conceptual emphasis on direct stormwater inputs to urban streams. Stream and groundwater NO3– concentrations remained consistently low during snowmelt and baseflow in most montane and urban stream reaches, indicating effective subsurface N retention or denitrification and minimal impact of fertilizer or deposition N sources. Rather, NO3– concentrations increased 50-fold following urban groundwater inputs, showing that subsurface flow paths potentially impact nutrient loading more than surficial land use. Isotopic composition of H2O and NO3– suggested that snowmelt-derived urban groundwater intercepted NO3– from leaking sewers. Sewer maintenance could potentially mitigate hotspots of stream N inputs at mountain/valley transitions, which have been largely overlooked in semiarid urban ecosystems.

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