Geologic, Climate and Anthropogenic Influences on Long‐Term and Seasonal Nitrate Dynamics in Two Adjacent Montane Catchments
Gracie Neher, Keira Johnson, Nicholas Bouskill, Kenneth H. Williams, Pamela L. SullivanABSTRACT
Stream nitrate (NO 3 − ) concentrations reflect complex interactions among atmospheric, biologic, hydrologic, geologic and anthropogenic processes, which are changing rapidly with ongoing climate and land use change. Montane watersheds are especially vulnerable to these changes, and are critical sources of freshwater to downstream ecosystems and water users. Thus, understanding the drivers of stream NO 3 − variability in mountain systems is essential for predicting future water quality and ecosystem responses. In this study, we evaluated the climate, geologic and land use controls on long‐term, seasonal and spatial patterns of riverine NO 3 − concentrations in two nested high‐elevation watersheds in the Upper Colorado River Basin: the East River and the Slate River. Despite their geographic proximity, the two watersheds exhibit contrasting NO 3 − dynamics, reflecting differences in dominant sources of NO 3 − . The East River, underlain by nitrogen‐rich marine shale, showed a decline in mean annual NO 3 − concentrations between 1993 and 2023; however, this trend was not present when evaluating shorter, more recent time scales (2003–2023). We attribute the historic decline to increased vegetation NO 3 − uptake associated with longer growing seasons and declining NO 3 − deposition, which is offset in more recent years by increased contributions from N‐rich groundwater. The East and Slate Rivers showed drastically different seasonal patterns, with the highest concentrations in the East observed during the spring freshet, while the Slate River, influenced by wastewater effluent and anthropogenic land use, exhibited the highest concentrations in the winter associated with a time of moderate wastewater effluent but low instream uptake. Despite point source inputs, however, NO 3 − concentrations in the Slate River were quickly attenuated along the stream channel, eventually acting as a diluting source of NO 3 − at its confluence with the East River. These findings highlight the importance of geologic and anthropogenic factors in shaping NO 3 − variability and underscore the need for continued monitoring to understand how montane watersheds will respond to ongoing environmental change.