Subsurface Flow Path and Stream Chemistry Response to Warming in a High Elevation Mountain Watershed in Colorado

Abstract:

High-elevation mountain watersheds are undergoing rapid warming and declining snow fractions worldwide, causing earlier and quicker snowmelt. Understanding how this hydrologic shift affects subsurface flow paths, biogeochemical reactions, and solute export has been challenging due to the entanglement of hydrological and biogeochemical processes. Temperature and topography have been identified as main controls of biogeochemical reactions but understanding how variations in these parameters change hydrologic and biogeochemical processes and how each process propels dissolved organic carbon (DOC) increase remains opaque. This work aims to understand the impacts of shifting climate on water partitioning and stream chemistry in Coal Creek, a high-elevation catchment (2,700 – 3,700 m, 53 km²) in Colorado. Notably, in the last four years, Coal Creek has seen an increase of >3x in stream DOC concentration compared to historical USGS data, whereas geogenic cations and dissolved inorganic carbon (DIC) remain relatively unchanged. Analysis of annual averages indicates flow-weighted concentrations of DOC correlate positively to daily minimum air temperature, especially as minimum temperatures approach 0°C. We hypothesize temperature has an oversized effect on OC decomposition and DOC production in the shallow subsurface, while weathering and geogenic solutes are more influenced by water partitioning between shallow versus deeper groundwater. We will use multiple years of stable isotope (¹⁸O and ²H) and reactive solute data together with the reactive transport model BioRT-Flux-PIHM to quantify water age and fluxes in shallow and deep zones. Rates of organic matter decomposition and weathering in different years were quantified using reactive solute data (e.g., DOC, DIC, and cations). Preliminary results indicate as warming occurs a higher fraction of meltwater routes through the deeper subsurface and C decomposition rates depend more on temperature than soil moisture. This work shows complex hydrological and biogeochemical coupling at the watershed scale. These findings imply water flow path and water quality alterations are responding to changing climate in high-elevation mountain watersheds in the western United States and potentially in high-elevation mountain watersheds worldwide.