Snowmelt controls on concentration‐discharge relationships and the balance of oxidative and acid‐base weathering fluxes in an alpine catchment, East River, Colorado

Abstract

Although important for riverine solute and nutrient fluxes, the connections between biogeochemical processes and subsurface hydrology remain poorly characterized. We investigate these couplings in the East River, CO, a high‐elevation shale‐dominated catchment in the Rocky Mountains, using concentration‐discharge (C‐Q) relationships for major cations, anions, and organic carbon. Dissolved organic carbon (DOC) displays a positive C‐Q relationship with clockwise hysteresis, indicating mobilization and depletion of DOC in the upper soil horizons and emphasizing the importance of shallow flow paths during snowmelt. Cation and anion concentrations demonstrate that carbonate weathering, which dominates solute fluxes, is promoted by both sulfuric acid derived from pyrite oxidation in the shale bedrock and carbonic acid derived from subsurface respiration. Sulfuric acid weathering dominates during base flow conditions when waters infiltrate below the inferred pyrite oxidation front, whereas carbonic acid weathering plays a dominant role during snowmelt as a result of shallow flow paths. Differential C‐Q relationships between solutes suggest that infiltrating waters approach calcite saturation before reaching the pyrite oxidation front, after which sulfuric acid reduces carbonate alkalinity. This reduction in alkalinity results in CO2 outgassing when waters equilibrate to surface conditions, and reduces the riverine export of carbon and alkalinity by roughly 33% annually. Future changes in snowmelt dynamics that control the balance of carbonic and sulfuric acid weathering may substantially alter carbon cycling in the East River. Ultimately, we demonstrate that differential C‐Q relationships between major solutes can provide unique insights into the complex subsurface flow and biogeochemical dynamics that operate at catchment scales.

Sensitivity analysis of hydraulic conductivity and Manning's n parameters lead to new method to scale effective hydraulic conductivity across model resolutions

“…With about 0.8‐m of precipitation each year (1985–2014 average), steep topography leading to steep temperature and humidity gradients, heterogeneous landcover, and complex geology, it is considered representative of most headwater basins in the western Rocky Mountains that feed the Colorado River (Battaglin, Hay, & Markstrom, ). Additional site details can be found in McCabe and Hay (), Winnick et al (), and Carroll, Bearup, Brown, and Willlams ().…”

“…Mountains that feed the Colorado River (Battaglin, Hay, & Markstrom, 2012). Additional site details can be found in McCabe and Hay (1995), Winnick et al (2017), and Carroll, Bearup, Brown, and Willlams (2018).…”

Hyporheic Zone Microbiome Assembly Is Linked to Dynamic Water Mixing Patterns in Snowmelt‐Dominated Headwater Catchments

“…These fluids (groundwater and “older” river water) likely contain organic molecules derived from the underlying Mancos shale and entrained plant DOC, which may explain the diversity of detected compounds. Conversely, although a higher concentration of river water DOC was measured May 2017 during snowmelt‐driven elevated flow (Winnick et al, ), the carbon pools within these depth‐resolved samples were less diverse, with lower NOSC values. While this observation might suggest that DOC derived from the surrounding terrestrial catchment is more compositionally uniform than groundwater carbon pools (Figure ), the results may also reflect preferential usage and rapid turnover of the energetically favorable, higher NOSC value carbon compounds at a time point where streambed microbial metabolism is the highest (Figure ; LaRowe & Van Cappellen, ).…”

Critical zone structure controls concentration‐discharge relationships and solute generation in forested tropical montane watersheds

“…A large body of literature describes spatial variability in the sources of solutes in different geologic terrains and watershed settings, as originally articulated by Gibbs [1970]. Well-documented influences that can leave an imprint on stream chemistry include the differential weathering of various bedrock types [e.g., Holloway et al, 1998;Ibarra et al, 2016], vertical gradients in the weathering zone [Winnick et al, 2017], anthropogenic inputs of nutrients or salts into a watershed [Howarth et al, 1996;Daley et al, 2009], atmospheric deposition of sea salts [McDowell and Asbury, 1994], or changes in water table elevation [Hornberger et al, 1994[Hornberger et al, , 2001.…”