Shale weathering controls on hydrochemical fluxes and nutrients cycling in a mountainous watershed
Abstract
Bedrock weathering exerts a major control on solute exports and nutrients cycling and significantly impacts the water quality of streams and aquifers. In sub-alpine catchments, subsurface geochemical exports vary seasonally and long-term changes are increasingly reported as a consequence of climate change. Therefore, the understanding of the physical and biogeochemical processes controlling the weathering at the soil-atmosphere interface is of pivotal importance to evaluate the impact of climate on the evolution of watershed hydrochemistry. In this study, we explore the coupling and feedback between shale weathering and the exports of aqueous solutes and nutrients. We consider a well-instrumented hillslope-to-river transect underlain by N-rich Mancos shale sedimentary rocks and located at the East River study site, a snow-dominated pristine mountainous watershed within the Upper Colorado River basin. Based on the field measurements, we implemented a multidimensional and multiphase reactive transport model that simulates the variably saturated flow, gas exchange, microbially-mediated reactions, and abiotic water-rock reactions. We specifically focused on the impacts of snowmelt-driven infiltration events, water table fluctuation, and water residence time on the biogeochemical cycling of carbon and nitrogen by performing transient simulations. The comprehensive field dataset combined with the reactive transport model allowed us to mechanistically interpret the impacts of the external forcings on the weathering of shale, the export of solutes, and the emission of greenhouse gases from the subsurface. Our results demonstrate that aerobic respiration is a key mechanism controlling the mobilization of nutrients and major ions, and that seasonal forcing events significantly impact the exchange of reactants between the atmosphere and the subsurface, which in turn influence the microbial respiration and the mineral reactions. The model-based calculation of biogeochemical budgets show that such perturbations not only drive rapid change in hydrochemical fluxes, but are markedly contributing to the total annual exports from hillslope to river.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.H24F..01S