Understanding the Hydrogeochemical Response of a Mountainous Watershed Using Integrated Surface‐Subsurface Flow and Reactive Transport Modeling

• Published in: Water resources research 2022-08, Vol.58 (8), p.n/a
• Main Authors: Xu, Zexuan, Molins, Sergi, Özgen‐Xian, Ilhan, Dwivedi, Dipankar, Svyatsky, Daniil, Moulton, J. David, Steefel, Carl
• Format: Article
• Language: English
• Subjects: ATS; Calcite; Calcite dissolution; Calcium; Calcium content; Catchment area; Catchments; Chemical reactions; Chemical speciation; Climate change; Climate change models; Climate models; Climate variability; Climatic conditions; concentration‐Discharge; Dilution; Dissolution; Dissolving; Disturbances; ENVIRONMENTAL SCIENCES; Evapotranspiration; Evapotranspiration processes; Exports; Groundwater; Headwater catchments; Headwaters; high‐performance computing; Hydrogeochemistry; Hydrologic models; Hydrologic processes; Hydrology; Mathematical models; Mineral composition; Minerals; Modelling; mountainous watershed; Mountains; Pyrite; Rainfall; reactive transport modeling; River basin development; River basins; River valleys; Rivers; Saturation; Snowmelt; Snowpack; Solutes; Stream discharge; Streams; Subsurface flow; Sulfates; Temporal variability; Temporal variations; Tracers; Transport; Variability; Water quality; Water resources; Water supply; Watersheds
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2022WR032075

Climate change and other disturbances significantly impact hydrogeochemical exports from mountainous headwater catchments such as the Upper Colorado River Basin. Developing a mechanistic understanding of how the physical and chemical processes interact in time and space in an integrated manner is key to quantifying the future impacts of such disturbances. The hydrogeochemical response of a mountainous catchment in the 2010–2019 period is evaluated quantitatively using a high‐resolution model that simulates integrated hydrology, and transport and reactions for selected solutes and minerals. The model assumes that pyrite is present only at depth while calcite is distributed uniformly, and captures the observed C‐Q reasonably well. Distinct C‐Q dynamics are observed in an average (WY16), a wet (WY17), and a dry (WY18) water year. The model also quantifies the water fraction from surface, shallow and deep groundwater compartments using tracers, and suggests greater groundwater contributions to peak stream discharge in the dry WY18. Results demonstrate that calcium concentrations do not change significantly from year to year, while sulfate shows significant temporal variability. Pyrite dissolution is affected by the changing hydrological drivers where it is enhanced in the dry WY18; calcite dissolution supplements calcium dilution under high flow conditions. The model simulates the reaction hotspots controlled by hydrological conditions, and the spatially‐resolved results show that higher soil saturation and less snowpack occur earlier on the south‐facing side than on the north‐facing side. This is a first‐of‐its‐kind demonstration of a model that integrates hydrologic processes, including evapotranspiration, and reactive transport to enable a predictive understanding of hydrogeochemical exports. Plain Language Summary Climate change significantly impacts water resources, including water quantity and quality, particularly in mountainous headwater catchments such as the Upper Colorado River Basin that are key for water supply in downstream regions of the western U.S. In this work, we used a mathematical model to quantify the movement of water and chemical species under changing weather and climate conditions. Changing rainfall and early snowmelt affects both the volume of water and the amount of minerals that dissolve, impacting water quality observed at the gauged stations downstream. Consistent with the available measurements, the three‐dimensional model makes