Hydrogeochemical model development and advanced numerical simulation of alpine hillslope geochemical response to temperature-induced hydrologic changes

Abstract

Subsurface hydrogeochemical systems are complex components of the terrestrial environment, playing host to a multitude of interacting processes. Reactive transport modeling provides a method to numerically integrate our mechanistic, process-based understanding of the phenomena that dictate terrestrial hydrogeochemistry, and can help to further refine and test our understanding of these systems. Here, the development of a new hydrogeochemical reactive transport model, ParCrunchFlow, is detailed. This model provides a continuum-scale approximation of subsurface geochemical reactions in porous media under transient, surface-subsurface flow conditions. An integrated land-surface model allows the representation of snow accumulation/melt and evapotranspiration, driven by realistic meteorological forcing data. A novel representation of atmosphere-subsurface diffusion is described, and is used to simulate atmospheric oxygen diffusion into the vadose zone. The code runs in parallel, and is highly scalable, allowing the representation of field-scale systems at high resolutions. Several higher-order positivity-preserving advection schemes are described, and the effects of scheme accuracy on mixing-induced kinetic and equilibrium reactions are explored. The response of an alpine hillslope containing pyrite to a two degree Celsius temperature increase is examined. The temperature increase causes hillslope discharge to decrease and water table elevation to decline, exposing more of the reactive subsurface to atmospheric oxygen and increasing pyrite oxidation rates. Concentration-discharge plots, commonly used in field studies to analyze geochemical controls, are generated from model outputs and are a useful tool with which to interpret model behavior. The results suggest that recently observed increasing concentrations of pyrite oxidation proxies in alpine catchments, thought to be caused by increasing temperatures, may be more attributable to the effects of reduced dilution than to increasing reaction rates, though both appear to play a role.