While the vadose zone is hypothesized to be an active region of solute generation, direct observations to verify this is lacking. Here, we report an integrated approach to characterize chemical weathering reactions in the vadose zone using a novel Vadose Monitoring System (VMS) installed at the Eel River CZO in combination with laboratory experiments and reactive transport modeling. The VMS allows for the direct observation of chemical and gas signatures in the vadose zone for the comparison of chemical evolution of fluids in lab experimental setting. We sampled water and gases across 18 meters of thick regolith at high spatiotemporal resolution over two years, observing a dynamic range of major cation concentrations and ion ratios that vary primarily with depth rather than time despite large seasonal changes in water storage. To validate our simulations, we combine direct observations of solute chemistry from the VMS with the results of batch dissolution experiments using bedrock samples collected from a range of depths across the weathering profile. Using a set of reactive transport models, we evaluate the extent to which these coupled water-gas-rock reactions drive weathering and the observed solute fluxes across a lithological gradient of regolith. Our unique reactive transport model shows that this subtle difference of changing regolith material through depth is crucial to accurately predicting the evolution of solute fluxes. Furthermore, our model results suggest that effective flow rate for the yearly resolution is not the principle factor governing solute concentrations. In total, our work shows that the influence of chemical reactivity across this gradient of weathered solid is a nuance important to accurately portraying solute fluxes from across the profile of the vadose zone.
Wang, Jia (2019): Linkages between fluid flow paths, reactive gases, and chemical weathering across a shale bedrock hillslope. University of Illinois at Urbana-Champaign.
This Paper/Book acknowledges NSF CZO grant support.