No abstract

Dissolved Oxygen (DO) plays a key role in reactive processes and microbial dynamics in the critical zone. While the general view is that oxygen is rapidly depleted in soils and that deeper compartments are anoxic, recent observations showed that fractures can provide rapid pathways for deep oxygen penetration, triggering unexpected biogeochemical processes. As it is transported in the subsurface, DO reacts with electron donors, such as $Feˆ{2+}$ coming from mineral dissolution, hence influencing rockweathering. Yet, little is known about the factors controlling the spatial heterogeneity and distribution of oxygen with depth. Here we present analytical expressions describing the coupled evolution of DO and $Feˆ{2+}$ as a function of fluid travel time in crystalline rocks. Our model, validated with reactive transport simulations, predicts a linear decay of DO with time, followed by a rapid non-linear increase of $Feˆ{2+}$ concentrations up to an equilibrium state. Relative effects of the reducing capacity of the bedrock and of transport velocity are quantified through a Damkohler number, capturing key hydrological and geological

Dissolved Oxygen (DO) plays a key role in reactive processes and microbial dynamics in the critical zone. Recent observations showed that fractures can provide rapid pathways for oxygen penetration in aquifers, triggering unexpected biogeochemical processes. In the shallow subsurface, DO reacts with electron donors, such as Fe2+ coming from mineral dissolution. Yet, little is known about the factors controlling the spatial heterogeneity and distribution of oxygen with depth. Here we present a reduced analytical model describing the coupled evolution of DO and Fe2+ as a function of fluid travel time in silicate catchments. Our model, validated from fully resolved reactive transport simulations, predicts a linear decay of DO with time, followed by a rapid non‐linear increase of Fe2+ concentrations up to a far‐from‐equilibrium steady‐state. The relative effects of geological and hydrological forcings are quantified through a Damköhler number (Da) and a lithological number (Λ). We use this framework to investigate the depth distribution of DO and Fe2+ in two catchments with similar environmental contexts but contrasted hydrochemical properties. We show that hydrochemical differences are explained by small variations in Da but orders of magnitude variations in Λ. Therefore, we demonstrate that the hydrological and geological drivers controlling hydrochemistry in silicate catchments can be discriminated by analyzing jointly the O2 and Fe2+ evolution with depth. These findings provide a new conceptual framework to understand and predict the evolution of DO in modern groundwater, which plays an important role in critical zone processes.