Hydrological and Geological Controls for the Depth Distribution of Dissolved Oxygen and Iron in Silicate Catchments

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 Damkohler number (Da) and a lithological number (Lambda). 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 Lambda. Therefore, we demonstrate that the hydrological and geological drivers controlling hydrochemistry in silicate catchments can be discriminated by analyzing jointly the O-2 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. Plain Language Summary Dissolved Oxygen (DO) is involved in important biogeochemical reactions, such as the aerobic respiration of microbes, rock-weathering or contaminant degradation. The general view is that the deeper subsurface of continents is mostly anoxic. However, recent observations have shown that cracks in rocks can allow oxygen to reach deeper down, thus extending deeper the influence of oxygen in reactions. Here, we present a modeling framework validated with field data to understand and predict the hydrological and geological controls on DO evolution in silicate catchments, shedding new light on its influence on rock-weathering and microbial life in the continents.