Conservative and reactive transport of oxygen at the groundwater/atmosphere interface: experiments and model-based interpretation

Oxygen is an important component in the subsurface because it significantly influences biogeochemical processes such as subsurface redox conditions, geochemical reactions rate, metabolic activity of aerobic microbes, and, consequently, the overall groundwater quality. The atmosphere represents an unlimited source of oxygen that triggers the migration of O2 toward terrestrial environments. The atmosphere also provides a sink for water vapor that drives water flux from the soil surface (i.e. soil water evaporation), leading to soil drying. This process influences the exchange of components like oxygen at the soil/atmosphere interface and control their migration mechanisms in the subsurface. This study aims to address the oxygen transport in the subsurface by considering the dynamics and interactions occurring in a coupled atmosphere/subsurface compartment. To this end, a set of column experiments were performed to study the diffusive transport of oxygen under conservative and reactive conditions in three different soils with and without considering the dynamic forcing from the atmosphere. Such forcing includes the imposing of water vapor concentration and temperature gradients at the soil/atmosphere interface, causing evaporation from the soil surface. The reactive experiments were conducted by including a layer of pyrite at the top of the soil, leading to the oxidative dissolution of pyrite. A non-invasive optode technique was used to track O2 concentration and temperature in the porous media at high resolution. To simulate the diffusion experiments under fully water-saturated conditions and quantitatively describe the conservative/reactive transport process and the geochemical reactions, numerical models were developed. The simulation of the conservative experiments (without evaporation) showed a good agreement between the model and data and allowed to capture the temporal and spatial evolution of oxygen in three different soils containing initially anoxic pore water. The outcome of the reactive experiments demonstrated that pyrite oxidation led to a notable retardation of the oxygen diffusion in comparison with the conservative case due to the strong consumption of O2 in the presence of pyrite. The evaporation experiments were performed by exposing the sample to the atmosphere under well-controlled temperature and relative humidity conditions, allowing a “natural” evaporation. Another set of evaporation experiments was performed by applying a heat source on the top of the column, causing an “enhanced” evaporation of pore water. In both last cases, pore water vaporization involves the appearance of a gaseous phase inside the soil, responsible for an increased oxygen diffusion rate, both in the conservative and in the reactive case, thanks to a bigger surface area at the liquid/gas interface as well as a higher oxygen diffusion coefficient in the gas phase. The influence of grain size on such interactive processes was also investigated, highlighting the influence of capillary forces on reorganization of remaining water in the pores and thus oxygen dissolution and vertical diffusion. The research findings show the importance of land/atmosphere interactions on the spatiotemporal dynamics of components like oxygen and water in the subsurface. This study can be further developed to understand the exchange of other gas like CO2 and CH4 at the groundwater/atmosphere boundary.