Oxygen and carbon dioxide transport at the groundwater/atmosphere interface

Diffusive mass transfer of volatile compounds between the atmosphere and the subsurface is a key process both for the supply of O2 into anoxic groundwater and for the exsolution of CO2 in the atmosphere. On one hand, the oxygen transport influences many biogeochemical processes in the subsurface, and, consequently, it is important for the groundwater quality. On the other hand, the soil and shallow subsurface represent key compartments for CO2 production due to root respiration and microbially-mediated organic compound degradation processes. The aim of the study is to quantitatively describe the O2 and CO2 mass transfer at the groundwater/atmosphere interface taking into account the initially anoxic pore water in the subsurface and the impact of evaporation. A set of 1-D column experiments were performed to study the diffusive transport of oxygen and carbon dioxide in three different soils with and without evaporation. The O2 and CO2 concentrations were measured over time (breakthrough curves) and over space (spatial profiles) with a non-invasive optode technique. Firstly, a diffusion experiments without evaporation have been carried out to study the oxygen and carbon dioxide mass transfer. Then, the impact of evaporation on the oxygen diffusion have been evaluated under different boundary conditions (temperature and relative humidity); with natural and enhanced evaporation. Evaporation of water permitted air to penetrate into the media, increasing significantly the velocity of the O2 mass transfer between the atmosphere and the subsurface Since O2 and CO2 fluxes are vital for many biogeochemical processes in the subsurface, future research are recommended: these approaches should investigate the oxygen and carbon dioxide transfer dynamics under the variation of hydrogeological conditions, including evaporation, in order to predict the exchange of volatile compounds between the interacting gaseous and liquid phase of atmosphere and groundwater respectively.