High-resolution monitoring of CO2/O2 transport in recharge wells

The Earth's surface forms a dynamic boundary characterized by continuous gas exchanges between the critical zone and the overlying atmosphere. As global concern grows over climate change driven by increasing levels of greenhouse gases – such as carbon dioxide (CO₂) and methane (CH₄) – abandoned oil, gas, and even groundwater wells can be significant sources of these emissions. Here, we monitor CO₂ and oxygen (O₂) and quantify the CO₂ flux in two different recharge wells – one that extends below the groundwater level (wet well) and one that reaches into the unsaturated zone above the groundwater level (dry well). Novel monitoring systems that measure CO₂, O₂, temperature, and relative humidity were installed at the top and bottom of each well, enabling high-resolution, continuous data collection at 1-min time intervals. Using atmospheric measurements taken from a nearby meteorological station, we investigate the mechanisms that influence the air transport between the wells and the atmosphere. The high-resolution measurements indicate different air transport mechanisms between the two wells. In the wet well, there was stratification during the summer, with consistently high CO₂ values ​​measured at the bottom of the well while low values ​​were measured at the top of the well. In the dry well, two daily outflow cycles were observed, with high CO₂ concentrations and fluxes from the well to the atmosphere. These findings highlight the potential contribution of recharge wells to CO₂ emissions and the importance of understanding their transport mechanisms.