Barometric Pumping as a Driver of Subsurface CO₂ Emissions: A Comparative Study of Vadose Zone vs. Groundwater Wells

Abstract. Wells can serve as conduits for gas exchange between the subsurface and the atmosphere. Yet, the controlling gas-transport mechanisms as a function of well type remain insufficiently understood. In this study, we identify and compare the controlling air transport mechanism between two infiltration well types located at the same site under similar soil and climatic conditions – a vadose zone well (i.e., dry well) and a groundwater well (i.e., wet well). During a 1-year experiment, high-resolution CO₂ and O₂ concentrations were measured at 1-minute intervals in each well, and atmospheric forcing was investigated using barometric pressure and temperature data. The dry well exhibited clear semi-diurnal cycles in CO₂ and O₂ concentrations that were coupled with fluctuations in atmospheric pressure, confirming barometric pumping (BP) as the dominant air transport mechanism. In contrast, the wet well showed no persistent diurnal oscillations. Instead, air transport was governed by seasonal processes: diffusion-driven stratification during summer and thermal-induced convection (TIC) combined with recharge-related dilution during winter. The groundwater table in the wet well acted as an impermeable boundary to BP, effectively suppressing advective gas exchange. From an environmental perspective, the dry well exhibits higher CO₂ emissions than the wet well, primarily due to the dominance of advective transport driven by BP, compared to the more diffusion-dominated transport (with a minor seasonal TIC addition) in the wet well. These results demonstrate that well type strongly controls air transport mechanisms and associated CO₂ emissions.