| 04 May 2026
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.
This study investigates seasonal dynamics of gas transport and exchange between subsurface wells and the atmosphere. The research was conducted at a field site where different types of wells (i.e., dry and wet wells) are used for managed aquifer recharge. The study compares gas transport in a dry well that extends through the unsaturated zone with a wet well that penetrates below the groundwater table. The aim was to evaluate the role of barometric pumping and thermal convection in driving subsurface CO₂ emissions in the dry and wet wells. The study provides high-resolution sensor-based measurements of gaseous O2 and CO2 for temporal dynamics, along with in situ depth-specific monthly CO2 measurements. The authors concluded that CO2 fluxes are controlled by barometric pressure in dry wells, whereas the contribution of this mechanism to CO2 exchange in wet wells that are below groundwater is minimal. Instead, the CO2 fluxes in such wells are controlled by thermal dynamics. I found this study interesting and relevant, and I believe the dataset is valuable, but I cannot recommend the current version of the manuscript for publication in HESS. I believe the analysis needs to be considerably extended and become more quantitative, perhaps by including a process-based model to test the hypothesis presented in the paper.
In the following, I list the major points that the authors need to carefully address:
Additionally, below I list some minor points that authors need to consider: