the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1910', Anonymous Referee #1, 07 Jul 2026
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RC2: 'Comment on egusphere-2026-1910', Anonymous Referee #2, 04 Aug 2026
The manuscript titled “Barometric Pumping as a Driver of Subsurface CO2 Emissions: A Comparative Study of Vadose Zone vs. Ground Water Wells” aims to determine the effect of the Well type (dry or wet) on the gas exchange mechanisms for CO2 via high-resolution experiments. The study team has conducted extended in situ monitoring at selected depths in two wells. Studies have shown that dry-well CO2 emissions depend on barometric pumping, and wet-well CO2 emissions are seasonally dependent on diffusion and thermally induced convection. The study shows novelty, careful experimental design, and clear presentation of results and understanding. I only have a limited number of comments. The manuscript can be accepted for publication after the authors address the following comments.
Abstract: The abstract is well written and provides a good qualitative explanation. However, a quantitative index between dry- and wet-well gas transport mechanisms is required.
Methods:
Line 131: How do the authors validate the barometric pumping impacts at ~47 m depth of a 0.2m narrow pipe? Further, how does this study relate to wells of larger diameter, for example, ~2 m wide wet wells of similar depths?
Line 157: T and RH should be spelled out at first
Line 180: What is the basis for the assumption of “assuming advective transport, where CO2 moves as part of bulk mass…”?
Line 186: What is the source of ~420 ppm atmospheric concentration?
Line 209: Explain 30-point rolling mean
Line 295: The graph should be improved with a legend, and the vertical axes should be given in black text
Conclusions: It is clear that the higher depth of the wet well is to reach the groundwater table. However, I am wondering if ~42 m depth has a relatively low impact of the BP relative to the 22 m depth of the dry well. At the same time, if the wet well has the same shallow depth, will there be increased impact from the BP?. Therefore, how will the authors take this depth difference into account?
Further, the conclusions are well written based on the observations from the experiments. I am wondering how these understandings could be used in application. Therefore, an implications section would improve the paper.
Citation: https://doi.org/10.5194/egusphere-2026-1910-RC2 -
RC3: 'Comment on egusphere-2026-1910', Anonymous Referee #3, 06 Aug 2026
This manuscript presents a valuable one-year, minute-resolution dataset of CO₂ and O₂ concentrations from two infiltration wells at the same site. The observations are potentially useful for understanding gas transport in open wells. However, several central conclusions currently exceed what can be supported by the experimental design and analyses. The manuscript therefore requires substantial methodological clarification and revision of its causal claims.
Major comments
- The manuscript attributes the differences between the wells primarily to dry versus wet well type. However, the wells also differ in total depth, screened interval, sensor spacing, gas-column length, saturation conditions, and operational history. In addition, only the wet well received controlled infiltration and several months of operational recharge. Consequently, the effects of saturation status cannot be separated from those of well geometry, depth, screened interval, and recharge disturbance. The present study compares two specific wells rather than providing a replicated comparison of dry and wet well types. Statements such as “well type strongly controls” should therefore be substantially weakened, and this limitation should be clearly acknowledged throughout the manuscript.
- The manuscript explicitly states that air velocity and CO₂ flux were calculated only for the dry well. The decision not to calculate wet-well flux was based on low CO₂ concentrations near the wellhead and the inferred absence of strong advective transport. However, low concentration does not demonstrate low total flux. The wet well may still exchange CO₂ through diffusion, weak or intermittent convection, groundwater degassing, or recharge-induced gas displacement. Therefore, the manuscript does not currently support the conclusion that CO₂ emissions from the dry well were greater than those from the wet well. Unless wet-well flux is independently quantified, the authors should limit their conclusion to stating that periodic advective transport was quantified in the dry well, whereas no comparable periodic advective signal was detected in the wet well. Comparative emission claims in the title, abstract, discussion, and conclusions should be revised accordingly.
- Equation (1) appears dimensionally and numerically inconsistent. The standard conversion from ppm to mole fraction is not represented correctly, and the unit-conversion factor shown in the equation is unclear. The magnitude shown in Figure 5 may indicate that the code used the correct conversion and that the problem is limited to the printed equation. Nevertheless, this must be explicitly demonstrated.
- The dry-well velocity is reported as approximately 0.2–0.3 m/min, but the comparison with the previous study is expressed in m/s. This represents a factor-of-60 discrepancy and directly affects the interpretation of why the measured flux was lower than previously reported values. The authors should verify the units used in the text, Figure 5, Equation (2), the Python code, and the cited study. All velocity and time units used in the flux calculation must be consistent.
Minor comments
- Explain why the selected five-day period is considered representative and provide objective annual statistics for comparison.
- The distance between the two wells is reported as approximately 5 m in one section and approximately 7 m in another.
- Use “semi-diurnal” consistently when referring to two cycles per day.
- The in-text citation for Forde et al. gives 2017, whereas the reference list gives 2019.
- Correct “Perriaer” to “Perrier.”
- Correct the incomplete NGWA citation.
- Correct capitalization and spacing inconsistencies in subsection headings.
- Distinguish the two authors with the initials “EL” in the author-contribution statement.
- Report saturated measurements as at least 40,000 ppm.
- Revise the Figure 5 caption to specify the time and velocity units, treatment of atmospheric background, treatment of inflow, and handling of missing periods.
Citation: https://doi.org/10.5194/egusphere-2026-1910-RC3
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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: