| 06 Feb 2026
Downstream export dominates the fate of groundwater-derived CO2 in a boreal stream
Abstract. Groundwater inflow is increasingly recognized as a major source of carbon dioxide (CO2) to streams. Yet, its fate – whether it is emitted to the atmosphere or exported downstream – remains poorly characterized, partly due to the challenges of quantifying groundwater inflow rates at high spatial (meter) and temporal (days) resolutions. In this study, we assessed the fate of groundwater-derived CO2 in a 400 m boreal headwater stream reach by combining fine-scale measurements of groundwater inputs, emissions and downstream export of CO2. Spatial patterns in groundwater-derived CO2 inputs were primarily driven by the magnitude of groundwater inflows, which were controlled by catchment characteristics, such as stream slope and localized aquifer properties. Temporally, peaks in groundwater CO2 inputs during snowmelt were primarily driven by increased groundwater discharge rather than elevated CO2 concentrations in the groundwater, whereas peaks during summer and early autumn were associated with rainfall events and higher CO2 concentrations in groundwater, likely resulting from enhanced soil respiration. Overall, groundwater CO2 inputs exceeded CO2 emissions by up to fourfold, with 40–60 % of terrestrial CO2 transported downstream. This indicates that a substantial portion bypasses immediate atmospheric emission and may contribute to CO2 emission further along the stream network or be cycled through in–stream processes downstream. Our results demonstrate how and to what extent groundwater inflows contribute to the variability of CO2 fluxes from headwater streams. These findings highlight the importance of integrative assessments of CO2 fluxes (i.e. groundwater inputs, emissions, and downstream export), which consider both in-stream processes and catchment-scale dynamics. This is particularly important in the context of climate-driven changes in hydrology and terrestrial carbon cycling.
This study quantified the influence of groundwater inflow on CO2 emissions and export from a headwater stream in northern Sweden, with a cold, humid boreal climate, over 13 sampling campaigns in 2020. For this, the authors used radon 222 as a hydrologic tracer to obtain direct estimates of groundwater flow and groundwater CO2 fluxes. In general this is a well written manuscript and could represent an important contribution to our understanding of the C cycle in fluvial systems; however, there is room for improvement. Not enough care and attention was put towards the methods section which I found confusing at times. In addition, I recommend the authors expand the sensitivity/uncertainty analysis for the groundwater CO2 inputs to the other fluxes too (downstream export and emissions to the atmosphere), this will help better understand the magnitude of the sources of error when building mass balances for CO2. Also, I’d like to see a comparison between the direct estimations of CO2 inputs via Rn222 and by mass balance of CO2. Does the budget add up? This is particularly concerning because the authors observed that their estimates of groundwater flow based on Rn222 were only a fraction of the groundwater inputs when estimated based on upstream/downstream differences in stream discharge. More detailed comments as follows:
Either in the intro or somewhere in the discussion I’d like to see some discussion on the role of CO2 vs other GHGs in the emissions of GHGs from boreal systems, just to have an idea of why the focus on CO2 and what we are missing by not measuring CH4 or N2O emissions.
METHODS
Mind the organization in the methods, make sure you have consistency in the order of CO2, and Rn222 sampling/processing for all the water sources. Be consistent in the use of the equation terms throughout the method descriptions to facilitate understanding exactly which pool or flux the authors are referring to. Consider creating a simple diagram that shows the equation terms and simple version of how they were measured.
L143—What is the reasoning behind acidifying the sample, if the interest is only in the CO2 concentration? Why not to measure the CO2 in the original sample? Is it to fix the sample (avoid microbial metabolism) I’d be a bit concerned about the kind of error introduced from the back-calculation of the original CO2 concentration from the CO2 concentration of an acidified sample and pH, simply because pH is so sensitive and prone to error. Perhaps the error is small because of the low pH…?
L168 – This is confusing. The previous paragraph explains how Rn222 is measured in the water. But this line then refers to the concentration in the air, and how the concentrations in the water are determined based on the concentrations in the air…?
L186 – Why were concentrations of Rn222 in the groundwater measured using a different technique/equipment than stream water (or air, I am not sure, see comment for L168 above)?
L189 – Do the authors mean “to account for radioactive decay” as in L172? If so make sure wording is consistent.
L189 – Why sampling soil air instead of the actual water for groundwater CO2 concentrations?
L201—Was discharge measured during every sampling campaign?
L202 – There are other more general citations for salt pulse that could be used here instead of a paper in revision.
L204-207 – This section about gas transfer should be moved below to the section on CO2 emissions. Was k measured during every sampling campaign?
L221 – Are these incubation experiments what is explained in the following paragraph? Is it soils or is it sediments? Is the diffusing radon input Fdiff mentioned below?
L222 – Did the authors mean sediment instead or soil?
L222, L232 – Why/how concentration of Rn222 in the groundwater again? How are these different from the concentration of Rn222 in the groundwater described in L186? Is one advective and one diffusive? If so, make that very clear.
L232—It is unclear to me where the Rlab and Rfield come from. If Rlab (bottle) refer to the bottle experiments described in L223? If so, it would really help to refer to the Rlab and (Rfield) terms in their corresponding sections.
L246—Ci-1, I assume concentrations in the stream water? Add “streamwater” as a qualifier.\
L257 – Equation 2?
L258 – The description of the uncertainty estimations should be in a different section, perhaps under “statistics and sensitivity analysis”
L297 – How good was this model? This is important to know, to understand the accuracy of the CO2 emissions.
STATISTICS
Why is the sensitivity analysis only conducted for Fgw? This same approach could be used for most other measurements to obtain associated errors for CO2 emissions and export.
Sound is not a well-known method, perhaps expand on the error associated with this type of measurement? This is in part why I suggest a sensitivity analysis for CO2 emissions.
RESULTS
Some of the figures could be joined to reduce the large number of figures and to better allow for comparisons of concentrations and fluxes of the different pathways. For example, consider moving Figure S3a and S3b to the main document and combine it with Figure 2, so that stream and gw concentrations are plotted side by side. Same applies to Figure S3c and S3d, combine with Figure 3 but use the same type of graph, either box plot or line, but consistent.
There is lots of inconsistencies with the number of significant digits used within the different rates. Make sure the significant digits are also consistent between text and figures too.
L329 – Constant is a strong word for such variability. I do not see anything constant in the longitudinal trends.
L419 – What drives the somewhat less variable CO2 emissions? How did gas exchange vary longitudinally and temporally?
DISCUSSION
A few things I’d like to see discussed: how does the “conservative” (L375) nature of your methodology affects your results and conclusions?; the study is based on a relatively limited number of wells, how does this affect the overall results and conclusions? (besides not capturing potentially large spatial variability); how do gas exchange rates and CO2 emissions compare to other studies/methods in similar systems and how does the uncertainty on those values affect the relative importance of downstream export vs emissions to the atmosphere?; does the budget close? Or could the authors compare their gw input results with values obtained from the difference between upstream inputs and (downstream export+emissions)?
L496—It is interesting that there is less variability in gw CO2 concentrations than in gw fluxes. I’d imagine the hydrologic patterns would have some effect on the biological factors controlling CO2 concentrations…?
L557—focus on published work that supports your findings.
L572—Residence time?