Review comments on EGUSPHERE-2025-4567 (for Hydrology and Earth System Sciences)

This study focuses on identifying upwelling groundwater fluxes that could feed patterned pools in raised northern peat bogs in Maine, USA. It also considers the potential impact of these fluxes on the development of patterned pools. This topic is scientifically original and relevant to the readership of HESS. The methodological approaches include geophysical measurements (GPR and transient electromagnetic surveys), thermal imaging, in situ measurements of conductivity and temperature, and water sampling for Fe and Mn concentration analysis. These approaches are relevant to answering the research question. Overall, this is original and scientifically relevant work. However, I believe that some improvements are still needed before publication to maximise the manuscript's scientific impact.

General comment 1: I believe more details are needed in section 2 (Material and methods) to better explain the choices for sampling and in situ measurement points:

• It is stated (L213) that specific conductivity and temperature measurements were conducted at a 25 cm depth a few centimeters from the pools perimeter. This choice is likely justified by the enhanced accessibility of the sites close to pool margins. However, it is important to consider the possibility that measurements taken in the immediate vicinity of the pool margins may not be representative of the conditions prevailing towards the centre of the pools, especially at greater depths. Could thermal stratification occur in some pools, with cold water at the bottom and warmer water at the surface?
• A variety of measurements were conducted on different days at multiple sites. It is to be considered whether there has been a change in the temperature of surface water in pools during the period of measurement. To illustrate this point, if measurements are conducted in August, can stable temperatures be expected between the early morning and the afternoon across all measurement points? The potential impact of temporal variability, i.e. variations in surface water temperature during the day of sampling, on the comparisons between different sampling points merits consideration.
• The selection of Fe and Mn as analytical parameters could be expanded upon. Indeed, the hydrogeochemical behaviour of these two chemical elements is very sensitive to Eh-pH conditions and the presence of organic colloids. Consequently, utilising these two elements as tracers of groundwater flows without also considering Eh-pH values and the presence of organic colloids can result in a high degree of uncertainty. The use of other tracers, such as stable isotopes of the water molecule, could facilitate a more precise analysis of the sources of water feeding the pools.
• Here, the focus is set on advective transport but could diffusion from the underlaying inorganic sediments explain part of the spatial variability in pool water conductivity?

General comment 2: Should it be feasible, it would be of interest to present the precise topographic contours for the peatlands under study. Some figures illustrate upgradient and downgradient areas (e.g., Figure 2); however, it is probable that the flow configuration is more intricate than what is visible in the image. The incorporation of topographic contours (e.g., derived from Lidar data) would facilitate enhanced comprehension of the surface flow paths between pools. This would further facilitate the comprehension of the hydraulic head data presented in Figure S16 and the general pattern illustrated in the conceptual model (Figure 10).

General comment 3: The conceptual model provided in Figure 10 is of particular interest. Nevertheless, further information could be pertinent:

• If possible, it would be relevant to provide an estimate of the topographic gradient between the esker and the lagg and between the central part of the peatland and the lagg.
• If the necessary data are available, it would also be relevant to illustrate the hydraulic gradient between the esker and the peatland. Are there data confirming the higher hydraulic heads in eskers near the studied peatlands?
• The available figures (e.g. 6, S7, S9) do not provide unequivocal evidence to determine whether the eskers are solely buried formations or if they are connected to topographically higher recharge areas. Can this be explained in more details or illustrated on a figure?

General comment 4: While the text on the methodological approaches and results is interesting, it can be difficult for readers unfamiliar with the study sites to follow. This is because several methods were used at different times for three sites. To optimise readability, I would suggest adding a summary table of the work carried out at the different sites to the Materials and Methods section. For example, in such a table, the rows could represent the methods and the columns could represent the sites. The information in the cells could provide information such as the number of measurements and the dates of the measurements. Similarly, a table could be added to the results section, summarising the highlights of the results (using a similar table structure).

Some additional specific comments on the manuscript:

L28 : I believe there is a missing apostrophe

L111 + 112 : the expressions “glacial esker” and “glacial till” are used. Please double check if the term “glacial” is really needed. I believe both are, by definition, of glacial origin. The comment applies to the other appearances of these expressions.

L230 (Eq. 2) : I believe this could be an oversimplification. Is the thermal regime of the pools dependent on their size and depth, as well as groundwater fluxes? 

Figure 2 : Would it be relevant to apply a topographic correction to the profiles? This also applies to other profiles shown.

L380 (Figure 6) :à I suggest adding the date of measurements in the figure caption.

L441 : Please explain with more details the reported temperatures. Are all zones associated with temperatures below zero frozen?

L618 : Can you please better explain point c)? Are there measurements/results directly representing this?

L634 (+figure 10) : It would be interesting to add information on the image (conceptual model) on how the parameters you have measured (with geophysics, geochemistry and temperature) help for constraining the model.

L677 : Towards the end of the discussion, you would likely have an opportunity to recommend a series of measures to prioritize in future studies in order to test your conceptual model. What would you recommend to future researchers working on this issue? Such recommendations could inform future research.

Figures S5, S6, S8  : I recommend adding a line between both images and adding labels (A for temperature, B for Conductance).

Tables S1, S2, S3, S4,  : Please carefully check how many decimals should be kept for the different values in these tables.

Figure S9 : I am having difficulty understanding how the arrow is oriented in relation to the up- and down-gradient areas. Please could you double-check this?

Figure S15 : I recommend revising the text that appears on the images “3D Surface Plot of Hydraulic Gradient...”. I believe the text in the caption should suffice.

Figure S17 : The hydraulic head contours proposed therein appear to be oversimplified.

Final comment:

Overall, I believe this is a very interesting study, scientifically original and of interest to HESS readers. The suggestions presented above are intended to be constructive, and it is hoped that they will be useful to the authors.

This manuscript, titled “Proposing Sources for Discrete Groundwater Discharges to Patterned Pools in Three Regional Raised Northern Peat Bogs” addresses an important and timely topic by exploring the sources of discrete groundwater discharges to patterned pools in raised northern peat bogs using a combination of geophysical surveys (GPR and TEM), in-situ measurements (temperature and specific conductance), and geochemistry. The integration of geophysics with hydrochemical indicators is a promising approach, and the regional perspective across multiple bogs adds value to the study. The findings on discrete minerogenous groundwater upwelling linked to sub-peat glacial eskers are significant and contribute novel insights to peatland hydrology and carbon cycle science. However, the current version of the manuscript raises several methodological and interpretational concerns, particularly regarding temporal consistency, justification of measurement choices, and the strength of the linkage between geophysical responses and groundwater upwelling processes. While the results suggest differences between upgradient and downgradient zones, additional analysis and clarification are needed to strengthen the conclusions and improve the scientific rigor of the study.

Major Comments

I have highlighted some comments below that I think would strengthen the paper.

1. Justification for Multiple Conductivity Meters

The study uses three different conductivity meters [Hach sensION 5 Conductivity Meter (+/- 0.86 μS/cm), YSI Pro30 Conductivity Meter (+/- 1.0 μS/cm), and EcoSense EC300A Conductivity Meter (+/- 2.0 μS/cm)] to measure specific conductance (SpC). The rationale for using multiple instruments instead of a single, standardized meter is not adequately explained. This raises concerns regarding data consistency, calibration differences, and potential measurement bias. The authors should clearly justify this choice and describe any intercalibration procedures used to ensure comparability across instruments.

1. Temporal Mismatch Between Datasets

A major limitation of the study is the lack of temporal alignment between datasets. The GPR surveys were conducted in August and November 2023 and June 2024, the TEM survey in May 2025, while the in-situ temperature and SpC data were collected only in August 2023. Given the strong seasonal and yearly variability in peatland hydrology, this temporal mismatch could introduce significant uncertainty into the interpretation of groundwater discharge processes. The authors should more explicitly discuss how seasonal dynamics and year-to-year variability may affect the geophysical and in-situ signals. Ideally, the study would benefit from co-located, same-day measurements of TEM, temperature, and SpC to better constrain interpretations. At minimum, this limitation should be clearly acknowledged and its implications for the conclusions carefully addressed.

1. Quantification of Upgradient vs. Downgradient Differences

The manuscript demonstrates that upgradient and downgradient populations differ based on values; however, the magnitude of this difference is not quantified. Reporting effect sizes or percentage differences would strengthen the argument and allow readers to better assess the hydrological significance of the findings, beyond statistical separation alone.

1. Strength of Evidence for Minerogenous versus Ombrogenous Sources

L510-515: The discussion effectively presents evidence for upwelling but is cautious in attributing it solely to minerogenous groundwater, rightly noting the potential for peat pore water flow cells. This ambiguity is a strength scientifically, but could be sharpened. The aqueous geochemistry results (Fe, Mn) are particularly ambiguous in this regard. Strengthen this section by weighing the evidence for each source more directly. For example, the spatial correlation with eskers (GPR/TEM) strongly suggests a geological control. The T/SpC/TIR signals confirm vertical fluid movement. Do the Fe/Mn levels uniquely indicate a glacial aquifer, or could they be mobilized from peat? A paragraph explicitly comparing the likelihood of each source model, perhaps using the conceptual model (Fig. 10) as a framework, would be very helpful.

1. Role of Geophysics: Structure vs. Process

As presented, the GPR and TEM data primarily provide a structural framework of the subsurface. While this framework is valuable, the manuscript sometimes implies a stronger process-based linkage between geophysical responses and groundwater upwelling than is currently supported by the data. The authors should clarify whether the geophysical results are intended to directly identify groundwater discharge pathways or simply constrain plausible structural controls. Future research directions should be explicitly recommended, particularly studies that quantitatively link geophysical parameters (e.g., bulk electrical conductivity) to groundwater fluxes or upwelling processes, rather than relying solely on point-scale in-situ measurements.

1. Interpretation of Equation (2) and TEM Conductivity

Equation (2) suggests a relationship involving specific conductance; however, the manuscript does not clearly demonstrate how SpC relates to the electrical conductivity derived from the TEM survey. Given that the TEM and in-situ datasets were not collected concurrently, it remains unclear which physical properties primarily control the TEM-derived conductivity (e.g., pore-water conductivity, saturation, lithology, or organic matter content). The authors should clarify the conceptual basis linking TEM conductivity to groundwater upwelling and patterned pool formation. If such a linkage cannot be robustly established with the available data, this limitation should be explicitly acknowledged.