Continuous Electrical Resistivity Tomography Monitoring in Waste Landfill Sites with Different Properties and Visualization of Water Channels

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The purpose of this article is to disseminate a monitoring study carried out using electrical resistivity tomography (ERT) technology, with the aim of analyzing the internal behavior of water, as well as leaching due to stormwater infiltration, in municipal solid waste (MSW) landfills.
It is important to note that it is of great interest to understand how wastewater behaves in the presence of groundwater infiltration, so this study is a fundamental element in understanding the interaction of non-combustible waste and ash once deposited in the landfill. Stormwater infiltration directly affects these deposits, seeping into the subsoil and potentially reaching groundwater levels that could become contaminated. This is why it is so essential to know their status in real time.
First, we must focus on the appropriateness of real-time monitoring with ERT. Compared to other tomographic techniques, ERT provides high-quality visualization, is competitively priced, and is easy to use. In this regard, we agree with the chosen technique. Second, we analyze the methodology used. Two sites (A and B) were chosen, representing two different landfills. Site A is a public landfill located in Saitama, Japan, where non-combustible municipal solid waste (MSW) has been deposited since 2019. Site B is also located in Saitama, at a distance of 52 km from Site A. In this case, MSW ash has been deposited. We also consider the choice of both sites for study and comparison to be highly appropriate.
Third, the study contains a detailed description of the theoretical basis, the evaluation of time-dependent resistivity changes, and the water injection procedures to analyze the changes caused by its presence at both landfills. This section is crucial for drawing the conclusions presented, allowing the external researcher to fully monitor the process. We therefore emphasize our agreement with the study. The conclusions presented are clearly deduced from the procedures carried out and demonstrate their validity. Thus, it is noted that the effect of washing was more pronounced in MSW discharges in the form of ash, as is evident, compared to a lesser effect in non-combustible waste.
Finally, we note that the work is well-oriented overall, clearly presented and structurally developed, with sufficient figures (8) that provide valuable complementary information to the text on the procedures performed. The bibliography is also complete and up-to-date.

Author Response

Dear Reviewer 1

We would like to appreciate for your peer review.

Your constructive and positive comments are an encouragement and motivation for our continuous research activities.

Now we have revised our manuscript according to all of reviewer comments and suggestions as follows. Corrections have been underlined. We believe that we have made more meaningful and significant progress. Thank you very much for your kindly support.

Reviewer 2 Report

Comments and Suggestions for Authors

This research utilizes continuous Electrical Resistivity Tomography monitoring, together with X-ray CT imaging, in the study of water flow and washout behavior in Municipal Solid Waste landfills of varying composition. Although this sequential setup provides a comprehensive description of the water migration and washout behavior, the lack of any quantitative comparison or statistical analyses between the two sites diminishes the ability to make stronger conclusions regarding the water migration and washout processes.

 

additional comments
The main question that is addressed is: Rationale for using continuous electrical resistivity tomography (ERT) and X-ray CT-imaging for monitoring and analyzing internal water dynamics and washout behaviour in municipal solid waste landfills.

 

The paper idea is novel and relevant to the literature, and the combination of ERT and X-ray CT imaging provides explicit insight into water flow dynamics. 

 

One specific enhancement could be to conduct more-in-depth quantitative statistical treatment on the data to add validity to the reliability of the results. 

 

Overall, the conclusions are aligned with the evidence and rationale; however, the authors must add more quantified evidence to support the identified results. With respect to references, there were not sufficient papers, would suggest adding additional citations in the introduction and discuss them there to improve the flow of this work.

 

The tables and figures are generally clear and provide useful visual summaries of the findings of the study. Overall, an interesting paper that can be reconsidered after major revisions.

 

 

 

 

 

Author Response

Dear Reviewer 2

We would like to appreciate for your peer review.Your comment that our explanation lacks quantitative comparisons is very important indication.

We are here to respond to your comments and suggestions as follows.

And we have revised our manuscript according to all of reviewer comments and suggestions. Corrections have been underlined. We believe that we have made more meaningful and significant progress. Thank you very much for your kindly support.

[Comment 1 and 2]

• The main question that is addressed is: Rationale for using continuous electrical resistivity tomography (ERT) and X-ray CT-imaging for monitoring and analyzing internal water dynamics and washout behavior in municipal solid waste landfills.
• The paper idea is novel and relevant to the literature, and the combination of ERT and X-ray CT imaging provides explicit insight into water flow dynamics. 

[Response 1 and 2]

One of the advantages of continuous ERT monitoring is the ability to estimate subsurface structure and physical changes nondestructively without excavation. ERT was adopted in this study as an appropriate method to understand the moisture dynamics inside a final disposal site, because the resistivity value has a relationship with the variation of water content and electrical conductivity of pore water. It is important to understand the physical properties of landfill waste as a basis for interpreting the results of ERT monitoring. In this study, the pore structure of waste samples obtained by excavation was investigated by X-ray CT-imaging. We believe that a comprehensive survey combining both methods is highly novel.

[Comment 3]

• One specific enhancement could be to conduct more-in-depth quantitative statistical treatment on the data to add validity to the reliability of the results. 

[Response 3]

According to the suggestion, we added sentence in Section 2.2.1 (Line 121-123, 128-131).

[Comment 4]

• Overall, the conclusions are aligned with the evidence and rationale; however, the authors must add more quantified evidence to support the identified results. With respect to references, there were not sufficient papers, would suggest adding additional citations in the introduction and discuss them there to improve the flow of this work.

[Response 4]

According to the suggestion, we added some sentence in Section 1 and Section 3.1, respectively, (Line 64-65, 69-70, 287-289 and 295-296) with additional citations [ref. 13, 14, 21, 23 and 25].

[Comment 5]

• The tables and figures are generally clear and provide useful visual summaries of the findings of the study. Overall, an interesting paper that can be reconsidered after major revisions.

[Response 5]

According to the comments of Reviewer 2 and other reviewers, figures and captions were revised for better clarity.

Reviewer 3 Report

Comments and Suggestions for Authors

The comments are included in the attached file.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Language should be improved

Author Response

Dear Reviewer 3

We would like to appreciate for your peer review. Your comment about our lack of novelty aspects and your kindly suggestion of previously papers have given us great insight and knowledge.

We are here to respond to your comments and suggestions as follows.

And we have revised our manuscript according to all of reviewer comments and suggestions. Corrections have been underlined. We believe that we have made more meaningful and significant progress. Thank you very much for your kindly support.

[Comment 1]

The main drawback I observe in the prepared documents is the lack of a justified novelty aspects.

[Response 1]

Thank you for referring us to the papers in the previous studies by Helene et al. and Dumont et al. These were all important studies for our research.

The work of Helene et al. was a great motivation for us. Based on their results, we believe that the novelty aspects are to search for water channels inside the repository under temporal and spatial highly density conditions, and we have added some sentence in Section 1 with reference. (Line 69-70, Ref. 21)

We were convinced that the results of the Dumont et al. study strongly support the finding that the gas vent pipe is an important drainage pathway in our SiteA findings. We added some sentences in Section 3.1 with reference. (Line 295-296, Ref. 25)

[Comment 2] Line 18: X-ray CT has been employed without previous introduction of the meaning of CT.

[Response 2] We revised the abstract according to the comment (Line 10-14)

[Comment 3] Line 47: The format of the reference is not correct (point before parentheses).

[Response 3] We removed the point and corrected according to the format (Line 48).

[Comment 4] Line 132: The symbol ρ has been defined twice, while the symbol w has not been defined.

[Response 4] We corrected second symbol ρ to ρw (Line 142).

[Comment 5] Section 2.2.2: The theoretical background of ERT can be included in the introduction.

[Response 5] We added some sentence in Section 1 (Line 61-63).

[Comment 6] Figures 1 and 2: A specific description of each image included in the figures should be provided.

[Response 6] We revised the caption of Figure 1 and 2.

[Comment 7] Figure 3: Additional information about the axis of the graphs should be provided.

[Response 7] We added the information about the axis in Figure 3, 5 and 6.

[Comment 8] Figure 4: This figure is more useful if presented before Figure 3.

[Response 8] We put Figure 4 before Figure 3. So, the number of Figure 3 and 4 were exchanged.

[Comment 9] Figure 6: The trench is marked in this figure, while there is no marked trench in Figure 3.

[Response 9] We added the information of the trench in Figure 5 and 6.

[Comment 10] References [5-10]: The number of references by Loke et al can be excessive.

[Response 10] We removed three references of Loke et al. [5,7,10].

[Comment 11] Furthermore, the language of the manuscript should be improved, for instance, in Lines 23, 92, 176, 194 or 196.

[Response 11] We corrected several sentences in Lines 23-25, 99, 191, 209-213.

[Comment 12] Much more examples of the application of ERT in landfills

[Response 12] We cited some papers of Helene et al. [Ref. 21], Dumont et al.[Ref. 25], in addition, Suntoro et al. [Ref. 13] and Feng et al. [Ref. 14], that you referred to us.

Reviewer 4 Report

Comments and Suggestions for Authors

This study explores the behavior of water infiltration in waste landfill sites with varying properties, aiming to elucidate the formation of water channels, the washout processes, and the influence of landfill design factors on water flow and waste stabilization. The research holds significant value. However, I have the following concerns:

1. In Section 2.1, it is recommended to clearly differentiate between the two landfill sites by using a table format.
2. Detailed explanations regarding the mapping basis (data sources, measurement methods, etc.) for Figures 8 and 9 should be provided.
3. It is suggested to add a scale bar to Figure 4.

Author Response

Dear Reviewer 4

We would like to appreciate for your peer review. Your valuable comments have lead us to make our manuscript more visually informative.

We are here to respond to your comments and suggestions as follows.

And we have revised our manuscript according to all of reviewer comments and suggestions. Corrections have been underlined. We believe that we have made more meaningful and significant progress. Thank you very much for your kindly support.

[Comment 1]

In Section 2.1, it is recommended to clearly differentiate between the two landfill sites by using a table format.

[Response 1]

We summarized the information of two landfill sites in Table 1.

[Comment 2]

Detailed explanations regarding the mapping basis (data sources, measurement methods, etc.) for Figures 8 and 9 should be provided.

[Response 2]

We added additional explanations for Figure 8 and 9 in Line 357–369.

[Comment 3]

It is suggested to add a scale bar to Figure 4.

[Response 3]

We added a scale bar to Figure 4.

Reviewer 5 Report

Comments and Suggestions for Authors

Dear Authors：

This study conducts a valuable investigation into the application of Electrical Resistivity Tomography (ERT) technology for monitoring water migration and scouring processes in landfills with different waste compositions. The combination of ERT monitoring, water injection tests, and X-ray Computed Tomography (X-ray CT) analysis provides new insights into subsurface water dynamics, which is of clear guiding significance for environmental engineering and waste management. Although the study is reasonably structured and methodologically reliable, some concerns should be noted before this paper can be accepted:

1. Line18 (Page 1): When X-ray CT first appears, it is recommended to expand it to X-ray computed tomography (X-ray CT) instead of using the abbreviation directly.
2. Section 2.2 (page 5-6): The theoretical background of Electrical Resistivity Tomography (ERT) can be further supported by relevant cases. Zhao et al. (2024) conducted a simulation study on the response characteristics of water-bearing zones in metal mines monitored by the resistivity method, which shares the same principle with the ERT monitoring of water infiltration in this study. Their research verified the effectiveness of resistivity methods in capturing fluid distribution and seepage paths in complex media, providing a practical application example for the application of ERT technology in heterogeneous landfill environments.（e. g. Doi: 10.1016/j.jsasus.2024.11.006）
3. Figure 3 (Page 6): It is advisable to eschew the use of sentences for annotations. Instead, employ concise phrases. Specific details can be explained in the main text for better clarity and to maintain the figure's simplicity.
4. Line284 (Page 8): There might be a spelling error: "mail components" should be "main components".
5. Section 4 Conclusions (Page 12): It is recommended to clarify the on-site application limitations of the ERT technology and supplement future research directions.
6. Section 4 Discussion (Page 12): When envisioning future technical integration, the combination of ERT and AI can be further emphasized. Lu et al. (2025) developed an AI-driven complex network system for real-time prediction of coal mine gas explosions, which integrates multi-modal data fusion and dynamic modeling. The time-series analysis of ERT in this study, such as the resistivity change rate Δρ, shares methodological similarities with the multi-modal data processing in Lu’s research. Citing this literature (e. g. Doi: 10.1016/j.jsasus.2025.2.002 ) can provide valuable methodological references for combining AI with ERT to improve the accuracy of water channel prediction in landfills.
7. Section 4 Discussion (Page 12): The methodology employed in the manuscript to track the evolution of water channels through analyzing time-dependent resistivity distributions aligns with the slope instability warning approach proposed by Wang et al. (2025). The latter utilizes time-series data and Sen’s trend analysis to detect dynamic changes in rock mass stability. Citing this literature in the conclusion can further validate the rationality of using ERT-derived time-dependent trends to assess water migration risks and highlight the value of time-series analysis in risk early warning (e. g. DOI: 10.1016/j.tust.2025.106514)

Author Response

Dear Reviewer 5

We would like to appreciate for your peer review. Your valuable comments and showing several previously papers have given us great insight and knowledge.

We are here to respond to your comments and suggestions as follows.

And we have revised our manuscript according to all of reviewer comments and suggestions. Corrections have been underlined. We believe that we have made more meaningful and significant progress. Thank you very much for your kindly support.

[Comment 1]

Line18 (Page 1): When X-ray CT first appears, it is recommended to expand it to X-ray computed tomography (X-ray CT) instead of using the abbreviation directly.

[Response 1]

We revised the abstract according to the comment (Line 10-14).

[Comment 2]

Section 2.2 (page 5-6): The theoretical background of Electrical Resistivity Tomography (ERT) can be further supported by relevant cases. Zhao et al. (2024) conducted a simulation study on the response characteristics of water-bearing zones in metal mines monitored by the resistivity method, which shares the same principle with the ERT monitoring of water infiltration in this study. Their research verified the effectiveness of resistivity methods in capturing fluid distribution and seepage paths in complex media, providing a practical application example for the application of ERT technology in heterogeneous landfill environments.（e. g. Doi: 10.1016/j.jsasus.2024.11.006）

[Response 2]

Thank you for the useful references. The numerical simulation results by Zhao et al. showing that resistivity decreases with high sensitivity with water penetration into pore structures such as fractures, and we added sentence in (section 3.1, Line 287-289, Ref. 23) as supporting our results that the resistivity decrease in a landfill is due to water penetration into the water channel.

[Comment 3]

Figure 3 (Page 6): It is advisable to eschew the use of sentences for annotations. Instead, employ concise phrases. Specific details can be explained in the main text for better clarity and to maintain the figure's simplicity.

[Response 3]

According to your suggestion, we have revised the caption of Figure 4 and 5. We have changed the order of Figures 3 and 4 in accordance with the comments of other reviewers.

[Comment 4]

Line284 (Page 8): There might be a spelling error: "mail components" should be "main components".

[Response 4]

We corrected to “main components” (Line 308).

[Comment 5]

Section 4 Conclusions (Page 12): It is recommended to clarify the on-site application limitations of the ERT technology and supplement future research directions.

[Response 5]

We added the sentence of the limitation of the ERT and future research directions in Line 418-423.

[Comment 6 and 7]

• Section 4 Discussion (Page 12):When envisioning future technical integration, the combination of ERT and AI can be further emphasized. Lu et al. (2025) developed an AI-driven complex network system for real-time prediction of coal mine gas explosions, which integrates multi-modal data fusion and dynamic modeling. The time-series analysis of ERT in this study, such as the resistivity change rate Δρ, shares methodological similarities with the multi-modal data processing in Lu’s research. Citing this literature (e. g. Doi: 10.1016/j.jsasus.2025.2.002 ) can provide valuable methodological references for combining AI with ERT to improve the accuracy of water channel prediction in landfills.
• Section 4 Discussion (Page 12):The methodology employed in the manuscript to track the evolution of water channels through analyzing time-dependent resistivity distributions aligns with the slope instability warning approach proposed by Wang et al. (2025). The latter utilizes time-series data and Sen’s trend analysis to detect dynamic changes in rock mass stability. Citing this literature in the conclusion can further validate the rationality of using ERT-derived time-dependent trends to assess water migration risks and highlight the value of time-series analysis in risk early warning (e. g. DOI: 10.1016/j.tust.2025.106514)

[Response 6 and 7]

Thank you for the powerful advice. It is exactly what we are hoping for in the future development of AI-assisted ERT. And we also appreciate for sharing the useful two references. We found additional paper on ERT and AI collaboration by Aversana et al., and we have cited all three paper in section 4 (Line 423-432, Ref. 26-28).

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This version is better. I want to again refer to the earlier comment on the conclusions. "Overall, the conclusions are aligned with the evidence and rationale; however, the authors must add more quantified evidence to support the identified results." Be sure to include some specified numerical values/ranges of the data to substantiate the primary findings, which, in turn, will strengthen the conclusions based on the evidence. In addition, I have also noticed that references were included in the conclusion section. This is typically not done unless a reference is to provide context. In this case, the conclusions should collate and synthesize the study's findings without using or relying on context from an outside reference. Please reconsider the references to this section, or provide clarification on the necessity of these references if they are essential to substantiate a point.

Author Response

[Comment]

This version is better. I want to again refer to the earlier comment on the conclusions. "Overall, the conclusions are aligned with the evidence and rationale; however, the authors must add more quantified evidence to support the identified results." Be sure to include some specified numerical values/ranges of the data to substantiate the primary findings, which, in turn, will strengthen the conclusions based on the evidence. In addition, I have also noticed that references were included in the conclusion section. This is typically not done unless a reference is to provide context. In this case, the conclusions should collate and synthesize the study's findings without using or relying on context from an outside reference. Please reconsider the references to this section, or provide clarification on the necessity of these references if they are essential to substantiate a point.

 

[Response]

Dear reviewer 2.

We sincerely appreciate your thorough and detailed reviews.

All corrections and text additions have been highlighted in blue.

We will now address the first comment. First, we added numerical data on infiltration rates from the water injection test to illustrate the qualitative differences in landfill quality between Sites A and B (see Section 3.2, lines 316–320, and Section 4, Lines 429-432, 434-439, and 443-445). In detail, the water infiltration rate during the first water injection test was 3.10 cm/min at Site A and 0.77 cm/min at Site B. The landfill at Site A consists primarily of non-combustible materials, which are difficult to compact. This results in high permeability. In contrast, the landfill at Site B consists solely of incineration residues, which are easier to compact. This results in lower permeability and makes washout difficult. This observation is consistent with Fig. 8, which shows that the non-combustible material landfill layer has a porosity of 0.328 and the incineration ash landfill layer has a smaller porosity of 0.195.

Regarding the second comment, we added a new subsection, 3.4 "Further Study," to address it rather than including it in the conclusion.

Reviewer 3 Report

Comments and Suggestions for Authors

The comments are included in the attached file.

Comments for author File: Comments.pdf

Author Response

[Comment 1 (Answer to Response 1)]

• The novelty statement of the revised manuscript is insufficient. The authors mention “the application of temporal and spatial high-resolution ERT monitoring for exploring a water　channel in landfill.” However, as introduced when the work by Helene et al. is cited, that work　demonstrated that the ERT monitoring at a landfill could describe leachate infiltration and　their flow pathway. Therefore, the new aspects provided by this work when compared to the　one published by Helene et al. are unclear. Moreover, other relevant works about the　application of ERT to landfill characterization, including leachate channel have not been　cited in the manuscript (see answer to Comment 12).

[Response 1]

Dear reviewer 3.

We sincerely appreciate your thorough and detailed reviews.

All corrections and text additions have been highlighted in blue.

We believe the novel aspects of our study are as follows: First, we compared two landfill sites—one dominated by non-combustible residues and the other by incineration ash—using the same resistivity survey. Second, we confirmed that the pore structure influenced these results through undisturbed sample collection and X-ray CT analysis. We then characterized the pore structure further to investigate its relationship with the resistivity survey results.

This explanation is already described in lines 10–14 of the abstract, lines 72 and following of the introduction, and line 195 of the methods section. However, in accordance with your comments, we have reemphasized the explanation of the novelty and revised the text for clarity. Specifically, we added the following text to lines 75–78: “To investigate the influence of the pore structure of landfill layers on ERT results at different sites, this study collected undisturbed samples and performed X-ray CT analysis. This study is novel in that continuous ERT and X-ray CT analysis were conducted at two sites with different types of waste.”

NOTE: other novelty elements

Other novel elements:

We believe the following three points demonstrate novelty as well. However, to avoid redundancy, we have not made comparisons with individual papers. Although you may find some parts of our response unsatisfactory, we believe that no similar studies exist when considering the background of the research, such as practical experience and constraints, rather than the research methodology and technology. Conducting ERT at a landfill site can address various challenges, such as obtaining the cooperation of site managers and avoiding disruptions to operations. Therefore, we believe that the results of conducting ERT and X-ray CT analysis at different landfill sites are important administrative findings and should be shared. This is why we submitted this paper.

• Most previous studies had electrode intervals of 1 m or more and a temporal resolution of approximately three measurements. In this study, however, we enhanced the resolution to 0.5 m and 66 measurements, respectively.
• While previous studies aimed to identify leakage points after closure, this study aimed to investigate water pathways for maintenance and closure purposes.
• While some studies observe flow using ERT, few have conducted water injection tests to clarify flow. Dumont et al. performed water injection tests; however, since they reinjected the leachate, the resulting resistivity distributions and temporal variations differ from those in this study.

 

[Comment 2 (Answer to Response 12)]

• Although additional references have been included, some relevant ones are still　unmentioned in the manuscript, for instance:

- Study on Resistivity Imaging of Urban Landfill Leachate Channel Using 3D　Electrical Resistivity Tomography with Annular Electrode Configuration

Cheng, J., Zhang, Y., Shen, Y., Li, Y., Huang, J.

Journal of Physics: Conference Series, 2651(1), 012136 2023

- Characterizing landfill extent, composition, and biogeochemical activity using　electrical resistivity tomography and induced polarization under varying　geomembrane coverage

Ma, X., Zhang, J., Schwartz, N., ... Meng, J., Mao, D.　Geophysics, 89(4), pp. E151–E164　2024

- Three-Dimensional Electrical Resistivity Tomography for Leachate Imaging　Considering Thin Impermeable Layers of Landfills

Li, Y., Liu, Y., Yin, C., ... Ren, X., Zhang, B.

IEEE Transactions on Geoscience and Remote Sensing, 62, 4511514 2024

• Therefore, the review of the state of the art regarding the application of ERT to landfills is not adequate.
• Taking into account the poor novelty aspects of the manuscript and the poor analysis of the state of the art, which lead to unoriginal conclusions and no noteworthy contributions, I consider the manuscript should be rejected.

[Response 2]

Our initial response was insufficient. We are making every effort to collect as many references as possible in the limited time that we have to respond to the review. However, as you know, there are restrictions on the references that can be obtained depending on the affiliated institution's policies. Unfortunately, we were unable to collect the papers by Ma et al. and Li et al. in time for the first revision.

This time, we have reviewed the additional papers you requested. Li et al.'s study you recommended is important and we have cited it in Section 1, Lines 54–55 (Ref. 4). On the other hand, we did not cite Cheng et al.'s paper, which was a study on numerical simulation, or Ma et al.'s paper, which is still unavailable.

According to MDPI's guidelines, authors should not include all suggested reference papers, but only those they consider necessary, if instructed to add reference papers based on the review results. Therefore, we omitted references that could not be verified in the main text. However, we will keep the references you pointed out in mind and incorporate them into future research.

Again, we sincerely appreciate the valuable information you have provided.

Reviewer 5 Report

Comments and Suggestions for Authors

All my concerns have been addressed well. It can be accepted as its present form.

Author Response

[Comment]

All my concerns have been addressed well. It can be accepted as its present form.

 

[Response]

Dear Reviewer 5.

Thank you very much for your repeated and thoughtful reviews.

 

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

The last revised version of the manuscript has not modified my opinion: the manuscrispt should be rejected.