Multiscale Geophysical Characterization of Leachate and Gas Plumes in a Tropical Landfill Using Electrical Resistivity Tomography for Environmental Analysis and Diagnosis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of the manuscript with title "Multiscale geophysical characterizations of leachate and gas Plumes in a Tropical Landfill using electrical Resistivity Tomography for Environmental Analysis and Diagnosis"  reports interesting research results. However, there are several drawbacks in the manuscript, the manuscript can be revised taking into consideration comments of the reviewer.

1. The abstract section needs further improvement with details regarding reasons for the selection of the Electrical Resistivity Tomography in this research.
2. Introduction section should be thoroughly revised with inclusion of the research summary in open literature about the application of Electrical Resistivity Tomography for Environmental Analysis and Diagnosis.
3. Authors are advised to explain in detail regarding samplings of the leachate and gas plumes from different places in the landfills.
4.  2.3. Operational Principles and Data Acquisition, Have the authors studied the variations in the electrical current while measuring with the ERT method ?
5.   Figure 3. Electrical resistivity ranges for various geological materials, minerals and chemical compunds, it is suggested that authors explain the criteria for selection of different types of minerals and chemicals.
6. 2.8 Volumetric Estimation of Gas from Resistivity Anomalies, authors are advised to write in detail regarding the calibration method applied for measurement of potential gas accumulation.
7. Figure 5. Electrical resistivity model obtained using Dipole-Dipole method along Tomography Line 1. Authors have mentioned that Dipole-Dipole method is is highly sensitive, Can the authors explain what is the accuracy of this method ?. Furthermore they are advised to include some other relevant methods for comparison purposes.
8. Conclusion section should be rewritten with most relevant research results.
9. Quality of figures and tables should be improved.
10. Layout of the manuscript should be revised according to the recommendations of the Environment. 

Author Response

Best regards

We have thoroughly revised the manuscript to address each of the reviewers' comments. Below, we provide a detailed response to each point, outlining the changes implemented in the revised version.

Reviewer 1:

Reviewer 1, Comment 1:
The abstract section needs further improvement with details regarding reasons for the selection of the Electrical Resistivity Tomography in this research.

Response:
We appreciate the reviewer’s comment. The abstract has been revised to explicitly justify the selection of Electrical Resistivity Tomography (ERT) in this study. Additional details were incorporated to highlight its advantages for post-closure landfill monitoring in tropical environments, including: (i) its ability to discriminate between highly conductive leachate and resistive gas/dry waste; (ii) its suitability for achieving robust depth control in heterogeneous and moisture-rich contexts; and (iii) its cost-effectiveness compared to intrusive borehole drilling and other geophysical methods that are limited in conductive environments. These clarifications have been added to the abstract (lines 13–20).

Reviewer 1, Comment 2:

Introduction section should be thoroughly revised with inclusion of the research summary in open literature about the application of Electrical Resistivity Tomography for Environmental Analysis and Diagnosis.

Response:
We sincerely thank the reviewer for this valuable suggestion. Accordingly, the Introduction has been revised to provide a broader summary of the open literature on the application of Electrical Resistivity Tomography (ERT) for environmental analysis and diagnosis.

In the revised manuscript, we incorporated a new paragraph (see Introduction, lines 98–108) that highlights representative studies demonstrating the diverse applications of ERT: the detection of conductive leachate plumes (Wilkinson et al., 2010), detailed mapping of leachate migration using 2D and 3D modeling correlated with hydrogeochemical data (Maurya et al., 2017), the identification of infiltration pathways and leachate flow directions (Helene et al., 2020), and the spatial characterization of leachate plumes in mixed-age landfill deposits (Bichet et al., 2016). Additionally, recent advances include large-scale applications for detecting leachate and gas distributions in landfill cells (Zhan et al., 2019) and the geophysical monitoring of leachate injection in pretreated landfills (Godio & Chiampo, 2023).

This expanded literature review reinforces the consolidation of ERT as a robust and replicable methodology for environmental risk assessment, integrating statistical inversion, 3D volumetric modeling, and hydrogeochemical validation. We believe this addition directly addresses the reviewer’s concern and strengthens the scientific foundation of our study.

Reviewer 1, Comment 3:
Authors are advised to explain in detail regarding samplings of the leachate and gas plumes from different places in the landfills.

Response:
We sincerely thank the reviewer for this important recommendation. We clarify that the present study was designed as a non-invasive geophysical survey, and therefore no new intrusive sampling campaigns of leachate or gas columns were performed. Instead, we relied on the existing monitoring infrastructure of the landfill, which includes leachate collection wells and gas extraction points routinely monitored by the operator.

To address the reviewer’s concern, we revised Section 2.7, Resistivity Interpretation Criteria (lines 321–333) to explicitly describe how operational monitoring wells were used as a qualitative reference for anomaly validation. Specifically, leachate wells located near low-resistivity zones confirmed the interpretation of saturated and conductive materials, while gas extraction points in the vicinity of high-resistivity anomalies were consistent with potential biogas accumulations.

Furthermore, in the Study Limitations and Scope (Section 4.6, lines 709–730), we emphasized that future research should incorporate systematic hydrogeochemical and compositional sampling to achieve full cross-validation of geophysical results. This addition clarifies the role of indirect validation in strengthening the reliability of anomaly classification, while also acknowledging the methodological need for direct sampling in future studies.

Reviewer 1, Comment 4:

2.3. Operational Principles and Data Acquisition, Have the authors studied the variations in the electrical current while measuring with the ERT method?

Response:
We sincerely thank the reviewer for this pertinent observation. We clarify that variations of the injected current were continuously recorded by the multi-electrode system during each measurement cycle. These values were monitored in real time to ensure measurement stability and to control electrode–ground contact resistance. Measurements showing deviations greater than ±5% from the nominal current were systematically discarded prior to inversion processing.

To address this comment, we have included an explicit statement in Section 2.3, Operational Principles and Data Acquisition (lines 218–229), describing this quality-control procedure. This addition clarifies that the apparent resistivity values used in the inversion reliably represented subsurface conditions and improved the robustness of the geophysical dataset.

Reviewer 1, Comment 5:

Figure 3. Electrical resistivity ranges for various geological materials, minerals and chemical compunds, it is suggested that authors explain the criteria for selection of different types of minerals and chemicals.

Response:
We sincerely thank the reviewer for this valuable suggestion. To clarify this point, we have expanded Section 2.7 (Resistivity Interpretation Criteria) by adding an explanatory note below Figure 3. This addition, incorporated in the revised manuscript on lines 293–303, specifies that the materials and compounds included in the figure were selected based on classical geophysical references (Telford et al., 1990; Sharma, 1997; Dahlin & Zhou, 2004) as well as recent landfill-related studies. The selection criteria prioritized materials directly relevant to landfill environments under tropical conditions: clay-rich soils, sands, and gravels (representing common geological backgrounds); saline and organic solutions (representing the high ionic content typically found in landfill leachates); and dry or hydrocarbon-bearing phases (representing resistive conditions associated with landfill gas accumulations). This clarification ensures that Figure 3 functions as a practical reference framework for correlating field-measured resistivity anomalies with plausible subsurface conditions.

 

 

Reviewer 1, Comment 6:

2.8 Volumetric Estimation of Gas from Resistivity Anomalies, authors are advised to write in detail regarding the calibration method applied for measurement of potential gas accumulation.

Response:
We sincerely thank the reviewer for this valuable recommendation. In response, we have expanded Section 2.8 to provide a detailed description of the calibration method applied during the volumetric estimation process. Specifically, the revised text (see lines 357–361) explains that the triaxial ellipsoid fitting procedure was calibrated against reference resistivity thresholds (>100 Ω·m) reported in the literature (Dajnov, 1982; Zhan et al., 2019; Godio & Chiampo, 2023). Semi-axis measurements were constrained to orthogonal cross-sections within RES3DINV and Surfer v25, with a fixed spatial resolution of ±0.5 m, ensuring that the ellipsoidal morphology reflected subsurface anisotropy rather than interpolation artifacts. Additionally, high-resistivity bodies were cross-checked against lithological and operational information (e.g., cover soils and gas extraction wells) to avoid misclassification of dry waste or gravel-rich horizons as gas accumulations. Although no direct gas sampling was available, this indirect calibration provided a robust methodological framework to approximate the volumetric extent of gas anomalies in closed landfill cells.

Reviewer 1, Comment 7:

Figure 5. Electrical resistivity model obtained using Dipole-Dipole method along Tomography Line 1. Authors have mentioned that Dipole-Dipole method is is highly sensitive, Can the authors explain what is the accuracy of this method ?. Furthermore they are advised to include some other relevant methods for comparison purposes.

Response:

We sincerely thank the reviewer for this insightful comment. To address it, we have expanded Section 3.1.2 to include a detailed explanation of the accuracy of the Dipole–Dipole method. In our study, this configuration achieved an RMS error of 8.7%, which falls within the typical range (5–12%) reported for landfill surveys (Dahlin & Zhou, 2004; Zhan et al., 2019), thereby confirming its reliability for lateral detection. For comparison, the Wenner and Gradient arrays yielded RMS errors of 21.1% and 14.0%, respectively, which further highlights the higher lateral precision of the Dipole–Dipole method.

In addition, we now reference other configurations such as pole–dipole and unconventional 3D arrays (Martorana et al., 2023), which have demonstrated complementary strengths in landfill studies. These additions, incorporated in lines 391–394 and 579-586, provide a more comprehensive comparative framework and reinforce the methodological robustness of our work.

 

Reviewer 1, Comment 8:

Conclusion section should be rewritten with most relevant research results.

Response:

We sincerely thank the reviewer for this constructive observation. Accordingly, we have thoroughly rewritten Section 5 (Conclusions) to emphasize the most relevant outcomes of our study. The revised section (see lines 733–754) explicitly highlights the detection of extensive low-resistivity zones (<2.1 Ω·m) in the southeastern sector, interpreted as leachate accumulations; the identification of high-resistivity anomalies (>154 Ω·m) in the southwestern sector, consistent with potential biogas pockets, with a volumetric estimation of 20,208 m³ (≈1.1% of the modeled landfill cell volume); the comparative accuracy of the Wenner, Dipole–Dipole, and Gradient arrays, showing that Dipole–Dipole achieved the highest lateral precision (RMS error of 8.7%); and the geoenvironmental implications of localized pressure build-up in gas-rich zones and preferential leachate migration pathways. In addition, the rewritten section underscores the replicable and cost-effective nature of the proposed methodological framework, which can be applied to strengthen post-closure landfill management under tropical conditions.

Reviewer 1, Comment 9:

Quality of figures and tables should be improved.

Response:

We sincerely thank the reviewer for this valuable observation. In response, we have carefully revised all figures and tables to meet the publication standards of Environments. Specifically, figures were reprocessed and exported in high resolution (TIFF/PNG, ≥600 dpi), with optimized logarithmic and continuous color scales, improved contrast, and harmonized font sizes (minimum 10 pt) for legends, scales, and titles to ensure legibility. Scale bars and clear references were added consistently across all 2D, 2.5D, and 3D sections, while redundant or misaligned labels were removed to enhance visual coherence. Tables were reformatted for clarity and consistency, with uniform typography and alignment.

Reviewer 1, Comment 10:

Layout of the manuscript should be revised according to the recommendations of the Environment. Response:

We thank the reviewer for this valuable observation. In response, we revised the manuscript to align with the recommendations of Environments. Specifically, explicit references to the Colombian Ministry of Environment and Sustainable Development regulations—Decree 838/2005, Resolution 1541/2013, and the Technical Guidelines for Leachate Management in Sanitary Landfills (MinAmbiente, 2010)—were added in the Introduction ( lines 125–132) to emphasize the regulatory framework governing landfill management in Colombia. In addition, a paragraph was incorporated into Section 4.5 (Environmental Diagnosis and Monitoring in Hydrogeological Risk Contexts, lines 693–698) to underscore how the study’s findings are consistent with these national requirements, particularly in identifying leachate migration pathways, gas accumulation zones, and potential containment failures. These modifications ensure that the manuscript is explicitly aligned with the Colombian environmental regulatory framework.

We sincerely appreciate the valuable comments and suggestions provided by the reviewers. Their insights have significantly contributed to the improvement of our manuscript, and we are grateful for the opportunity to refine our work. Thank you for your time and consideration.

Reviewer 2 Report

Comments and Suggestions for Authors

General Assessment
I read the manuscript “Multiscale Geophysical Characterization of Leachate and Gas Plumes in a Tropical Landfill Using Electrical Resistivity Tomography for Environmental Analysis and Diagnosis” with great interest and high expectations. The topic is relevant and timely, and the application of geophysical methods to landfill monitoring is of considerable scientific and societal value. However, in its current form, the manuscript requires substantial revisions before it can be considered for publication. In particular, the introduction, methodology, and presentation of results need to be strengthened and clarified in order to highlight the novelty and robustness of the study.

 

Major Comments

1. Introduction
• Lines 55–57: Electrical resistivity also depends on the specific geoelectrical signature of the subsurface materials. The authors should briefly discuss the variability of resistivity data and the geoeletrical signatures of shallow deposits, which adds uncertainty to the interpretation of anomalies. Appropriate bibliographic references should be included.
• The introduction should also mention other geophysical techniques that have been applied to landfill characterization, with relevant references.
• The existing knowledge gaps in the use of ERT for landfill investigations should be clearly articulated, together with an explanation of how the present study aims to address them.
• The innovative contribution of this work compared to previous studies should be explicitly highlighted at the end of the introduction.
2. Materials and Methods
• Lines 84–85: Please use the acronym ERT consistently throughout the manuscript, after introducing the full term once in the abstract and again at its first occurrence in the main text.
• Line 101: The software employed (e.g., Golden Software Grapher 20.2) should be properly cited.
• The electrode spacing, data acquisition procedures, and inversion methods need to be reported in detail.
• Line 104: A scale bar should be added to Figure 1.
• Line 148–149: The current sentence reads more like a result than a methodological description. It should be reformulated to clearly describe materials and methods.
• Line 181–182: The technical literature cited must be properly referenced. A wide range of studies relate resistivity to sediment grain size, and these should enrich the introductory section.
• A description of the geological setting of the study area is missing. This is essential to support the interpretation of the results.
3. Results and Discussion
• Figure 5c: There appears to be an error in the inversion results, as the caption mentions 8.7% whereas the figure shows 87%. This discrepancy should be carefully checked.
• The objectives of the study are not fully clear: is the main goal to analyze results from different acquisition geometries, or to delineate leachate and gas plumes? If the latter, it may be appropriate to merge data from different acquisition arrays and remove anomalous datasets before inversion.
• The construction of Figures 7–11 is not clearly explained. Do these figures correspond to a single acquisition geometry or a combination of all? This needs clarification in the methodology and results sections.
• Results and discussion should be separated:
• The Results section should objectively describe the geophysical evidence obtained.
• The Discussion section should compare these findings with the literature, noting both similarities and differences, and discussing the limitations of the adopted electrode spacing in capturing highly site-specific processes.

 

Minor Comments

• Ensure consistency in terminology and acronyms throughout the text.
• Improve figure captions so that they are self-explanatory.
• Revise the language for clarity and conciseness in several parts of the manuscript.

 

Recommendation
In summary, the manuscript presents an interesting application of ERT to landfill monitoring. However, significant revisions are needed to strengthen the scientific framework, clarify the methodology, and improve the presentation of results. I recommend major revisions before the manuscript can be reconsidered for publication.

 

Author Response

Best regards

We have thoroughly revised the manuscript to address each of the reviewers' comments. Below, we provide a detailed response to each point, outlining the changes implemented in the revised version.

Reviewer 2, Comment 1:

Lines 55–57: Electrical resistivity also depends on the specific geoelectrical signature of the subsurface materials. The authors should briefly discuss the variability of resistivity data and the geoeletrical signatures of shallow deposits, which adds uncertainty to the interpretation of anomalies. Appropriate bibliographic references should be included.

Response:
We appreciate the reviewer’s insightful comment regarding the variability of resistivity data and the influence of geoelectrical signatures of shallow deposits. In response, we have revised the Introduction (lines 79–90) to emphasize that electrical resistivity values are not absolute but strongly conditioned by lithology, porosity, grain size, and fluid saturation. We now highlight that these factors produce overlapping resistivity ranges, which introduce uncertainty into anomaly interpretation, particularly in heterogeneous tropical landfill environments. To strengthen this discussion, we incorporated both classical and recent references that address the role of geoelectrical signatures in resistivity variability and their implications for landfill studies (Palacky, 1987; Nobes, 1996; Alam et al., 2024; Ciampi et al., 2024).

Reviewer 2, Comment 2:
The introduction should also mention other geophysical techniques that have been applied to landfill characterization, with relevant references.

Response:
We thank the reviewer for this valuable suggestion. In response, we have expanded the Introduction (lines 63–78) to include a concise overview of other geophysical techniques that have been applied to landfill characterization. Specifically, we now discuss the use of Ground Penetrating Radar (GPR) for mapping shallow stratigraphy and buried waste deposits [Orlando & Marchesi, 2001; Porsani et al., 2004], electromagnetic induction methods for detecting leachate-impacted zones [Deidda et al., 2022; Bavusi et al., 2006], and seismic surveys to evaluate waste thickness and geomechanical stability [Darvasi & Agnon, 2023]. In addition, recent studies on magnetotellurics [Martí et al., 2024], induced polarization [Martorana et al., 2023], and magnetometry [Prezzi et al., 2005] were incorporated to highlight their complementary role to ERT. These references strengthen the contextual framework of our study and demonstrate that the integration of multiple geophysical methods provides a more robust characterization of landfill heterogeneity.

Reviewer 2, Comment 3:

The existing knowledge gaps in the use of ERT for landfill investigations should be clearly articulated, together with an explanation of how the present study aims to address them.

Response:
We thank the reviewer for highlighting the importance of clearly articulating existing knowledge gaps in the application of ERT to landfill investigations. In response, we have expanded the Introduction (lines 110–123) to explicitly identify the main limitations of previous research, namely the predominance of two-dimensional surveys, the scarce integration of volumetric models and statistical validation, and the lack of long-term monitoring approaches. We also emphasize the persistent uncertainties in differentiating leachate, biogas, and lithological heterogeneities, particularly in tropical environments where high rainfall and unconsolidated sediments complicate geophysical interpretation. Furthermore, we note the limited incorporation of ERT results into regulatory monitoring frameworks in Latin America, which leaves a gap between geophysical evidence and operational management practices. To address these gaps, our study combines multiple array configurations (Wenner, Dipole–Dipole, and Gradient) with 2D, 2.5D, and 3D inversion models, supported by statistical analyses and indirect validation from existing monitoring infrastructure. This multiscale approach strengthens the methodological framework by reducing interpretive uncertainty and providing actionable insights for post-closure environmental management in tropical landfill settings

Reviewer 2, Comment 4:

The innovative contribution of this work compared to previous studies should be explicitly highlighted at the end of the introduction.

Response:
We thank the reviewer for emphasizing the need to highlight the innovative aspects of this study. Accordingly, we revised the final part of the Introduction (lines 144–154) to clearly state that the novelty lies in developing a multiscale and replicable geophysical framework tailored to tropical landfill conditions. Unlike most previous works limited to single-array or 2D surveys, our approach integrates Wenner, Dipole–Dipole, and Gradient arrays with 2D, 2.5D, and 3D inversion models, supported by statistical validation and indirect calibration with monitoring infrastructure. This advancement improves the discrimination between leachate, gas, and lithological heterogeneities and provides volumetric estimates directly applicable to post-closure management in Latin American tropical landfills.

Reviewer 2, Comment 5:

Lines 84–85: Please use the acronym ERT consistently throughout the manuscript, after introducing the full term once in the abstract and again at its first occurrence in the main text.

Response:
We thank the reviewer for pointing out the need to use the acronym consistently. We have revised the manuscript to ensure that the full term “Electrical Resistivity Tomography (ERT)” appears only once in the Abstract and once in its first mention in the Introduction. From that point onward, the acronym ERT is used uniformly throughout the text, including section titles, figure captions, and tables.

Reviewer 2, Comment 6:

Line 101: The software employed (e.g., Golden Software Grapher 20.2) should be properly cited.

Response:
We appreciate the reviewer’s comment regarding the correct citation of the software used. The manuscript has been revised to include the proper reference to Grapher, Version 20.2 (Golden Software, LLC, Golden, CO, USA) in the Methods section. In addition, a full citation has been added to the reference list.

Reviewer 2, Comment 7:

The electrode spacing, data acquisition procedures, and inversion methods need to be reported in detail.

Response:
We thank the reviewer for this valuable observation. The Materials and Methods section has been revised to include detailed information on electrode spacing, data acquisition procedures, and inversion methods. Specifically, we now report that each survey line consisted of 48 electrodes with a uniform spacing of 10 m, providing an average depth of investigation of 60–70 m. The acquisition protocol incorporated three to five current stacks per measurement to improve the signal-to-noise ratio, while real-time monitoring of injected current and contact resistance ensured data stability; measurements deviating more than ±5% from the nominal current were discarded. Data inversion was performed using RES2DINV v4.10 and RES3DINV v3.14 (Geotomo Software, Malaysia), applying a smoothness-constrained least-squares algorithm with 5–8 iterations. These additions strengthen methodological transparency and reproducibility (lines 189–194) and lines 218–229.

Reviewer 2, Comment 8:

Line 104: A scale bar should be added to Figure 1.

Response:
We thank the reviewer for this observation. A graphical scale bar (0–100 m, with 25 m intervals) has been added to Figure 1. This ensures that distances can be directly interpreted from the map, in accordance with cartographic standards. The revised figure now includes this feature (line 204 -205).

Reviewer 2, Comment 9:

Line 148–149: The current sentence reads more like a result than a methodological description. It should be reformulated to clearly describe materials and methods.

Response:
We thank the reviewer for this valuable comment. The sentence in lines 254–257 has been revised to avoid presenting results within the Materials and Methods section. It now clearly describes the methodological procedure, indicating that the integration of 2D models through spatial interpolation was used to construct 2.5D slices and 3D volumetric models, which were parameterized to differentiate zones of contrasting resistivity values (low-resistivity for potential leachate-saturated areas and high-resistivity for possible biogas accumulations).

Reviewer 2, Comment 10:

Line 181–182: The technical literature cited must be properly referenced. A wide range of studies relate resistivity to sediment grain size, and these should enrich the introductory section.

Response:
We thank the reviewer for this important observation. The text in lines 294–303 has been revised to include proper references to technical and geophysical literature that explicitly relate resistivity values to grain size, porosity, and water content. Classical works (Telford et al., 1990; Sharma, 1997; Griffiths & Barker, 1993; Dahlin & Zhou, 2004) and recent landfill studies (Vaudelet et al., 2011; Maurya et al., 2017; Zhan et al., 2019; Morita et al., 2023; Zaini & Hasan, 2024) have been added to strengthen the methodological justification. This modification enriches the introduction of interpretation criteria and ensures that the thresholds used are technically well supported

Reviewer 2, Comment 11:

A description of the geological setting of the study area is missing. This is essential to support the interpretation of the results.

Response:
We thank the reviewer for this observation. A geological description of the study area has been added to Section 2.1 (lines 172–183). This addition details the presence of Quaternary alluvial sediments and the underlying Las Perdices Formation (sandstones and claystones), emphasizing their contrasting permeability and resistivity responses. The geological context now provides essential support for the interpretation of the anomalies detected in the landfill cells.

Reviewer 2, Comment 12:

Figure 5c: There appears to be an error in the inversion results, as the caption mentions 8.7% whereas the figure shows 87%. This discrepancy should be carefully checked.

Response:
We thank the reviewer for pointing out this inconsistency. The discrepancy was due to a typographical error in the exported inversion figure, where the software output incorrectly displayed “87.0%” instead of the correct RMS error value of 8.7%. The figure has been corrected to accurately report the error, and the caption has been updated to clarify this correction (Figure 5c).

Reviewer 2, Comment 13:

The objectives of the study are not fully clear: is the main goal to analyze results from different acquisition geometries, or to delineate leachate and gas plumes? If the latter, it may be appropriate to merge data from different acquisition arrays and remove anomalous datasets before inversion.

Response:
We thank the reviewer for this observation. To avoid ambiguity, the objectives of the study have been clarified at the end of the Introduction. The main objective is now explicitly stated as the delineation of leachate and biogas plumes under tropical landfill conditions using ERT as a non-invasive diagnostic tool. The use of Wenner, Dipole–Dipole, and Gradient arrays is described not as a comparative analysis of acquisition geometries, but as a complementary strategy to enhance subsurface resolution and reduce interpretive uncertainty (lines 254–257).

Reviewer 2, Comment 14:

The construction of Figures 7–11 is not clearly explained. Do these figures correspond to a single acquisition geometry or a combination of all? This needs clarification in the methodology and results sections.

Response:
We thank the reviewer for this important observation. The manuscript has been revised to clarify how Figures 7–11 were constructed. In the Methods section (2.6, lines 284–288), we now explicitly state that the 2.5D and 3D volumetric models were generated by integrating inversion results from Wenner, Dipole–Dipole, and Gradient arrays, with anomalous or unstable measurements removed prior to interpolation. Furthermore, at the beginning of the Results section (3.2, lines 437–441), we added a clarification indicating that Figures 7–11 correspond to these integrated multiconfigurational models, which combine the complementary sensitivities of the three electrode arrays to improve vertical and lateral resolution. These modifications ensure methodological transparency and provide a clear basis for the interpretation of the results.

Reviewer 2, Comment 15:

Results and discussion should be separated:

The Results section should objectively describe the geophysical evidence obtained.

The Discussion section should compare these findings with the literature, noting both similarities and differences, and discussing the limitations of the adopted electrode spacing in capturing highly site-specific processes.

Response:
We thank the reviewer for this valuable observation. The manuscript has been revised to clearly separate the Results and Discussion sections. The Results section (lines 362–545) now presents only the objective description of the geophysical evidence obtained, including resistivity values, inversion models, volumetric estimations, and identified anomalies. The Discussion section (lines 548–731) has been restructured to compare these findings with the relevant literature, highlighting both similarities and differences with previous studies. Furthermore, it now includes a critical analysis of the limitations imposed by the adopted electrode spacing for capturing site-specific processes such as preferential flow pathways or microfractures. This restructuring improves the clarity and rigor of the manuscript, ensuring a more systematic presentation of empirical results and their interpretation.

Reviewer 2, Comment 16:

Ensure consistency in terminology and acronyms throughout the text.

Response:
We thank the reviewer for this valuable observation. A comprehensive revision of the manuscript was conducted to ensure consistency in both terminology and acronyms. Specifically, the expression Electrical Resistivity Tomography (ERT) is now uniformly employed, with the acronym ERT used in subsequent mentions. The electrode configurations are consistently referred to as Wenner, Dipole–Dipole, and Gradient arrays upon their first mention, after which they are simplified to Wenner, Dipole–Dipole, or Gradient for readability. The unit of resistivity has been standardized to the SI format Ω·m across the text and figures. Likewise, leachate is used as a singular, uncountable noun, while landfill gas (biogas) is introduced at first mention and subsequently referred to as biogas for precision and clarity.

Reviewer 2, Comment 17:

Improve figure captions so that they are self-explanatory.

Response:
We appreciate this valuable comment. All figure captions have been thoroughly revised to ensure that they are self-explanatory and provide sufficient context for independent interpretation. Each caption now specifies the ERT configuration employed, the study setting (closed tropical landfill in the Colombian Caribbean), and the key features illustrated (e.g., low-resistivity zones associated with leachate, high-resistivity anomalies interpreted as biogas pockets, or stratigraphic contrasts). Technical details such as inversion parameters, error metrics, and depth or spatial resolution have also been included where appropriate.

Reviewer 2, Comment 18:

Revise the language for clarity and conciseness in several parts of the manuscript.

Response:
We thank the reviewer for this observation. The entire manuscript was carefully revised to enhance clarity and conciseness. Long and complex sentences were restructured into shorter and more direct statements, redundant expressions were removed (e.g., repetitions in the description of data acquisition), and technical terminology was standardized to ensure coherence throughout the text. These adjustments improve readability while maintaining the scientific rigor and precision required for accurate geophysical and environmental interpretation.

We sincerely appreciate the valuable comments and suggestions provided by the reviewers. Their insights have significantly contributed to the improvement of our manuscript, and we are grateful for the opportunity to refine our work. Thank you for your time and consideration.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept.

Thank you very much for sending the revised version of the manuscript and reply of the authors submitted for publication in Environment.I am pleased to inform you that authors have taken into consideration comments of the reviewer. The scientific quality of the manuscript is substantially improved after the revision of the manuscript.The manuscript is recommended for publication in Environment.

Author Response

Reviewer 1 – General Comment
We would like to express our gratitude to Reviewer 1 for the careful evaluation of our manuscript and for the positive recommendation for publication.

Reviewer 1, Comment 1:
Thank you very much for sending the revised version of the manuscript and reply of the authors submitted for publication in Environment. I am pleased to inform you that authors have taken into consideration comments of the reviewer. The scientific quality of the manuscript is substantially improved after the revision of the manuscript. The manuscript is recommended for publication in Environment.

Response:
We sincerely appreciate your positive evaluation of our revised manuscript and your recommendation for publication. Your thoughtful feedback during the review process was invaluable in improving the scientific quality and clarity of our work. We are grateful for your recognition and support.

Once again, we sincerely thank Reviewer 1 for the constructive input and positive assessment of our work. We are honored by your recognition and look forward to contributing to Environments with this study.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made commendable efforts to improve the manuscript “Multiscale Geophysical Characterization of Leachate and Gas Plumes in a Tropical Landfill Using Electrical Resistivity Tomography for Environmental Analysis and Diagnosis”, which is now substantially enhanced compared to the previous version. The overall quality has improved, and the manuscript is closer to meeting the standards for publication. Nevertheless, a few aspects still require refinement to further strengthen the contribution.

1. Abstract
   The abstract should remain focused on the work actually performed in this study. References to techniques not employed, such as GPR and EM, should be removed in order to maintain clarity and precision.

2. Validation of Geophysical Data
   In lines 302–304, the authors highlight that resistivity data are in some way validated through stratigraphic logs. This is a particularly relevant point that should be presented more explicitly and convincingly in the text. The geological validation of geophysical results is a critical strength of the study and deserves clear and incisive emphasis.

3. Results and Discussion
   Although the separation and restructuring of the Results and Discussion sections represent a notable improvement, further elaboration is still needed. Specifically, the discussion should address in greater depth the uncertainties associated with the interpretation of the Integrated Resistivity Models, particularly in the absence of direct geological and/or chemical investigations. Moreover, the limitations imposed by acquisition geometry and resolution should be explicitly discussed.

Author Response

Dear Reviewer 2,

On behalf of all co-authors, we would like to extend our sincere gratitude for your thorough evaluation of our manuscript and for the constructive comments provided during this second round of review. Your feedback has been invaluable in guiding us to further refine the manuscript, improve methodological clarity, and strengthen the scientific contributions of this study. Below, we provide a detailed, point-by-point response to each of your comments.

Reviewer 2, Comment 1:

The abstract should remain focused on the work actually performed in this study. References to techniques not employed, such as GPR and EM, should be removed in order to maintain clarity and precision.

Response:
We thank the reviewer for this valuable observation. In response, we have removed references to Ground Penetrating Radar (GPR) and electromagnetic (EM) methods from the abstract. The revised version is now fully focused on the work actually performed in this study, thereby ensuring greater clarity and precision.

Reviewer 2, Comment 2:

In lines 302–304, the authors highlight that resistivity data are in some way validated through stratigraphic logs. This is a particularly relevant point that should be presented more explicitly and convincingly in the text. The geological validation of geophysical results is a critical strength of the study and deserves clear and incisive emphasis.

Response:
We sincerely thank the reviewer for highlighting this important aspect. In response, we have revised the manuscript to present the geological validation of the geophysical results more explicitly and convincingly. Specifically, we expanded the Materials and Methods (Section 2.7, lines 300–311) to clarify how stratigraphic information from the Las Perdices Formation (gray mudstones interbedded with fine-grained sandstones, Oligocene–Miocene) was systematically integrated into the interpretation framework, providing an independent basis for validating resistivity anomalies. In addition, we emphasize in the Discussion (Section 4.2.2, lines 645–650) that stratigraphic logs from the site confirm the presence of clay-rich and sandstone-rich horizons consistent with the resistivity contrasts observed. This correspondence directly supports the classification of anomalies, reduces interpretive ambiguity, and underscores the reliability of the integrated resistivity models. Furthermore, a complementary statement was added in the Conclusions (line 785) to reinforce that geological validation constitutes a critical strength of the study.

Reviewer 2, Comment 3:

Although the separation and restructuring of the Results and Discussion sections represent a notable improvement, further elaboration is still needed. Specifically, the discussion should address in greater depth the uncertainties associated with the interpretation of the Integrated Resistivity Models, particularly in the absence of direct geological and/or chemical investigations. Moreover, the limitations imposed by acquisition geometry and resolution should be explicitly discussed.

Response:
We sincerely thank the reviewer for this valuable observation. In response, we have expanded the Discussion to explicitly address the uncertainties associated with the interpretation of the integrated resistivity models. A new paragraph was added in (lines 658–669) emphasizing that electrical resistivity is an indirect parameter influenced by multiple factors (e.g., lithological variability, compaction, ionic strength, and seasonal hydrological dynamics), which generate overlapping ranges and complicate the discrimination between leachate-impacted horizons, natural clay layers, and dry waste deposits. We also highlight that, despite the complementary perspectives obtained from Wenner, Dipole–Dipole, and Gradient arrays combined with 2.5D and 3D inversions, residual uncertainties remain due to the absence of systematic hydrogeochemical and lithological validation. Furthermore, the Study Limitations and Scope (lines 738–740, lines 746-748) was expanded to underline that electrode spacing and survey geometry constrain the resolution of microfractures and thin preferential pathways, and that volumetric models represent static snapshots unable to capture seasonal variability. These additions make the uncertainties and methodological constraints more explicit, thereby reinforcing the transparency and robustness of the study.

We believe that the revisions incorporated in this second round have further strengthened the scientific depth, methodological rigor, and practical relevance of the manuscript. We are confident that the revised version now fully addresses the reviewers’ comments and meets the high standards of Environments. It would be an honor to have it accepted for publication.