Environmental Monitoring of Pig Slurry Ponds Using Geochemical and Geoelectrical Techniques

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for giving me the opportunity to review the paper entitled: ‘Environmental monitoring of pig slurry ponds using geochemical and electrical resistivity tomography techniques.

In the title, instead of electrical resistivity tomography, they should use the term Geoelectrical: …using geochemical and geoelectrical techniques.

Line 71: this paragraph starts very abruptly, there should be at least one sentence that is used as a segway between the slurry pond and ERT.

Line 72: I do not agree in that ERT is extensively applied in marine surveys, but it is certainly one of its applications.

Line 76: and composition of the soil!

Line 88: this is untrue, there are other ways in which ERT interpretations can be cross-checked, geochemical analysis being only one possible option.

Lines 88 to 94: the text seems to contradict itself. On one hand it is saying that geochemical analysis is necessary and on the other hand it says that is only produces limited results. This paragraph should be rewritten.

Lines 71 to 117: The authors of this study already published 2 papers in ‘Water’ and ‘Data in Brief’ in 2022 covering this topic. The findings of those previous publications should be acknowledged and summarized in this section, together with any other previous work in this topic. Then, they should highlight what makes this more recent study different (i.e., the monitoring part).

Lines 120 to 128: What are the hydrogeological characteristics of the bedrock under the slurry? At what depth is the water table? These seem like relevant factors in this study.

Line 122: (Little Sea, in English), this is irrelevant, delete.

Line 154: Why was dipole-dipole array selected? Given that the main changes in resistivity are vertical, Wenner-Schlumberger or similar seems like a better option.

Line 168: There should be more information about the reference model used. Given the overall low resistivity of the models, the resistivity variations could have been represented with absolute values instead of % of change.

Line 212: I find it very hard to identify 3 clearly differentiated layers in the resistivity section within the 1.5 m of depth of the slurry. This should be represented in a separate figure, with either a zoom in or a graph showing resistivity values within the slurry pond at different times. Does the moisture content in the slurry pond decrease over time, thus increasing the overall resistivity?

Line 240: Do environmental factors (weather patterns, temperature, etc.) play a role in the resistivity measured during the monitoring at different times of the year?

Line 258: Does this mean that the lining being used is not effective in waterproofing the slurry pit?

Line 259: What is the role of these salts? How are they added to the pit?

Figure 3: I don’t find this figure easy to read and interpret. There seem to be some considerable changes in resistivity of the bedrock below the slurry pit, but this isn’t really addressed in the text. As I mentioned before, I think that representing the actual resistivity variations rather than % would be better. Also, in the graphs on the left the highlighting of data points seems somewhat arbitrary, with the same percentage of negative resistivity change being highlighted in some instances and not in others. Overall, I don’t find this data to be presented nor discussed in an enlightening way.

Figure 4: Where in the slurry pit where the samples taken? Without that spatial information it is impossible to correlate these results with the resistivity sections.

In general terms I miss having a 3D visualization of the pit, not as in Capa-Camacho et al. 2022, but with horizontal slices of the model at different depths. An example of this visualization can be found in “Characterization of legacy landfills with electrical resistivity tomography; a comparative study. Journal of Applied Geophysics, Volume 203, 2022, 104716, ISSN 0926-985”. Also, the analysis of the resistivity variations should be improved and expanded, including what is happening underneath the slurry pit; why does the resistivity increase in many areas under the pit? What are the hydrogeological dynamics in the site?

In any case, I find the manuscript interesting as a follow up of the previous publication and I believe that it should be considered for publication if and when the issues mentioned above are addressed.

Comments on the Quality of English Language

It is generally well written, but it does require some review

Author Response

Dear reviewer,

 

I would like to express our gratitude for the opportunity to submit our manuscript, "Environmental monitoring of pig slurry ponds using geochemical and geoelectrical techniques."

 

We appreciate your time and effort in reviewing our work and providing feedback. Your insightful comments and valuable suggestions have greatly helped us improve our manuscript. We have taken your suggestions into account and made changes accordingly. You will find these changes highlighted in the revised manuscript. Furthermore, we have also provided a detailed response to your comments and concerns in blue.

 

Review 1:

• “In the title, instead of electrical resistivity tomography, they should use the term Geoelectrical: …using geochemical and geoelectrical techniques.

Thanks a lot for the observation. This point has been corrected.

• Line 71: This paragraph starts very abruptly. There should be at least one sentence that is used as a segway between the slurry pond and ERT. In the title, instead of electrical resistivity tomography, they should use the term Geoelectrical: …using geochemical and geoelectrical techniques.

Thanks a lot for the observation. This point has been corrected, and lines 70-74 have been added.

• Line 72: I do not agree that ERT is extensively applied in marine surveys, but it is certainly one of its applications.

We thank the reviewers for this comment. This point has been corrected. Lines 76.

• Line 76: and composition of the soil!

We thank the reviewers for this mention. This item has been added. Line 77.

• Line 88: This is untrue. There are other ways in which ERT interpretations can be cross-checked, geochemical analysis being only one possible option.

We thank the reviewers for this comment. This point has been corrected. Line 100

• Lines 88 to 94: the text seems to contradict itself. On one hand it is saying that geochemical analysis is necessary and on the other hand it says that is only produces limited results. This paragraph should be rewritten.

We would like to express our gratitude to the reviewers for their valuable feedback. We have made the necessary corrections to address the point mentioned in their comment. Lines 100-112.

• Lines 71 to 117: The authors of this study already published 2 papers in ‘Water’ and ‘Data in Brief’ in 2022 covering this topic. The findings of those previous publications should be acknowledged and summarized in this section, together with any other previous work in this topic. Then, they should highlight what makes this more recent study different (i.e., the monitoring part).

We thank the reviewers for bringing this matter to our attention. We have now included this item as you suggested, and lines 90-99 have been added.

• Lines 120 to 128: What are the hydrogeological characteristics of the bedrock under the slurry? At what depth is the water table? These seem like relevant factors in this study.

We thank the reviewers for this mention. Lines 139-145 have been added to this item.

• Line 122: (Little Sea, in English), this is irrelevant, delete.

Thanks a lot for the observation. This point has been corrected.

• Line 154: Why was the dipole-dipole array selected? Given that the main changes in resistivity are vertical, Wenner-Schlumberger or similar seems like a better option.

We thank the reviewers for this mention. We chose to use the dipole-dipole configuration due to its high sensitivity to lateral horizontal variations, seawater intrusion, and contamination. Since the slurry ponds were very thin (only 1.6m) and compacted at the bottom, we anticipated a higher contamination plume on the sides than at depth. Line 171.

• Line 168: There should be more information about the reference model used. Given the overall low resistivity of the models, the resistivity variations could have been represented with absolute values instead of % of change.

We thank the reviewers for this mention. Lines 184-192 have been added to this item. Figure 2 displays the actual resistivity values obtained for each survey in each profile.

• Line 212: I find it very hard to identify 3 clearly differentiated layers in the resistivity section within the 1.5 m of depth of the slurry. This should be represented in a separate figure, with either a zoom in or a graph showing resistivity values within the slurry pond at different times. Does the moisture content in the slurry pond decrease over time, thus increasing the overall resistivity?

We thank the reviewers for this mention. This point has been corrected; Figure 3 shows the resistivity values inside the slurry pond at different times and depths. This item has been added in lines 243-254.

Regarding the question, yes, the humidity of the pig slurry decreases with time, mainly due to the effect of the temperature, which favors evaporation. Because it is a pond without any coating, the resistivity value is much higher when more crust is observed on the surface, which is evident in Survey 3. 

• Line 240: Do environmental factors (weather patterns, temperature, etc.) play a role in the resistivity measured during the monitoring at different times of the year?

We thank the reviewers for this mention. Yes, environmental factors such as temperature influence, since in hot periods such as summer, the slurry, being in a pond without any cover, presents a more significant loss of moisture content, leading to the formation of a crust on the surface. However, in this case, we do not consider the temperature to be such; we focus on the formation of the crust on the surface.  Line 274

• Line 258: Does this mean that the lining being used is not effective in waterproofing the slurry pit?

We thank the reviewers for this mention. This item has been added. Yes, this suggests that the change in soil moisture content and, therefore, in the value of electrical resistivity is attributed to the infiltration of pig slurry. Line 302

• Line 259: What is the role of these salts? How are they added to the pit?

We thank the reviewers for this mention. The salts are added by slurry infiltration, as this by-product is rich in salts in the form of Nitrates (NO₃^-) and Phosphates (PO₄^3-). Line 302

• Figure 3: I don’t find this figure easy to read and interpret. There seem to be some considerable changes in resistivity of the bedrock below the slurry pit, but this isn’t really addressed in the text. As I mentioned before, I think that representing the actual resistivity variations rather than % would be better. Also, in the graphs on the left the highlighting of data points seems somewhat arbitrary, with the same percentage of negative resistivity change being highlighted in some instances and not in others. Overall, I don’t find this data to be presented nor discussed in an enlightening way.

We thank the reviewers for this mention. This point has been corrected; we have added Figure 4, which shows the Differences in electrical resistivity values at the surface and at the bottom of the slurry pond measured at each electrode between surveys 1, 2, and 3 for each of the profiles.

• Figure 4: Where in the slurry pit where the samples taken? Without that spatial information it is impossible to correlate these results with the resistivity sections.

We thank the reviewers for this mention. Sampling was performed at the points close to the location of the profiles.

• In general terms I miss having a 3D visualization of the pit, not as in Capa-Camacho et al. 2022, but with horizontal slices of the model at different depths. An example of this visualization can be found in “Characterization of legacy landfills with electrical resistivity tomography, a comparative study. Journal of Applied Geophysics, Volume 203, 2022, 104716, ISSN 0926-985”. Also, the analysis of the resistivity variations should be improved and expanded, including what is happening underneath the slurry pit; why does the resistivity increase in many areas under the pit? What are the hydrogeological dynamics in the site?

We thank the reviewers for this mention. This article has been added regarding the increase in resistivity under the pond, lines 322-328. The hydrogeological dynamics at the site have not been considered for this study.

 

Again, thank you for all your suggestions, comments, and consideration of this manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper is well written and despite some minor mistakes that do not affect the quality of the article, it can be published in this journal after these revisions.

- The paper presents a new method for monitoring slurry ponds using electrical resistivity tomography (ERT). I propose that the authors delete the geochemical techniques in the title and concentrate on the ERT techniques which constitute the main method for monitoring pig slurry ponds.

- Brief details of the local geology should be mentioned in the method section. Still in this part, it will be important in my opinion to give more details on the nature of the soil on which the slurry pond is built

- Figure 1a is not only the geographical location of the studied pond in southeast Spain, please give more details on this figure. It would also be important to add geographic coordinates to the map of Spain. Still in this figure, the position of the North is curiously identical on the same pond in a), b) and c) while the orientation of the shots (photos of the slurry pond) is different. I don’t understand the importance of specifying the perimeter of slurry pig in the legend of this figure.

- In Figures 2 and 3, what do the numbers on the x-axis from 0 to 27 represent?

Author Response

Dear reviewer,

 

I would like to express our gratitude for the opportunity to submit our manuscript, "Environmental monitoring of pig slurry ponds using geochemical and geoelectrical techniques."

 

We appreciate your time and effort in reviewing our work and providing feedback. Your insightful comments and valuable suggestions have greatly helped us improve our manuscript. We have taken your suggestions into account and made changes accordingly. You will find these changes highlighted in the revised manuscript. Furthermore, we have also provided a detailed response to your comments and concerns in blue.

 

Review 2:

• I propose that the authors delete the geochemical techniques in the title and concentrate on the ERT techniques which constitute the main method for monitoring pig slurry ponds.

We thank the reviewers for this mention. We consider that we could retain the geochemistry part since it represents an important part.

• Brief details of the local geology should be mentioned in the method section. Still in this part, it will be important in my opinion to give more details on the nature of the soil on which the slurry pond is built

We thank the reviewers for this mention. This item has been added in lines 139-146.

• Figure 1a is not only the geographical location of the studied pond in southeast Spain, please give more details on this figure. It would also be important to add geographic coordinates to the map of Spain. Still in this figure, the position of the North is curiously identical on the same pond in a), b) and c) while the orientation of the shots (photos of the slurry pond) is different. I don’t understand the importance of specifying the perimeter of slurry pig in the legend of this figure.

We thank the reviewers for this mention. The figure was corrected. We consider the importance of specifying the perimeter as the quality of the satellite image is not the best, so there could be confusion.

• In Figures 2 and 3, what do the numbers on the x-axis from 0 to 27 represent?

We thank the reviewers for this comment. The measurement profile encompasses a total of 28 electrodes, each of which is numbered from 0 to 27.

 

Again, thank you for all your suggestions, comments, and consideration of this manuscript.

Sincerely,

 

Marcos Antonio Martínez Segura

Universidad Politécnica de Cartagena

Paseo Alfonso XIII 30203, Spain

Tel.: +34968327073

E-mail : [email protected]

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I consider that the changes made to the paper successfully address the comments in my initial review, so I support the publication of this manuscript in its current form.