Geoelectrical Measurements to Monitor a Hydrocarbon Leakage in the Aquifer: Simulation Experiment in the Lab

Round 1

Reviewer 1 Report

The paper addresses an important topic that has been little studied so far: the use of DC geoelectrical methods to trace leaks of petroleum-derived contaminants in groundwater.  The laboratory approach is quite interesting in contrast with previous studies on gas stations that should be mentioned in the introduction (e.g., Rosales et al., 2012; Delgado-Rodriguez et al., 2014).

Although the results of these experiments are interesting, the obvious question that arises from reading this work is how these results extrapolate in a large-scale real-life situation (i.e., geoelectrical surveys in a gas station).

Moreover, what kind of behavior would be expected in the case of contamination by different carcinogenic Petroleum Hydrocarbons (PHC), Polycyclic aromatic hydrocarbons (PAH), and volatile organic compounds (VOC)?  Are you planning further experiments with this kind of compound?

What are the minimum concentrations of diesel in groundwater to produce detectable signals in a large-scale geoelectrical survey.

More specific comments:

What is the reason for the resistivity spike for an 80-day time-lapse in EF (Figure 7)?

The abstract does not reflect this work's results, conclusions and implications.  Please rewrite it completely.  

The English is very rough, although most of the article is quite understandable.  Before it is published, it requires proofreading by a native speaker.

The following are just a few examples of the bad English used all through the manuscript:

Lines 19-20 "With the aim to investigate the capability of electrical resistivity methods for monitoring physical variations induced by a contamination in the subsoil." This seems to be a fragment of a sentence.

Lines 341-343 "The first phase is obviously characterized by an increase of the electrical resistivity imputable to the extremely resistive behavior of the LNAPL that works as an insulator layer covering the sand with interacting." with interacting what?

Lines 343-345: "What happens in the second phase, which starts after a few days of the contamination, appears to be very interesting." Please avoid these kinds of comments that sound like value judgments.

Line 345: "A strong and fast increase of the electrical conductivity characterizes the sandbox" Be careful; it does not characterize the sandbox but the resistivity signal inside the sandbox

Lines 347-349 "As noted also by Serrano et al. [55] in a controlled test, a dramatic decrease in pollutant concentrations in the soil in only 18 days after the spill." Do you mean that: it takes place in only 18 days?

Line 384: "it is gives evident" what do you mean by that?

The word "imputable" widely used throughout the manuscript, is not common in English.  In the few cases it is used, it has an undesirable and negative implication.  Use other words such as "due to" instead.

References

Delgado-Rodríguez, Omar; Flores-Hernández, David; Amezcua-Allieri, Myriam A.; Rosas-Molina, Andrés; Marín-Córdova, Salvador; Shevnin, Vladimir (2014). Joint interpretation of geoelectrical and volatile organic compounds data: a case study in a hydrocarbons contaminated urban site. Geofísica Internacional, 53(2), 183–198. doi:10.1016/s0016-7169(14)71499-0

Rosales, R. M., Martínez-Pagan, P., Faz, A., & Moreno-Cornejo, J. (2012). Environmental Monitoring Using Electrical Resistivity Tomography (ERT) in the Subsoil of Three Former Petrol Stations in SE of Spain. Water, Air, & Soil Pollution, 223(7), 3757–3773. doi:10.1007/s11270-012-

Author Response

Reply to Reviewer 1

Dear Reviewer thank you very much for your revision. We rewrote part of the paper as you suggested us.

Below you can find in red our comments to your suggestions.

Comments and Suggestions for Authors

The paper addresses an important topic that has been little studied so far: the use of DC geoelectrical methods to trace leaks of petroleum-derived contaminants in groundwater.  The laboratory approach is quite interesting in contrast with previous studies on gas stations that should be mentioned in the introduction (e.g., Rosales et al., 2012; Delgado-Rodriguez et al., 2014).

Although the results of these experiments are interesting, the obvious question that arises from reading this work is how these results extrapolate in a large-scale real-life situation (i.e., geoelectrical surveys in a gas station).

Authors: From our point of view, the laboratory experiments are necessary in order to define analogue models of the real field sites and to observe the occurred phenomena under controlled conditions. In this way, you can obtain useful knowledges that are important when we have to work in real field site and to set up the best approach for the field work. In example, our experiment highlights useful information on the behaviour of the subsoil when a natural attenuation happens on hydrocarbon contaminants and, therefore, this is important to think about when we have to carried out measurements on a gas station as the cited paper. We defined this experiment because we did ERT measurements on the field site close to oil spill incident from a tanker without knowing the starting time point and the final results confused us a little bit.

Moreover, what kind of behavior would be expected in the case of contamination by different carcinogenic Petroleum Hydrocarbons (PHC), Polycyclic aromatic hydrocarbons (PAH), and volatile organic compounds (VOC)?  Are you planning further experiments with this kind of compound?

Authors: We discuss this aspect at the end of the paper. We are planning new experiments with different kinds of LNAPL (and DNAPL) contaminant in the next lab project.

What are the minimum concentrations of diesel in groundwater to produce detectable signals in a large-scale geoelectrical survey.

Authors: This question is very important and, a new experiment focused on contaminant concentration is planned, in order to verify  the capability to detect and observe, via DC methods, the  biodegradation phenomena. This point is crucial  for the remediation actions as it is necessary to estimate the quantitative of the chemical product to use for obtaining a good stimulation of the contaminants. In this case, the DC method should give a good contribute on a monitoring of this phenomena. 

More specific comments:

What is the reason for the resistivity spike for an 80-day time-lapse in EF (Figure 7)?

Authors: Your question was our question. We don’t know what happened at that moment. The spike was localized on all three sections but the farthest one, between the E and F boreholes, highlights a very large value. At this moment, we are not able to explain all the spikes, but we were able to depict the four phases.

The abstract does not reflect this work's results, conclusions and implications.  Please rewrite it completely. 

Authors: In our head the abstract described our work, but may be we got it wrong and, therefore,, rewrote it. We hope now it reflects better our work.

The English is very rough, although most of the article is quite understandable.  Before it is published, it requires proofreading by a native speaker.

Authors: We asked to native speaker to read it and now the paper should be improved.

The following are just a few examples of the bad English used all through the manuscript:

Lines 19-20 "With the aim to investigate the capability of electrical resistivity methods for monitoring physical variations induced by a contamination in the subsoil." This seems to be a fragment of a sentence.

Authors: We rewrote the sentence.

Lines 341-343 "The first phase is obviously characterized by an increase of the electrical resistivity imputable to the extremely resistive behavior of the LNAPL that works as an insulator layer covering the sand with interacting." with interacting what?

Authors: We rewrote the sentence.

Lines 343-345: "What happens in the second phase, which starts after a few days of the contamination, appears to be very interesting." Please avoid these kinds of comments that sound like value judgments.

Authors: We deleted the sentence.

Line 345: "A strong and fast increase of the electrical conductivity characterizes the sandbox" Be careful; it does not characterize the sandbox but the resistivity signal inside the sandbox

Authors: We rewrote the sentence.

Lines 347-349 "As noted also by Serrano et al. [55] in a controlled test, a dramatic decrease in pollutant concentrations in the soil in only 18 days after the spill." Do you mean that: it takes place in only 18 days?

Authors: Also Serrano et al., in their research, observe that after only 18 days the TPH concentrations vary dramatically and come from 100% to 20%.

Line 384: "it is gives evident" what do you mean by that?

Authors: We rewrote the sentence.

The word "imputable" widely used throughout the manuscript, is not common in English.  In the few cases it is used, it has an undesirable and negative implication.  Use other words such as "due to" instead.

Authors: We changed the term.

References

Delgado-Rodríguez, Omar; Flores-Hernández, David; Amezcua-Allieri, Myriam A.; Rosas-Molina, Andrés; Marín-Córdova, Salvador; Shevnin, Vladimir (2014). Joint interpretation of geoelectrical and volatile organic compounds data: a case study in a hydrocarbons contaminated urban site. Geofísica Internacional, 53(2), 183–198. doi:10.1016/s0016-7169(14)71499-0

Rosales, R. M., Martínez-Pagan, P., Faz, A., & Moreno-Cornejo, J. (2012). Environmental Monitoring Using Electrical Resistivity Tomography (ERT) in the Subsoil of Three Former Petrol Stations in SE of Spain. Water, Air, & Soil Pollution, 223(7), 3757–3773. doi:10.1007/s11270-012-

Authors: Thank you very much for your suggested papers that we added in our references. Moreover, we introduced recent papers about the topic.

Reviewer 2 Report

The manuscript entitled “Geoelectrical measurements to monitor a gasoline leakage in the aquifer: simulation experiment in the lab” presents preliminary the laboratory tests results for the purposes of the possibility of using geoelectrical method in a gasoline pollution monitoring. The paper presents the lab scale test of Cross Borehole Electrical Resistivity Tomography used for non-aqueous liquids contamination detection. The topic is with the tendency of research conducted in the world aimed to describe the behavior and fate of these compounds in the environment, however, the tests were conducted in a very limited scope and under homogeneous conditions.

 The specific remarks are:

·  The paper is well prepared and well structured.

.   The aim of the study needs to be formulated more precisely and more specific.     

.   There are no major issues related to the investigation/testing approach, results in analyses, or interpretation. The authors need to present the novelty of the research since the method used in the study is well known and established. 

.   Please be more specific when defining the aim of the study in the Introduction, similar work has already been published, so the authors should try to find a more accurately formulated aim. l. 87-92 should be moved to materials and methods. Hydrogeological parameters from tab.1 should be explained. Values in fig. 2 should be enlarged, to be visible. 

.  In my opinion the Abstract should be revised and modified. It should be stated that the tests are carried out under specific conditions and for application purposes the scope should be greatly extended. Do not use abbreviations in the abstract. Sentence 19 is not clear.  The abstract is missing clearly specified aims and objectives. The results also need to be briefly presented in the abstract. Sentence 36 needs revision. 

·    The literature review seems to be very limited. I suggest authors to elaborate more on the topic and refer to many more scientific research related to the issue of hydrocarbons contamination and parameters its conductivity.

·     The literature review should be based on the most recent scientific data. Now, more than 90% of cited references are very old. There are only a few new items. Please improve that, this would definitely increase the value of the paper. The authors should more focus on exposing the contribution of the research to existing knowledge and science. 

·     It is not clear what is the hypothesis of performed study.

·    What procedure did the authors follow during the soil preparation? The reference is needed.

·    The authors should enhance the discussion and highlight the importance of performed study in terms of the sustainability. The discussion part is sufficient and critical, which is appreciated, although the conclusion part needs a more scientific-sounding presenting the results (findings) more accurately. 

·   The novelty and limitations of the performed research should be delineated.

.  In general, the paper needs revision in terms of style and grammar, there are sentences that are not clear.

Author Response

Reply to Reviewer 2

Dear Reviewer thank you very much for your revision. We rewrote part of the paper as you suggested us.

Below you can find in red our comments to your suggestions.

The manuscript entitled “Geoelectrical measurements to monitor a gasoline leakage in the aquifer: simulation experiment in the lab” presents preliminary the laboratory tests results for the purposes of the possibility of using geoelectrical method in a gasoline pollution monitoring. The paper presents the lab scale test of Cross Borehole Electrical Resistivity Tomography used for non-aqueous liquids contamination detection. The topic is with the tendency of research conducted in the world aimed to describe the behavior and fate of these compounds in the environment, however, the tests were conducted in a very limited scope and under homogeneous conditions.

 Authors: How the reviewer wrote, our lab test was conducted in homogeneous conditions, but in order to detect the correlations between the electrical resistivity signal and the biodegradation, we have to be sure about the origin of the signal. Moreover, the main aim was to have a continuous (1 year) data set for the monitoring of a dynamic phenomenon that is still followed by several researchers in the world, but each test highlights new aspects, even if the main one is well known.

 The specific remarks are:

• The paper is well prepared and well structured.

Author: thank you very much for your comments

.   The aim of the study needs to be formulated more precisely and more specific.

Author: we rewrote the abstract and several part of the manuscript in order to explain better our aims and originality.

     .   There are no major issues related to the investigation/testing approach, results in analyses, or interpretation. The authors need to present the novelty of the research since the method used in the study is well known and established. 

Author: We known that the geoelectrical technique is already used in the past on this topic with successful results. On the contrary, we did not find papers with a long monitoring approach (1 year) in a controlled site. Therefore, we think that our results define well the behaviour of the electrical resistivity during a natural biodegradation that it is still unexplored. Moreover, despite there are papers that confirm the importance of monitoring natural biodegradation for optimize the remediation works, there are few scientific experiences for analyse the capability of ad hoc geophysical monitoring systems.  Here, there is the originality of our experiment.

.   Please be more specific when defining the aim of the study in the Introduction, similar work has already been published, so the authors should try to find a more accurately formulated aim. 

Author: we rewrote the Introduction trying to improve the text in order to highlight the aim of our work.

1. 87-92 should be moved to materials and methods. Hydrogeological parameters from tab.1 should be explained. Values in fig. 2 should be enlarged, to be visible. 

Author: we did the suggested revisions

.  In my opinion the Abstract should be revised and modified. It should be stated that the tests are carried out under specific conditions and for application purposes the scope should be greatly extended. Do not use abbreviations in the abstract. Sentence 19 is not clear.  The abstract is missing clearly specified aims and objectives. The results also need to be briefly presented in the abstract. Sentence 36 needs revision. 

Author: we did the suggested revisions

•   The literature review seems to be very limited. I suggest authors to elaborate more on the topic and refer to many more scientific research related to the issue of hydrocarbons contamination and parameters its conductivity.

Author: we improved the manuscript with new references

•    The literature review should be based on the most recent scientific data. Now, more than 90% of cited references are very old. There are only a few new items. Please improve that, this would definitely increase the value of the paper. The authors should more focus on exposing the contribution of the research to existing knowledge and science. 

Author: we improved the manuscript introducing new references

•    It is not clear what is the hypothesis of performed study.

Author: we rewrote several part of the manuscript in order to improve the originality and the main aims of our work.

•   What procedure did the authors follow during the soil preparation? The reference is needed.

Author: we improved the manuscript with new references

•   The authors should enhance the discussion and highlight the importance of performed study interms of the sustainability. The discussion part is sufficient and critical, which is appreciated, although the conclusion part needs a more scientific-sounding presenting the results (findings) more accurately.

Author: we improved the discussion and conclusion chapter following your suggestions.

•  The novelty and limitations of the performed research should be delineated.

Author: we define our experiment as a first level approach and more test we have to make with different contaminants and complex subsoil.

.  In general, the paper needs revision in terms of style and grammar, there are sentences that are not clear.

Author: we make a strong revision of the manuscript and we hope now it is improved.

Round 2

Reviewer 1 Report

Although this revised version has improved considerably from the previous one, the English is still quite bumpy.  I would suggest having it proofread by a native speaker who knows about the subject matter to smooth it down.

Author Response

Dear Reviewer,

thank you very much for appreciating our efforts to improve the paper.

Reviewer 2 Report

The Authors made significant additions and corrections to the manuscript, with reference to the comments of the reviewers. The article is clearer and the research results are described correctly. The objectives and description of the research methodology were improved. It is also possible to include articles from recent years in the field of geoelectrical research in polluted areas, also published in mdpi journals (e.g. in Applied Sciences). Nevertheless, it is also possible to accept the article for publication in its current form. Good luck!

Author Response

Dear Reviewer,

thank you very much for appreciating our efforts to improve the work.