Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography
and Thomas Hermans 1
Round 1
Reviewer 1 Report
The submitted manuscript tries to use electrical resistivity tomography "to evaluate the extent of SI [Saltwater Intrusion] in heterogeneous aquifers by proposing a new methodology for the selection of the saline boundary". As a final product of the research, the authors provide a "new freah-saline groundwater map" for the study area.
The abstract describes the case study too specifically and does not place it in the general issue, i.e. the use of electrical resistivity tomography as as tool for the study of the coastal saltwater intrusion. Please, carefully follow Water's guidelines to edit the Abstract.
The use of electrical resistivity tomography in saltwater intrusion is a topic that has a wide scientific literature. Authors must clarify the originality of their methodology, what is the current state of the issue and how their research contributes to better address it.
The conclusions are challenging but remain rather generic, almost nullifying the effort made and casting doubts on the validity of the methodology used and proposed. First, are the authors sure that in literature ([21] and [22], I suppose) the river is considered as "the only driver for SI in the area"? Or is the cause less categorically indicated?
Again, with a complex hydrogeological setting, it is easy to indicate more than one cause ("paleo-hydrological conditions, geological heterogeneity, and agricultural practices" see 548-549 lines), if not all possible causes at the origin of the SI process. And how can one determine what is, or what are the main causes? Difficult to do only with the use of geophysics, and even more with the ERT alone.
The discussion and conclusions should instead also underline the limitations and shortcomings of the proposed methodology, and indicate which other activities are necessary to better diagnose the causes of the phenomenon under study.
Minor Comments
I have not found the following final text added: Authors contributions; Funding; Data availability statements; Conflict of interest. Acknowledgments is inappropriately listed as a final Section, soon after Conclusions. Please, apply the "Instruction for Authors".
There are some shortcomings in the References, such as [22] and [23]. Who are they besides Cao, D.G. the other authors? Please complete by including the other identifying information. What is APAC? On which pages is the paper in the "Proceedings of the 10th International Conference on Asian and Pacific Coasts"?
In References, journal names (Advances in Water Resources, Hydrogeology Journal, Nordic Hydrology, ..., Journal of Hydrology, ... Journal of Applied Geophysics, Near Surface Geophysics, ... Environmental Research Letters, ..., Journal of Environmental Engineering Geophysics and so on ...) are not shortened.
Author Response
Dear Reviewer,
We would like to thank the reviewer for your detailed comments and the time spent on the manuscript. We would like to send the explanatory file and the revised manuscript. Please find them out in the attached file.
Best,
On behalf of the Authors,
Cong-Thi Diep
Author Response File: 
Author Response.docx
Reviewer 2 Report
The main objective of the present paper is to the lateral and vertical extent of SI in the Luy River (Vietnam) unconfined coastal aquifers of Pleistocene and Holocene. The authors evaluate the current state of salinity in the shallow coastal aquifer of the Luy River, using 21 electrical resistivity tomography (ERT) profiles, collected along the downstream part. ERT profiles reveal the depth of the rock basement and the thickness of the aquifers. Physical imaging through ERT coupled with regionally specific petrophysics revealed that the zone affected by SI is much larger than previously expected from shallow water samples.
The study is interesting and I recommend for publication after a moderate language review.
Author Response
Dear Reviewer,
Thank you for the positive feedback. We made a thorough language editing of the manuscript.
Best,
On behalf of the Authors,
Cong-Thi Diep
Author Response File: Author Response.docx
Reviewer 3 Report
- The paper presents an application of electric resistivity tomography (ERT) to map the salinity of groundwater in the shallow coastal aquifer of the Luy River, Vietnam.
- It is clear from the paper that one of the causes of the high salinity in the aquifer are tidal waves that progress upstream in the river, specially during droughts. The river recharges salty water to the aquifer. The ERT results are consistent with this hydrologic regime.
- However, high groundwater salinity beyond the tidally affected area must have another explanation. Could it be the sea transgressions in the region during the Quaternary might have created saline rocks which continue to leak salt into groundwater? What about the effect of groundwater withdrawal? The paper should include a hydraulic-head map of the study region.
- Also, I was intrigued by the statement of dryness in Binh Thuan province. What is the average annual precipitation? I believe all of Vietnam is affected by the monsoons.
Author Response
Dear Reviewer,
We would like to thank the reviewer for your detailed comments and the time spent on the manuscript. We would like to send the explanatory file and the revised manuscript. Please find them out in the attached file.
Best,
On behalf of the Authors,
Cong-Thi Diep
Author Response File: Author Response.docx
Reviewer 4 Report
Dear Authors:
Review of the paper
Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) using Electrical Resistivity Tomography
Authors:
Cong-Thi Diep1,3,* , Pham Dieu Linh1,3, Robin Thibaut1, Marieke Paepen1, Hieu Huu Ho3, Frédéric Nguyen2, Thomas Hermans1
In the abstract
What do you mean by a hyper arid climate. Provide values for precipitation.
ERT data clearly reveal (data plural)
Line 24: is this perched water table?
In the introduction line 34: what do you mean by density effects?
Line 45: please explain what you mean by relevant scale.
Line 53-54: Are you talking about the resolution of airborne methods or in general?
Please explain the difficulties in time-lapse studies with electrical methods.
Line 74. What is the magnitude of the tides?
Line 87/88 can the clay lenses act as impermeable media? Have you identified the presence of aquitards in the area?
Line 90: don’t understand “collect co located data”
Lines 145-148: authors say that the “storage capacity and productivity of the Pleistocene aquifer is proven”. Questions: what is the research system for this aquifer? Do you have hydric data/ balances to justify this?
Is there a hydraulic connection between the Holocene and the Pleistocene aquifers?
Furthermore, Fig. 2b shows some wells crossing Holocene and Pleistocene formations. Do these wells provide hydraulic connection between the two formations?
Line 156 : please consider to add a figure of the field layout for resistivity measurements.
Line 161: why?
Line 163 needs a reference for the roll along technique.
Line 163-164 please justify.
Line 172: a reference is needed for the “reciprocal error” or for the principle of reciprocity. From the text in line 173 I assume that reciprocity was not checked for all data. Why?
You don’t need line 174-175 if you give a reference for the principle of reciprocity.
Line 176: the references come too late.
Line 177. What factors? Please explain.
In Fig. 3a what is the relative resistance reciprocal error?
Why did you choose profile 29 and not all data (Legend of fig 3)
Line 195: why did you change to mOhm? Everything so far is in Ohm.
I don’t understand the need to explain the principles for the inversion. Nowadays there is software available and it would have been enough to mention the software and the method used for inversion.
Lines 227-230 Is the ground reasonably flat? What is the topography like if you feel you need to add these? To consider this there must be problems with the topography. If it is so what is the accuracy of the GPS and field observations?
Line 247-255 needs a reference.
Line 289-290: do you mean observed resistivities or modelled resistivities? There is a huge difference in the concepts.
Line 290 Why do you use log scale for the resistivities in Fig. 5? It is commoner to use a normal scale with the RERS2Dinv and it is easier to read.
Line 292-293 Do you have any samples and measurements to confirm this?
Line 307 Fig. 4 is called after fig. 5? Please review.
Please give a reference for the values in lines 311 and 312 or confirm you have done experiments.
Line 316- 319 I would be more comfortable if the authors provide literature values or call adequate references. Alternatively, authors should provide values measured in situ to prove their assumptions.
Line 389-390 Authors don’t have any evidence of this.
Fig. 6 Qh?Qp?K? in the other figures the same happens.
Line 408 Fig. 8 does not show the high salinity observed in all profiles. What I can see in Fig. 8 are resistivity zones. Please rewrite as it is not acceptable as it is.
Line 410 /ohm.m and 35 Ohm.m are not high resistivity values. Please rephrase as this is not acceptable.
Line 411 – 412 Saline water occupies both aquifers is in contradiction with lines 145-148 and justifies my note about these lines. Please rethink and rewrite. This is not acceptable.
Line 412-413 must be confirmed with data. I think the hydrogeological model must be revisited.
Line 416 -419 needs data to justify. This is a recurrent issue and follows in the discussion.
Line 428 Fig. 7 is introduced here after the discussion of Figs. 8 and 9. This in not acceptable. Please reorganize.
Line 434 when you refer that 500Ohm.m should be unaltered granite some authors should be mentioned to justify this classification. Resistivity variation in granites is very large and your assumptions must be compared with values in the literature.
Line 435-440 Can you prove this or these are just assumptions? If so needs rewriting.
Line 443 400 Ohm.m is not a very high resistivity. Please rewrite.
Line 447 needs a reference
Line 453 Why do you justify saline water? What hydrogeological model do you have in mind to justify this?
Line 459 ok but if you start extracting and do not care about recharge saline intrusion will come in. Please rephrase.
Figues 6, 8, 9 and 10 display resistivity in a logarithm scale. Why? Usually RES2Dinv uses a numeric scale. If you have no justification I recommend (strongly) to change to leave the logarithm scale. This is a commoner procedure and easier to follow. The figures and discussion must be rearranged as fig. 7 if brought into the text after figs. 8 and 9 and, so far, there is no deep discussion about figure 7.
Fig. 11 why did you choose these three profiles? With no adequate explanation you need to show them all. Above 0.8 there is no difference in the colours.
Line 490-491 please refer to my previous note.
Line 494-495 if the figure represents the whole thickness of the aquifer so it is not possible to distinguish between the Holocene and the Pleistocene aquifers. If this is so the previous text must be rewritten as I mentioned before.
Line 499 yes because of overexploitation. I mentioned this before. Please comment as well as on the need of establishing an exploitation sustainable model.
Line 505 Aquitards?
Fig. 12 3D view?
Fig 12 needs a proper discussion. The legend is too long. Please discuss thoroughly the yellow, purple lines, etc. The results of the ERT are not easy to follow. Perhaps it would have been better to draw a map of the interpreted depths for the bedrock and for the depths of the aquifers. Fig 12 is very difficult to follow and understand.
I mentioned before the tides and their amplitudes. Is there an influence of tides on ERT data? Did authors check for eventual seasonal effects (Dry wet season)?
Final notes:
Authors must revisit the hydrogeological model, ERT data should/must be presented using a linear scale and not a logarithm scale, and the text must be improved so that the figures are introduced and discussed properly.
The English and its use must be checked out.
Therefore, it is my final opinion that is a case study, as it is the aim of the journal special issue, that could be of interest to the local community. The technique is known, the data are reliable and care was taken in their interpretation.
Authors could exploit this study with a proposition for the sustainable exploitation of the water resources along with the UN SGDs. This would be a huge contribution for this area and an example of the use of Applied Geophysics in this area. They need recharge studies, infiltration, runoff and evaporation estimates, water exploitation rates or estimates, etc,
There are several assumptions that must be checked with hydrochemical data, and many assumptions must have references as pointed out in the previous points.
It is my final opinion that this contribution must go through a deep revision before been accepted for publication.
Author Response
Dear Reviewer,
We would like to thank the reviewer for your detailed comments and the time spent on the manuscript. We would like to send the explanatory file and the revised manuscript. Please find them out in the attached file.
Best,
On behalf of the Authors,
Cong-Thi Diep
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The changes made by the authors in the revised version of the manuscript are satisfactory.
However, several journal are not cited shortened in References. They are: Hydrology and Earth System Sciences [8], Bollettino di Geofisica Teorica ed Applicata [10], Vlaamse Milieumaatschappij & Deltares [11], International Journal of Geotechnical Engineering [15], Environmental Research Letters [19], Journal of Geophysical Research [23], Geophysical Prospecting [45], Journal of Environmental Engineering Geophysics [53], Environmental monitoring and assessment [60], Earth Science and Environment [62], Natural Resources Research [64], Journal of Earth System Science [65], Translational Psychiatry [68].
Author Response
Dear Reviewer,
We deeply thank the reviewer for your detailed comments. We modified them already!
On behalf co-authors
First author
Diep Cong-Thi
Author Response File: Author Response.docx
Reviewer 4 Report
Please go through the following notes:
Dear Authors,
Thank you very much for the alterations and for the cover letter you provided.
Most of the problems are addressed and the document was improved. However I must point out:
The comments:
In the introduction line 34: what do you mean by density effects?
It was explained as expected. However, gravity is a Geophysical method which is not used commonly in the investigation of the problems addressed by this paper and it DEALS WITH DENSITY CONTRASTS. When you are writing a paper you should have in mind the general public and be careful with contents. Recently some gravity experimental observations were done in water reservoirs using an absolute gravimeter. These experiments are able to address problems related with water table variations. So be careful with the terms you use.
Line 156 : please consider to add a figure of the field layout for resistivity measurements.
What I meant, and in my opinion will help the reader, if a figure showing the gradient array and its use to obtain anERT was shown.
You are using the multiple gradient array that is not as popular as the Wenner-Schlumberger or Dipole-dipole arrays, for instance.
I don’t understand the need to explain the principles for the inversion. Nowadays there is software available and it would have been enough to mention the software and the method used for inversion.
I understand the black box problems. However you don’t provide any novelty in this section so you could refer one or two references and skip it. This is known and common among geophysicists and has been developed since the last decade of the last century.
Lines 227-230 Is the ground reasonably flat? What is the topography like if you feel you need to add these? To consider this there must be problems with the topography. If it is so what is the accuracy of the GPS and field observations?
Expected answer but you did not provide information on the accuracy of the measurements.
Line 290 Why do you use log scale for the resistivities in Fig. 5? It is commoner to use a normal scale with the RERS2Dinv and it is easier to read.
Your answer is not entirely true. The scales in RES2DINV are chosen. Furthermore, you are right that resistivity can vary several orders of magnitude. The contrast between contaminated and non-contaminated aquifers can be 1 to 10. So, to see contrasts we don’t want to smooth them. OK you can argue that transitions, particularly in SI, can be diffuse but we want boundaries and, more important, we want to see boundary change with time and space. Anyway, I respect your opinion and decision but I would not do it.
Line 434 when you refer that 500Ohm.m should be unaltered granite some authors should be mentioned to justify this classification. Resistivity variation in granites is very large and your assumptions must be compared with values in the literature.
You could add in the text: “possibly…”
I mentioned before the tides and their amplitudes. Is there an influence of tides on ERT data? Did authors check for eventual seasonal effects (Dry wet season)?
The explanation is reasonable but something on this could be said in the text. Just a remark saying that you are aware of this and that all data are from the same season.
FINAL NOTES: I don’t have objections to the publication of this work. It is my opinion that the above comments could improve the paper but I leave to the authors to consider them.
Author Response
Dear Reviewer,
We deeply thank the reviewer for your further comments. They greatly helped to improve the quality of the paper.
On behalf of co-authors
First author
Diep Cong-Thi
Author Response File: Author Response.docx
