3D Characterization of a Coastal Freshwater Aquifer in SE Malta (Mediterranean Sea) by Time-Domain Electromagnetics

Round 1

Reviewer 1 Report

This was an overall well written paper using TDEM methods in a coastal zone.  I liked the use of soundings in a geometric pattern to discern the variations in the thickness of freshwater, brackish water, and saltwater.  My major issues with the paper start with the misleading title.  Your data and analyses clearly indicate brackish water along the coast or seawater based upon the resistivities measured.  Where is the offshore connectivity demonstrated in the paper?  This is a classic example of fresh groundwater flow in fractured rock discharging to the near coast as the heads decrease from inland to coastline.  I suggest a rewrite changing the title and expanding the values of what saltwater with respect to resistivities to better match the established values from most authors.  I also did not agree with disregarding TDEM soundings that did not fit nearby curves.  Only one curve had anomalous values indicating interference.  The rest appeared smooth curves potentially indicating the variations in the fractured-rock permeability or fractures. These disregarded soundings should have been inverted and the smooth models shown.  I suspect these would have shown the fractures controlled the migration of saltwater into the carbonate aquifer.

Line 2 to 4 Misleading title since your own conclusions and data indicate you measured saltwater and brackish groundwater along the coastline of the island.  No offshore data was collected or presented in this paper.  To try to connect this land based work with offshore data which the authors admit is within a higher porosity low permeable unit is not an aquifer and clearly is not connected to the carbonate aquifer they are describing as being studied in this paper.  Change the title to better reflect what is presented data wise and analyzed land based TDEM along a coastline mapping the saltwater interface.

Line 59 You define freshwater as 10 to 100 ohm-m which is misleading.  In fact many references describe freshwater as closer to greater than 40 ohm-meters. Brackish water is less than about 40 ohm-meters to about 10 ohm-meters and saltwater less than 10 ohm-meters.

Line 57 Not sure what offshore methods have to do with this study since none were collected and were outside the study area.

Line 60 Saltwater is not 0.3 ohm-meters in my own experience and those of many other workers it's closer to 10 ohm-meters. 

Line 62 I disagree that land based EM soundings can determine hydraulic connections to offshore aquifers by themselves no since geology and hydrology data on and offshore needs to be measured and delineated to make such a determination.

Lines 78 to 91 Lack of any references for the geology and precipitation is concerning. Also the authors did not mention the high heterogeneity of the formation related to freshwater aquifer and the presence of mudstones.

Line 99 No reference for the geologic map provided and should be numbered but none listed in references list.

Line 115 The term normally used to avoid confusion is groundwater pumpage not abstraction.

Line 155 The study is hampered by the small areal extent the soundings were made within only 150 m.  My issues are the large amount of heterogeneity of the formation requires larger study areas to make conclusions on the island's hydrogeology and freshwater-saltwater interface.

Line273 I do not understand why the authors did not plot all the 1D models and see how the interface varied.  Instead they threw out soundings that deviated from the majority.  I have to object to that method since they assume interference of some kind but in fact this variation or deviation may indicate the heterogeneities in the aquifer but do not make for smooth cross sections.

Line 281 This curve is smooth and does not indicate anomalous measurements but rather may show variation in the depth to the interface due to geology variations.

Line 365 Why use base 10 logarithm for the resistivity scale?  Because 2 ohm-meters is saltwater and there isn't enough contrast.  The saltwater interface is onshore here and should be noted.  Probably due to groundwater withdrawals exceeding precipitation recharge.  Another proof that there is no freshwater offshore in an aquifer capable of being pumped.

Line 390 Making a statement about a regional part of the island when only 150 meters of land was surveyed is an overreach. 

Line 399 The authors state for the first time the low elevation of the site at only 10m this should have been mentioned in the description of the study area earlier in the paper.

Line 405 Use of the Ghyben-Hertzberg ration should have a reference, and it does not apply to the hydrogeology of this island.  The ration applies only to unconsolidated, homogeneous, and isotropic aquifers.  Since the authors present NO water level elevations from any observation wells how can they make such a statement about the island?  And second the fractured-rock aquifer system would not conform to such a definition.  Had the authors measured the depth to the interface further inland and presented such data along with the hydrology data such a statement would be supported.

Line 412 The authors statement that no evidence of an offshore extension to the freshwater was found supports my argument that the title of this paper is misleading and should be changed to reflect what was actually accomplished a coastal TDEM survey.

Line 413 The authors state there are indications of saltwater intrusion.  The fact that such low resistivities were measured in saturated carbonates indicates saltwater has intruded the aquifer and despite the fact that the land surface is 10m above sea-level is in fact intruded when one considers groundwater is being pumped for supply and the arid climate.

Line 418 and 419  The authors claim that offshore EM surveys found resistive units which fresh water is controversial.  Their own coastal data indicates saltwater has intruded the aquifer system and the unit they theorize is fresh or freshened is a lower permeable unit. Coastal borehole EM and resistivity logging along the eastern part of the United States in coastal communities indicates aquifers when intruded with saltwater show a reversal of the normal resistivity response so that aquifers are now electrically conductive and clay units are fresher or resistivity in comparison.

 

 

 

Author Response

Please see the attachment.

P. Pondthai, et al.

Author Response File: Author Response.docx

Reviewer 2 Report

In general, I can conclude that the attached scientific paper presents an interesting topic for the determination of non-saline aquifers in the coastal zone. The content of the article is well organized and the geophysical TDEM method is clearly described. Nevertheless, I have some questions and suggestions for the authors:

1) I think a lithological profile is needed to make the geological description more comprehensible.

2) In Figure 1, the breaks should be marked with a color that is more visible. The Maghlaq fracture is poorly seen.

3) I wonder if it is possible to include data from potential wells drilled for other purposes in a given area.

4) I have to say that the greater lateral variability in the direction perpendicular to the coast is not surprising to me, so I think more frequent measurements should be made in this direction.

5) Why did you not compare the results with another geophysical method (VES)?

6) Given that we are dealing with indirect interpretation, would I expect some more attention to assess the reliability of the model? (Model calibration)

Author Response

Please see the attachment.

P. Pondthai, et al.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

See the attached file.  

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

Please see the attachment.

 

Best regards,

Potpreecha Pondthai

Author Response File: Author Response.docx

Reviewer 2 Report

I have no more remarks!

Author Response

Dear Reviewer, 

We appreciate your effort for reviewing our revised manuscript.

 

Best regards,

P. Pondthai, et al. 

Round 3

Reviewer 1 Report

I appreciate the author's response to my comments and their effort rewriting the manuscript.  My only comment is reference #39 is a personal communication and I do not believe that is allowed.  I suggest removing it. I noticed a handful of grammer issues perhaps a quick editorial review and you should be good to proceed.