Review of Effective Porosity in Sandstone Aquifers: Insights for Representation of Contaminant Transport

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article explores the characterization of effective porosity in sandstone aquifers and its impact on contaminant transport. It highlights that both the matrix and fractures in sandstone exhibit hydraulic conductivity, necessitating the integration of multiple datasets—from core to wellbore to field scales—for hydraulic property characterization. The hydraulic properties of the matrix serve as the foundation, as contaminants diffuse within fractured porous media. Fractures, being highly conductive, require the identification of hydraulically active rock discontinuities to ensure the reliability of discrete fracture network models. The article summarizes recent advancements in sandstone hydrogeology, such as active line-source temperature logging and tracer tests, emphasizing the importance of combining core-scale physics, wellbore hydrogeology, and tracer testing to accurately characterize contaminant transport in dual-porosity sandstones. Overall, the paper is well-structured and logically sound. I recommend acceptance after minor revisions. Below are some minor suggestions:​​
​​（1）The paper should include more relevant references from the past five years.​​
​​（2）Some equations are not formatted properly. For example, in Equation (1), the "s" in "Ts" should be a subscript.​

Author Response

This article explores the characterization of effective porosity in sandstone aquifers and its impact on contaminant transport. It highlights that both the matrix and fractures in sandstone exhibit hydraulic conductivity, necessitating the integration of multiple datasets—from core to wellbore to field scales—for hydraulic property characterization. The hydraulic properties of the matrix serve as the foundation, as contaminants diffuse within fractured porous media. Fractures, being highly conductive, require the identification of hydraulically active rock discontinuities to ensure the reliability of discrete fracture network models. The article summarizes recent advancements in sandstone hydrogeology, such as active line-source temperature logging and tracer tests, emphasizing the importance of combining core-scale physics, wellbore hydrogeology, and tracer testing to accurately characterize contaminant transport in dual-porosity sandstones.

Overall, the paper is well-structured and logically sound. I recommend acceptance after minor revisions.

Below are some minor comments:

（1）The paper should include more relevant references from the past five years.​​

REPLY: New references of the past 5 years have been added in the Paragraphs 3 and 4 (Lines 103-110).

​​（2）Some equations are not formatted properly. For example, in Equation (1), the "s" in "Ts" should be a subscript.​

REPLY: We deleted the “s” in “Ts” to avoid formatting issues. All the 3 references have also been placed in the central part to fit the formatting guidelines of Sustainability (Lines 198-204).

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is about the insights of effective porosity in sandstone aquifers on the transport of pollutants. The overall quality of the thesis is relatively good, but the following problems still exist:

1. It is necessary to specifically explain the key or uncertain parameters in the pollutant diffusion model, such as effective porosity, fracture network structure, etc.
2. Discussions on the complementarity among different testing methods need to be strengthened. For example, it is possible to explore how core physical tests, borehole geophysical tests and tracer tests complement each other, and how to integrate the results of these tests to improve the accuracy of pollutant diffusion characterization.
3. Further emphasize the contribution of the conclusion to solving practical problems.
4. Discuss the applicable scope of various detection methods, etc., and put forward corresponding insights.
5. It is suggested that the future research direction be further improved.

Author Response

This paper is about the insights of effective porosity in sandstone aquifers on the transport of pollutants. The overall quality of the thesis is relatively good, but the following problems still exist:

1. It is necessary to specifically explain the key or uncertain parameters in the pollutant diffusion model, such as effective porosity, fracture network structure, etc.

REPLY: We have now specified the key parameters difficult to determine. Note that, we now use the words “key” and “uncertainties” that improve clarity in the text (Lines 208-210, 375, 494).

2. Discussions on the complementarity among different testing methods need to be strengthened. For example, it is possible to explore how core physical tests, borehole geophysical tests and tracer tests complement each other, and how to integrate the results of these tests to improve the accuracy of pollutant diffusion characterization.

REPLY: We clarified the reason for the combination of core physical tests, borehole geophysical tests and tracer tests (Lines 508-517). Each test provides different hydraulic information.

3. Further emphasize the contribution of the conclusion to solving practical problems.

REPLY: We have empathized how this research finds practical application in the workflow of environmental agencies and industries worldwide in the discussion (Lines 504-516) and conclusion (Lines 557-565).

4. Discuss the applicable scope of various detection methods, etc., and put forward corresponding insights.

REPLY: We have provided detail on how the new insight of combining different methods can provide three different information by characterizing only two wells (Lines 531-536). Practical applications of the presented research has also been empathized in the text in other parts of the manuscript (Lines 504-516, 557-565)

5. It is suggested that the future research direction be further improved.

REPLY: We have now (i) modified the text, and (ii) added new references in the Paragraph 4 that describes the future research pathways (Lines 491-536).

 

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for the well written and interesting review article manuscript. I've attached my comments as a separate file, and I hope they're helpful.

 

Comments for author File: Comments.pdf

Author Response

I would like to thank the authors for an interesting manuscript (Review) that uses discusses various methods to characterize the hydraulic properties of fractures and matrix areas in sandstone aquifers. The manuscript was well written and interesting. My comments are separated into two sections. First, I present some comments that deal with general topics. My second section is a list of line-by-line comments that are intended to help the authors improve their manuscript. Overall, I feel that the paper only needs minor revisions to be considered ready for publication.

REPLY: Thank you for the positive comment, no action required here.

General comments

- First, I’d like to compliment the authors on the writing. They have generated a very written manuscript with beautiful figures. Also, in most cases the captions are very good and thorough enough to understand. I’ve made a few small suggestions for clarity and typographical errors.

REPLY: Thank you for this further positive comment, no action required here.

- There are a lot of sloppy errors with units and variables. Here are some examples: There are two significant errors in Line 285, which reads, “In each borehole the number of hydraulically active fractures vary from 5.8 ×10−5 to 5.8 ×10−5 ms−1 ”. First, it looks like the same numerical value has been repeated here, and therefore one of the values needs to be corrected. However, the units also don’t make sense. The number of fractures would not have units. The frequency of fractures would have units of m-1 perhaps, but ms-1 does not make sense to me here. There are other places in the manuscript where the numbers or units don’t make sense with the variable mentioned.

REPLY: Thank you, we have applied a correction (Line 292). No need of units we’re dealing with a percentage.

Line 367 states that travel times are ms-1. This is a rate, not a time. Do the authors mean flow velocities, and if so, are they Darcy or average linear velocities? Please read these carefully to see if they are what is intended.

REPLY: Average linear velocities. We have clarified the point (Lines 378-404).

- Lines 321-336 (section on turbidites of the Chatsworth Sandstone). Here the authors claim that the higher proportion of hydraulically active fractures found by [36] is related to the better accuracy of the active line source temperature sensor. However, the other sandstones compared (e.g., the Mt. Simon of Wisconsin and elsewhere) are completely different lithologies. It is unfair (and a confounding factor) to compare densely compacted turbiditic sandstones with loosely cemented quartz arenites. In turbidites, the intergranular spaces between framework sand grains are filled by mud. In contrast, quartz arenites may be variably cemented, but the cement doesn’t completely occlude the pore networks like the depositional clays do in turbidites. Isn’t it just as likely that the difference here is due to two completely different rock types (i.e., comparing apples & oranges), rather than the testing? In my sedimentological experience, I would not ignore this important confounding factor. The only way to confirm that the testing method is better is to actually compare the methods on the same rock formations. Some readers might consider this to be an attempt to mislead them. I suggest either explaining the confounding factors or drop the method comparison if it’s relying on this type of difference. Some clarification might be helpful.

REPLY: Seven new lines added to clarify the very good point highlighted by the reviewer (Lines 344-349)

- Line 350, equation 4, and elsewhere (e.g., lines 381, 386, etc.). The authors are unclear about the velocity here. Only in Line 373 do the authors describe “average linear velocity”. Yes, equation 4 is clear for the experts, but because Darcy velocity and average linear velocity might be confused by the reader, it should be explicitly defined this way in equation 4 and elsewhere where it is used (lines 381, 386, 394-395, etc.). In some cases, like line 387, the units are missing!

REPLY: We have specified that is average linear velocities (Lines 378-404). Several authors (e.g., Worthington et al. 2012 J. Hydrol.) use the average linear velocity of the tracer tests to extrapolate the effective porosity using the Equation 4.

- This is only a suggestion, but I believe MDPI journals use the Oxford comma. I suggest adding it (e.g., Line 49 and elsewhere), but I know it’s controversial for some reason.

REPLY: We have inserted the Oxford comma with consistency before “and” when a third element is mentioned (Lines e.g., 50, 56).

- Regarding other punctuation, a Google Scholar search for “St. Bees Sandstone” vs. “St Bees Sandstone” reveals that the variant with “St.” is far more common. I suggest using this to conform with the literature.

REPLY: We now stick with “St. Bees Sandstone” as suggsted by the reviewer (Lines 69, 81, 187, 232, 246, 269, 279, 287, 297, 307).

- There are several places where the authors put parenthetical expressions before the noun in which they are describing. For example, line 136-137 reads “… defined for fractured sandstone (35 x 35 x 35 m; [26]) aquifers.” In this case, “aquifers” is the noun, and “fractured sandstone” is the adjective. I suggest reordering this as “… defined for fractured sandstone aquifers (35 x 35 x 35 m; [26]).”

REPLY: Correction made as suggested by the reviewer (Line 138).

- According to the template for the journal Sustainability, the equation should be in the center of the page, with its comma, and the (equation number) should be right justified. The equations here are not like that, and Equation 2 is confusing. Is the first (2) multiplied by phi or a duplicate numbering of the equation?

REPLY: All the 4 equations are now in the centre as reqested by the reviewer. The issue on the Equation 2 has non been fixed (Lines 200, 219, 261, 367).

 

Line by line comments.

- Line 44: Please define UCSnat<20 MPa. Many hydrogeologists will be interested in reading the paper, but many will not have a geotechnical background related to fracture testing, etc. It will help the reader to briefly explain this.

REPLY: We have provided clarification in the new version (Line 40).

- Lines 85-86: I suggest using the word “article” instead of “manuscript” because once your paper is published, it’s no longer a manuscript.

REPLY: Correction made as requested by the reviewer (Line 87, 88, 138).

- Line 97: I suggest “aquifers, this”.

REPLY: Correction made as requested by the reviewer (Line 99).

- Figure 5 caption has an apparent error. Line 186 states horizontal hydraulic conductivity, but uses (Kv). This doesn’t seem correct. Also, there could be a space between Kh and (m/day) in panel d, but it’s not critical.

REPLY: I have applied the correction in the caption (Line 188).

- Lines 199-200. The v in the equation is in italics, but it’s not in the text body.

REPLY: We have now removed the Italics from all the 4 formulas.

- Line 216. Most other cases of localities mentioned have the country name. While people familiar with the United States might know where Wisconsin and New Jersey are, many people in the world don’t know. (In fact, many in the easter US don’t know where Wisconsin is, from my experience – one person though it was part of Canada!). It might be worth considering listing this, at least in the table or at the first instance.

REPLY: We have now insterted details of country / province / state in the Table 1

- Line 255 (Equation 3): Why is it Vmax’ in the equation, but there is no ‘ (prime) mark in the text body. That makes it a different variable.

REPLY: We changed the text to clarify the Vmax meaning (Lines 259-266). The key and unknown parameter of the equation is Ti and not Vmax.

- Line 271. I suggest “The same approach…”

REPLY: Change made as requested by the reviewer (Line 278).

- Table 1. I think the reader would benefit from two changes here. First, the Sustainability template document from MDPI clearly shows the table may stretch across the entire page. If the left-hand margin is reduced, there won’t be so many wrapped formation names. In addition, the location of each of these should be added to help the reader reconcile what is not visible in Figure 1 due to the scale. The locations are mentioned throughout, but not in the table? Seems like an odd omission.

REPLY: The Table 1 has been modified. The first column is now smaller and details of state / province / country has been added.

- Table 1. Spelling error for Neogene in entry 3.

REPLY: Change made as requested by the reviewer in the Table 1.

- Line 321 and elsewhere: This type of citation is not standard for MDPI. When referring directly to authors, their names should be mentioned. Therefore, the line should read “The key finding is that Bairos et al. [36] found that…”. The [36] is only a parenthetical expression, and the sentence should still read correctly if removed.

REPLY: Change made as requested the reviewer (Line 322, 328, 335, 342, 482, 513)

- This is also true for Line 447 and elsewhere.

REPLY: Change made as requested the reviewer (Line 482), as well as elsewhere (Lines 322, 328, 335, 342, 513).

- Line 381: I suggest using “to” instead of the “–” to avoid confusion and to be consistent with what is done elsewhere in the manuscript.

REPLY: Change made as requested the reviewer (Line 382).

- Line 383: I suggest using “to” instead of “and” between the values.

REPLY: Change made as requested the reviewer (Line 393).

- Line 387: What are the units for the velocity? Is this Darcy velocity or average linear velocity?

REPLY: We have specified average linear velocities and inserted the units (Lines 400-401).

- Line 396: Remove the extra space.

REPLY: Change made as requested the reviewer (Line 410).

- Line 442: Why have the exponent? Isn’t 10 sufficient?

REPLY: Change made (Line 457).

- Line 452: The parenthetical expression applies to the noun “scale” and should go after it to read “… smaller spatial scale (1-5 km2) to represent…”

REPLY: Change made as requested the reviewer (Line 467).

- Line 464: The Mt. Simon in Wisconsin is mostly marine, but earlier in the paper it was mentioned that some was fluvial.

REPLY: Clarification made as requested by the reviewer. Yes, the Mt. Simon Sandstone has abundant near-shore marine facies (Lines 270, 345).

- Line 530: I suggest “can be high in lithified and turbiditic sandstones”.

REPLY: Change made as requested by the reviewer (Line 557).

- Line 563: Why is 6. Patents listed here? Should this be deleted?

REPLY: List 6 deleted (Line 611).

- Line 708: The word “Minnelusa” should be capitalized in Reference 64.

REPLY: Change made as requested by the reviewer (Line 737).