Nitrate–Conductivity Correlations in Aqueous Environments: From Standard Solutions to Natural Water Bodies

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Graphical Abstract

• Provide a graphical abstract

 

Abstract

• Include key quantitative relationships (e.g., R² values or correlation strength) for seawater, groundwater, and rainwater to clarify the practical significance.

 

1. Introduction

• Lines 85–90: The motivation for the study is mentioned, but the primary research objective is not clearly or explicitly stated. Consider revising these lines to include a clear sentence.
• Lines 29-34: The Introduction discusses general nitrate pollution and health impacts well but does not emphasize the importance of the study in the context of water-scarce or arid regions like Saudi Arabia, where low-cost proxy monitoring could be especially valuable.
• Lines 77–84: The discussion on prior work establishing EC-NO₃⁻ correlations is comprehensive. However, the specific limitations or gaps in previous studies are not well defined.

 

2. Materials and Methods

• Lines 91–122: The rationale for selecting sample locations (e.g., Wadi Al Batin vs. Hafr Al Batin City) is well-described, but lacks justification for the number of samples (e.g., why 5 seawater or 32 shallow wells?). Consider adding a sentence to clarify how these numbers were determined (e.g., statistical power, representativeness).
• Lines 123–129: Good calibration method for ISE, but there's no mention of inter-instrument agreement or cross-validation between the ISE and YSI 9500. This weakens confidence in consistency. Please clarify.
• Lines 130–142: The photometric procedure is detailed, but the chemical preparation steps (e.g., Nitratest reagents) could benefit from clearer references or protocols, especially since they are key to nitrate determination accuracy.

 

3. Results and Discussion

3.2. Standard and Reference Samples

• Lines 216–240: This section presents a strong case for linear correlation. However, it's missing standard error or confidence interval data on the linear regression—include these to better demonstrate reliability.

3.3. Seawater

• Lines 243–272: The nonlinear nitrate behavior in seawater is a critical insight. However, the explanation (e.g., “non-ideal mixing behavior”) is speculative without supporting ionic activity data. Including ionic speciation or salinity data would strengthen this interpretation.

3.4. Rainwater

• Lines 274–298: While the influence of atmospheric CO₂ and organics is mentioned, these were not measured. Please clarify whether any TOC (Total Organic Carbon) or sulfate/chloride levels were analyzed to support this interpretation.

3.5. Groundwater: Lines 300–344: This section needs significant clarification:

• Were the shallow and deep wells sampled from the same region?
• Were any wells treated before testing? If so, how?
• Line 333–334 mentions "ion exchange" and treatment, but the methodology section does not state this.

3.6–3.8. Comparative Analysis and TDS/ORP

• Lines 346–422: These sections are strong in data presentation, but lack statistical comparison (e.g., ANOVA or t-tests) across water types. The “moderate” or “weak” classifications should be quantified (i.e., R² values with interpretation thresholds)..

 

4. Figures and Tables

• Figure 3b, 4, and 5: Legends and axis labels need improvement. For example, add units and sample IDs where relevant.
• Table 2 (Lines 363 onward): Excellent summary. However, include numerical correlation coefficients (R or R²) alongside qualitative terms like "strong" or "weak."

 

5. Future Works and Conclusion

• Lines 424–435 (Future Works): Excellent suggestions.
• Specify what type of machine learning models might be useful (e.g., regression trees, ANN).
• Mention potential datasets or sources for training models.
• Lines 437–460 (Conclusion): Strong and concise. Could be improved by emphasizing practical applications (e.g., rapid nitrate screening tools for municipalities).

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The relationship between conductivity and TDS is well studied, not too many studied the correlation between conductivity and Nitrate, as Nitrate is normally a minor portion of the aquatic environment. 

 

Here, it shows some interesting results, but the correlation relationship between conductivity and nitrate remains doubts, as in real water bodies, nitrate only accounts for less than 10% of the TDS, so what is the real impacts on applications?

 

The experimental results show some good regression relationship, but how to justify or avoid the interfacial impact from Natural water bodies. Did not see any strong evidence to justify this. Especially the result in 3~~5, can not really support this statement.

 

Dilution method is designed as main experimental method, but is this method really valid or suitable for this kind of experiment? Did not see its rationale.

 

 

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Kindly find the attachment.

Comments for author File: Comments.pdf

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Dear author,

your paper "Nitrate-Conductivity Correlations in Aqueous Environments: From Standard Solutions to Natural Water Bodies" deals with the relationship between nitrate concentration and the electrical conductivity in different solutions, e.g. standard solutions, seawater, rainwater, and groundwater. The established relationships describe the applicability of an approach for calibration and nitrate estimation using the EC parameter.

Unfortunately, the relationship in standard solutions is the only valid to confirm the approach, as expected. The relationships are not valid for the environmental samples, especially seawater (as expected), because of the complex water matrix, including the ionic composition of the solutions.

Although not stated in the title, abstract, and keywords, relationships between TDS and EC and ORP and EC were also included for completeness, just to confirm the lack of such relationships in environmental samples.

General comments:

1. Please include the equations and R2 in all the figures - e.g. Fig 1 and Fig 2a.
2. Please omit the "et al." and the year of publication when citing previous works. Please rephrase where necessary.
3. Paragraph 3.6. represents a comparative analysis, but it essentially repeats the conclusions drawn in all the previous 5 paragraphs (3.1. - 3.5.)
4. Table 2 should be formatted in the same font and size as the text.
5. Paragraph 3.7. and Figure 6: As newly introduced, the relationship between TDS and EC should be presented for each water matrix - standard solutions, seawater, rainwater, and groundwater. In such a way, more meaningful R2 will be obtained individually, not collectively.
6. The same applies to paragraph 3.8. and Figure 7, where the worst relationship was drawn - between ORP and EC.

Comments on the Quality of English Language

Dear Editor,

The paper "Nitrate-Conductivity Correlations in Aqueous Environments: From Standard Solutions to Natural Water Bodies" needs some minor corrections, if accepted.

Best regards,

Tony

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The paper is generally well organized and well written, and the figures and tables are clear and appropriate.  And the paper appears to include numerous new analyses of the nitrate content of some shallow and deeper groundwater, though there is no supporting information on the samples.  However, there is no reason to believe that conductivity could be a good measure of nitrate content of most waters and the author provides no information to support this idea.  Nitrate is not an important ion in most “natural” water, and when nitrate content is high, so are the concentrations of numerous other cations and anions that forms the basis of solution conductivity. I do not believe that this paper could be revised to be suitable for publication.  However, the groundwater samples might form the basis for a publication.

 

  I have made comments and suggested rewording  in an annotated .pdf, which I attach.

Comments for author File: Comments.pdf

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript meets the basic criteria 

Author Response

We are sincerely thankful for your thoughtful evaluation and for recommending that “the revised manuscript meets the basic criteria.” We greatly appreciate your constructive feedback throughout the review process, which has significantly contributed to improving the quality and clarity of our work.

Reviewer 3 Report

Comments and Suggestions for Authors

Authors have improved their manuscript and can be accepted in its current form.

Author Response

We sincerely thank the reviewer for their time, constructive feedback, and thoughtful evaluation throughout the review process. We are especially grateful for your positive recommendation:

"Authors have improved their manuscript and can be accepted in its current form."

We truly appreciate your support and are pleased that the revisions have met your expectations. Your comments have contributed significantly to enhancing the clarity and quality of our manuscript.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors,

The paper has been sufficiently revised and can be considered for publication in Nitrogen.

Author Response

We sincerely thank you for your time and effort in reviewing our manuscript. We are truly grateful for your positive assessment and for recommending that “the paper has been sufficiently revised and can be considered for publication in Nitrogen.”

Your constructive feedback throughout the revision process was highly valuable and has significantly contributed to improving the clarity and quality of the manuscript.